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Sample records for crystal field

  1. Crystal Field Handbook

    NASA Astrophysics Data System (ADS)

    Newman, D. J.; Ng, Betty

    2007-09-01

    List of contributors; Preface; Introduction; 1. Crystal field splitting mechanisms D. J. Newman and Betty Ng; 2. Empirical crystal fields D. J. Newman and Betty Ng; 3. Fitting crystal field parameters D. J. Newman and Betty Ng; 4. Lanthanide and actinide optical spectra G. K. Liu; 5. Superposition model D. J. Newman and Betty Ng; 6. Effects of electron correlation on crystal field splitting M. F. Reid and D. J. Newman; 7. Ground state splittings in S-state ions D. J. Newman and Betty Ng; 8. Invariants and moments Y. Y. Yeung; 9. Semiclassical model K. S. Chan; 10. Transition intensities M. F. Reid; Appendix 1. Point symmetry D. J. Newman and Betty Ng; Appendix 2. QBASIC programs D. J. Newman and Betty Ng; Appendix 3. Accessible program packages Y. Y. Yeung, M. F. Reid and D. J. Newman; Appendix 4. Computer package CST Cz. Rudowicz; Bibliography; Index.

  2. Crystal field and magnetic properties

    NASA Technical Reports Server (NTRS)

    Flood, D. J.

    1977-01-01

    Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at theta = 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) x 10 to the -6th Weber m/kg Tesla. The saturation moment is 3.84 + or - 1 - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is on the order of 160 to 180 K.

  3. Crystal-field effects in fluoride crystals for optical refrigeration

    SciTech Connect

    Hehlen, Markus P

    2010-01-01

    The field of optical refrigeration of rare-earth-doped solids has recently seen an important breakthrough. The cooling of a YLiF{sub 4} (YLF) crystal doped with 5 mol% Yb3+ to 155 K by Seletskiy et al [NPhot] has surpassed the lowest temperatures ({approx}170 K for {approx}100 mW cooling capacity) that are practical with commercial multi-stage thermoelectric coolers (TEC) [Glaister]. This record performance has advanced laser cooling into an application relevant regime and has put first practical optical cryocoolers within reach. The result is also relevant from a material perspective since for the first time, an Yb3+-doped crystal has outperformed an Yb3+-doped glass. The record temperature of 208 K was held by the Yb3+-doped fluorozirconate glass ZBLAN. Advanced purification and glass fabrication methods currently under development are expected to also advance ZBLAN:Yb3+ to sub-TEC temperatures. However, recent achievements with YLF:Yb3+ illustrate that crystalline materials may have two potentially game-changing advantajes over glassy materials. First, the crystalline environment reduces the inhomogeneous broadening of the Yb3+ electronic transitions as compared to a glassy matrix. The respective sharpening of the crystal-field transitions increases the peak absorption cross section at the laser excitation wavelength and allows for more efficient pumping of the Yb3+ ions, particularly at low temperatures. Second, many detrimental impurities present in the starting materials tend to be excluded from the crystal during its slow growth process, in contrast to a glass where all impurities present in the starting materials are included in the glass when it is formed by temperature quenching a melt. The ultra high purity required for laser cooling materials [PRB] therefore may be easier to realize in crystals than in glasses. Laser cooling occurs by laser excitation of a rare-earth ion followed by anti-Stokes luminescence. Each such laser-cooling cycle extracts

  4. Crystal growth under external electric fields

    SciTech Connect

    Uda, Satoshi; Koizumi, Haruhiko; Nozawa, Jun; Fujiwara, Kozo

    2014-10-06

    This is a review article concerning the crystal growth under external electric fields that has been studied in our lab for the past 10 years. An external field is applied electrostatically either through an electrically insulating phase or a direct injection of an electric current to the solid-interface-liquid. The former changes the chemical potential of both solid and liquid and controls the phase relationship while the latter modifies the transport and partitioning of ionic solutes in the oxide melt during crystallization and changes the solute distribution in the crystal.

  5. Unifying the crystallization behavior of hexagonal and square crystals with the phase-field-crystal model

    NASA Astrophysics Data System (ADS)

    Tao, Yang; Zheng, Chen; Jing, Zhang; Yongxin, Wang; Yanli, Lu

    2016-03-01

    By employing the phase-field-crystal models, the atomic crystallization process of hexagonal and square crystals is investigated with the emphasis on the growth mechanism and morphological change. A unified regime describing the crystallization behavior of both crystals is obtained with the thermodynamic driving force varying. By increasing the driving force, both crystals (in the steady-state) transform from a faceted polygon to an apex-bulged polygon, and then into a symmetric dendrite. For the faceted polygon, the interface advances by a layer-by-layer (LL) mode while for the apex-bulged polygonal and the dendritic crystals, it first adopts the LL mode and then transits into the multi-layer (ML) mode in the later stage. In particular, a shift of the nucleation sites from the face center to the area around the crystal tips is detected in the early growth stage of both crystals and is rationalized in terms of the relation between the crystal size and the driving force distribution. Finally, a parameter characterizing the complex shape change of square crystal is introduced. Project supported by the National Natural Science Foundation of China (Grant Nos. 54175378, 51474176, and 51274167), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2014JM7261), and the Doctoral Foundation Program of Ministry of China (Grant No. 20136102120021).

  6. Consistent Hydrodynamics for Phase Field Crystals.

    PubMed

    Heinonen, V; Achim, C V; Kosterlitz, J M; Ying, See-Chen; Lowengrub, J; Ala-Nissila, T

    2016-01-15

    We use the amplitude expansion in the phase field crystal framework to formulate an approach where the fields describing the microscopic structure of the material are coupled to a hydrodynamic velocity field. The model is shown to reduce to the well-known macroscopic theories in appropriate limits, including compressible Navier-Stokes and wave equations. Moreover, we show that the dynamics proposed allows for long wavelength phonon modes and demonstrate the theory numerically showing that the elastic excitations in the system are relaxed through phonon emission. PMID:26824543

  7. Phase-Field Simulations of Crystal Growth

    NASA Astrophysics Data System (ADS)

    Plapp, Mathis

    2010-07-01

    This course gives an elementary introduction to the phase-field method and to its applications for the modeling of crystal growth. Two different interpretations of the phase-field variable are given and discussed. It can be seen as a physical order parameter that characterizes a phase transition, or as a smoothed indicator function that tracks domain boundaries. Elementary phase-field models for solidification and epitaxial growth are presented and are applied to the dendritic growth of a pure substance and the step-flow growth on a vicinal surface.

  8. Phase-Field Crystals with Elastic Interactions

    SciTech Connect

    Stefanovic, Peter; Provatas, Nikolas; Haataja, Mikko

    2006-06-09

    We report on a novel extension of the recently introduced phase-field crystal (PFC) method [Elder et al., Phys. Rev. Lett. 88, 245701 (2002)], which incorporates elastic interactions as well as crystal plasticity and diffusive dynamics. In our model, elastic interactions are mediated through wave modes that propagate on time scales many orders of magnitude slower than atomic vibrations but still much faster than diffusive time scales. This allows us to preserve the quintessential advantage of the PFC model: the ability to simulate atomic-scale interactions and dynamics on time scales many orders of magnitude longer than characteristic vibrational time scales. We demonstrate the two different modes of propagation in our model and show that simulations of grain growth and elastoplastic deformation are consistent with the microstructural properties of nanocrystals.

  9. Oxidation and crystal field effects in uranium

    SciTech Connect

    Tobin, J. G.; Booth, C. H.; Shuh, D. K.; van der Laan, G.; Sokaras, D.; Weng, T. -C.; Yu, S. W.; Bagus, P. S.; Tyliszczak, T.; Nordlund, D.

    2015-07-06

    An extensive investigation of oxidation in uranium has been pursued. This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide (UO2), uranium trioxide (UO3), and uranium tetrafluoride (UF4). As a result, a discussion of the role of non-spherical perturbations, i.e., crystal or ligand field effects, will be presented.

  10. Oxidation and crystal field effects in uranium

    NASA Astrophysics Data System (ADS)

    Tobin, J. G.; Yu, S.-W.; Booth, C. H.; Tyliszczak, T.; Shuh, D. K.; van der Laan, G.; Sokaras, D.; Nordlund, D.; Weng, T.-C.; Bagus, P. S.

    2015-07-01

    An extensive investigation of oxidation in uranium has been pursued. This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide (U O2) , uranium trioxide (U O3) , and uranium tetrafluoride (U F4) . A discussion of the role of nonspherical perturbations, i.e., crystal or ligand field effects, will be presented.

  11. Effects of magnetic fields on dissolution of arthritis causing crystals

    NASA Astrophysics Data System (ADS)

    Takeuchi, Y.; Iwasaka, M.

    2015-05-01

    The number of gout patients has rapidly increased because of excess alcohol and salt intake. The agent responsible for gout is the monosodium urate (MSU) crystal. MSU crystals are found in blood and consist of uric acid and sodium. As a substitute for drug dosing or excessive water intake, physical stimulation by magnetic fields represents a new medical treatment for gout. In this study, we investigated the effects of a magnetic field on the dissolution of a MSU crystal suspension. The white MSU crystal suspension was dissolved in an alkaline solution. We measured the light transmission of the MSU crystal suspension by a transmitted light measuring system. The magnetic field was generated by a horizontal electromagnet (maximum field strength was 500 mT). The MSU crystal suspension that dissolved during the application of a magnetic field of 500 mT clearly had a higher dissolution rate when compared with the control sample. We postulate that the alkali solution promoted penetration upon diamagnetic rotation and this magnetic field orienting is because of the pronounced diamagnetic susceptibility anisotropy of the MSU crystal. The results indicate that magnetic fields represent an effective gout treatment approach.

  12. Enhancement of crystal homogeneity of protein crystals under application of an external alternating current electric field

    SciTech Connect

    Koizumi, H.; Uda, S.; Fujiwara, K.; Nozawa, J.; Tachibana, M.; Kojima, K.

    2014-10-06

    X-ray diffraction rocking-curve measurements were performed on tetragonal hen egg white (HEW) lysozyme crystals grown with and without the application of an external alternating current (AC) electric field. The crystal quality was assessed by the full width at half maximum (FWHM) value for each rocking curve. For two-dimensional maps of the FWHMs measured on the 440 and the 12 12 0 reflection, the crystal homogeneity was improved under application of an external electric field at 1 MHz, compared with that without. In particular, the significant improvement of the crystal homogeneity was observed for the 12 12 0 reflection.

  13. Crystal growth under microgravity conditions with using of magnetic fields

    NASA Astrophysics Data System (ADS)

    Feonychev, A.; Bondareva, N.

    The peculiarities of melt flows and crystal growth by the Bridgman and floating zone methods aboard spacecrafts under the action of steady axial or rotating magnetic field are considered. Steady magnetic field can minimize adverse effect of residual accelerations and vibrations on dopant segregation in crystals growing by the Bridgman method but it requires using strong magnetic fields, which induces specific oscillations. Under strong convection in terrestrial conditions steady magnetic field gives positive effect. Under growth of small-sized crystals by the floating zone method in microgravity conditions an use of steady magnetic field brings into dramatic increase of radial segregation due to convective vortex to free fluid surface. The flows being created by rotating magnetic field and resultant under combination of Marangoni convection with rotating magnetic field were studied for wide range of parameters including the regimes of oscillatory (turbulent) convection. Mathematical model and computer program was tested by published results of two experiments. The dependence of transition from laminar to oscillatory flow was obtained for different boundary conditions, geometric parameters of fluid and intensity of magnetic field. Specific oscillations with very low frequency and oscillations of the beating type had been discovered under the action rotating magnetic field on Marangoni convection. The mutual influence of rotating magnetic field and thermocapillary convection on flow stability was noted. Use of rotating magnetic field under crystal growth by floating zone method leads to reduction of azimuth velocity which is responsible for origin of oscillatory convection and striation of crystals. It was shown on concrete examples that there is a possibility to reduce radial segregation under optimization of rotating velocity and intensity of magnetic field. For the Bridgman method (in general for ampoule methods of crystal growth), the use of rotating magnetic

  14. Energy levels fitting and crystal-field calculations of Nd3+ doped in GYSGG crystal

    NASA Astrophysics Data System (ADS)

    Gao, Jinyun; Zhang, Qingli; Sun, Dunlu; Luo, Jianqiao; Liu, Wenpeng; Yin, Shaotang

    2012-10-01

    The single crystal Nd3+-doped in GdY2Sc2Ga3O12 (Nd3+:GYSGG) was grown by Czochralski method successfully, and its absorption spectra was analyzed in a wider spectral wavelength range at 7.6 K and 300 K, respectively. The free-ions and crystal-field parameters were fitted to the experimental energy levels at 7.6 K and 300 K with the root mean square deviation of 11.25 and 12.48 cm-1, respectively. According to the crystal-field calculations, 116 levels of Nd3+ at 7.6 K and 114 levels of Nd3+ at 300 K were assigned. The fitting results of free-ions and crystal-field parameters were compared with those already reported of Nd3+:GSGG and Nd3+:YSAG. The results indicated that the free-ions parameters are similar to those of the Nd3+ in GYSGG, GSGG and YSAG crystals, and the crystal-field interaction of GSGG and YSAG is stronger than that of GYSGG, which results in the dual-wavelength properties of Nd3+:GYSGG crystal.

  15. Electric-field-assisted convective assembly of colloidal crystal coatings.

    PubMed

    Kleinert, Jairus; Kim, Sejong; Velev, Orlin D

    2010-06-15

    A new technique that combines evaporative convective deposition of colloidal crystal coatings with an electric field to achieve more rapid assembly and reduce the defects in the crystal structure is reported. When an ac voltage is applied across the particle suspension and the substrate in the convective assembly process, a longer film spreads from the meniscus by the electrowetting-on-dielectric (EWOD) effect. The data suggest that the EWOD-increased liquid surface area results in increased evaporation-driven particle flux and crystal assembly that is up to five times more rapid. The extended drying film also provides more time for particle rearrangement before the structure becomes fixed, resulting in formation of crystal domains an order of magnitude larger than those deposited by convective assembly alone. The results demonstrate that EWOD is a facile tool for controlling particle assembly processes in wetting films. The technique could be used in improved large-scale colloidal crystal coating processes. PMID:20465234

  16. Crystal field spectra of lunar pyroxenes.

    NASA Technical Reports Server (NTRS)

    Burns, R. G.; Abu-Eid, R. M.; Huggins, F. E.

    1972-01-01

    Absorption spectra in the visible and near infrared regions have been obtained for pyroxene single crystals in rocks from the Apollo 11, 12, 14, and 15 missions. The polarized spectra are compared with those obtained from terrestrial calcic clinopyroxenes, subcalcic augites, pigeonites, and orthopyroxenes. The lunar pyroxenes contain several broad, intense absorption bands in the near infrared, the positions of which are related to bulk composition, Fe(2+) site occupancy and structure type of the pyroxene. The visible spectra contain several sharp, weak peaks mainly due to spin-forbidden transitions in Fe(2+). Additional weak bands in this region in Apollo 11 pyroxenes are attributed to Ti(3+) ions. Spectral features from Fe(3+), Mn(2+), Cr(3+), and Cr(2+) were not observed.

  17. Magnetic Field Applications in Semiconductor Crystal Growth and Metallurgy

    NASA Technical Reports Server (NTRS)

    Mazuruk, Konstantin; Ramachandran, Narayanan; Grugel, Richard; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    The Traveling Magnetic Field (TMF) technique, recently proposed to control meridional flow in electrically conducting melts, is reviewed. In particular, the natural convection damping capability of this technique has been numerically demonstrated with the implication of significantly improving crystal quality. Advantages of the traveling magnetic field, in comparison to the more mature rotating magnetic field method, are discussed. Finally, results of experiments with mixing metallic alloys in long ampoules using TMF is presented

  18. Electromagnetic Field Effects in Semiconductor Crystal Growth

    NASA Technical Reports Server (NTRS)

    Dulikravich, George S.

    1996-01-01

    This proposed two-year research project was to involve development of an analytical model, a numerical algorithm for its integration, and a software for the analysis of a solidification process under the influence of electric and magnetic fields in microgravity. Due to the complexity of the analytical model that was developed and its boundary conditions, only a preliminary version of the numerical algorithm was developed while the development of the software package was not completed.

  19. Ferromagnetic Switching of Knotted Vector Fields in Liquid Crystal Colloids.

    PubMed

    Zhang, Qiaoxuan; Ackerman, Paul J; Liu, Qingkun; Smalyukh, Ivan I

    2015-08-28

    We experimentally realize polydomain and monodomain chiral ferromagnetic liquid crystal colloids that exhibit solitonic and knotted vector field configurations. Formed by dispersions of ferromagnetic nanoplatelets in chiral nematic liquid crystals, these colloidal ferromagnets exhibit spontaneous long-range alignment of magnetic dipole moments of individual platelets, giving rise to a continuum of the magnetization field M(r). Competing effects of surface confinement and chirality prompt spontaneous formation and enable the optical generation of localized twisted solitonic structures with double-twist tubes and torus knots of M(r), which exhibit a strong sensitivity to the direction of weak magnetic fields ∼1  mT. Numerical modeling, implemented through free energy minimization to arrive at a field-dependent three-dimensional M(r), shows a good agreement with experiments and provides insights into the torus knot topology of observed field configurations and the corresponding physical underpinnings. PMID:26371682

  20. Control over colloidal crystallization by shear and electric fields

    NASA Astrophysics Data System (ADS)

    Wu, Y. L.

    2007-05-01

    We used shear flow and an electric field to control colloidal crystallization. The structures were examined in situ with confocal microscopy. For experiments under shear, a new parallel plate shear cell was designed. It had a zero-velocity plane that was stationary with respect to the microscope. The plates were microscopy slides of a few square centimeters. They both had a maximum travel of 1 cm and for the systems that we study an oscillatory shear with such a large amplitude can be regarded as a continuous shear. The gap width was variable between 20- 200 m. The plates were parallel within 1-2 m. The flow profile of a dilute suspension was linear, but that of a crystallizing dispersion appeared to be non-linear. At the walls a sliding layer structure formed of which the local viscosity was 1.5 times smaller than that of the liquid-like structure in the middle of the gap. At low shear rates hexagonal layers of a crystal showed a zigzag motion. We also examined shear melting and crystallization. Shear melting appeared to be a local process in which the local order sometimes increased and sometimes decreased while the average order decreased. Shear induced crystallization occurred uniformly over the sample and the order increased monotonously. By spin coating colloidal crystalline films were fabricated that consisted of randomly stacked hexagonal layers. The structures could be made permanent by using a dispersing medium that could be polymerized. The interparticle spacing was not fixed, but probably depended subtly on the surface charge of the particles and the ionic strength of the medium. Different from crystals formed by sedimentation, spin coated crystals were crystalline to the top. This is one of the indications that crystal formation started at the air interface with a 2D layer that grew into a 3D multilayer structure while it was sedimenting. From these spin coated crystals freestanding colloidal crystalline films could be made. Without crack

  1. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Schweizer, M.; Cobb, S. D.; Walker, J. S.; Szofran, F. R.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    A series of (100)-oriented gallium-doped germanium crystals have been grown by the Bridgman method and under the influence of a rotating magnetic field (RMF). The RMF has a marked affect on the interface shape, changing it from concave to nearly flat. The onset of time-dependent flow instabilities occurs when the critical magnetic Taylor number is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. The critical magnetic Taylor number is a sensitive function of the aspect ratio and, as the crystal grows under a constant applied magnetic field, the induced striations change from nonperiodic to periodic, undergo a period-doubling transition, and then cease to exist. Also, by pulsing the RMF on and off, it is shown that intentional interface demarcations can be introduced.

  2. Tailor-made force fields for crystal-structure prediction.

    PubMed

    Neumann, Marcus A

    2008-08-14

    A general procedure is presented to derive a complete set of force-field parameters for flexible molecules in the crystalline state on a case-by-case basis. The force-field parameters are fitted to the electrostatic potential as well as to accurate energies and forces generated by means of a hybrid method that combines solid-state density functional theory (DFT) calculations with an empirical van der Waals correction. All DFT calculations are carried out with the VASP program. The mathematical structure of the force field, the generation of reference data, the choice of the figure of merit, the optimization algorithm, and the parameter-refinement strategy are discussed in detail. The approach is applied to cyclohexane-1,4-dione, a small flexible ring. The tailor-made force field obtained for cyclohexane-1,4-dione is used to search for low-energy crystal packings in all 230 space groups with one molecule per asymmetric unit, and the most stable crystal structures are reoptimized in a second step with the hybrid method. The experimental crystal structure is found as the most stable predicted crystal structure both with the tailor-made force field and the hybrid method. The same methodology has also been applied successfully to the four compounds of the fourth CCDC blind test on crystal-structure prediction. For the five aforementioned compounds, the root-mean-square deviations between lattice energies calculated with the tailor-made force fields and the hybrid method range from 0.024 to 0.053 kcal/mol per atom around an average value of 0.034 kcal/mol per atom. PMID:18642947

  3. Birefringence of the antiferromagnetic crystals linear in a magnetic field

    NASA Astrophysics Data System (ADS)

    Eremenko, V. V.; Kharchenko, N. F.; Beliy, L. I.; Tutakina, O. P.

    1980-01-01

    The new linear magneto-optical effect-birefringence-of a linear polarized light which is directly proportional to the magnetic field strength has been observed. This effect is permitted in crystals which allow piezo-magnetic properties. One was studied in antiferromagnet CoF 2 and CoCO 3 for the longitudinal geometry of an experiment.

  4. Reflective liquid crystal light valve with hybrid field effect mode

    NASA Technical Reports Server (NTRS)

    Boswell, Donald D. (Inventor); Grinberg, Jan (Inventor); Jacobson, Alexander D. (Inventor); Myer, Gary D. (Inventor)

    1977-01-01

    There is disclosed a high performance reflective mode liquid crystal light valve suitable for general image processing and projection and particularly suited for application to real-time coherent optical data processing. A preferred example of the device uses a CdS photoconductor, a CdTe light absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The non-coherent light image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the AC voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal thus modulates the polarization of the coherent read-out or projection light responsively to the non-coherent image. An analyzer is used to create an intensity modulated output beam.

  5. Field Stability of Piezoelectric Shear Properties in PIN-PMN-PT Crystals Under Large Drive Field

    PubMed Central

    Zhang, Shujun; Li, Fei; Luo, Jun; Xia, Ru; Hackenberger, Wesley; Shrout, Thomas R.

    2013-01-01

    The coercive fields (EC) of Pb(In0.5Nb0.5)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) ternary single crystals were found to be 5 kV/cm, double the value of binary Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMNT) crystals, further increased to 6 to 9 kV/cm using Mn modifications. In addition to an increased EC, the acceptor modification resulted in the developed internal bias (Eint), on the order of ~1 kV/cm. The piezoelectric shear properties of unmodified and Mn-modified PIN-PMN-PT crystals with various domain configurations were investigated. The shear piezoelectric coefficients and electromechanical coupling factors for different domain configurations were found to be >2000 pC/N and >0.85, respectively, with slightly reduced properties observed in Mn-modified tetragonal crystals. Fatigue/cycling tests performed on shear-mode samples as a function of ac drive field level demonstrated that the allowable ac field levels (the maximum applied ac field before the occurrence of depolarization) were only ~2 kV/cm for unmodified crystals, less than half of their coercive field. Allowable ac drive levels were on the order of 4 to 6 kV/cm for Mn-modified crystals with rhombohedral/orthorhombic phase, further increased to 5 to 8 kV/cm in tetragonal crystals, because of their higher coercive fields. It is of particular interest that the allowable ac drive field level for Mn-modified crystals was found to be ≥60% of their coercive fields, because of the developed Eint, induced by the acceptor-oxygen vacancy defect dipoles. PMID:21342812

  6. TOPICAL REVIEW: Organic field-effect transistors using single crystals

    NASA Astrophysics Data System (ADS)

    Hasegawa, Tatsuo; Takeya, Jun

    2009-04-01

    Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for 'plastic electronics'. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20-40 cm2 Vs-1, achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps.

  7. Low-field susceptibility anisotropy of some biotite crystals

    NASA Astrophysics Data System (ADS)

    Zapletal, Karel

    1990-10-01

    The low-field magnetic susceptibility anisotropy (LMA) of weakly magnetic rocks is dominated by paramagnetic minerals among which micas, and mainly biotite, is important. For this reason, the LMA of biotite crystals was investigated in detail. Natural biotite crystals (from ten localities) having a wide range of iron concentration were also studied by other methods, including optical microscopy, X-ray microanalysis, Mössbauer spectroscopy and induced isothermal remanent magnetization. Ferromagnetic inclusions disturbing the magnetic properties of biotite were revealed in some crystals. The measured mean bulk susceptibility of pure crystals (four localities) ranges from 1.0 × 10 -3 to 1.8 × 10 -3 SI and agrees with the susceptibility calculated from the iron concentration (ranging from 12 to 20 wt.%) determined for each specimen. The susceptibility ellipsoid of pure biotite crystals is rotational about the minimum susceptibility direction parallel to the crystallographic c'-axis, and the anisotropy degree ranges from 1.34 to 1.36.

  8. Magnetic field controlled FZ single crystal growth of intermetallic compounds

    NASA Astrophysics Data System (ADS)

    Hermann, R.; Behr, G.; Gerbeth, G.; Priede, J.; Uhlemann, H.-J.; Fischer, F.; Schultz, L.

    2005-02-01

    Intermetallic rare-earth-transition-metal compounds with their coexistence of magnetic ordering and superconductivity are still of great scientific interest. The crystal growth of bulk single crystals is very often unsuccessful due to an unfavorable solid-liquid interface geometry enclosing concave fringes. The aim of the work is the contactless control of heat and material transport during floating-zone single crystal growth of intermetallic compounds. This control is provided by a tailored design of the electromagnetic field and the resulting electromagnetically driven convection. Numerical simulations for the determination of the electromagnetic field configuration induced by the RF heater coil and the solution of the coupled heat and hydrodynamic equations were done for the model substance Ni with and without additional magnetic field. As a result, an innovative magnetic two-phase stirrer system has been developed which enables the controlled influence on the melt ranging from intensive inwards/outwards flows to flows almost at rest. The selection of parameters necessary for the desired fluid flow is determined from numerical simulation. The basis for the calculations are the process-related fluid flow conditions which are determined by the mode of heating, heat radiation at the free surface and material parameters. This treatment of the problem leads to the customised magnetic field for the special intermetallic compound. The application of the new magnetic system leads to a distinct improvement of the solid-liquid interface validated on experiments with the model substance Nickel.

  9. Crystal field and magnetic properties of ErH3

    NASA Technical Reports Server (NTRS)

    Flood, D. J.

    1977-01-01

    Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) times 10 to the minus 6 Weber m/kg Tesla. The saturation moment is 3.84 + or - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is of the order of 160 to 180 K.

  10. Magnetic-field tunable defect modes in a photonic-crystal/liquid-crystal cell.

    PubMed

    Zyryanov, Victor Ya; Myslivets, Sergey A; Gunyakov, Vladimir A; Parshin, Alexander M; Arkhipkin, Vasily G; Shabanov, Vasily F; Lee, Wei

    2010-01-18

    Light transmission spectrum of a multilayer photonic crystal with a central liquid-crystal defect layer placed between crossed polarizers has been studied. Transmittance was varied due to the magnetically induced reorientation of the nematic director from homeotropic to planar alignment. Two notable effects were observed for this scheme: the spectral shift of defect modes corresponding to the extraordinary light wave and its superposition with the ordinary one. As a result, the optical cell allows controlling the intensity of interfering defect modes by applied magnetic field. PMID:20173953

  11. 3D crack tip fields for FCC single crystals

    SciTech Connect

    Cuitino, A.M.; Ortiz, M.

    1995-12-31

    Cracks in single crystals are of concern in a number of structural and non-structural applications, ranging form single-crystal turbine blades and rotors to metal interconnect lines in microcircuits. In this paper we present 3D numerical simulations of the crack-tip fields of a Cu single crystal, including stress, strain and slip activity patterns. The orientation of the crack tip is along the crystallographic orientation (101), while the crack plane is (010). A material model based on dislocation mechanics is used in these simulations. This model correctly predicts the observed behavior of Cu, including the basic hardening characteristics of single crystals, orientation dependence and stage I-II-III structure of the stress-strain curves, the observed levels of latent hardening and their variation with orientation and deformation in the primary system and slip activities and dislocation densities. We use the FEM within the context of finite deformation plasticity. In the figure below, we show the finite element mesh composed by 12-noded tetrahedrons with 6-noded triangular faces. The model simulates half of a beam, which is subjected to a concentrated load at 1/8 of total length from the support. Detailed results of the stress, deformation and slip activity are presented at different radii from crack tip and at different depths from the surface. In general, the results show a strong difference in the slip activity pattern form the interior to the exterior, while smaller differences are encountered in the stress and strain fields.

  12. Magnetic Field Measurements Based on Terfenol Coated Photonic Crystal Fibers

    PubMed Central

    Quintero, Sully M. M.; Martelli, Cicero; Braga, Arthur M. B.; Valente, Luiz C. G.; Kato, Carla C.

    2011-01-01

    A magnetic field sensor based on the integration of a high birefringence photonic crystal fiber and a composite material made of Terfenol particles and an epoxy resin is proposed. An in-fiber modal interferometer is assembled by evenly exciting both eigenemodes of the HiBi fiber. Changes in the cavity length as well as the effective refractive index are induced by exposing the sensor head to magnetic fields. The magnetic field sensor has a sensitivity of 0.006 (nm/mT) over a range from 0 to 300 mT with a resolution about ±1 mT. A fiber Bragg grating magnetic field sensor is also fabricated and employed to characterize the response of Terfenol composite to the magnetic field. PMID:22247655

  13. Low-frequency electromagnetic field in a Wigner crystal

    SciTech Connect

    Stupka, Anton

    2013-03-15

    Long-wave low-frequency oscillations are described in a Wigner crystal by generalization of the reverse continuum model for the case of electronic lattice. The internal self-consistent long-wave electromagnetic field is used to describe the collective motions in the system. The eigenvectors and eigenvalues of the obtained system of equations are derived. The velocities of longitudinal and transversal sound waves are found.

  14. Order by virtual crystal field fluctuations in pyrochlore XY antiferromagnets

    NASA Astrophysics Data System (ADS)

    Rau, Jeffrey G.; Petit, Sylvain; Gingras, Michel J. P.

    2016-05-01

    Conclusive evidence of order by disorder is scarce in real materials. Perhaps one of the strongest cases presented has been for the pyrochlore XY antiferromagnet Er2Ti2O7 , with the ground state selection proceeding by order by disorder induced through the effects of quantum fluctuations. This identification assumes the smallness of the effect of virtual crystal field fluctuations that could provide an alternative route to picking the ground state. Here we show that this order by virtual crystal field fluctuations is not only significant, but competitive with the effects of quantum fluctuations. Further, we argue that higher-multipolar interactions that are generically present in rare-earth magnets can dramatically enhance this effect. From a simplified bilinear-biquadratic model of these multipolar interactions, we show how the virtual crystal field fluctuations manifest in Er2Ti2O7 using a combination of strong-coupling perturbation theory and the random-phase approximation. We find that the experimentally observed ψ2 state is indeed selected and the experimentally measured excitation gap can be reproduced when the bilinear and biquadratic couplings are comparable while maintaining agreement with the entire experimental spin-wave excitation spectrum. Finally, we comment on possible tests of this scenario and discuss implications for other order-by-disorder candidates in rare-earth magnets.

  15. Semiconductor Crystal Growth in Static and Rotating Magnetic fields

    NASA Technical Reports Server (NTRS)

    Volz, Martin

    2004-01-01

    Magnetic fields have been applied during the growth of bulk semiconductor crystals to control the convective flow behavior of the melt. A static magnetic field established Lorentz forces which tend to reduce the convective intensity in the melt. At sufficiently high magnetic field strengths, a boundary layer is established ahead of the solid-liquid interface where mass transport is dominated by diffusion. This can have a significant effect on segregation behavior and can eliminate striations in grown crystals resulting from convective instabilities. Experiments on dilute (Ge:Ga) and solid solution (Ge-Si) semiconductor systems show a transition from a completely mixed convective state to a diffusion-controlled state between 0 and 5 Tesla. In HgCdTe, radial segregation approached the diffusion limited regime and the curvature of the solid-liquid interface was reduced by a factor of 3 during growth in magnetic fields in excess of 0.5 Tesla. Convection can also be controlled during growth at reduced gravitational levels. However, the direction of the residual steady-state acceleration vector can compromise this effect if it cannot be controlled. A magnetic field in reduced gravity can suppress disturbances caused by residual transverse accelerations and by random non-steady accelerations. Indeed, a joint program between NASA and the NHMFL resulted in the construction of a prototype spaceflight magnet for crystal growth applications. An alternative to the suppression of convection by static magnetic fields and reduced gravity is the imposition of controlled steady flow generated by rotating magnetic fields (RMF)'s. The potential benefits of an RMF include homogenization of the melt temperature and concentration distribution, and control of the solid-liquid interface shape. Adjusting the strength and frequency of the applied magnetic field allows tailoring of the resultant flow field. A limitation of RMF's is that they introduce deleterious instabilities above a

  16. Crystallization of insulin and lysozyme under reduced convection condition in a large gradient magnetic field

    NASA Astrophysics Data System (ADS)

    Yin, D. C.; Wakayama, N. I.; Fujiwara, M.; Harata, K.; Xue, X. P.; Fu, Z. X.; Zhang, S. W.; Shang, P.; Tanimoto, Y.

    The crystallization of protein from solution is governed by the process of transport phenomenon Any reason affecting the process of solute transport will impose effects on the crystallization process thus further affects the crystal quality Recent advancement in superconducting magnet technology makes it possible to provide a low cost long-time durable low effective gravity environment for the control of convection which is similar to the environment in the space As an ideal means to damp natural convection in a non-conductive solution on the Earth it may find applications in the field of protein crystallization In this presentation the authors investigated the crystallization of orthorhombic lysozyme crystals tetragonal lysozyme crystals and insulin crystals in a large gradient magnetic field Three effective gravity levels were used milli-gravity around 0G normal gravity 1G and hypergravity 1 8G Comparisons of the crystal quality obtained inside and outside the magnetic field showed that both the magnetic field and the effective gravity could affect the crystal quality But the effect also depends on the crystal and protein type For lysozyme crystals in tetragonal form the magnetic field and effective gravity showed no obvious effect on the quality whereas for the crystals in orthorhombic form both the magnetic field and effective gravity improved the crystal quality For insulin crystal which is highly symmetrical magnetic field and effective gravity showed no strong effect on the crystal quality It is well known that

  17. Plasticity and dislocation dynamics in a phase field crystal model.

    PubMed

    Chan, Pak Yuen; Tsekenis, Georgios; Dantzig, Jonathan; Dahmen, Karin A; Goldenfeld, Nigel

    2010-07-01

    The critical dynamics of dislocation avalanches in plastic flow is examined using a phase field crystal model. In the model, dislocations are naturally created, without any ad hoc creation rules, by applying a shearing force to the perfectly periodic ground state. These dislocations diffuse, interact and annihilate with one another, forming avalanche events. By data collapsing the event energy probability density function for different shearing rates, a connection to interface depinning dynamics is confirmed. The relevant critical exponents agree with mean field theory predictions. PMID:20867460

  18. Discrete dislocation plasticity and crack tip fields in single crystals

    NASA Astrophysics Data System (ADS)

    Van der Giessen, E.; Deshpande, V. S.; Cleveringa, H. H. M.; Needleman, A.

    2001-09-01

    Small-scale yielding around a stationary plane strain mode I crack is analyzed using discrete dislocation plasticity. The dislocations are all of edge character, and are modeled as line singularities in a linear elastic material. Superposition is used to represent the solution in terms of analytical fields for edge dislocations in a half-space and a numerical image solution that enforces the boundary conditions. The description of the dislocation dynamics includes the lattice resistance to dislocation motion, dislocation nucleation, interaction with obstacles and annihilation. A model planar crystal with three slip systems is considered. Two slip system orientations are analyzed that differ by a 90° rotation. The non-hardening, single crystal plasticity continuum slip solution of Rice (Mech. Mater. 6 (1987) 317) for this model crystal predicts that slip and kink bands emerge for both crystal geometries, while Drugan (J. Mech. Phys. Solids 49 (2001) 2155) has obtained kink band free solutions. For a reference set of parameter values, kink band free solutions are found in one orientation while the emergence of kink bands is seen in the other orientation. However, lowering the dislocation source density suppresses the formation of kink bands in this orientation as well. In all calculations, the opening stress in the immediate vicinity of the crack tip is much larger than predicted by continuum slip theory.

  19. Subsurface Stress Fields In Single Crystal (Anisotropic) Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.; Knudsen, Erik C.; Duke, Greg; Battista, Gilda; Swanson, Greg

    2004-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent HCF failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades. An analytical procedure is , presented, for evaluating the subsurface stresses in the elastic half-space, using a complex potential method outlined by Lekhnitskii. Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis. Effects of crystal orientation on stress response and fatigue life are examined.

  20. Subsurface Stress Fields in FCC Single Crystal Anisotropic Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.; Knudsen, Erik; Swanson, Gregory R.; Duke, Gregory; Ham-Battista, Gilda

    2004-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and non-crystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii. Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis (FEA). Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single crystal contact problems require extremely refined three-dimensional (3-D) finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses (RSS) on the principal slip planes also involves

  1. Portable piezoelectric crystal detector for field monitoring of environmental pollutants

    SciTech Connect

    Ho, M.H.; Guilbault, G.G.; Rietz, B.

    1983-09-01

    A portable field monitor was constructed by using a coated piezoelectric crystal for direct monitoring of toluene in a Danish printing plant. Toluene vapor was adsorbed onto the Pluronic F-68 coating on a quartz crystal and a decrease in frequency was observed. Various substances which could interfere with toluene determination were tested. No interference from CO, NH/sub 3/, SO/sub 2/, HCl at 100 ppm are expected. Water vapor interfered and was selectively removed using a Nafion permeation tube. The readings from the piezoelectric detector were compared to two accepted procedures for monitoring toluene, the photoionization detector and the Drager tube. Results indicate that the piezoelectric detector gave data consistent with both other methods and with better relative standard deviations than the other two. 8 references, 2 figures, 1 table.

  2. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Szofran, F. R.; Cobb, S. D.; Schweizer, M.; Walker, J. S.

    2005-01-01

    A series of (100)-oriented gallium-doped germanium crystals has been grown by the vertical Bridgman method and under the influence of a rotating magnetic field (RMF). Time-dependent flow instabilities occur when the critical magnetic Taylor number (Tm(sup c)) is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. Tm(sup c) decreases as the aspect ratio of the melt increases, and approaches the theoretical limit expected for an infinite cylinder. Intentional interface demarcations are introduced by pulsing the RMF on and off The RMF has a marked affect on the interface shape, changing it from concave to nearly flat as the RMF strength is increased.

  3. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Walker, J. S.; Schweizer, M.; Cobb, S. D.; Szofran, F. R.

    2004-01-01

    A series of (100)-oriented gallium-doped germanium crystals have been grown by the Bridgman method and under the influence of a rotating magnetic field (RMF). Time-dependent flow instabilities occur when the critical magnetic Taylor number (Tm(sup c) is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. The experimental data indicate that Tm(sup c) increases as the aspect ratio of the melt decreases. Modeling calculations predicting Tm(sup c) as a function of aspect ratio are in reasonable agreement with the experimental data. The RMF has a marked affect on the interface shape, changing it from concave to nearly flat as the RMF strength is increased. Also, by pulsing the RMF on and off, it is shown that intentional interface demarcations can be introduced.

  4. Field expulsion and reconfiguration in polaritonic photonic crystals.

    PubMed

    Huang, Kerwyn Casey; Bienstman, Peter; Joannopoulos, John D; Nelson, Keith A; Fan, Shanhui

    2003-05-16

    We uncover a rich set of optical phenomena stemming from the incorporation of polar materials exhibiting transverse phonon polariton excitations into a photonic crystal structure. We identify in the frequency spectrum two regimes in which the dielectric response of the polaritonic medium can induce extreme localization of the electromagnetic energy. Our analysis of the effect of polarization and the interaction between the polariton and photonic band gaps on the Bloch states leads to a pair of mechanisms for sensitive frequency-controlled relocation and/or reconfiguration of the fields. PMID:12785962

  5. Vibronic spectra of U4 in octahedral crystal fields

    NASA Astrophysics Data System (ADS)

    Flint, C. D.; Tanner, P. A.

    The absorption spectra of UCl2-6 diluted into the cubic lattices Cs2ZrCl6 and Cs2SnCl6, and of UBr2-6 diluted into Cs2ZrBr6 have been measured at liquid helium temperatures. The derived energy levels, together with earlier luminescence measurements have been used to evaluate the crystal-field, spinorbit coupling, inter-electron repulsion and Trees correction parameters and the corresponding eigenvalues. Several of the states are derived from an almost equal admixture of two or more Russell-Saunders terms. Some magnetic properties are also computed and discussed.

  6. Faceting transitions in crystal growth and heteroepitaxial growth in the anisotropic phase-field crystal model

    NASA Astrophysics Data System (ADS)

    Chen, Cheng; Chen, Zheng; Zhang, Jing; Yang, Tao; Du, Xiu-Juan

    2012-11-01

    We modify the anisotropic phase-field crystal model (APFC), and present a semi-implicit spectral method to numerically solve the dynamic equation of the APFC model. The process results in the acceleration of computations by orders of magnitude relative to the conventional explicit finite-difference scheme, thereby, allowing us to work on a large system and for a long time. The faceting transitions introduced by the increasing anisotropy in crystal growth are then discussed. In particular, we investigate the morphological evolution in heteroepitaxial growth of our model. A new formation mechanism of misfit dislocations caused by vacancy trapping is found. The regular array of misfit dislocations produces a small-angle grain boundary under the right conditions, and it could significantly change the growth orientation of epitaxial layers.

  7. Phase-field-crystal methodology for modeling of structural transformations.

    PubMed

    Greenwood, Michael; Rottler, Jörg; Provatas, Nikolas

    2011-03-01

    We introduce and characterize free-energy functionals for modeling of solids with different crystallographic symmetries within the phase-field-crystal methodology. The excess free energy responsible for the emergence of periodic phases is inspired by classical density-functional theory, but uses only a minimal description for the modes of the direct correlation function to preserve computational efficiency. We provide a detailed prescription for controlling the crystal structure and introduce parameters for changing temperature and surface energies, so that phase transformations between body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal-close-packed (hcp), and simple-cubic (sc) lattices can be studied. To illustrate the versatility of our free-energy functional, we compute the phase diagram for fcc-bcc-liquid coexistence in the temperature-density plane. We also demonstrate that our model can be extended to include hcp symmetry by dynamically simulating hcp-liquid coexistence from a seeded crystal nucleus. We further quantify the dependence of the elastic constants on the model control parameters in two and three dimensions, showing how the degree of elastic anisotropy can be tuned from the shape of the direct correlation functions. PMID:21517507

  8. Subsurface Stress Fields in Single Crystal (Anisotropic) Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.

    2003-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and fatigue stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent HCF failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. Techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts are presented in this report. Figure 1 shows typical damper contact locations in a turbine blade. The subsurface stress results are used for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades.

  9. Crystal Field Studies on MgGa2O4:Ni2+

    NASA Astrophysics Data System (ADS)

    Andreici, L.; Stanciu, M.; Avram, N. M.

    2010-08-01

    The energy levels scheme of octahedrally coordinated Ni2+ ion in single crystal, powder nano-single crystal, ceramics and glass-ceramics of MgGa2O4 host matrix, has been calculated in the exchange charge model of crystal field. The parameters of the crystal field acting on the Ni2+ ion are calculated from the crystal structure data, after optimization of the geometry of the system. The energy level schemes have been calculated by diagonalization of the crystal field Hamiltonian of this system. The obtained results were compared with experimental data; a good agreement were demonstrated, which confirm the validity of the model and used method.

  10. Convection patterns and temperature fields of ammonothermal GaN bulk crystal growth process

    NASA Astrophysics Data System (ADS)

    Masuda, Yoshio; Sato, Osamu; Tomida, Daisuke; Yokoyama, Chiaki

    2016-05-01

    The natural convection heat transfer in an ammonothermal process for growing GaN bulk single crystals has been examined numerically. We consider only one crystal to simplify the calculation and discuss the relationship between convection patterns and temperature fields. Two types of convection patterns are observed owing to the difference in the crystal radius. When the convection pattern is transformed, the crystal surface temperature decreases as the crystal radius increases.

  11. Control of active liquid crystals with a magnetic field.

    PubMed

    Guillamat, Pau; Ignés-Mullol, Jordi; Sagués, Francesc

    2016-05-17

    Living cells sense the mechanical features of their environment and adapt to it by actively remodeling their peripheral network of filamentary proteins, known as cortical cytoskeleton. By mimicking this principle, we demonstrate an effective control strategy for a microtubule-based active nematic in contact with a hydrophobic thermotropic liquid crystal. By using well-established protocols for the orientation of liquid crystals with a uniform magnetic field, and through the mediation of anisotropic shear stresses, the active nematic reversibly self-assembles with aligned flows and textures that feature orientational order at the millimeter scale. The turbulent flow, characteristic of active nematics, is in this way regularized into a laminar flow with periodic velocity oscillations. Once patterned, the microtubule assembly reveals its intrinsic length and time scales, which we correlate with the activity of motor proteins, as predicted by existing theories of active nematics. The demonstrated commanding strategy should be compatible with other viable active biomaterials at interfaces, and we envision its use to probe the mechanics of the intracellular matrix. PMID:27140604

  12. The Effects of a Magnetic Field on the Crystallization of a Fluorozirconate Glass

    NASA Technical Reports Server (NTRS)

    Tucker, Dennis S.; Lapointe, Michael R.; Jia, Zhiyong

    2006-01-01

    An axial magnetic field of 0.1T was applied to ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fibers during heating to the glass crystallization temperature. Scanning electron microscopy and x-ray diffraction were used to identify crystal phases. It was shown that fibers exposed to the magnetic field did not crystallize while fibers not exposed to the field did crystallize. A hypothesis based on magnetic work was proposed to explain the results and tested by measuring the magnetic susceptibilities of the glass and crystal.

  13. Influence of magnetic field on the morphology of the andrographolide crystal from supercritical carbon dioxide extraction crystallization

    NASA Astrophysics Data System (ADS)

    Chen, Kexun; Zhang, Xingyuan; Pan, Jian; Zhang, Wencheng; Yong, Ji; Yin, Wenhong

    2003-10-01

    In this paper, a supercritical fluid extraction-crystallization of andrographolide, a kind of Chinese traditional medicine, was investigated. We have studied the extraction-crystallization process with or without magnet in the extractor, respectively. It was found that the presence of magnetic field is an important factor influencing the quality of the products. SEM images showed that the crystal was slice-like in shape, and many slices reunited together in the absence of magnet. Further research showed that pressure had a certain effect on the morphology of the crystal.

  14. An efficient algorithm for solving the phase field crystal model

    SciTech Connect

    Cheng Mowei Warren, James A.

    2008-06-01

    We present and discuss the development of an unconditionally stable algorithm used to solve the evolution equations of the phase field crystal (PFC) model. This algorithm allows for an arbitrarily large algorithmic time step. As the basis for our analysis of the accuracy of this algorithm, we determine an effective time step in Fourier space. We then compare our calculations with a set of representative numerical results, and demonstrate that this algorithm is an effective approach for the study of the PFC models, yielding a time step effectively 180 times larger than the Euler algorithm for a representative set of material parameters. As the PFC model is just a simple example of a wide class of density functional theories, we expect this method will have wide applicability to modeling systems of considerable interest to the materials modeling communities.

  15. Magnetic rotations of uric acid crystals and uratic crystals by static magnetic fields of up to 500 mT.

    PubMed

    Takeuchi, Yuka; Mizukawa, Yuri; Iwasaka, Masakazu

    2013-01-01

    In recent years, the disease concerning ureteral calculus is increasing possibly due to the changing lifestyles. For example, it is well known that the urinary calculi have a large impact to gout. As eating habitual diseases, gout and the hyper-uricemia are related to the formation of urinary calculus. In the previous studies, therapeutic agents were developed to enhance the uric acid excretion. From the viewpoint of side effects induction by the chemical agents, we are motivated to explore an alternative method to control the formation of ureteral crystals stimulator by physical stimulations. Therefore in the present study, we focused on the behaviors of uric acid crystals under magnetic fields of several hundreds of mT (Tesla). The uric acid crystals were re-crystallized from a suspension of uric acid powder, and the micro-crystals were prepared to be floating in the solution. We generated horizontal magnetic fields of maximum 500 mT by an electromagnet which contained a CCD microscope. A permanent magnet with magnetic fields of 200∼400 mT was also utilized. During the magnetic fields were applied to the uric acid crystals, we observed that the uric acid crystals were oriented by the magnetic fields down to 200 mT at the room temperature. It was speculated that the dimagnetic anisotropy in the uric acid crystals exhibited the rotational responses. The results indicate the possible remote control of the uric acid crystals in living body by the magnetic fields of 200 mT to 500 mT. PMID:24110424

  16. Crystal-field levels in UBr 3 determined by neutron spectroscopy

    NASA Astrophysics Data System (ADS)

    Murasik, A.; Furrer, A.; Szczepaniak, W.

    1980-03-01

    Inelastic neutron scattering experiments have been performed in UBr 3 in order to determine the crystal-field levels. Four transitions between the ground state and all excited states have been observed, thus the crystal-field level scheme could be unambigously assigned. The Russell-Saunders coupling scheme has been used to derive the crystal-field parameters which cannot be accounted for by a simple point charge calculation.

  17. Inelastic neutron scattering investigation of crystal-field splittings in UBr 3

    NASA Astrophysics Data System (ADS)

    Murasik, A.; Furrer, A.

    1980-10-01

    An inelastic neutron scattering technique was used to measure the crystal-field splittings in UBr 3 at various temperatures and momentum transfers. In the interpretation of the observed energy spectra it turns out that the Russell-Saunders coupling scheme is a reasonable approximation. The crystal-field level scheme could be unambiguously assigned. The detailed nature of the crystal-field transition from the ground state to the first-excited state is not yet fully understood.

  18. Modeling nuclear field shift isotope fractionation in crystals

    NASA Astrophysics Data System (ADS)

    Schauble, E. A.

    2013-12-01

    In this study nuclear field shift fractionations in solids (and chemically similar liquids) are estimated using calibrated density functional theory calculations. The nuclear field shift effect is a potential driver of mass independent isotope fractionation(1,2), especially for elements with high atomic number such as Hg, Tl and U. This effect is caused by the different shapes and volumes of isotopic nuclei, and their interactions with electronic structures and energies. Nuclear field shift isotope fractionations can be estimated with first principles methods, but the calculations are computationally difficult, limiting most theoretical studies so far to small gas-phase molecules and molecular clusters. Many natural materials of interest are more complex, and it is important to develop ways to estimate field shift effects that can be applied to minerals, solutions, in biomolecules, and at mineral-solution interfaces. Plane-wave density functional theory, in combination with the projector augmented wave method (DFT-PAW), is much more readily adapted to complex materials than the relativistic all-electron calculations that have been the focus of most previous studies. DFT-PAW is a particularly effective tool for studying crystals with periodic boundary conditions, and may also be incorporated into molecular dynamics simulations of solutions and other disordered phases. Initial calibrations of DFT-PAW calculations against high-level all-electron models of field shift fractionation suggest that there may be broad applicability of this method to a variety of elements and types of materials. In addition, the close relationship between the isomer shift of Mössbauer spectroscopy and the nuclear field shift isotope effect makes it possible, at least in principle, to estimate the volume component of field shift fractionations in some species that are too complex even for DFT-PAW models, so long as there is a Mössbauer isotope for the element of interest. Initial results

  19. Effect of an electric field on nucleation and growth of crystals

    NASA Astrophysics Data System (ADS)

    Yurov, V. M.; Guchenko, S. A.; Gyngazova, M. S.

    2016-02-01

    The effect of the electric field strength on nucleation and growth of the crystals of ammonium halides and alkali metal sulfates has been studied. The optimal electric field strength for NH4Cl and NH4Br crystals was found to be 15 kV/cm, and for NH4I, it equaled 10 kV/cm. No effect of the electric field strength on the crystal growth was found for alkali metal sulfates. This difference is analyzed in terms of the crystal growth thermodynamics. In case, when the electric field is small and the Gibbs energy is of a significant value, the influence of the electric field at the crystal growth is negligible. A method to estimate the critical radius of homogeneous nucleation of the crystal is suggested.

  20. Rashba coupling amplification by a staggered crystal field

    PubMed Central

    Santos-Cottin, David; Casula, Michele; Lantz, Gabriel; Klein, Yannick; Petaccia, Luca; Le Fèvre, Patrick; Bertran, François; Papalazarou, Evangelos; Marsi, Marino; Gauzzi, Andrea

    2016-01-01

    There has been increasing interest in materials where relativistic effects induce non-trivial electronic states with promise for spintronics applications. One example is the splitting of bands with opposite spin chirality produced by the Rashba spin-orbit coupling in asymmetric potentials. Sizable splittings have been hitherto obtained using either heavy elements, where this coupling is intrinsically strong, or large surface electric fields. Here by means of angular resolved photoemission spectroscopy and first-principles calculations, we give evidence of a large Rashba coupling of 0.25 eV Å, leading to a remarkable band splitting up to 0.15 eV with hidden spin-chiral polarization in centrosymmetric BaNiS2. This is explained by a huge staggered crystal field of 1.4 V Å−1, produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering. PMID:27089869

  1. Rashba coupling amplification by a staggered crystal field

    NASA Astrophysics Data System (ADS)

    Santos-Cottin, David; Casula, Michele; Lantz, Gabriel; Klein, Yannick; Petaccia, Luca; Le Fèvre, Patrick; Bertran, François; Papalazarou, Evangelos; Marsi, Marino; Gauzzi, Andrea

    2016-04-01

    There has been increasing interest in materials where relativistic effects induce non-trivial electronic states with promise for spintronics applications. One example is the splitting of bands with opposite spin chirality produced by the Rashba spin-orbit coupling in asymmetric potentials. Sizable splittings have been hitherto obtained using either heavy elements, where this coupling is intrinsically strong, or large surface electric fields. Here by means of angular resolved photoemission spectroscopy and first-principles calculations, we give evidence of a large Rashba coupling of 0.25 eV Å, leading to a remarkable band splitting up to 0.15 eV with hidden spin-chiral polarization in centrosymmetric BaNiS2. This is explained by a huge staggered crystal field of 1.4 V Å-1, produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering.

  2. Electric-field-assisted position and orientation control of organic single crystals.

    PubMed

    Kotsuki, Kenji; Obata, Seiji; Saiki, Koichiro

    2014-12-01

    We have investigated the motion of growing pentacene single crystals in solution under various electric fields. The pentacene single crystals in 1,2,4-trichlorobenzene responded to the electric field as if they were positively charged. By optimizing the strength and frequency of an alternating electric field, the pentacene crystals automatically bridged the electrodes on SiO2. The pentacene crystal with a large aspect ratio tended to direct the [1̅10] orientation parallel to the conduction direction, which will be suitable from a viewpoint of anisotropy in mobility. The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric field, which leads to high throughput and low cost industrial manufacturing of the single crystal array from solution. PMID:25360544

  3. Crystal-field calculations for transition-metal ions by application of an opposing potential

    DOE PAGESBeta

    Zhou, Fei; Aberg, Daniel

    2016-02-16

    We propose a fully ab initio method, the opposing crystal potential (OCP), to calculate the crystal-field parameters of transition-metal impurities in insulator hosts. Through constrained density functional calculations, OCP obtains the constraining Lagrange multipliers, which act as a cancellation potential against the crystal field and lead to spherical d-electron distribution. Furthermore, the method is applied to several insulators doped with Mn4+ and Mn2+ ions and shown to be in good agreement with experiment.

  4. Crystal-field transitions in f-electron oxides

    NASA Astrophysics Data System (ADS)

    Kern, S.; Loong, C.-K.; Lander, G. H.

    1985-09-01

    Neutron inelastic scattering has been used to measure for the first time the ground- to excited-state crystal-field transitions in PrO2 (130 meV), BaPrO3 (255 meV), and UO2 (~160 meV). Details of these neutron experiments using the epithermal neutrons from the Argonne National Laboratory spallation source are given. From the observed transitions the following values of V4=A4 are deduced: PrO2 (-66 meV), BaPrO3 (119 meV), and UO2 (-385 meV). Comparisons are made with V4 values deduced for metallic systems and those determined by optical techniques for dilute lanthanides in transparent hosts. In the case of UO2, two peaks are seen, one at 155 meV and the other at 172 meV. This structure exists both below and above the Néel temperature TN (30.8 K) and is discussed in terms of mechanisms that might exist in UO2. Several further neutron experiments are suggested now that energy transfers above ~100 meV may be measured at small (i.e., <~5 Å-1) values of the momentum transfer.

  5. Using Magnetic Fields to Control Convection during Protein Crystallization: Analysis and Validation Studies

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F. W.

    2004-01-01

    The effect of convection during the crystallization of proteins is not very well understood. In a gravitational field, convection is caused by crystal sedimentation and by solutal buoyancy induced flow and these can lead to crystal imperfections. While crystallization in microgravity can approach diffusion limited growth conditions (no convection), terrestrially strong magnetic fields can be used to control fluid flow and sedimentation effects. In this work, we develop the analysis for magnetic flow control and test the predictions using analog experiments. Specifically, experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented. This phenomenon suggests that magnetic fields may be useful in mimicking the microgravity environment of space for some crystal growth ana biological applications where fluid convection is undesirable.

  6. Effect of intense magnetic fields on the convection of biogenic guanine crystals in aqueous solution

    NASA Astrophysics Data System (ADS)

    Iwasaka, M.; Mizukawa, Y.

    2015-05-01

    In this study, the basic magneto-optic properties of biogenic microcrystals in aqueous media were investigated. Microcrystals, mica plates, silica, and microcrystals from a diatom cell and biogenic guanine crystals from goldfish showed light scattering inhibition when the crystals were observed in water under a 5 T magnetic field and dark-field illumination. In particular, in 50% ethanol/water medium, convection of the biogenic guanine particle aggregates was reversibly inhibited when the microcrystal suspension was exposed to a 5 T magnetic field. Microscopic observation comparing the biogenic guanine crystals in water with 95% ethanol or 99% acetone revealed that light flickering on the surface of the crystals was affected by the surface interaction of the crystal with the surrounding medium. By considering both the magnetic orientation of the microcrystals and the possible interactions of crystals with the surrounding medium, a magnetically controllable fluidic tracer was suggested.

