Self-Assembly of Cubes into 2D Hexagonal and Honeycomb Lattices by Hexapolar Capillary Interactions
NASA Astrophysics Data System (ADS)
Soligno, Giuseppe; Dijkstra, Marjolein; van Roij, René
2016-06-01
Particles adsorbed at a fluid-fluid interface induce capillary deformations that determine their orientations and generate mutual capillary interactions which drive them to assemble into 2D ordered structures. We numerically calculate, by energy minimization, the capillary deformations induced by adsorbed cubes for various Young's contact angles. First, we show that capillarity is crucial not only for quantitative, but also for qualitative predictions of equilibrium configurations of a single cube. For a Young's contact angle close to 90°, we show that a single-adsorbed cube generates a hexapolar interface deformation with three rises and three depressions. Thanks to the threefold symmetry of this hexapole, strongly directional capillary interactions drive the cubes to self-assemble into hexagonal or graphenelike honeycomb lattices. By a simple free-energy model, we predict a density-temperature phase diagram in which both the honeycomb and hexagonal lattice phases are present as stable states.
Self-Assembly of Cubes into 2D Hexagonal and Honeycomb Lattices by Hexapolar Capillary Interactions.
Soligno, Giuseppe; Dijkstra, Marjolein; van Roij, René
2016-06-24
Particles adsorbed at a fluid-fluid interface induce capillary deformations that determine their orientations and generate mutual capillary interactions which drive them to assemble into 2D ordered structures. We numerically calculate, by energy minimization, the capillary deformations induced by adsorbed cubes for various Young's contact angles. First, we show that capillarity is crucial not only for quantitative, but also for qualitative predictions of equilibrium configurations of a single cube. For a Young's contact angle close to 90°, we show that a single-adsorbed cube generates a hexapolar interface deformation with three rises and three depressions. Thanks to the threefold symmetry of this hexapole, strongly directional capillary interactions drive the cubes to self-assemble into hexagonal or graphenelike honeycomb lattices. By a simple free-energy model, we predict a density-temperature phase diagram in which both the honeycomb and hexagonal lattice phases are present as stable states. PMID:27391753
Hexagonal structure of baby Skyrmion lattices
Hen, Itay; Karliner, Marek
2008-03-01
We study the zero-temperature crystalline structure of baby Skyrmions by applying a full-field numerical minimization algorithm to baby Skyrmions placed inside different parallelogramic unit cells and imposing periodic boundary conditions. We find that within this setup, the minimal energy is obtained for the hexagonal lattice, and that in the resulting configuration the Skyrmion splits into quarter Skyrmions. In particular, we find that the energy in the hexagonal case is lower than the one obtained on the well-studied rectangular lattice, in which splitting into half Skyrmions is observed.
HEXAGONAL ARRAY STRUCTURE FOR 2D NDE APPLICATIONS
Dziewierz, J.; Ramadas, S. N.; Gachagan, A.; O'Leary, R. L.
2010-02-22
This paper describes a combination of simulation and experimentation to evaluate the advantages offered by utilizing a hexagonal shaped array element in a 2D NDE array structure. The active material is a 1-3 connectivity piezoelectric composite structure incorporating triangular shaped pillars--each hexagonal array element comprising six triangular pillars. A combination of PZFlex, COMSOL and Matlab has been used to simulate the behavior of this device microstructure, for operation around 2.25 MHz, with unimodal behavior and low levels of mechanical cross-coupling predicted. Furthermore, the application of hexagonal array elements enables the array aperture to increase by approximately 30%, compared to a conventional orthogonal array matrix and hence will provide enhanced volumetric coverage and SNR. Prototype array configurations demonstrate good corroboration of the theoretically predicted mechanical cross-coupling between adjacent array elements (approx23 dB).
Discrete breathers in hexagonal dusty plasma lattices
Koukouloyannis, V.; Kourakis, I.
2009-08-15
The occurrence of single-site or multisite localized vibrational modes, also called discrete breathers, in two-dimensional hexagonal dusty plasma lattices is investigated. The system is described by a Klein-Gordon hexagonal lattice characterized by a negative coupling parameter epsilon in account of its inverse dispersive behavior. A theoretical analysis is performed in order to establish the possibility of existence of single as well as three-site discrete breathers in such systems. The study is complemented by a numerical investigation based on experimentally provided potential forms. This investigation shows that a dusty plasma lattice can support single-site discrete breathers, while three-site in phase breathers could exist if specific conditions, about the intergrain interaction strength, would hold. On the other hand, out of phase and vortex three-site breathers cannot be supported since they are highly unstable.