  7. Which charge definition for describing the crystal polarizing field and the χ((1)) and χ((2)) of organic crystals?

    PubMed

    Seidler, Tomasz; Champagne, Benoît

    2015-07-15

    The impact of atomic charge definition for describing the crystal polarizing electric field has been assessed in view of predicting the linear and nonlinear optical susceptibilities of molecular crystals. In this approach, the chromophores are embedded in the electric field of its own point charges, which are evaluated through a self-consistent procedure including charge scaling to account for the screening of the dielectric. Once the crystal field is determined, dressed molecular polarizabilities and hyperpolarizabilities are calculated and used as input of an electrostatic interaction scheme to evaluate the crystal linear and nonlinear optical responses. It is observed that many charge definitions (i) based on partitioning the electron density (QTAIM), (ii) obtained by analyzing the quantum-chemical wavefunction (Mulliken, MBS, and NBO), and (iii) derived by fitting to the electrostatic potential (MK, CHelpG, and HLYGAt) give very consistent results and are equally valid whereas Hirshfeld partitioning and CM5 charge parametrizations underestimate the refractive indices and second-order nonlinear optical susceptibilities. An alternative approach omitting charge scaling is demonstrated to overestimate the different crystal optical properties. On the other hand, the molecule embedding approach provides results in close agreement with those calculated with a charge field obtained from periodic boundary condition calculations. PMID:26144533

  8. Neutron study of crystal field excitations in single crystal CeCu2Ge2

    NASA Astrophysics Data System (ADS)

    Loewenhaupt, Michael; Faulhaber, Enrico; Schneidewind, Astrid; Deppe, Micha; Hradil, Klaudia

    2010-03-01

    CeCu2Ge2 is the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 is a magnetically ordering (TN= 4.1 K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/molK^2 [1]. Inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet at 16.5 meV [1] though a splitting of the 4f^1 (J=5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce^3+ ions. We performed detailed inelastic neutron scattering experiments on the thermal triple-axis spectrometer PUMA at FRM II at temperatures between 10 K and 300 K and for different crystallographic directions from low to high momentum transfers. In this way we obtained a reliable separation of magnetic and phonon contributions. From our results we infer that the quasi-quartet consists in fact of two doublets at 17 and 18 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally we will present a new set of crystal field parameters and their implications on other magnetic properties. [1] G. Knopp et al., Z. Physik B 77 (1989) 95

  9. Spatial Distribution of -Crystals in Metallocene-Made Isotactic Polypropylene Crystallized under Combined Thermal and Flow Fields

    SciTech Connect

    Wang, Y.; Pan, J; Mao, Y; Li, Z; Li, L; Hsiao, B

    2010-01-01

    The present Article reports the relationships between molecular orientation, formation, and spatial distribution of {gamma}-crystals in metallocene-made isotactic polypropylene (m-iPP) samples prepared by two industrial processes: conventional injection molding (CIM) and oscillatory shear injection molding (OSIM), in which combined thermal and flow fields typically exist. In particular, spatial distributions of crystallinity, fraction of {gamma}-crystal (f{gamma}) with respect to {alpha}-crystal, and lamella-branched shish-kebab structure in the shaped samples were characterized by synchrotron two-dimensional (2D) wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The results showed that the crystallinity in any given region of OSIM samples was always higher than that of CIM samples. The value of f{gamma} increased monotonously from skin to core in CIM samples, whereas the corresponding f{gamma} increased nonmonotonically in OSIM samples. The spatial distribution of {gamma}-crystal in OSIM samples can be explained by the epitaxial arrangement between {gamma}- and {alpha}-crystal in a lamella-branched shish-kebab structure. In the proposed model, the parent lamellae of {alpha}-crystal provide secondary nucleation sites for daughter lamellae of {alpha}-crystal and {gamma}-crystal, and the different content of parent lamellae results in varying amounts of {gamma}-crystal. In OSIM samples, the smallest parent-daughter ratio ([R] = 1.38) in the core region led to the lowest fraction of {gamma}-crystal (0.57), but relatively higher {gamma}-crystal content (0.69) at 600 and 1200 {micro}m depth of the samples (corresponding to [R] of 4.5 and 5.8, respectively). This is consistent with the proposed model where more parent lamellae provide more nucleation sites for crystallization, thus resulting in higher content of {gamma}-crystal. The melting behavior of CIM and OSIM samples was studied by differential scanning calorimetery (DSC). The

  10. Magnetic field sensor based on selectively magnetic fluid infiltrated dual-core photonic crystal fiber

    NASA Astrophysics Data System (ADS)

    Gangwar, Rahul Kumar; Bhardwaj, Vanita; Singh, Vinod Kumar

    2016-02-01

    We reported the modeling result of selectively magnetic fluid infiltrated dual-core photonic crystal fiber based magnetic field sensor. Inside the cross-section of the designed photonic crystal fiber, the two fiber cores filled with magnetic fluid (Fe3O4) form two independent waveguides with mode coupling. The mode coupling under different magnetic field strengths is investigated theoretically. The sensitivity of the sensor as a function of the structural parameters of the photonic crystal fiber is calculated. The result shows that the proposed sensing device with 1 cm photonic crystal fiber length has a large sensitivity of 305.8 pm/Oe.

  11. Large area mode field photonic crystal fiber design

    NASA Astrophysics Data System (ADS)

    Guo, Shuqin; An, Wensheng; Wang, Kang; Zhu, Guangxin; Le, Zichun

    2005-11-01

    A novel design method about photonic crystal fiber (PCF) with large area model field (LAMF) is demonstrated. Different from ordinarily design that the core of PCF formed by missing one air holes in the center of section, many air holes distributed in heartland all together come into being the core region. Air holes are arranged regularly in core region and outer cladding regions according to different periodical character, respectively. The effective refractive index (n eff ) of core region should be higher than cladding region because of total internal reflection (TIR) requirement. In this paper, two kinds of typical scheme are offered to realize LAMF-PCF. First, Λ, the spacing of neighboring air holes in whole section is fixed, once the radius of air holes in the core region r c is smaller than the cladding air holes r cla, LAMF-PCF will be formed. The modal area only lessens a little as r c is reduced. Especially, optimal size of r c can nearly make MFA insensitive to wavelength. On the contrary, dispersion parameter of PCF will take place visible change along with r c reduced, and ultra-flattened dispersion character can be realized when r c is optimized. Another method of designing LAMF-PCF is keeping all air holes uniform in the whole section of PCF, but the space of neighboring air holes in the core region Λ c is longer than the cladding region Λ cla, so n eff of core region is higher than the cladding region and TIR can take place.

  12. Polymer crystallization in a temperature gradient field with controlled crystal growth rate

    NASA Technical Reports Server (NTRS)

    Hansen, D.; Taskar, A. N.; Casale, O.

    1971-01-01

    A method is described for studying the influence of a temperature gradient on the crystallization of quiescent polymer melts. The apparatus used consists of two brass plates with embedded electrical resistance heaters and cooling coils. The crystallizations experiments were conducted by placing polymer specimens between the paltes, and manually adjusting heaters and cooling fluids for temperature control. Linear polyethylene, isotactic polyprophylene, and a high density polyethylene were used. It is concluded that the role of a temperature gradient in producing oriented crystallization is in producing conditions which lead the spherulitic growth pattern to proceed primarily in one direction. Steep gradients diminish the penetration of supercooling and favors oriented growth.

  13. Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores.

    PubMed

    Malkin, B Z; Lummen, T T A; van Loosdrecht, P H M; Dhalenne, G; Zakirov, A R

    2010-07-14

    The experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R(2)Ti(2)O(7) (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed. PMID:21399269

  14. Formation of temperature fields in doped anisotropic crystals under spatially inhomogeneous light beams passing through them

    SciTech Connect

    Zaitseva, E. V.; Markelov, A. S.; Trushin, V. N. Chuprunov, E. V.

    2013-12-15

    The features of formation of thermal fields in potassium dihydrophosphate crystal doped with potassium permanganate under a 532-nm laser beam passing through it have been investigated. Data on the influence of birefringence on the temperature distribution in an anisotropic crystal whose surface is illuminated by a spatially modulated light beam are presented.

  15. Dynamic control over the heat field during LBO crystal growth by High temperature solution method

    NASA Astrophysics Data System (ADS)

    Kokh, A.; Vlezko, V.; Kokh, K.; Kononova, N.; Villeval, Ph.; Lupinski, D.

    2012-12-01

    The paper presents LiB3O5 crystal growth under oscillating temperature regime provided by sequential switching of the heaters placed around the crucible. First results have demonstrated the ability to grow high-quality crystals under dynamicaly changed (rotating) heat field confirming the possibility to control over heat-mass-transfer processes by proposed contact free method.

  16. Formation of temperature fields in doped anisotropic crystals under spatially inhomogeneous light beams passing through them

    NASA Astrophysics Data System (ADS)

    Zaitseva, E. V.; Markelov, A. S.; Trushin, V. N.; Chuprunov, E. V.

    2013-12-01

    The features of formation of thermal fields in potassium dihydrophosphate crystal doped with potassium permanganate under a 532-nm laser beam passing through it have been investigated. Data on the influence of birefringence on the temperature distribution in an anisotropic crystal whose surface is illuminated by a spatially modulated light beam are presented.

  17. Imposed Orientation of Dye Molecules by Liquid Crystals and an Electric Field.

    ERIC Educational Resources Information Center

    Sadlej-Sosnowska, Nina

    1980-01-01

    Describes experiments using dye solutions in liquid crystals in which polar molecules are oriented in an electrical field and devices are constructed to change their color in response to an electric signal. (CS)

  18. Transverse magnetic field impact on waveguide modes of photonic crystals.

    PubMed

    Sylgacheva, Daria; Khokhlov, Nikolai; Kalish, Andrey; Dagesyan, Sarkis; Prokopov, Anatoly; Shaposhnikov, Alexandr; Berzhansky, Vladimir; Nur-E-Alam, Mohammad; Vasiliev, Mikhail; Alameh, Kamal; Belotelov, Vladimir

    2016-08-15

    This Letter presents a theoretical and experimental study of waveguide modes of one-dimensional magneto-photonic crystals magnetized in the in-plane direction. It is shown that the propagation constants of the TM waveguide modes are sensitive to the transverse magnetization and the spectrum of the transverse magneto-optical Kerr effect has resonant features at mode excitation frequencies. Two types of structures are considered: a non-magnetic photonic crystal with an additional magnetic layer on top and a magneto-photonic crystal with a magnetic layer within each period. We found that the magneto-optical non-reciprocity effect is greater in the first case: it has a magnitude of δ∼10-4, while the second structure type demonstrates δ∼10-5 only, due to the higher asymmetry of the claddings of the magnetic layer. Experimental observations show resonant features in the optical and magneto-optical Kerr effect spectra. The measured dispersion properties are in good agreement with the theoretical predictions. An amplitude of light intensity modulation of up to 2.5% was observed for waveguide mode excitation within the magnetic top layer of the non-magnetic photonic crystal structure. The presented theoretical approach may be utilized for the design of magneto-optical sensors and modulators requiring pre-determined spectral features. PMID:27519096

  19. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  20. Method of bonding single crystal quartz by field-assisted bonding

    DOEpatents

    Curlee, R.M.; Tuthill, C.D.; Watkins, R.D.

    1991-04-23

    The method of producing a hermetic stable structural bond between quartz crystals includes providing first and second quartz crystals and depositing thin films of borosilicate glass and silicon on portions of the first and second crystals, respectively. The portions of the first and second crystals are then juxtaposed in a surface contact relationship and heated to a temperature for a period sufficient to cause the glass and silicon films to become electrically conductive. An electrical potential is then applied across the first and second crystals for creating an electrostatic field between the adjoining surfaces and causing the juxtaposed portions to be attracted into an intimate contact and form a bond for joining the adjoining surfaces of the crystals. 2 figures.

  1. Method of bonding single crystal quartz by field-assisted bonding

    DOEpatents

    Curlee, Richard M.; Tuthill, Clinton D.; Watkins, Randall D.

    1991-01-01

    The method of producing a hermetic stable structural bond between quartz crystals includes providing first and second quartz crystals and depositing thin films of borosilicate glass and silicon on portions of the first and second crystals, respectively. The portions of the first and second crystals are then juxtaposed in a surface contact relationship and heated to a temperature for a period sufficient to cause the glass and silicon films to become electrically conductive. An electrical potential is then applied across the first and second crystals for creating an electrostatic field between the adjoining surfaces and causing the juxtaposed portions to be attracted into an intimate contact and form a bond for joining the adjoining surfaces of the crystals.

  2. Directional growth by low electric-field-controlled crystallization of bulk amorphous lithium tetraborate

    NASA Astrophysics Data System (ADS)

    Kim, S. J.; Kim, J. E.; Yang, Y. S.

    2004-12-01

    Highly oriented rod-shaped crystals were grown during crystallization of bulk amorphous Li2B4O7 under a low ac electric field of ˜5V/mm. The crystal c axis that is the long direction of rods and perpendicular to the flat surface of the sample is parallel to the applied electric-field direction. The oriented crystals, with an alignment within a declined angle of ˜15°, are so long along the c direction compared with those of other directions that the geometrical structure of each rod is quasi-one-dimensional. The measured electromechanical coupling coefficient of kt=0.47 is comparable to that of single-crystal Li2B4O7.

  3. A test of improved force field parameters for urea: molecular-dynamics simulations of urea crystals.

    PubMed

    Özpınar, Gül Altınbaş; Beierlein, Frank R; Peukert, Wolfgang; Zahn, Dirk; Clark, Timothy

    2012-08-01

    Molecular-dynamics (MD) simulations of urea crystals of different shapes (cubic, rectangular prismatic, and sheet) have been performed using our previously published force field for urea. This force field has been validated by calculating values for the cohesive energy, sublimation temperature, and melting point from the MD data. The cohesive energies computed from simulations of cubic and rectangular prismatic urea crystals in vacuo at 300 K agreed very well with the experimental sublimation enthalpies reported at 298 K. We also found very good agreement between the melting points as observed experimentally and from simulations. Annealing the crystals just below the melting point leads to reconstruction to form crystal faces that are consistent with experimental observations. The simulations reveal a melting mechanism that involves surface (corner/edge) melting well below the melting point, and rotational disordering of the urea molecules in the corner/edge regions of the crystal, which then facilitates the translational motion of these molecules. PMID:22281810

  4. Field localization and enhancement near the Dirac point of a finite defectless photonic crystal

    NASA Astrophysics Data System (ADS)

    D'Aguanno, Giuseppe; Mattiucci, Nadia; Conti, Claudio; Bloemer, Mark J.

    2013-02-01

    We use a rigorous electromagnetic approach to show the existence of strongly localized modes in the stop band of a linear, two-dimensional, finite photonic crystal near its Dirac point. At normal incidence, the crystal exhibits a Dirac point with 100% transmission. At angles slightly off the normal, where the crystal is 100% reflective, instead of exponentially decaying fields as in a photonic stop band, the field becomes strongly localized and enhanced inside the crystal. We explain that this anomalous localization is due to guided mode resonances that are the foundation of the Dirac point itself and also shape its adjacent band gap. Besides shedding new light on the physical origin of Dirac points in finite photonic crystals, our results could have applications in many nonlinear light-matter interaction phenomena in which it is crucial to achieve a high degree of light localization.

  5. Models of Mass Transport During Microgravity Crystal Growth of Alloyed Semiconductors in a Magnetic Field

    NASA Technical Reports Server (NTRS)

    Ma, Nancy

    2003-01-01

    Alloyed semiconductor crystals, such as germanium-silicon (GeSi) and various II-VI alloyed crystals, are extremely important for optoelectronic devices. Currently, high-quality crystals of GeSi and of II-VI alloys can be grown by epitaxial processes, but the time required to grow a certain amount of single crystal is roughly 1,000 times longer than the time required for Bridgman growth from a melt. Recent rapid advances in optoelectronics have led to a great demand for more and larger crystals with fewer dislocations and other microdefects and with more uniform and controllable compositions. Currently, alloyed crystals grown by bulk methods have unacceptable levels of segregation in the composition of the crystal. Alloyed crystals are being grown by the Bridgman process in space in order to develop successful bulk-growth methods, with the hope that the technology will be equally successful on earth. Unfortunately some crystals grown in space still have unacceptable segregation, for example, due to residual accelerations. The application of a weak magnetic field during crystal growth in space may eliminate the undesirable segregation. Understanding and improving the bulk growth of alloyed semiconductors in microgravity is critically important. The purpose of this grant to to develop models of the unsteady species transport during the bulk growth of alloyed semiconductor crystals in the presence of a magnetic field in microgravity. The research supports experiments being conducted in the High Magnetic Field Solidification Facility at Marshall Space Flight Center (MSFC) and future experiments on the International Space Station.

  6. Imaging of director fields in liquid crystals using stimulated Raman scattering microscopy.

    PubMed

    Lee, Taewoo; Mundoor, Haridas; Gann, Derek G; Callahan, Timothy J; Smalyukh, Ivan I

    2013-05-20

    We demonstrate an approach for background-free three-dimensional imaging of director fields in liquid crystals using stimulated Raman scattering microscopy. This imaging technique is implemented using a single femtosecond pulsed laser and a photonic crystal fiber, providing Stokes and pump frequencies needed to access Raman shifts of different chemical bonds of molecules and allowing for chemically selective and broadband imaging of both pristine liquid crystals and composite materials. Using examples of model three-dimensional structures of director fields, we show that the described technique is a powerful tool for mapping of long-range molecular orientation patterns in soft matter via polarized chemical-selective imaging. PMID:23736433

  7. A Navier-Stokes phase-field crystal model for colloidal suspensions.

    PubMed

    Praetorius, Simon; Voigt, Axel

    2015-04-21

    We develop a fully continuous model for colloidal suspensions with hydrodynamic interactions. The Navier-Stokes Phase-Field Crystal model combines ideas of dynamic density functional theory with particulate flow approaches and is derived in detail and related to other dynamic density functional theory approaches with hydrodynamic interactions. The derived system is numerically solved using adaptive finite elements and is used to analyze colloidal crystallization in flowing environments demonstrating a strong coupling in both directions between the crystal shape and the flow field. We further validate the model against other computational approaches for particulate flow systems for various colloidal sedimentation problems. PMID:25903907

  8. Influence of Acoustic Field Structure on Polarization Characteristics of Acousto-optic Interaction in Crystals

    NASA Astrophysics Data System (ADS)

    Muromets, A. V.; Trushin, A. S.

    Influence of acoustic field structure on polarization characteristics of acousto-optic interaction is investigated. It is shown that inhomogeneity of acoustic field and mechanism of ultrasound excitation causes changes in values of acousto-optic figure of merit for ordinary and extraordinary light beams in comparison with theoretic values. The theoretic values were derived under assumption that acoustic wave is homogeneous. Experimental analysis was carried out in acousto-optic cell based on lithium niobate crystal where the acoustic wave propagates at the angle 13 degrees to Z axis of the crystal. We used three different methods of ultrasound generation in the crystal: by means of external piezotransducer, by interdigital transducer and by two sets of electrodes placed on top of the crystal surface. In the latter case, the first pair of the electrodes was directed along X crystal axis, while the second pair of the electrodes was directed orthogonally to X crystal axis and the direction of ultrasound. Obtained values for diffraction efficiencies for ordinary and extraordinary polarized optical beams were qualitatively different which may be caused by spatial inhomogeneity of the generated acoustic waves in the crystal. Structure of acoustic field generated by these sets of electrodes was examined by laser probing. We performed the analysis of the acoustic field intensity using acousto-optic method. A relation of diffraction efficiencies for ordinary and extraordinary light waves was measured during each iteration of the laser probing.

  9. Near-field studies of microwave three-dimensional photonic crystals with waveguides.

    PubMed

    Liu, Rong-Juan; Li, Zhi-Yuan; Zhou, Fei; Zhang, Dao-Zhong

    2007-11-12

    By utilizing a vector network analyzer, the field distributions on the surface of a three-dimensional woodpile photonic crystal with a straight waveguide or a bend waveguide buried under the surface were measured in the microwave regime. The information of field profile and propagation characteristics of the guided modes can be successfully extracted from the surface near-field measurement. This work indicates that the near-field detection can become a promising means for experimental characterization of three-dimensional photonic crystal devices in supplement to the usual transmission spectrum measurement. PMID:19550839

  10. Hidden local symmetry of Eu{sup 3+} in xenotime-like crystals revealed by high magnetic fields

    SciTech Connect

    Han, Yibo; Ma, Zongwei; Zhang, Junpei; Wang, Junfeng; Du, Guihuan; Xia, Zhengcai; Han, Junbo Li, Liang; Yu, Xuefeng

    2015-02-07

    The excellent optical properties of europium-doped crystals in visible and near infrared wavelength regions enable them to have broad applications in optoelectronics, laser crystals and sensing devices. The local site crystal fields can affect the intensities and peak positions of the photo-emission lines strongly, but they are usually difficult to be clarified due to magnetically degenerate 4f electronic levels coupling with the crystal fields. Here, we provide an effective way to explore the hidden local symmetry of the Eu{sup 3+} sites in different hosts by taking photoluminescence measurements under pulsed high magnetic fields up to 46 T. The zero-field photoluminescence peaks split further at high magnetic fields when the Zeeman splitting energy is comparable to or larger than that of the crystal field induced zero-field splitting. In particular, a magnetic field induced crossover of the local crystal fields has been observed in the GdVO{sub 4}:Eu{sup 3+} crystal, which resulted from the alignment of Gd{sup 3+} magnetic moment in high magnetic fields; and a hexagonally symmetric local crystal fields was observed in the YPO{sub 4} nanocrystals at the Eu{sup 3+} sites characterized by the special axial and rhombic crystal field terms. These distinct Zeeman splitting behaviors uncover the crystal fields-related local symmetry of luminescent Eu{sup 3+} centers in different hosts or magnetic environments, which are significant for their applications in optics and optoelectronics.

  11. Effect of far-field flow on a columnar crystal in the convective undercooled melt

    NASA Astrophysics Data System (ADS)

    Ji, Xiao-Jian; Chen, Ming-Wen; Xu, Xiao-Hua; Wang, Zi-Dong

    2015-01-01

    The growth behavior of a columnar crystal in the convective undercooled melt affected by the far-field uniform flow is studied and the asymptotic solution for the interface evolution of the columnar crystal is derived by means of the asymptotic expansion method. The results obtained reveal that the far-field flow induces a significant change of the temperature around the columnar crystal and the convective flow caused by the far-field flow accelerates the growth velocity of the interface of the growing columnar crystal in the upstream direction and inhibits its growth velocity in the downstream direction. Our results are similar to the experimental data and numerical simulations. Project supported by the Overseas Distinguished Scholar Program by the Ministry of Education of China (Grant No. MS2010BJKJ005), the National Natural Science Foundation of China (Grant No. 10972030), and the Science and Technology Support Project of Jiangxi, China (Grant No. 20112BBE50006).

  12. Phase diagrams of mixtures of a polymer and a cholesteric liquid crystal under an external field

    SciTech Connect

    Matsuyama, Akihiko

    2014-11-14

    We present a mean field theory to describe phase behaviors in mixtures of a polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between a polymer and a liquid crystal under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. It is found that a cholesteric-nematic phase transition can be induced by not only the external field but also concentration and temperature. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch+pN), nematic-paranematic (N+pN), cholesteric-nematic (Ch+N) phase separations, etc., on the temperature-concentration plane. We also discuss mixtures of a non-chiral nematic liquid crystal and a chiral dopant.

  13. Controlled deposition or organic semiconductor single crystals and its application in field-effect transistors

    NASA Astrophysics Data System (ADS)

    Liu, Shuhong

    The search for low-cost, large area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors. Single-crystal organic field-effect transistors (OFETs) are ideal device structures for studying fundamental science associated with charge transport in organic materials and have demonstrated high mobility and outstanding electrical characteristics. For example, an exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated for rubrene single crystal field effect transistors. However, it remains a technical challenge to integrate single-crystal devices into practical electronic applications. A key difficulty is that organic single-crystal devices are usually fabricated one device at a time by handpicking a single crystal and placing it onto the device substrate. This makes it impossible to mass-produce at high density with reasonable throughput. Therefore, there is a great need for a high-throughput method for depositing large arrays of organic semiconductor single crystals directly onto device structures. In this dissertation, I develop several approaches towards realizing this goal. The first approach is a solution-processing technique, which relies on solvent wetting and de-wetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. The assembly of different organic crystals over centimeter-squared areas on Au, SiO 2 and flexible plastic substrates is demonstrated. By designing line features on the substrate, alignment of needle-like crystals is also achieved. As a demonstration of the potential application of this approach, arrays of organic single crystal FETs are fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes. Besides organic single crystals, this self-assembly strategy is also applicable for patterning other objects such as metallic nanowires. In the second technique, organic

  14. Magnetic field controlled single crystal growth and surface modification of titanium alloys exposed for biocompatibility

    NASA Astrophysics Data System (ADS)

    Hermann, Regina; Uhlemann, Margitta; Wendrock, Horst; Gerbeth, Gunter; Büchner, Bernd

    2011-03-01

    The aim of this work is growth and characterisation of Ti55Nb45 (wt%) single crystals by floating-zone single crystal growth of intermetallic compounds using two-phase radio-frequency (RF) electromagnetic heating. Thereby, the process and, in particular, the flow field in the molten zone is influenced by additional magnetic fields. The growth of massive intermetallic single crystals is very often unsuccessful due to an unfavourable solid-liquid interface geometry enclosing concave fringes. It is generally known that the crystallization process stability is enhanced if the crystallization interface is convex. For this, a tailored magnetic two-phase stirrer system has been developed, which enables a controlled influence on the melt ranging from intensive inwards to outwards flows. Since Ti is favourably light, strong and biocompatible, it is one of the few materials that naturally match the requirements for implantation in the human body. Therefore, the magnetic system was applied to crystal growth of Ti alloys. The grown crystals were oriented and cut to cubes with the desired crystallographic orientations [1 0 0] and [1 0 1] normally on a plane. The electron backscatter diffraction (EBSD) technique was applied to clearly determine crystal orientation and to localize grain boundaries. The formation of oxidic nanotubes on Ti surfaces in dependence of the grain orientation was investigated, performed electrochemically by anodic oxidation from fluoride containing electrolyte.

  15. Three-dimensional electric field visualization utilizing electric-field-induced second-harmonic generation in nematic liquid crystals

    NASA Astrophysics Data System (ADS)

    Chen, I.-Hsiu; Chu, Shi-Wei; Bresson, Francois; Tien, Ming-Chun; Shi, Jin-Wei; Sun, Chi-Kuang

    2003-08-01

    An electric-field-induced second-harmonic-generation signal in a nematic liquid crystal is used to map the electric field in an integrated-circuit-like sample. Since the electric-field-induced second-harmonic-generation signal intensity exhibits a strong dependence on the polarization of the incident laser beam, both the amplitude and the orientation of the electric field vectors can be measured. Combined with scanning second-harmonic-generation microscopy, three-dimensional electric field distribution can be easily visualized with high spatial resolution of the order of 1 μm.

  16. Spontaneous emission control of quantum dots embedded in photonic crystals: Effects of external fields and dimension

    NASA Astrophysics Data System (ADS)

    Vaseghi, B.; Hashemi, H.

    2016-06-01

    In this paper simultaneous effects of external electric and magnetic fields and quantum confinement on the radiation properties of spherical quantum dot embedded in a photonic crystal are investigated. Under the influence of photonic band-gap, effects of external static fields and dot dimension on the amplitude and spectrum of different radiation fields emitted by the quantum dot are studied. Our results show the considerable effects of external fields and quantum confinement on the spontaneous emission of the system.

  17. Monitoring of hydroxyapatite crystal formation using field-effect transistor

    NASA Astrophysics Data System (ADS)

    Kajisa, Taira; Sakata, Toshiya

    2016-04-01

    The biomineralization process of hydroxyapatite (HAp) in simulated body fluid (SBF) was monitored in realtime using extended-gate FETs whose gate electrode was modified with a variety of alkanethiol self-assembled monolayers (SAMs). It was found that the gate surface potential of the carboxyl- and amino-group-terminated SAM-coated gate FET was increased in SBF as HAp crystals grew on the gate surface. Moreover, in the carboxyl-group-terminated SAM-coated gate FET, the rate of increase and the shift of gate surface potential of the FET were found to depend on the concentration of calcium ions in the SBF. It was concluded that the process of HAp crystallization at a SAM-modified surface can be detected using FETs. Thus, a FET device that enables the easy detection of ionic charges in a real-time and label-free manner, will be useful for evaluating biomaterials based on biomineralization such as those in the bone regeneration process.

  18. Study of Fluid Flow Control In Protein Crystallization Using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F.; Ciszak, E.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in 'microgravity', researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  19. Phase field modelling of strain induced crystal growth in an elastic matrix.

    PubMed

    Laghmach, Rabia; Candau, Nicolas; Chazeau, Laurent; Munch, Etienne; Biben, Thierry

    2015-06-28

    When a crystal phase grows in an amorphous matrix, such as a crystallisable elastomer, containing cross-links and/or entanglements, these "topological constraints" need to be pushed away from the crystal phase to allow further crystallization. The accumulation of these topological constraints in the vicinity of the crystal interface may store elastic energy and affect the phase transition. To evaluate the consequences of such mechanism, we introduce a phase field model based on the Flory theory of entropic elasticity. We show that the growth process is indeed sensibly affected, in particular, an exponential increase of the surface energy with the displacement of the interface is induced. This explains the formation of stable nano-crystallites as it is observed in the Strain Induced Crystallization (SIC) of natural rubber. Although simple, the model developed here is able to account for many interesting features of SIC, for instance, the crystallite shapes and their sizes which depend on the applied deformation. PMID:26133455

  20. Diffraction and fringing field effects in small pixel liquid crystal devices with homeotropic alignment

    NASA Astrophysics Data System (ADS)

    Vanbrabant, Pieter J. M.; Beeckman, Jeroen; Neyts, Kristiaan; Willman, Eero; Fernandez, F. Anibal

    2010-10-01

    Reducing the pixel dimensions of liquid crystal microdisplays in search of high resolution has a fundamental impact on their electro-optic behavior. The liquid crystal director orientation becomes distorted due to fringing fields and diffraction effects influence the optical characteristics of the device once the structure features approach the wavelength of the incident light. Three-dimensional finite element simulation of the liquid crystal dynamics with a variable order approach is combined with a full-vector beam propagation analysis to investigate how elasticity and diffraction limit the resolution as a function of the pixel size for transmissive and reflective architectures with vertical liquid crystal alignment. The key liquid crystal properties are considered and the importance of materials with high birefringence is confirmed for small pixel devices as these improve the contrast for a fixed pixel size.

  1. Phase field modelling of strain induced crystal growth in an elastic matrix

    NASA Astrophysics Data System (ADS)

    Laghmach, Rabia; Candau, Nicolas; Chazeau, Laurent; Munch, Etienne; Biben, Thierry

    2015-06-01

    When a crystal phase grows in an amorphous matrix, such as a crystallisable elastomer, containing cross-links and/or entanglements, these "topological constraints" need to be pushed away from the crystal phase to allow further crystallization. The accumulation of these topological constraints in the vicinity of the crystal interface may store elastic energy and affect the phase transition. To evaluate the consequences of such mechanism, we introduce a phase field model based on the Flory theory of entropic elasticity. We show that the growth process is indeed sensibly affected, in particular, an exponential increase of the surface energy with the displacement of the interface is induced. This explains the formation of stable nano-crystallites as it is observed in the Strain Induced Crystallization (SIC) of natural rubber. Although simple, the model developed here is able to account for many interesting features of SIC, for instance, the crystallite shapes and their sizes which depend on the applied deformation.

  2. Reorientation of single-wall carbon nanotubes in negative anisotropy liquid crystals by an electric field

    PubMed Central

    García-García, Amanda; Vergaz, Ricardo; Algorri, José F; Zito, Gianluigi; Cacace, Teresa; Marino, Antigone; Otón, José M

    2016-01-01

    Summary Single-wall carbon nanotubes (SWCNT) are anisotropic nanoparticles that can cause modifications in the electrical and electro-optical properties of liquid crystals. The control of the SWCNT concentration, distribution and reorientation in such self-organized fluids allows for the possibility of tuning the liquid crystal properties. The alignment and reorientation of CNTs are studied in a system where the liquid crystal orientation effect has been isolated. Complementary studies including Raman spectroscopy, microscopic inspection and impedance studies were carried out. The results reveal an ordered reorientation of the CNTs induced by an electric field, which does not alter the orientation of the liquid crystal molecules. Moreover, impedance spectroscopy suggests a nonnegligible anchoring force between the CNTs and the liquid crystal molecules. PMID:27547599

  3. Reorientation of single-wall carbon nanotubes in negative anisotropy liquid crystals by an electric field.

    PubMed

    García-García, Amanda; Vergaz, Ricardo; Algorri, José F; Zito, Gianluigi; Cacace, Teresa; Marino, Antigone; Otón, José M; Geday, Morten A

    2016-01-01

    Single-wall carbon nanotubes (SWCNT) are anisotropic nanoparticles that can cause modifications in the electrical and electro-optical properties of liquid crystals. The control of the SWCNT concentration, distribution and reorientation in such self-organized fluids allows for the possibility of tuning the liquid crystal properties. The alignment and reorientation of CNTs are studied in a system where the liquid crystal orientation effect has been isolated. Complementary studies including Raman spectroscopy, microscopic inspection and impedance studies were carried out. The results reveal an ordered reorientation of the CNTs induced by an electric field, which does not alter the orientation of the liquid crystal molecules. Moreover, impedance spectroscopy suggests a nonnegligible anchoring force between the CNTs and the liquid crystal molecules. PMID:27547599

  4. Electric field effects on phase transitions in the 8CB liquid crystal doped with ferroelectric nanoparticles

    NASA Astrophysics Data System (ADS)

    Lin, Y.; Daoudi, A.; Segovia-Mera, A.; Dubois, F.; Legrand, C.; Douali, R.

    2016-06-01

    The influence of a low ac electric field on phase transitions is discussed in the case of a nematic liquid crystal 4 -n -octyl-4 '-cyanobiphenyl (8CB) doped with Sn2P2S6 ferroelectric nanoparticles. The phase-transition temperatures obtained from temperature-dependent dielectric measurements were higher than those determined by the calorimetric method. This difference is explained by the presence of the measuring electric field which induces two effects. The first one is the amplification of the interactions between the nanoparticle polarization and the liquid-crystal order parameter. The second one is the field-induced disaggregation or aggregation process at high nanoparticle concentrations.

  5. Far-field detection system for laser beam and crystal alignment

    NASA Astrophysics Data System (ADS)

    Zhang, Jiachen; Liu, Daizhong; Zhu, Baoqiang; Tang, Shunxing; Gao, Yanqi

    2016-03-01

    Laser beam far-field alignment as well as frequency-doubling and frequency-tripling crystal adjustment is very important for high-power laser facility. Separate systems for beam and crystal alignment are generally used while the proposed approach by off-axial grating sampling share common optics for these two functions, reducing both space and cost requirements. This detection system has been demonstrated on the National Laser Facility of Israel. The experimental results indicate that the average far-field alignment error is <5% of the spatial filter pinhole diameter, average autocollimation angle error of crystals is <10 μrad, and average frequency-tripling conversion efficiency is 69.3%, which meet the alignment system requirements on the beam direction and crystals.

  6. Heat transport in polymer-dispersed liquid crystals under electric field

    NASA Astrophysics Data System (ADS)

    Hadj Sahraoui, Abdelhak; Delenclos, Sylvain; Longuemart, Stéphane; Dadarlat, Dorin

    2011-08-01

    The concepts of effective thermal conductivity and interfacial thermal contact resistance in composite media are applied to study heat transport in polymer-dispersed liquid crystals (PDLC). In these systems, the thermal properties of liquid crystal inclusions are changed by an imposed electric field. The photopyroelectric (PPE) technique with a cell allowing the application of an electric field to the sample is used to measure the thermal parameters. A model based on effective medium approximation is used to assess the impact of interfaces on the flow of heat through the determination of the Kapitza radius. It was found that the effect of interfaces becomes dominant compared to the volume conduction of the droplet when the liquid crystal (LC) droplet radius becomes smaller than 1 micron. The comparison of the thermal behavior of LC in the droplets with that of bulk liquid crystal allowed to evaluate the effect of confinement on the LC nematic phase.

  7. Self-Aligned Growth of Organic Semiconductor Single Crystals by Electric Field.

    PubMed

    Kotsuki, Kenji; Obata, Seiji; Saiki, Koichiro

    2016-01-19

    We proposed a novel but facile method for growing organic semiconductor single-crystals via solvent vapor annealing (SVA) under electric field. In the conventional SVA growth process, nuclei of crystals appeared anywhere on the substrate and their crystallographic axes were randomly distributed. We applied electric field during the SVA growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) on the SiO2/Si substrate on which a pair of electrodes had been deposited beforehand. Real-time observation of the SVA process revealed that rodlike single crystals grew with their long axes parallel to the electric field and bridged the prepatterned electrodes. As a result, C8-BTBT crystals automatically formed a field effect transistor (FET) structure and the mobility reached 1.9 cm(2)/(V s). Electric-field-assisted SVA proved a promising method for constructing high-mobility single-crystal FETs at the desired position by a low-cost solution process. PMID:26695105

  8. Separating different contributions to the crystal-field parameters using Wannier functions.

    PubMed

    Scaramucci, A; Ammann, J; Spaldin, N A; Ederer, C

    2015-05-01

    We discuss the calculation of crystal-field splittings using Wannier functions and show how contributions to the crystal-field splitting that are due to hybridization with different ligand states can be separated from the bare Coulomb contribution by constructing sets of Wannier functions incorporating different levels of hybridization. We demonstrate this method using SrVO3 as a generic example of a transition metal oxide. We then calculate trends in the crystal-field splitting for two series of hypothetical tetragonally distorted perovskite oxides and discuss the relation between the calculated 'electrostatic' contribution to the crystal field and the simple point charge model. Finally, we apply our method to the charge disproportionated 5d electron system CsAuCl3. The proposed procedure elucidates the way in which the negative charge transfer energy in this material leads to a reversal of the p-d ligand contribution to the crystal-field splitting such that the eg states of the nominally Au(3+) cation are energetically lower than the corresponding t2g states. PMID:25872527

  9. Internal bias field in triglycine sulphate crystals with L-, α-alanine grown at negative temperatures

    NASA Astrophysics Data System (ADS)

    Milovidova, S. D.; Rogazinskaya, O. V.; Sidorkin, A. S.; Ionova, E. V.; Kirichenko, A. P.; Bavykin, S. A.

    2010-09-01

    The dielectric and pyroelectric properties of triglycine sulphate (TGS) crystals with L, α-alanine impurities grown at negative temperatures have been investigated. It is shown that a lower impurity concentration (2 mol % in solution) in this temperature range leads to the formation of internal bias fields of the same order of magnitude (˜800 V/cm) as for TGS crystals grown at T ⩽ 50°C but with an L, α-alanine concentration of 20 mol % in solution.

  10. Application of a rotating magnetic field to semiconductor crystal growth in Space

    NASA Astrophysics Data System (ADS)

    Senchenkov, A. S.; Barmin, I. V.

    2003-12-01

    To eliminate the tremendous influence of the residual accelerations on homogeneity of the crystal growing in a space experiment, a rotating magnetic field (RMF) is used. A number of the experiments have been performed in space within the RMF both in the frame of the Russian national program and together with European scientists. In the paper some theoretical and experimental results illustrating the effectiveness of RMF application to crystal growth under microgravity conditions are presented. Tables 2, Figs 5, Refs 8.

  11. Electron correlations in semiconductors: Bulk cohesive properties and magnetic-field-induced Wigner crystal at heterojunctions

    SciTech Connect

    Louie, S.G.; Zhu, X.

    1992-08-01

    A correlated wavefunction variational quantum Monte Carlo approach to the studies of electron exchange and correlation effects in semiconductors is presented. Applications discussed include the cohesive and structural properties of bulk semiconductors, and the magnetic-field-induced Wigner electron crystal in two dimensions. Landau level mixing is shown to be important in determining the transition between the quantum Hall liquid and the Wigner crystal states in the regime of relevant experimental parameters.

  12. Nonlinear optical tuning of photonic crystal microcavities by near-field probe

    SciTech Connect

    Vignolini, Silvia; Zani, Margherita; Riboli, Francesco; Vinattieri, Anna; Wiersma, Diederik S.; Gurioli, Massimo; Intonti, Francesca; Balet, Laurent; Li, Lianhe; Colocci, Marcello; Francardi, Marco; Gerardino, Annamaria; Fiore, Andrea

    2008-07-14

    We report on a nonlinear way to control and tune the dielectric environment of photonic crystal microcavities exploiting the local heating induced by near-field laser excitation at different excitation powers. The temperature gradient due to the optical absorption results in an index of refraction gradient which modifies the dielectric surroundings of the cavity and shifts the optical modes. Reversible tuning can be obtained either by changing the excitation power density or by exciting in different points of the photonic crystal microcavity.

  13. Role of the crystal field stabilization energy in the formation of metal(II) formate mixed crystals

    NASA Astrophysics Data System (ADS)

    Balarew, Christo; Stoilova, Donka; Vassileva, Violeta

    A relationship between the distribution coefficient values and the factors determining the isomorphous substitution of some metal(II) formates (Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd) has been found, given by D=[exp⁡{aṡf[ΔR/R]+bṡϕ(Δɛ)+cṡψ(Δs)}/{RT}, where Δ R/R is the relative difference in the ionic radii of the intersubstituting ions, Δɛ is the difference in the Me sbnd O bond energy, Δ s is the difference in the crystal field stabilization energy. The pre-exponential term represents the balance in bonding factors between the ions in the crystal and in the aqueous solution, in the case of ideally mixing in the solid state. The exponential term takes into account the enthalpy of mixing in the solid state. For the isostructural formate salts in which the substitution of a given cation by another one occurs in equivalent octahedral positions, the difference in the crystal field stabilization energy exerts the most important influence on the enthalpy of mixing.

  14. Atomic density functional and diagram of structures in the phase field crystal model

    NASA Astrophysics Data System (ADS)

    Ankudinov, V. E.; Galenko, P. K.; Kropotin, N. V.; Krivilyov, M. D.

    2016-02-01

    The phase field crystal model provides a continual description of the atomic density over the diffusion time of reactions. We consider a homogeneous structure (liquid) and a perfect periodic crystal, which are constructed from the one-mode approximation of the phase field crystal model. A diagram of 2D structures is constructed from the analytic solutions of the model using atomic density functionals. The diagram predicts equilibrium atomic configurations for transitions from the metastable state and includes the domains of existence of homogeneous, triangular, and striped structures corresponding to a liquid, a body-centered cubic crystal, and a longitudinal cross section of cylindrical tubes. The method developed here is employed for constructing the diagram for the homogeneous liquid phase and the body-centered iron lattice. The expression for the free energy is derived analytically from density functional theory. The specific features of approximating the phase field crystal model are compared with the approximations and conclusions of the weak crystallization and 2D melting theories.

  15. A phase-field model coupled with lattice kinetics solver for modeling crystal growth in furnaces

    SciTech Connect

    Lin, Guang; Bao, Jie; Xu, Zhijie; Tartakovsky, Alexandre M.; Henager, Charles H.

    2014-02-02

    In this study, we present a new numerical model for crystal growth in a vertical solidification system. This model takes into account the buoyancy induced convective flow and its effect on the crystal growth process. The evolution of the crystal growth interface is simulated using the phase-field method. Two novel phase-field models are developed to model the crystal growth interface in vertical gradient furnaces with two temperature profile setups: 1) fixed wall temperature profile setup and 2) time-dependent temperature profile setup. A semi-implicit lattice kinetics solver based on the Boltzmann equation is employed to model the unsteady incompressible flow. This model is used to investigate the effect of furnace operational conditions on crystal growth interface profiles and growth velocities. For a simple case of macroscopic radial growth, the phase-field model is validated against an analytical solution. Crystal growth in vertical gradient furnaces with two temperature profile setups have been also investigated using the developed model. The numerical simulations reveal that for a certain set of temperature boundary conditions, the heat transport in the melt near the phase interface is diffusion dominant and advection is suppressed.

  16. Giant Magnetic Field-induced Phase Transitions in Dimeric Liquid Crystals

    NASA Astrophysics Data System (ADS)

    Salili, Seyyed Muhammad; Salamonczyk, Miroslaw; Tamba, Maria-Gabriela; Sprunt, Samuel; Mehl, Georg; Jakli, Antal; Gleeson, James; Kent Group Collaboration; Hull Group Collaboration

    Liquid crystals are responsive to external fields such as electric, magnetic fields. The first experimental observation of dependence of isotropic to nematic phase transition on the applied magnetic field was done using a strong magnetic field on bent-core nematogens and the phase transition temperature exhibited an upshift of 0.7 C at B =30 T. We report on measurements of giant magnetic field-induced isotropic-nematic transition of chainsticks (nunchuks) type dimeric liquid crystals. Upon using the B =25 T split-helix resistive solenoid magnet at NHMFL, we have observed up to 18 C upshift of the isotropic to nematic phase transition temperature at B =22T. We discuss the results within the context of differential thermodynamic potential and the two basic mean-field theories. To our knowledge, this is the first observation of such huge shifts in the phase transitions of thermotropic liquid ctystals

  17. Semiconductor crystal growth in crossed electric and magnetic fields: Center Director's Discretionary Fund

    NASA Technical Reports Server (NTRS)

    Mazuruk, K.; Volz, M. P.

    1996-01-01

    A unique growth cell was designed in which crossed electric and magnetic fields could be separately or simultaneously applied during semiconductor crystal growth. A thermocouple was inserted into an InSb melt inside the growth cell to examine the temperature response of the fluid to applied electromagnetic fields. A static magnetic field suppressed time-dependent convection when a destabilizing thermal field was applied. The simultaneous application of electric and magnetic fields resulted in forced convection in the melt. The InSb ingots grown in the cell were polycrystalline. An InGaSb crystal, 0.5 cm in diameter and 23-cm long, was grown without electromagnetic fields applied. The axial composition results indicated that complete mixing in the melt occurred for this large aspect ratio.

  18. Control of Convection by Dynamic Magnetic Fields for VB, FZ and THM Crystal Growth Application

    NASA Technical Reports Server (NTRS)

    Mazuruk, Konstantin

    2000-01-01

    The Traveling Magnetic Field (TMF) technique, recently proposed to control meridional flow in electrically conducting melts is reviewed. In particular, the natural convection damping capability of this technique has been numerically demonstrated with the implication of significantly improving crystal quality. Advantages of the traveling magnetic field, in comparison to the more mature rotating magnetic field method, are discussed. Finally, a detailed one-dimensional planar TMF model has been developed and is presented.

  19. Fabrication of specimens of metamorphic magnetite crystals for field ion microscopy and atom probe microanalysis.

    PubMed

    Kuhlman, K R; Martens, R L; Kelly, T F; Evans, N D; Miller, M K

    2001-10-01

    Field ion specimens have been successfully fabricated from samples of metamorphic magnetite crystals (Fe3O4) extracted from a polymetamorphosed, granulite-facies marble with the use of a focused ion beam. These magnetite crystals contain nanometer-scale, disk-shaped inclusions making this magnetite particularly attractive for investigating the capabilities of atom probe field ion microscopy (APFIM) for geological materials. Field ion microscope images of these magnetite crystals were obtained in which the observed size and morphology of the precipitates agree with previous results. Samples were analyzed in the energy compensated optical position-sensitive atom probe. Mass spectra were obtained in which peaks for singly ionized 16O, 56Fe and 56FeO and doubly ionized 54Fe, 56Fe and 57Fe peaks were fully resolved. Manganese and aluminum were observed in a limited analysis of a precipitate in an energy compensated position sensitive atom probe. PMID:11770743

  20. Determination of optimal ionic liquid for organic single-crystal field-effect transistors

    NASA Astrophysics Data System (ADS)

    Ono, S.; Miwa, K.; Seki, S.

    2016-02-01

    We investigate organic single-crystal field-effect transistors with various ionic liquids as gate dielectric. We find that the mobility of the field-effect transistors for both p-type and n-type organic semiconductors increases with decreasing total capacitance of the ionic liquid. However, it does not depend on the ion species at the interface between the organic semiconductor and the ionic liquid. By choosing an appropriate ionic liquid, a high carrier mobility of 12.4 cm2/V s in rubrene single crystals (p-type) and 0.13 cm2/V s in 7.7.8.8-Tetracyanoquinodimethane single crystals (n-type) are achieved. This study clarifies the influence of ionic liquids on the device performance of organic field-effect transistors and shows a way to maximize carrier mobility at the solid/liquid interface.

  1. Crystal structure analysis of LiTaO3 under electric field

    NASA Astrophysics Data System (ADS)

    Aoyagi, Shinobu; Osawa, Hitoshi; Sugimoto, Kunihisa; Iwata, Makoto; Takeda, Shoichi; Moriyoshi, Chikako; Kuroiwa, Yoshihiro

    2015-10-01

    Structural changes of a stoichiometric LiTaO3 single crystal accompanied by polarization switching are investigated using high-energy X-ray diffraction under static electric fields. The electric field dependence of the c-axis lattice constant depicts a small hysteresis and shows a butterfly curve. Inversion of the crystal structure accompanied by polarization switching is clearly detected with changes in the diffraction intensities of Friedel pairs. The electric field dependences of the atomic positions and volume ratio of the ferroelectric domains are obtained by crystal structure analyses. The results are fully consistent with the bulk properties and ensure that the present experimental techniques are applicable to detecting the transient atomic motions in the nucleation and growth of the ferroelectric domains during polarization switching.

  2. Crystal field parameters and energy levels scheme of trivalent chromium doped BSO

    SciTech Connect

    Petkova, P.; Andreici, E.-L.; Avram, N. M.

    2014-11-24

    The aim of this paper is to give an analysis of crystal field parameters and energy levels schemes for the above doped material, in order to give a reliable explanation for experimental data. The crystal field parameters have been modeled in the frame of Exchange Charge Model (ECM) of the crystal field theory, taken into account the geometry of systems, with actually site symmetry of the impurity ions. The effect of the charges of the ligands and covalence bonding between chromium cation and oxygen anions, in the cluster approach, also were taken into account. With the obtained values of the crystal field parameters we simulated the scheme of energy levels of chromium ions by diagonalizing the matrix of the Hamiltonian of the doped crystal. The obtained energy levels and estimated Racah parameters B and C were compared with the experimental spectroscopic data and discussed. Comparison with experiment shows that the results are quite satisfactory which justify the model and simulation scheme used for the title system.

  3. Crystal field parameters and energy levels scheme of trivalent chromium doped BSO

    NASA Astrophysics Data System (ADS)

    Petkova, P.; Andreici, E.-L.; Avram, N. M.

    2014-11-01

    The aim of this paper is to give an analysis of crystal field parameters and energy levels schemes for the above doped material, in order to give a reliable explanation for experimental data. The crystal field parameters have been modeled in the frame of Exchange Charge Model (ECM) of the crystal field theory, taken into account the geometry of systems, with actually site symmetry of the impurity ions. The effect of the charges of the ligands and covalence bonding between chromium cation and oxygen anions, in the cluster approach, also were taken into account. With the obtained values of the crystal field parameters we simulated the scheme of energy levels of chromium ions by diagonalizing the matrix of the Hamiltonian of the doped crystal. The obtained energy levels and estimated Racah parameters B and C were compared with the experimental spectroscopic data and discussed. Comparison with experiment shows that the results are quite satisfactory which justify the model and simulation scheme used for the title system.

  4. Long-range orientational order, local-field anisotropy, and mean molecular polarizability in liquid crystals

    SciTech Connect

    Aver'yanov, E. M.

    2009-01-15

    The problems on the relation of the mean effective molecular polarizability {gamma}-bar to the long-range orientational order of molecules (the optical anisotropy of the medium) in uniaxial and biaxial liquid crystals, the local anisotropy on mesoscopic scales, and the anisotropy of the Lorentz tensor L and the local-field tensor f are formulated and solved. It is demonstrated that the presence of the long-range orientational order of molecules in liquid crystals imposes limitations from below on the molecular polarizability {gamma}-bar, which differs for uniaxial and biaxial liquid crystals. The relation between the local anisotropy and the molecular polarizability {gamma}-bar is investigated for calamitic and discotic uniaxial liquid crystals consisting of lath- and disk-shaped molecules. These liquid crystals with identical macroscopic symmetry differ in the local anisotropy and the relationships between the components L{sub parallel} < L{sub perpendicular} , f{sub parallel} < f{sub perpendicular} (calamitic) and L{sub parallel} > L{sub perpendicular} , f{sub parallel} > f{sub perpendicular} (discotic) for an electric field oriented parallel and perpendicular to the director. The limitations from below and above on the molecular polarizability {gamma}-bar due to the anisotropy of the tensors L and f are established for liquid crystals of both types. These limitations indicate that the molecular polarizability {gamma}-bar depends on the phase state and the temperature. The factors responsible for the nonphysical consequences of the local-field models based on the approximation {gamma}-bar = const are revealed. The theoretical inferences are confirmed by the experimental data for a number of calamitic nematic liquid crystals with different values of birefringence and the discotic liquid crystal Col{sub ho}.

  5. An optimized intermolecular force field for hydrogen-bonded organic molecular crystals using atomic multipole electrostatics.

    PubMed

    Pyzer-Knapp, Edward O; Thompson, Hugh P G; Day, Graeme M

    2016-08-01

    We present a re-parameterization of a popular intermolecular force field for describing intermolecular interactions in the organic solid state. Specifically we optimize the performance of the exp-6 force field when used in conjunction with atomic multipole electrostatics. We also parameterize force fields that are optimized for use with multipoles derived from polarized molecular electron densities, to account for induction effects in molecular crystals. Parameterization is performed against a set of 186 experimentally determined, low-temperature crystal structures and 53 measured sublimation enthalpies of hydrogen-bonding organic molecules. The resulting force fields are tested on a validation set of 129 crystal structures and show improved reproduction of the structures and lattice energies of a range of organic molecular crystals compared with the original force field with atomic partial charge electrostatics. Unit-cell dimensions of the validation set are typically reproduced to within 3% with the re-parameterized force fields. Lattice energies, which were all included during parameterization, are systematically underestimated when compared with measured sublimation enthalpies, with mean absolute errors of between 7.4 and 9.0%. PMID:27484370

  6. An optimized intermolecular force field for hydrogen-bonded organic molecular crystals using atomic multipole electrostatics

    PubMed Central

    Pyzer-Knapp, Edward O.; Thompson, Hugh P. G.; Day, Graeme M.

    2016-01-01

    We present a re-parameterization of a popular intermolecular force field for describing intermolecular interactions in the organic solid state. Specifically we optimize the performance of the exp-6 force field when used in conjunction with atomic multipole electrostatics. We also parameterize force fields that are optimized for use with multipoles derived from polarized molecular electron densities, to account for induction effects in molecular crystals. Parameterization is performed against a set of 186 experimentally determined, low-temperature crystal structures and 53 measured sublimation enthalpies of hydrogen-bonding organic molecules. The resulting force fields are tested on a validation set of 129 crystal structures and show improved reproduction of the structures and lattice energies of a range of organic molecular crystals compared with the original force field with atomic partial charge electrostatics. Unit-cell dimensions of the validation set are typically reproduced to within 3% with the re-parameterized force fields. Lattice energies, which were all included during parameterization, are systematically underestimated when compared with measured sublimation enthalpies, with mean absolute errors of between 7.4 and 9.0%. PMID:27484370

  7. Metal electrode dependent field effect transistors made of lanthanide ion-doped DNA crystals

    NASA Astrophysics Data System (ADS)

    Reddy Dugasani, Sreekantha; Hwang, Taehyun; Kim, Jang Ah; Gnapareddy, Bramaramba; Kim, Taesung; Park, Sung Ha

    2016-03-01

    We fabricated lanthanide ion (Ln3+, e.g. Dy3+, Er3+, Eu3+, and Gd3+)-doped self-assembled double-crossover (DX) DNA crystals grown on the surface of field effect transistors (FETs) containing either a Cr, Au, or Ni electrode. Here we demonstrate the metal electrode dependent FET characteristics as a function of various Ln3+. The drain-source current (I ds), controlled by the drain-source voltage (V ds) of Ln3+-doped DX DNA crystals with a Cr electrode on an FET, changed significantly under various gate voltages (V g) due to the relative closeness of the work function of Cr to the energy band gap of Ln3+-DNA crystals compared to those of Au and Ni. For Ln3+-DNA crystals on an FET with either a Cr or Ni electrode at a fixed V ds, I ds decreased with increasing V g ranging from  -2 to 0 V and from 0 to  +3 V in the positive and negative regions, respectively. By contrast, I ds for Ln3+-DNA crystals on an FET with Au decreased with increasing V g in only the positive region due to the greater electronegativity of Au. Furthermore, Ln3+-DNA crystals on an FET exhibited behaviour sensitive to V g due to the appreciable charge carriers generated from Ln3+. Finally, we address the resistivity and the mobility of Ln3+-DNA crystals on an FET with different metal electrodes obtained from I ds-V ds and I ds-V g curves. The resistivities of Ln3+-DNA crystals on FETs with Cr and Au electrodes were smaller than those of pristine DNA crystals on an FET, and the mobility of Ln3+-DNA crystals on an FET with Cr was relatively higher than that associated with other electrodes.

  8. Twist-bend nematic liquid crystals in high magnetic fields.

    PubMed

    Challa, P K; Borshch, V; Parri, O; Imrie, C T; Sprunt, S N; Gleeson, J T; Lavrentovich, O D; Jákli, A

    2014-06-01

    We present magneto-optic measurements on two materials that form the recently discovered twist-bend nematic (N_{tb}) phase. This intriguing state of matter represents a fluid phase that is orientationally anisotropic in three directions and also exhibits translational order with periodicity several times larger than the molecular size. N_{tb} materials may also spontaneously form a visible, macroscopic stripe texture. We show that the optical stripe texture can be persistently inhibited by a magnetic field, and a 25T external magnetic field depresses the N-N_{tb} phase transition temperature by almost 1{∘}C. We propose a quantitative mechanism to account for this shift and suggest a Helfrich-Hurault-type mechanism for the optical stripe formation. PMID:25019707

  9. Field-induced phase transitions in chiral smectic liquid crystals studied by the constant current method

    NASA Astrophysics Data System (ADS)

    H, Dhaouadi; R, Zgueb; O, Riahi; F, Trabelsi; T, Othman

    2016-05-01

    In ferroelectric liquid crystals, phase transitions can be induced by an electric field. The current constant method allows these transition to be quickly localized and thus the (E,T) phase diagram of the studied product can be obtained. In this work, we make a slight modification to the measurement principles based on this method. This modification allows the characteristic parameters of ferroelectric liquid crystal to be quantitatively measured. The use of a current square signal highlights a phenomenon of ferroelectric hysteresis with remnant polarization at null field, which points out an effect of memory in this compound.

  10. Accurate force fields and methods for modelling organic molecular crystals at finite temperatures.

    PubMed

    Nyman, Jonas; Pundyke, Orla Sheehan; Day, Graeme M

    2016-06-21

    We present an assessment of the performance of several force fields for modelling intermolecular interactions in organic molecular crystals using the X23 benchmark set. The performance of the force fields is compared to several popular dispersion corrected density functional methods. In addition, we present our implementation of lattice vibrational free energy calculations in the quasi-harmonic approximation, using several methods to account for phonon dispersion. This allows us to also benchmark the force fields' reproduction of finite temperature crystal structures. The results demonstrate that anisotropic atom-atom multipole-based force fields can be as accurate as several popular DFT-D methods, but have errors 2-3 times larger than the current best DFT-D methods. The largest error in the examined force fields is a systematic underestimation of the (absolute) lattice energy. PMID:27230942

  11. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  12. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-11-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  13. Dark-field transmission electron microscopy of cortical bone reveals details of extrafibrillar crystals.

    PubMed

    Schwarcz, Henry P; McNally, Elizabeth A; Botton, Gianluigi A

    2014-12-01

    In a previous study we showed that most of the mineral in bone is present in the form of "mineral structures", 5-6nm-thick, elongated plates which surround and are oriented parallel to collagen fibrils. Using dark-field transmission electron microscopy, we viewed mineral structures in ion-milled sections of cortical human bone cut parallel to the collagen fibrils. Within the mineral structures we observe single crystals of apatite averaging 5.8±2.7nm in width and 28±19nm in length, their long axes oriented parallel to the fibril axis. Some appear to be composite, co-aligned crystals as thin as 2nm. From their similarity to TEM images of crystals liberated from deproteinated bone we infer that we are viewing sections through platy crystals of apatite that are assembled together to form the mineral structures. PMID:25449316

  14. An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth

    PubMed Central

    Potticary, Jason; Terry, Lui R.; Bell, Christopher; Papanikolopoulos, Alexandros N.; Christianen, Peter C. M.; Engelkamp, Hans; Collins, Andrew M.; Fontanesi, Claudio; Kociok-Köhn, Gabriele; Crampin, Simon; Da Como, Enrico; Hall, Simon R.

    2016-01-01

    The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal. PMID:27161600

  15. Electric field generation of Skyrmion-like structures in a nematic liquid crystal.

    PubMed

    Cattaneo, Laura; Kos, Žiga; Savoini, Matteo; Kouwer, Paul; Rowan, Alan; Ravnik, Miha; Muševič, Igor; Rasing, Theo

    2016-01-21

    Skyrmions are particle-like topological objects that are increasingly drawing attention in condensed matter physics, where they are connected to inversion symmetry breaking and chirality. Here we report the generation of stable Skyrmion-like structures in a thin nematic liquid crystal film on chemically patterned patchy surfaces. Using the interplay of material elasticity and surface boundary conditions, we use a strong electric field to quench the nematic liquid crystal from a fully aligned phase to vortex-like nematic liquid crystal structures, centered on patterned patches, which carry two different sorts of topological defects. Numerical calculations reveal that these are Skyrmion-like structures, seeded from the surface boojum topological defects and swirling towards the second confining surface. These observations, supported by numerical methods, demonstrate the possibility to generate, manipulate and study Skyrmion-like objects in nematic liquid crystals on patterned surfaces. PMID:26549212

  16. An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth

    NASA Astrophysics Data System (ADS)

    Potticary, Jason; Terry, Lui R.; Bell, Christopher; Papanikolopoulos, Alexandros N.; Christianen, Peter C. M.; Engelkamp, Hans; Collins, Andrew M.; Fontanesi, Claudio; Kociok-Köhn, Gabriele; Crampin, Simon; da Como, Enrico; Hall, Simon R.

    2016-05-01

    The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal.

  17. A full field, 3-D velocimeter for microgravity crystallization experiments

    NASA Technical Reports Server (NTRS)

    Brodkey, Robert S.; Russ, Keith M.

    1991-01-01

    The programming and algorithms needed for implementing a full-field, 3-D velocimeter for laminar flow systems and the appropriate hardware to fully implement this ultimate system are discussed. It appears that imaging using a synched pair of video cameras and digitizer boards with synched rails for camera motion will provide a viable solution to the laminar tracking problem. The algorithms given here are simple, which should speed processing. On a heavily loaded VAXstation 3100 the particle identification can take 15 to 30 seconds, with the tracking taking less than one second. It seeems reasonable to assume that four image pairs can thus be acquired and analyzed in under one minute.

  18. Electric-field-induced weakly chaotic transients in ferroelectric liquid crystals.

    PubMed

    Śliwa, I; Jeżewski, W; Kuczyński, W

    2016-01-01

    Nonlinear dynamics induced in surface stabilized ferroelectric liquid crystals by strong alternating external electric fields is studied both theoretically and experimentally. As has already been shown, molecular reorientations induced by sufficiently strong fields of high-enough frequencies can reveal a long transient behavior that has a weakly chaotic character. The resulting complex dynamics of ferroelectric liquid crystals can be considered not only as a consequence of irregular motions of particular molecules but also as a repercussion of a surface-enforced partial decorrelation of nonlinear molecular motions within smectic layers. To achieve more insight into the nature of this phenomenon and to show that the underlying complex field-induced behavior of smectic liquid crystals is not exceptional, ranges of system parameters for which the chaotic behavior occurs are determined. It is proved that there exists a large enough set of initial phase trajectory points, for which weakly chaotic long-time transitory phenomena occur, and, thereby, it is demonstrated that such a chaotic behavior can be regarded as being typical for strongly field-driven thin liquid crystal systems. Additionally, the influence of low-amplitude random noise on the duration of the transient processes is numerically studied. The strongly nonlinear contribution to the electro-optic response, experimentally determined for liquid crystal samples at frequencies lower than the actual field frequency, is also analyzed for long-time signal sequences. Using a statistical approach to distinguish numerically response signals of samples from noise generated by measuring devices, it is shown that the distribution of sample signals distinctly differs from the device noise. This evidently corroborates the occurrence of the nonlinear low-frequency effect, found earlier for different surface stabilized liquid crystal samples. PMID:26871130

  19. Pulsed field actuation of Ni-Mn-Ga ferromagnetic shape memory alloy single crystal

    NASA Astrophysics Data System (ADS)

    Marioni, M.; Bono, D.; Banful, A. B.; del Rosario, M.; Rodriguez, E.; Peterson, B. W.; Allen, S. M.; O'Handley, R. C.

    2003-10-01

    Ferromagnetic Shape Memory Alloy Ni-Mn-Ga has twin boundaries in the martensitic phase that move when a suitable magnetic field is applied. In this fashion strains of up to 6% have been observed for static fields in single crystals [1]. Recently 2.5% strain has been demonstrated [2] in Ni-Mn-Ga single crystals for oscillating fields up to frequencies of 75 Hz (150 Hz actuation). This work studies the actuation of single crystals when pulsed fields are applied. Fields in the 0.4-1.5MA/m-range were generated in an air coil with rise times of the order of 1ms and below. The elongation of the samples is measured with a light beam reflected off the tip of the crystal. Single twin boundaries have been observed to advance 0.16 mm during 600 μsec-ong pulses. Actuation has been shown to be possible at least up to frequencies of 1700 Hz.

  20. Polymorphism, crystal nucleation and growth in the phase-field crystal model in 2D and 3D.

    PubMed

    Tóth, Gyula I; Tegze, György; Pusztai, Tamás; Tóth, Gergely; Gránásy, László

    2010-09-15

    We apply a simple dynamical density functional theory, the phase-field crystal (PFC) model of overdamped conservative dynamics, to address polymorphism, crystal nucleation, and crystal growth in the diffusion-controlled limit. We refine the phase diagram for 3D, and determine the line free energy in 2D and the height of the nucleation barrier in 2D and 3D for homogeneous and heterogeneous nucleation by solving the respective Euler-Lagrange (EL) equations. We demonstrate that, in the PFC model, the body-centered cubic (bcc), the face-centered cubic (fcc), and the hexagonal close-packed structures (hcp) compete, while the simple cubic structure is unstable, and that phase preference can be tuned by changing the model parameters: close to the critical point the bcc structure is stable, while far from the critical point the fcc prevails, with an hcp stability domain in between. We note that with increasing distance from the critical point the equilibrium shapes vary from the sphere to specific faceted shapes: rhombic dodecahedron (bcc), truncated octahedron (fcc), and hexagonal prism (hcp). Solving the equation of motion of the PFC model supplied with conserved noise, solidification starts with the nucleation of an amorphous precursor phase, into which the stable crystalline phase nucleates. The growth rate is found to be time dependent and anisotropic; this anisotropy depends on the driving force. We show that due to the diffusion-controlled growth mechanism, which is especially relevant for crystal aggregation in colloidal systems, dendritic growth structures evolve in large-scale isothermal single-component PFC simulations. An oscillatory effective pair potential resembling those for model glass formers has been evaluated from structural data of the amorphous phase obtained by instantaneous quenching. Finally, we present results for eutectic solidification in a binary PFC model. PMID:21386517

  1. Polymorphism, crystal nucleation and growth in the phase-field crystal model in 2D and 3D

    NASA Astrophysics Data System (ADS)

    Tóth, Gyula I.; Tegze, György; Pusztai, Tamás; Tóth, Gergely; Gránásy, László

    2010-09-01

    We apply a simple dynamical density functional theory, the phase-field crystal (PFC) model of overdamped conservative dynamics, to address polymorphism, crystal nucleation, and crystal growth in the diffusion-controlled limit. We refine the phase diagram for 3D, and determine the line free energy in 2D and the height of the nucleation barrier in 2D and 3D for homogeneous and heterogeneous nucleation by solving the respective Euler-Lagrange (EL) equations. We demonstrate that, in the PFC model, the body-centered cubic (bcc), the face-centered cubic (fcc), and the hexagonal close-packed structures (hcp) compete, while the simple cubic structure is unstable, and that phase preference can be tuned by changing the model parameters: close to the critical point the bcc structure is stable, while far from the critical point the fcc prevails, with an hcp stability domain in between. We note that with increasing distance from the critical point the equilibrium shapes vary from the sphere to specific faceted shapes: rhombic dodecahedron (bcc), truncated octahedron (fcc), and hexagonal prism (hcp). Solving the equation of motion of the PFC model supplied with conserved noise, solidification starts with the nucleation of an amorphous precursor phase, into which the stable crystalline phase nucleates. The growth rate is found to be time dependent and anisotropic; this anisotropy depends on the driving force. We show that due to the diffusion-controlled growth mechanism, which is especially relevant for crystal aggregation in colloidal systems, dendritic growth structures evolve in large-scale isothermal single-component PFC simulations. An oscillatory effective pair potential resembling those for model glass formers has been evaluated from structural data of the amorphous phase obtained by instantaneous quenching. Finally, we present results for eutectic solidification in a binary PFC model.

  2. Thermoelectric Magnetohydrodynamic Flow During Crystal Growth with a Moderate or Weak Magnetic Field

    NASA Technical Reports Server (NTRS)

    Khine, Y. Y.; Walker, John S.; Szofran, Frank R.; Rose, M. Franklin (Technical Monitor)

    2000-01-01

    This paper treats a steady, axisymmetric melt motion in a cylindrical ampoule with a uniform, axial magnetic field and with an electric current due to a radial temperature variation along the crystal-melt interface, where the values of the absolute thermoelectric power for the crystal and melt are different. The radial component of the thermoelectric current in the melt produces an azimuthal body force, and the axial variation of the centrifugal force due to the azimuthal motion drives a meridional circulation with radial and axial velocities. For moderate magnetic field strengths, the azimuthal velocity and magnetic field produce a radial induced electric field which partially cancels the Seebeck electromotive force in the melt, so that the thermoelectric current and the melt motion are coupled. For weak magnetic fields, the thermoelectric current is decoupled from the melt motion, which is an ordinary hydrodynamic flow driven by a known azimuthal body force. The results show how the flow varies with the strength of the magnetic field and with the magnitude of the temperature variation along the crystal-melt interface. They also define the parameter ranges for which the simpler weak-field decoupled analysis gives accurate predictions.

  3. Deuterium NMR investigations of field-induced director alignment in nematic liquid crystals.

    PubMed

    Sugimura, Akihiko; Luckhurst, Geoffrey R

    2016-05-01

    There have been many investigations of the alignment of nematic liquid crystals by either a magnetic and/or an electric field. The basic features of the important hydrodynamic processes for low molar mass nematics have been characterized for the systems in their equilibrium and non-equilibrium states. These have been created using electric and magnetic fields to align the director and deuterium nuclear magnetic resonance ((2)H NMR) spectroscopy has been used to explore this alignment. Theoretical models based on continuum theory have been developed to complement the experiments and found to describe successfully the static and the dynamic phenomena observed. Such macroscopic behaviour has been investigated with (2)H NMR spectroscopy, in which an electric field in addition to the magnetic field of the spectrometer is used to rotate the director and produce a non-equilibrium state. This powerful technique has proved to be especially valuable for the investigation of nematic liquid crystals. Since the quadrupolar splitting for deuterons observed in the liquid crystal phase is determined by the angle between the director and the magnetic field, time-resolved and time-averaged (2)H NMR spectroscopies can be employed to investigate the dynamic director alignment process in a thin nematic film following the application or removal of an electric field. In this article, we describe some seminal studies to illustrate the field-induced static and dynamic director alignment for low molar mass nematics. PMID:27247284

  4. Two-prism crystal structures for far-field imaging of subwavelength features at terahertz frequencies

    NASA Astrophysics Data System (ADS)

    Barros, D. A.; Dumelow, T.

    2016-08-01

    We investigate how a system of two single crystals can be used for far field imaging of subwavelength features. We make use of the phonon response to induce canalization of narrow collimated beams in crystals with suitably high anisotropy at the appropriate transverse optical phonon frequency. By cutting the crystals into suitably designed prisms, we show that an magnified image can be obtained and projected into the far field by a two-prism structure, noting that a single prism does not give a faithful reproduction of the object and will usually result in total internal reflection of most of the radiation. We show simulations using triglycine sulphate, which is both highly anisotropic and has very low absorption, at low temperature.

  5. Crystal field effect on EPR and optical absorption properties of natural green zoisite.

    PubMed

    Javier-Ccallata, Henry; Watanabe, Shigueo

    2013-03-01

    In this study the electron paramagnetic resonance (EPR) and optical absorption (OA) of natural crystal of zoisite were investigated after γ ((60)Co) irradiation and high temperature annealing. EPR measurements show that the zoisite from Tefilo Otoni MG Brazil contain Cr(3+), Fe(3+) and Mn(2+) ions and occupy distorted Al(3+) octahedral and tetrahedral sites which are subjected to the action of a strong crystal field in axial direction. Absorption bands which in principle give rise to sets of EPR lines between 500 and 2500 G were found using the deconvolution method. The application of high doses of gamma ray and high temperature annealing has shown no significant effects on EPR and OA spectra. Spin-allowed, spin-forbidden and crystal field parameters were calculated for 3d(3) configuration and interpreted using the spin Hamiltonian formalism containing axial and rhombic terms in low symmetries. PMID:23291113

  6. Electric Field-Controlled Crystallizing CaCO3 Nanostructures from Solution

    NASA Astrophysics Data System (ADS)

    Qi, Jian Quan; Guo, Rui; Wang, Yu; Liu, Xuan Wen; Chan, Helen Lai Wah

    2016-03-01

    The role of electric field is investigated in determining the structure, morphology, and crystallographic characteristics of CaCO3 nanostructures crystallized from solution. It is found that the lattice structure and crystalline morphology of CaCO3 can be tailed by the electric field applied to the solution during its crystallization. The calcite structure with cubic-like morphology can be obtained generally without electric field, and the vaterite structure with the morphology of nanorod is formed under the high electric field. The vaterite nanorods can be piled up to the petaliform layers. Both the nanorod and the petaliform layer can have mesocrystal structures which are piled up by much fine units of the rods with the size of several nanometers. Beautiful rose-like nanoflowers can be self-arranged by the petaliform layers. These structures can have potential application as carrier for medicine to involve into metabolism of living cell.

  7. Electric Field-Controlled Crystallizing CaCO3 Nanostructures from Solution.

    PubMed

    Qi, Jian Quan; Guo, Rui; Wang, Yu; Liu, Xuan Wen; Chan, Helen Lai Wah

    2016-12-01

    The role of electric field is investigated in determining the structure, morphology, and crystallographic characteristics of CaCO3 nanostructures crystallized from solution. It is found that the lattice structure and crystalline morphology of CaCO3 can be tailed by the electric field applied to the solution during its crystallization. The calcite structure with cubic-like morphology can be obtained generally without electric field, and the vaterite structure with the morphology of nanorod is formed under the high electric field. The vaterite nanorods can be piled up to the petaliform layers. Both the nanorod and the petaliform layer can have mesocrystal structures which are piled up by much fine units of the rods with the size of several nanometers. Beautiful rose-like nanoflowers can be self-arranged by the petaliform layers. These structures can have potential application as carrier for medicine to involve into metabolism of living cell. PMID:26932759

  8. Fringing field-induced monodomain of a polymer-stabilized blue phase liquid crystal

    NASA Astrophysics Data System (ADS)

    Li, Wei-Huan; Hu, De-Chun; Li, Yan; Chen, Chao Ping; Lee, Yung-Jui; Lien, Alan; Lu, Jian-Gang; Su, Yikai

    2015-12-01

    The influence of fringe electric field applied during photopolymerization on the electro-optic properties of polymer-stabilized blue phase liquid crystals (PS-BPLCs) was investigated. It has been found that the thermal stability would not degrade if the electric field was less than a critical value. The contrast ratio of PS-BPLC can be improved significantly because the uniformity of blue phase liquid crystal domain was enhanced by the electric fields, which were applied during photopolymerization. Meanwhile, with the electric filed, the potential energy of the BPLC molecules may lower the anchoring energy of the polymer network resulting in the improvement of electro-optic response properties. With optimized electric field during polymerization, the contrast ratio and the Kerr constant of PS-BPLC can be improved by 4.1 times and 15%, respectively, and the hysteresis can be decreased by 10%, while the response time and residual birefringence have no degradation.

  9. Tuning the colloidal crystal structure of magnetic particles by external field.

    PubMed

    Pal, Antara; Malik, Vikash; He, Le; Erné, Ben H; Yin, Yadong; Kegel, Willem K; Petukhov, Andrei V

    2015-02-01

    Manipulation of the self-assembly of magnetic colloidal particles by an externally applied magnetic field paves a way toward developing novel stimuli responsive photonic structures. Using microradian X-ray scattering technique we have investigated the different crystal structures exhibited by self-assembly of core-shell magnetite/silica nanoparticles. An external magnetic field was employed to tune the colloidal crystallization. We find that the equilibrium structure in absence of the field is random hexagonal close-packed (RHCP) one. External field drives the self-assembly toward a body-centered tetragonal (BCT) structure. Our findings are in good agreement with simulation results on the assembly of these particles. PMID:25510837

  10. Particle trapping and transport achieved via an adjustable acoustic field above a phononic crystal plate

    NASA Astrophysics Data System (ADS)

    Wang, T.; Ke, M.; Qiu, C.; Liu, Z.

    2016-06-01

    We present the design for an acoustic system that can achieve particle trapping and transport using the acoustic force field above a phononic crystal plate. The phononic crystal plate comprised a thin brass plate with periodic slits alternately embedded with two kinds of elastic inclusions. Enhanced acoustic transmission and localized acoustic fields were achieved when the structure was excited by external acoustic waves. Because of the different resonant frequencies of the two elastic inclusions, the acoustic field could be controlled via the working frequency. Particles were transported between adjacent traps under the influence of the adjustable acoustic field. This device provides a new and versatile avenue for particle manipulation that would complement other means of particle manipulation.

  11. Synthesis and field emission behaviour of well faceted In2Se3 micro-crystals

    NASA Astrophysics Data System (ADS)

    Kolhe, Panakj S.; Suryawanshi, Sachin R.; Shisode, Raju T.; More, Mahendra A.

    2015-06-01

    Here in, we report synthesis of crystalline Indium Selenide (In2Se3) elegant microcrystals on Au coated Si substrates using one-step facile thermal evaporation route and their field emission investigations. The as-synthesized In2Se3 micro-crystals were subjected to structural and morphological analysis prior to the field emission studies. The XRD spectrum of the as-synthesized product reveals formation of crystalline hexagonal phase of In2Se3 under the prevailing experimental conditions. Under optimized process variables, the morphology of the as-synthesized product is characterized by presence of well facetted micron size particles of In2Se3. Furthermore, the EDAX analysis confirms the presence of In and Se in the as-synthesized sample. The field emission characteristic of the In2Se3 micro-crystal emitter is found to be superior to the other metal chalcogenides micro-crystal based emitters. The synthesized In2Se3 micro-crystals emitter delivers current density of ˜ 225 µA/cm2 at an applied electric field of ˜ 7.44 V/µm. The emission current stability investigated at pre-set value of ˜ 3 µA is observed to be fairly good. These observed results demonstrate potential of the In2Se3 cathode as an electron source for practical applications in vacuum microelectronic devices.

  12. Crystal field splitting on D<-->S transitions of atomic manganese isolated in solid krypton

    NASA Astrophysics Data System (ADS)

    Byrne, O.; Collier, M. A.; Ryan, M. C.; McCaffrey, J. G.

    2010-05-01

    Narrow excitation features present on the [Ar]3d64s1aD(J=9/2-1/2)6←[Ar]3d54s2aS1/26 transitions of manganese atoms isolated in solid Kr are analyzed within the framework of weak crystal field splitting. Use of the Wp optical lineshape function allowed identification of multiple zero-phonon lines for individual spin-orbit J states of the a aD6←aS6 transition recorded with laser-induced excitation spectroscopy. Excellent agreement exists between the predicted crystal field splitting patterns for the J levels of the aD6 state isolated in the «red» tetravacancy site of solid Kr. The tetrahedral crystal field of the «red» trapping site splits J >3/2 levels of the aDJ6 and aD7/24 states by approximately 30cm-1. This report represents the first definitive evidence of crystal field splitting, induced by the weak van der Waals interactions between a neutral metal atom and the rare gas atoms surrounding it in a well-defined solid-state site.

  13. Double perovskite structure: a vibrational and luminescence investigation providing a perspective on crystal field strength.

    PubMed

    Li, Wenyu; Ning, Lixin; Tanner, Peter A

    2012-07-12

    The luminescence spectra of Eu(3+) doped in a series of double perovskite lattices Ba(2)LnMO(6) (Ln = Y, Gd; M = Nb, Ta) have been recorded at room temperature and 10 K. Together with FT-IR and FT-Raman spectra and aided by DFT vibrational energy calculations, assignments have been made for the crystal field levels of the (5)D(J) (J = 0,1) and (7)F(J) (J = 0-2) multiplets. The luminescence spectra are consistent with monoclinic symmetry of these systems. The crystal field parameters from the fitting of the energy level data set of Ba(2)YNbO(6):Eu(3+) enable the crystal field strength to be calculated, and the order of magnitude is Cl(-) < O(2-) < F(-) for the EuX(6)(n-) (n = 6 for halogen, 9 for oxide) moieties. For these systems, an empirical linear relationship between crystal field strength and electronegativity of ligand X has been found. By contrast, the nephelauxetic series from the depression of the Slater parameter F(2) is Cl(-) ≈ O(2-) > F(-) > free ion for these systems. PMID:22703165

  14. Crystal Field Theory and the Angular Overlap Model Applied to Hydrides of Main Group Elements.

    ERIC Educational Resources Information Center

    Moore, E. A.

    1990-01-01

    Described is how crystal field theory and the angular overlap model can be applied to very simple molecules which can then be used to introduce such concepts as bonding orbitals, MO diagrams, and Walsh diagrams. The main-group compounds are used as examples and a switch to the transition metal complexes. (KR)

  15. Electronic transitions, crystal field effects and phonons in UO 2

    NASA Astrophysics Data System (ADS)

    Schoenes, J.

    1980-08-01

    An extensive optical study of the 5f magnetic semiconductor UO 2 is presented. The experimental data include near normal incidence reflectivity measurements from 0.0025 to 13 eV, absorption and Faraday rotation measurements as function of temperature and of magnetic fields up to 100 kOe and photoemission results. From the data in the fundamental absorption region an energy level scheme is derived. This level scheme differs markedly from an earlier model but it is quantitatively supported by a calculation using the thermochemical Haber-Born process and also by cluster calculations. The localized nature of the 5f electrons is demonstrated. The absorption edge at 2 eV shows an abrupt shift to lower energies at the first order phase transition of UO 2 to the antiferromagnetic state. This shift is shown to be larger than expected from the lattice contraction indicating a magnetic order induced contribution to the total red shift. Below the absorption edge, intra-5f transitions and multiphonon excitations are reported, showing striking order induced effects at and below TN = 30.8 K. New results are presented for ε st, ε opt, ω TO and ω LO which fulfill the Lyddane-Sachs-Teller relation.

  16. Measurement of temperature and velocity fields of freezing water using liquid crystal tracers

    NASA Astrophysics Data System (ADS)

    Kowalewski, Tomasz A.

    A new experimental technique based on a computational analysis of the colour and displacement of thermochromic liquid crystal tracers was applied to determine both the temperature and velocity fields of freezing water. The technique combines Digital Particle Image Thermometry and Digital Particle Image Velocimetry. Full 2-D temperature and velocity fields are determined from a pair or a longer sequence, of colour images taken for the selected cross-section of the flow.

  17. Electric-field modulation of liquid crystal structures in contact with structured surfactant monolayers.

    PubMed

    Guillamat, Pau; Sagués, Francesc; Ignés-Mullol, Jordi

    2014-05-01

    We present experiments in which we use an electric field to switch between different configurations in the cellular patterns induced in a confined nematic liquid crystal by the contact with a surfactant monolayer that features lateral order and surface defects. By using different combinations of far-field alignment and mesogen dielectric anisotropy, we unravel the nature and stability of point defects and disclinations resulting from the hybrid boundary conditions. PMID:25353818

  18. Electro-optical field sensor using single total internal reflection in electro-optical crystals

    NASA Astrophysics Data System (ADS)

    Kijima, K.; Abe, O.; Shimizu, A.; Nakamura, T.; Kono, H.; Hagihara, S.; Torikai, E.; Hori, H.

    2015-08-01

    A novel electro-optical radio frequency field sensor with simple structure and high sensitivity is realized using single total internal reflection in electro-optical crystals. Without employing any waveguide structures, the minimum detectable electric field strength of the total internal reflection electro-optical-sensor is estimated to 86.52 dB μV/m (21.18 mV/m) at a resolution band width of 100 Hz for a short interaction length.

  19. High Field Magnetization measurements of uranium dioxide single crystals (P08358- E003-PF)

    SciTech Connect

    Gofryk, K.; Harrison, N.; Jaime, M.

    2014-12-01

    Our preliminary high field magnetic measurements of UO2 are consistent with a complex nature of the magnetic ordering in this material, compatible with the previously proposed non-collinear 3-k magnetic structure. Further extensive magnetic studies on well-oriented (<100 > and <111>) UO2 crystals are planned to address the puzzling behavior of UO2 in both antiferromagnetic and paramagnetic states at high fields.

  20. Far-field detection system for laser beams alignment and crystals alignment

    NASA Astrophysics Data System (ADS)

    Liu, D.; Qin, H.; Zhu, B.

    2015-08-01

    Laser beams far-field alignment is very important for the high power laser facility as well as the frequency doubling crystals adjustment. Traditional beams alignment system and crystals alignment system are separated. That means, they use different optical image systems and CCD cameras, which will occupy larger space and use more money. A new farfield detection system of laser beams is presented with a big diffraction grating (37mm*37mm), a set of optical imaging components and a high resolution CCD camera. This detection system, which is fully demonstrated on the National Laser Facility of Israel, can align high power laser facility beams' direction as well as the frequency doubling crystals. The new system occupies small space in the spatial filter through off-axial grating sampling. The experimental results indicate that the average far-field alignment error is less than 5% of spatial filter pinhole diameter, and the average crystals' matching angle error is less than 10urad, which meet the alignment system requirements for beams and crystals.

  1. Self-Action of Light Fields in Waveguide Photon Structures Based on Electro-Optic Crystals

    NASA Astrophysics Data System (ADS)

    Shandarov, V. M.

    2016-02-01

    Special features of spatial self-action of light fields in nonlinear optical photonic waveguide structures formed in strontium barium niobate and lithium niobate electro-optic crystals are discussed. The main methods of forming such structures including photorefractive waveguide elements and systems are briefly considered. The formation of spatial optical solitons in planar waveguides based on lithium niobate and strontium barium niobate crystals as well as in one-dimensional photonic lattices in lithium niobate is demonstrated experimentally for light beams of microwatt power. In regimes of spatial optical solitons, channel optical waveguides are formed not only in the planar waveguides, but also in the volume of photorefractive lithium niobate.

  2. Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

    PubMed

    Ranjini, Radhakrishnan; Matham, Murukeshan Vadakke; Nguyen, Nam-Trung

    2014-05-01

    This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states. PMID:24921859

  3. Creating physically-based three-dimensional microstructures: Bridging phase-field and crystal plasticity models.

    SciTech Connect

    Lim, Hojun; Owen, Steven J.; Abdeljawad, Fadi F.; Hanks, Byron; Battaile, Corbett Chandler

    2015-09-01

    In order to better incorporate microstructures in continuum scale models, we use a novel finite element (FE) meshing technique to generate three-dimensional polycrystalline aggregates from a phase field grain growth model of grain microstructures. The proposed meshing technique creates hexahedral FE meshes that capture smooth interfaces between adjacent grains. Three dimensional realizations of grain microstructures from the phase field model are used in crystal plasticity-finite element (CP-FE) simulations of polycrystalline a -iron. We show that the interface conformal meshes significantly reduce artificial stress localizations in voxelated meshes that exhibit the so-called "wedding cake" interfaces. This framework provides a direct link between two mesoscale models - phase field and crystal plasticity - and for the first time allows mechanics simulations of polycrystalline materials using three-dimensional hexahedral finite element meshes with realistic topological features.

  4. Electric field effects in nematic liquid crystals doped with carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Cîrtoaje, Cristina; Petrescu, Emil; Moţoc, Cornelia

    2013-12-01

    The aim of this paper was to investigate electric field induced effects in mixtures of nematic liquid crystals (NLCs) with positive electric anisotropies (MCL 6601 Merck) with carbon nanotubes (MWCNT from Aldrich). In planar alignment, the current-electric field dependence and the current-temperature dependence were explained by assuming a Poole-Frenkel effect (i.e. a tunnelling mechanism) and good agreement with the experimental data was obtained. Within this high field range it resulted that in planar aligned NLC-CNTs mixture the conductivity decreases when the temperature was increased. In homeotropic aligned mixture, the conduction mechanism is similar to the one occurring in a semiconductor: the conductivity increases when increasing temperature. This happens because in thin liquid crystal cells there is a possibility to realize an inner contact between nanotubes and electrodes so the mixture behaves like a semiconductor.

  5. Dislocation dynamics, plasticity and avalanche statistics using the phase-field crystal model

    NASA Astrophysics Data System (ADS)

    Angheluta, Luiza

    2013-03-01

    The plastic deformation of stressed crystalline materials is characterized by intermittency and scaling behavior. The sudden strain bursts arise from collective interactions between depinned crystal defects such as dislocations. Recent experiments on sheared nanocrystals provide insights into the connection between the crystal plasticity and the mean field theory of the depinning transition, based on the similar power-law statistics of avalanche events. However, a complete theoretical formulation of this connection is still lacking, as are high quality numerical data. Phase field crystal modelling provides an efficient numerical approach to simulating the dynamics of dislocations in plastic flows at finite temperature. Dislocations are naturally created as defects in a periodic ground state that is being sheared, without any ad hoc creation and annihilation rules. These crystal defects interact and annihilate with one another, generating a collective effect of avalanches in the global plastic strain rate. We examine the statistics of plastic avalanches both at finite and zero temperatures, and find good agreement with the predictions of the mean field interface depinning theory. Moreover, we predict universal scaling forms for the extreme statistics of avalanches and universal relations between the power-law exponents of avalanche duration, size and extreme value. These results account for the observed power-law distribution of the maximum amplitudes in acoustic emission experiments of crystal plasticity, but are also broadly applicable to other systems in the mean-field interface depinning universality class, ranging from magnets to earthquakes. The work reported here was performed in collaboration with: Georgios Tsekenis, Michael LeBlanc, Patrick Y Chan, Jon Dantzig, Karin Dahmen, and Nigel Goldenfeld. The work was supported by the Center for Physics of Geological Processes (Norway) through a post-doctoral grant, the National Science Foundation through grant NSF

  6. Paramagnetic Meissner effect at high fields in YCaBaCuO single crystal

    NASA Astrophysics Data System (ADS)

    Dias, F. T.; Vieira, V. N.; Falck, A. L.; da Silva, D. L.; Pureur, P.; Schaf, J.

    2012-12-01

    We report on systematic magnetization experiments in an Y1-xCaxBa2Cu3O7-δ (x = 0.25 at%) single crystal. The magnetization experiments were made using a superconducting quantum interference device magnetometer (SQUID). Magnetic moments were measured as functions of the temperature according to the zero-field cooling (ZFC), field-cooled cooling (FCC), and field-cooled warming (FCW) prescriptions. The time-dependence of the FC magnetization at fixed magnetic fields was studied. Magnetic fields up to 50 kOe were applied and a paramagnetic response related to the superconducting state was observed when strong enough fields were applied parallel to the c axis. The magnitude of the high field paramagnetic moment (HFPME) increases when the field is augmented. The effect shows strong and anomalous time dependence, such that the paramagnetic moment increases as a function of the time. An YBa2Cu3O7-δ single crystal exhibiting the same effect was used for comparison. We discuss our results in terms of the flux compression scenario into the sample modulated by Ca concentration.

  7. Crystal field excitations in CeCu2Ge2: Revisited employing a single crystal and inelastic neutron scattering

    NASA Astrophysics Data System (ADS)

    Loewenhaupt, Michael; Faulhaber, Enrico; Schneidewind, Astrid; Deppe, Micha; Hradil, Klaudia

    2012-04-01

    The intermetallic compound, CeCu2Ge2, is the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 is a magnetically ordering (TN = 4.1K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/ molK2. Earlier inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet excited state at 16.5 meV, although a splitting of the 4f1 (J = 5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce3+ ions. We performed detailed inelastic neutron scattering experiments on a single crystal at the thermal triple-axis spectrometer PUMA at FRM II for different crystallographic directions. From our results we infer that the quasi-quartet, in fact, consists of two doublets at 17.0 and 18.3 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally, we will present a new set of crystal field parameters.

  8. Resistive memory effects in BiFeO3 single crystals controlled by transverse electric fields

    NASA Astrophysics Data System (ADS)

    Kawachi, S.; Kuroe, H.; Ito, T.; Miyake, A.; Tokunaga, M.

    2016-04-01

    The effects of electric fields perpendicular to the c-axis of the trigonal cell in single crystals of BiFeO3 are investigated through magnetization and resistance measurements. Magnetization and resistance exhibit hysteretic changes under applied electric fields, which can be ascribed to the reorientation of the magnetoelectric domains. Samples are repetitively switched between high- and low-resistance states by changing the polarity of the applied electric fields over 20 000 cycles at room temperature. These results demonstrate the potential of BiFeO3 for use in non-volatile memory devices.

  9. Solitary waves in two-dimensional dusty plasma crystal: Effects of weak magnetic field

    SciTech Connect

    Ghosh, Samiran; Gupta, M. R.

    2010-03-15

    It is shown that in the presence of weak magnetic field, the dust lattice solitary wave in two-dimensional (2D) hexagonal dusty plasma crystal is governed by a gyration-modified 2D Korteweg-de Vries equation due to the action of Lorentz force on the dust particles. Numerical solutions reveal that only for weak magnetic field an apparently single hump solitary wave solution exist. But, for strong magnetic field dust lattice solitary wave becomes unstable showing repetitive solitary hump of increasing magnitude with time.

  10. Accurate Full-Field Thermochromic Liquid Crystal Thermography for the Study of Instantaneous Turbulent Heat Transfer

    NASA Astrophysics Data System (ADS)

    Sabatino, D. R.; Praisner, T. J.; Smith, C. R.

    1998-11-01

    The color change of thermochromic liquid crystals with temperature can be effectively utilized as full-field surface temperature sensors to investigate the fundamental structure of wall turbulence. In order to accurately quantify turbulent heat transfer behavior, a new technique has been developed for the calibration of wide-band micro-encapsulated thermochromic liquid crystals. Lighting/viewing arrangements are described and evaluated for ease of implementation and accuracy of the displayed color. This new technique employs images recorded in-situ with the test surface systematically exposed to a series of uniform temperature conditions spanning the bandwidth of the liquid crystals. This sequence of images is used to generate point-wise color/temperature calibration curves for the entire surface. Experimental results will be presented illustrating the application of the technique for assessment of spatial/temporal surface heat transfer behavior due to selected turbulent flows in a water channel

  11. A coupled ductile fracture phase-field model for crystal plasticity

    NASA Astrophysics Data System (ADS)

    Hernandez Padilla, Carlos Alberto; Markert, Bernd

    2015-08-01

    Nowadays crack initiation and evolution play a key role in the design of mechanical components. In the past few decades, several numerical approaches have been developed with the objective to predict these phenomena. The objective of this work is to present a simplified, nonetheless representative phenomenological model to predict the crack evolution of ductile fracture in single crystals. The proposed numerical approach is carried out by merging a conventional elasto-plastic crystal plasticity model and a phase-field model modified to predict ductile fracture. A two-dimensional initial boundary value problem of ductile fracture is introduced considering a single-crystal setup and Nickel-base superalloy material properties. The model is implemented into the finite element context subjected to a quasi-static uniaxial tension test. The results are then qualitatively analyzed and briefly compared to current benchmark results in the literature.

  12. Photorefractive Bragg gratings in nematic liquid crystals aligned by a magnetic field

    SciTech Connect

    Wiederrecht, G.P.; Wasielewski, M.R. |

    1999-06-01

    Photorefractive Bragg gratings are observed in low-molar-mass nematic liquid crystals doped with electron donor and acceptor molecules. This is accomplished by alignment of the nematic liquid crystals in a 0.3 T magnetic field, which produces thicker homeotropic aligned samples than traditional surfactant techniques. Grating fringe spacings as low as 3.7 {mu}m are achieved with 176-{mu}m-thick samples, producing grating {ital Q} values of 33. Up to this point, low molar mass nematic liquid crystals have exhibited photorefractive gratings with Q{le}1. Asymmetric two-beam coupling and photoconductivity experiments are performed to verify the photorefractive origin of the gratings. {copyright} {ital 1999 American Institute of Physics.}

  13. Light scattering from liquid crystal director fluctuations in steady magnetic fields up to 25 tesla

    NASA Astrophysics Data System (ADS)

    Challa, Pavan K.; Curtiss, O.; Williams, J. C.; Twieg, R.; Toth, J.; McGill, S.; Jákli, A.; Gleeson, J. T.; Sprunt, S. N.

    2012-07-01

    We report on homodyne dynamic light scattering measurements of orientational fluctuation modes in both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 tesla, which is consistent with strictly lowest order coupling of the tensor order parameter Q to field (QαβBαBβ) in the nematic free energy. However, we also observe evidence of field dependence of certain nematic material parameters, an effect which may be expected from the mean field scaling of these quantities with the magnitude of Q and the predicted variation of Q with field.

  14. Light scattering from liquid crystal director fluctuations in steady magnetic fields up to 25 tesla.

    PubMed

    Challa, Pavan K; Curtiss, O; Williams, J C; Twieg, R; Toth, J; McGill, S; Jákli, A; Gleeson, J T; Sprunt, S N

    2012-07-01

    We report on homodyne dynamic light scattering measurements of orientational fluctuation modes in both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 tesla, which is consistent with strictly lowest order coupling of the tensor order parameter Q to field (Q(αβ)B(α)B(β)) in the nematic free energy. However, we also observe evidence of field dependence of certain nematic material parameters, an effect which may be expected from the mean field scaling of these quantities with the magnitude of Q and the predicted variation of Q with field. PMID:23005438

  15. Magnetic-field tunable multichannel filter in a plasma photonic crystal at microwave frequencies.

    PubMed

    Chang, Tsung-Wen; Chien, Jia-Ren Chang; Wu, Chien-Jang

    2016-02-01

    The microwave magnetic-field tunable filtering properties in a multichannel filter based on use of a one-dimensional finite magnetized plasma photonic crystal (PPC) are theoretically investigated. The considered PPC has a structure of air/(AB)N/air, where A is a dielectric layer, B is a plasma layer, and N is the stack number. First, in the absence of an externally applied magnetic field, the structure can work as a multichannel filter whose channel number is equal to N-1 for N>1. Next, in the presence of an externally applied field, the filtering properties become tunable, i.e., the channel frequencies can be shifted as a function of the applied magnetic field. We find that the effect of the magnetic field will cause the channel frequencies to be blue-shifted or red-shifted depending on the orientation of the applied magnetic field. PMID:26836104

  16. Fiber optic dynamic electric field sensor based on nematic liquid crystal Fabry-Perot etalon

    NASA Astrophysics Data System (ADS)

    Ko, Myeong Ock; Kim, Sung-Jo; Kim, Jong-Hyun; Jeon, Min Yong

    2014-05-01

    We propose a fiber-optic dynamic electric field sensor using a nematic liquid crystal (NLC) Fabry-Perot etalon and a wavelength-swept laser. The transmission wavelength of the NLC Fabry-Perot etalon depends on the applied electric field intensity. The change in the effective refractive index of the NLC is measured while changing the applied electric field intensity. It decreases from 1.67 to 1.51 as the applied the electric field intensity is increased. Additionally, we successfully measure the dynamic variation of the electric field using the high-speed wavelength-swept laser. By measuring the modulation frequency of the transmission peaks in the temporal domain, the frequency of the modulated electric field can be estimated.

  17. Selective Precipitation and Concentrating of Perovskite Crystals from Titanium-Bearing Slag Melt in Supergravity Field

    NASA Astrophysics Data System (ADS)

    Gao, Jintao; Zhong, Yiwei; Guo, Zhancheng

    2016-06-01

    Selective precipitation and concentrating of perovskite crystals from titanium-bearing slag melt in the supergravity field was investigated in this study. Since perovskite was the first precipitated phase from the slag melt during the cooling process, and a greater precipitation quantity and larger crystal sizes of perovskite were obtained at 1593 K to 1563 K (1320 °C to 1290 °C), concentrating of perovskite crystals from the slag melt was carried out at this temperature range in the supergravity field, at which the perovskite transforms into solid particles while the other minerals remain in the liquid melt. The layered structures appeared significantly in the sample obtained by supergravity treatment, and all the perovskite crystals moved along the supergravity direction and concentrated as the perovskite-rich phase in the bottom area, whereas the molten slag concentrated in the upper area along the opposite direction, in which it was impossible to find any perovskite crystals. With the gravity coefficient of G = 750, the mass fraction of TiO2 in the perovskite-rich phase was up to 34.65 wt pct, whereas that of the slag phase was decreased to 12.23 wt pct, and the recovery ratio of Ti in the perovskite-rich phase was up to 75.28 pct. On this basis, an amplification experimental centrifugal apparatus was exploited and the continuous experiment with larger scale was further carried out, the results confirming that selective precipitation and concentrating of perovskite crystals from the titanium-bearing slag melt by supergravity was a feasible method.

  18. Selective Precipitation and Concentrating of Perovskite Crystals from Titanium-Bearing Slag Melt in Supergravity Field

    NASA Astrophysics Data System (ADS)

    Gao, Jintao; Zhong, Yiwei; Guo, Zhancheng

    2016-08-01

    Selective precipitation and concentrating of perovskite crystals from titanium-bearing slag melt in the supergravity field was investigated in this study. Since perovskite was the first precipitated phase from the slag melt during the cooling process, and a greater precipitation quantity and larger crystal sizes of perovskite were obtained at 1593 K to 1563 K (1320 °C to 1290 °C), concentrating of perovskite crystals from the slag melt was carried out at this temperature range in the supergravity field, at which the perovskite transforms into solid particles while the other minerals remain in the liquid melt. The layered structures appeared significantly in the sample obtained by supergravity treatment, and all the perovskite crystals moved along the supergravity direction and concentrated as the perovskite-rich phase in the bottom area, whereas the molten slag concentrated in the upper area along the opposite direction, in which it was impossible to find any perovskite crystals. With the gravity coefficient of G = 750, the mass fraction of TiO2 in the perovskite-rich phase was up to 34.65 wt pct, whereas that of the slag phase was decreased to 12.23 wt pct, and the recovery ratio of Ti in the perovskite-rich phase was up to 75.28 pct. On this basis, an amplification experimental centrifugal apparatus was exploited and the continuous experiment with larger scale was further carried out, the results confirming that selective precipitation and concentrating of perovskite crystals from the titanium-bearing slag melt by supergravity was a feasible method.

  19. A phase-field/Monte-Carlo model describing organic crystal growth from solution. Investigation of the diffusion-influenced growth of hydroquinone crystals

    NASA Astrophysics Data System (ADS)

    Kundin, J.; Yürüdü, C.; Ulrich, J.; Emmerich, H.

    2009-08-01

    In this paper work we present a phase-field/Monte-Carlo hybrid algorithm for the simulation of solutal growth of organic crystals. The algorithm is subsequently used for an investigation of diffusion effects on the growth mechanisms. This method combines a two-scale phase-field model of the liquid phase epitaxial growth and a Monte-Carlo algorithm of the 2D nucleation and thus is faster than previous purely Monte Carlo simulations of crystal growth. The inclusion of supersaturation and diffusion in the method allows the study of crystal growth under various growth conditions. Parameters used in the hybrid algorithm are bound to the energetic parameters of crystal faces, which can be estimated from a detailed study of the actual crystal structure based on a connected nets analysis, which allows the prediction of the shape and morphology of real crystals. The study of the diffusion effect is carried out based on an example of a hydroquinone crystal, which grows from the water solution at various supersaturations. The dependencies of the growth rate and the nucleation rate on the supersaturation indicate the change of the growth mechanism from spiral growth to 2D nucleation. The difference in the growth rate for various faces is in agreement with the crystal morphologies derived from the attachment energy method and observed experimentally. The main result of the simulation is the evaluation of engineering limits for choosing appropriate external process conditions.

  20. Vertically-Aligned Single-Crystal Nanocone Arrays: Controlled Fabrication and Enhanced Field Emission.

    PubMed

    Duan, Jing Lai; Lei, Dang Yuan; Chen, Fei; Lau, Shu Ping; Milne, William I; Toimil-Molares, M E; Trautmann, Christina; Liu, Jie

    2016-01-13

    Metal nanostructures with conical shape, vertical alignment, large ratio of cone height and curvature radius at the apex, controlled cone angle, and single-crystal structure are ideal candidates for enhancing field electron-emission efficiency with additional merits, such as good mechanical and thermal stability. However, fabrication of such nanostructures possessing all these features is challenging. Here, we report on the controlled fabrication of large scale, vertically aligned, and mechanically self-supported single-crystal Cu nanocones with controlled cone angle and enhanced field emission. The Cu nanocones were fabricated by ion-track templates in combination with electrochemical deposition. Their cone angle is controlled in the range from 0.3° to 6.2° by asymmetrically selective etching of the ion tracks and the minimum tip curvature diameter reaches down to 6 nm. The field emission measurements show that the turn-on electric field of the Cu nanocone field emitters can be as low as 1.9 V/μm at current density of 10 μA/cm(2) (a record low value for Cu nanostructures, to the best of our knowledge). The maximum field enhancement factor we measured was as large as 6068, indicating that the Cu nanocones are promising candidates for field emission applications. PMID:26666466

  1. Spectral and polarization structure of field-induced photonic bands in cholesteric liquid crystals

    NASA Astrophysics Data System (ADS)

    Palto, S. P.; Barnik, M. I.; Geivandov, A. R.; Kasyanova, I. V.; Palto, V. S.

    2015-09-01

    Transmission of planar layers of cholesteric liquid crystals is studied in pulsed electric fields perpendicular to the helix axis at normal incidence of both linearly polarized and unpolarized light. Spectral and light polarization properties of the primary photonic band and the field-induced bands up to fourth order of Bragg selective reflection are studied in detail. In our experiments we have achieved an electric field strength several times higher than the theoretical values corresponding to the critical field of full helix unwinding. However, the experiments show that despite the high strength of the electric field applied the helix does not unwind, but strongly deforms, keeping its initial spatial period. Strong helix deformation results in distinct spectral band splitting, as well as very high field-induced selective reflectance that can be applied in lasers and other optoelectronic devices. Peculiarities of inducing and splitting the bands are discussed in terms of the scattering coefficient approach. All observed effects are confirmed by numerical simulations. The simulations also show that liquid crystal surface anchoring is not the factor that prevents the helix unwinding. Thus, the currently acknowledged concept of continuous helix unwinding in the electric field should be reconsidered.

  2. Recent developments in Liquid Phase Electroepitaxial growth of bulk crystals under magnetic field

    NASA Astrophysics Data System (ADS)

    Dost, Sadik; Lent, Brian; Sheibani, Hamdi; Liu, Yongcai

    2004-05-01

    This review article presents recent developments in Liquid Phase Electroepitaxial (LPEE) growth of bulk single crystals of alloy semiconductors under an applied static magnetic field. The growth rate in LPEE is proportional to the applied electric current. However, at higher electric current levels the growth becomes unstable due to the strong convection occurring in the liquid zone. In order to address this problem, a significant body of research has been performed in recent years to suppress and control the natural convection for the purpose of prolonging the growth process to grow larger crystals. LPEE growth experiments show that the growth rate under an applied static magnetic field is also proportional and increases with the field intensity level. The modeling of LPEE growth under magnetic field was also the subject of interest. Two-dimensional mathematical models developed for the LPEE growth process predicted that the natural convection in the liquid zone would be suppressed almost completely with increasing the magnetic field level. However, experiments and also three-dimensional models have shown that there is an optimum magnetic field level below which the growth process is stable and the convection in the liquid zone is suppressed, but above such a field level the convective flow becomes very strong and leads to unstable growth with unstable interfaces. To cite this article: S. Dost et al., C. R. Mecanique 332 (2004).

  3. Influence of electromagnetic field intensity on the metastable zone width of CaCO3 crystallization in circulating water

    NASA Astrophysics Data System (ADS)

    Wang, Jianguo; Liang, Yandong; Chen, Si

    2016-09-01

    In this study, changes in the metastable zone width of CaCO3 crystallization was determined through conductivity titration by altering electromagnetic field parameters applied to the circulating water system. The critical conductivity value and metastable zone curves of CaCO3 crystallization were determined under different solution concentrations and electromagnetic field intensities. Experimental results indicate that the effect of the electromagnetic field intensity on the critical conductivity value intensifies with the increase of solution concentration. Moreover, the metastable zone width of CaCO3 crystallization increases with the increase of electromagnetic field intensity within 200 Gs, thereby prolonging the induction period of nucleation.

  4. Temperature dependence of Fe/++/ crystal field spectra - Implications to mineralogical mapping of planetary surfaces

    NASA Technical Reports Server (NTRS)

    Sung, C.-M.; Singer, R. B.; Parkin, K. M.; Burns, R. G.; Osborne, M.

    1977-01-01

    Results are reported of Fe(++) crystal field spectral measurements for olivines and pyroxenes up to 400 C. The results are correlated with crystal structure data at elevated temperatures, and the validity of remote-sensed identifications of minerals on hot surfaces of the moon and Mercury is assessed. Two techniques were used to obtain spectra of minerals at elevated temperatures using a spectrophotometer. One employed a diamond cell assembly or a specially designed sample holder to measure polarized absorption spectra of heated single crystals. For the other technique, a sample holder was designed to attach to a diffuse reflectance accessory to produce reflectance spectra of heated powdered samples. Polarized absorption spectra of forsterite at 20-400 C are shown in a graph. Other graphs show the temperature dependence of Fe(++) crystal field bands in olivines, the diffuse reflectance spectra of olivine at 40-400 C, the polarization absorption spectra of orthopyroxene at 30-400 C, the diffuse reflectance spectra of pigeonite at 40-400 C, and unpolarized absorption spectra of lunar pyroxene from Apollo 15 rock 15058.

  5. Interfacial free energy adjustable phase field crystal model for homogeneous nucleation.

    PubMed

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Huang, Yunhao

    2016-05-18

    To describe the homogeneous nucleation process, an interfacial free energy adjustable phase-field crystal model (IPFC) was proposed by reconstructing the energy functional of the original phase field crystal (PFC) methodology. Compared with the original PFC model, the additional interface term in the IPFC model effectively can adjust the magnitude of the interfacial free energy, but does not affect the equilibrium phase diagram and the interfacial energy anisotropy. The IPFC model overcame the limitation that the interfacial free energy of the original PFC model is much less than the theoretical results. Using the IPFC model, we investigated some basic issues in homogeneous nucleation. From the viewpoint of simulation, we proceeded with an in situ observation of the process of cluster fluctuation and obtained quite similar snapshots to colloidal crystallization experiments. We also counted the size distribution of crystal-like clusters and the nucleation rate. Our simulations show that the size distribution is independent of the evolution time, and the nucleation rate remains constant after a period of relaxation, which are consistent with experimental observations. The linear relation between logarithmic nucleation rate and reciprocal driving force also conforms to the steady state nucleation theory. PMID:27117814

  6. Travelling magnetic fields applied to bulk crystal growth from the melt: The step from basic research to industrial scale

    NASA Astrophysics Data System (ADS)

    Rudolph, Peter

    2008-04-01

    After introduction of various types of magnetic fields in crystal growth, their main pros and cons for crystallization processes are discussed. It is shown that their further developments towards industrial maturity are bound up with the cardinal demands—increase of the process output, improvement of the crystal quality, and reduction of costs. In a further section, the advantages of travelling magnetic fields are presented. The central chapter is devoted to the target of the current KRISTMAG˜ project—the development of an internal heater-magnet module for coupled generation of temperature and a travelling magnetic field, suitable for incorporation into industrial Czochralski pullers and vertical gradient freeze equipments. Amplitude, frequency and phase shift of the three-phase current are all adjustable and are combined with a dc component to control the crystallization process effectively. Results of accompanying numeric modelling are presented. The current state of crystal growth experiments in travelling magnetic field and first encouraging results are given.

  7. Magnetostrictive behaviors of Fe-Si(001) single-crystal films under rotating magnetic fields

    NASA Astrophysics Data System (ADS)

    Kawai, Tetsuroh; Aida, Takuya; Ohtake, Mitsuru; Futamoto, Masaaki

    2015-05-01

    Magnetostrictive behaviors under rotating magnetic fields are investigated for bcc(001) single-crystal films of Fe100-x-Six(x = 0, 6, 10 at. %). The magnetostriction observation directions are along bcc[100] and bcc[110] of the films. The magnetostriction waveform varies greatly depending on the observation direction. For the observation along [100], the magnetostriction waveform of all the films is bathtub-like and the amplitude stays at almost constant even when the magnetic field is increased up to the anisotropy field. On the other hand, the waveform along [110] is triangular and the amplitude increases with increasing magnetic field up to the anisotropy field and then saturates. In addition, the waveform of Fe90Si10 film is distorted triangular when the applied magnetic fields are less than its anisotropy field. These magnetostrictive behaviors under rotating magnetic fields are well explained by employing a proposed modified coherent rotation model where the anisotropy field and the magnetization reversal field are determined by using measured magnetization curves. The results show that magnetocrystalline anisotropy plays important role on magnetostrictive behavior under rotating magnetic fields.

  8. Flow and temperature field measurements of thermal convection in a small vertical gap using liquid crystals

    NASA Astrophysics Data System (ADS)

    Heiland, Hans Georg; Wozniak, Günter; Wozniak, Klaus

    2007-07-01

    Thermal convection in a small vertical gap is studied experimentally applying digital particle image velocimetry/thermometry. This optical method enables the simultaneous measurement of two-dimensional flow and temperature fields in a liquid. The principle is based on seeding the liquid flow medium with thermochromic liquid crystal particles. The temperature is measured by the crystal particles which change their reflected colour as function of temperature. The flow velocity is measured by using the same particles as flow tracers. The investigation shall contribute to the understanding of the fluid mechanical behaviour of biological liquids within micro reactor systems. However, the problem is also of fundamental interest as far as heat and mass transfer is concerned. Measured temperature and flow velocity fields are presented and discussed.

  9. Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet.

    PubMed

    Rechkemmer, Yvonne; Fischer, Julia E; Marx, Raphael; Dörfel, María; Neugebauer, Petr; Horvath, Sebastian; Gysler, Maren; Brock-Nannestad, Theis; Frey, Wolfgang; Reid, Michael F; van Slageren, Joris

    2015-10-14

    The electronic structure of a novel lanthanide-based single-ion magnet, {C(NH2)3}5[Er(CO3)4]·11H2O, was comprehensively studied by means of a large number of different spectroscopic techniques, including far-infrared, optical, and magnetic resonance spectroscopies. A thorough analysis, based on crystal field theory, allowed an unambiguous determination of all relevant free ion and crystal field parameters. We show that inclusion of methods sensitive to the nature of the lowest-energy states is essential to arrive at a correct description of the states that are most relevant for the static and dynamic magnetic properties. The spectroscopic investigations also allowed for a full understanding of the magnetic relaxation processes occurring in this system. Thus, the importance of spectroscopic studies for the improvement of single-molecule magnets is underlined. PMID:26394012

  10. Glassy phases and driven response of the phase-field-crystal model with random pinning.

    PubMed

    Granato, E; Ramos, J A P; Achim, C V; Lehikoinen, J; Ying, S C; Ala-Nissila, T; Elder, K R

    2011-09-01

    We study the structural correlations and the nonlinear response to a driving force of a two-dimensional phase-field-crystal model with random pinning. The model provides an effective continuous description of lattice systems in the presence of disordered external pinning centers, allowing for both elastic and plastic deformations. We find that the phase-field crystal with disorder assumes an amorphous glassy ground state, with only short-ranged positional and orientational correlations, even in the limit of weak disorder. Under increasing driving force, the pinned amorphous-glass phase evolves into a moving plastic-flow phase and then, finally, a moving smectic phase. The transverse response of the moving smectic phase shows a vanishing transverse critical force for increasing system sizes. PMID:22060323

  11. Phase field crystal study of nano-crack growth and branch in materials

    NASA Astrophysics Data System (ADS)

    Yingjun, Gao; Zhirong, Luo; Lilin, Huang; Hong, Mao; Chuanggao, Huang; Kui, Lin

    2016-06-01

    The phase field crystal (PFC) method is a new multiscale method, which can reproduce physical phenomena on an atomic level and on a diffusion time scale for the microstructure evolution of materials. The morphology of microcrack propagation and the branch of single crystal materials under tensile strain with a fixed grip condition are simulated by using PFC coupling with an external field method. The results show that microcrack propagation depends a lot on the applied strain. The crack starts to grow and branch when the strain reaches a critical value for biaxial tension. The temperature parameter may also have an effect on crack propagation and the branch. In order to indicate the connection between the PFC results and materials behavior, the energy balance approach is used to analyze the mechanism of crack extension, and also the critical value of the strain for crack extension is obtained. The simulated results are in good agreement with other simulation results and experimental results.

  12. Emergence of foams from the breakdown of the phase field crystal model

    NASA Astrophysics Data System (ADS)

    Guttenberg, Nicholas; Goldenfeld, Nigel; Dantzig, Jonathan

    2010-06-01

    The phase field crystal (PFC) model captures the elastic and topological properties of crystals with a single scalar field at small undercooling. At large undercooling, new foamlike behavior emerges. We characterize this foam phase of the PFC equation and propose a modified PFC equation that may be used for the simulation of foam dynamics. This minimal model reproduces von Neumann’s rule for two-dimensional dry foams and Lifshitz-Slyozov coarsening for wet foams. We also measure the coordination number distribution and find that its second moment is larger than previously reported experimental and theoretical studies of soap froths, a finding that we attribute to the wetness of the foam increasing with time.

  13. Direct mapping of local director field of nematic liquid crystals at the nanoscale.

    PubMed

    Xia, Yu; Serra, Francesca; Kamien, Randall D; Stebe, Kathleen J; Yang, Shu

    2015-12-15

    Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies. However, it remains challenging to directly probe the molecular-scale organization of nonglassy nematic LC molecules without altering the LC directors. We design and synthesize a new type of nematic liquid crystal monomer (LCM) system with strong dipole-dipole interactions, resulting in a stable nematic phase and strong homeotropic anchoring on silica surfaces. Upon photopolymerization, the director field can be faithfully "locked," allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) in real space with 100-nm resolution. Using this technique, we study the nematic textures in more complex LC/colloidal systems and calculate the extrapolation length of the LCM. PMID:26621729

  14. Direct mapping of local director field of nematic liquid crystals at the nanoscale

    PubMed Central

    Xia, Yu; Serra, Francesca; Kamien, Randall D.; Stebe, Kathleen J.; Yang, Shu

    2015-01-01

    Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies. However, it remains challenging to directly probe the molecular-scale organization of nonglassy nematic LC molecules without altering the LC directors. We design and synthesize a new type of nematic liquid crystal monomer (LCM) system with strong dipole–dipole interactions, resulting in a stable nematic phase and strong homeotropic anchoring on silica surfaces. Upon photopolymerization, the director field can be faithfully “locked,” allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) in real space with 100-nm resolution. Using this technique, we study the nematic textures in more complex LC/colloidal systems and calculate the extrapolation length of the LCM. PMID:26621729

  15. Novel magnetic field sensor based on magnetic fluids infiltrated dual-core photonic crystal fibers

    NASA Astrophysics Data System (ADS)

    Li, Jianhua; Wang, Rong; Wang, Jingyuan; Zhang, Baofu; Xu, Zhiyong; Wang, Huali

    2014-03-01

    Novel magnetic field sensor based on magnetic fluids infiltrated dual-core Photonic Crystal Fibers (PCFs) is proposed in this paper. Inside the cross-section of the designed PCFs, the two fiber cores filled with magnetic fluids (Fe3O4) are separated by an air hole, and then form two independent waveguides with mode coupling. The mode coupling under different magnetic field strength is investigated theoretically. A novel and simple magnetic field sensing system is proposed and its sensing performances have been studied numerically. The results show that the magnetic field sensor with 15-cm PCFs has a large sensing range and high sensitivity of 4.80 pm/Oe. It provides a new feasible method to design PCF-based magnetic field sensor.