Ultracold Quantum Gases in Hexagonal Optical Lattices
NASA Astrophysics Data System (ADS)
Sengstock, Klaus
2010-03-01
Hexagonal structures occur in a vast variety of systems, ranging from honeycombs of bees in life sciences to carbon nanotubes in material sciences. The latter, in particular its unfolded two-dimensional layer -- Graphene -- has rapidly grown to one of the most discussed topics in condensed-matter physics. Not only does it show proximity to various carbon-based materials but also exceptional properties owing to its unusual energy spectrum. In quantum optics, ultracold quantum gases confined in periodic light fields have shown to be very general and versatile instruments to mimic solid state systems. However, so far nearly all experiments were performed in cubic lattice geometries only. Here we report on the first experimental realization of ultracold quantum gases in a state-dependent, two-dimensional, Graphene-like optical lattice with hexagonal symmetry. The lattice is realized via a spin-dependent optical lattice structure with alternating σ^+ and σ^- -sites and thus constitutes a so called `magnetic'-lattice with `antiferromagnetic'-structure. Atoms with different spin orientation can be loaded to specific lattice sites or -- depending on the parameters -- to the whole lattice. As a consequence e.g. superpositions of a superfluid spin component with a different spin component in the Mott-insulating phase can be realized as well as spin-dependent transport properties, disorder etc. After preparing an antiferromagnetically ordered state we e.g. measure sustainable changes of the transport properties of the atoms. This manifests in a significant reduction of the tunneling as compared to a single-component system. We attribute this observation to a partial tunneling blockade for one spin component induced by population in another spin component localized at alternating lattice sites. Within a Gutzwiller-Ansatz we calculate the phase diagrams for the mixed spin-states and find very good agreement with our experimental results. Moreover, by state-resolved recording
2D metamaterials with hexagonal structure: spatial resonances and near field imaging.
Zhuromskyy, O; Shamonina, E; Solymar, L
2005-11-14
The current and field distribution in a 2D metamaterial consisting of resonant elements in a hexagonal arrangement are found assuming magnetic interaction between the elements. The dispersion equation of magnetoinductive (MI) waves is derived with the aid of the direct and reciprocal lattice familiar from solid state theory. A continuous model for the current variation in the elements is introduced leading to the familiar wave equation in the form of a second order differential equation. The current distributions are shown to exhibit a series of spatial resonances for rectangular, circular and hexagonal boundaries. The axial and radial components of the resulting magnetic field are compared with previously obtained experimental results on a Swiss Roll metamaterial with hexagonal boundaries. Experimental and theoretical results are also compared for the near field image of an object in the shape of the letter M followed by a more general discussion of imaging. It is concluded that a theoretical formulation based on the propagation of MI waves can correctly describe the experimental results. PMID:19503131
Multilayer DNA origami packed on hexagonal and hybrid lattices.
Ke, Yonggang; Voigt, Niels V; Gothelf, Kurt V; Shih, William M
2012-01-25
"Scaffolded DNA origami" has been proven to be a powerful and efficient approach to construct two-dimensional or three-dimensional objects with great complexity. Multilayer DNA origami has been demonstrated with helices packing along either honeycomb-lattice geometry or square-lattice geometry. Here we report successful folding of multilayer DNA origami with helices arranged on a close-packed hexagonal lattice. This arrangement yields a higher density of helical packing and therefore higher resolution of spatial addressing than has been shown previously. We also demonstrate hybrid multilayer DNA origami with honeycomb-lattice, square-lattice, and hexagonal-lattice packing of helices all in one design. The availability of hexagonal close-packing of helices extends our ability to build complex structures using DNA nanotechnology. PMID:22187940
Stanene: Atomically Thick Free-standing Layer of 2D Hexagonal Tin.