  16. Effect of vertical magnetic field on convection and segregation in vertical Bridgman crystal growth

    NASA Technical Reports Server (NTRS)

    Kim, Do Hyun; Adornato, Peter M.; Brown, Robert A.

    1988-01-01

    A previous finite-element analysis of vertical Bridgman growth for dilute and nondilute alloys is extended to include the effect of a vertically-aligned magnetic field in the limit of zero magnetic Reynolds number. Calculations are presented for growth of a dilute gallium-germanium alloy in a vertically stabilized Bridgman-Stockbarger system and in a furnace with a uniform temperature gradient imposed along the ampoule. Steady cellular convection driven by radial temperature gradients causes good axial and radial mixing in both systems without a magnetic field. A weak magnetic field decreases the intensity of convection and the effectiveness of solute mixing. The radial nonuniformity is greatest for an intermediate field strength. Stronger fields suppress flow recirculation completely, and lead to uniform solute segregation across the crystal and to diffusion-controlled axial segregation.

  17. Low-field magnetic torque of a single crystal MgB 2

    NASA Astrophysics Data System (ADS)

    Atsumi, Toshiyuki; Tsuji, Mitsuyuki; Xu, Mingxiang; Kitazawa, Hideaki; Ishida, Takekazu

    2003-10-01

    We have investigated the magnetic torque of MgB 2 single crystal synthesized by the vapor transport method. We use a torque magnetometer consisting of a 4-K closed cycle refrigerator and a variable field permanent magnet up to 10 kG. The torque can be measured as an off-balance signal of the Wheatstone bridge of the four piezoresistors on a Si cantilever. The torque curves are analyzed by the Kogan model. The superconducting anisotropy γ is rather independent of temperature, but is dependent on field. As expected from the BCS theory, the prefactor of the Kogan formula, which corresponds to the lower critical field Hc1, changes smoothly as a function of temperature. We consider that the field dependence of γ comes from the multiple superconducting gaps and their different upper critical fields Hc2.

  18. Electric Field-Induced Skyrmion Crystals via Charged Monopoles in Insulating Helimagets

    NASA Astrophysics Data System (ADS)

    Watanabe, Haruki; Vishwanath, Ashvin

    2016-06-01

    Electrons propagating in a magnetically ordered medium experience an additional gauge field associated with the Berry phase of their spin following the local magnetic texture. In contrast to the usual electromagnetic field, this gauge field admits monopole excitations, corresponding to hedgehog defects of the magnetic order. In an insulator, these hedgehogs carry a well-defined electric charge allowing for them to be controlled by electric fields. One particularly robust mechanism that contributes to the charge is the orbital magnetoelectric effect, captured by a θ angle, which leads to a charge of eθ/2π on hedgehogs. This is a direct consequence of the Witten effect for magnetic monopoles in a θ medium. A physical consequence is that external electric fields can induce skyrmion crystal phases in insulating helimagnets.

  19. Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals

    NASA Astrophysics Data System (ADS)

    Wang, Xiang; Pu, Shengli; Ji, Hongzhu; Yu, Guojun

    2012-05-01

    Ferronematic materials composed of 4-cyano-4'-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δ n) and figure of merit of optical properties ( Q = Δ n/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of Q R exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field.

  20. Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals.

    PubMed

    Wang, Xiang; Pu, Shengli; Ji, Hongzhu; Yu, Guojun

    2012-01-01

    Ferronematic materials composed of 4-cyano-4'-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δn) and figure of merit of optical properties (Q = Δn/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of QR exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field. PMID:22587542

  1. Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals

    PubMed Central

    2012-01-01

    Ferronematic materials composed of 4-cyano-4′-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δn) and figure of merit of optical properties (Q = Δn/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of QR exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field. PMID:22587542

  2. Rheology of Pure Glasses and Crystal Bearing Melts: from the Newtonian Field to the Brittle Onset

    NASA Astrophysics Data System (ADS)

    Cordonnier, B.; Caricchi, L.; Pistone, M.; Castro, J. M.; Hess, K.; Dingwell, D. B.

    2010-12-01

    The brittle-ductile transition remains a central question of modern geology. If rocks can be perceived as a granular flow on geological time-scale, their behavior is brittle in dynamic areas. Understanding rock failure conditions is the main parameter in mitigating geological risks, more specifically the eruptive style transitions from effusive to explosive. If numerical simulations are the only way to fully understanding the physical processes involved, we are in a strong need of an experimental validation of the proposed models. here we present results obtained under torsion and uni-axial compression on both pure glasses and crystal bearing melts. We characterized the brittle onset of two phases magmas from 0 to 65% crystals. The strain-rates span a 5 orders magnitude range, from the Newtonian flow to the Brittle field (10-5 - 100 s-1). We particularly emphasize the time dependency of the measured rheology. The materials tested are a borosilicate glass from the National Bureau of Standards, a natural sample from Mt Unzen volcano and a synthetic sample. The lattest is an HPG8 melt with 7% sodium mole excess. The particles are quasi-isometric corundum crystalschosen for their shape and integrity under the stress range investigated. The crystal fraction ranges from 0 to 0.65. Concerning pure magmas, we recently demonstrated that the material passes from a Newtonian to a non-Nemtonian behavior with increasing strain-rate. This onset can mostly be explained by viscous-heating effects. However, for even greater strain-rates, the material cracks and finally fail. The brittle onset is here explained with the visco-elastic theory and corresponds to a Deborah number greater than 10-2. Concerning crystal bearing melts the departure from the Newtonian state is characterized by two effects: a shear-thinning and a time weakening effect. The first one is instantaneous and loading-unloading cyclic tests suggest an elastic contribution of the crystal network. The second one

  3. Dust Lattice Waves in Two-Dimensional Hexagonal Dust Crystals with an External Magnetic Field

    SciTech Connect

    Farokhi, B.; Shahmansouri, M.

    2008-09-07

    The influence of a constant magnetic field on the propagation of dust-lattice (DL) modes in a two-dimensional hexagonal strongly coupled plasma crystal formed by paramagnetic particles is considered. The expression for the wave dispersion relation clearly shows that high-frequency and low-frequency branches exist as a result of the coupling of longitudinal and transverse modes due to the Lorentz force acting on the dust particles.

  4. Consideration of the condensation processes of thin films in the crystal substrate's potential field

    NASA Astrophysics Data System (ADS)

    Tupik, V. A.; Margolin, V. I.; Trong Su, Chu

    2016-07-01

    The condensation process of a single particle in an ideal crystal substrate's potential field is considered. The optimal deposition path and the potential barrier of deposited particle's motion are shown. Some computer modeling examples of thin film's growth process were carried out on the basis of the implemented programs. A fractal analysis of obtained thin films was made, on the basis of which the possibility of estimating the performance of thin film's growth process will be discussed.

  5. Extended phase diagram of the three-dimensional phase field crystal model.

    PubMed

    Jaatinen, A; Ala-Nissila, T

    2010-05-26

    We determine the phase diagram of the phase field crystal model in three dimensions by using numerical free energy minimization methods. Previously published results, based on single mode approximations, have indicated that in addition to the uniform (liquid) phase, there would be regions of stability of body-centered cubic, hexagonal and stripe phases. We find that in addition to these, there are also regions of stability of face-centered cubic and hexagonal close packed structures in this model. PMID:21393705

  6. Hyperspectral optical near-field imaging: Looking graded photonic crystals and photonic metamaterials in color

    NASA Astrophysics Data System (ADS)

    Dellinger, Jean; Van Do, K.; Le Roux, Xavier; de Fornel, Frédérique; Cassan, Eric; Cluzel, Benoît

    2012-10-01

    Using a scanning near-field optical microscope operating with a hyperspectral detection scheme, we report the direct observation of the mirage effect within an on-chip integrated artificial material made of a two dimensional graded photonic crystal. The light rainbow due to the material dispersion is quantified experimentally and quantitatively compared to three dimensional plane wave assisted Hamiltonian optics predictions of light propagation.

  7. Fabrication of Crystals and Devices for Studies of Field Induced Superconductivity

    NASA Astrophysics Data System (ADS)

    Kloc, Christian

    2002-03-01

    It was demonstrated that injection of electrons or holes into materials using field effect transistor can transform the surface layer into a metal or at low enough temperatures even into a superconductor. Which substances could exhibit electric field induced superconductivity is currently not well know. Superconductivity has been successfully induced in single crystals of arenes, (pentacene Tc = 2 K, tetracene Tc = 2.7K, anthracene Tc = 4 K) oligophenylenevinylenes (trimer Tc =4.2K , tetramer Tc=2.9 K and pentamer, Tc= 2 K) sexithiophene, polymer thin film (regioregular polythiophene, Tc = 2.35 K) and single crystals of pure and intercalated fullerenes (hole and electron doped C60 Tc = 52 and 11K, C70, Tc = 7K and substituted C60, Tc = up to 117 K). Additional, Schon et al. have reported the observation of field induced superconductivity in copper oxide compounds. Despite of relative simple device structures, they consist of semiconducting single crystals or well oriented thin films, metal source and drain electrodes, an aluminum oxide dielectric layer and a conducting gate electrode, the fabrication is onerous and the significance of particular technological steps to functionality of a whole device is not well explored. In this presentation I would like to discuss these technological procedures leading to superconducting devices and further developments in search for high temperature superconducting organics.

  8. Crystal-Field and Covalency Effects in Uranates: An X-ray Spectroscopic Study.

    PubMed

    Butorin, Sergei M; Kvashnina, Kristina O; Smith, Anna L; Popa, Karin; Martin, Philippe M

    2016-07-01

    The electronic structure of U(V) - and U(VI) -containing uranates NaUO3 and Pb3 UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U 3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U 4f X-ray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3 UO6 , respectively, which indicates a significant covalent character for these compounds. PMID:27257782

  9. A molecular-field approximation for quantum crystals. Ph.D. Thesis; [considering ground state properties

    NASA Technical Reports Server (NTRS)

    Danilowicz, R.

    1973-01-01

    Ground-state properties of quantum crystals have received considerable attention from both theorists and experimentalists. The theoretical results have varied widely with the Monte Carlo calculations being the most successful. The molecular field approximation yields ground-state properties which agree closely with the Monte Carlo results. This approach evaluates the dynamical behavior of each pair of molecules in the molecular field of the other N-2 molecules. In addition to predicting ground-state properties that agree well with experiment, this approach yields data on the relative importance of interactions of different nearest neighbor pairs.

  10. A photonic-crystal optical antenna for extremely large local-field enhancement.

    PubMed

    Chang, Hyun-Joo; Kim, Se-Heon; Lee, Yong-Hee; Kartalov, Emil P; Scherer, Axel

    2010-11-01

    We propose a novel design of an all-dielectric optical antenna based on photonic-band-gap confinement. Specifically, we have engineered the photonic-crystal dipole mode to have broad spectral response (Q~70) and well-directed vertical-radiation by introducing a plane mirror below the cavity. Considerably large local electric-field intensity enhancement~4,500 is expected from the proposed design for a normally incident planewave. Furthermore, an analytic model developed based on coupled-mode theory predicts that the electric-field intensity enhancement can easily be over 100,000 by employing reasonably high-Q (~10,000) resonators. PMID:21164762

  11. Capacitance changes in ferronematic liquid crystals induced by low magnetic fields

    NASA Astrophysics Data System (ADS)

    Tomašovičová, Natália; Timko, Milan; Mitróová, Zuzana; Koneracká, Martina; Rajňak, Michal; Éber, Nándor; Tóth-Katona, Tibor; Chaud, Xavier; Jadzyn, Jan; Kopčanský, Peter

    2013-01-01

    The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH), and SWCNT functionalized with Fe3O4 nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was ϕ1=10-4 and ϕ2=10-3. Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic.

  12. Imprint electric field controlled electronic transport in TlGaSe2 crystals

    NASA Astrophysics Data System (ADS)

    Seyidov, MirHasan Yu; Suleymanov, Rauf A.; Balaban, Ertan; Şale, Yasin

    2013-09-01

    The effect of built-in electric field onto the dc electrical conductivity, photoconductivity, and electrical switching phenomenon were investigated in TlGaSe2 layered semiconductor within the temperature range of 77-300 K. We have used different types of electrodes for different TlGaSe2 samples in both parallel and perpendicular directions to the plane of layers. The effect of electric field was investigated by cooling the samples from the room temperature under the electric field and then removing it at ˜80 K. After the procedure, it was found that a built-in internal electric field which strongly affects transport properties appears in TlGaSe2 crystals. Substantial increasing of both dark currents and photo-conductivities were observed predominantly at low temperatures, where hopping was the main conductivity mechanism. The anomalous decrease of the activation energy in the low temperature region and the switching effect are also the main experimental findings of the present work. Such behavior can be understood by assuming that the built-in electric field greatly increases the contribution of the hopping conductivity at low temperatures. Obtained results are discussed on the basis of the models widely used for disordered semiconductors. It was shown that TlGaSe2 crystal demonstrates the peculiar behavior that is typical to such type of semiconductors.

  13. Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets

    SciTech Connect

    Johnson, Phillip S.; Boukahil, Idris; Himpsel, F. J.; García-Lastra, J. M.; Kennedy, Colton K.; Jersett, Nathan J.; Cook, Peter L.

    2014-03-21

    Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

  14. In-situ observation of electric-field-induced acceleration in crystal growth of tetrathiafulvalene-tetracyanoquinodimethane

    NASA Astrophysics Data System (ADS)

    Sakai, Masatoshi; Kuniyoshi, Shigekazu; Yamauchi, Hiroshi; Iizuka, Masaaki; Nakamura, Masakazu; Kudo, Kazuhiro

    2013-04-01

    In-situ observations of vapor-phase growth of tetrathiafulvalene (TTF)-tetracyanoquinodimethane (TCNQ) crystals under an electric field were conducted without influencing the actual crystal growth process. The shortest incubation time of TTF-TCNQ nuclei and the highest initial growth rate of the crystals are obtained on the anode side and in high electric field regions. It is demonstrated that the distribution of molecules thermally diffusing on the substrate surface is controlled by an external electric field. These results indicate the potential for selective growth of highly conductive organic wires for micro- and nanoscale wiring in organic nanodevices.

  15. Crystal structure and high-field magnetism of La{sub 2}CuO{sub 4}

    SciTech Connect

    Reehuis, M.; Ulrich, C.; Keimer, B.; Prokes, K.; Gozar, A.; Blumberg, G.; Komiya, Seiki; Ando, Yoichi; Pattison, P.

    2006-04-01

    Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La{sub 2}CuO{sub 4} single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined to drive a continuous reorientation of the copper spins from the orthorhombic b axis to the c axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for nonstoichiometric La{sub 2}CuO{sub 4} is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.

  16. Photonic crystal cavities for resonant evanescent field trapping of single bacteria

    NASA Astrophysics Data System (ADS)

    van Leest, Thijs; Heldens, Jeroen; van der Gaag, Bram; Caro, Jaap

    2012-06-01

    In monitoring the quality of drinking water with respect to the presence of hazardous bacteria there is a strong need for on-line sensors that allow quick identification of bacterium species at low cost. In this respect, the combination of photonics and microfluidics is promising for lab-on-a-chip sensing of these contaminants. Photonic crystal slabs have proven to form a versatile platform for controlling the flow of light and creating resonant cavities on a wavelength scale. The goal of our research is to use photonic crystal cavities for optical trapping of microorganisms in water, exploiting the enhanced evanescent field of the cavity mode. We optimize the H0, H1 and L3 cavities for optical trapping of bacteria in water, by reducing out-of-plane losses and taking into account the trapping-induced resonance shift and the in-plane coupling with photonic crystal waveguides. The cavities are fabricated on silicon-on-insulator material, using e-beam lithography and dry etching. A fluidic channel is created on top of the photonic crystal using dry film resist techniques. Transmission measurements show clear resonances for the cavities in water. In the present state of our research, we demonstrate optical trapping of 1 μm diameter polystyrene beads for the three cavities, with estimated trapping forces on the order of 0.7 pN.

  17. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields

    SciTech Connect

    Armas-Pérez, Julio C.; Londono-Hurtado, Alejandro; Guzmán, Orlando; Hernández-Ortiz, Juan P.; Pablo, Juan J. de

    2015-07-28

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate.

  18. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields.

    SciTech Connect

    Armas-Perez, Julio C.; Londono-Hurtado, Alejandro; Guzman, Orlando; Hernandez-Ortiz, Juan P.; de Pablo, Juan J.

    2015-07-27

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate.

  19. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields.

    PubMed

    Armas-Pérez, Julio C; Londono-Hurtado, Alejandro; Guzmán, Orlando; Hernández-Ortiz, Juan P; de Pablo, Juan J

    2015-07-28

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate. PMID:26233107

  20. Magnetic Field Tunable Small-scale Mechanical Properties of Nickel Single Crystals Measured by Nanoindentation Technique

    PubMed Central

    Zhou, Hao; Pei, Yongmao; Fang, Daining

    2014-01-01

    Nano- and micromagnetic materials have been extensively employed in micro-functional devices. However, measuring small-scale mechanical and magnetomechanical properties is challenging, which restricts the design of new products and the performance of smart devices. A new magnetomechanical nanoindentation technique is developed and tested on a nickel single crystal in the absence and presence of a saturated magnetic field. Small-scale parameters such as Young's modulus, indentation hardness, and plastic index are dependent on the applied magnetic field, which differ greatly from their macroscale counterparts. Possible mechanisms that induced 31% increase in modulus and 7% reduction in hardness (i.e., the flexomagnetic effect and the interaction between dislocations and magnetic field, respectively) are analyzed and discussed. Results could be useful in the microminiaturization of applications, such as tunable mechanical resonators and magnetic field sensors. PMID:24695002

  1. Nematic liquid crystals in a spatially step-wise magnetic field.

    PubMed

    Napoli, Gaetano; Scaraggi, Michele

    2016-01-01

    We study the molecular reorientation induced by a textured external field in a nematic liquid crystal (nLC). In particular, we consider an infinitely wide cell with strong planar anchoring boundary conditions, subjected to a spatially periodic piecewise magnetic field. In the framework of the Frank's continuum theory, we use the perturbation analysis to study in detail the field-induced splay-bend Fréedericksz transition. A numerical approach, based on the finite differences method, is instead employed to solve the fully nonlinear equations. At high field strengths, an analytic approach allows us to draw the bulk profile of the director in terms of elliptic integrals. Finally, through the application of the Bruggeman texture hydrodynamics theory, we qualitatively discuss on the LCs piecewise director configuration under sliding interfaces, which can be adopted to actively regulate friction. Our study opens the pathway for the application of highly controlled nLC texturing for tribotronics. PMID:26871129

  2. A high-field (30 Tesla) pulsed magnet instrument for single-crystal scattering studies

    NASA Astrophysics Data System (ADS)

    Islam, Zahirul; Nojiri, Hiroyuki; Narumi, Yasuo; Lang, Jonathan

    2010-03-01

    Pulsed magnets have emerged as a viable approach at synchrotron x-ray facilities for studying materials in high magnetic fields. We are developing a new high-field (30 Tesla) pulsed magnet system for single-crystal x-ray diffraction studies. It consists of a single 18mm-bore solenoid, designed and built at Tohoku University using high-tensile-strength and high conductivity CuAg wires. A dual-cryostat scheme has been developed at Advanced Photon Source in order to cool the coil using liquid nitrogen and the sample using a closed-cycle cryostat independently. Liquid nitrogen cooling allows repetition rate of a few minutes for peak fields near 30 Tesla. This scheme is unique in that it allows the applied magnetic field to be parallel to the scattering plane. Time-resolved scattering data are typically collected using a fast one-dimensional strip detector. Opportunities and challenges for experiments and instrumentation will be discussed.

  3. Nematic liquid crystals in a spatially step-wise magnetic field

    NASA Astrophysics Data System (ADS)

    Napoli, Gaetano; Scaraggi, Michele

    2016-01-01

    We study the molecular reorientation induced by a textured external field in a nematic liquid crystal (nLC). In particular, we consider an infinitely wide cell with strong planar anchoring boundary conditions, subjected to a spatially periodic piecewise magnetic field. In the framework of the Frank's continuum theory, we use the perturbation analysis to study in detail the field-induced splay-bend Fréedericksz transition. A numerical approach, based on the finite differences method, is instead employed to solve the fully nonlinear equations. At high field strengths, an analytic approach allows us to draw the bulk profile of the director in terms of elliptic integrals. Finally, through the application of the Bruggeman texture hydrodynamics theory, we qualitatively discuss on the LCs piecewise director configuration under sliding interfaces, which can be adopted to actively regulate friction. Our study opens the pathway for the application of highly controlled nLC texturing for tribotronics.

  4. Magnetic-field-driven surface electromagnetic states in the graphene-antiferromagnetic photonic crystal system

    SciTech Connect

    Averkov, Yu. O. Tarapov, S. I.; Yakovenko, V. M.; Yampol’skii, V. A.

    2015-04-15

    The surface electromagnetic states (SEMSs) on graphene, which has a linear carrier dispersion law and is placed in an antiferromagnetic photonic crystal, are theoretically studied in the terahertz frequency range. The unit cell of such a crystal consists of layers of a nonmagnetic insulator and a uniaxial antiferromagnet, the easy axis of which is parallel to the crystal layers. A dc magnetic field is parallel to the easy axis of the antiferromagnet. An expression that relates the SEMS frequencies to the structure parameters is obtained. The problem of SEMS excitation by an external TE-polarized electromagnetic wave is solved, and the dependences of the transmission coefficient on the dc magnetic field and the carrier concentration are constructed. These dependences are shown to differ substantially from the case of a conventional two-dimensional electron gas with a quadratic electron dispersion law. Thus, the positions of the transmission coefficient peaks related to resonance SEMS excitation can be used to determine the character of carrier dispersion law in a two-dimensional electron gas.

  5. Directed peptide amphiphile assembly using aqueous liquid crystal templates in magnetic fields.

    PubMed

    van der Asdonk, Pim; Keshavarz, Masoumeh; Christianen, Peter C M; Kouwer, Paul H J

    2016-08-21

    An alignment technique based on the combination of magnetic fields and a liquid crystal (LC) template uses the advantages of both approaches: the magnetic fields offer non-contact methods that apply to all sample sizes and shapes, whilst the LC templates offer high susceptibilities. The combination introduces a route to control the spatial organization of materials with low intrinsic susceptibilities. We demonstrate that we can unidirectionally align one such material, peptide amphiphiles in water, on a centimeter scale at a tenfold lower magnetic field by using a lyotropic chromonic liquid crystal as a template. We can transform the aligned supramolecular assemblies into optically active π-conjugated polymers after photopolymerization. Lastly, by reducing the magnetic field strength needed for addressing these assemblies, we are able to create more complex structures by initiating self-assembly of our supramolecular materials under competing alignment forces between the magnetically induced alignment of the assemblies (with a positive diamagnetic anisotropy) and the elastic force dominated alignment of the template (with a negative diamagnetic anisotropy), which is directed orthogonally. Although the approach is still in its infancy and many critical parameters need optimization, we believe that it is a very promising technique to create tailor-made complex structures of (aqueous) functional soft matter. PMID:27320385

  6. Imaging the oblique propagation of electrons in germanium crystals at low temperature and low electric field

    NASA Astrophysics Data System (ADS)

    Moffatt, R. A.; Cabrera, B.; Corcoran, B. M.; Kreikebaum, J. M.; Redl, P.; Shank, B.; Yen, J. J.; Young, B. A.; Brink, P. L.; Cherry, M.; Tomada, A.; Phipps, A.; Sadoulet, B.; Sundqvist, K. M.

    2016-01-01

    Excited electrons in the conduction band of germanium collect into four energy minima, or valleys, in momentum space. These local minima have highly anisotropic mass tensors which cause the electrons to travel in directions which are oblique to an applied electric field at sub-Kelvin temperatures and low electric fields, in contrast to the more isotropic behavior of the holes. This experiment produces a full two-dimensional image of the oblique electron and hole propagation and the quantum transitions of electrons between valleys for electric fields oriented along the [0,0,1] direction. Charge carriers are excited with a focused laser pulse on one face of a germanium crystal and then drifted through the crystal by a uniform electric field of strength between 0.5 and 6 V/cm. The pattern of charge density arriving on the opposite face is used to reconstruct the trajectories of the carriers. Measurements of the two-dimensional pattern of charge density are compared in detail with Monte Carlo simulations developed for the Cryogenic Dark Matter Search (SuperCDMS) to model the transport of charge carriers in high-purity germanium detectors.

  7. Magnetostrictive behaviors of Fe-Al(001) single-crystal films under rotating magnetic fields

    NASA Astrophysics Data System (ADS)

    Kawai, Tetsuroh; Abe, Tatsuya; Ohtake, Mitsuru; Futamoto, Masaaki

    2016-05-01

    Magnetostrictive behaviors of Fe100-x - Alx(x = 0 - 30 at.%)(001) single-crystal films under rotating magnetic fields are investigated along the two different crystallographic orientations, [100] and [110]. The behaviors of Fe and Fe90Al10 films show bath-tub like waveform along [100], easy magnetization axis, and triangular waveform along [110], hard magnetization axis, with respect to their four-fold magnetic anisotropy. On the other hand, the behaviors of Fe80Al20 film are different from those of Fe or Fe90Al10 film. The output of the film along [100] shows a strong magnetic field dependence. The Fe70Al30 film shows similar magnetostrictive behaviors along both [100] and [110] reflecting its magnetic properties, which are almost same for the both directions. The growth of ordered phase (B2) in Fe80Al20 and Fe70Al30 films is considered to have affected their magnetostrictive behaviors. The Al content dependence on λ100 and λ111 values shows similar tendency to that reported for the bulk samples but the values are slightly different. The Fe90Al10(001) single-crystal film shows a large magnetostriction along [100] under a very small magnetic field of 0.02 kOe, which is comparable to the saturated one, and changes the value abruptly in relation to the angle of applied magnetic field.

  8. Pressure dependence of upper critical fields in FeSe single crystals

    NASA Astrophysics Data System (ADS)

    Kang, Ji-Hoon; Jung, Soon-Gil; Lee, Sangyun; Park, Eunsung; Lin, Jiunn-Yuan; Chareev, Dmitriy A.; Vasiliev, Alexander N.; Park, Tuson

    2016-03-01

    We investigate the pressure dependence of the upper critical fields (μ 0 H c2) for FeSe single crystals with pressure up to 2.57 GPa. The superconducting (SC) properties show a disparate behavior across a critical pressure where the pressure-induced antiferromagnetic phase coexists with superconductivity. The magnetoresistance for H//ab and H//c is very different: for H//c, magnetic field induces and enhances a hump in the resistivity close to the T c for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured μ 0 H c2 for FeSe samples is smaller than the orbital limited upper critical field ({{{H}}{{orb}}}{{c}2}) estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (α) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of μ 0 H c2(T). Interestingly, the α value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the μ 0 H c2(0) for the FeSe single crystals is found to be almost similar to that of T c(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature.

  9. Phase-field crystal modeling of shape transition of strained islands in heteroepitaxy

    NASA Astrophysics Data System (ADS)

    Chen, Cheng; Chen, Zheng; Zhang, Jing; Du, XiuJuan

    2012-11-01

    The phase-field crystal (PFC) model is employed to study the shape transition of strained islands in heteroepitaxy on vicinal substrates. The influences of both substrate vicinal angles β and the lattice mismatch ζ are discussed. The increase of substrate vicinal angles is found to be capable of significantly changing the surface nanostructures of epitaxial films. The surface morphology of films undergoes a series of transitions that include Stranski-Krastonov (SK) islands, the couple growth of islands and the step flow as well as the formation of step bunching. In addition, the larger ζ indicates an increased strained island density after coarsening, and results in the incoherent growth of strained islands with the creation of misfit dislocations. Coarsening, coalescence and faceting of strained islands are also observed. Some facets in the shape transition of strained islands are found to be stable and can be determined by β and crystal symmetry of the film.

  10. General equations for the motions of ice crystals and water drops in gravitational and electric fields

    NASA Technical Reports Server (NTRS)

    Nisbet, John S.

    1988-01-01

    General equations for the Reynolds number of a variety of types of ice crystals and water drops are given in terms of the Davies, Bond, and Knudsen numbers. The equations are in terms of the basic physical parameters of the system and are valid for calculating velocities in gravitational and electric fields over a very wide range of sizes and atmospheric conditions. The equations are asymptotically matched at the bottom and top of the size spectrum, useful when checking large computer codes. A numerical system for specifying the dimensional properties of ice crystals is introduced. Within the limits imposed by such variables as particle density, which have large deviations, the accuracy of velocities appears to be within 10 percent over the entire range of sizes of interest.

  11. Temperature and Field Induced Strain Measurements in Single Crystal Gd5Si2Ge2

    NASA Astrophysics Data System (ADS)

    McCall, S. K.; Nersessian, N.; Carman, G. P.; Pecharsky, V. K.; Schlagel, D. L.; Radousky, H. B.

    2016-06-01

    The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of -8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of -8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.

  12. Crystal size of epidotes: A potentially exploitable geothermometer in geothermal fields

    SciTech Connect

    Patrier, P.; Beaufort, D.; Touchard, G. ); Fouillac, A.M. )

    1990-11-01

    Crystal size of epidotes crystallized in quartz + epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of Saint Martin (Lesser Antilles). The epidote-bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220-350C. The length/width ratio of the epidote grains is constant for all the analyzed samples and suggests isotropic growth environments. However, the length and width of the grains vary exponentially with temperature. The obtained results offer new perspectives for simple grain-size geothermomentry but must be extended to other geologic environments to clarify the influence of different rock types.

  13. Three-Dimensional Stress Fields and Slip Systems for Single Crystal Superalloy Notched Specimens

    NASA Technical Reports Server (NTRS)

    Magnan, Shannon M.; Throckmorton, David (Technical Monitor)

    2002-01-01

    Single crystal superalloys have become increasingly popular for turbine blade and vane applications due to their high strength, and creep and fatigue resistance at elevated temperatures. The crystallographic orientation of a single crystal material greatly affects its material properties, including elastic modulus, shear modulus, and ductility. These directional properties, along with the type of loading and temperature, dictate an anisotropic response in the yield strength, creep resistance, creep rupture ductility, fatigue resistance, etc. A significant amount of research has been conducted to determine the material properties in the <001> orientation, yet the material properties deviating from the <001> orientation have not been assessed for all cases. Based on the desired application and design criteria, a crystal orientation is selected to yield the maximum properties. Currently, single crystal manufacturing is able to control the primary crystallographic orientation within 15 of the target orientation, which is an acceptable deviation to meet both performance and cost guidelines; the secondary orientation is rarely specified. A common experiment is the standard load-controlled tensile test, in which specimens with different orientations can be loaded to observe the material response. The deformation behavior of single-crystal materials under tension and compression is known to be a function of not only material orientation, but also of varying microdeformation (i.e. dislocation) mechanisms. The underlying dislocation motion causes deformation via slip, and affects the activation of specific slip systems based on load and orientation. The slip can be analyzed by observing the visible traces left on the surface of the specimen from the slip activity within the single crystal material. The goal of this thesis was to predict the slip systems activated in three-dimensional stress fields of a notched tensile specimen, as a function of crystal orientation, using

  14. AOM reconciling of crystal field parameters for UCl sub 3 , UBr sub 3 , Ul sub 3 series

    SciTech Connect

    Gajek, Z.; Mulak, J. )

    1990-07-01

    Available inelastic neutron scattering interpretations of crystal field effect in the uranium trihalides have been verified in terms of Angular Overlap Model. For UCl{sub 3} a good reconciling of both INS and optical interpretations of crystal field effect has been obtained. On the contrary, the parameterizations for UBr{sub 3} and UI{sub 3} were found to be highly artificial and suggestion is given to experimentalists to reinterpret their INS spectra.

  15. Patterns driven by combined ac and dc electric fields in nematic liquid crystals.

    PubMed

    Krekhov, Alexei; Decker, Werner; Pesch, Werner; Eber, Nándor; Salamon, Péter; Fekete, Balázs; Buka, Agnes

    2014-05-01

    The effect of superimposed ac and dc electric fields on the formation of electroconvection and flexoelectric patterns in nematic liquid crystals was studied. For selected ac frequencies, an extended standard model of the electrohydrodynamic instabilities was used to characterize the onset of pattern formation in the two-dimensional parameter space of the magnitudes of the ac and dc electric field components. Numerical as well as approximate analytical calculations demonstrate that depending on the type of patterns and on the ac frequency, the combined action of ac and dc fields may either enhance or suppress the formation of patterns. The theoretical predictions are qualitatively confirmed by experiments in most cases. Some discrepancies, however, seem to indicate the need to extend the theoretical description. PMID:25353815

  16. Field-induced periodic distortions in a nematic liquid crystal: deuterium NMR study and theoretical analysis.

    PubMed

    Sugimura, A; Zakharov, A V

    2011-08-01

    The peculiarities in the dynamic of the director reorientation in a liquid crystal (LC) film under the influence of the electric E field directed at an angle α to the magnetic B field have been investigated both experimentally and theoretically. Time-resolved deuterium NMR spectroscopy is employed to investigate the field-induced director dynamics. Analysis of the experimental results, based on the predictions of hydrodynamic theory including both the director motion and fluid flow, provides an evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4'-cyanobiphenyl LC film, at the angles α>60∘, in response to the suddenly applied E. These periodic distortions produce a lower effective rotational viscosity. This gives a faster response of the director rotation than for a uniform mode, as observed in our NMR experiment. PMID:21929001

  17. Strain mapping in nanocrystalline grains simulated by phase field crystal model

    NASA Astrophysics Data System (ADS)

    Guo, Yaolin; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Tang, Sai; Liu, Feng; Zhou, Yaohe

    2015-03-01

    In recent years, the phase field crystal (PFC) model has been confirmed as a good candidate to describe grain boundary (GB) structures and their nearby atomic arrangement. To further understand the mechanical behaviours of nanocrystalline materials, strain fields near GBs need to be quantitatively characterized. Using the strain mapping technique of geometric phase approach (GPA), we have conducted strain mapping across the GBs in nanocrystalline grains simulated by the PFC model. The results demonstrate that the application of GPA in strain mapping of low and high angles GBs as well as polycrystalline grains simulated by the PFC model is very successful. The results also show that the strain field around the dislocation in a very low angle GB is quantitatively consistent with the anisotropic elastic theory of dislocations. Moreover, the difference between low angle GBs and high angle GBs is revealed by the strain analysis in terms of the strain contour shape and the structural GB width.

  18. Electric field sensor based on cholesteric liquid crystal Fabry-Perot etalon

    NASA Astrophysics Data System (ADS)

    Ko, Myeong Ock; Kim, Sung-Jo; Kim, Jong-Hyun; Lee, Bong Wan; Jeon, Min Yong

    2015-09-01

    We propose an electric field sensor using a cholesteric liquid crystal (CLC) Fabry-Perot etalon and a broadband optical source. The CLC cell consists of glass substrates, polyimide layers, electrodes, and CLC layer. There is a threshold behavior for CLC cell and no change in the transmitted wavelength occurs until a threshold value. The threshold value is 0.8 V/μm for fabricated CLC cell in this experiment. The transmitted or reflected wavelength from the CLC Fabry-Perot etalon depends on the applied electric field. The valley wavelengths of the transmitted light from the CLC device are linearly increased from 1303 nm to 1317 nm as the applied electric field to the CLC device is increased from 0.8 V/μm to 1.9 V/μm.

  19. Melt Motion Due to Peltier Marking During Bridgman Crystal Growth with an Axial Magnetic Field

    NASA Technical Reports Server (NTRS)

    Sellers, C. C.; Walker, John S.; Szofran, Frank R.; Motakef, Shariar

    2000-01-01

    This paper treats a liquid-metal flow inside an electrically insulating cylinder with electrically conducting solids above and below the liquid region. There is a uniform axial magnetic field, and there is an electric current through the liquid and both solids. Since the lower liquid-solid interface is concave into the solid and since the liquid is a better electrical conductor than the adjacent solid, the electric current is locally concentrated near the centerline. The return to a uniform current distribution involves a radial electric current which interacts with the axial magnetic field to drive an azimuthal flow. The axial variation of the centrifugal force due to the azimuthal velocity drives a meridional circulation with radial and axial velocities. This problem models the effects of Peltier marking during the vertical Bridgman growth of semiconductor crystals with an externally applied magnetic field, where the meridional circulation due to the Peltier Current may produce important mixing in the molten semiconductor.

  20. Molecular simulation of model liquid crystals in a strong aligning field

    NASA Astrophysics Data System (ADS)

    de Miguel, Enrique; Blas, Felipe J.; Martín Del Río, Elvira

    We report a computer simulation study of systems of perfectly aligned molecules interacting through the Gay-Berne (GB) potential model for two different values of the molecular anisotropy parameter κ, namely 3 and 4.4. The models are appropriate to gauge the effects of strong aligning fields on the thermodynamics and structural properties of thermotropic liquid crystals. According to our results, one of the main effects of the external field is to increase the range of stability of the smectic A phase, which indicates the existence of a strong coupling between orientational and translational order. For the κ = 3 GB model the smectic phase, which is not stable in the absence of the field, is promoted when the molecules are constrained to be parallel. According to the simulation results, the smectic A-nematic transition is, in general, continuous; however, this transition appears to be first order at low pressure for the κ = 4.4 GB fluid model.

  1. Field-induced phase transitions in an antiferroelectric liquid crystal using the pyroelectric effect

    PubMed

    Shtykov; Vij; Lewis; Hird; Goodby

    2000-08-01

    The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical polar phases. Measurements of pyroelectric response of these phases as a function of temperature and bias field have elucidated the ability of this method for investigating the nature of antiferroelectric phases and phase transitions under the bias field. The pyroelectric signal as a function of the bias field at fixed temperatures and as a function of temperature for fixed bias fields was measured for different phases of the investigated AFLC material. A theoretical model describing the pyroelectric response in different phases of AFLC is given, and the experimental results are interpreted. The threshold fields for field induced phase transitions are determined. The type of field induced phase transition from the AF phase in particular is found to be dependent on the temperature within its range. The properties of an unusual ferrielectric phase existing between ferrielectric chiral smectic-C (SmC*) and antiferroelectric AF phases are studied in a great detail. The results confirm that this phase is one of the incommensurate phases, predicted by the axial next-nearest neighbor Ising model and Landau model for this temperature region. PMID:11088695

  2. Crystal morphology and thermal EMF of pyrites in the western flank of Sukholozhsky gold ore field (Lenski ore area)

    NASA Astrophysics Data System (ADS)

    Gavrilov, R.; Pshenichkin, A.; Ponamarenko, M.; Abramova, R.

    2015-11-01

    The investigated crystal morphology and thermal EMF of pyrites in the western flank of Sukholozsky ore field showed that the pyrite crystals have cubic habitus with a weakly-developed face {210}. The crystal faces {100} and {210} are covered with multiple irregular-oriented growth laminae. It has been determined that pyrites have such a property as p-type conduction and embrace insignificant thermal EMF variations. The results of the research indicated the fact of upper ore zone erosion in the western flank of Sukholozhsky ore field and its area potential at depth.

  3. Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals.

    PubMed

    Tarduno, John A; Cottrell, Rory D; Watkeys, Michael K; Bauch, Dorothy

    2007-04-01

    The strength of the Earth's early geomagnetic field is of importance for understanding the evolution of the Earth's deep interior, surface environment and atmosphere. Palaeomagnetic and palaeointensity data from rocks formed near the boundary of the Proterozoic and Archaean eons, some 2.5 Gyr ago, show many hallmarks of the more recent geomagnetic field. Reversals are recorded, palaeosecular variation data indicate a dipole-dominated morphology and available palaeointensity values are similar to those from younger rocks. The picture before 2.8 Gyr ago is much less clear. Rocks of the Archaean Kaapvaal craton (South Africa) are among the best-preserved, but even they have experienced low-grade metamorphism. The variable acquisition of later magnetizations by these rocks is therefore expected, precluding use of conventional palaeointensity methods. Silicate crystals from igneous rocks, however, can contain minute magnetic inclusions capable of preserving Archaean-age magnetizations. Here we use a CO2 laser heating approach and direct-current SQUID magnetometer measurements to obtain palaeodirections and intensities from single silicate crystals that host magnetite inclusions. We find 3.2-Gyr-old field strengths that are within 50 per cent of the present-day value, indicating that a viable magnetosphere sheltered the early Earth's atmosphere from solar wind erosion. PMID:17410173

  4. Inversion of absorption anisotropy and bowing of crystal field splitting in wurtzite MgZnO

    NASA Astrophysics Data System (ADS)

    Neumann, M. D.; Esser, N.; Chauveau, J.-M.; Goldhahn, R.; Feneberg, M.

    2016-05-01

    The anisotropic optical properties of wurtzite MgxZn1-xO thin films (0 ≤x ≤0.45 ) grown on m-plane ZnO substrates by plasma assisted molecular beam epitaxy are studied using spectroscopic ellipsometry at room temperature. The data analysis provides the dielectric functions for electric field polarizations perpendicular and parallel to the optical axis. The splitting between the absorption edges of the two polarization directions decreases between x = 0 and x = 0.24, while an inverted absorption anisotropy is found at higher Mg content, indicating a sign change of the crystal field splitting Δcr as for the spin orbit parameter. The characteristic energies such as exciton binding energies and band gaps are determined from the analysis of the imaginary parts of the dielectric functions. In particular, these data reveal a bowing parameter of b =-283 meV for describing the compositional dependence of the crystal field splitting and indicate Δcr=-327 meV for wurtzite MgO. The inverted valence band ordering of ZnO ( Γ7-Γ9-Γ7 ) is found to be preserved with increasing Mg content, while the optical selection rules interchange.

  5. Crystal growth of pure substances: Phase-field simulations in comparison with analytical and experimental results

    NASA Astrophysics Data System (ADS)

    Nestler, B.; Danilov, D.; Galenko, P.

    2005-07-01

    A phase-field model for non-isothermal solidification in multicomponent systems [SIAM J. Appl. Math. 64 (3) (2004) 775-799] consistent with the formalism of classic irreversible thermodynamics is used for numerical simulations of crystal growth in a pure material. The relation of this approach to the phase-field model by Bragard et al. [Interface Science 10 (2-3) (2002) 121-136] is discussed. 2D and 3D simulations of dendritic structures are compared with the analytical predictions of the Brener theory [Journal of Crystal Growth 99 (1990) 165-170] and with recent experimental measurements of solidification in pure nickel [Proceedings of the TMS Annual Meeting, March 14-18, 2004, pp. 277-288; European Physical Journal B, submitted for publication]. 3D morphology transitions are obtained for variations in surface energy and kinetic anisotropies at different undercoolings. In computations, we investigate the convergence behaviour of a standard phase-field model and of its thin interface extension at different undercoolings and at different ratios between the diffuse interface thickness and the atomistic capillary length. The influence of the grid anisotropy is accurately analyzed for a finite difference method and for an adaptive finite element method in comparison.

  6. The Structure, Thermodynamics and Solubility of Organic Crystals from Simulation with a Polarizable Force Field

    PubMed Central

    Schnieders, Michael J.; Baltrusaitis, Jonas; Shi, Yue; Chattree, Gaurav; Zheng, Lianqing; Yang, Wei; Ren, Pengyu

    2012-01-01

    An important unsolved problem in materials science is prediction of the thermodynamic stability of organic crystals and their solubility from first principles. Solubility can be defined as the saturating concentration of a molecule within a liquid solvent, where the physical picture is of solvated molecules in equilibrium with their solid phase. Despite the importance of solubility in determining the oral bioavailability of pharmaceuticals, prediction tools are currently limited to quantitative structure–property relationships that are fit to experimental solubility measurements. For the first time, we describe a consistent procedure for the prediction of the structure, thermodynamic stability and solubility of organic crystals from molecular dynamics simulations using the polarizable multipole AMOEBA force field. Our approach is based on a thermodynamic cycle that decomposes standard state solubility into the sum of solid-vapor sublimation and vapor-liquid solvation free energies ΔGsolubilityo=ΔGsubo+ΔGsolvo, which are computed via the orthogonal space random walk (OSRW) sampling strategy. Application to the n-alkylamides series from aeetamide through octanamide was selected due to the dependence of their solubility on both amide hydrogen bonding and the hydrophobic effect, which are each fundamental to protein structure and solubility. On average, the calculated absolute standard state solubility free energies are accurate to within 1.1 kcal/mol. The experimental trend of decreasing solubility as a function of n-alkylamide chain length is recapitulated by the increasing stability of the crystalline state and to a lesser degree by decreasing favorability of solvation (i.e. the hydrophobic effect). Our results suggest that coupling the polarizable AMOEBA force field with an orthogonal space based free energy algorithm, as implemented in the program Force Field X, is a consistent procedure for predicting the structure, thermodynamic stability and solubility of

  7. Optical electric-field sensor based on angular optical bias using single β-BaB2O4 crystal.

    PubMed

    Li, Changsheng; Shen, Xiaoli; Zeng, Rong

    2013-11-01

    A novel optical electric-field sensor is proposed and demonstrated in experiment by use of a single beta barium borate (β-BaB2O4, BBO) crystal. The optical sensing unit is only composed of one BBO crystal and two polarizers. An optical phase bias of 0.5π is provided by using natural birefringence in the BBO crystal itself. A small angle (e.g., 0.6°) between the sensing light beam and principal axis of the crystal is required in order to produce the above optical bias. Thus the BBO crystal is used as the electric-field-sensing element and quarter waveplate. The ac electric field in the range of (1.4-703.2) kV/m has been measured with measurement sensitivity of 1.39 mV/(kV/m) and nonlinear error of 0.6%. Compared with lithium niobate crystal used as an electric-field sensor, main advantages of the BBO crystal include higher measurement sensitivity, compact configuration, and no ferroelectric ringing effect. PMID:24216661

  8. The crystallization of apo-form UMP kinase from Xanthomonas campestris is significantly improved in a strong magnetic field

    SciTech Connect

    Tu, Jhe-Le; Chin, Ko-Hsin; Wang, Andrew H.-J.; Chou, Shan-Ho

    2007-05-01

    A bacterial UMP kinase from the plant pathogen X. campestris pathovar campestris has been overexpressed in E. coli, purified and crystallized in a strong magnetic field. The crystals diffracted to 2.35 Å. Bacterial UMP kinases (UMPKs) are crucial enzymes that are responsible for microbial UTP biosynthesis. Interestingly, eukaryotic and prokaryotic cells use different enzymes for UMP-phosphorylation reactions. Prokaryotic UMPKs are thus believed to be potential targets for antimicrobial drug development. Here, the cloning, expression and crystallization of SeMet-substituted XC1936, a bacterial UMPK from Xanthomonas campestris pathovar campestris, are reported. The crystallization of the apo-form UMPK was found to be significantly improved in a strong magnetic field; the crystals diffracted to a resolution of 2.35 Å, a dramatic improvement over the original value of 3.6 Å. Preliminary structural analyses of apo-form XC1936 using crystals grown in a strong magnetic field clearly reveal well defined loop regions involved in substrate-analogue binding that were previously not visible. Crystallization in a strong magnetic field thus was found to be indispensable in determining the flexible region of the XC1936 UMPK structure.

  9. Magnetic properties of Dy3+ ions and crystal field characterization in YF3:Dy3+ and DyF3 single crystals

    NASA Astrophysics Data System (ADS)

    Savinkov, A. V.; Korableva, S. L.; Rodionov, A. A.; Kurkin, I. N.; Malkin, B. Z.; Tagirov, M. S.; Suzuki, H.; Matsumoto, K.; Abe, S.

    2008-12-01

    The dc magnetic susceptibilities of the orthorhombic DyF3 single crystals have been measured in the temperature range between 1.8 and 300 K. The susceptibility along the b-axis does not depend on temperature below TC = 2.55 K and is equal to the demagnetizing factor of the sample, that gives evidence for the ferromagnetic phase induced by the magnetic dipole-dipole interactions between the Dy3+ ions. The saturation moment of 8.5 μB/Dy3+ along the b-axis was determined from magnetization measurements. The observed strong anisotropy of the magnetic susceptibility at low temperatures agrees with the measured anisotropic g-tensor of the ground state of impurity Dy3+ ions in YF3 single crystals. The results of measurements are interpreted in the frameworks of the crystal field theory and the mean magnetic field approximation.

  10. Electrical Impact of SiC Structural Crystal Defects on High Electric Field Devices (Invited)

    NASA Technical Reports Server (NTRS)

    Neudeck, Philip G.

    1999-01-01

    As illustrated by the invited paper at this conference and other works, SiC wafers and epilayers contain a variety of crystallographic imperfections, including micropipes, closed-core screw dislocations, grain boundaries, basal plane dislocations, heteropolytypic inclusions, and surfaces that are often damaged and contain atomically rough features like step bunching and growth pits or hillocks. Present understanding of the operational impact of various crystal imperfections on SiC electrical devices is reviewed, with an emphasis placed on high-field SiC power devices and circuits.

  11. Fluoride Binding and Crystal-Field Analysis of Lanthanide Complexes of Tetrapicolyl-Appended Cyclen.

    PubMed

    Blackburn, Octavia A; Kenwright, Alan M; Jupp, Andrew R; Goicoechea, Jose M; Beer, Paul D; Faulkner, Stephen

    2016-06-20

    Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal-field parameter, B20 . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes. PMID:27167830

  12. Oxygen content and crystal field effects in RBa sub 2 Cu sub 3 O sub x

    SciTech Connect

    Podlesnyak, A.A.; Mirmelstein, A.V.; Bobrovskii, V.I.; Zhadhin, I.L.; Blinovskov, Y.N.; Kozhenvnikov, V.L.; Goshchitskii, B.N. )

    1991-05-10

    Effects of crystal electrical field (CEF) in the orthorhombic phase of RBa{sub 2}Cu{sub 3}O{sub x} (R = Ho, Er; x = 6.9, 6.4) are investigated by inelastic magnetic neutron scattering (IMNS). The level schemes of the lowest multiplet of 4f-shell of the rare-earth ion are found. In this paper it is shown that the experimental scattering spectra and their variation depending on the oxygen content may be explained by spacial distribution of the negative charge within the unit cell.

  13. Light quasiparticles dominate electronic transport in molecular crystal field-effect transistors

    SciTech Connect

    Li, Z. Q.; Podzorov, V.; Sai, N.; Martin, Michael C.; Gershenson, M. E.; Di Ventra, M.; Basov, D. N.

    2007-03-01

    We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m[small star, filled]comparable to free electron mass. Furthermore, the m[small star, filled]values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids.

  14. Tailoring of spectral response and spatial field distribution with corrugated photonic crystal slab.

    PubMed

    Gad, Raanan; Lau, Wah Tung; Nicholaou, Costa; Ahmadi, Soroosh; Sigal, Iliya; Levi, Ofer

    2015-08-15

    We report a new physical mechanism for simultaneous tuning of quality factors, spectral responses, and field distributions in photonic crystal slabs through removal of polarization mode degeneracy using a lattice of elliptical nano-holes. The quality factors in these structures can become higher than those obtained with much smaller circular nano-holes. Furthermore, the modes can be superimposed by either rotating or morphing the elliptical nano-holes into a corrugated grating. These findings will enable improved radiation-matter interaction in optical, microwave, and THZ frequencies along with enhanced opto-acoustic coupling. PMID:26274642

  15. Molecular simulations elucidate electric field actuation in swollen liquid crystal elastomers

    PubMed Central

    Skačej, Gregor; Zannoni, Claudio

    2012-01-01

    Swollen elastomer liquid crystals undergo significant deformations by application of an electric field perpendicular to their alignment axis, as shown in experiments by Urayama et al. [Urayama K, Honda S, Takigawa T (2006) Macromolecules 39:1943–1949]. Here we clarify this surprising effect at the molecular level using large-scale Monte Carlo simulations of an off-lattice model based on a soft Gay–Berne potential. We provide the internal change of molecular organization, as well as the key observables during the actuation cycle. PMID:22679288

  16. Molecular simulations elucidate electric field actuation in swollen liquid crystal elastomers.

    PubMed

    Skačej, Gregor; Zannoni, Claudio

    2012-06-26

    Swollen elastomer liquid crystals undergo significant deformations by application of an electric field perpendicular to their alignment axis, as shown in experiments by Urayama et al. [Urayama K, Honda S, Takigawa T (2006) Macromolecules 39:1943-1949]. Here we clarify this surprising effect at the molecular level using large-scale Monte Carlo simulations of an off-lattice model based on a soft Gay-Berne potential. We provide the internal change of molecular organization, as well as the key observables during the actuation cycle. PMID:22679288

  17. Crystal field analysis of Dy and Tm implanted silicon for photonic and quantum technologies.

    PubMed

    Hughes, Mark A; Lourenço, Manon A; Carey, J David; Murdin, Ben; Homewood, Kevin P

    2014-12-01

    We report the lattice site and symmetry of optically active Dy3+ and Tm3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence. PMID:25606863

  18. Two-dimensional liquid crystalline growth within a phase-field-crystal model.

    PubMed

    Tang, Sai; Praetorius, Simon; Backofen, Rainer; Voigt, Axel; Yu, Yan-Mei; Wang, Jincheng

    2015-07-01

    By using a two-dimensional phase-field-crystal (PFC) model, the liquid crystalline growth of the plastic triangular phase is simulated with emphasis on crystal shape and topological defect formation. The equilibrium shape of a plastic triangular crystal (PTC) grown from an isotropic phase is compared with that grown from a columnar or smectic-A (CSA) phase. While the shape of a PTC nucleus in the isotropic phase is almost identical to that of the classical PFC model, the shape of a PTC nucleus in CSA is affected by the orientation of stripes in the CSA phase, and irregular hexagonal, elliptical, octagonal, and rectangular shapes are obtained. Concerning the dynamics of the growth process, we analyze the topological structure of the nematic order, which starts from nucleation of +1/2 and -1/2 disclination pairs at the PTC growth front and evolves into hexagonal cells consisting of +1 vortices surrounded by six satellite -1/2 disclinations. It is found that the orientational and the positional order do not evolve simultaneously; the orientational order evolves behind the positional order, leading to a large transition zone, which can span over several lattice spacings. PMID:26274192

  19. Two-dimensional liquid crystalline growth within a phase-field-crystal model

    NASA Astrophysics Data System (ADS)

    Tang, Sai; Praetorius, Simon; Backofen, Rainer; Voigt, Axel; Yu, Yan-Mei; Wang, Jincheng

    2015-07-01

    By using a two-dimensional phase-field-crystal (PFC) model, the liquid crystalline growth of the plastic triangular phase is simulated with emphasis on crystal shape and topological defect formation. The equilibrium shape of a plastic triangular crystal (PTC) grown from an isotropic phase is compared with that grown from a columnar or smectic-A (CSA) phase. While the shape of a PTC nucleus in the isotropic phase is almost identical to that of the classical PFC model, the shape of a PTC nucleus in CSA is affected by the orientation of stripes in the CSA phase, and irregular hexagonal, elliptical, octagonal, and rectangular shapes are obtained. Concerning the dynamics of the growth process, we analyze the topological structure of the nematic order, which starts from nucleation of +1/2 and -1/2 disclination pairs at the PTC growth front and evolves into hexagonal cells consisting of +1 vortices surrounded by six satellite -1/2 disclinations. It is found that the orientational and the positional order do not evolve simultaneously; the orientational order evolves behind the positional order, leading to a large transition zone, which can span over several lattice spacings.

  20. Rigorous analysis of an electric-field-driven liquid crystal lens for 3D displays

    NASA Astrophysics Data System (ADS)

    Kim, Bong-Sik; Lee, Seung-Chul; Park, Woo-Sang

    2014-08-01

    We numerically analyzed the optical performance of an electric field driven liquid crystal (ELC) lens adopted for 3-dimensional liquid crystal displays (3D-LCDs) through rigorous ray tracing. For the calculation, we first obtain the director distribution profile of the liquid crystals by using the Erickson-Leslie motional equation; then, we calculate the transmission of light through the ELC lens by using the extended Jones matrix method. The simulation was carried out for a 9view 3D-LCD with a diagonal of 17.1 inches, where the ELC lens was slanted to achieve natural stereoscopic images. The results show that each view exists separately according to the viewing position at an optimum viewing distance of 80 cm. In addition, our simulation results provide a quantitative explanation for the ghost or blurred images between views observed from a 3D-LCD with an ELC lens. The numerical simulations are also shown to be in good agreement with the experimental results. The present simulation method is expected to provide optimum design conditions for obtaining natural 3D images by rigorously analyzing the optical functionalities of an ELC lens.

  1. Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures

    NASA Astrophysics Data System (ADS)

    Popa-Nita, V.; van der Schoot, P.; Kralj, S.

    2006-11-01

    The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combining the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.

  2. Direct mapping of local director field of nematic liquid crystals at the nano-scale

    NASA Astrophysics Data System (ADS)

    Xia, Yu; Serra, Francesca; Yang, Shu; Kamien, Randall

    2015-03-01

    The director field in liquid crystals (LCs) has been characterized mainly via polarized optical microscopy, fluorescence confocal microscopy, and Raman spectroscopy, all of which are limited by optical wavelengths - from hundreds of nanometers to several micrometers. Since LC orientation cannot be resolved directly by these methods, theory is needed to interpret the local director field of LC alignment. In this work, we introduce a new approach to directly visualize the local director field of a nematic LC (NLC) at the nano-scale using scanning electron microscopy (SEM). A new type of NLC monomer bearing crosslinkable groups was designed and synthesized. It can be well-oriented at particle surfaces and patterned polymer substrates, including micron-sized silica colloids, porous membranes, micropillar arrays, and 1D channels. After carefully crosslinking, the molecular orientation of NLCs around the particles or within the patterns could be directly visualized by SEM, showing oriented nanofibers representing LC director from the fractured samples. Here, we could precisely resolve not only the local director field by this approach, but the defect structures of NLCs, including hedgehogs and line defects. The direct mapping of LC directors at the nanoscale using this method will improve our understanding of NLC local director field, and thus their manipulation and applications. More importantly, a theoretical interpretation will no longer be a necessity to resolve a new material system in this field.

  3. Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric interface control and very thin single crystal

    NASA Astrophysics Data System (ADS)

    Dong, Ji; Yu, Peng; Atika Arabi, Syeda; Wang, Jiawei; He, Jun; Jiang, Chao

    2016-07-01

    A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a ‘seed-controlled’ pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm2 V‑1 s‑1—more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices.

  4. Determination of forced convection parameters by interferometric imaging of the concentration field during growth of KDP crystals

    NASA Astrophysics Data System (ADS)

    Verma, Sunil; Muralidhar, K.

    2011-07-01

    Growth of a potassium dihydrogen phosphate (KDP) crystal from its aqueous solution has been considered under forced convection conditions. The KDP crystal is grown in a conventional top hanging geometry. Forced convection conditions are created by rotating the crystal about a vertical axis. The rotational RPM is varied in a cycle, creating an accelerated rotation (AR) paradigm. The effect of varying the rotational RPM on the concentration field around the crystal was investigated. Mach-Zehnder interferometry was adopted as an optical technique to image the evolving concentration fields. Six different experiments were performed to obtain the specific set of time periods and rotation rates of the acceleration cycle that result in a uniform concentration field around the growing crystal. The Reynolds number, an index of the strength of forced convection, was optimized through the experiments. The optimized parameters of the accelerated rotation cycle were found to be as follows: maximum rotation rate of 32 RPM, spin up period=40 s, spin down period=40 s, steady period=40 s, and stationary period=40 s. The parametric study further revealed that concentration was highly sensitive to the maximum rotation rate adopted during the AR cycle. It did not depend crucially on the time periods that could be varied by as much as ±25% around the respective average values. Finally, a KDP crystal was grown using the optimized forced convection parameters and the crystal quality was found to be good.

  5. Reconstruction of crystal band structure from the power spectrum of strong-field generated high harmonics

    NASA Astrophysics Data System (ADS)

    Wang, Chang-Ming; Ho, Tak-San; Chu, Shih-I.

    2016-05-01

    The study of high harmonic generation in solid driven by intense laser fields is a subject of much current interest. Recently we introduce a new optimization method to directly reconstruct the band structure of the crystal from the power spectrum of strong-field generated high harmonics. Without loss of generality, the reconstruction is formulated for a one-dimensional single band model as a minimization problem and solved by a derivative-free unconstrained optimization algorithm-NEWUOA. The method can be readily generalized to treat multi-band problems. Numerical simulations are presented to demonstrate the applicability of the method, and the reconstructed band structure is found to be in excellent agreement with the exact one. It is also shown that our optimization method remains robust and efficient even starting from the poorly guessed band structure.

  6. Optical properties and electric field enhancement in cholesteric liquid crystal containing different periodicities

    NASA Astrophysics Data System (ADS)

    Ozaki, Ryotaro; Matsuhisa, Yuko; Yoshida, Hiroyuki; Yoshino, Katsumi; Ozaki, Masanori

    2006-07-01

    We study a defect mode in a one-dimensional photonic band gap of a cholesteric liquid crystal (CLC) consisting of two helicoidal periodicities. The optical properties of this CLC are analyzed using 4×4 transfer matrix and finite difference time domain (FDTD) methods. In calculated transmission spectra of this CLC, one of the defect modes always appears at the band edge wavelength of the inner CLC having a different helix to that of two sides of CLCs. Furthermore, the electric field analysis of this CLC has also been demonstrated by the FDTD method. At the defect mode wavelength, the electric field enhancement is found to be significant larger than a normal CLC.

  7. Self-consistent-field studies of core-level shifts in ionic crystals: LiF

    SciTech Connect

    Broughton, J.Q.; Bagus, P.S.

    1984-10-15

    Restricted-Hartree-Fock, self-consistent-field calculations have been performed for the ground and ionic states of clusters of lithium and fluorine atoms. These clusters are appropriately charged to represent an Li/sup +/F/sup -/ ionic crystal and point charges are used to represent the Madelung field due to ions not explicitly included in the cluster. Factors giving rise to core- and valence-level binding-energy shifts have been examined and separated into compressional, relaxational, and effective Madelung contributions. Many tests of ionicity are shown to be well satisfied, although this is often caused by fortuitous cancellations. Extra-ionic relaxation energies on both the cation and anion are shown to be small, and shakeup probabilities are calculated.

  8. Interaction of intrinsic point defects with dislocation stress fields in hcp zirconium crystal

    SciTech Connect

    Chernov, V. M. Chulkin, D. A.; Sivak, A. B.

    2010-01-15

    The crystallographic, energetic, and kinetic characteristics of intrinsic point defects (vacancy-self-interstitial atom) in stable, metastable, and saddle configurations in hcp zirconium crystal have been calculated by the molecular-statics method. The spatial dependences of the interaction energies of intrinsic point defects and stress fields of rectilinear dislocations with Burgers vectors of 1/3[112 bar 0], 1/3 [112 bar 3], and [0001] have been found within the anisotropic linear theory of elasticity. The most likely trajectories of intrinsic point defects in dislocation stress fields (trajectories with minimum energy barriers for motion) have been constructed. Such trajectories result in dislocation only for the interaction of self-interstitial atoms with an edge dislocation that has a Burgers vector of 1/3 [112 bar 3].

  9. Transferred large area single crystal MoS2 field effect transistors

    NASA Astrophysics Data System (ADS)

    Lee, Choong Hee; McCulloch, William; Lee, Edwin W.; Ma, Lu; Krishnamoorthy, Sriram; Hwang, Jinwoo; Wu, Yiying; Rajan, Siddharth

    2015-11-01

    Transfer of epitaxial, two-dimensional (2D) MoS2 on sapphire grown via synthetic approaches is a prerequisite for practical device applications. We report centimeter-scale, single crystal, synthesized MoS2 field effect transistors (FETs) transferred onto SiO2/Si substrates, with a field-effect mobility of 4.5 cm2 V-1 s-1, which is among the highest mobility values reported for the transferred large-area MoS2 transistors. We demonstrate simple and clean transfer of large-area MoS2 films using deionized water, which can effectively avoid chemical contamination. The transfer method reported here allows standard i-line stepper lithography process to realize multiple devices over the entire film area.

  10. Intrinsic dipole-field-driven mesoscale crystallization of core-shell ZnO mesocrystal microspheres.

    PubMed

    Liu, Z; Wen, X D; Wu, X L; Gao, Y J; Chen, H T; Zhu, J; Chu, P K

    2009-07-01

    Novel uniform-sized, core-shell ZnO mesocrystal microspheres have been synthesized on a large scale using a facile one-pot hydrothermal method in the presence of the water-soluble polymer poly(sodium 4-styrenesulfonate). The mesocrystal forms via a nonclassical crystallization process. The intrinsic dipole field introduced by the nanoplatelets as a result of selective adsorption of the polyelectrolyte on some polar surfaces of the nanoparticles acts as the driving force. In addition, it plays an important role throughout the mesoscale assembly process from the creation of the bimesocrystalline core to the apple-like structure and finally the microsphere. Our calculation based on a dipole model confirms the dipole-field-driven mechanism forming the apple-like structure. PMID:19518047

  11. Long-wavelength properties of phase-field-crystal models with second-order dynamics

    NASA Astrophysics Data System (ADS)

    Heinonen, V.; Achim, C. V.; Ala-Nissila, T.

    2016-05-01

    The phase-field-crystal (PFC) approach extends the notion of phase-field models by describing the topology of the microscopic structure of a crystalline material. One of the consequences is that local variation of the interatomic distance creates an elastic excitation. The dynamics of these excitations poses a challenge: pure diffusive dynamics cannot describe relaxation of elastic stresses that happen through phonon emission. To this end, several different models with fast dynamics have been proposed. In this article we use the amplitude expansion of the PFC model to compare the recently proposed hydrodynamic PFC amplitude model with two simpler models with fast dynamics. We compare these different models analytically and numerically. The results suggest that in order to have proper relaxation of elastic excitations, the full hydrodynamical description of the PFC amplitudes is required.

  12. Electrostimulation of the magnetoplastic effect in LiF crystals by an "internal" electric field induced during indentation

    NASA Astrophysics Data System (ADS)

    Galustashvili, M. V.; Driaev, D. G.; Akopov, F. Kh.; Tsakadze, S. D.

    2013-08-01

    Indented LiF crystals demonstrate a change in the length of the dislocation rosette rays during their exposure to jointly acting dc magnetic and electric fields. It is shown that magnetic field with induction B = 1 T causes the electrostimulation or electrosuppression depending on the magnitude and direction of the external electric field with respect to the "internal" electric field induced by the charge transfer due to dislocations moving during the indentation.

  13. Deposition and field emission properties of highly crystallized silicon films on aluminum-coated polyethylene napthalate

    NASA Astrophysics Data System (ADS)

    Li, Junshuai; Wang, Jinxiao; Yin, Min; Gao, Pingqi; He, Deyan; Chen, Qiang; Shirai, Hajime

    2007-08-01

    Highly crystallized silicon films were deposited on aluminum-coated polyethylene napthalate (PEN) substrates by inductively coupled plasma (ICP-) chemical vapor deposition (CVD) at room temperature. The films with uniform grains about 50 nm have the (1 1 1) preferred orientation. By studying the relation of the silicon film crystallinity to the flow ratio of SiH 4 to H 2, it was found that the interaction between precursors and aluminum layers plays an important role in the crystallization process. The surface roughness of the resultant films was analyzed by atomic force microscopy (AFM). The results reveal that the roughness of the silicon films on aluminum-coated PEN substrates, compared to the films on bare PEN substrates, is dependent on the film phase rather than the substrate morphology. The measurement of field electron emission of the crystalline silicon film indicates that the threshold field is about 8.3 V/μm and the emission is reproducible in the emission region.

  14. Phase-field crystal model for a diamond-cubic structure.

    PubMed

    Chan, V W L; Pisutha-Arnond, N; Thornton, K

    2015-05-01

    We present a structural phase-field crystal model [M. Greenwood et al., Phys. Rev. Lett. 105, 045702 (2010)] that yields a stable dc structure. The stabilization of a dc structure is accomplished by constructing a two-body direct correlation function (DCF) approximated by a combination of two Gaussian functions in Fourier space. A phase diagram containing a dc-liquid phase coexistence region is calculated for this model. We examine the energies of solid-liquid interfaces with normals along the [100], [110], and [111] directions. The dependence of the interfacial energy on a temperature parameter, which controls the heights of the peaks in the two-body DCF, is described by a Gaussian function. Furthermore, the dependence of the interfacial energy on the peak widths of the two-body DCF, which controls the excess energy associated with interfaces, defects, and strain, is described by an inverse power law. These relationships can be used to parametrize the phase-field crystal model for the dc structure to match solid-liquid interfacial energies to those measured experimentally or calculated from atomistic simulations. PMID:26066277

  15. Rubrene crystal field-effect mobility modulation via conducting channel wrinkling

    PubMed Central

    Reyes-Martinez, Marcos A.; Crosby, Alfred J.; Briseno, Alejandro L.

    2015-01-01

    With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure–function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices. PMID:25939864

  16. Modified phase-field-crystal model for solid-liquid phase transitions

    NASA Astrophysics Data System (ADS)

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Tang, Sai

    2015-07-01

    A modified phase-field-crystal (PFC) model is proposed to describe solid-liquid phase transitions by reconstructing the correlation function. The effects of fitting parameters of our modified PFC model on the bcc-liquid phase diagram, numerical stability, and solid-liquid interface properties during planar interface growth are examined carefully. The results indicate that the increase of the correlation function peak width at k =km will enhance the stability of the ordered phase, while the increase of peak height at k =0 will narrow the two-phase coexistence region. The third-order term in the free-energy function and the short wave-length of the correlation function have significant influences on the numerical stability of the PFC model. During planar interface growth, the increase of peak width at k =km will decrease the interface width and the velocity coefficient C , but increase the anisotropy of C and the interface free energy. Finally, the feasibility of the modified phase-field-crystal model is demonstrated with a numerical example of three-dimensional dendritic growth of a body-centered-cubic structure.

  17. Modified phase-field-crystal model for solid-liquid phase transitions.

    PubMed

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Tang, Sai

    2015-07-01

    A modified phase-field-crystal (PFC) model is proposed to describe solid-liquid phase transitions by reconstructing the correlation function. The effects of fitting parameters of our modified PFC model on the bcc-liquid phase diagram, numerical stability, and solid-liquid interface properties during planar interface growth are examined carefully. The results indicate that the increase of the correlation function peak width at k=k(m) will enhance the stability of the ordered phase, while the increase of peak height at k=0 will narrow the two-phase coexistence region. The third-order term in the free-energy function and the short wave-length of the correlation function have significant influences on the numerical stability of the PFC model. During planar interface growth, the increase of peak width at k=k(m) will decrease the interface width and the velocity coefficient C, but increase the anisotropy of C and the interface free energy. Finally, the feasibility of the modified phase-field-crystal model is demonstrated with a numerical example of three-dimensional dendritic growth of a body-centered-cubic structure. PMID:26274309

  18. Structural phase field crystal approach for modeling graphene and other two-dimensional structures

    NASA Astrophysics Data System (ADS)

    Seymour, Matthew; Provatas, Nikolas

    2016-01-01

    This paper introduces a new structural phase field crystal (PFC) type model that expands the PFC methodology to a wider class of structurally complex crystal structures than previously possible. Specifically, our approach allows for stabilization of graphene, as well as its coexistence with a disordered phase. It also preserves the ability to model the usual triangular and square lattices previously reported in two-dimensional (2D) PFC studies. Our approach is guided by the formalism of classical field theory, wherein the free-energy functional is expanded to third order in PFC density correlations. It differs from previous PFC approaches in two main features. First, it utilizes a hard-sphere repulsion to describe two-point correlations. Second, and more important, is that it uses a rotationally invariant three-point correlation function that provides a unified way to control the formation of crystalline structures that can be described by a specific bond angle, such as graphene, triangular, or square symmetries. Our approach retains much of the computational simplicity of previous PFC models and allows for efficient simulation of nucleation and growth of polycrystalline 2D materials. In preparation for future applications, this paper details the mathematical derivation of the model and its equilibrium properties and uses dynamical simulations to demonstrate defect structures produced by the model.

  19. Flow-induced Crystallization of Long Chain Aliphatic Polyamides under a Complex Flow Field

    NASA Astrophysics Data System (ADS)

    Dong, Xia; Gao, Yunyun; Wang, Lili; Wang, Dujin

    The present work deals with the flow-induced multiple orientations and crystallization structure of polymer melts under a complex flow field. This complex flow field is characteristic of the consistent coupling of extensional ``pulse'' and closely followed shear flow in a narrow channel. Utilizing an ingenious combination of an advanced micro-injection device and long chain aliphatic polyamides, the flow-induced crystallization morphology was well preserved for ex-situ synchrotron micro-focused wide angle X-ray scattering as well as small angle X-ray scattering. The experimental results clearly indicate that the effect of extensional pulse on the polymer melt is restrained and further diminished due to either the transverse tumble of fountain flow or the rapid retraction of stretched high molecular weight tails. However, the residual shish-kebab structures in the core layer of the far-end of channel suggest that the effect of extensional pulse should be considered in the small-scaled geometries or under the high strain rate condition. The authors thank the financial support from MOST (2013BAE02B02, 2014CB643600) and NSFC(21574140).

  20. Rubrene crystal field-effect mobility modulation via conducting channel wrinkling.

    PubMed

    Reyes-Martinez, Marcos A; Crosby, Alfred J; Briseno, Alejandro L

    2015-01-01

    With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure-function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices. PMID:25939864

  1. Single molecule spectroscopy of conjugated polymer chains in an electric field-aligned liquid crystal.

    PubMed

    Chang, Wei-Shun; Link, Stephan; Yethiraj, Arun; Barbara, Paul F

    2008-01-17

    Using single molecule polarization spectroscopy, we investigated the alignment of a polymer solute with respect to the liquid crystal (LC) director in an LC device while applying an external electric field. The polymer solute is poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (or MEH-PPV), and the LC solvent is 5CB. The electric field induces a change in the LC director orientation from a planar alignment (no electric field) to a perpendicular (homeotropic) alignment with an applied field of 5.5 x 103 V/cm. We find that the polymer chains align with the LC director in both planar and homeotropic alignment when measured in the bulk of the LC solution away from the device interface. Single molecule polarization distributions measured as a function of distance from the LC device interface reveal a continuous change of the MEH-PPV alignment from planar to homeotropic. The observed polarization distributions are modeled using a conventional elastic model that predicts the depth profile of the LC director orientation for the applied electric field. The excellent agreement between experiment and simulations shows that the alignment of MEH-PPV follows the LC director throughout the LC sample. Furthermore, our results suggest that conjugated polymers such as MEH-PPV can be used as sensitive local probes to explore complex (and unknown) structures in anisotropic media. PMID:17975912

  2. Field-driven dynamics of microcapillaries filled with nematic liquid crystal

    NASA Astrophysics Data System (ADS)

    Fu, Fred; Khayyatzadeh, Pouya; Abukhdeir, Nasser M.

    Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of study for their unique electro-optical properties and the feasibility of manufacturing them on a large scale, resulting in applications such as switchable windows. LC domains within PDLCs are typically spheroidal, as opposed to rectangular in LCD technology, and thus exhibit substantially different behaviour in the presence of an external field. In this work, continuum simulations were performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. A simplified elliptic cylinder (microcapillary) geometry is used and the effects of varying aspect ratio, surface anchoring, and external field strength were studied using the Landau-de Gennes model. The observed nematic formation and reorientation dynamics were found to be governed by the presence and motion of defects within the domain. Aspect ratio was found to strongly influence domain texture by providing regions of high curvature to which defects are attracted. Simulations also predict the presence of a geometry-controlled transition from nematic order enhanced by an external field (low aspect ratio) to nematic order frustrated by an external field (high aspect ratio). This work was made possible by the Natural Sciences and Engineering Research Council of Canada and Compute Ontario.

  3. Motion of polymer cholesteric liquid crystal flakes in an electric field

    NASA Astrophysics Data System (ADS)

    Kosc, Tanya Zoriana

    Polymer cholesteric liquid crystal (PCLC) flakes suspended in a host fluid can be manipulated with an electric field. Controlling a flake's orientation provides the opportunity to change and control the amount of selective reflection from the flake surface. Flake motion results from charge accumulation and an induced dipole moment established due to Maxwell-Wagner polarization. The type of flake behavior, whether random motion or uniform reorientation, depends upon the dielectric properties of the host fluid, which in turn dictate whether a DC or an AC electric field must be applied. PCLC flakes suspended in highly dielectric silicone oil host fluids tend to move randomly in the presence of a DC electric field, and no motion is seen in AC fields. Flakes suspended in a moderately conductive host fluid reorient 90° in the presence of an AC field within a specific frequency range. The flake shape and size are also important parameters that need to be controlled in order to produce uniform motion. Several methods for patterning flakes were investigated and identical square flakes were produced. Square PCLC flakes (80 mum sides) suspended in propylene carbonate reorient in 400 ms when a 40mVrms/mum field at 70 Hz is applied to the test device. Theoretical modeling supported experimental observations well, particularly in identifying the inverse quadratic dependence on the applied electric field and the electric field frequency dependence that is governed by the host fluid conductivity. Future goals and suggested experiments are provided, as well as an explanation and comparison of possible commercial applications for PCLC flakes. This research has resulted in one patent application and a series of invention disclosures that could place this research group and any industrial collaborators in a strong position to pursue commercial applications, particularly in the area of displays, and more specifically, electronic paper.

  4. A three-dimensional phase field model coupled with lattice kinetics solver for modeling crystal growth in furnaces with accelerated crucible rotation and traveling magnetic field

    SciTech Connect

    Lin, Guang; Bao, Jie; Xu, Zhijie

    2014-11-01

    In this study, which builds on other related work, we present a new three-dimensional numerical model for crystal growth in a vertical solidification system. This model accounts for buoyancy, accelerated crucible rotation technique (ACRT), and traveling magnetic field (TMF) induced convective flow and their effect on crystal growth and the chemical component's transport process. The evolution of the crystal growth interface is simulated using the phase field method. A semi-implicit lattice kinetics solver based on the Boltzmann equation is employed to model the unsteady incompressible flow. A one-way coupled concentration transport model is used to simulate the component fraction variation in both the liquid and solid phases, which can be used to check the quality of the crystal growth.

  5. Change in the microhardness of nonmagnetic crystals after their exposure to the Earth's magnetic field and AC pump field in the EPR scheme

    NASA Astrophysics Data System (ADS)

    Alshits, V. I.; Darinskaya, E. V.; Koldaeva, M. V.; Petrzhik, E. A.

    2012-02-01

    Changes in the microhardness of ZnO, triglycine sulfate (TGS), and potassium acid phthalate (KAP) crystals after their exposure to crossed ultralow magnetic fields, i.e., the Earth's field B Earth ≈ 50 μT and the alternating-current field tilde B ≈ 3 μ T orthogonal to it, have been revealed. In ZnO crystals, the microhardness increases, whereas in TGS and KAP, it decreases. A maximum change (10-15%) is reached within 1-3 h after magnetic treatment; then, the microhardness gradually recovers to its initial value for the first day. After a sufficient pause, the effect is completely reproduced under the same conditions. The resonant frequency of the pump field tilde B corresponds to the EPR condition with a g-factor close to two. The magnetic memory exhibits a strong anisotropy: for each of the crystals, a direction is found, which, being coincident with the Earth's magnetic field vector B Earth, causes complete or partial suppression of the effect. In ZnO and TGS crystals, these are symmetry axes 6 and 2, respectively. In the KAP crystal, it is the direction in the cleavage plane orthogonal the 2 axis. Possible physical mechanisms of the observed phenomena have been discussed.

  6. Effects of the biaxial transverse crystal-field on the phase diagrams of a spin-1 nanowire

    NASA Astrophysics Data System (ADS)

    Magoussi, H.; Zaim, A.; Boughrara, M.; Kerouad, M.

    2016-09-01

    By using the effective field theory based on a probability distribution method, the phase diagrams and the magnetic properties of an Ising nanowire in the presence of the biaxial transverse crystal-field are investigated. The effects of the biaxial transverse crystal field, the interfacial coupling and the exchange interaction in the surface on the phase diagram, the magnetization and the internal energy are examined. Some characteristic phenomena are found such as the tricritical behavior, the critical end point and the re-entrant phenomenon.

  7. Photonic crystal fiber injected with Fe{sub 3}O{sub 4} nanofluid for magnetic field detection

    SciTech Connect

    Thakur, Harneet V.; Nalawade, Sandipan M.; Gupta, Swati; Kitture, Rohini; Kale, S. N.

    2011-10-17

    We report a magnetic field sensor having advantages of both photonic crystal fiber and optofluidics, combining them on a single platform by infiltrating small amount of Fe{sub 3}O{sub 4} magnetic optofluid/nanofluid in cladding holes of polarization-maintaining photonic crystal fiber. We demonstrated that magnetic field of few mT can be easily and very well detected with higher sensitivity of 242 pm/mT. The change in the birefringence values has been correlated to the response of nanofluid to applied field.

  8. Synthesis and superconducting properties of FeTe1-xSe Single Crystals under high magnetic fields

    NASA Astrophysics Data System (ADS)

    Gebre, Tesfaye; Li, G.; Whalen, J.; Conner, B.; Kostov, M.; Siegrist, T.; Balicas, L.

    2011-03-01

    Single crystals of superconductor FeTe 1-x Se x (0.1 <= x <= 0.5) were synthesized using optical floating zone, Bridgeman technique, and solid stat reaction. The samples were synthesized under various temperature gradients and cooling rates. Crystals were characterized via EDX, X-ray scattering, magnetization and transport measurements. Upper critical fields Hc 2 as estimated through the Werthamer-Hohenberg-Helfand (WHH) formalism indicate that these materials strongly surpass the weak coupling Pauli limiting field indicating that the shape of their phase diagram under field is essentially controlled by the Pauli effect. Annealing, leads to a metallic temperature dependence of the resistivity, and to sharper superconducting transitions. Despite the relatively small increase in single crystallinity, as quantified by single crystal x-ray diffraction measurements, we observe a different phase diagram under high magnetic fields when compared to non-annealed samples.

  9. Tunable Lamb wave band gaps in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field.

    PubMed

    Zhou, Changjiang; Sai, Yi; Chen, Jiujiu

    2016-09-01

    This paper theoretically investigates the band gaps of Lamb mode waves in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field. With the assumption of uniformly oriented magnetization, an equivalent piezomagnetic material model is used. The effects of magnetostatic field on phononic crystals are considered carefully in this model. The numerical results indicate that the width of the first band gap is significantly changed by applying the external magnetic field with different amplitude, and the ratio between the maximum and minimum gap widths reaches 228%. Further calculations demonstrate that the orientation of the magnetic field obviously affects the width and location of the first band gap. The contactless tunability of the proposed phononic crystal slabs shows many potential applications of vibration isolation in engineering. PMID:27281285

  10. Spectroscopic characterisation and crystal field calculations of varicoloured kyanites from Loliondo, Tanzania

    NASA Astrophysics Data System (ADS)

    Wildner, Manfred; Beran, Anton; Koller, Friedrich

    2013-04-01

    Orange, ochre-coloured, light green and dark blue varieties of kyanite, ideally Al2SiO5, from Loliondo, Tanzania, have been characterised by electron microprobe analysis and polarised infrared and optical absorption spectroscopy. All colour varieties show elevated Fe contents of 0.39 to 1.31 wt.% FeO, but Ti contents only in the range of the EMP detection limit. Orange and ochre-coloured crystals have Mn contents of 0.23 and 0.06 wt.% MnO, respectively, the dark blue kyanite contains 0.28 wt.% Cr2O3, while the light green sample is nearly free from transition metal cations other than Fe. Polarised infrared spectra reveal OH defect concentrations of 3 to 17 wt.ppm H2O with structural OH defects partially replacing the OB (O2) oxygen atoms. Polarised optical absorption spectra show that the colour of all four varieties is governed by crystal field d-d transitions of trivalent cations, i.e. Fe3+ (all samples), Mn3+ (orange and ochre) and Cr3+ (blue kyanite), replacing Al in sixfold coordinated triclinic sites of the kyanite structure. Intervalence charge transfer, the prevalent colour-inducing mechanism in `usual' (Cr-poor) blue kyanites, seems to play a very minor, if any, role in the present samples. Crystal field calculations in both a `classic' tetragonal and in the semiempirical Superposition Model approach, accompanied by distance- and angle-least-squares refinements, indicate that Fe3+ preferably occupies the Al4 site, Cr3+ prefers the Al1 and Al2 sites, and Mn3+ predominantly enters the Al1 site. In each case specific local relaxation effects were observed according to the crystal chemical preferences of these transition metal cations. Furthermore, the high values obtained in the calculations for the interelectronic repulsion parameter Racah B correspond to a high ionic contribution to Me3+-O bonding in the kyanite structure. In the particular case of the blue sample, band positions specifically related to the high Racah B value enable this `unusual' type of

  11. Effect of temperature and electric field on 2D nematic colloidal crystals stabilised by vortex-like topological defects.

    PubMed

    Zuhail, K P; Dhara, Surajit

    2016-08-10

    We report experimental studies on 2D colloidal crystals of dimers stabilized by vortex-like defects in planar nematic and π/2 twisted nematic cells. The dimers are prepared and self-assembled using a laser tweezer. We study the effect of temperature and electric field on the lattice parameters of the colloidal crystals. The lattice parameters vary with the temperature in the nematic phase and a discontinuous structural change is observed at the nematic to smectic-A phase transition. In the nematic phase, we observed a large change in the lattice parameters (≃30%) by applying an external electric field perpendicular to the plane of the 2D crystals. The idea and the active control of the lattice parameters could be useful for designing tunable colloidal crystals. PMID:27445255

  12. Multilayer thin film growth on crystalline and quasicrystalline surfaces: A phase-field crystal study

    NASA Astrophysics Data System (ADS)

    Muralidharan, Srevatsan; Khodadad, Raika; Sullivan, Ethan; Haataja, Mikko

    2012-06-01

    In this paper, we explore the effects of misfit strain fields on both heterogeneous nucleation behavior and anisotropic growth of islands at submonolayer coverages and compositional patterning at complete monolayer coverage via simulations of a phase-field crystal model. In particular, deposition on top of a herringbone structure and quasicrystalline (QC) substrate are considered, the former representing a system with spatially periodic misfit strain fields arising from the presence of surface dislocations, and the latter representing a system which inherently possesses a wide range of local, aperiodic misfit patterns. In the case of single-component systems, we demonstrate that misfit strain fields lead to heterogeneous nucleation behavior and anisotropic island growth. In the case of QC substrate, a wide range of morphologies, such as coexistence of locally hexagonally ordered atomic clusters within a larger scale arrangement with overall QC symmetry and so-called “starfish” aggregates, is observed in a pure system at submonolayer coverages when the adlayer-substrate interaction strength and lattice mismatch are tuned. In the case of bulk-immiscible binary systems at complete monolayer coverage, strain-stabilized compositional domains emerge at low line tension values for both substrates. Interestingly, the compositional domains on the QC substrate inherit their symmetries at sufficiently low line tension values, while at larger line tension values, the domain structure begins to resemble the classical spinodal microstructure. Such domain structures should be readily observable in colloidal systems in which attractive interparticle and particle-substrate interactions can be tuned.

  13. Ferroelectric Single-Crystal Gated Graphene/Hexagonal-BN/Ferroelectric Field-Effect Transistor.

    PubMed

    Park, Nahee; Kang, Haeyong; Park, Jeongmin; Lee, Yourack; Yun, Yoojoo; Lee, Jeong-Ho; Lee, Sang-Goo; Lee, Young Hee; Suh, Dongseok

    2015-11-24

    The effect of a ferroelectric polarization field on the charge transport in a two-dimensional (2D) material was examined using a graphene monolayer on a hexagonal boron nitride (hBN) field-effect transistor (FET) fabricated using a ferroelectric single-crystal substrate, (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT). In this configuration, the intrinsic properties of graphene were preserved with the use of an hBN flake, and the influence of the polarization field from PMN-PT could be distinguished. During a wide-range gate-voltage (VG) sweep, a sharp inversion of the spontaneous polarization affected the graphene channel conductance asymmetrically as well as an antihysteretic behavior. Additionally, a transition from antihysteresis to normal ferroelectric hysteresis occurred, depending on the V(G) sweep range relative to the ferroelectric coercive field. We developed a model to interpret the complex coupling among antihysteresis, current saturation, and sudden conductance variation in relation with the ferroelectric switching and the polarization-assisted charge trapping, which can be generalized to explain the combination of 2D structured materials with ferroelectrics. PMID:26487348

  14. Competition between local disordering and global ordering fields in nematic liquid crystals

    PubMed Central

    Ambrožič, Milan; Kralj, Samo

    2010-01-01

    Summary We study the influence of external electric or magnetic field B on orientational ordering of nematic liquid crystals or of other rod-like objects (e.g. nanotubes immersed in a liquid) in the presence of random anisotropy field type of disorder. The Lebwohl–Lasher lattice type of semi-microscopic approach is used at zero temperature. Therefore, results are valid well below the transition into the isotropic phase. We calculate the correlation function of systems as a function of B, concentration p of impurities imposing random anisotropy field disorder, the disorder strength W and system dimensionality (2D and 3D systems). In order to probe memory effects we calculate correlation length ξ for random and homogeneous initial configurations. We determine the crossover fields B c(p) separating roughly the ordered and disordered regime. Memory effects are apparent only in the latter case, i.e. for B < B c. PACS numbers: 47.51.+a, 47.54.-r, 07.05.Tp, 61.30.-v PMID:20502609

  15. Terahertz probes of magnetic field induced spin reorientation in YFeO{sub 3} single crystal

    SciTech Connect

    Lin, Xian; Jiang, Junjie; Ma, Guohong; Jin, Zuanming; Wang, Dongyang; Tian, Zhen; Han, Jiaguang; Cheng, Zhenxiang

    2015-03-02

    Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO{sub 3} single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO{sub 3} (R = Y{sup 3+} or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching in the orthoferrite antiferromagnetic systems.

  16. Study of the local field distribution on a single-molecule magnet by a single paramagnetic crystal: A DPPH crystal on the surface of an Mn12-acetate crystal

    NASA Astrophysics Data System (ADS)

    Žilić, Dijana; Rakvin, Boris; Dalal, Naresh S.

    2011-11-01

    The local magnetic field distribution on the surface of a single-molecule magnet crystal (SMM) above blocking temperature (T ≫Tb) detected for a very short time interval (~10-10s), has been investigated. Electron paramagnetic resonance (EPR) spectroscopy, using a local paramagnetic probe, was employed as a simple alternative detection method. An SMM crystal of [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2-diphenyl-1-picrylhydrazyl (DPPH), as the paramagnetic probe, were chosen for this study. The EPR spectra of DPPH deposited on Mn12-acetate show additional broadening and shifting in the magnetic field in comparison to the spectra of the DPPH in the absence of the SMM crystal. The additional broadening of the DPPH linewidth was considered in terms of the two dominant electron spin interactions (dipolar and exchange) and the local magnetic field distribution on the crystal surface. The temperature dependence of the linewidth of the Gaussian distribution of local fields at the SMM surface was extrapolated for the low-temperature interval (70-5 K).

  17. Phase-field crystal approach for modeling the role of microstructure in multiferroic composite materials

    NASA Astrophysics Data System (ADS)

    Seymour, Matthew; Sanches, F.; Elder, Ken; Provatas, Nikolas

    2015-11-01

    This paper introduces a phase-field crystal (PFC) approach that couples the atomic-scale PFC density field to order parameters describing ferromagnetic and ferroelectric ordering, as well to a solute impurity field. This model extends the magnetic PFC model introduced by Faghihi et al. [N. Faghihi, Ph.D. Thesis, The University of Western Ontario, 2012; N. Faghihi, N. Provatas, K. R. Elder, M. Grant, and M. Karttunen, Phys. Rev. E 88, 032407 (2013), 10.1103/PhysRevE.88.032407] to incorporate polarization and concentration fields, as well as anisotropic ordering of the magnetization and polarization fields as determined by the local crystalline orientation. Magnetoelectric coupling is incorporated through the elastic coupling. Analytic calculations for a body centered-cubic (BCC) system are presented to illustrate that the model reduces to the standard multiferroic phase-field models when only a single crystal is considered. Two special cases of the model are then studied, the first focusing on magnetocrystalline interactions in a system described by the two-point correlation function of the XPFC model developed by Greenwood et al. [M. Greenwood, N. Provatas, and J. Rottler, Phys. Rev. Lett. 105, 045702 (2010), 10.1103/PhysRevLett.105.045702; M. Greenwood, J. Rottler, and N. Provatas, Phys. Rev. E 83, 031601 (2011), 10.1103/PhysRevE.83.031601], and the second focusing on electrocrystalline interactions in a system described by the original PFC kernel developed by Elder et al. K. R. Elder, M. Katakowski, M. Haataja, and M. Grant, Phys. Rev. Lett. 88, 245701 (2002), 10.1103/PhysRevLett.88.245701; K. R. Elder and M. Grant, Phys. Rev. E 70, 051605 (2004), 10.1103/PhysRevE.70.051605]. We examine the small deformation properties of these two realizations of the model . Numerical simulations are performed to illustrate how magnetocrystalline coupling can be exploited to design a preferential grain texture and how defects and grain boundaries influence the ferroelectric

  18. Faraday isolator based on a TSAG single crystal with compensation of thermally induced depolarization inside magnetic field

    NASA Astrophysics Data System (ADS)

    Snetkov, Ilya; Palashov, Oleg

    2015-04-01

    A Faraday isolator based on a terbium scandium aluminum garnet (TSAG) single crystal with compensation of thermally induced depolarization inside magnetic field was demonstrated. An isolation ratio of 32 dB at 350 W cw laser radiation power was achieved. Thermally induced depolarization and thermal lens were studied and compared with similar thermal effects arising in the widely used terbium gallium garnet crystal (TGG) for the first time.

  19. Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors

    NASA Astrophysics Data System (ADS)

    Takeya, J.; Nishikawa, T.; Takenobu, T.; Kobayashi, S.; Iwasa, Y.; Mitani, T.; Goldmann, C.; Krellner, C.; Batlogg, B.

    2004-11-01

    The surface conductivity is measured by a four-probe technique for pentacene and rubrene single crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ˜1012cm-2 at the surface of the organic single crystals.

  20. Controlling electromagnetic fields using periodic structures: Gratings, metamaterials, and photonic crystals

    NASA Astrophysics Data System (ADS)

    Memarian, Mohammad

    This thesis presents novel devices and techniques that enable new methods for enhancement, concentration, refraction, shaping, collimation, and directive beaming of electromagnetic fields. These unprecedented methods to control electromagnetic fields are achieved by exploring and harnessing the unique wave-interactions in periodic gratings, metamaterials, and photonic crystals, with emphasis on Epsilon-Near-Zero (ENZ) metamaterials and zero-index media. The presented solutions impact a wide variety of applications ranging from microwave to optical frequencies. A discovery of dramatic radiation enhancement of an invisible array of sources next to a sub-wavelength periodic metal strip grating is reported, both theoretically and experimentally. The phenomenon is first systematically theorized by introducing the 'spectral impulse response' approach for the aperiodic excitation problem, followed by the 'spectral array factor' approach for designing the near-field of array of sources. Such radiation enhancement has applications in sensing, detection, and accurate measurement of distance. The shaping and collimation of radiation of a simple dipole source near or buried inside a general anisotropic ENZ half-space is then systematically studied using the Lorentz reciprocity method. Various elliptic and hyperbolic anisotropic ENZ media are considered, showing how the air-side radiation can be enhanced and shaped using certain ENZs. A novel device and technique is proposed for collecting, refracting and concentrating incident waves into an area of high power concentration, at extremely short distances. This flat low-profile light-concentrator comprises a hetero-junction of anisotropic ENZ metamaterials (hyperbolic or elliptic), and is realized with plasmonic layered media at optical frequencies. By harnessing an extremely oblique refraction process in ENZs, the light-concentrator significantly outperforms the size requirements of existing thick high curvature lenses, useful

  1. Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis

    PubMed Central

    Zhang, Yibo; Lee, Seung Yoon Celine; Zhang, Yun; Furst, Daniel; Fitzgerald, John; Ozcan, Aydogan

    2016-01-01

    Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient’s joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient’s tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm2), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings. PMID:27356625

  2. Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis

    NASA Astrophysics Data System (ADS)

    Zhang, Yibo; Lee, Seung Yoon Celine; Zhang, Yun; Furst, Daniel; Fitzgerald, John; Ozcan, Aydogan

    2016-06-01

    Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient’s joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient’s tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm2), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings.

  3. Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis.

    PubMed

    Zhang, Yibo; Lee, Seung Yoon Celine; Zhang, Yun; Furst, Daniel; Fitzgerald, John; Ozcan, Aydogan

    2016-01-01

    Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient's joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient's tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm(2)), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings. PMID:27356625

  4. Crystal-field excitations and magnetic properties of Ho{sup 3+} in HoVO{sub 4}

    SciTech Connect

    Skanthakumar, S.; Loong, C.; Soderholm, L.; Abraham, M.M.; Boatner, L.A.

    1995-05-01

    The magnetic excitations in HoVO{sub 4} were studied by neutron scattering and susceptibility techniques. Well-defined transitions between the crystal-field-split states of the Ho{sup 3+} ions were observed at 15, 40, and 100 K. The magnetic spectra were analyzed using a single-ion crystal-field model which includes intermediate coupling of the LS states of Ho. A quantitative comparison of the observed energies and intensities with the model was made and used to refine the five crystal-field parameters needed to calculate the Ho ionic wave functions and other magnetic properties. The nonmagnetic {Gamma}{sub 1}-singlet ground state (containing about 90% pure {vert_bar}8,0{r_angle} component) of the Ho ions, in conjunction with the next higher doublet state situated at 2.5 meV, strongly influences the low-temperature magnetic behavior. The calculated magnetic susceptibility, which exhibits an easy plane coinciding with the crystallographic {ital a}-{ital b} plane at low temperatures, agrees very well with the experimental data obtained from single-crystal measurements. The magnetic properties of HoVO{sub 4} are contrasted with those of an isostructural compound HoPO{sub 4} which has a 98% pure {vert_bar}8,7{r_angle}-doublet ground state. The difference in the crystal-field-level structure between these two compounds is reflected in a sign change of the {ital B}{sub 0}{sup 2} crystal-field parameter. Despite the overall tetragonal crystal structure of HoVO{sub 4}, which predicts double degeneracy for each {Gamma}{sub 5} state, a small splitting in the first-excited doublet was clearly observed at low temperatures.

  5. A high-field magnetization study of a Nd(2)Fe(14)Si(3) single crystal.

    PubMed

    Andreev, A V; Yoshii, S; Kuz'min, M D; de Boer, F R; Kindo, K; Hagiwara, M

    2009-04-01

    Magnetization study of a single crystal of Nd(2)Fe(14)Si(3) (with the rhombohedral Th(2)Zn(17)-type structure) reveals that the compound is a ferromagnet with a spontaneous magnetic moment of 32.3μ(B) per formula unit (at T = 2 K) and a Curie temperature equal to 495 K. The easy-magnetization direction lies close to the b-axis, tilting slightly towards the c-axis. (The b-axis [120] is not a high-symmetry direction in the crystallographic class D(3d).) The observed strong magnetic anisotropy is attributed almost entirely to the Nd sublattice, as concluded from comparison with a Y(2)Fe(14)Si(3) single crystal. A magnetic field applied along the c-axis induces a first-order spin reorientation transition at B(FOMP) = 20 T. In the process of magnetization the Nd and Fe sublattices behave as essentially non-collinear. This is manifest particularly in the downward curvature of the first (pre-FOMP) stage of the magnetization curve. It is proposed to regard this curvature as a validity criterion for the single-sublattice approximation. PMID:21825352

  6. Anisotropic magnetic behavior of PrAg2Ge2—a crystal field study

    NASA Astrophysics Data System (ADS)

    Chatterji, A.; Chatterjee, Tania; Mitra, S.

    2012-02-01

    A crystal field (CF) analysis of the experimental, single crystal magnetic susceptibility data (300-1.8 K) along and perpendicular to the [0 0 1] axis, of the Ag based rare earth intermetallic compound PrAg 2Ge 2, has been carried out for the first time, thus yielding first reliable set of CF parameters for the system. The susceptibility feature at 12 K is possibly due to CF effects rather than the magnetic order as proposed earlier. This removes the issue of the transition temperature being too large to scale properly with the de Gennes factor. We have used the set of CF parameters to find the Stark energies of the ground state and the excited states together with their corresponding eigenvectors, and the thermal variation of the magnetic specific heat. Possible explanation of the absence of magnetic ordering of the Pr sublattice and the nature of variation of the CF parameters with the substitution of transition-metal ion in PrAg 2Ge 2 is studied and discussed in relation to PrAu 2Ge 2. All computations have been carried out using the intermediate coupling scheme including the J-mixing.

  7. Temperature and field induced strain measurements in single crystal Gd5Si2Ge2

    DOE PAGESBeta

    McCall, S. K.; Nersessian, N.; Carman, G. P.; Pecharsky, V. K.; Schlagel, D. L.; Radousky, H. B.

    2016-03-29

    The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of –8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of –8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Furthermore,more » using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.« less

  8. Phase field crystal modeling as a unified atomistic approach to defect dynamics

    NASA Astrophysics Data System (ADS)

    Berry, Joel; Provatas, Nikolas; Rottler, Jörg; Sinclair, Chad W.

    2014-06-01

    Material properties controlled by evolving defect structures, such as mechanical response, often involve processes spanning many length and time scales which can not be modeled using a single approach. We present a variety of results that demonstrate the ability of phase field crystal (PFC) models to describe complex defect evolution phenomena on atomistic length scales and over long, diffusive time scales. Primary emphasis is given to the unification of conservative and nonconservative dislocation creation mechanisms in three-dimensional fcc and bcc materials. These include Frank-Read-type glide mechanisms involving closed dislocation loops or grain boundaries as well as Bardeen-Herring-type climb mechanisms involving precipitates, inclusions, and/or voids. Both source classes are naturally and simultaneously captured at the atomistic level by PFC descriptions, with arbitrarily complex defect configurations, types, and environments. An unexpected dipole-to-quadrupole source transformation is identified, as well as various complex geometrical features of loop nucleation via climb from spherical particles. Results for the strain required to nucleate a dislocation loop from such a particle are in agreement with analytic continuum theories. Other basic features of fcc and bcc dislocation structure and dynamics are also outlined, and initial results for dislocation-stacking fault tetrahedron interactions are presented. These findings together highlight various capabilities of the PFC approach as a coarse-grained atomistic tool for the study of three-dimensional crystal plasticity.

  9. Vectorial near-field imaging of a GaN based photonic crystal cavity

    SciTech Connect

    La China, F. Intonti, F.; Caselli, N.; Lotti, F.; Vinattieri, A.; Gurioli, M.; Vico Triviño, N.; Carlin, J.-F.; Butté, R.; Grandjean, N.

    2015-09-07

    We report a full optical deep sub-wavelength imaging of the vectorial components of the electric local density of states for the confined modes of a modified GaN L3 photonic crystal nanocavity. The mode mapping is obtained with a scanning near-field optical microscope operating in a resonant forward scattering configuration, allowing the vectorial characterization of optical passive samples. The optical modes of the investigated cavity emerge as Fano resonances and can be probed without the need of embedded light emitters or evanescent light coupling into the nanocavity. The experimental maps, independently measured in the two in-plane polarizations, turn out to be in excellent agreement with numerical predictions.

  10. Photonic crystal membrane reflectors by magnetic field-guided metal-assisted chemical etching

    SciTech Connect

    Balasundaram, Karthik; Mohseni, Parsian K.; Li, Xiuling E-mail: xiuling@illinois.edu; Shuai, Yi-Chen; Zhao, Deyin; Zhou, Weidong E-mail: xiuling@illinois.edu

    2013-11-18

    Metal-assisted chemical etching (MacEtch) is a simple etching method that uses metal as the catalyst for anisotropic etching of semiconductors. However, producing nano-structures using MacEtch from discrete metal patterns, in contrast to interconnected ones, has been challenging because of the difficulties in keeping the discrete metal features in close contact with the semiconductor. We report the use of magnetic field-guided MacEtch (h-MacEtch) to fabricate periodic nanohole arrays in silicon-on-insulator (SOI) wafers for high reflectance photonic crystal membrane reflectors. This study demonstrates that h-MacEtch can be used in place of conventional dry etching to produce ordered nanohole arrays for photonic devices.

  11. Changes in crystal structure and physicochemical properties of potato starch treated by induced electric field.

    PubMed

    Li, Dandan; Yang, Na; Jin, Yamei; Zhou, Yuyi; Xie, Zhengjun; Jin, Zhengyu; Xu, Xueming

    2016-11-20

    The effects of induced electric field (IEF) on the crystal structure and physicochemical properties of potato starch were investigated by subjecting identically treated control and electrically-modified samples to the same temperature history. Additionally, a method of combining IEF with heating for efficient modification of native polymer was also proposed. Results showed that the application of IEF at an electric voltage of 75V has a statistically significant effect on starch gelatinization and pasting properties, especially when combined with heating at 50°C. After treatment by the combination method for 96h, the gelatinization temperatures increased, which can be explained by the slight increase in the ratio of 1044/1015cm(-1) and relative crystallinity. Furthermore, IEF reduced granular swelling and therefore contributed to decreasing the peak, breakdown, and setback viscosity of potato starch. This study explores the potential of IEF as innovative technology for starch modification. PMID:27561526

  12. First and second order operator splitting methods for the phase field crystal equation

    SciTech Connect

    Lee, Hyun Geun; Shin, Jaemin; Lee, June-Yub

    2015-10-15

    In this paper, we present operator splitting methods for solving the phase field crystal equation which is a model for the microstructural evolution of two-phase systems on atomic length and diffusive time scales. A core idea of the methods is to decompose the original equation into linear and nonlinear subequations, in which the linear subequation has a closed-form solution in the Fourier space. We apply a nonlinear Newton-type iterative method to solve the nonlinear subequation at the implicit time level and thus a considerably large time step can be used. By combining these subequations, we achieve the first- and second-order accuracy in time. We present numerical experiments to show the accuracy and efficiency of the proposed methods.

  13. Comparison of Phase Field Crystal and Molecular Dynamics Simulations for a Shrinking Grain

    SciTech Connect

    Radhakrishnan, Balasubramaniam; Gorti, Sarma B; Nicholson, Don M

    2012-01-01

    The Phase-Field Crystal (PFC) model represents the atomic density as a continuous function, whose spatial distribution evolves at diffusional, rather than vibrational time scales. PFC provides a tool to study defect interactions at the atomistic level but over longer time scales than in molecular dynamics (MD). We examine the behavior of the PFC model with the goal of relating the PFC parameters to physical parameters of real systems, derived from MD simulations. For this purpose we model the phenomenon of the shrinking of a spherical grain situated in a matrix. By comparing the rate of shrinking of the central grain using MD and PFC we obtain a relationship between PFC and MD time scales for processes driven by grain boundary diffusion. The morphological changes in the central grain including grain shape and grain rotation are also examined in order to assess the accuracy of the PFC in capturing the evolution path predicted by MD.

  14. Phase-field study of crystal growth in three-dimensional capillaries: Effects of crystalline anisotropy.

    PubMed

    Debierre, Jean-Marc; Guérin, Rahma; Kassner, Klaus

    2016-07-01

    Phase-field simulations are performed to explore the thermal solidification of a pure melt in three-dimensional capillaries. Motivated by our previous work for isotropic or slightly anisotropic materials, we focus here on the more general case of anisotropic materials. Different channel cross sections are compared (square, hexagonal, circular) to reveal the influence of geometry and the effects of a competition between the crystal and the channel symmetries. In particular, a compass effect toward growth directions favored by the surface energy is identified. At given undercooling and anisotropy, the simulations generally show the coexistence of several growth modes. The relative stability of these growth modes is tested by submitting them to a strong spatiotemporal noise for a short time, which reveals a subtle hierarchy between them. Similarities and differences with experimental growth modes in confined geometry are discussed qualitatively. PMID:27575207

  15. Phase-field study of crystal growth in three-dimensional capillaries: Effects of crystalline anisotropy

    NASA Astrophysics Data System (ADS)

    Debierre, Jean-Marc; Guérin, Rahma; Kassner, Klaus

    2016-07-01

    Phase-field simulations are performed to explore the thermal solidification of a pure melt in three-dimensional capillaries. Motivated by our previous work for isotropic or slightly anisotropic materials, we focus here on the more general case of anisotropic materials. Different channel cross sections are compared (square, hexagonal, circular) to reveal the influence of geometry and the effects of a competition between the crystal and the channel symmetries. In particular, a compass effect toward growth directions favored by the surface energy is identified. At given undercooling and anisotropy, the simulations generally show the coexistence of several growth modes. The relative stability of these growth modes is tested by submitting them to a strong spatiotemporal noise for a short time, which reveals a subtle hierarchy between them. Similarities and differences with experimental growth modes in confined geometry are discussed qualitatively.

  16. Liquid Crystal Switching Response by Localized Surface Plasmon Induced Electric Fields

    NASA Astrophysics Data System (ADS)

    Nuno, Zachary; Hirst, Linda; Ghosh, Sayantani

    2013-03-01

    We investigate the effect of electric fields induced by localized surface plasmons (LSPs) from gold nanoparticles (AuNPs) on the director of a nematic liquid crystal (LC). We deposit LC thin films on a self-assembled AuNP layer and excite the LSPs in the AuNPs using 530 nm excitation light. Using polarized optical microscopy we follow the birefringence of the LC film as the excitation is turned on and off and observe the homeotropic alignment of the LC change to planar. This realignment response is observed to be dependent on the excitation wavelength, excitation power, and temperature; occurring only within 1 degree Celsius of the LC phase transition from nematic to isotropic. This work was funded by UC Merced GRC Summer Fellowship.

  17. Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

    SciTech Connect

    Hossain, M. A.; Hu, Z.; Haverkort, M. W.; Burnus, T.; Chang, C. F.; Klein, S.; Denlinger, J. D.; Lin, H.-J.; Chen, C. T.; Mathieu, R.; Kaneko, Y.; Tokura, Y.; Satow, S.; Yoshida, Y.; Takagi, H.; Tanaka, A.; Elfimov, I. S.; Sawatzky, G. A.; Tjeng, L. H.; Damascelli, A.

    2008-01-15

    Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.

  18. A binary phase field crystal study for liquid phase heteroepitaxial growth

    NASA Astrophysics Data System (ADS)

    Lu, Yanli; Peng, Yingying; Chen, Zheng

    2016-09-01

    The liquid phase heteroepitaxial growth on predefined crystalline substrate is studied with binary phase field crystal (PFC) model. The purpose of this paper focuses on changes of the morphology of epitaxial films, influences of substrate vicinal angles on epitaxial growth, characteristics of islands growth on both sides of the substrate as well. It is found that the morphology of epitaxial films undergoes the following transitions: layer-by-layer growth, islands formation, mismatch dislocations nucleation and climb towards the film-substrate interface. Meanwhile, the density of steps and islands has obviously direct ratio relations with the vicinal angles. Also, preferential regions are found when islands grow on both sides of the substrate. For thinner substrate, the arrangement of islands is more orderly and the appearance of preferential growth is more obvious than that of thicker substrate. Also, the existing of preferential regions is much more valid for small substrate vicinal angles in contrast for big substrate vicinal angles.

  19. Inductive crystal field control in layered metal oxides with correlated electrons

    SciTech Connect

    Balachandran, P. V.; Cammarata, A.; Rondinelli, J. M.; Nelson-Cheeseman, B. B.; Bhattacharya, A.

    2014-07-01

    We show that the NiO{sub 6} crystal field energies can be tailored indirectly via heterovalent A cation ordering in layered (La,A)NiO{sub 4} Ruddlesden–Popper (RP) oxides, where A = Sr, Ca, or Ba, using density functional calculations. We leverage as a driving force the electrostatic interactions between charged [LaO]{sup 1+} and neutral [AO]{sup 0} planes to inductively tune the Ni–O bond distortions, without intentional doping or epitaxial strain, altering the correlated d-orbital energies. We use this strategy to design cation ordered LaCaNiO{sub 4} and LaBaNiO{sub 4} with distortions favoring enhanced Ni e{sub g} orbital polarization, and find local electronic structure signatures analogous to those in RP La-cuprates, i.e., parent phases of the high-temperature superconducting oxides.

  20. An adaptive time-stepping strategy for solving the phase field crystal model

    SciTech Connect

    Zhang, Zhengru; Ma, Yuan; Qiao, Zhonghua

    2013-09-15

    In this work, we will propose an adaptive time step method for simulating the dynamics of the phase field crystal (PFC) model. The numerical simulation of the PFC model needs long time to reach steady state, and then large time-stepping method is necessary. Unconditionally energy stable schemes are used to solve the PFC model. The time steps are adaptively determined based on the time derivative of the corresponding energy. It is found that the use of the proposed time step adaptivity cannot only resolve the steady state solution, but also the dynamical development of the solution efficiently and accurately. The numerical experiments demonstrate that the CPU time is significantly saved for long time simulations.

  1. Crystal-field levels of terbium in pyrochlore compounds R2Ti2O7

    NASA Astrophysics Data System (ADS)

    Klimin, S. A.

    2015-06-01

    Temperature-dependent infrared spectroscopic study of Tb3+ f-f transitions has been performed for pyrochlore compounds Tb2Ti2O7, (Y0.99Tb0.01)2Ti2O7, and (Gd0.9Tb0.1)2Ti2O7. The peculiarities of terbium absorption were explained taking into account the presence of inversion symmetry at the rare-earth site. The energies of crystal-field levels for the ground Tb3+ multiplet were obtained. Spectral anomaly in Tb2Ti2O7 observed in the temperature range T<20 K was assigned to a strong ion-ion interaction between rare-earth ions, which leads to collective CF excitations in the terbium subsystem.

  2. Low Field Electronic Behavior and Contact Impedance of Organic Single Crystal Transistors

    NASA Astrophysics Data System (ADS)

    Bittle, Emily; Basham, James; Jackson, Thomas; Jurchescu, Oana; Gundlach, David

    2015-03-01

    Organic electronic devices are attractive for a range of existing and emerging electronic applications. Most technological demonstrations of organic transistors rely on their large signal response for pixel control or logic. However, considerable application space requires analog circuits, e.g. distributed signal conditioning in sensor arrays. Charge transport and trapping mechanisms differ significantly in organic as compared to inorganic transistors, and as a result commonly used analogies to inorganic band transport theory can break down in response to small signal stimulus and at high frequencies required in some analog circuit applications. Therefore, a detailed investigation of organic transistor behavior at small signals is needed and is critical to developing design models for analog circuit applications. In this study, we look at the small signal AC impedance of small molecule, single crystal transistors to investigate ``ideal'' low field, high frequency electronic behavior. Using a transmission line model to fit the transistor channel coupled with a parallel resistor-capacitor model of the contact impedance, we are able to observe the behavior of the transistor channel and contacts separately at low field and high frequency. We determine the low field mobility of the device independent of contact resistance and show that rapidly changing contact resistance dominates the current flow at low gate voltage in DC current-voltage measurements.

  3. Field Control of the Surface Electroclinic Effect in Liquid Crystal Displays II

    NASA Astrophysics Data System (ADS)

    Zappitelli, Kara; Hipolite, Dana; Saunders, Karl

    2012-11-01

    As previously introduced in the presentation by Dana Hipolite, chiral, smectic liquid crystal molecules aligned in layers can be controlled by the application of an electric field, which has a variety of implications for the quality of LCD displays. Both the bulk electroclinic effect (BECE) and surface electroclinic effect (SECE) impact the angle at which the molecules tilt with respect to the director in different areas of the cell. Certain LC's exhibit a continuous Sm-A* to Sm-C* transition, where the angle of the surface and bulk molecules change continually with the electric field. Other LC's exhibit first order transitions where we see jumps in the tilt at different values of the applied electric field for the bulk and surface molecules respectively. The difference in angle of the bulk and surface molecules in both of these situations causes discrepancies in the layer spacing within the LC cell. These discrepancies lead to frustrations within the cell, which can be quantified by the strain (?). These frustrations can be relieved in multiple ways, however the method of relief may lead to negative impacts on the alignment quality of the display itself.

  4. The onset of layer undulations in smectic A liquid crystals due to a strong magnetic field

    NASA Astrophysics Data System (ADS)

    Contreras, A.; Garcia-Azpeitia, C.; García-Cervera, C. J.; Joo, S.

    2016-08-01

    We investigate the effect of a strong magnetic field on a three dimensional smectic A liquid crystal. We identify a critical field above which the uniform layered state loses stability; this is associated to the onset of layer undulations. In a previous work García-Cervera and Joo (2012 Arch. Ration. Mech. Anal. 203 1–43), García-Cervera and Joo considered the two dimensional case and analyzed the transition to the undulated state via a simple bifurcation. In dimension n  =  3 the situation is more delicate because the first eigenvalue of the corresponding linearized problem is not simple. We overcome the difficulties inherent to this higher dimensional setting by identifying the irreducible representations for natural actions on the functional that take into account the invariances of the problem thus allowing for reducing the bifurcation analysis to a subspace with symmetries. We are able to describe at least two bifurcation branches, highlighting the richer landscape of energy critical states in the three dimensional setting. Finally, we analyze a reduced two dimensional problem, assuming the magnetic field is very strong, and are able to relate this to a model in micromagnetics studied in Alouges et al (2002 ESAIM Control Optim. Calc. Var. 8 31–68), from where we deduce the periodicity property of minimizers.