Saxena, Sumit; Chaudhary, Raghvendra Pratap; Shukla, Shobha
2016-01-01
Stanene is one of most important of 2D materials due to its potential to demonstrate room temperature topological effects due to opening of spin-orbit gap. In this pursuit we report synthesis and investigation of optical properties of stanene up to few layers, a two-dimensional hexagonal structural analogue of graphene. Atomic scale morphological and elemental characterization using HRTEM equipped with SAED and EDAX detectors confirm the presence of hexagonal lattice of Sn atoms. The position of Raman peak along with the inter-planar 'd' spacing obtained from SAED for prepared samples are in good agreement with that obtained from first principles calculations and confirm that the sheets are not (111) α-Sn sheets. Further, the optical signature calculated using density functional theory at ~191 nm and ~233 nm for low buckled stanene are in qualitative agreement with the measured UV-Vis absorption spectrum. AFM measurements suggest interlayer spacing of ~0.33 nm in good agreement with that reported for epitaxial stanene sheets. No traces of oxygen were observed in the EDAX spectrum suggesting the absence of any oxidized phases. This is also confirmed by Raman measurements by comparing with oxidized stanene sheets. PMID:27492139
Stanene: Atomically Thick Free-standing Layer of 2D Hexagonal Tin
Saxena, Sumit; Chaudhary, Raghvendra Pratap; Shukla, Shobha
2016-01-01
Stanene is one of most important of 2D materials due to its potential to demonstrate room temperature topological effects due to opening of spin-orbit gap. In this pursuit we report synthesis and investigation of optical properties of stanene up to few layers, a two-dimensional hexagonal structural analogue of graphene. Atomic scale morphological and elemental characterization using HRTEM equipped with SAED and EDAX detectors confirm the presence of hexagonal lattice of Sn atoms. The position of Raman peak along with the inter-planar ‘d’ spacing obtained from SAED for prepared samples are in good agreement with that obtained from first principles calculations and confirm that the sheets are not (111) α-Sn sheets. Further, the optical signature calculated using density functional theory at ~191 nm and ~233 nm for low buckled stanene are in qualitative agreement with the measured UV-Vis absorption spectrum. AFM measurements suggest interlayer spacing of ~0.33 nm in good agreement with that reported for epitaxial stanene sheets. No traces of oxygen were observed in the EDAX spectrum suggesting the absence of any oxidized phases. This is also confirmed by Raman measurements by comparing with oxidized stanene sheets. PMID:27492139
Stanene: Atomically Thick Free-standing Layer of 2D Hexagonal Tin
NASA Astrophysics Data System (ADS)
Saxena, Sumit; Chaudhary, Raghvendra Pratap; Shukla, Shobha
2016-08-01
Stanene is one of most important of 2D materials due to its potential to demonstrate room temperature topological effects due to opening of spin-orbit gap. In this pursuit we report synthesis and investigation of optical properties of stanene up to few layers, a two-dimensional hexagonal structural analogue of graphene. Atomic scale morphological and elemental characterization using HRTEM equipped with SAED and EDAX detectors confirm the presence of hexagonal lattice of Sn atoms. The position of Raman peak along with the inter-planar ‘d’ spacing obtained from SAED for prepared samples are in good agreement with that obtained from first principles calculations and confirm that the sheets are not (111) α-Sn sheets. Further, the optical signature calculated using density functional theory at ~191 nm and ~233 nm for low buckled stanene are in qualitative agreement with the measured UV-Vis absorption spectrum. AFM measurements suggest interlayer spacing of ~0.33 nm in good agreement with that reported for epitaxial stanene sheets. No traces of oxygen were observed in the EDAX spectrum suggesting the absence of any oxidized phases. This is also confirmed by Raman measurements by comparing with oxidized stanene sheets.
NASA Astrophysics Data System (ADS)
Ma, Yandong; Kou, Liangzhi; Li, Xiao; Dai, Ying; Heine, Thomas
2016-01-01
So far, several transition metal dichalcogenide (TMDC)-based two-dimensional (2D) topological insulators (TIs) have been discovered, all of them based on a tetragonal lattice. However, in 2D crystals, the hexagonal rather than the tetragonal symmetry is the most common motif. Here, based on first principles calculations, we propose a class of stable 2D TMDCs of composition MX2(M =Mo ,W ;X =S ,Se ,Te ) with a hexagonal lattice. They are all in the same stability range as other 2D TMDC allotropes that have been demonstrated experimentally, and they are identified to be practical 2D TIs with large band gaps ranging from 41 to 198 meV, making them suitable for applications at room temperature. Besides, in contrast to tetragonal 2D TMDCs, their hexagonal lattice will greatly facilitate the integration of theses novel TI state van der Waals crystals with other hexagonal or honeycomb materials and thus provide a route for 2D material-based devices for wider nanoelectronic and spintronic applications. The nontrivial band gaps of both WS e2 and WT e2 2D crystals are 198 meV, which are larger than that in any previously reported TMDC-based TIs. These large band gaps entirely stem from the strong spin orbit coupling strength within the d orbitals of Mo/W atoms near the Fermi level. Our findings broaden the scientific and technological impact of both 2D TIs and TMDCs.
Analysis of photonic logic gates based on single hexagonal-lattice photonic crystal ring resonator
NASA Astrophysics Data System (ADS)
Jiang, Junzhen; Qiang, Zexuan; Xu, Xiaofu; Chen, Xiyao
2011-01-01
We devised a new configuration of optical logic gates based on a single hexagonal-lattice photonic crystal ring resonator (PCRR) composed of two-dimensional (2D) cylindrical silicon rods in air. The modal behavior of the proposed logic gates was comprehensively analyzed with a topology optimization technique based on the principle of beam interference and perturbation theory. It was then numerically verified by using a 2D finite-difference time-domain technique. The predictions have a very good agreement with the numerical results. This new single PCRR can really function as NOT and NOR gates. And the logic ``0'' and ``1'' of the hexagonal ring can be defined as less than 8% and greater than 86%, respectively, much better than earlier reported square-lattice results.