  5. Spectroscopy of Charge Carriers and Traps in Field-Doped Single Crystal Organic Semiconductors

    SciTech Connect

    Zhu, Xiaoyang

    2014-12-10

    The proposed research aims to achieve quantitative, molecular level understanding of charge carriers and traps in field-doped crystalline organic semiconductors via in situ linear and nonlinear optical spectroscopy, in conjunction with transport measurements and molecular/crystal engineering. Organic semiconductors are emerging as viable materials for low-cost electronics and optoelectronics, such as organic photovoltaics (OPV), organic field effect transistors (OFETs), and organic light emitting diodes (OLEDs). Despite extensive studies spanning many decades, a clear understanding of the nature of charge carriers in organic semiconductors is still lacking. It is generally appreciated that polaron formation and charge carrier trapping are two hallmarks associated with electrical transport in organic semiconductors; the former results from the low dielectric constants and weak intermolecular electronic overlap while the latter can be attributed to the prevalence of structural disorder. These properties have lead to the common observation of low charge carrier mobilities, e.g., in the range of 10-5 - 10-3 cm2/Vs, particularly at low carrier concentrations. However, there is also growing evidence that charge carrier mobility approaching those of inorganic semiconductors and metals can exist in some crystalline organic semiconductors, such as pentacene, tetracene and rubrene. A particularly striking example is single crystal rubrene (Figure 1), in which hole mobilities well above 10 cm2/Vs have been observed in OFETs operating at room temperature. Temperature dependent transport and spectroscopic measurements both revealed evidence of free carriers in rubrene. Outstanding questions are: what are the structural features and physical properties that make rubrene so unique? How do we establish fundamental design principles for the development of other organic semiconductors of high mobility? These questions are critically important but not comprehensive, as the nature of

  6. Transferability of coarse-grained force field for nCB liquid crystal systems.

    PubMed

    Zhang, Jianguo; Guo, Hongxia

    2014-05-01

    In this paper, the transferability of the coarse-grained (CG) force field originally developed for the liquid crystal (LC) molecule 5CB ( Zhang et al. J. Phys. Chem. B 2012 , 116 , 2075 - 2089 ) was investigated by its homologues 6CB and 8CB molecules. Note that, to construct the 5CB CG force field, we combined the structure-based and thermodynamic quantities-based methods and at the same time attempted to use several fragment molecular systems to derive the CG nonbonded interaction parameters. The resultant 5CB CG force field exhibits a good transferability to some extent. For example, not only the experimental densities, the local packing of atom groups, and the antiparallel arrangements of nearest neighboring molecules, but also the unique LC mesophases as well as the nematic-isotropic phase transition temperatures of 6CB and 8CB were reproduced. Meanwhile, the limitations of this 5CB CG force field were also observed, such as the phase transition from nematic to smectic was postponed to the lower temperature and the resulting smectic phase structure is single-layer-like instead of partially interdigitated bilayer-like as observed in underlying atomistic model. Apparently, more attention should be paid when applying a CG force field to the state point which is quite different from which the force field is explicitly parametrized for. The origin of the above limitations can be potentially traced back to the inherent simplifications and some approximations often adopted in the creation process of CG force field, for example, choosing symmetric CG potentials which do not explicitly include electrostatic interactions and are parametrized by reproducing the target properties of the specific nematic 5CB phase at 300 K and 1 atm, as well as using soft nonbonded potential and excluding torsion barriers. Moreover, although by construction this CG force field could inevitably incorporate both thermodynamic and local structural information on the nematic 5CB phase, the

  7. Investigation of quasi lateral-field-excitation on (yxl)-17° LiNbO3 single crystal.

    PubMed

    Ma, Tingfeng; Wang, Ji; Du, Jianke; Yuan, Lili; Qian, Zhenghua; Zhang, Zhitian; Zhang, Chao

    2014-04-01

    Quasi lateral-field-excitation (LFE) on LiNbO3 crystal is investigated both theoretically and experimentally. It is found that when the driving electric field direction is parallel to the crystallographic X-axis of the piezoelectric substrate, (yxl)-17° LiNbO3 LFE bulk acoustic wave devices work on quasi-LFE mode. The experimental results agreed with the theoretical prediction well. The results provide the cut of LiNbO3 crystal for quasi-LFE bulk acoustic wave devices, which is important for designing high performance LFE sensors on LiNbO3 substrates. PMID:24485749

  8. Linear magnetoresistance and zero-field anomalies in HfNiSn single crystals

    NASA Astrophysics Data System (ADS)

    Steinke, Lucia; Kistner-Morris, Jedediah J.; Deng, Haiming; Geschwind, Gayle; Aronson, Meigan C.

    The Half-Heusler compound HfNiSn is probably best known as a candidate material for thermoelectric applications, and studies of its properties have mainly focused on polycrystalline samples and thin films. However, magnetotransport studies of HfNiSn show unusual transport properties like linear magnetoresistance (LMR), where single-crystalline samples of HfNiSn exhibit unexpected LMR at very low fields. In this work, we optimized the solution growth of HfNiSn to obtain high-quality single crystals, where electrical transport measurements show that it is a compensated semimetal below ~ 200 K, where the Hall voltage is zero. At higher temperatures, we see a finite Hall contribution from activated excess carriers. In the semimetallic regime, we observe transport anomalies like resistive signals that strongly depend on contact configuration, and LMR below 5 K. Both low-field DC and low frequency AC magntization measurements show pronounced diamagnetic behavior and the onset of paramagnetism below 4 K. High-frequency diamagnetic screening may be attributed to a decreased skin depth with decreased resistance, but this scenario seems unlikely in HfNiSn since the measured resistance increases steeply at the lowest temperatures This research was supported by the Army Research Office.

  9. An endoscopic system adopting a liquid crystal lens with an electrically tunable depth-of-field.

    PubMed

    Chen, Hung-Shan; Lin, Yi-Hsin

    2013-07-29

    Conventional endoscopic systems consisting of several solid lenses suffer from a fixed and limited depth-of-field (DOF). For practical applications, conventional endoscopes mechanically change the distance between the solid lenses of a lens module in order to change the focusing plane and DOF to see clearly in a scene. In this paper, we demonstrate an electrically tunable endoscopic system adopting a liquid crystal lens. By means of tunable focusing properties of the LC lens as a positive lens and a negative lens, the object at different objective distances can be imaged to the image sensor clearly and the corresponding depth-of-field can also help to enlarge the total spatial depth perception in a scene. The optical mechanism is discussed. In the experiments, under adjustment of three discrete lens powers of the LC lens, the viewing range or total spatial depth perception of the endoscopic system is from 76.4 mm to 12.4 mm which is 2x improved compared to the conventional one without LC lens. We believe this study can be extended to the applications of industrial and medical endoscopes. PMID:23938679

  10. Large Upper Critical Field of Superconductivity in the Single Crystal U6Co

    NASA Astrophysics Data System (ADS)

    Aoki, Dai; Nakamura, Ai; Honda, Fuminori; Li, DeXin; Homma, Yoshiya

    2016-07-01

    We grew single crystals of U6Co by the self-flux method and measured the magnetic susceptibility, resistivity, and specific heat. The magnetic susceptibility shows very small anisotropy and weak temperature dependence, indicating small spin-susceptibility. Superconductivity was clearly observed in the resistivity, susceptibility, and specific heat at Tc ˜ 2.3 K. The upper critical field was remarkably large, 7.9 and 6.6 T for H || [001] and [110], respectively, in the tetragonal structure, indicating that the ellipsoidal Fermi surface is slightly suppressed along the [001] direction according to the effective mass model. The specific heat shows a large jump at Tc with ΔC/γTc = 1.58, and the field dependence of the specific heat at low temperatures shows an almost linear increase. These experimental results are well explained by the BCS model in the dirty limit condition. U6Co is most likely a conventional s-wave superconductor with a full superconducting gap.

  11. Growth, spectral property and crystal field analysis of Cr3+-doped Na2Mg5(MoO4)6 crystal

    NASA Astrophysics Data System (ADS)

    Zhang, Lizhen; Li, Linyun; Huang, Yisheng; Sun, Shijia; Lin, Zhoubin; Wang, Guofu

    2015-11-01

    This paper reports the growth and spectral properties of Cr3+:Na2Mg5(MoO4)6 crystals. The Na2Mg5(MoO4)6 crystal was grown from a flux of Na2Mo2O7 by the top seeded solution growth method. The absorption cross-sections are 0.692 × 10-19 cm2 at 507 nm and 1.151 × 10-19 cm2 at 736 nm, respectively. The emission cross-section is 1.62 × 10-19 cm2 at 914 nm with FWHM of 192 nm. Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, effective phonon energy ℏω and the Huang-Rhys factor S were calculated. The spectral properties of Cr3+:Na2Mg5(MoO4)6 crystal demonstrate a good potential for tunable laser material.

  12. Glass bead size and morphology characteristics in support of Crystal Mist field experiments

    SciTech Connect

    Einfeld, W.

    1995-03-01

    One of the tasks of the Lethality Group within US Army Space and Strategic Defense Command (USASSDC) is the development of a capability to simulate various missile intercept scenarios using computer codes. Currently under development within USASSDC and its various contractor organizations is a group of codes collected under a master code called PEGEM for Post Event Ground Effects Model. Among the various components of the code are modules which are used to predict atmospheric dispersion and transport of particles or droplets following release at the altitude specified in the missile intercept scenario. The atmospheric transport code takes into account various source term data from the intercept such as: initial cloud size; droplet or particle size distribution; and, total mass of agent released. An ongoing USASSDC experimental program termed Crystal Mist involved release of precision glass beads under various altitude and meteorological conditions to assist in validation and refinement of various codes that are components of PEGEM used to predict particle atmospheric transport and diffusion following a missile intercept. Here, soda-lime glass beads used in the Crystal Mist series of atmospheric transport and diffusion tests were characterized by scanning electron microscopy and automated image processing routines in order to fully define their size distributions and morphology. Four bead size classifications ranging from a median count diameter of 45 to 200 micrometers were found to be approximately spherical and to fall within the supplier`s sizing specifications. Log-normal functions fit to the measured size distributions resulted in geometric standard deviations ranging from 1.08 to 1.12, thereby fulfilling the field trial requirements for a relatively narrow bead size distribution.

  13. Effects of three-dimensional polymer networks in vertical alignment liquid crystal display controlled by in-plane field.

    PubMed

    Lim, Young Jin; Choi, Young Eun; Lee, Jun Hee; Lee, Gi-Dong; Komitov, Lachezar; Lee, Seung Hee

    2014-05-01

    Polymer network in vertical alignment liquid crystal cell driven by in-plane field (VA-IPS) is formed in three dimensions to achieve fast response time and to keep the liquid crystal alignment even when an external pressure is applied to the cell. The network formed by UV irradiation to vertically aligned liquid crystal cell with reactive mesogen does not disturb a dark state while exhibiting very fast decaying response time less than 2ms in all grey scales and almost zero pooling mura. The proposed device has a strong potential to be applicable to field sequential display owing to super-fast response time and flexible display owing to polymer network in bulk which supports a gap between two substrates. PMID:24921764

  14. Diffusion-Controlled Anisotropic Growth of Stable and Metastable Crystal Polymorphs in the Phase-Field Crystal Model

    NASA Astrophysics Data System (ADS)

    Tegze, G.; Gránásy, L.; Tóth, G. I.; Podmaniczky, F.; Jaatinen, A.; Ala-Nissila, T.; Pusztai, T.

    2009-07-01

    We use a simple density functional approach on a diffusional time scale, to address freezing to the body-centered cubic (bcc), hexagonal close-packed (hcp), and face-centered cubic (fcc) structures. We observe faceted equilibrium shapes and diffusion-controlled layerwise crystal growth consistent with two-dimensional nucleation. The predicted growth anisotropies are discussed in relation with results from experiment and atomistic simulations. We also demonstrate that varying the lattice constant of a simple cubic substrate, one can tune the epitaxially growing body-centered tetragonal structure between bcc and fcc, and observe a Mullins-Sekerka-Asaro-Tiller-Grinfeld-type instability.

  15. LABORATORY AND FIELD EVALUATION OF CRYSTALLIZED DOW 704 OIL ON THE PERFORMANCE OF THE PM2.5 WINS FRACTIONATOR

    EPA Science Inventory

    Subsequent to the PM2.5 FRM's 1997 promulgation, technicians at the CT Dept. of Env. Protection observed that the DOW 704 diffusion oil used in the method's WINS fractionator would occasionally crystallize during field use - particularly under wintertime conditions. While the f...

  16. Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

    NASA Astrophysics Data System (ADS)

    Aksenova, E. V.; Karetnikov, A. A.; Karetnikov, N. A.; Kovshik, A. P.; Ryumtsev, E. I.; Sakhatskii, A. S.; Svanidze, A. V.

    2016-05-01

    The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Fréedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.

  17. On the influence of a high-frequency electric field on the exchange interaction in magnetic crystals

    NASA Astrophysics Data System (ADS)

    Gladkov, S. O.

    1992-03-01

    It is shown that the presence of a high-frequency electric field E( t) strongly influences the exchange interaction magnitude Jex. It is noted that due to this circumstance the liquid crystallization temperature or the phase transition temperature in magnetic substances can be changed, by varying the frequency ω and the amplitude of the external electric field E0. The dependence Jex(ω, E0) is determined.

  18. Magma Differentiation and Storage Inferred from Crystal Textures at Harrat Rahat Volcanic Field, Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Witter, M. R.; Mahood, G. A.; Stelten, M. E.; Downs, D. T.; Zahran, H. M.

    2015-12-01

    We present results of a petrographic study of Harrat Rahat volcanic field in western Saudi Arabia as part of a collaborative project between the U.S.G.S. and the Saudi Geological Survey. Lavas range in composition from alkali basalt to trachyphonolite. Basalts have <2-10 vol.% phenocrysts of euhedral olivine and plagioclase (± minor clinopyroxene). In intermediate lavas, phenocrysts (<5 vol.%) of olivine and plagioclase are resorbed, and plagioclase also exhibits sieve textures and strong zoning, indicative of complex magmatic histories. Trachyphonolite lavas have 0-35 vol.% large phenocrysts of anorthoclase and trace fayalitic olivine but are characterized by a size distribution of crystals that is seriate in hand specimen, so that most exceeded 45% crystals at the time of eruption. Some contain groundmass alkali amphibole. Crystal size distributions (CSD) of crystal-rich trachyphonolites produce simple linear trends (see below), which are interpreted as signifying that all the crystals are related through a common nucleation and growth history, at more or less constant pressure. Linear CSDs indicate no loss of small crystals due to reheating of magmas by recharge, no gain of small crystals due to late-stage nucleation on ascent or degassing, and no addition of large phenocrysts by crystal accumulation or magma mixing. Experimental studies demonstrate that silica-undersaturated evolved magmas like those erupted at Harrat Rahat can form by fractionation of alkali basalts at crustal depths greater than ~25 km. The observed phenocryst assemblage in the trachyphonolites, however, forms at shallow depths, ~2-4 km, according to MELTS modeling. Coupled with CSD data, this suggests that deep extraction events yield crystal-poor trachyphonolite magmas that rise to the upper crust where they undergo crystallization. Extensive shallow crystallization of trachyphonolites may have triggered eruptions by causing vapor saturation, which lowers magma density via vesiculation and

  19. Dynamics of three-dimensional convection in microgravity crystal growth: g-jitter with steady magnetic fields

    NASA Astrophysics Data System (ADS)

    Yeckel, Andrew; Derby, Jeffrey J.

    2004-03-01

    We present results from three-dimensional simulations of the flow induced by transient acceleration (g-jitter) in microgravity crystal growth. Transient accelerations in both axial and transverse directions are considered for a simple prototype of a vertical Bridgman crystal growth system. We also consider the effects of applying a steady magnetic field in axial or transverse directions to suppress the flow. In most cases, application of a magnetic field suppresses flow oscillations, but for transverse jitter at intermediate frequencies, flow oscillations are increased. This counter-intuitive effect is a dynamic one, in which boundary layer formation under the influence of a magnetic field shortens the time scale of momentum transfer, allowing the flow to respond more quickly to the time variation of acceleration. The effect of the magnetic field on an enclosed flow with electrically insulating boundary conditions is to preferentially suppress the velocity component tangential to the magnetic field. The ability to filter a single velocity component by application of a specified magnetic field could be useful for simultaneously improving both axial and radial segregation in semiconductor crystal growth.

  20. Investigating the role of oriented nucleus in polymer shish-kebab crystal growth via phase-field method

    NASA Astrophysics Data System (ADS)

    Wang, Xiaodong; Ouyang, Jie; Su, Jin; Zhou, Wen

    2014-03-01

    The phase-field method has been developed to simulate the shish-kebab crystal growth in polymer crystallization by introducing the oriented nucleus. With the help of this developed phase-field model, the role of oriented nucleus in polymer shish-kebab crystal growth has been investigated. It appears that the growth mechanisms of shish-kebab crystal on a preformed oriented nucleus may be attributed to epitaxial growth and lattice match. First the oriented nucleus (early shish) further grows into stable shish entity through epitaxial growth, and then lattice match supplies the sites for kebabs and epitaxial lateral growth from these sites forms the kebabs. It also has been verified that kebabs can be grown on oriented nucleus in the total absence of any flow. Therefore, with regard to flow induced shish-kebab crystal, the oriented nucleus plays a major role in the growth of shish-kebab morphology and the flow mainly helps to generate the oriented nucleus. Besides, when the nucleus possesses a rod-like profile, the kebabs are generally parallel and equidistantly distributed, and the well-defined interval between adjacent kebabs is strongly influenced by the orientation angle of the rod-like nucleus. On the other hand, when the nucleus is slightly curved and presents a thread-like profile, the distribution of kebabs on the shish is no longer equidistant and the influence of orientation angle on the kebab density becomes weak.

  1. Influence of the three dimensionality of the HF electromagnetic field on resistivity variations in Si single crystals during FZ growth

    NASA Astrophysics Data System (ADS)

    Ratnieks, G.; Muiznieks, A.; Buligins, L.; Raming, G.; Mühlbauer, A.; Lüdge, A.; Riemann, H.

    2000-06-01

    Three-dimensional numerical modelling is carried out to analyse the floating zone crystal growth with the needle-eye technique used for the production of high-quality silicon single crystals with large diameters ( ⩾100 mm ). Since the pancake inductor has only one turn, the EM field and the distribution of heat sources and EM forces are only roughly axisymmetric. The non-symmetry together with crystal rotation reflects itself on the hydrodynamic, thermal and dopant concentration fields in the molten zone and causes variations of resistivity in the grown single crystal, which are known as the so-called rotational striations. The non-symmetric high-frequency electromagnetic field of the pancake inductor is calculated by boundary element method. The obtained non-symmetric power distribution on the free melt surface and the corresponding EM forces are used for the coupled calculation of the 3D steady-state hydrodynamic and temperature fields in the molten zone on a body fitted structured 3D grid by a commercial program package with control volume approach. The buoyancy, Marangoni and EM forces are considered. The afterwards calculated corresponding 3D dopant concentration field is used to derive the variations of resistivity in a longitudinal cut of the grown crystal. The results are compared with experimental measurements (photo-scanning method) and with results of 2D transient flow calculations. Rotational striations are found in both 3D-calculated and experimental resistivity distributions and show a qualitative agreement. A Fourier analysis for the resistivity variations is performed and the observed differences are explained by modelling limitations.

  2. Appearance of singularities of optical fields under torsion of crystals containing threefold symmetry axes.

    PubMed

    Skab, Ihor; Vasylkiv, Yurij; Zapeka, Bohdan; Savaryn, Viktoriya; Vlokh, Rostyslav

    2011-07-01

    We present an analysis of the effect of torsion stresses on the spatial distribution of optical birefringence in crystals of different point symmetry groups. The symmetry requirements needed so that the optical beam carries dislocations of the phase front are evaluated for the case when the crystals are twisted and the beam closely corresponds to a plane wave. It is shown that the torsion stresses can produce screw-edge, pure screw, or pure edge dislocations of the phase front in the crystals belonging to cubic and trigonal systems. The conditions for appearance of canonical and noncanonical vortices in the conditions of crystal torsion are analyzed. PMID:21734730

  3. Heteromorphism and crystallization paths of katungites, Navajo volcanic field, Arizona, USA

    SciTech Connect

    Laughlin, A.W.; Charles, R.W.; Aldrich, M.J. Jr.

    1986-01-01

    A swarm of thin, isochemical but heteromorphic dikes crops out in the valley of Hasbidito Creek in NE Arizona. The swarm is part of the dominantly potassic, mid-Tertiary Navajo volcanic field of the Colorado Plateau. Whole-rock chemical analyses of five samples from four of the dikes indicate that they are chemically identical to the katungites of Uganda. These dikes show the characteristic seriate-porphyritic texture of lamprophyres. Samples of an olivine-melilitite dike from the same swarm lack this texture and the chemical analysis, while similar to those of the other dikes, shows effects from the incorporation of xenocrystic olivine. Over 20 mineral phases have been identified in the Arizona samples and as many as 18 phases may occur in a single sample. The major phases are phlogopite, olivine, perovskite, opaque oxides, +- melilite and +- clinopyroxene. Based upon the modal mineralogies and textures of ten dike samples, we recognize five general non-equilibrium assemblages. Comparison of these assemblages with recent experimental results shows that they represent various combinations of complete and incomplete reactions. Reaction relations were determined by entering melt and phase compositions into the computer program GENMIX to obtain balanced reactions. By combining petrographic observations with mineral chemical data, balanced reactions from GENMIX, and the recently determined phase diagrams we are able to trace crystallization paths for the katungite magma.

  4. Minimizing eyestrain on a liquid crystal display considering gaze direction and visual field of view

    NASA Astrophysics Data System (ADS)

    Lee, Won Oh; Heo, Hwan; Lee, Eui Chul; Park, Kang Ryoung

    2013-07-01

    Recently, it has become necessary to evaluate the performance of display devices in terms of human factors. To meet this requirement, several studies have been conducted to measure the eyestrain of users watching display devices. However, these studies were limited in that they did not consider precise human visual information. Therefore, a new eyestrain measurement method is proposed that uses a liquid crystal display (LCD) to measure a user's gaze direction and visual field of view. Our study is different in the following four ways. First, a user's gaze position is estimated using an eyeglass-type eye-image capturing device. Second, we propose a new eye foveation model based on a wavelet transform, considering the gaze position and the gaze detection error of a user. Third, three video adjustment factors-variance of hue (VH), edge, and motion information-are extracted from the displayed images in which the eye foveation models are applied. Fourth, the relationship between eyestrain and three video adjustment factors is investigated. Experimental results show that the decrement of the VH value in a display induces a decrease in eyestrain. In addition, increased edge and motion components induce a reduction in eyestrain.

  5. Velocity profiles of electric-field-induced backflows in liquid crystals confined between parallel plates

    NASA Astrophysics Data System (ADS)

    Tsuji, Tomohiro; Chono, Shigeomi; Matsumi, Takanori

    2015-02-01

    For the purpose of developing liquid crystalline microactuators, we visualize backflows induced between two parallel plates for various parameters such as the twist angle, cell gap, applied voltage, and molecular configuration mode. We use 4-cyano-4'-pentyl biphenyl, a typical low-molar-mass nematic liquid crystal. By increasing the twist angle from 0° to 180°, the velocity component parallel to the anchoring direction of the lower plate changes from an S-shaped profile to a distorted S-shaped profile before finally becoming unidirectional. In contrast, the velocity component perpendicular to the anchoring direction evolves from a flat profile at 0° into an S-shaped profile at 180°. Because both an increase in the applied voltage and a decrease in the cell gap increase the electric field intensity, the backflow becomes large. The hybrid molecular configuration mode induces a larger backflow than that for the planar aligned mode. The backflow develops in two stages: an early stage with a microsecond time scale and a later stage with a millisecond time scale. The numerical predictions are in qualitative agreement with the measurements, but not quantitative agreement because our computation ignores the plate edge effect of surface tension.

  6. An energy stable, hexagonal finite difference scheme for the 2D phase field crystal amplitude equations

    NASA Astrophysics Data System (ADS)

    Guan, Zhen; Heinonen, Vili; Lowengrub, John; Wang, Cheng; Wise, Steven M.

    2016-09-01

    In this paper we construct an energy stable finite difference scheme for the amplitude expansion equations for the two-dimensional phase field crystal (PFC) model. The equations are formulated in a periodic hexagonal domain with respect to the reciprocal lattice vectors to achieve a provably unconditionally energy stable and solvable scheme. To our knowledge, this is the first such energy stable scheme for the PFC amplitude equations. The convexity of each part in the amplitude equations is analyzed, in both the semi-discrete and fully-discrete cases. Energy stability is based on a careful convexity analysis for the energy (in both the spatially continuous and discrete cases). As a result, unique solvability and unconditional energy stability are available for the resulting scheme. Moreover, we show that the scheme is point-wise stable for any time and space step sizes. An efficient multigrid solver is devised to solve the scheme, and a few numerical experiments are presented, including grain rotation and shrinkage and grain growth studies, as examples of the strength and robustness of the proposed scheme and solver.

  7. Incorporating physically-based microstructures in materials modeling: Bridging phase field and crystal plasticity frameworks

    NASA Astrophysics Data System (ADS)

    Lim, Hojun; Abdeljawad, Fadi; Owen, Steven J.; Hanks, Byron W.; Foulk, James W.; Battaile, Corbett C.

    2016-05-01

    The mechanical properties of materials systems are highly influenced by various features at the microstructural level. The ability to capture these heterogeneities and incorporate them into continuum-scale frameworks of the deformation behavior is considered a key step in the development of complex non-local models of failure. In this study, we present a modeling framework that incorporates physically-based realizations of polycrystalline aggregates from a phase field (PF) model into a crystal plasticity finite element (CP-FE) framework. Simulated annealing via the PF model yields ensembles of materials microstructures with various grain sizes and shapes. With the aid of a novel FE meshing technique, FE discretizations of these microstructures are generated, where several key features, such as conformity to interfaces, and triple junction angles, are preserved. The discretizations are then used in the CP-FE framework to simulate the mechanical response of polycrystalline α-iron. It is shown that the conformal discretization across interfaces reduces artificial stress localization commonly observed in non-conformal FE discretizations. The work presented herein is a first step towards incorporating physically-based microstructures in lieu of the overly simplified representations that are commonly used. In broader terms, the proposed framework provides future avenues to explore bridging models of materials processes, e.g. additive manufacturing and microstructure evolution of multi-phase multi-component systems, into continuum-scale frameworks of the mechanical properties.

  8. Crystal electric field excitations in quasicrystal approximant TbCd6 studied by inelastic neutron scattering

    NASA Astrophysics Data System (ADS)

    Das, Pinaki; Flint, R.; Kong, T.; Canfield, P. C.; Kreyssig, A.; Goldman, A. I.; de Boissieu, M.; Lory, P.-F.; Beutier, G.; Hiroto, T.

    All of the known quasicrystals with local moments exhibit frustration and spin glass-like behavior at low temperature. The onset of the spin freezing temperature is believed to be affected by the crystal electric field (CEF) splitting of the local moments. The quasicrystal approximant TbCd6 and its related icosahedral quasicrystal phase, i-Tb-Cd, form a set of model systems to explore how magnetism evolves from a conventional lattice (approximant phase) to an aperiodic quasicrystal. Though TbCd6 shows long-range antiferromagnetic ordering (TN = 24 K), only spin glass like behavior is observed in i-Tb-Cd with a spin freezing temperature of TF = 6 K. To investigate further, we have performed inelastic neutron scattering measurements on powder samples of TbCd6 and observed two distinct CEF excitations at low energies which points to a high degeneracy of the CEF levels related to the Tb surrounding with almost icosahedral symmetry. Work at Ames Laboratory was supported by the DOE, BES, Division of Materials Sciences & Engineering, under Contract No. DE-AC02-07CH11358. This research used resources at Institut Laue-Langevin, France.

  9. Incorporating physically-based microstructures in materials modeling: Bridging phase field and crystal plasticity frameworks

    DOE PAGESBeta

    Lim, Hojun; Abdeljawad, Fadi; Owen, Steven J.; Hanks, Byron W.; Foulk, James W.; Battaile, Corbett C.

    2016-04-25

    Here, the mechanical properties of materials systems are highly influenced by various features at the microstructural level. The ability to capture these heterogeneities and incorporate them into continuum-scale frameworks of the deformation behavior is considered a key step in the development of complex non-local models of failure. In this study, we present a modeling framework that incorporates physically-based realizations of polycrystalline aggregates from a phase field (PF) model into a crystal plasticity finite element (CP-FE) framework. Simulated annealing via the PF model yields ensembles of materials microstructures with various grain sizes and shapes. With the aid of a novel FEmore » meshing technique, FE discretizations of these microstructures are generated, where several key features, such as conformity to interfaces, and triple junction angles, are preserved. The discretizations are then used in the CP-FE framework to simulate the mechanical response of polycrystalline α-iron. It is shown that the conformal discretization across interfaces reduces artificial stress localization commonly observed in non-conformal FE discretizations. The work presented herein is a first step towards incorporating physically-based microstructures in lieu of the overly simplified representations that are commonly used. In broader terms, the proposed framework provides future avenues to explore bridging models of materials processes, e.g. additive manufacturing and microstructure evolution of multi-phase multi-component systems, into continuum-scale frameworks of the mechanical properties.« less

  10. Phase field crystal modelling of the order-to-disordered atomistic structure transition of metallic glasses

    NASA Astrophysics Data System (ADS)

    Zhang, W.; Mi, J.

    2016-03-01

    Bulk metallic glass composites are a new class of metallic alloy systems that have very high tensile strength, ductility and fracture toughness. This unique combination of mechanical properties is largely determined by the presence of crystalline phases uniformly distributed within the glassy matrix. However, there have been very limited reports on how the crystalline phases are nucleated in the super-cooled liquid and their growth dynamics, especially lack of information on the order-to-disordered atomistic structure transition across the crystalline-amorphous interface. In this paper, we use phase field crystal (PFC) method to study the nucleation and growth of the crystalline phases and the glass formation of the super cooled liquid of a binary alloy. The study is focused on understanding the order-to-disordered transition of atomistic configuration across the interface between the crystalline phases and amorphous matrix of different chemical compositions at different thermal conditions. The capability of using PFC to simulate the order-to-disorder atomistic transition in the bulk material or across the interface is discussed in details.

  11. On a phase field approach for martensitic transformations in a crystal plastic material at a loaded surface

    NASA Astrophysics Data System (ADS)

    Schmitt, Regina; Kuhn, Charlotte; Müller, Ralf

    2015-06-01

    A continuum phase field model for martensitic transformations is introduced, including crystal plasticity with different slip systems for the different phases. In a 2D setting, the transformation-induced eigenstrain is taken into account for two martensitic orientation variants. With aid of the model, the phase transition and its dependence on the volume change, crystal plastic material behavior, and the inheritance of plastic deformations from austenite to martensite are studied in detail. The numerical setup is motivated by the process of cryogenic turning. The resulting microstructure qualitatively coincides with an experimentally obtained martensite structure. For the numerical calculations, finite elements together with global and local implicit time integration scheme are employed.

  12. Terahertz field enhancement via coherent superposition of the pulse sequences after a single optical-rectification crystal

    SciTech Connect

    Sajadi, Mohsen Wolf, Martin; Kampfrath, Tobias

    2014-03-03

    Terahertz electromagnetic pulses are frequently generated by optical rectification of femtosecond laser pulses. In many cases, the efficiency of this process is known to saturate with increasing intensity of the generation beam because of two-photon absorption. Here, we demonstrate two routes to reduce this effect in ZnTe(110) crystals and enhance efficiency, namely, by (i) recycling the generation pulses and by (ii) splitting each generation pulse into two pulses before pumping the crystal. In both methods, the second pulse arrives ∼1 ns after the first one, sufficiently long for optically generated carriers to relax. Enhancement is achieved by coherently superimposing the two resulting terahertz fields.

  13. Statistics of Dislocation Slip Avalanches in Nanosized Single Crystals Show Tuned Critical Behavior Predicted by a Simple Mean Field Model

    NASA Astrophysics Data System (ADS)

    Friedman, Nir; Jennings, Andrew T.; Tsekenis, Georgios; Kim, Ju-Young; Tao, Molei; Uhl, Jonathan T.; Greer, Julia R.; Dahmen, Karin A.

    2012-08-01

    We show that slowly sheared metallic nanocrystals deform via discrete strain bursts (slips), whose size distributions follow power laws with stress-dependent cutoffs. We show for the first time that plasticity reflects tuned criticality, by collapsing the stress-dependent slip-size distributions onto a predicted scaling function. Both power-law exponents and scaling function agree with mean-field theory predictions. Our study of 7 materials and 2 crystal structures, at various deformation rates, stresses, and crystal sizes down to 75 nm, attests to the universal characteristics of plasticity.

  14. Ground-state zero-field splitting of Mn 2+ ions in ZnO and CdSe crystals

    NASA Astrophysics Data System (ADS)

    Kuang, Xiao-Yu

    1996-02-01

    ZnO and CdSe crystals have similar hexagonal wurtzite structures with a contraction along the c-axis of the crystal, but contrary electronic fine structures for ZnO:Mn 2+ ( D < 0) and CdSe:Mn 2+ ( D > 0) have been found in EPR experiments. We demonstrate that the ground-state splitting in ZnO:Mn 2+ is due to a trigonal ligand field, whereas the main physical mechanism of the splitting in CdSe:Mn 2+ can be attributed to the combined effect of a slight trigonal distortion and a covalence spin-orbit coupling interaction.

  15. Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

    NASA Astrophysics Data System (ADS)

    Choi, Tae-Hoon; Oh, Seung-Won; Park, Young-Jin; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-06-01

    We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure.

  16. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2

    PubMed Central

    Haravifard, S.; Graf, D.; Feiguin, A. E.; Batista, C. D.; Lang, J. C.; Silevitch, D. M.; Srajer, G.; Gaulin, B. D.; Dabkowska, H. A.; Rosenbaum, T. F.

    2016-01-01

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems. PMID:27320787

  17. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2.

    PubMed

    Haravifard, S; Graf, D; Feiguin, A E; Batista, C D; Lang, J C; Silevitch, D M; Srajer, G; Gaulin, B D; Dabkowska, H A; Rosenbaum, T F

    2016-01-01

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems. PMID:27320787

  18. Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

    PubMed Central

    Choi, Tae-Hoon; Oh, Seung-Won; Park, Young-Jin; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-01-01

    We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure. PMID:27301651

  19. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2

    NASA Astrophysics Data System (ADS)

    Haravifard, S.; Graf, D.; Feiguin, A. E.; Batista, C. D.; Lang, J. C.; Silevitch, D. M.; Srajer, G.; Gaulin, B. D.; Dabkowska, H. A.; Rosenbaum, T. F.

    2016-06-01

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems.

  20. X-ray dynamical diffraction from single crystals with arbitrary shape and strain field: A universal approach to modeling

    NASA Astrophysics Data System (ADS)

    Yan, Hanfei; Li, Li

    2014-01-01

    The effects of dynamical diffraction in single crystals engender many unique diffraction phenomena that cannot be interpreted by the kinematical-diffraction theory, yet knowledge of them is vital to resolving a vast variety of scientific problems ranging from crystal optics to strain measurements in crystalline specimens. Although the fundamental dynamical-diffraction theory was established decades ago, modeling it remains a challenge in a general case wherein the crystal has complex boundaries and mixed diffraction geometries (Bragg or Laue). Here, we propose a universal approach for modeling x-ray dynamical diffraction from a single crystal with arbitrary shape and strain field that is based on the integral representation of the Takagi-Taupin equations. Using it, we can construct the solution iteratively via a converging series, independent of the diffraction geometry. Moreover, the integral equations offer additional insights into the diffraction physics that are not readily apparent in its differential counterparts. To demonstrate this approach, we studied the dynamical diffraction from a slab of single crystal with both Bragg and Laue diffraction excited on the entrance boundaries, a problem that is difficult to model by other methods. We also explored the mirage effect caused by the presence of a linear strain field and compared it to the Eikonal theory. Lastly, we derived a dynamical-diffraction equation correlating the structural properties of a particle to its far-field Bragg-diffraction pattern, shedding light on how dynamical diffraction affects these kinematical-diffraction-based inverse techniques for reconstructing the shape and the strain field.

  1. Modeling local structure using crystal field and spin Hamiltonian parameters: the tetragonal FeK3+-OI2- defect center in KTaO3 crystal

    NASA Astrophysics Data System (ADS)

    Gnutek, P.; Y Yang, Z.; Rudowicz, C.

    2009-11-01

    The local structure and the spin Hamiltonian (SH) parameters, including the zero-field-splitting (ZFS) parameters D and (a+2F/3), and the Zeeman g factors g_{\\parallel } and g_{\\perp } , are theoretically investigated for the FeK3+-OI2- center in KTaO3 crystal. The microscopic SH (MSH) parameters are modeled within the framework of the crystal field (CF) theory employing the CF analysis (CFA) package, which also incorporates the MSH modules. Our approach takes into account the spin-orbit interaction as well as the spin-spin and spin-other-orbit interactions omitted in previous studies. The superposition model (SPM) calculations are carried out to provide input CF parameters for the CFA/MSH package. The combined SPM-CFA/MSH approach is used to consider various structural models for the FeK3+-OI2- defect center in KTaO3. This modeling reveals that the off-center displacement of the Fe3+ ions, Δ1(Fe3+), combined with an inward relaxation of the nearest oxygen ligands, Δ2(O2-), and the existence of the interstitial oxygen OI2- give rise to a strong tetragonal crystal field. This finding may explain the large ZFS experimentally observed for the FeK3+-OI2- center in KTaO3. Matching the theoretical MSH predictions with the available structural data as well as electron magnetic resonance (EMR) and optical spectroscopy data enables predicting reasonable ranges of values of Δ1(Fe3+) and Δ2(O2-) as well as the possible location of OI2- ligands around Fe3+ ions in KTaO3. The defect structure model obtained using the SPM-CFA/MSH approach reproduces very well the ranges of the experimental SH parameters D, g_{\\parallel } and g_{\\perp } and importantly yields not only the correct magnitude of D but also the sign, unlike previous studies. More reliable predictions may be achieved when experimental data on (a+2F/3) and/or crystal field energy levels become available. Comparison of our results with those arising from alternative models existing in the literature indicates

  2. Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field.

    PubMed

    Satoh, Katsuhiko

    2006-04-14

    The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field. PMID:16626239

  3. Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field

    NASA Astrophysics Data System (ADS)

    Satoh, Katsuhiko

    2006-04-01

    The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter ⟨P2⟩ for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the ⟨P2⟩ is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The ⟨P2⟩ dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field.

  4. Effect of an electric field on the magnetization of a SmFe3(BO3)4 single crystal

    NASA Astrophysics Data System (ADS)

    Freidman, A. L.; Balaev, A. D.; Dubrovskii, A. A.; Eremin, E. V.; Shaikhutdinov, K. A.; Temerov, V. L.; Gudim, I. A.

    2015-07-01

    A change in the magnetization of a SmFe3(BO3)4 single crystal in response to an applied alternating electric field has been experimentally observed for the first time. The measurements have demonstrated that the magnetization oscillates not only at a frequency of the applied electric field but also at twice the frequency. The dependences of the magnetoelectric effect on the magnetic and electric fields and temperature have been measured. It has been assumed that the existence of the second harmonic of the magnetoelectric effect is due to the electrostriction.

  5. Magnetic anisotropy and crystalline electric field effects in RRh{sub 4}B{sub 4} single crystals.

    SciTech Connect

    Zhou, H.; Lambert, S. E.; Maple, M. B.; Dunlap, B. D.; Materials Science Division; Univ. of California at San Diego

    2009-08-01

    Research on polycrystalline RRh{sub 4}B{sub 4} samples has shown that crystalline electric field (CEF) effects play an important role in these compounds. The successful synthesis of single crystal samples of RRh{sub 4}B{sub 4} with R = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu has provided an opportunity to further investigate CEF effects in these materials. Magnetization and magnetic susceptibility measurements on the RRh{sub 4}B{sub 4} single crystals revealed strong magnetic anisotropy, and the experimental results could be described well by CEF calculations based on the parameters derived from an analysis of experimental data for ErRh{sub 4}B{sub 4} single crystals. The easy directions of magnetization of these compounds are consistent with the signs of the Stevens factor {alpha}J of the CEF Hamiltonian. A strong influence of magnetic anisotropy on superconductivity was also observed.

  6. Solidification behavior of high-density polyethylene (HDPE) during injection molding: Correlation between crystallization kinetics and thermal gradient field

    NASA Astrophysics Data System (ADS)

    Yang, Bin; Deng, Yan-Li; Li, Gui-Jing; Miao, Ji-Bin; Xia, Ru; Qian, Jia-Sheng; Chen, Peng; Liu, Jing-Wang

    2015-07-01

    This work mainly investigated the effect of thermal field on the crystallization kinetics of high-density polyethylene (HDPE) during injection molding (IM) process. The thickness X = 0.4 was found to be a crucial location heavily influenced by thermal conduction. The temperature decay tended to be stable, with limited variation of the crystallization rate when X > 0.4. It was observed that the crystallization rate was in good proportion to the cooling rate (ϕ). Our experimental finding showed that the consequence of relative crystallinity (χ) was in agreement with that of the secondary temperature difference (STD). This study is practically significant to the further investigation on the relationship among “processing-structure-property” of polymeric materials.

  7. Far-Field Patterns from Dye-Doped Planar-Aligned Nematic Liquid Crystals Under nanosecond Laser Irradiation

    SciTech Connect

    Lukishova, S.G.; Lepeshkin, N.; Boyd, R.W.; Marshall, K.L.

    2006-08-18

    High-definition patterns were observed under 10-Hz-pulse-repetition-rate, nanosecond laser irradiation of azodye-doped planar-nematic liquid crystal layers at incident intensities I ~ 5-10 MW/cm^2 in a single beam configuration and without any feedback involved. An incident polarization parallel to the nematic director was used. Under periodic pulsed laser irradiation, far-field beam patterns at the output of a dye-doped liquid crystal layer changed kaleidoscopically from rings and stripes to multiple hexagons. This pattern-formation regime had a buildup time of several seconds to minutes. We explain the observed effect by diffraction of the laser beam on light-induced micrometer-size inhomogeneities inside the liquid crystal layer with absorption and refraction properties different from the surrounding area. Possible mechanisms of the formation of the inhomogeneities are discussed.

  8. Thermal Characterization, Crystal Field Analysis and In-Band Pumped Laser Performance of Er Doped NaY(WO4)2 Disordered Laser Crystals

    PubMed Central

    Serrano, María Dolores; Cascales, Concepción; Han, Xiumei; Zaldo, Carlos; Jezowski, Andrzej; Stachowiak, Piotr; Ter-Gabrielyan, Nikolay; Fromzel, Viktor; Dubinskii, Mark

    2013-01-01

    Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er3+ levels up to 4G7/2 multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the 4I13/2↔4I15/2 laser related transition have been determined at 77 K. The 4I13/2 Er3+ lifetime (τ) was measured in the temperature range of 77–300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the 4I13/2 multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal 4I15/2 → 4I13/2 absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration. PMID:23555664

  9. Combining a rotating magnetic field and crystal rotation in the floating-zone process with a needle-eye induction coil

    NASA Astrophysics Data System (ADS)

    Ma, N.; Walker, J. S.; Lüdge, A.; Riemann, H.

    2001-08-01

    This paper presents numerical solutions for the steady, axisymmetric melt motions in a simplified model of the floating-zone process with a needle-eye induction coil. With only buoyant and thermocapillary convections, there are two meridional circulations, and the stronger, outer circulation involves an undesirable radially inward flow near the crystal-melt interface. Adding crystal rotation alone can only decrease the magnitudes of the meridional circulations. Adding a rotating magnetic field alone has the undesirable effect of increasing the magnitude and extent of the radially inward flow near the crystal-melt interface. Combining a rotating magnetic field in one azimuthal direction with crystal rotation in the opposite azimuthal direction overwhelms the buoyant and thermocapillary convections and produces the desired radially outward flow over the entire crystal-melt interface. The magnitude of this radially outward flow is easily controlled by changing either the angular velocity of the crystal rotation or the strength of the rotating magnetic field.

  10. Doping-driven orbital-selective Mott transition in multi-band Hubbard models with crystal field splitting

    NASA Astrophysics Data System (ADS)

    Yilin, Wang; Li, Huang; Liang, Du; Xi, Dai

    2016-03-01

    We have studied the doping-driven orbital-selective Mott transition in multi-band Hubbard models with equal band width in the presence of crystal field splitting. Crystal field splitting lifts one of the bands while leaving the others degenerate. We use single-site dynamical mean-field theory combined with continuous time quantum Monte Carlo impurity solver to calculate a phase diagram as a function of total electron filling N and crystal field splitting Δ. We find a large region of orbital-selective Mott phase in the phase diagram when the doping is large enough. Further analysis indicates that the large region of orbital-selective Mott phase is driven and stabilized by doping. Such models may account for the orbital-selective Mott transition in some doped realistic strongly correlated materials. Project supported by the National Natural Science Foundation of China (Grant No. 2011CBA00108) and the National Basic Research Program of China (Grant No. 2013CB921700).

  11. Effect of crystal-field splitting and interband hybridization on the metal-insulator transitions of strongly correlated systems

    NASA Astrophysics Data System (ADS)

    Poteryaev, Alexander I.; Ferrero, Michel; Georges, Antoine; Parcollet, Olivier

    2008-07-01

    We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts the orbital degeneracy as well as an interorbital hopping (interband hybridization). Both terms are relevant to the realistic description of correlated materials such as transition-metal oxides. The nature of the Mott metal-insulator transition is clarified and is found to depend on the magnitude of the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band metal is first found as the on-site repulsion is increased and is followed by a Mott transition for the remaining band, which follows the single-band (Brinkman-Rice) scenario well documented previously within dynamical mean-field theory. At small values of the crystal-field splitting, a direct transition from a two-band metal to a Mott insulator with partial orbital polarization is found, which takes place simultaneously for both orbitals. This transition is characterized by a vanishing of the quasiparticle weight for the majority orbital but has a first-order character for the minority orbital. It is pointed out that finite-temperature effects may easily turn the metallic regime into a bad metal close to the orbital polarization transition in the metallic phase.

  12. Numerical simulation of the flow fields around falling ice crystals with inclined orientation and the hydrodynamic torque

    NASA Astrophysics Data System (ADS)

    Hashino, Tempei; Chiruta, Mihai; Polzin, Dierk; Kubicek, Alexander; Wang, Pao K.

    2014-12-01

    The flow field and orientation of ice particles are fundamental information to understand cloud microphysical processes, optical phenomena, and electric-field induced orientation and to improve remote sensing of ice clouds. The purpose of this study is to investigate the flow fields and hydrodynamic torques of falling ice columns and hexagonal plates with their largest dimension inclined with respect to the airflow. The Reynolds numbers range from 2 to 70 for columns and 2 to 120 for plates. The flow fields are obtained by numerically solving the relevant Navier-Stokes equations under the assumption of air incompressibility. It was found that for the intermediate Reynolds number the streamlines around the inclined crystals exhibit less spiral rotation behind them than those around the stable posture. The vorticity magnitude was larger in the upstream side and broader in the downstream than the one without inclination. For plates, a high-pressure dome on the center of the lower basal face disappears with inclination, possibly leading to an increase of riming there. The torques acting on the crystals have a local maximum over the inclined angle and exhibit almost symmetric around 45° over the range of Reynolds numbers. The torque parameterization was performed under pressures of 300, 500, and 800 hPa as a function of Reynolds number and aspect ratio. It was found that the time scale of rotation for plates is smaller than the one for columns. Furthermore, the torque formula was applied to assess alignment of crystals along electric fields. It was found that these crystals of millimeter size require 120 kV/m for the electrical alignment, which agrees with previous studies.

  13. Particles at fluid-fluid interfaces: A new Navier-Stokes-Cahn-Hilliard surface- phase-field-crystal model.

    PubMed

    Aland, Sebastian; Lowengrub, John; Voigt, Axel

    2012-10-01

    Colloid particles that are partially wetted by two immiscible fluids can become confined to fluid-fluid interfaces. At sufficiently high volume fractions, the colloids may jam and the interface may crystallize. The fluids together with the interfacial colloids form an emulsion with interesting material properties and offer an important route to new soft materials. A promising approach to simulate these emulsions was presented in Aland et al. [Phys. Fluids 23, 062103 (2011)], where a Navier-Stokes-Cahn-Hilliard model for the macroscopic two-phase fluid system was combined with a surface phase-field-crystal model for the microscopic colloidal particles along the interface. Unfortunately this model leads to spurious velocities which require very fine spatial and temporal resolutions to accurately and stably simulate. In this paper we develop an improved Navier-Stokes-Cahn-Hilliard-surface phase-field-crystal model based on the principles of mass conservation and thermodynamic consistency. To validate our approach, we derive a sharp interface model and show agreement with the improved diffuse interface model. Using simple flow configurations, we show that the new model has much better properties and does not lead to spurious velocities. Finally, we demonstrate the solid-like behavior of the crystallized interface by simulating the fall of a solid ball through a colloid-laden multiphase fluid. PMID:23214691

  14. Particles at fluid-fluid interfaces: A new Navier-Stokes-Cahn-Hilliard surface- phase-field-crystal model

    NASA Astrophysics Data System (ADS)

    Aland, Sebastian; Lowengrub, John; Voigt, Axel

    2012-10-01

    Colloid particles that are partially wetted by two immiscible fluids can become confined to fluid-fluid interfaces. At sufficiently high volume fractions, the colloids may jam and the interface may crystallize. The fluids together with the interfacial colloids form an emulsion with interesting material properties and offer an important route to new soft materials. A promising approach to simulate these emulsions was presented in Aland [Phys. FluidsPHFLE61070-663110.1063/1.3584815 23, 062103 (2011)], where a Navier-Stokes-Cahn-Hilliard model for the macroscopic two-phase fluid system was combined with a surface phase-field-crystal model for the microscopic colloidal particles along the interface. Unfortunately this model leads to spurious velocities which require very fine spatial and temporal resolutions to accurately and stably simulate. In this paper we develop an improved Navier-Stokes-Cahn-Hilliard-surface phase-field-crystal model based on the principles of mass conservation and thermodynamic consistency. To validate our approach, we derive a sharp interface model and show agreement with the improved diffuse interface model. Using simple flow configurations, we show that the new model has much better properties and does not lead to spurious velocities. Finally, we demonstrate the solid-like behavior of the crystallized interface by simulating the fall of a solid ball through a colloid-laden multiphase fluid.

  15. Twisted nematic liquid crystal cell characterization using rotating polarizers including full-field cell gap thickness measurement

    NASA Astrophysics Data System (ADS)

    Dev, Kapil; Prakarsa, Andy; Jiang, Yin Xi; Lee, Hooi Leng; Asundi, Anand

    2009-12-01

    Liquid crystal cells have always been an important part of commercially available displays, modulators and projectors. In this paper, Jones matrix representation including four independent parameters for twisted nematic liquid crystal (TN-LC) cell has been demonstrated. The physical parameters of the TN-LC cell such as twist angle, birefringence and director orientation at the input face of cell has been calculated using intensity transmittance of an experimental set-up that includes circularly polarized light and TN-LC cell sandwiched between two polarizers. The physical parameters have been calculated without any ambiguity using three different wavelengths. The knowledge from above measurement gives the general information about the birefringence and hence liquid crystal cell gap thickness. To acquire full-field liquid crystal cell gap thickness measurement phase shift polariscope has been adopted. The four phase shifted images through the rotating analyzer gives the full-field cell gap thickness measurement. The experimental results have been compared with the commercial point wise measurement and are in good agreement.

  16. Twisted nematic liquid crystal cell characterization using rotating polarizers including full-field cell gap thickness measurement

    NASA Astrophysics Data System (ADS)

    Dev, Kapil; Prakarsa, Andy; Jiang, Yin Xi; Lee, Hooi Leng; Asundi, Anand

    2010-03-01

    Liquid crystal cells have always been an important part of commercially available displays, modulators and projectors. In this paper, Jones matrix representation including four independent parameters for twisted nematic liquid crystal (TN-LC) cell has been demonstrated. The physical parameters of the TN-LC cell such as twist angle, birefringence and director orientation at the input face of cell has been calculated using intensity transmittance of an experimental set-up that includes circularly polarized light and TN-LC cell sandwiched between two polarizers. The physical parameters have been calculated without any ambiguity using three different wavelengths. The knowledge from above measurement gives the general information about the birefringence and hence liquid crystal cell gap thickness. To acquire full-field liquid crystal cell gap thickness measurement phase shift polariscope has been adopted. The four phase shifted images through the rotating analyzer gives the full-field cell gap thickness measurement. The experimental results have been compared with the commercial point wise measurement and are in good agreement.

  17. Magnetic behavior and crystal field of Dy(BrO3)3.9H2O

    NASA Astrophysics Data System (ADS)

    Neogy, D.; Purohit, T.

    1987-04-01

    Magnetic susceptibility measurements have been made on dysprosium bromate enneahydrate crystals from room temperature down to 90 K. An analysis of our observed data and the magnetic measurements on the crystal below liquid-He temperatures by Simizu, Bellesis, and Friedberg [in 2 Proceedings of the 17th International Conference on Low Temperature Physics, Karlsruhe, Germany, 1984, edited by U. Eckern, A. Schmid, W. Weber, and H. Wühl (North-Holland, Amsterdam, 1984), p.187; J. Appl. Phys. 55, 2333 (1984)] using a superconducting quantum-interference device magnetometer, was undertaken for the first time invoking an effective electrostatic interaction of the Dy3+ ion with its diamagnetic neighbors. Expressions giving magnetic susceptibilities between 300 and 1 K were obtained taking into account the crystal field J-J mixing and the intermediate-coupling effects. The interpretation of the magnetic data is very good down to ~1 K, below which the effects of the dipolar interaction take over, leading to the onset of ferromagnetism at 170 mK. Available information from heat-capacity and optical-absorption measurements is also explained. The influence of the crystal field on the thermal variation of the Mössbauer spectra, and hyperfine heat capacity CN for the two isotopes 161Dy and 163Dy is discussed.

  18. Dependence of Pentacene Crystal Growth on Dielectric Roughness for Fabrication of Flexible Field-Effect Transistors

    SciTech Connect

    Yang, H.; Yang, C; Kim, S; Jang, M; Park, C

    2010-01-01

    The dependence of pentacene nanostructures on gate dielectric surfaces were investigated for flexible organic field-effect transistor (OFET) applications. Two bilayer types of polymer/aluminum oxide (Al{sub 2}O{sub 3}) gate dielectrics were fabricated on commercial Al foils laminated onto a polymer back plate. Some Al foils were directly used as gate electrodes, and others were smoothly polished by an electrolytic etching. These Al surfaces were then anodized and coated with poly({alpha}-methyl styrene) (PAMS). For PAMS/Al{sub 2}O{sub 3} dielectrics onto etched Al foils, surface roughness up to 1 nm could be reached, although isolated dimples with a lateral diameter of several micrometers were still present. On PAMS/Al{sub 2}O{sub 3} dielectrics (surface roughness >40 nm) containing mechanical grooves of Al foil, average hole mobility ({mu}FET) of 50 nm thick pentacene-FETs under the low operating voltages (|V| < 6 V) was {approx}0.15 cm{sup 2} V{sup -1} s{sup -1}. In contrast, pentacene-FETs employing the etched Al gates exhibited {mu}FET of 0.39 cm{sup 2} V{sup -1} s{sup -1}, which was comparable to that of reference samples with PAMS/Al{sub 2}O{sub 3} dielectrics onto flat sputtered Al gates. Conducting-probe atomic force microscopy and two-dimensional X-ray diffraction of pentacene films with various thicknesses revealed different out-of-plane and in-plane crystal orderings of pentacene, depending on the surface roughness of the gate dielectrics.

  19. Touch sensors based on planar liquid crystal-gated-organic field-effect transistors

    SciTech Connect

    Seo, Jooyeok; Lee, Chulyeon; Han, Hyemi; Lee, Sooyong; Nam, Sungho; Kim, Youngkyoo; Kim, Hwajeong; Lee, Joon-Hyung; Park, Soo-Young; Kang, Inn-Kyu

    2014-09-15

    We report a tactile touch sensor based on a planar liquid crystal-gated-organic field-effect transistor (LC-g-OFET) structure. The LC-g-OFET touch sensors were fabricated by forming the 10 μm thick LC layer (4-cyano-4{sup ′}-pentylbiphenyl - 5CB) on top of the 50 nm thick channel layer (poly(3-hexylthiophene) - P3HT) that is coated on the in-plane aligned drain/source/gate electrodes (indium-tin oxide - ITO). As an external physical stimulation to examine the tactile touch performance, a weak nitrogen flow (83.3 μl/s) was employed to stimulate the LC layer of the touch device. The LC-g-OFET device exhibited p-type transistor characteristics with a hole mobility of 1.5 cm{sup 2}/Vs, but no sensing current by the nitrogen flow touch was measured at sufficiently high drain (V{sub D}) and gate (V{sub G}) voltages. However, a clear sensing current signal was detected at lower voltages, which was quite sensitive to the combination of V{sub D} and V{sub G}. The best voltage combination was V{sub D} = −0.2 V and V{sub G} = −1 V for the highest ratio of signal currents to base currents (i.e., signal-to-noise ratio). The change in the LC alignment upon the nitrogen flow touch was assigned as the mechanism for the present LC-g-OFET touch sensors.

  20. Assessment of phase-field-crystal concepts using long-time molecular dynamics

    NASA Astrophysics Data System (ADS)

    Baker, K. L.; Curtin, W. A.

    2015-01-01

    The ability of the phase-field-crystal (PFC) model to quantitatively predict atomistic defect structures in crystalline solids is addressed. First, general aspects of the PFC model are discussed within the context of obtaining quantitative results in solid materials. Then a specific example is used to illustrate major points. Specifically, accelerated molecular dynamics is used to compute the one-particle probability density ρ(1 )(r ) in a complex atomistic defect consisting of a Lomer dislocation with an equilibrium distribution of vacancies in the core, and the results are considered within the general framework of the PFC model. As expected, ρ(1 )(r ) shows numerous spatially localized peaks with integrated densities smaller than unity, as would arise in a PFC computation. However, the ρ(1 )(r ) actually corresponds to a time-averaged superposition of a few well-defined atomic configurations each having a well-defined energy. The deconvolution of ρ(1 )(r ) to obtain the actual distinct atomic configurations is not feasible. Using a potential energy functional that accurately computes the energies of distinct configurations, the potential energy computed using ρ(1 )(r ) differs from the actual average atomistic energy by ˜50 eV divided among approximately 46 atoms in the core of the defect. Attempts to rectify this deviation by introducing correlations cannot significantly reduce this error. The simulations show energy barriers between distinct configurations varying by up to 0.5 eV, indicating that the simple kinetic evolution law used in PFC cannot accurately capture the true time evolution in this problem. Overall, these results demonstrate, in one nontrivial case, that the PFC model is probably unable to predict atomistic defect structures, energies, or kinetic barriers at the quantitative levels needed for application to problems in materials science.

  1. Atomistic study of diffusion-mediated plasticity and creep using phase field crystal methods

    NASA Astrophysics Data System (ADS)

    Berry, Joel; Rottler, Jörg; Sinclair, Chad W.; Provatas, Nikolas

    2015-10-01

    The nonequilibrium dynamics of diffusion-mediated plasticity and creep in materials subjected to constant load at high homologous temperatures is studied atomistically using phase field crystal (PFC) methods. Creep stress and grain size exponents obtained for nanopolycrystalline systems, m ≃1.02 and p ≃1.98 , respectively, closely match those expected for idealized diffusional Nabarro-Herring creep. These exponents are observed in the presence of significant stress-assisted diffusive grain boundary migration, indicating that Nabarro-Herring creep and stress-assisted boundary migration contribute in the same manner to the macroscopic constitutive relation. When plastic response is dislocation-mediated, power-law stress exponents inferred from dislocation climb rates are found to increase monotonically from m ≃3 , as expected for generic climb-mediated natural creep, to m ≃5.8 as the dislocation density ρd is increased beyond typical experimental values. Stress exponents m ≳3 directly measured from simulations that include dislocation nucleation, climb, glide, and annihilation are attributed primarily to these large ρd effects. Extrapolation to lower ρd suggests that m ≃4 -4.5 should be obtained from our PFC description at typical experimental ρd values, which is consistent with expectations for power-law creep via mixed climb and glide. The anomalously large stress exponents observed in our atomistic simulations at large ρd may nonetheless be relevant to systems in which comparable densities are obtained locally within heterogeneous defect domains such as dislocation cell walls or tangles.

  2. Crystal-field parameters of the rare-earth pyrochlores R2Ti2O7 (R =Tb , Dy, and Ho)

    NASA Astrophysics Data System (ADS)

    Ruminy, M.; Pomjakushina, E.; Iida, K.; Kamazawa, K.; Adroja, D. T.; Stuhr, U.; Fennell, T.

    2016-07-01

    In this work, we present inelastic neutron scattering experiments which probe the single ion ground states of the rare-earth pyrochlores R2Ti2O7 (R =Tb , Dy, Ho). Dy2Ti2O7 and Ho2Ti2O7 are dipolar spin ices, now often described as hosts of emergent magnetic monopole excitations; the low-temperature state of Tb2Ti2O7 has features of both spin liquids and spin glasses, and strong magnetoelastic coupling. We measured the crystal-field excitations of all three compounds and obtained a unified set of crystal-field parameters. Additional measurements of a single crystal of Tb2Ti2O7 clarified the assignment of the crystal-field levels in this material and also revealed an example of a bound state between a crystal-field level and an optical phonon mode.

  3. Metastable Copper-Phthalocyanine Single-Crystal Nanowires and Their Use in Fabricating High-Performance Field-Effect Transistors

    SciTech Connect

    Xiao, Kai; Li, Rongjin; Tao, Jing; Payzant, E Andrew; Ivanov, Ilia N; Puretzky, Alexander A; Hu, Wenping; Geohegan, David B

    2009-01-01

    This paper describes a simple, vapor-phase route to the synthesis of metastable α-phase copper-phthalocyanine (CuPc) single-crystal nanowires through control of the growth temperature. The influence of the growth temperature on the crystal structures, morphology, and size of the CuPc nanostructures was explored by XRD, optical absorption and Transmission Electron Microscopy (TEM). α-CuPc nanowires were successfully incorporated as active semiconductors in field-effect transistors (FETs). Single nanowire devices exhibited the carrier mobilities and current on/off ratios as high as 0.4 cm2/Vs and > 104, respectively, rendering them useful for organic photovoltaic cells, organic light-emitting diodes, field-effect transistors, memories and gas sensors

  4. Tuning the band structures of a one-dimensional width-modulated magnonic crystal by a transverse magnetic field

    SciTech Connect

    Di, K.; Lim, H. S. Zhang, V. L.; Ng, S. C.; Kuok, M. H.; Nguyen, H. T.; Cottam, M. G.

    2014-02-07

    Theoretical studies, based on three independent techniques, of the band structure of a one-dimensional width-modulated magnonic crystal under a transverse magnetic field are reported. The band diagram is found to display distinct behaviors when the transverse field is either larger or smaller than a critical value. The widths and center positions of bandgaps exhibit unusual non-monotonic and large field-tunability through tilting the direction of magnetization. Some bandgaps can be dynamically switched on and off by simply tuning the strength of such a static field. Finally, the impact of the lowered symmetry of the magnetic ground state on the spin-wave excitation efficiency of an oscillating magnetic field is discussed. Our finding reveals that the magnetization direction plays an important role in tailoring magnonic band structures and hence in the design of dynamic spin-wave switches.

  5. 4D STUDY OF STRAIN GRADIENTS EVOLUTION IN TWINNED NiMnGa SINGLE CRYSTALS UNDER MAGNETIC FIELD

    SciTech Connect

    Barabash, Rozaliya; Xu, Ruqing; Barabash, Oleg M; Sozinov, Alexei

    2014-01-01

    Time-resolved 3D X-ray microscopy with a submicron beam size was used to follow the evolution of strains in off-stoichiometric NiMnGa twinned crystals near type I (hard) twin boundary under magnetic field. Laminate A/B microstructure was revealed near the twin boundaries in A variant. Large strain gradients are observed in the C variant in the immediate vicinity of the type I twin boundary: the lattice is under large tensile strains ~0.4% along the c- axes within first micron. Distinct a and b lattice parameter evolution with temperature and magnetic field is demonstrated. In an applied magnetic field the strain field was observed at larger distances from the twin boundary and becomes more complex. Stochastic twin boundary motion was observed after the magnetic field reaches a certain critical value.

  6. A Monte Carlo study of the Blume-Capel thin film in the presence of a random crystal field

    NASA Astrophysics Data System (ADS)

    Boughrara, M.; Kerouad, M.; Zaim, A.

    2016-07-01

    A Monte Carlo simulation with heat bath algorithm is used to study the effect of random crystal field and surface exchange interactions on the critical behavior and the magnetic properties of a spin-1 Ising ferromagnetic thin film having the simple cubic symmetry. The phase diagram exhibits a rich variety of behaviors such as the double reentrant phenomena and the existence of tricritical points. Thermal magnetization behavior and phase diagrams have been discussed in detail.

  7. Synthesis and field emission behaviour of well faceted In{sub 2}Se{sub 3} micro-crystals

    SciTech Connect

    Kolhe, Panakj S.; Suryawanshi, Sachin R.; More, Mahendra A.; Shisode, Raju T.

    2015-06-24

    Here in, we report synthesis of crystalline Indium Selenide (In{sub 2}Se{sub 3}) elegant microcrystals on Au coated Si substrates using one-step facile thermal evaporation route and their field emission investigations. The as-synthesized In{sub 2}Se{sub 3} micro-crystals were subjected to structural and morphological analysis prior to the field emission studies. The XRD spectrum of the as-synthesized product reveals formation of crystalline hexagonal phase of In{sub 2}Se{sub 3} under the prevailing experimental conditions. Under optimized process variables, the morphology of the as-synthesized product is characterized by presence of well facetted micron size particles of In2Se3. Furthermore, the EDAX analysis confirms the presence of In and Se in the as-synthesized sample. The field emission characteristic of the In{sub 2}Se{sub 3} micro-crystal emitter is found to be superior to the other metal chalcogenides micro-crystal based emitters. The synthesized In{sub 2}Se{sub 3} micro-crystals emitter delivers current density of ∼ 225 µA/cm{sup 2} at an applied electric field of ∼ 7.44 V/µm. The emission current stability investigated at pre-set value of ∼ 3 µA is observed to be fairly good. These observed results demonstrate potential of the In{sub 2}Se{sub 3} cathode as an electron source for practical applications in vacuum microelectronic devices.

  8. Self-action of a light beam in nematic liquid crystals in the presence of a DC electric field

    SciTech Connect

    Budagovsky, I. A.; Zolot'ko, A. S. Smayev, M. P.; Barnik, M. I.

    2010-07-15

    The results of experimental study of the light beam self-action in a nematic liquid crystal placed in a dc electric field are presented, and a theory of this effect is developed. This self-action of a light beam is shown to cause a hyperbolic umbilic caustic. The intensity distribution and caustics calculated in the far diffraction zone of the light beam agree well with the experimental data.

  9. The easy magnetization directions in R{sub 6}Fe{sub 23} intermetallic compounds: A crystal field analysis

    SciTech Connect

    Bowden, G.J.; Cadogan, J.M.; de Leon, H.; Ryan, D.H.

    1997-04-01

    X-ray diffraction patterns on magnetically aligned powder samples of R{sub 6}Fe{sub 23} (R=Dy, Er, Ho, and Tm) show that these compounds all magnetize along a [111] easy direction. At first sight it is difficult to reconcile the common easy magnetization direction of Er{sub 6}Fe{sub 23} and Tm{sub 6}Fe{sub 23} on one hand, with Dy{sub 6}Fe{sub 23} and Ho{sub 6}Fe{sub 23} on the other, since the respective B{sub 20} rare earth crystal field parameters of these pairs of compounds are opposite in sign. In this article we show that the crystal field stabilization energy of the [111] direction, relative to either [100] or [110], varies as the square of the crystal field term B{sub 20}, thereby providing an explanation for the common [111] direction of easy magnetization. {copyright} {ital 1997 American Institute of Physics.}

  10. Crystal field and magnetism of Pr³⁺ and Nd³⁺ ions in orthorhombic perovskites.

    PubMed

    Novák, P; Knížek, K; Maryško, M; Jirák, Z; Kuneš, J

    2013-11-01

    Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO3 perovskites (R=Pr, Nd, M=Ga, Co) are calculated using a first-principles electronic structure and the Wannier projection. The method contains a single adjustable parameter that characterizes the hybridization of R(4f) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of the trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the 4f electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within a few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO3 the theory is also compared with the magnetic measurements on a single crystal presented by Luis et al (1998 Phys. Rev. B 58 798). Good agreement between the experiment and theory is found. PMID:24113417

  11. Effect of Magnetic Fields on g-jitter Induced Convection and Solute Striation During Space Processing of Single Crystals

    NASA Technical Reports Server (NTRS)

    deGroh, H. C.; Li, K.; Li, B. Q.

    2002-01-01

    A 2-D finite element model is presented for the melt growth of single crystals in a microgravity environment with a superimposed DC magnetic field. The model is developed based on the deforming finite element methodology and is capable of predicting the phenomena of the steady and transient convective flows, heat transfer, solute distribution, and solid-liquid interface morphology associated with the melt growth of single crystals in microgravity with and without an applied magnetic field. Numerical simulations were carried out for a wide range of parameters including idealized microgravity conditions, the synthesized g-jitter and the real g-jitter data taken by on-board accelerometers during space flights. The results reveal that the time varying g-jitter disturbances, although small in magnitude, cause an appreciable convective flow in the liquid pool, which in turn produces detrimental effects during the space processing of single crystal growth. An applied magnetic field of appropriate strength, superimposed on microgravity, can be very effective in suppressing the deleterious effects resulting from the g-jitter disturbances.

  12. Investigation of Three-Dimensional Stress Fields and Slip Systems for FCC Single Crystal Superalloy Notched Specimens

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.; Magnan, Shannon; Ebrahimi, Fereshteh; Ferroro, Luis

    2004-01-01

    Metals and their alloys, except for a few intermetallics, are inherently ductile, i.e. plastic deformation precedes fracture in these materials. Therefore, resistance to fracture is directly related to the development of the plastic zone at the crack tip. Recent studies indicate that the fracture toughness of single crystals depends on the crystallographic orientation of the notch as well as the loading direction. In general, the dependence of crack propagation resistance on crystallographic orientation arises from the anisotropy of (i) elastic constants, (ii) plastic deformation (or slip), and (iii) the weakest fracture planes (e.g. cleavage planes). Because of the triaxial stress state at the notch tips, many slip systems that otherwise would not be activated during uniaxial testing, become operational. The plastic zone formation in single crystals has been tackled theoretically by Rice and his co-workers and only limited experimental work has been conducted in this area. The study of the stresses and strains in the vicinity of a FCC single crystal notch tip is of relatively recent origin. We present experimental and numerical investigation of 3D stress fields and evolution of slip sector boundaries near notches in FCC single crystal tension test specimens, and demonstrate that a 3D linear elastic finite element model that includes the effect of material anisotropy is shown to predict active slip planes and sectors accurately. The slip sector boundaries are shown to have complex curved shapes with several slip systems active simultaneously near the notch. Results are presented for surface and mid-plane of the specimens. The results demonstrate that accounting for 3D elastic anisotropy is very important for accurate prediction of slip activation near FCC single crystal notches loaded in tension. Results from the study will help establish guidelines for fatigue damage near single crystal notches.

  13. Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single crystals

    NASA Astrophysics Data System (ADS)

    Felner, I.; Tsindlekht, M. I.; Drachuck, G.; Keren, A.

    2013-02-01

    Optimally doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H-T phase diagram is presented. Close to TC = 35 K, for the c-crystal, {\\boldsymbol{\\gamma}}^{c}={H}_{{C3}}^{c}/{H}_{{C2}}^{c}=1.8 0(2), whereas for the a-crystal the {\\boldsymbol{\\gamma}}^{a}={H}_{{C3}}^{a}/{H}_{{C2}}^{a}=4.0(2) obtained is much higher than 1.69, predicted by the ideal mathematical model. At low applied dc fields, positive field-cooled branches known as the ‘paramagnetic Meissner effect’ (PME) are observed; their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γa and the PME are connected to each other.

  14. Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single crystals.

    PubMed

    Felner, I; Tsindlekht, M I; Drachuck, G; Keren, A

    2013-02-13

    Optimally doped La(1.85)Sr(0.15)CuO(4) single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (H(C2)) and the surface critical field (H(C3)) were measured. The H-T phase diagram is presented. Close to T(C) = 35 K, for the c-crystal, γ(C) = H(C3)(c)/H(C2)(c) = 1.80(2), whereas for the a-crystal the γ(a) = H(C3)(a)/H(C2)(a) = 4.0(2) obtained is much higher than 1.69, predicted by the ideal mathematical model. At low applied dc fields, positive field-cooled branches known as the 'paramagnetic Meissner effect' (PME) are observed; their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γ(a) and the PME are connected to each other. PMID:23315336

  15. Giant rotating magnetocaloric effect at low magnetic fields in multiferroic TbMn2O5 single crystals

    NASA Astrophysics Data System (ADS)

    Balli, M.; Jandl, S.; Fournier, P.; Dimitrov, D. Z.

    2016-03-01

    In conventional magnetocaloric refrigeration systems, the magnetocaloric effect is exploited by moving the active material in and out of the magnetic field source. Here, we demonstrate that a large and reversible magnetocaloric effect (-ΔSR, max = 6.4 J/kg K and ΔTR, max = 8 K under 2 T) can be generated simply by rotating the multiferroic TbMn2O5 single crystal around its b axis in a relatively low constant magnetic field applied in the ac plane. Our results should inspire and open ways toward the implementation of compact, efficient and embedded magnetocaloric devices for low temperature and space applications.

  16. Effects of composition and temperature on the large-field behavior of [001]C relaxor single crystals.

    PubMed

    Gallagher, John; Lynch, Christopher; Tian, Jian

    2014-12-01

    The compositional dependence of the large-field behavior of [001]C-cut relaxor ferroelectric xPb(In1/2Nb1/2) O3-(1-x-y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 (PIN-PMN-PT) single crystals that are on the rhombohedral side of the morphotropic phase boundary was characterized under electrical, mechanical, and thermal loading. The effects of varying the concentrations of PIN and PT are discussed. Composition was found to impact the material constants and the field-induced phase transformation threshold in the piezoelectric d333-mode configuration. PMID:25474790

  17. Realization of Field Sequential Color in Simple Matrix Antiferroelectric Liquid Crystal Displays by Utilizing Fast Pretransitional Response

    NASA Astrophysics Data System (ADS)

    Suzuki, Yasushi; Chen, Guo-Ping; Manna, Uttam; Vij, Jagdish K.; Fukuda, Atsuo

    2009-07-01

    Simple matrix antiferroelectric liquid crystal displays (SM-AFLCDs) are prototyped to realize field sequential color (FSC) by utilizing the fast pretransitional response. The developed FSC-SM-AFLCDs will lead to the replacement of existing static driven FSC-SM-nematic-LCDs. Bright and clear color can be given to already market-acquired, black-and-white SM-LCDs of up to 1/64-duty and 3-in. diagonal size. To optimize the display performance, we analyze two important factors, the large pretransitional effect and the appropriate reset pulse, in terms of the interlayer interaction potential used in describing the field-induced transition of the antiferroelectric smectic phase.

  18. Determining point charge arrays that produce accurate ionic crystal fields for atomic cluster calculations

    SciTech Connect

    Derenzo, Stephen E.; Klintenberg, Mattias K.; Weber, Marvin J.

    2000-02-01

    In performing atomic cluster calculations of local electronic structure defects in ionic crystals, the crystal is often modeled as a central cluster of 5-50 ions embedded in an array of point charges. For most crystals, however, a finite three-dimensional repeated array of unit cells generates electrostatic potentials that are in significant disagreement with the Madelung (infinite crystal) potentials computed by the Ewald method. This is illustrated for the cubic crystal CaF{sub 2}. We present a novel algorithm for solving this problem for any crystal whose unit cell information is known: (1) the unit cell is used to generate a neutral array containing typically 10 000 point charges at their normal crystallographic positions; (2) the array is divided into zone 1 (a volume defined by the atomic cluster of interest), zone 2 (several hundred additional point charges that together with zone 1 fill a spherical volume), and zone 3 (all other point charges); (3) the Ewald formula is used to compute the site potentials at all point charges in zones 1 and 2; (4) a system of simultaneous linear equations is solved to find the zone 3 charge values that make the zone 1 and zone 2 site potentials exactly equal to their Ewald values and the total charge and dipole moments equal to zero, and (5) the solution is checked at 1000 additional points randomly chosen in zone 1. The method is applied to 33 different crystal types with 50-71 ions in zone 1. In all cases the accuracy determined in step 5 steadily improves as the sizes of zones 2 and 3 are increased, reaching a typical rms error of 1 {mu}V in zone 1 for 500 point charges in zone 2 and 10 000 in zone 3. (c) 2000 American Institute of Physics.

  19. Faceted growth of primary Al{sub 2}Cu crystals during directional solidification in high magnetic field

    SciTech Connect

    Li, Chuanjun; Ren, Zhongming; Shen, Yu; Wang, Qiuliang; Dai, Yinming; Wang, Hui

    2013-10-21

    The high magnetic field is widely used to modify the crystal morphology. In this work, the effect of the magnetic field on growing behavior of faceted crystals in the Al-40 wt. %Cu alloy was investigated using directional solidification technique. It was found that the faceted growth of primary Al{sub 2}Cu phase was degraded and the primary spacing was reduced upon applying the magnetic field. Additionally, the length of the mushy zone first decreased and then increased with increase of the magnetic field intensity. The quantitative analysis reveals that the shear stress induced by the fluid motion is insufficient to break the atom bonds at the solid-liquid interface. However, both of the thermoelectric magnetic convection (TEMC) and the thermoelectric magnetic force (TEMF) cause dendrites to fracture and reduce the primary spacing. The two effects also weaken the faceting growth. Moreover, the instability of the solid-liquid interface is generated by the TEMF, which further leads to degrade the faceted growth. The length of mushy zone was changed by the TEMC and reached the minimum in the magnetic field of 0.5 T, which is in good agreement with the predicted value (0.83 T)

  20. Magnetic field induced extraordinary photoluminescence enhancement in Er{sup 3+}:YVO{sub 4} single crystal

    SciTech Connect

    Zhang, Junpei; Wang, Xia; Tang, Chaoqun; Zhong, Zhiqiang; Ma, Zongwei; Wang, Shaoliang; Han, Yibo; Han, Jun-Bo Li, Liang

    2015-08-28

    A bright green photoluminescence (PL) from {sup 4}S{sub 3∕2} → {sup 4}I{sub 15∕2} emission band in Er{sup 3+}:YVO{sub 4} single crystal has been observed with the excitation of an argon laser at 488.0 nm. More than two orders of PL enhancement have been obtained under the effect of magnetic fields, and the enhancement factor f reaches 170 when the applied magnetic field is 7.7 T under the sample temperature of 4.2 K. Unusually, the PL enhancements only happen at some certain magnetic fields (B{sub c}s), and a decrease of sample temperature will lead to the increase of f and decrease of B{sub c}. The results confirm that this PL enhancement originates from the resonance excitation of the electron transitions induced by the cross of the laser energy and the absorption energy modulated by both the magnetic field and temperature. This special PL enhancement in Er{sup 3+}:YVO{sub 4} single crystal can be applied in the calibration of pulsed high magnetic field, detection of material fine energy structures, and modulation of magneto-optical devices.

  1. Thickness-Dependent and Magnetic-Field-Driven Suppression of Antiferromagnetic Order in Thin V5S8 Single Crystals.

    PubMed

    Hardy, Will J; Yuan, Jiangtan; Guo, Hua; Zhou, Panpan; Lou, Jun; Natelson, Douglas

    2016-06-28

    With materials approaching the 2D limit yielding many exciting systems with intriguing physical properties and promising technological functionalities, understanding and engineering magnetic order in nanoscale, layered materials is generating keen interest. One such material is V5S8, a metal with an antiferromagnetic ground state below the Néel temperature TN ∼ 32 K and a prominent spin-flop signature in the magnetoresistance (MR) when H∥c ∼ 4.2 T. Here we study nanoscale-thickness single crystals of V5S8, focusing on temperatures close to TN and the evolution of material properties in response to systematic reduction in crystal thickness. Transport measurements just below TN reveal magnetic hysteresis that we ascribe to a metamagnetic transition, the first-order magnetic-field-driven breakdown of the ordered state. The reduction of crystal thickness to ∼10 nm coincides with systematic changes in the magnetic response: TN falls, implying that antiferromagnetism is suppressed; and while the spin-flop signature remains, the hysteresis disappears, implying that the metamagnetic transition becomes second order as the thickness approaches the 2D limit. This work demonstrates that single crystals of magnetic materials with nanometer thicknesses are promising systems for future studies of magnetism in reduced dimensionality and quantum phase transitions. PMID:27163511

  2. Susceptibility of single molecule magnet Mn12-acetate single crystals as a function of temperature and transverse field

    NASA Astrophysics Data System (ADS)

    Subedi, Pradeep; Wen, Bo; Bo, Lin; Sarachik, Myriam; Yeshurun, Yosi; Kent, Andrew; Lampropoulos, Christos; Christou, George

    2010-03-01

    The longitudinal susceptibility of Mn12-acetate single crystals has been measured in a magnetic field applied transverse to the Ising axis using micro-Hall effect magnetometry in a He^3 cryostat with a 3D vector superconducting magnet. We have investigated the blocking temperature as a function of longitudinal-field-sweep-rate and as a function of the magnitude of the transverse field. We find that the transverse field accelerates the relaxation to equilibrium and lowers the blocking temperature, as expected based on the Mn12-ac spin-Hamiltonian and a classical model of single domain uniaxial nanomagnets. The susceptibility is found to obey a Curie-Weiss law, indicating a low temperature transition to a ferromagnetic phase due to dipolar interactions. We discuss these experiments as well as experiments in which an array of Hall-bars is used to spatially resolve the longitudinal susceptibility above the blocking temperature.

  3. Simultaneous negative refraction and focusing of fundamental frequency and second-harmonic fields by two-dimensional photonic crystals

    SciTech Connect

    Zhang, Jun; Zhang, Xiangdong

    2015-09-28

    Simultaneous negative refraction for both the fundamental frequency (FF) and second-harmonic (SH) fields in two-dimensional nonlinear photonic crystals have been found through both the physical analysis and exact numerical simulation. By combining such a property with the phase-matching condition and strong second-order susceptibility, we have designed a SH lens to realize focusing for both the FF and SH fields at the same time. Good-quality non-near field images for both FF and SH fields have been observed. The physical mechanism for such SH focusing phenomena has been disclosed, which is different from the backward SH generation as has been pointed out in the previous investigations. In addition, the effect of absorption losses on the phenomena has also been discussed. Thus, potential applications of these phenomena to biphotonic microscopy technique are anticipated.

  4. Propagation characteristics of a focused laser beam in a strontium barium niobate photorefractive crystal under reverse external electric field.

    PubMed

    Guo, Q L; Liang, B L; Wang, Y; Deng, G Y; Jiang, Y H; Zhang, S H; Fu, G S; Simmonds, P J

    2014-10-01

    The propagation characteristics of a focused laser beam in a SBN:75 photorefractive crystal strongly depend on the signal-to-background intensity ratio (R=Is/Ib) under reverse external electric field. In the range 20>R>0.05, the laser beam shows enhanced self-defocusing behavior with increasing external electric field, while it shows self-focusing in the range 0.03>R>0.01. Spatial solitons are observed under a suitable reverse external electric field for R=0.025. A theoretical model is proposed to explain the experimental observations, which suggest a new type of soliton formation due to "enhancement" not "screening" of the external electrical field. PMID:25322227

  5. Partial Dissolution of Charge Order Phase Observed in β-(BEDT-TTF)2PF6 Single Crystal Field Effect Transistor.

    PubMed

    Sakai, Masatoshi; Moritoshi, Norifumi; Kuniyoshi, Shigekazu; Yamauchi, Hiroshi; Kudo, Kazuhiro; Masu, Hyuma

    2016-04-01

    The effect of an applied gate electric field on the charge-order phase in β-(BEDT-TTF)2PF6 single-crystal field-effect transistor structure was observed at around room temperature by technical improvement with respect to sample preparation and electrical measurements. A relatively slight but systematic increase of the electrical conductance induced by the applied gate electric field and its temperature dependence was observed at around the metal-insulator transition temperature (TMI). The temperature dependence of the modulated electrical conductance demonstrated that TMI was shifted toward the lower side by application of a gate electric field, which corresponds to partial dissolution of the charge-order phase. The thickness of the partially dissolved charge order region was estimated to be several score times larger than the charge accumulation region. PMID:27451615

  6. DNA- and AC electric field-assisted assembly of two-dimensional colloidal photonic crystals and their controlled defect insertion

    NASA Astrophysics Data System (ADS)

    Kim, Sejong

    single crystal monolayer in microfluidic chamber. Dielectrophoretic (DEP) force with high frequency electric field induced compression of colloidal microspheres to form colloidal crystal domain at the center of hexapolar shape electrode. DEP-compression/relaxation-cycle-induced aging process significantly facilitated crystal growth of 10 mum monodispersed polystyrene microsphere, allowing grain boundary-free single crystalline monolayer domain of c.a. 200 mum size. Microsphere size as well as size distribution affected the formation of such a single crystalline domain. Utilizing non-ionic polyacrylamide, such a single crystalline domain was successfully immobilized onto the glass substrate without loosing its crystallinity.

  7. The Design of a Transparent Vertical Multizone Furnace: Application to Thermal Field Tuning and Crystal Growth

    NASA Technical Reports Server (NTRS)

    Duvual, Walter M. B.; Batur, Celal; Bennett, Robert J.

    1998-01-01

    We present an innovative design of a vertical transparent multizone furnace which can operate in the temperature range of 25 C to 750 C and deliver thermal gradients of 2 C/cm to 45 C/cm for the commercial applications to crystal growth. The operation of the eight zone furnace is based on a self-tuning temperature control system with a DC power supply for optimal thermal stability. We show that the desired thermal profile over the entire length of the furnace consists of a functional combination of the fundamental thermal profiles for each individual zone obtained by setting the set-point temperature for that zone. The self-tuning system accounts for the zone to zone thermal interactions. The control system operates such that the thermal profile is maintained under thermal load, thus boundary conditions on crystal growth ampoules can be predetermined prior to crystal growth. Temperature profiles for the growth of crystals via directional solidification, vapor transport techniques, and multiple gradient applications are shown to be easily implemented. The unique feature of its transparency and ease of programming thermal profiles make the furnace useful in scientific and commercial applications for determining the optimized process parameters for crystal growth.

  8. Influence of rotating magnetic fields on THM growth of CdZnTe crystals under microgravity and ground conditions

    NASA Astrophysics Data System (ADS)

    Stelian, Carmen; Duffar, Thierry

    2015-11-01

    The influence of rotating magnetic fields (RMF) on species transport and interface stability during the growth of Cd0.96Zn0.04Te:In crystals by using the traveling heater method (THM), under microgravity and terrestrial conditions, is numerically investigated. The numerical results are compared to ground and space experiments. The modeling of THM under ground conditions shows very deleterious effects of the natural convection on the morphological stability of the growth interface. The vertical flow transports the liquid of low Te concentration from the dissolution interface to the growth interface, which is consequently destabilized. The suppression of this flow, in low-gravity conditions, results in higher morphological stability of the growth interface. Application of RMF induces a two flow cell pattern, which has a destabilizing effect on the growth interface. Simulations performed by varying the magnetic field induction in the range of 1 - 3 mT show optimal conditions for the growth with a stable interface at low strength of the magnetic field (B = 1 mT). Computations of indium distribution show a better homogeneity of crystals grown under purely diffusive conditions. Rotating magnetic fields of B = 1 mT induce low intensity convection, which generates concentration gradients near the growth interface. These numerical results are in agreement with experiments performed during the FOTON M4 space mission, showing good structural quality of Cd0.96Zn0.04Te crystals grown at very low gravity level. Applying low intensity rotating magnetic fields in ground experiments has no significant influence on the flow pattern and solute distribution. At high intensity of RMF (B = 50 mT), the buoyancy convection is damped near the growth front, resulting in a more stable advancing interface. However, convection is strengthening in the upper part of the liquid zone, where the flow becomes unsteady. The multi-cellular unsteady flow generates temperature oscillations, having

  9. Synthesis, crystal structure, vibrational spectral analysis and Z-scan studies of a new organic crystal N,N‧dimethylurea ninhydrin: A scaled quantum mechanical force field study

    NASA Astrophysics Data System (ADS)

    John, Jerin Susan; Sajan, D.; Umadevi, T.; Chaitanya, K.; Sankar, Pranitha; Philip, Reji

    2015-10-01

    A new organic material, N,N‧dimethylurea ninhydrin (3a,8a-dihydroxy-1,3-dimethyl-1,3,3a,8a-tetrahydroindeno[2,1-d]imidazole-2,8-dione) (NDUN) was synthesized. Structural details were obtained from single crystal X-ray diffraction (XRD) data. A detailed interpretation of the vibrational spectra is carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. TG/DTA and 1H NMR studies were carried out. Linear optical properties were studied from UV-Vis spectra. From the open aperture Z-scan data, it is found that the molecule shows third order nonlinear optical behaviour due to two photon absorption (2PA) mechanism.

  10. Crystal-field analysis and calculation of two-photon absorption line strengths of dicesium sodium hexachlorogadolinate(III).

    PubMed

    Duan, Chang-Kui; Tanner, Peter A

    2010-03-31

    The crystal-field energy level calculation of the 4f(7) ion Gd(3+) in the crystal Cs(2)NaGdCl(6) has fitted 45 levels with standard deviation 12 cm(-1), with the energy parameters being consistent with those from other studies. The resulting eigenvectors have been employed in the calculation of two-photon absorption (TPA) intensities of transitions from the electronic ground state (8)S(7/2) to the crystal-field levels of excited (6)P, (6)I and (6)D multiplet terms. The TPA line strengths are highly polarization dependent and exhibit striking differences for linearly polarized incident radiation compared with circularly polarized radiation. The relative intensities are compared with those available from previous experimental studies and some reassignments have been made. Good agreement of calculated and experimental TPA spectra is found, except for the intensity ratio of the transitions to (6)P(7/2) or (6)P(5/2) compared with that to (6)P(3/2), for linear and circular polarizations, where the calculation overestimates the ratio. Reasons for this disagreement are presented. PMID:21389490

  11. Single-crystal field-effect transistors of new Cl2-NDI polymorph processed by sublimation in air

    NASA Astrophysics Data System (ADS)

    He, Tao; Stolte, Matthias; Burschka, Christian; Hansen, Nis Hauke; Musiol, Thomas; Kälblein, Daniel; Pflaum, Jens; Tao, Xutang; Brill, Jochen; Würthner, Frank

    2015-01-01

    Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (β-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (α-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm2 V-1 s-1 (α-phase) and up to 3.5 cm2 V-1 s-1 (β-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on β-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm2 V-1 s-1. The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.

  12. Far-field disentanglement of modes in hybrid plasmonic-photonic crystals by fluorescence nano-reporters

    NASA Astrophysics Data System (ADS)

    Ungureanu, Simona; Kolaric, Branko; Chen, Jianing; Hillenbrand, Rainer; Vallée, Renaud A. L.

    2013-07-01

    In this paper, the resonance modes exhibited by a hybrid nanostructure have been disentangled in the far-field owing to narrow-band fluorescence nano-reporters. Hybrid plasmonic-photonic crystals were fabricated using large (457 nm) monodisperse polystyrene spheres self-assembled into 2D photonic crystals and subsequently coated by a 30 nm thick silver layer. Such structures exhibit a complex resonance pattern, which has been elucidated owing to numerical simulations and electric near-field patterns obtained with a scattering type scanning near-field optical microscope (s-SNOM). For the sake of disentangling the resonance modes of the hybrid structure in the far-field, different types of semiconductor quantum dots (QDs), acting as nano-reporters of the local interactions, were dispersed on top of distinct structures. Depending on the relative overlap of the emission spectrum of a particular type of QDs with the resonance features of the hybrid structure, we affect their emission rate in a unique way, as a consequence of the complex interaction occurring between the plasmo-photonic modes and the excitons. Such plasmonic structures appear to be particularly relevant for fluorescence-based sensing devices.

  13. Large low-field positive magnetoresistance in nonmagnetic half-Heusler ScPtBi single crystal

    NASA Astrophysics Data System (ADS)

    Hou, Zhipeng; Wang, Yue; Liu, Enke; Zhang, Hongwei; Wang, Wenhong; Wu, Guangheng

    2015-11-01

    High-quality nonmagnetic half-Heusler ScPtBi single crystals were synthesized by a Bi self-flux method. This compound was revealed to be a hole-dominated semimetal with a large low-field magnetoresistance up to 240% at 2 K in a magnetic field of 1 T. Magneto-transport measurements demonstrated that the large low-field magnetoresistance effect resulted from the coexistence of field-induced metal-semiconductor transition and weak-antilocalization effect. Moreover, Hall measurements indicated that ScPtBi single crystal showed a high mobility over a wide temperature region even up to room temperature (4050 cm2V-1s-1 at 2 K-2016 cm2V-1s-1 at 300 K). These findings not only suggest the nonmagnetic ScPtBi semimetal a potential material candidate for applications in high-sensitivity magnetic sensors but also are of great significance to comprehensively understand the rare-earth based half-Heusler compounds.

  14. Lithium ion diffusion in Li β-alumina single crystals measured by pulsed field gradient NMR spectroscopy

    SciTech Connect

    Chowdhury, Mohammed Tareque Takekawa, Reiji; Iwai, Yoshiki; Kuwata, Naoaki; Kawamura, Junichi

    2014-03-28

    The lithium ion diffusion coefficient of a 93% Li β-alumina single crystal was measured for the first time using pulsed field gradient (PFG) NMR spectroscopy with two different crystal orientations. The diffusion coefficient was found to be 1.2 × 10{sup −11} m{sup 2}/s in the direction perpendicular to the c axis at room temperature. The Li ion diffusion coefficient along the c axis direction was found to be very small (6.4 × 10{sup −13} m{sup 2}/s at 333 K), which suggests that the macroscopic diffusion of the Li ion in the β-alumina crystal is mainly two-dimensional. The diffusion coefficient for the same sample was also estimated using NMR line narrowing data and impedance measurements. The impedance data show reasonable agreement with PFG-NMR data, while the line narrowing measurements provided a lower value for the diffusion coefficient. Line narrowing measurements also provided a relatively low value for the activation energy and pre-exponential factor. The temperature dependent diffusion coefficient was obtained in the temperature range 297–333 K by PFG-NMR, from which the activation energy for diffusion of the Li ion was estimated. The activation energy obtained by PFG-NMR was smaller than that obtained by impedance measurements, which suggests that thermally activated defect formation energy exists for 93% Li β-alumina single crystals. The diffusion time dependence of the diffusion coefficient was observed for the Li ion in the 93% Li β-alumina single crystal by means of PFG-NMR experiments. Motion of Li ion in fractal dimension might be a possible explanation for the observed diffusion time dependence of the diffusion coefficient in the 93% Li β–alumina system.

  15. Theoretical modeling of the linear and nonlinear optical properties of organic crystals within the rigorous local field theory (RLFT)

    SciTech Connect

    Seidler, T.; Stadnicka, K.; Champagne, B.

    2015-03-30

    This contribution summarizes our current findings in the field of calculating and predicting the linear and second-order nonlinear electric susceptibility tensor components of organic crystals. The methodology used for this purpose is based on a combination of the electrostatic interaction scheme developed by Munn and his coworkers (RLFT) with high-level electronic structure calculations. We compare the results of calculations with available experimental data for several examples of molecular crystals. We show the quality of the final results is influenced by i) the chromophore geometry, ii) the method used for molecular properties calculations and iii) the partitioning scheme used. In conclusion we summarize further plans to improve the reliability and predictability of the method.

  16. Nuclear spin-lattice relaxation at field-induced level crossings in a Cr8F8 pivalate single crystal

    NASA Astrophysics Data System (ADS)

    Yamamoto, Shoji

    2016-01-01

    We construct a microscopic theory for the proton spin-lattice relaxation-rate 1 / T1 measurements around field-induced level crossings in a single crystal of the trivalent chromium ion wheel complex [Cr8F8(OOCtBu)16] at sufficiently low temperatures [E. Micotti et al., Phys. Rev. B 72 (2005) 020405(R)]. Exactly diagonalizing a well-equipped spin Hamiltonian for the individual clusters and giving further consideration to their possible interactions, we reveal the mechanism of 1 / T1 being single-peaked normally at the first level crossing but double-peaked intriguingly around the second level crossing. We wipe out the doubt about poor crystallization and find out a solution-intramolecular alternating Dzyaloshinsky-Moriya interaction combined with intermolecular coupling of antiferromagnetic character, each of which is so weak as several tens of mK in magnitude.

  17. Superposition model calculation of zero-field splitting of Fe3+ in LiTaO3 crystal

    NASA Astrophysics Data System (ADS)

    Yeom, T. H.

    2001-11-01

    The second-order zero-field splitting (ZFS) parameter b20 of the Fe3+ ion centre at the Li site, the Ta site and the structural vacancy site in the LiTaO3 crystal are calculated using the empirical superposition model. The fourth-order ZFS parameters b40, b43 and b4-3 are also calculated at the Li and Ta site, respectively. The calculated b20 of Fe3+ ion at the Li site agrees well with the experimental one. It is concluded that the Fe3+ replaces the Li+ ion rather than the Ta5+ ion in the LiTaO3 crystal. This conclusion confirms the site assignment from the electron nuclear double resonance experiments.

  18. Research on temperature field of KDP crystal under ion beam cleaning.

    PubMed

    Li, Furen; Xie, Xuhui; Tie, Guipeng; Hu, Hao; Zhou, Lin

    2016-06-20

    KH2PO4 (KDP) crystal is a kind of excellent nonlinear optical component used as a laser frequency conversion unit in a high-power laser system. However, KDP crystal has raised a huge challenge in regards to its fabrication for high precision: KDP crystal has special physical and chemical characteristics. Abrasive-free water-dissolution magnetorheological finishing is used in KDP figuring in our lab. But the iron powders of MRF fluid are easily embedded into the soft surface of KDP crystal, which will greatly decrease the laser-induced damage resistance. This paper proposes to utilize ion beam figuring (IBF) technology to figure and clean the surface of a KDP component. Although IBF has many good performances, the thermal effect control is a headachy problem for the KDP process. To solve this problem, we have established its thermal effect models, which are used to calculate a component's surface temperature and thermal gradient in the whole process. By this way, we can understand how to control a temperature map and its gradient in the IBF process. Many experiments have been done to validate and optimize this method. Finally, a KDP component with the size of 200×200×12  mm is successfully processed by this method. PMID:27409114

  19. Numerical model of protein crystal growth in a diffusive field such as the microgravity environment.

    PubMed

    Tanaka, Hiroaki; Sasaki, Susumu; Takahashi, Sachiko; Inaka, Koji; Wada, Yoshio; Yamada, Mitsugu; Ohta, Kazunori; Miyoshi, Hiroshi; Kobayashi, Tomoyuki; Kamigaichi, Shigeki

    2013-11-01

    It is said that the microgravity environment positively affects the quality of protein crystal growth. The formation of a protein depletion zone and an impurity depletion zone due to the suppression of convection flow were thought to be the major reasons. In microgravity, the incorporation of molecules into a crystal largely depends on diffusive transport, so the incorporated molecules will be allocated in an orderly manner and the impurity uptake will be suppressed, resulting in highly ordered crystals. Previously, these effects were numerically studied in a steady state using a simplified model and it was determined that the combination of the diffusion coefficient of the protein molecule (D) and the kinetic constant for the protein molecule (β) could be used as an index of the extent of these depletion zones. In this report, numerical analysis of these depletion zones around a growing crystal in a non-steady (i.e. transient) state is introduced, suggesting that this model may be used for the quantitative analysis of these depletion zones in the microgravity environment. PMID:24121357

  20. Observation of nonlinear bands in near-field scanning optical microscopy of a photonic-crystal waveguide

    SciTech Connect

    Singh, A.; Huisman, S. R.; Ctistis, G. Mosk, A. P.; Pinkse, P. W. H.; Korterik, J. P.; Herek, J. L.

    2015-01-21

    We have measured the photonic bandstructure of GaAs photonic-crystal waveguides with high resolution in energy as well as in momentum using near-field scanning optical microscopy. Intriguingly, we observe additional bands that are not predicted by eigenmode solvers, as was recently demonstrated by Huisman et al. [Phys. Rev. B 86, 155154 (2012)]. We study the presence of these additional bands by performing measurements of these bands while varying the incident light power, revealing a non-linear power dependence. Here, we demonstrate experimentally and theoretically that the observed additional bands are caused by a waveguide-specific near-field tip effect not previously reported, which can significantly phase-modulate the detected field.

  1. Observation of nonlinear bands in near-field scanning optical microscopy of a photonic-crystal waveguide

    NASA Astrophysics Data System (ADS)

    Singh, A.; Ctistis, G.; Huisman, S. R.; Korterik, J. P.; Mosk, A. P.; Herek, J. L.; Pinkse, P. W. H.

    2015-01-01

    We have measured the photonic bandstructure of GaAs photonic-crystal waveguides with high resolution in energy as well as in momentum using near-field scanning optical microscopy. Intriguingly, we observe additional bands that are not predicted by eigenmode solvers, as was recently demonstrated by Huisman et al. [Phys. Rev. B 86, 155154 (2012)]. We study the presence of these additional bands by performing measurements of these bands while varying the incident light power, revealing a non-linear power dependence. Here, we demonstrate experimentally and theoretically that the observed additional bands are caused by a waveguide-specific near-field tip effect not previously reported, which can significantly phase-modulate the detected field.

  2. Specific features of attenuated light transmission by liquid-crystal twist cells in constant and alternating electric fields

    NASA Astrophysics Data System (ADS)

    Konshina, E. A.; Amosova, L. P.

    2012-07-01

    Optical transmission characteristics of dual-frequency nematic liquid crystal (NLC) twist cells with different alignment layers (rubbed polyimide and obliquely deposited cerium dioxide) have been studied in constant and alternating electric fields. It has been established that a change in the optical (twist effect) threshold and dynamic range of attenuated transmission depend both on the boundary conditions (that influence the screening of applied voltage) and on the parameters of the applied electric field. The maximum dynamic range (49.5 dB) has been obtained in the cell with a CeO2 alignment layer controlled by a constant potential. In the case of an alternating electric field, the dynamic range decreases because of reduced effective voltage.

  3. Study of crystal-field excitations and infrared active phonons in the multiferroic hexagonal DyMnO3

    NASA Astrophysics Data System (ADS)

    Jandl, S.; Mansouri, S.; Vermette, J.; Mukhin, A. A.; Ivanov, V. Yu; Balbashov, A.; Orlita, M.

    2013-11-01

    In hexagonal DyMnO3, Dy3+ crystal-field excitations are studied as a function of temperature and applied magnetic field. They are complemented with the measurements of infrared active phonon frequency shifts under applied magnetic field at T = 4.2 K. Between TN = 68 K and T = 10 K, the absence of Dy3+ Kramers doublet splittings at either the C3 or the C3v site symmetries indicates that the Mn3+ magnetic order effective exchange field has no component parallel to the c-axis at either site. Below T = 10 K, the ground state Kramers doublet splits under the Dy3+ internal effective field as well as the applied magnetic field. Also, relatively strong infrared active phonon energy shifts are observed in magneto-infrared reflectance measurements at T = 4.2 K, allowing the calculation of the induced electric polarization changes as a function of the applied magnetic field. Such changes are associated with a large magnetoelectric effect in DyMnO3, arising from a charge transfer between Dy3+ and apical oxygen ions.

  4. The Fano-type transmission and field enhancement in heterostructures composed of epsilon-near-zero materials and truncated photonic crystals

    SciTech Connect

    Zhang, Zhi-fang; Jiang, Hai-tao E-mail: jiang-haitao@tongji.edu.cn; Li, Yun-hui; Chen, Hong; Xue, Chun-hua E-mail: jiang-haitao@tongji.edu.cn; Lu, Hai

    2013-11-11

    The Fano-type interference effect is studied in the heterostructure composed of an epsilon-near-zero (ENZ) material and a truncated photonic crystal for transverse magnetic polarized light. In the Fano-type interference effect, the ENZ material provides narrow reflection pathway and the photonic crystal provides broadband reflection pathway. The boundary condition across the ENZ interface and the confinement effect provided by the photonic crystal can enhance the electric fields in the ENZ material greatly. The field enhancements, together with the asymmetric property of Fano-type spectrum, possess potential applications for significantly lowering the threshold of nonlinear processes such as optical switching and bistability.

  5. Difference-frequency generation in the field of a few-cycle laser pulse propagating in a GaAs crystal with a domain structure

    SciTech Connect

    Oganesyan, David L; Vardanyan, Aleksandr O; Oganesyan, G D

    2013-06-30

    Difference-frequency generation in a GaAs crystal with a periodic domain structure in the field of a few-cycle laser pulse is considered for the case of weakly pronounced material dispersion. The straight-line method is used to solve numerically the system of coupled nonlinear partial differential equations describing the evolution of the electric field of this laser pulse in GaAs crystals with periodic and chirped domain structures. It is shown that application of a GaAs crystal with a chirped domain structure makes it possible to control the frequency-modulation law for a broadband differencefrequency pulse. (nonlinear optical phenomena)

  6. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2

    DOE PAGESBeta

    Haravifard, S.; Graf, D.; Feiguin, A. E.; Batista, C. D.; Lang, J. C.; Silevitch, D. M.; Srajer, G.; Gaulin, B. D.; Dabkowska, H. A.; Rosenbaum, T. F.

    2016-06-20

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. In conclusion, themore » magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems.« less

  7. Control of photoluminescence of CdSe/ZnS quantum dots in a nematic liquid crystal by an electric field

    NASA Astrophysics Data System (ADS)

    Kurochkina, M. A.; Shcherbinin, D. P.; Konshina, E. A.

    2015-11-01

    Photoluminescence (PL) spectra of a nematic liquid crystal suspension doped with 10 wt % CdSe/ZnS semiconductor quantum dots (QDs) with a core size of 3.5 nm in a 20-µm thick plane-parallel cell have been investigated. The spectrum of a homogeneously oriented suspension layer exhibits a significant shift of the PL peak by ~83 nm to a wavelength of 676 nm as compared with the QD spectrum in toluene. The application of a dc electric field with a strength of 0.25 V/µm to the cell leads to a slight hypsochromic shift of this peak, the position of which does not change with a further increase in the field strength up to 2 V/µm. The PL intensity changes exponentially and decreases by a factor of two at the field strength of 1 V/µm. The mechanism of PL quenching of QDs in a homogeneously oriented layer of nematic liquid crystal is discussed.

  8. GV /m Single-Cycle Terahertz Fields from a Laser-Driven Large-Size Partitioned Organic Crystal

    NASA Astrophysics Data System (ADS)

    Vicario, Carlo; Monoszlai, Balazs; Hauri, Christoph P.

    2014-05-01

    We report on compact and efficient laser-based THz generation in the terahertz frequency gap (1-10 THz). The radiation is generated by optical rectification of a midinfrared laser in a large-size, partitioned nonlinear organic crystal assembly. This enables up-scaling of presently field-limited tabletop THz sources to GV /m electric and several tesla magnetic field at millijoule pulse energy. In agreement with simulations, the THz beam properties at focus are shown to be not deteriorated by the discontinuity of the emitter surface. The high laser-to-THz energy conversion efficiency exceeds the Manley-Rowe limit and is explained by a cascaded χ(2) process in the organic crystals accompanied by a significant redshift of the pump spectrum. The scheme provides a compact, tabletop THz source for single-cycle transients at field strength equivalent or even higher to linear accelerator and FEL-based THz sources. This opens an avenue toward novel nonlinear THz applications.

  9. Magnetization behavior of RE123 bulk magnets bearing twin seed-crystals in pulsed field magnetization processes

    NASA Astrophysics Data System (ADS)

    Oka, T.; Miyazaki, T.; Ogawa, J.; Fukui, S.; Sato, T.; Yokoyama, K.; Langer, M.

    2016-02-01

    Melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were fabricated by the cold seeding method with using single or twin-seed crystals composed of Nd-Ba-Cu-O thin films on MgO substrates. The behavior of the magnetic flux penetration into anisotropic-grown bulk magnets thus fabricated was precisely evaluated during and after the pulsed field magnetization operated at 35 K. These seed crystals were put on the top surfaces of the precursors to grow large grains during the melt-processes. Although we know the magnetic flux motion is restricted by the enhanced pinning effect in temperature ranges lower than 77 K, we observed that flux invasion occurred at applied fields of 3.3 T when the twin seeds were used. This is definitely lower than those of 3.7 T when the single-seeds were employed. This means that the magnetic fluxes are capable of invading into twin-seeded bulk magnets more easily than single-seeded ones. The twin seeds form the different grain growth regions, the narrow-GSR (growth sector region) and wide-GSR, according to the different grain growth directions which are parallel and normal to the rows of seed crystals, respectively. The invading flux measurements revealed that the magnetic flux invades the sample from the wide-GSR prior to the narrow-GSR. It suggests that such anisotropic grain growth leads to different distributions of pinning centers, variations of J c values, and the formation of preferential paths for the invading magnetic fluxes. Using lower applied fields definitely contributed to lowering the heat generation during the PFM process, which, in turn, led to enhanced trapped magnetic fluxes.

  10. Defining the Magnetic Field of the Early Earth Through Rock Magnetic and Paleomagnetic Analyses of Single Silicate Crystals

    NASA Astrophysics Data System (ADS)

    Bauch, D. G.; Tarduno, J. A.; Cottrell, R. D.; Watkeys, M. K.

    2005-12-01

    The current uncertainty on the age of the inner core, and its role in the geodynamo, highlights the need for improved paleomagnetic constraints based on Proterozoic to Archean-age rocks. However, most of the rocks available for sampling have seen low-grade metamorphic conditions; extreme care is needed in selecting suitable samples, conducting rock magnetic and paleomagnetic analyses, and interpreting the results. David Dunlop's many contributions in rock magnetism, from efforts to understand the time-temperature characteristics crucial for the preservation of magnetizations, to more recent work defining the domain state and recording characteristics of mafic minerals separated from dikes, have greatly assisted our efforts to learn more about the early magnetic field. Here we present new rock magnetic, paleomagnetic and paleointensity data from single silicate crystals separated from plutonic rocks of the Kaapvaal Craton of southern Africa. Magnetic hysteresis data demonstrates that different silicate minerals from these rocks have magnetic inclusions with vastly different magnetic domain states, suggesting that their potential to preserve primary magnetizations should vary considerably. In particular hornblende carries multidomain inclusions, whereas quartz and microcline have single to pseudo-single domain inclusions. Warming of an SIRM acquired at low temperatures (data acquired using the MPMS at the IRM) shows the Verwey transition for quartz and microcline crystals, indicating the presence of magnetite. We also will present joint paleomagnetic and paleointensity data derived from oriented crystals obtained using a stepwise CO2 laser heating approach, and field tests of the age of magnetization. These analyses will be used to discuss the strength of the mid-Archean field (3.0-3.6 Ga), its geometry and variation, and the implications for magnetic shielding in the early Earth.

  11. Optical characterization and crystal field calculations for some erbium based solid state materials for laser refrigeration

    NASA Astrophysics Data System (ADS)

    Hasan, Z.; Qiu, Z.; Johnson, Jackie; Homerick, Uwe

    2009-02-01

    The potential of three erbium based solids hosts has been investigated for laser cooling. Absorption and emission spectra have been studied for the low lying IR transitions of erbium that are relevant to recent reports of cooling using the 4I15/2-4I9/2 and4I15/2 -4I13/2 transitions. Experimental studies have been performed for erbium in three hosts; ZBLAN glass and KPb2Cl5 and Cs2NaYCl6 crystals. In order to estimate the efficiencies of cooling, theoretical calculations have been performed for the cubic Elpasolite (Cs2NaYCl6 ) crystal. These calculations also provide a first principle insight into the cooling efficiency for non-cubic and glassy hosts where such calculations are not possible.

  12. Structural field of K2Al2B2O7-family crystals

    NASA Astrophysics Data System (ADS)

    Atuchin, Victor V.; Bazarov, Bair G.; Grossman, Victoria G.; Molokeev, Maxim S.; Bazarova, Zhibzema G.

    2013-05-01

    The known crystal structures of borate compounds with general composition AI 2MIII 2B2O7 (AI: Na, K, Cs, Rb) have been considered. The structures in space groups P63/m, P-31c, P321 and P21/c have been found depending on cation combination. Criterion k = (r(MIII)+r(B)+2r(O))/r(AI) has been formulated and it controls the formation of trigonal K2Al2B2O7 (KABO) type structure. The existence of noncentrosymmetric KABO-type borates is possible over the range of k = 2.7 - 2.06. This range is promising for searching new borate crystals and solid solutions with pronounced nonlinear optical properties.

  13. Field-induced Bragg diffraction in polymer stabilized cholesteric liquid crystal bubbles

    NASA Astrophysics Data System (ADS)

    Varanytsia, Andrii; Chien, Liang-Chy

    2015-03-01

    Cholesteric liquid crystals (CLC) with a specific confinement conditions are known to form bubble domain (BD) texture. We have developed the CLC BD texture stabilized with a small amount of polymer. CLC bubbles of a BD texture self-assemble into domains with a hexagonal ordering and optically perform as a diffraction grating. By stabilization of the BD texture with a polymer we have improved optical quality of the diffractive CLC layer and have increased its mechanical stability. We discuss details about samples preparation, Bragg diffraction, electro-optical performance and present results of scanning electron microscopy (SEM) morphological study of the polymer network formed in the bulk of the diffractive liquid crystal layer.

  14. Organic crystal polymorphism: a benchmark for dispersion-corrected mean-field electronic structure methods.

    PubMed

    Brandenburg, Jan Gerit; Grimme, Stefan

    2016-08-01

    We analyze the energy landscape of the sixth crystal structure prediction blind test targets with various first principles and semi-empirical quantum chemical methodologies. A new benchmark set of 59 crystal structures (termed POLY59) for testing quantum chemical methods based on the blind test target crystals is presented. We focus on different means to include London dispersion interactions within the density functional theory (DFT) framework. We show the impact of pairwise dispersion corrections like the semi-empirical D2 scheme, the Tkatchenko-Scheffler (TS) method, and the density-dependent dispersion correction dDsC. Recent methodological progress includes higher-order contributions in both the many-body and multipole expansions. We use the D3 correction with Axilrod-Teller-Muto type three-body contribution, the TS based many-body dispersion (MBD), and the nonlocal van der Waals density functional (vdW-DF2). The density functionals with D3 and MBD correction provide an energy ranking of the blind test polymorphs in excellent agreement with the experimentally found structures. As a computationally less demanding method, we test our recently presented minimal basis Hartree-Fock method (HF-3c) and a density functional tight-binding Hamiltonian (DFTB). Considering the speed-up of three to four orders of magnitudes, the energy ranking provided by the low-cost methods is very reasonable. We compare the computed geometries with the corresponding X-ray data where TPSS-D3 performs best. The importance of zero-point vibrational energy and thermal effects on crystal densities is highlighted. PMID:27484372

  15. Te inclusion-induced electrical field perturbation in CdZnTe single crystals revealed by Kelvin probe force microscopy.

    PubMed

    Gu, Yaxu; Jie, Wanqi; Li, Linglong; Xu, Yadong; Yang, Yaodong; Ren, Jie; Zha, Gangqiang; Wang, Tao; Xu, Lingyan; He, Yihui; Xi, Shouzhi

    2016-09-01

    To understand the effects of tellurium (Te) inclusions on the device performance of CdZnTe radiation detectors, the perturbation of the electrical field in and around Te inclusions was studied in CdZnTe single crystals via Kelvin probe force microscopy (KPFM). Te inclusions were proved to act as lower potential centers with respect to surrounding CdZnTe matrix. Based on the KPFM results, the energy band diagram at the Te/CdZnTe interface was established, and the bias-dependent effects of Te inclusion on carrier transportation is discussed. PMID:27376976

  16. On a low-frequency relaxation in (K, Li)TaO3 crystals: an influence of the bias field

    NASA Astrophysics Data System (ADS)

    Turik, A. V.

    2002-01-01

    The low-frequency or π dielectric relaxation in (K, Li)TaO3 crystals is considered. An analysis of available experimental data on the influence of the bias field on the relaxation frequency enabled a value of the dipole moment of flipping polar elements to be determined for the first time. It is affirmed that they are most probably pairs of rigidly coupled dipoles (dimers) created by off-centre Li ions. The asymmetry changing the Arrhenius law for the mean relaxation time is emphasized and discussed. The dimer concentration and the dimer contribution to the dielectric permittivity are estimated.

  17. A Mean-Field Photoreaction Model for the Pretilt Generation of a Liquid Crystal on Photopolymer Layers upon Ultraviolet Exposure

    NASA Astrophysics Data System (ADS)

    Na, Jun-Hee; Pae, Hyungwu; Kim, Jinyool; Yu, Chang-Jae; Lee, Sin-Doo

    2011-03-01

    We present a mean-field photoreaction model for the pretilt generation of a nematic liquid crystal (NLC) on the surfaces of photopolymers upon the exposure of ultraviolet (UV) light. The angular distribution function of photopolymer side chains, calculated in the photoreaction model, is used for determining the surface anchoring energy within the Rapini-Papoular approximation. The pretilt angle transition from the homeotropic alignment to the planar alignment of the NLC is demonstrated in two photopolymers with different alkyl chain lengths as a function of the UV exposure time. The main features of the experimental results agree well with theoretical predictions.

  18. Crystalline sulfur dioxide: Crystal field splittings, absolute band intensities, and complex refractive indices derived from infra-red spectra

    NASA Astrophysics Data System (ADS)

    Khanna, R. K.; Zhao, Guizhi; Ospina, M. J.; Pearl, J. C.

    The infra-red absorption spectra of thin crystalline films of sulfur dioxide at 90 K are reported in the 2700-450 cm -1 region. The observed multiplicity of the bands in the regions of fundamental modes is attributed to crystal field effects, including factor group and LO—TO splittings, and naturally present minor 34S, 36S and 18O substituted isotopic species. Complex refractive indices determined by an iterative Kramers—Kronig analysis of the extinction data, and absolute band strengths derived from them, are also reported in this region.

  19. Near infrared Kerr effect and description of field-induced phase transitions in polymer-stabilized blue phase liquid crystals

    NASA Astrophysics Data System (ADS)

    Atorf, B.; Rasouli, H.; Nordendorf, G.; Wilkes, D.; Kitzerow, H.

    2016-02-01

    Studies of the influence of an electric field E on the effective refractive index of a polymer-stabilized blue phase in the near infrared spectral range reveal a considerable field-induced birefringence. At moderate voltages, the birefringence increases linearly with the square of the field strength as expected for the electro-optic Kerr effect, with an effective Kerr constant of K ≈ 6.3 - 6.9 × 10-10 m V-2. However, for E > ≈7.3 V/μm, the slope of the field-induced birefringence versus E2 increases abruptly, before saturation is reached at E > ≈8.5 V/μm. Based on previous observations on blue phases in the visible wavelength range, the discontinuous change can be attributed to a field-induced phase transition. A modification of the extended Kerr model introduced by Wu and coworkers is suggested to take this additional effect into account. In addition to the promising properties of blue phases for improved liquid crystal displays, the observed field-induced birefringence in the infrared region opens interesting perspectives for telecommunication and other non-display applications.

  20. Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric iInterface control and very thin single crystal.

    PubMed

    Dong, Ji; Yu, Peng; Arabi, Syeda Atika; Wang, Jiawei; He, Jun; Jiang, Chao

    2016-07-01

    A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a 'seed-controlled' pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm(2) V(-1) s(-1)-more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices. PMID:27211506

  1. 3D mathematical model system for melt hydrodynamics in the silicon single crystal FZ-growth process with rotating magnetic field

    NASA Astrophysics Data System (ADS)

    Lacis, K.; Muiznieks, A.; Ratnieks, G.

    2005-06-01

    A system of three-dimensional numerical models is described to analyse the melt hydrodynamics in the floating zone crystal growth by the needle-eye technique under a rotating magnetic field for the production of high quality silicon single crystals of large diameters big( 100dots 200 mm big). Since the pancake inductor has only one turn, the high frequency (HF) electromagnetic (EM) field and the distribution of heat sources and EM forces on the melt free surface have distinct asymmetric features. This asymmetry together with the displacement of the crystal and feed rod axis and crystal rotation manifests itself as three dimensional hydrodynamic, thermal and dopant concentration fields in the molten zone and causes variations of resistivity in the grown single crystal, which are known as the so-called rotational striations. Additionally, the rotating magnetic field can be used to influence the melt hydrodynamics and to reduce the flow asymmetry. In the present 3D model system, the shape of the molten zone is obtained from symmetric FZ shape calculations. The asymmetric HF EM field is calculated by the 3D boundary element method. The low-frequency rotating magnetic field and a corresponding force density distribution in the melt are calculated by the 3D finite element method. The obtained asymmetric HF field power distribution on the free melt surface, the corresponding HF EM forces and force density of the rotating magnetic field are used for the coupled calculation of 3D steady-state hydrodynamic and temperature fields in the molten zone on a body fitted structured 3D grid by a commercial program package with a control volume approach. Beside the EM forces, also the buoyancy and Marangoni forces are considered. After HD calculations a corresponding 3D dopant concentration field is calculated and used to derive the variations resistivity in the grown crystal. The capability of the system of models is illustrated by a calculation example of a realistic FZ system

  2. Crystal-field states of Kondo lattice heavy fermions CeRuSn3 and CeRhSn3

    NASA Astrophysics Data System (ADS)

    Anand, V. K.; Adroja, D. T.; Britz, D.; Strydom, A. M.; Taylor, J. W.; Kockelmann, W.

    2016-07-01

    Inelastic neutron scattering experiments have been carried out to determine the crystal-field states of the Kondo lattice heavy fermions CeRuSn3 and CeRhSn3. Both the compounds crystallize in LaRuSn3-type cubic structure (space group P m 3 ¯n ) in which the Ce atoms occupy two distinct crystallographic sites with cubic (m 3 ¯ ) and tetragonal (4 ¯m .2 ) point symmetries. The INS data of CeRuSn3 reveal the presence of a broad excitation centered around 6-8 meV, which is accounted by a model based on crystal electric field (CEF) excitations. On the other hand, the INS data of isostructural CeRhSn3 reveal three CEF excitations around 7.0, 12.2, and 37.2 meV. The neutron intensity sum rule indicates that the Ce ions at both cubic and tetragonal Ce sites are in Ce3 + state in both CeRuSn3 and CeRhSn3. The CEF level schemes for both the compounds are deduced. We estimate the Kondo temperature TK=3.1 (2 ) K for CeRuSn3 from neutron quasielastic linewidth in excellent agreement with that determined from the scaling of magnetoresistance which gives TK=3.2 (1 ) K. For CeRhSn3, the neutron quasielastic linewidth gives TK≈4.6 K. For both CeRuSn3 and CeRhSn3, the ground state of Ce3 + turns out to be a quartet for the cubic site and a doublet for the tetragonal site.

  3. Phase coexistence and transformations in field-cooled ternary piezoelectric single crystals near the morphotropic phase boundary

    NASA Astrophysics Data System (ADS)

    Luo, Chengtao; Wang, Yaojin; Wang, Zhiguang; Ge, Wenwei; Li, Jiefang; Luo, Haosu; Viehland, D.

    2014-12-01

    Structural phase transformations in (100)-oriented Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals have been investigated by X-ray diffraction. A cubic (C) → tetragonal (T) → monoclinic-C (MC) transformation sequence was observed in the field-cooled condition. Two phase coexistence regions of C + T and T + MC were found. In addition to an increase in the C → T phase transition temperature and a decrease of the T → MC one, a broadening of the coexistence regions was also found with increasing field. This broadening can be explained by the presence of polar nano regions within the C, T, and MC phase regions.

  4. Phase diagram of dipolar hard and soft spheres: manipulation of colloidal crystal structures by an external field.

    PubMed

    Hynninen, Antti-Pekka; Dijkstra, Marjolein

    2005-04-01

    Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field. PMID:15904046

  5. Domain Motion of Ferroelectricity of Bi2SrTa2O9 Single Crystals under an AC-Voltage Electric Field

    NASA Astrophysics Data System (ADS)

    Machida, Akio; Nagasawa, Naomi; Ami, Takaaki; Suzuki, Masayuki

    1999-02-01

    A novel phenomenon, which increases the remanent polarization of Bi2SrTa2O9 single crystals, a promising candidate for ferroelectric random access memories (FeRAM), has been identified. The single crystals, grown in vapor phases using the self-flux method, have a composition characterized asBixSryTa2O9 (x=2.08±0.09, y=1.04±0.06). Incontrast to BixSryTa2O9 (x=1.91±0.05, y=1.27±0.08) single crystals grown by the self-flux method, the coercive field of the present single crystals is smaller. Observing optical anisotropy in the c-plane, we found that this material has a paraelectric phase, which might originate from the partial distortion of the crystal. After voltage was applied, the paraelectric phase disappeared and the crystal became a ferroelectric domain structure. Measuring the electrical properties in the c-plane, the remanent polarization of the Bi2SrTa2O9 single crystal was increased by applying ac-voltage. One-hour annealing over the Curie temperature also produced a paraelectric phase in the crystal but it was confirmed that this paraelectric phase can also be decreased by applying ac-voltage. Using this ac-voltage application, we can clearly observe the domain structure of BiSTa single crystal for the first time.

  6. Resolving the spin reorientation and crystal-field transitions in TmFeO3 with terahertz transient

    NASA Astrophysics Data System (ADS)

    Zhang, Kailin; Xu, Kai; Liu, Xiumei; Zhang, Zeyu; Jin, Zuanming; Lin, Xian; Li, Bo; Cao, Shixun; Ma, Guohong

    2016-03-01

    Rare earth orthoferrites (RFeO3) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO3 single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61, and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (6H3) of Tm3+ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields.

  7. Resolving the spin reorientation and crystal-field transitions in TmFeO3 with terahertz transient

    PubMed Central

    Zhang, Kailin; Xu, Kai; Liu, Xiumei; Zhang, Zeyu; Jin, Zuanming; Lin, Xian; Li, Bo; Cao, Shixun; Ma, Guohong

    2016-01-01

    Rare earth orthoferrites (RFeO3) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO3 single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61, and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (6H3) of Tm3+ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields. PMID:27009361

  8. Resolving the spin reorientation and crystal-field transitions in TmFeO3 with terahertz transient.

    PubMed

    Zhang, Kailin; Xu, Kai; Liu, Xiumei; Zhang, Zeyu; Jin, Zuanming; Lin, Xian; Li, Bo; Cao, Shixun; Ma, Guohong

    2016-01-01

    Rare earth orthoferrites (RFeO3) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO3 single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61, and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets ((6)H3) of Tm(3+) ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields. PMID:27009361

  9. Random crystal field effect on the kinetic spin-3/2 Blume-Capel model under a time-dependent oscillating field

    NASA Astrophysics Data System (ADS)

    El Hachimi, A. G.; Dakir, O.; Sidi Ahmed, S.; Zaari, H.; El Yadari, M.; Benyoussef, A.; El Kenz, A.

    2016-09-01

    The effect of random crystal-field on the stationary states of the kinetic spin-3/2 Blume-Capel model is investigated within the framework of the mean-field approach. The Glauber-type stochastic dynamics is used to describe the time evolution of the system which is subject to a time-dependent oscillating external magnetic field. In addition to the well-known phase transitions and the appearance of the partly ferromagnetic phase characterized by the magnetization m = 1 in equilibrium case, a new dynamical regions between the ferromagnetic phases F1/2, F1 and F3/2 are found where F3/2 +F 1 / 2 ,F3/2 +F1, F1 +F1/2 phases coexist for a weak value of the reduced magnetic field (h). Whereas for higher value of h both solutions ordered F and disordered P phases coexist. Hence we present six types topologies of phase diagrams which exhibit dynamical first-order, second-order transition lines, dynamical tricritical and isolated critical end points. Furthermore, the dynamical thermal behavior magnetizations, susceptibilities and phase space trajectories are given and discussed.

  10. Crystal-face: A Field Experiment An Modelling Program Focused On Tropical Cirrus

    NASA Astrophysics Data System (ADS)

    Jensen, E.

    The Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) is a measurement campaign designed to investigate tropical cirrus cloud physical properties and formation processes. Understanding the production of upper tropospheric cirrus clouds is essential for the successful modeling of the Earth's climate. The deployment phase will occur in July, 2002 in southern Florida, USA. Several aircraft will be used, including the ER-2 and Proteus for cloud remote sensing, the WB-57 and Citation for in situ cloud measurements, the P-3 with a Doppler radar for characterization of convective systems, and the Twin Otter for sampling of inflow airmasses. In addition, numerous ground-based and satellite remote sensing measure- ments will be contributing. A central focus of the mission is improvement of our ability to model cirrus clouds with numerical models. Several research groups with a variety of model types (cloud- resolving models, mesoscale models, weather-prediction models, and general circu- lation models) will be participating. Our hope is to fully characterize several cumu- lonimbus/cirrus anvil systems that can be used as case studies for testing and improve- ment of the models. The models will be used for investigating cirrus generation and dissipation processes and the sensitivity of tropical cirrus to convective intensity and aerosol properties. Ultimately, we expect this effort to improve our ability to represent tropical cirrus in GCMs. A general description of the CRYSTAL-FACE program will be presented, with an emphasis on the cloud modeling approach.

  11. Electrical Impact of SiC Structural Crystal Defects on High Electric Field Devices

    NASA Technical Reports Server (NTRS)

    Neudeck, Philip G.

    1999-01-01

    Commercial epilayers are known to contain a variety of crystallographic imperfections. including micropipes, closed core screw dislocations. low-angle boundaries, basal plane dislocations, heteropolytypic inclusions, and non-ideal surface features like step bunching and pits. This paper reviews the limited present understanding of the operational impact of various crystal defects on SiC electrical devices. Aside from micropipes and triangular inclusions whose densities have been shrinking towards manageably small values in recent years, many of these defects appear to have little adverse operational and/or yield impact on SiC-based sensors, high-frequency RF, and signal conditioning electronics. However high-power switching devices used in power management and distribution circuits have historically (in silicon experience) demanded the highest material quality for prolonged safe operation, and are thus more susceptible to operational reliability problems that arise from electrical property nonuniformities likely to occur at extended crystal defects. A particular emphasis is placed on the impact of closed-core screw dislocations on high-power switching devices, because these difficult to observe defects are present in densities of thousands per cm,in commercial SiC epilayers. and their reduction to acceptable levels seems the most problematic at the present time.

  12. CRYSTAL-FACE: A Field Experiment and Modeling Program Focused on Tropical Anvils and Cirrus Layers

    NASA Technical Reports Server (NTRS)

    Jenson, Eric; Gore, Warren J. (Technical Monitor)

    2002-01-01

    The Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) is a measurement campaign designed to investigate tropical Cirrus cloud physical properties and formation processes. Understanding the production of upper tropospheric cirrus clouds is essential for the successful modeling of 'he Earth's climate. The deployment phase will occur in July, 2002 in southern Florida, USA. Several aircraft will be used, including the ER-2 and Proteus for cloud remote sensing, the WB-57 and Citation for in situ cloud measurements, the P-3 with a Doppler radar for characterization of convective systems, and the Twin otter for sampling of inflow airmasses. In addition, numerous ground-based and satellite remote sensing measurements will be contributing. A central focus of the mission is improvement of our ability to model cirrus clouds with numerical models. Several research groups with a variety of model types (cloud-resolving models, mesoscale models, weather-prediction models, and general circulation models) will be participating. Our hope is to fully characterize several mulonimbus/cirrus anvil systems that can be used as case studies for testing and improvement of the models. The models will be used for investigating cirrus generation and dissipation processes and the sensitivity of tropical cirrus to convective intensity and aerosol properties. Ultimately, we expect this effort to improve our ability to represent tropical cirrus in GCMs. A general description of the CRYSTAL-FACE program will be presented, with an emphasis on the cloud modeling approach.

  13. Hysteresis loop of a cubic nanowire in the presence of the crystal field and the transverse field

    NASA Astrophysics Data System (ADS)

    Jiang, Wei; Li, Xiao-Xi; Liu, Li-Mei; Chen, Jun-Nan; Zhang, Fan

    2014-03-01

    The effective-field theory with correlations (EFT) has been used to study the various shapes of the hysteresis loop for a ferromagnetic core of spin-1 and a ferromagnetic shell of spin-3/2 with ferrimagnetic interface coupling on a cubic nanowire. The magnetizations and phase diagrams of the nanowire have been investigated in the previous work (J. Magn. Magn. Mater. 324 (2012) 4034-4042). A number of characteristic behaviors are obtained especially for the triple and multiple hysteresis loop patterns for certain values of the system parameters at low temperature. We also examine the effect of the system parameters on coercivity of the nanowire.

  14. Orientational dynamics of a ferronematic liquid crystal in a rotating magnetic field

    SciTech Connect

    Boychuk, A. N. Zakhlevnykh, A. N.; Makarov, D. V.

    2015-09-15

    The behavior of the orientational structure of a ferronematic in a rotating uniform magnetic field is investigated using the continual theory. The time-dependent system of equations describing the dynamics of the ferronematic is derived. The dependences of the angles of rotation of the director and of the magnetization of the ferronematic on the velocity of field rotation are determined for various values of the material parameters. Two regimes (synchronous and asynchronous) of rotation of the ferronematic structure are detected. In the synchronous regime, the director rotates with the frequency of the magnetic field and a constant phase delay. The asynchronous regime is characterized by a time-dependent phase delay. The dependence of the critical angular velocity of magnetic field rotation, which determines the boundary between the synchronous and asynchronous regimes, on the magnetic field strength is derived.

  15. Magnetic assembly and field-tuning of ellipsoidal-nanoparticle-based colloidal photonic crystals.

    PubMed

    Wang, Mingsheng; He, Le; Xu, Wenjing; Wang, Xin; Yin, Yadong

    2015-06-01

    Anisotropic nanostructures provide an additional degree of freedom for tailoring the collective properties of their ensembles. Using Fe@SiO2 nanoellipsoids as anisotropic building blocks, herein we demonstrate a new class of magnetically responsive photonic structures whose photonic properties can be dynamically tuned by controlling the direction of the magnetic fields they are exposed to. These novel photonic structures diffract at a minimum wavelength when the field direction is perpendicular to the incident angle, and a maximum wavelength when the field is switched to parallel direction; and the diffraction intensity reaches maximum values when the fields are either parallel or perpendicular to the incident light, and decreases when the field direction is moved off-angle. PMID:25926272

  16. Upper critical fields in a FeSe0.5Te0.5 superconducting single crystal

    NASA Astrophysics Data System (ADS)

    Velasco-Soto, D.; Rivera-Gómez, F. J.; Santillán-Rodríguez, C. R.; Sáenz-Hernández, R. J.; Botello-Zubiate, M. E.; Matutes Aquino, J. A.

    2013-05-01

    A single crystal with a nominal composition FeSe0.5Te0.5 was obtained by the Bridgman method. A quartz ampulla with the sample inside was vacuum-sealed and maintained at 1050 °C for 37 h to homogenize the sample. Subsequently, the quartz ampulla with the sample was moved with a speed of 2.2 mm/h to a furnace which was at 450 °C. X-ray diffraction confirmed the tetragonal structure of the grown single crystal with the cleavage plane corresponding to the ab plane. Resistance measurements were carried out with magnetic fields from 0 to 9 T, applied parallel to the c axis and ab plane, respectively. A zero-field critical temperature Tc = 14 K was determined. The upper critical field vs. temperature phase diagram was built for temperatures where the resistance drops to 90%, 50%, and 10% of the normal state resistance. The linear extrapolation to T = 0 K gave upper critical fields of 57.2, 51.8, and 46.0 T for Hǁc axis and 109.6, 95.5, and 80.9 T for Hǁab. Applying the Werthamer-Helfand-Hohenberg (WHH) theory, upper critical fields of 39.6, 35.9, and 31.8 T and coherence lengths of 28.8, 30.3, and 32.1 Å were obtained for Hǁc; while for Hǁab, upper critical fields of 51.3, 40.7, and 37.5 T and coherence lengths of 22.3, 26.7, and 31.5 Å were obtained. The value of μ0Hc2/kBTc calculated by the WHH theory exceeds the Pauli limit (1.84 T/K) indicating the unconventional nature of superconductivity. The activation energy U0 has two different rates of change with the applied magnetic field probably due to two different thermal activation mechanisms; the origin of which requires further investigation. A similar behavior is observed in the irreversibility lines.

  17. Local magnetic field study of patterned Bi_2Sr_2CaCu_2Ox and YBa_2Cu_3Ox single crystals

    NASA Astrophysics Data System (ADS)

    Karapetrov, Goran; Kwok, W. K.; Olsson, R. J.; Paulius, L. M.; Crabtree, G. W.

    1998-03-01

    Local magnetic field measurements using GaAs Hall probe arrays were conducted on patterned heavy ion irradiated Bi_2Sr_2CaCu_2Ox and YBa_2Cu_3Ox single crystals. μ - size masks were used in order to form patterns of irradiated and unirradiated arreas on the crystals. Local field distributions and relaxation rates across the boundaries between irradiated and unirradiated areas were measured as a function of applied magnetic field and temperature. The effect of columnar disorder on the static and dynamic characteristics of the vortex phases will be presented.

  18. Narrowing the Zero-Field Tunneling Resonance by Decreasing the Crystal Symmetry of Mn12 Acetate.

    PubMed

    Espín, Jordi; Zarzuela, Ricardo; Statuto, Nahuel; Juanhuix, Jordi; Maspoch, Daniel; Imaz, Inhar; Chudnovsky, Eugene; Tejada, Javier

    2016-07-27

    We report the discovery of a less symmetric crystalline phase of Mn12 acetate, a triclinic phase, resulting from recrystallizing the original tetragonal phase reported by Lis in acetonitrile and toluene. This new phase exhibits the same structure of Mn12 acetate clusters and the same positions of tunneling resonances on the magnetic field as the conventional tetragonal phase. However, the width of the zero-field resonance is at least 1 order of magnitude smaller-can be as low as 50 Oe-indicating very small inhomogeneous broadening due to dipolar and nuclear fields. PMID:27383903

  19. Intrinsically incompatible crystal (ligand) field parameter sets for transition ions at orthorhombic and lower symmetry sites in crystals and their implications

    NASA Astrophysics Data System (ADS)

    Rudowicz, C.; Gnutek, P.

    2010-01-01

    Central quantities in spectroscopy and magnetism of transition ions in crystals are crystal (ligand) field parameters (CFPs). For orthorhombic, monoclinic, and triclinic site symmetry CF analysis is prone to misinterpretations due to large number of CFPs and existence of correlated sets of alternative CFPs. In this review, we elucidate the intrinsic features of orthorhombic and lower symmetry CFPs and their implications. The alternative CFP sets, which yield identical energy levels, belong to different regions of CF parameter space and hence are intrinsically incompatible. Only their ‘images’ representing CFP sets expressed in the same region of CF parameter space may be directly compared. Implications of these features for fitting procedures and meaning of fitted CFPs are categorized into negative: pitfalls and positive: blessings. As a case study, the CFP sets for Tm 3+ ions in KLu(WO 4) 2 are analysed and shown to be intrinsically incompatible. Inadvertent, so meaningless, comparisons of incompatible CFP sets result in various pitfalls, e.g., controversial claims about the values of CFPs obtained by other researchers as well as incorrect structural conclusions or faulty systematics of CF parameters across rare-earth ion series based on relative magnitudes of incompatible CFPs. Such pitfalls bear on interpretation of, e.g., optical spectroscopy, inelastic neutron scattering, and magnetic susceptibility data. An extensive survey of pertinent literature was carried out to assess recognition of compatibility problems. Great portion of available orthorhombic and lower symmetry CFP sets are found intrinsically incompatible, yet these problems and their implications appear barely recognized. The considerable extent and consequences of pitfalls revealed by our survey call for concerted remedial actions of researchers. A general approach based on the rhombicity ratio standardization may solve compatibility problems. Wider utilization of alternative CFP sets in the

  20. Energy levels and crystal field parameters for Nd3+ ions in BaY2F8, LiKYF5, and K2YF5 single crystals.

    PubMed

    Karbowiak, M; Gnutek, P; Rudowicz, C

    2012-02-15

    The available experimental energy levels of Nd(3+) ions doped into single crystals of BaY(2)F(8), LiKYF(5), and K(2)YF(5), which exhibit low site symmetry, are reanalyzed. A combined approach based on the ascent/descent in symmetry (ADS) method, the superposition model (SPM) analysis, and the pseudosymmetry axes method (PAM) is utilized to extract the crystal field (CF) parameters, B(kq), from experimental spectra. Corresponding sets of the free-ion parameters are also fitted. The crystallographic data are used to establish the axis systems most appropriate for approximation of the actual monoclinic C(2) site symmetry to higher orthorhombic D(2) and tetragonal D(4) symmetry used in CF calculations for BaY(2)F(8). Similarly, for triclinic C(1) site symmetry in LiKYF(5) and K(2)YF(5) approximation to monoclinic C(2) and orthorhombic D(2) symmetry for LiKYF(5), whereas the monoclinic C(s) symmetry for K(2)YF(5), are considered. It is shown that the C(2v) approximation used previously for K(2)YF(5):Nd(3+) is not suitable. SPM enables to calculate for the unapproximated and idealized polyhedrons YF(8) in a given ion-host system of the combined coordination factors Sg(k,q) expressed in the modified crystallographic axis system CAS* and approximated symmetry adapted axis systems, respectively. The quantities Sg(k,q) serve as input for PAM calculations for independent determination of the axis system appropriate for higher symmetry approximations. The pseudosymmetry axes represent the axis system that reflects most closely the approximated higher symmetry of the nearest ligands in a paramagnetic complex embodied in the 4th-rank CF parameters. The combined ADS/SPM/PAM approach provides sets of starting CF parameters (CFPs) in well-defined axis systems. Multiple fittings starting from different points in the CF parameter space yield converging solutions, thus increasing the reliability of the final optimized solutions, which may be then considered as the global minima. The

  1. Solidified liquid layer model expands the application fields of quartz crystal microbalance.

    PubMed

    Fu, Long; Chen, Yanyan; Ma, Hongwei

    2012-05-14

    The application of a quartz crystal microbalance (QCM) in liquid is hindered by the complexity of data analysis. Recently, a "solidified liquid layer" (SLL) model has been proposed to simplify the data analysis. Here, missing evidence to support the SLL model is provided: 1) the SLL model is responsive to the density change of the liquid environment, 2) thickness values from the SLL model (T(SLL) ) are in agreement with values measured by ellipsometry. The SLL model predicts that a 0.18 nm change of T(SLL) will lead to a 1 Hz signal, which is the resolution that most commercial QCMs could easily achieve. Using the SLL model, Au-S bond breakage has been successful. Biosensor applications are also being designed according to the SLL model. It is believed that with these results, the SLL model will bring QCM back to the radar screen of scientists. PMID:22492463

  2. Colloidal Stability and Magnetic Field-Induced Ordering of Magnetorheological Fluids Studied with a Quartz Crystal Microbalance

    PubMed Central

    Rodriguez-López, Jaime; Castro, Pedro; de Vicente, Juan; Johannsmann, Diethelm; Elvira, Luis; Morillas, Jose R.; Montero de Espinosa, Francisco

    2015-01-01

    This work proposes the use of quartz crystal microbalances (QCMs) as a method to analyze and characterize magnetorheological (MR) fluids. QCM devices are sensitive to changes in mass, surface interactions, and viscoelastic properties of the medium contacting its surface. These features make the QCM suitable to study MR fluids and their response to variable environmental conditions. MR fluids change their structure and viscoelastic properties under the action of an external magnetic field, this change being determined by the particle volume fraction, the magnetic field strength, and the presence of thixotropic agents among other factors. In this work, the measurement of the resonance parameters (resonance frequency and dissipation factor) of a QCM are used to analyze the behavior of MR fluids in static conditions (that is, in the absence of external mechanical stresses). The influence of sedimentation under gravity and the application of magnetic fields on the shifts of resonance frequency and dissipation factor were measured and discussed in the frame of the coupled resonance produced by particles touching the QCM surface. Furthermore, the MR-fluid/QCM system has a great potential for the study of high-frequency contact mechanics because the translational and rotational stiffness of the link between the surface and the particles can be tuned by the magnetic field. PMID:26690152

  3. Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field.

    PubMed

    Choi, Tae-Hoon; Woo, Jae-Hyeon; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-09-01

    We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs. PMID:27607702

  4. Noise-induced resonance-like phenomena in InP crystals embedded in fluctuating electric fields

    NASA Astrophysics Data System (ADS)

    Persano Adorno, D.; Pizzolato, N.; Spagnolo, B.

    2016-05-01

    We explore and discuss the complex electron dynamics inside a low-doped n-type InP bulk embedded in a sub-THz electric field, fluctuating for the superimposition of an external source of Gaussian correlated noise. The results presented in this study derive from numerical simulations obtained by means of a multi-valley Monte Carlo approach to simulate the nonlinear transport of electrons inside the semiconductor crystal. The electronic noise characteristics are statistically investigated by calculating the correlation function of the velocity fluctuations, its spectral density and the integrated spectral density, i.e. the total noise power, for different values of both amplitude and frequency of the driving oscillating electric field and for different correlation times of the field fluctuations. Our results show that the nonlinear response of electrons is strongly affected by the field fluctuations. In particular, crucially depending on the relationship between the correlation times of the external Gaussian noise and the timescales of complex phenomena involved in the electron dynamical behavior: (i) electrons self-organize among different valleys, giving rise to intrinsic noise suppression; (ii) this cooperative behavior causes the appearance of a resonance-like phenomenon in the noise spectra.

  5. Colloidal Stability and Magnetic Field-Induced Ordering of Magnetorheological Fluids Studied with a Quartz Crystal Microbalance.

    PubMed

    Rodriguez-López, Jaime; Castro, Pedro; de Vicente, Juan; Johannsmann, Diethelm; Elvira, Luis; Morillas, Jose R; Montero de Espinosa, Francisco

    2015-01-01

    This work proposes the use of quartz crystal microbalances (QCMs) as a method to analyze and characterize magnetorheological (MR) fluids. QCM devices are sensitive to changes in mass, surface interactions, and viscoelastic properties of the medium contacting its surface. These features make the QCM suitable to study MR fluids and their response to variable environmental conditions. MR fluids change their structure and viscoelastic properties under the action of an external magnetic field, this change being determined by the particle volume fraction, the magnetic field strength, and the presence of thixotropic agents among other factors. In this work, the measurement of the resonance parameters (resonance frequency and dissipation factor) of a QCM are used to analyze the behavior of MR fluids in static conditions (that is, in the absence of external mechanical stresses). The influence of sedimentation under gravity and the application of magnetic fields on the shifts of resonance frequency and dissipation factor were measured and discussed in the frame of the coupled resonance produced by particles touching the QCM surface. Furthermore, the MR-fluid/QCM system has a great potential for the study of high-frequency contact mechanics because the translational and rotational stiffness of the link between the surface and the particles can be tuned by the magnetic field. PMID:26690152

  6. Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure.

    PubMed

    Byrne, Peter J; Richardson, Patricia J; Chang, John; Kusmartseva, Anna F; Allan, David R; Jones, Anita C; Kamenev, Konstantin V; Tasker, Peter A; Parsons, Simon

    2012-06-18

    The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni⋅⋅⋅Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa. PMID:22615125

  7. Extended parametric gain range in photonic crystal fibers with strongly frequency-dependent field distributions.

    PubMed

    Petersen, Sidsel R; Alkeskjold, Thomas T; Olausson, Christina B; Lægsgaard, Jesper

    2014-08-15

    The parametric gain range of a degenerate four-wave mixing process is determined in the undepleted pump regime. The gain range is considered with and without taking the mode field distributions of the four-wave mixing components into account. It is found that the mode field distributions have to be included to evaluate the parametric gain correctly in dispersion-tailored speciality fibers and that mode profile engineering can provide a way to increase the parametric gain range. PMID:25121901

  8. Effective long-range interlayer interactions and electric-field-induced subphases in ferrielectric liquid crystals

    NASA Astrophysics Data System (ADS)

    Chandani, A. D. L.; Fukuda, Atsuo; Vij, Jagdish K.; Takanishi, Yoichi; Iida, Atsuo

    2016-04-01

    Microbeam resonant x-ray scattering experiments recently revealed the sequential emergence of electric-field-induced subphases (stable states) with exceptionally large unit cells consisting of 12 and 15 smectic layers. We explain the emergence of the field-induced subphases by the quasimolecular model based on the Emelyanenko-Osipov long-range interlayer interactions (LRILIs) together with our primitive way of understanding the frustration in clinicity using the qE number defined as qE=|[R ] -[L ] | /([R ] +[L ] ) ; here [R ] and [L ] refer to the numbers of smectic layers with directors tilted to the right and to the left, respectively, in the unit cell of a field-induced subphase. We show that the model actually stabilizes the field-induced subphases with characteristic composite unit cells consisting of several blocks, each of which is originally a ferrielectric three-layer unit cell stabilized by the LRILIs, but some of which would be modified to become ferroelectric by an applied electric field. In a similar line of thought, we also try to understand the puzzling electric-field-induced birefringence data in terms of the LRILIs.

  9. Magnetic Properties of a Highly Textured Barium Hexa-Ferrite Quasi-Single Crystal and Its Application in Low-Field Biased Circulators

    NASA Astrophysics Data System (ADS)

    Liu, Junliang; Zeng, Yanwei; Su, Zhijuan; Geiler, Michael; Chen, Yajie; Harris, Vincent G.

    2016-06-01

    A highly textured M-type barium hexa-ferrite (BaM) quasi-single crystal was fabricated by a magnetic forming plus liquid participation sintering technique. Its grain orientation degree was determined to be 97.3% with the tile angle no more that 5°. The magnetization behavior from its angular magnetic hysteresis loops was very similar to that of a BaM single crystal. Moreover, the feasibility of practical utilization of the as-fabricated BaM quasi-single crystal in low-field biased circulators was certificated by a simulation method.

  10. Effect of 10-T magnetic fields on structural colors in guanine crystals of fish scales

    NASA Astrophysics Data System (ADS)

    Iwasaka, M.; Miyashita, Y.; Kudo, M.; Kurita, S.; Owada, N.

    2012-04-01

    This work reports the magnetically modulated structural colors in the chromatophore of goldfish scales under static magnetic fields up to 10 T. A fiber optic system for spectroscopy measurements and a CCD microscope were set in the horizontal bore of a 10-T superconducting magnet. One leaf of a fish scale was set in a glass chamber, exposed to visible light from its side direction, and then static magnetic fields were applied perpendicular to the surface of the scale. In addition, an optical fiber for spectroscopy was directed perpendicular to the surface. During the magnetic field sweep-up, the aggregate of guanine thin plates partially showed a rapid light quenching under 0.26 to 2 T; however, most of the thin plates continued to scatter the side-light and showed changing iridescence, which was displayed individually by each guanine plate. For example, an aggregate in the chromatophore exhibited a dynamic change in structural color from white-green to dark blue when the magnetic fields changed from 2 to 10 T. The spectrum profile, which was obtained by the fiber optic system, confirmed the image color changes under magnetic field exposure. Also, a linearly polarized light transmission was measured on fish scales by utilizing an optical polarizer and analyzer. The transmitted polarized light intensities increased in the range of 500-550 nm compared to the intensity at 700 nm during the magnetic field sweep-up. These results indicate that the multi-lamella structure of nano-mirror plates in guanine hexagonal micro-plates exhibit diamagnetically modulated structure changes, and its light interference is affected by strong magnetic fields.

  11. RNA Crystallization

    NASA Technical Reports Server (NTRS)

    Golden, Barbara L.; Kundrot, Craig E.

    2003-01-01

    RNA molecules may be crystallized using variations of the methods developed for protein crystallography. As the technology has become available to syntheisize and purify RNA molecules in the quantities and with the quality that is required for crystallography, the field of RNA structure has exploded. The first consideration when crystallizing an RNA is the sequence, which may be varied in a rational way to enhance crystallizability or prevent formation of alternate structures. Once a sequence has been designed, the RNA may be synthesized chemically by solid-state synthesis, or it may be produced enzymatically using RNA polymerase and an appropriate DNA template. Purification of milligram quantities of RNA can be accomplished by HPLC or gel electrophoresis. As with proteins, crystallization of RNA is usually accomplished by vapor diffusion techniques. There are several considerations that are either unique to RNA crystallization or more important for RNA crystallization. Techniques for design, synthesis, purification, and crystallization of RNAs will be reviewed here.

  12. Unusual Kondo-hole effect and crystal-field frustration in Nd-doped CeRhIn5

    NASA Astrophysics Data System (ADS)

    Rosa, P. F. S.; Oostra, A.; Thompson, J. D.; Pagliuso, P. G.; Fisk, Z.

    2016-07-01

    We investigate single crystals of Ce1 -xNdxRhIn5 by means of x-ray-diffraction, microprobe, magnetic susceptibility, heat capacity, and electrical resistivity measurements. Our data reveal that the antiferromagnetic transition of CeRhIn5, at TNCe=3.8 K, is linearly suppressed with xNd. We associate this effect with the presence of a "Kondo hole" created by Nd substitution. The extrapolation of TNCe to zero temperature, however, occurs at xc˜0.3 , which is below the two-dimensional percolation limit found in Ce1 -xLaxRhIn5 . This result strongly suggests the presence of a crystal-field induced magnetic frustration. Near xNd˜0.2 , the Ising antiferromagnetic order from Nd3 + ions is stabilized and TNNd increases up to 11 K in NdRhIn5. Our results shed light on the effects of magnetic doping in heavy-fermion antiferromagnets and stimulate the study of such systems under applied pressure.

  13. Reservoir geology of Crystal Viking field, Lower Cretaceous estuarine tidal channel-bay complex, south-central Alberta

    SciTech Connect

    Reinson, G.E.; Clark, J.E.; Foscolos, A.E.

    1988-10-01

    The Lower Cretaceous Viking Formation in the Crystal field of south-central Alberta contains a linear sandstone body, as much as 30 m thick, that forms a complicated dual-pool hydrocarbon reservoir. This contrasts sharply with most other Viking reservoirs, which are much thinner, are commonly oriented northwest, and are composed of a single hydrocarbon pool. Thickness, orientation, and pool duality are attributed to the complicated depositional history of the Viking in the Crystal region. The sandstone body is interpreted as a multistage tidal channel-fill deposit within a larger estuarine channel-bay complex, which rests unconformably on inner shelf-lower shoreface facies. A major lowstand of sea level that occurred approximately 97 m.y. ago is believed to be responsible for incisement of the estuarine valley, which was filled during rising sea level. A depositional model of progressive estuarine valley fill under transgressive conditions readily accounts for the occurrence of two hydrodynamically separated oil pools, and also for differences in reservoir continuity and performance trends within the main oil-bearing A pool. Highly productive wells in the main pool correspond to specific channel-fill deposits, or are situated in areas where deposits of successive channel stages are highly superimposed. Conversely, marginally productive wells and poor reservoir communication between producing wells occur in areas where the different channel-stage deposits diverge. 22 figures, 2 tables.

  14. A Validation Study of the General Amber Force Field Applied to Energetic Molecular Crystals

    NASA Astrophysics Data System (ADS)

    Bergh, Magnus; Caleman, Carl

    2016-01-01

    Molecula dynamics is a well-established tool to computationally study molecules. However, to reach predictive capability at the level required for applied research and design, extensive validation of the available force fields is pertinent. Here we present a study of density, isothermal compressibility and coefficients of thermal expansion of four energetic materials (FOX-7, RDX, CL-20 and HMX) based on molecular dynamics simulations with the General Amber Force Field (GAFF), and compare the results to experimental measurements from the literature. Furthermore, we quantify the accuracy of the calculated properties through hydrocode simulation of a typical impact scenario. We find that molecular dynamics simulations with generic and computationally efficient force fields may be used to understand and estimate important physical properties of nitramine-like energetic materials.

  15. Effect of Crystal Fields in Ho1 - xDyxNi2B2 C

    NASA Astrophysics Data System (ADS)

    Lee, W. C.

    2013-03-01

    From the anisotropy and the temperature dependence of magnetic susceptibilities of Ho1 - x Dyx Ni2B2 C system with magnetic field H perpendicular or parallel to c-axis, the crystalline electric field (CEF) effect has been studied and the magnetic exchange interaction constant Jex of rare-earth ions perpendicular to the c-axis estimated for 0 <=x <=1. The crystalline electric field parameter, B02, the first Steven parameter and the most dominant term in this system, are determined from the high-temperature-limit anisotropic Weiss temperatures of the magnetic susceptibilities and there is a broad minimum around x ~ 0.3, where superconducting transition temperature, TC, and Néel temperature, TN, are almost same.

  16. Local hyperfine field vector of positive muon in mono-domain single crystal of antiferromagnetic La2CuO4

    NASA Astrophysics Data System (ADS)

    Torikai, E.; Ishihara, H.; Nagamine, K.; Kitazawa, H.; Tanaka, I.; Kojima, H.; Sulaiman, S. B.; Srinivas, S.; Das, T. P.

    1993-03-01

    By using a monodomain single crystal La2CuO4 grown by the travelling solvent floating zone method, the strength and direction of the local hyperfine field vector at positive muon sites were determined. Combining with the μ+ location determined separately, we found the importance of the local hyperfine field contributions with dipolar symmetry as well as the distant dipolar field from surrounding point dipoles of Cu atomic moments.

  17. Redshift in the optical absorption of ZnO single crystals in the presence of an intense midinfrared laser field.

    PubMed

    Ghimire, Shambhu; DiChiara, Anthony D; Sistrunk, Emily; Szafruga, Urszula B; Agostini, Pierre; DiMauro, Louis F; Reis, David A

    2011-10-14

    We report time-resolved electroabsorption of a weak probe in a 500 μm thick zinc-oxide crystal in the presence of a strong midinfrared pump in the tunneling limit. We observe a substantial redshift in the absorption edge that scales with the cube root of intensity up to 1 TW/cm(2) (0.38 eV cm(2/3) TW(-1/3)) after which it increases more slowly to 0.4 eV at a maximum applied intensity of 5 TW/cm(2). The maximum shift corresponds to more than 10% of the band gap. The change in scaling occurs in a regime of nonperturbative high-order harmonic generation where electrons undergo periodic Bragg scattering from the Brillouin zone boundaries. It also coincides with the limit where the electric field becomes comparable to the ratio of the band gap to the lattice spacing. PMID:22107430

  18. Calculating the role of composition in the anisotropy of solid-liquid interface energy using phase-field-crystal theory

    NASA Astrophysics Data System (ADS)

    Jugdutt, Bernadine A.; Ofori-Opoku, Nana; Provatas, Nikolas

    2015-10-01

    This work uses Ginzburg-Landau theory derived from a recent structural phase-field-crystal model of binary alloys developed by the authors to study the roles of concentration, temperature, and pressure on the interfacial energy anisotropy of a solid-liquid front. It is found that the main contribution to the change in anisotropy with concentration arises from a change in preferred crystallographic orientation controlled by solute-dependent changes in the two-point density correlation function of a binary alloy, a mechanism that leads to such phenomena as solute-induced elastic strain and dislocation-assisted solute clustering. Our results are consistent with experimental observations in recent studies by Rappaz et al. [J. Fife, P. Di Napoli, and M. Rappaz, Metall. Mater. Trans. A 44, 5522 (2013), 10.1007/s11661-013-1912-7]. This is the first PFC work, to our knowledge, to incorporate temperature, pressure, and density into the thermodynamic description of alloys.

  19. Calculating the role of composition in the anisotropy of solid-liquid interface energy using phase-field-crystal theory.

    PubMed

    Jugdutt, Bernadine A; Ofori-Opoku, Nana; Provatas, Nikolas

    2015-10-01

    This work uses Ginzburg-Landau theory derived from a recent structural phase-field-crystal model of binary alloys developed by the authors to study the roles of concentration, temperature, and pressure on the interfacial energy anisotropy of a solid-liquid front. It is found that the main contribution to the change in anisotropy with concentration arises from a change in preferred crystallographic orientation controlled by solute-dependent changes in the two-point density correlation function of a binary alloy, a mechanism that leads to such phenomena as solute-induced elastic strain and dislocation-assisted solute clustering. Our results are consistent with experimental observations in recent studies by Rappaz et al. [J. Fife, P. Di Napoli, and M. Rappaz, Metall. Mater. Trans. A 44, 5522 (2013)]. This is the first PFC work, to our knowledge, to incorporate temperature, pressure, and density into the thermodynamic description of alloys. PMID:26565255

  20. Effect of surface anchoring energy on electro-optic characteristics of a fringe-field switching liquid crystal cell

    NASA Astrophysics Data System (ADS)

    Kim, Jin Hyun; Kang, Wan Seok; Sol Choi, Han; Park, Kiwoong; Lee, Joong Hee; Yoon, Sangho; Yoon, Sukin; Lee, Gi-Dong; Lee, Seung Hee

    2015-11-01

    Surface anchoring strength of the alignment layer on liquid crystal (LC) determines electro-optic characteristics in the LC devices. This paper investigates how azimuthal and polar anchoring strength affects the electro-optic performance of a fringe-field switching (FFS) mode associated with electrode structure, cell gap and dielectric anisotropy of the LC by numerical simulation. Our important findings in the FFS mode are that both azimuthal and polar anchoring energy can considerably affect the operating voltage and also maximum transmittance when using a LC with positive dielectric anisotropy; however, when using a LC with negative dielectric anisotropy only azimuthal anchoring energy affects electro-optic characteristics. The study proposes an optimal design of an alignment layer for maximizing transmittance in the FFS mode.

  1. Combined crystal plasticity and phase-field method for recrystallization in a process chain of sheet metal production

    NASA Astrophysics Data System (ADS)

    Vondrous, Alexander; Bienger, Pierre; Schreijäg, Simone; Selzer, Michael; Schneider, Daniel; Nestler, Britta; Helm, Dirk; Mönig, Reiner

    2015-02-01

    In sheet metal production, a typical process chain contains hot rolling, cold rolling and annealing as a sequence of consecutive processing steps. We investigate the grain structure evolution of body centered cubic low carbon steel and focus on recrystallization, by employing different computational methods which operate across the process chain and across length scales. In particular, we combine finite element crystal plasticity with phase-field simulations to study the effect of deformation of the grain structure by hot and cold rolling on recrystallization during annealing. The overall goal is to represent the most important technological quantities such as texture evolution and the fraction of recrystallization. The results of grain quantities are validated by a comparison of the orientation distribution functions with experimental electron backscatter measurements. The coupling of the simulation methods has shown that the effects of recrystallization can be recovered well, depending on the preceding processing conditions.

  2. Phase-field-crystal modeling of glass-forming liquids: Spanning time scales during vitrification, aging, and deformation

    NASA Astrophysics Data System (ADS)

    Berry, Joel; Grant, Martin

    2014-06-01

    Two essential elements required to generate a glass transition within phase-field-crystal (PFC) models are outlined based on observed freezing behaviors in various models of this class. The central dynamic features of glass formation in simple binary liquids are qualitatively reproduced across 12 orders of magnitude in time by applying a physically motivated time scaling to previous PFC simulation results. New aspects of the equilibrium phase behavior of the same binary model system are also outlined, aging behavior is explored in the moderate and deeply supercooled regimes, and aging exponents are extracted. General features of the elastic and plastic responses of amorphous and crystalline PFC solids under deformation are also compared and contrasted.

  3. Crystal fields in YbInNi4 determined with magnetic form factor and inelastic neutron scattering

    NASA Astrophysics Data System (ADS)

    Severing, A.; Givord, F.; Boucherle, J.-X.; Willers, T.; Rotter, M.; Fisk, Z.; Bianchi, A.; Fernandez-Diaz, M. T.; Stunault, A.; Rainford, B. D.; Taylor, J.; Goremychkin, E.

    2011-04-01

    The magnetic form factor of YbInNi4 has been determined via the flipping ratios R with polarized neutron diffraction, and the scattering function S(Q,ω) was measured in an inelastic neutron scattering experiment. Both experiments were performed with the aim of determining the crystal-field scheme. The magnetic form factor clearly excludes the possibility of a Γ7 doublet as the ground state. The inelastic neutron data exhibit two almost equally strong peaks at 3.2 meV and 4.4 meV which points, in agreement with earlier neutron data, toward a Γ8 quartet ground state. Further possibilities such as a quasiquartet ground state are discussed.

  4. Multiscale modeling of polycrystalline graphene: A comparison of structure and defect energies of realistic samples from phase field crystal models

    NASA Astrophysics Data System (ADS)

    Hirvonen, Petri; Ervasti, Mikko M.; Fan, Zheyong; Jalalvand, Morteza; Seymour, Matthew; Vaez Allaei, S. Mehdi; Provatas, Nikolas; Harju, Ari; Elder, Ken R.; Ala-Nissila, Tapio

    2016-07-01

    We extend the phase field crystal (PFC) framework to quantitative modeling of polycrystalline graphene. PFC modeling is a powerful multiscale method for finding the ground state configurations of large realistic samples that can be further used to study their mechanical, thermal, or electronic properties. By fitting to quantum-mechanical density functional theory (DFT) calculations, we show that the PFC approach is able to predict realistic formation energies and defect structures of grain boundaries. We provide an in-depth comparison of the formation energies between PFC, DFT, and molecular dynamics (MD) calculations. The DFT and MD calculations are initialized using atomic configurations extracted from PFC ground states. Finally, we use the PFC approach to explicitly construct large realistic polycrystalline samples and characterize their properties using MD relaxation to demonstrate their quality.

  5. Elastic Softening in HoFe2Al10 due to the Quadrupole Interaction under an Orthorhombic Crystal Electric Field

    NASA Astrophysics Data System (ADS)

    Kamikawa, Shuhei; Ishii, Isao; Noguchi, Yoshihito; Goto, Hiroki; Fujita, Takahiro K.; Nakagawa, Fumiya; Tanida, Hiroshi; Sera, Masafumi; Suzuki, Takashi

    2016-07-01

    To investigate 4f electronic states in HoFe2Al10 under an orthorhombic crystal electric field (CEF), we measured the specific heat, magnetic susceptibility, magnetization, and elastic modulus of single-crystalline samples. We found elastic softening of the transverse elastic moduli C55 and C66 below 20 and 130 K, respectively. With further decreasing temperature, C66 shows further elastic softening below 5 K. We observed two Schottky peaks in the specific heat at 2.2 and 20 K and small anisotropy of the magnetic susceptibility and magnetization in the paramagnetic region. By analyzing these experimental data, we obtained the CEF parameters of HoFe2Al10. From the analysis, we clarified that the softening of C55 and C66 originates from indirect quadrupole interactions of Ozx and Oxy, and propose that the overall CEF splitting is about 85 K.

  6. Phase-field-crystal modeling of glass-forming liquids: spanning time scales during vitrification, aging, and deformation.

    PubMed

    Berry, Joel; Grant, Martin

    2014-06-01

    Two essential elements required to generate a glass transition within phase-field-crystal (PFC) models are outlined based on observed freezing behaviors in various models of this class. The central dynamic features of glass formation in simple binary liquids are qualitatively reproduced across 12 orders of magnitude in time by applying a physically motivated time scaling to previous PFC simulation results. New aspects of the equilibrium phase behavior of the same binary model system are also outlined, aging behavior is explored in the moderate and deeply supercooled regimes, and aging exponents are extracted. General features of the elastic and plastic responses of amorphous and crystalline PFC solids under deformation are also compared and contrasted. PMID:25019772

  7. An electrically tunable depth-of-field endoscope using a liquid crystal lens as an active focusing element

    NASA Astrophysics Data System (ADS)

    Chen, Hung-Shan; Chen, Ming-Syuan; Lin, Yi-Hsin

    2013-09-01

    An electrically tunable depth-of-field (DOF) endoscope using a liquid crystal lens (LC lens) as an active focusing element is demonstrated. The optical mechanism of the electrically-tunable DOF endoscope adopting a two-mode switching LC lens is introduced. The two-mode switching LC lens provides not only a positive lens power but also a negative lens power. Therefore, we could extend the range of DOF originally from 27 mm ~ 55 mm to 12.4 mm ~ 76.4 mm by using the two-mode switching LC lens as an active focusing element. The detail derivations of the optical mechanism of the endoscopic system adopting a LC lens are invistgated. The more detail experimental results are demonstrated. We believe this study can provide a more detail understanding of an endoscopic system adopting a tunable focusing lens.

  8. Effects of composition and temperature on the large field behavior of [011]{sub C} relaxor ferroelectric single crystals

    SciTech Connect

    Gallagher, John A.; Lynch, Christopher S.; Tian, Jian

    2014-08-04

    The large field behavior of [011]{sub C} cut relaxor ferroelectric lead indium niobate–lead magnesium niobate–lead titanate, xPb(In{sub 1/2}Nb{sub 1/2})O{sub 3}-(1-x-y)Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-yPbTiO{sub 3}, single crystals was experimentally characterized in the piezoelectric d{sub 322}-mode configuration under combined mechanical, electrical, and thermal loading. Increasing the concentration of lead indium niobate and decreasing the concentration of lead titanate in compositions near the morphotropic phase boundary resulted in a decrease of mechanical compliance, dielectric permittivity, and piezoelectric coefficients as well as a shift from a continuous to a discontinuous transformation.

  9. Field Ion Microscopy and Atom Probe Tomography of Metamorphic Magnetite Crystals

    NASA Technical Reports Server (NTRS)

    Kuhlman, K.; Martens, R. L.; Kelly, T. F.; Evans, N. D.; Miller, M. K.

    2001-01-01

    Magnetite has been analysed using Field Ion Microscopy (FIM) and Atom Probe Tomography (APT), highly attractive techniques for the nanoanalysis of geological materials despite the difficulties inherent in analyzing semiconducting and insulating materials. Additional information is contained in the original extended abstract.

  10. Fabrication of graphene field-effect transistor on top of ferroelectric single-crystal substrate

    NASA Astrophysics Data System (ADS)

    Park, Nahee; Kang, Haeyong; Lee, Yourack; Kim, Jeong-Gyun; Kim, Joong-Gyu; Yun, Yoojoo; Park, Jeongmin; Kim, Taesoo; Kim, Jung Ho; Jin, Youngjo; Shin, Yong Seon; Lee, Young Hee; Suh, Dongseok

    2015-03-01

    In the analysis of Graphene field-effect transistor, the substrate material which has the direct contact with Graphene layer plays an important in the device performance. In this presentation, we have tested PMN-PT(i.e.(1-x)Pb(Mg1/3Nb2/3) O3-xPbTiO3) substrate as a gate dielectric of Graphene field-effect transistor. Unlike the case of previously used substrates such as silicon oxide or hexagonal Boron-Nitride(h-BN), the PMN-PT substrate can induce giant amount of surface charge that is directly injected to the attached Graphene layer due to its ferroelectric property. And the hysteresis of polarization versus electric field of PMN-PT can cause the device to show the ferroelectric nonvolatile memory operation. We had successfully fabricated Graphene field-effect transistor using the mechanically exfoliated Graphene layer transferred on the PMN-PT(001) substrate. Unlike the case of mechanical exfoliation on the surface of silicon-oxide or the Poly(methyl methacrylate) (PMMA), the weak adhesion properties between graphene and PMNPT required the pretreatment on PMMA before the exfoliation process. The device performance is analyzed in terms of the effect of ferro- and piezo-electric effect of PMNPT substrate.

  11. Magnetic field induced quantum dot brightening in liquid crystal synergized magnetic and semiconducting nanoparticle composite assemblies

    SciTech Connect

    Amaral, Jose Jussi; Wan, Jacky; Rodarte, Andrea L.; Ferri, Christopher; Quint, Makiko T.; Pandolfi, Ronald J.; Scheibner, Michael; Hirst, Linda S.; Ghosh, Sayantani

    2014-10-22

    The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce ‘meta-materials’ has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.

  12. Magnetic field induced quantum dot brightening in liquid crystal synergized magnetic and semiconducting nanoparticle composite assemblies.

    PubMed

    Amaral, Jose Jussi; Wan, Jacky; Rodarte, Andrea L; Ferri, Christopher; Quint, Makiko T; Pandolfi, Ronald J; Scheibner, Michael; Hirst, Linda S; Ghosh, Sayantani

    2015-01-14

    The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce 'meta-materials' has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications. PMID:25354546

  13. Magnetic field induced quantum dot brightening in liquid crystal synergized magnetic and semiconducting nanoparticle composite assemblies

    DOE PAGESBeta

    Amaral, Jose Jussi; Wan, Jacky; Rodarte, Andrea L.; Ferri, Christopher; Quint, Makiko T.; Pandolfi, Ronald J.; Scheibner, Michael; Hirst, Linda S.; Ghosh, Sayantani

    2014-10-22

    The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce ‘meta-materials’ has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small appliedmore » magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.« less

  14. Enantioselective Crystallization of Sodium Chlorate in the Presence of Racemic Hydrophobic Amino Acids and Static Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Zorzano, María-Paz; Osuna-Esteban, Susana; Ruiz-Bermejo, Marta; Menor-Salván, Cesar; Veintemillas-Verdaguer, Sabino

    2014-06-01

    We study the bias induced by a weak (200 mT) external magnetic field on the preferred handedness of sodium chlorate crystals obtained by slow evaporation at ambient conditions of its saturated saline solution with 20 ppm of added racemic (dl) hydrophobic amino acids. By applying the Fisher test to pairs of experiments with opposing magnetic field orientation we conclude, with a confidence level of 99.7%, that at the water-air interface of this saline solution there is an enantioselective magnetic interaction that acts upon racemic mixtures of hydrophobic chiral amino acids. This interaction has been observed with the three tested racemic hydrophobic amino acids: dl-Phe, dl-Try and dl-Trp, at ambient conditions and in spite of the ubiquitous chiral organic contamination. This enantioselective magnetic dependence is not observed when there is only one handedness of added chiral amino-acid, if the added amino acid is not chiral or if there is no additive. This effect has been confirmed with a double blind test. This novel experimental observation may have implications for our view of plausible initial prebiotic scenarios and of the roles of the geomagnetic field in homochirality in the biosphere.

  15. Vortex Lattice Formation in High Magnetic Fields in an Underdoped Single Crystal of Hg1201 from 17O NMR

    NASA Astrophysics Data System (ADS)

    Lee, Jeongseop; Xin, Yizhou; Halperin, W. P.; Reyes, A. P.; Kuhns, P. L.

    The vortex lattice in HgBa2CuO4+δ forms at a vortex melting temperature, Tv, typically ~40K for underdoped crystals with a hole doping ~ 0.11. We present our results from 17O NMR for investigation of the vortex lattice as a function of external magnetic field up to 30 T and temperature as low as 5 K. The vortex contribution to the NMR linewidth can be separated from inhomogeneous broadening by deconvolution of the normal state spectra which was measured separately above, Tv. The vortex melting temperature was measured for two underdoped samples marked by the onset of extra linewidth broadening due to the inhomogeneous magnetic field distribution from the solid vortex lattice consistent with transverse relaxation measurements. We have found evidence for a change in the vortex lattice symmetry as a function of external fields. This work was supported by the DOE BES under Grant No. DE-FG02-05ER46248 and the NHMFL through the NSF and State of Florida.

  16. Integration of near-field probes and photonic crystal nanocavities for precise and low-loss resonance control

    NASA Astrophysics Data System (ADS)

    Chew, Xiongyeu; Zhou, Guangya; Chau, Fook Siong

    2011-03-01

    Research interest for silicon nanophotonics is a topic of heavy interest currently due to the requirements for high density communications of integrated devices with small footprints in the semiconductor industry. Silicon photonic crystals (PhC) are nanoscale subwavelength periodic structures that possess the capability to induce strong interaction between light and matter. PhC nanocavities utilizes the photonic bandgap effect to trap certain frequencies of light within a small confined region for a diverse range of applications such as enhancement and suppression of spontaneous emission, efficient and compact lasers, add/drop multiplexers, optical filters and sensing etc. In this paper, we describe a mechanically-perturbative near-field probe with a special design shape to achieve low-loss and precise resonance control of PhC nanocavities. One-dimensional (1D) PhC are chosen for our study due to the ease of integrating with low-loss SOI waveguide technology and easy integration with nanomechanical structures. Sub-micron microelectromechanical systems (MEMS/NEMS) technology is introduced as an ideal integration platform with such near-field probe designs due to its capabilities to accurately control fine displacements without the need of bulky equipment such as atomic force microscopy (AFM), scanning near field microscope (SNOM) or highly sensitive piezo-controlled micromanipulator stages. We propose that such near-field probe designs are capable of achieving large resonance spectral shift of up to few nm with high re-configurability, highly accurate actuation displacements, low power consumption, and portability. In this work, we propose an approach utilizing numerical methods to study and characterize the electromagnetic interaction between PhC nanocavities and nanomechanically displaced near-field nano-probes.

  17. Crystal field and magnetization of canted antiferromagnet CoCO3

    NASA Astrophysics Data System (ADS)

    Meshcheryakov, V. F.

    2007-11-01

    The magnetization of the canted antiferromagnet CoCO3 ( T N = 18.1 K) is calculated in the Weiss molecular field approximation taking into account the microscopic state of the Co2+ ion in the entire range of temperatures and magnetic fields. The values of T N, magnetic susceptibility in the basal plane, and ferromagnetic moment were used as parameters. It is shown that the anisotropy of the g factor and of the exchange interaction at low temperatures ( T < 30 K) including the magnetic ordering temperature is correctly described in the Abragam-Pryce approximation. At high temperatures, the g factor increases and becomes isotropic, but it cannot be described using the Abragam-Pryce approximation. The reasons for g factor variation and the magnitude of the magnetic moment are discussed.

  18. Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA]3+ in Aqueous Solution

    PubMed Central

    Blackburn, Octavia A.; Chilton, Nicholas F.; Keller, Katharina; Tait, Claudia E.; Myers, William K.; McInnes, Eric J. L.; Kenwright, Alan M.; Beer, Paul D.; Timmel, Christiane R.

    2015-01-01

    Abstract Yb⋅DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb⋅DTMA⋅F]2+ and [Yb⋅DTMA⋅OH2]3+ are in slow exchange on the relevant NMR timescale (<2000 s−1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the YbIII ground state from easy‐axis to easy‐plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride‐responsive complexes and contrast agents. PMID:27478267

  19. Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA]3+ in Aqueous Solution

    PubMed Central

    Blackburn, Octavia A; Chilton, Nicholas F; Keller, Katharina; Tait, Claudia E; Myers, William K; McInnes, Eric J L; Kenwright, Alan M; Beer, Paul D; Timmel, Christiane R; Faulkner, Stephen

    2015-01-01

    Yb⋅DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb⋅DTMA⋅F]2+ and [Yb⋅DTMA⋅OH2]3+ are in slow exchange on the relevant NMR timescale (<2000 s−1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the YbIII ground state from easy-axis to easy-plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride-responsive complexes and contrast agents. PMID:26223970

  20. High Magnetic Field-Induced Birefringence in Lyotropic Chromonic Liquid Crystals

    NASA Astrophysics Data System (ADS)

    Ostapenko, T.; Nastishin, Yu.; Gleeson, J. T.; Sprunt, S. N.; Lavrentovich, O. D.; Collings, P. J.

    2009-03-01

    We studied the effect of magnetic-field induced birefringence of a 14% solution of disodium cromoglycate (DSCG) in water at temperatures above the nematic-isotropic coexistence region. According to Landau-deGennes mean field theory, we expect to find a linear relationship between the inverse of the induced birefringence, δn, and the quantity (T-T*), where T* is the stability limit of the isotropic phase. Using the 31 T resistive magnet at the National High Magnetic Field Laboratory, we observed that, as we increase the temperature above the coexistence region, we deviate from this linear dependence. Our data shows that δn goes to zero, whereas Landau-deGennes predicts that δn should decrease asymptotically. This may be due to the lack of isodesmic aggregate formation at a finite temperature above the coexistence region.Supported by NSF (DMR-0710544 and DMR-0606160). Work performed at NHMFL, supported by NSF cooperative agreements DMR-0084173, the State of Florida and the DOE.