Optimization and Design of 2d Honeycomb Lattice Photonic Crystal Modulated by Liquid Crystals
NASA Astrophysics Data System (ADS)
Guo, Caihong; Zheng, Jihong; Gui, Kun; Zhang, Menghua; Zhuang, Songlin
2013-12-01
Photonic crystals (PCs) with infiltrating liquid crystals (LCs) have many potential applications because of their ability to continuously modulate the band-gaps. Using the plane-wave expansion method (PWM), we simulate the band-gap distribution of 2D honeycomb lattice PC with different pillar structures (circle, hexagonal and square pillar) and with different filling ratios, considering both when the LC is used as filling pillar material and semiconductors (Si, Ge) are used in the substrate, and when the semiconductors (Si, Ge) are pillar material and the LC is the substrate. Results show that unlike LC-based triangle lattice PC, optimized honeycomb lattice PC has the ability to generate absolute photonic band-gaps for fabricating optical switches. We provide optimization parameters for LC infiltrating honeycomb lattice PC structure based on simulation results and analysis.
Beam-Plasma Instabilities in a 2D Yukawa Lattice
Kyrkos, S.; Kalman, G. J.; Rosenberg, M.
2009-06-05
We consider a 2D Yukawa lattice of grains, with a beam of other charged grains moving in the lattice plane. In contrast to Vlasov plasmas, where the electrostatic instability excited by the beam is only longitudinal, here both longitudinal and transverse instabilities of the lattice phonons can develop. We determine and compare the transverse and longitudinal growth rates. The growth rate spectrum in wave number space exhibits remarkable gaps where no instability can develop. Depending on the system parameters, the transverse instability can be selectively excited.
Unitary quantum lattice gas representation of 2D quantum turbulence
NASA Astrophysics Data System (ADS)
Zhang, Bo; Vahala, George; Vahala, Linda; Soe, Min
2011-05-01
Quantum vortex structures and energy cascades are examined for two dimensional quantum turbulence (2D QT) using a special unitary evolution algorithm. The qubit lattice gas (QLG) algorithm, is employed to simulate the weakly-coupled Bose-Einstein condensate (BEC) governed by the Gross-Pitaevskii (GP) equation. A parameter regime is uncovered in which, as in 3D QT, there is a very short Poincare recurrence time. This short recurrence time is destroyed as the nonlinear interaction energy is increased. Energy cascades for 2D QT are considered to examine whether 2D QT exhibits the inverse cascades of 2D classical turbulence. In the parameter regime considered, the spectra analysis reveals no such dual cascades---dual cascades being a hallmark of 2D classical turbulence.
NASA Astrophysics Data System (ADS)
Venderbos, J. W. F.
2016-03-01
In this work we introduce a symmetry classification for electronic density waves which break translational symmetry due to commensurate wave-vector modulations. The symmetry classification builds on the concept of extended point groups: symmetry groups which contain, in addition to the lattice point group, translations that do not map the enlarged unit cell of the density wave to itself, and become "nonsymmorphic"-like elements. Multidimensional representations of the extended point group are associated with degenerate wave vectors. Electronic properties such as (nodal) band degeneracies and topological character can be straightforwardly addressed, and often follow directly. To further flesh out the idea of symmetry, the classification is constructed so as to manifestly distinguish time-reversal invariant charge (i.e., site and bond) order, and time-reversal breaking flux order. For the purpose of this work, we particularize to spin-rotation invariant density waves. As a first example of the application of the classification we consider the density waves of a simple single- and two-orbital square lattice model. The main objective, however, is to apply the classification to two-dimensional (2D) hexagonal lattices, specifically the triangular and the honeycomb lattices. The multicomponent density waves corresponding to the commensurate M -point ordering vectors are worked out in detail. To show that our results generally apply to 2 D hexagonal lattices, we develop a general low-energy SU(3 ) theory of (spinless) saddle-point electrons.
Lattice-Polarity-Driven Epitaxy of Hexagonal Semiconductor Nanowires.
Wang, Ping; Yuan, Ying; Zhao, Chao; Wang, Xinqiang; Zheng, Xiantong; Rong, Xin; Wang, Tao; Sheng, Bowen; Wang, Qingxiao; Zhang, Yongqiang; Bian, Lifeng; Yang, Xuelin; Xu, Fujun; Qin, Zhixin; Li, Xinzheng; Zhang, Xixiang; Shen, Bo
2016-02-10
Lattice-polarity-driven epitaxy of hexagonal semiconductor nanowires (NWs) is demonstrated on InN NWs. In-polarity InN NWs form typical hexagonal structure with pyramidal growth front, whereas N-polarity InN NWs slowly turn to the shape of hexagonal pyramid and then convert to an inverted pyramid growth, forming diagonal pyramids with flat surfaces and finally coalescence with each other. This contrary growth behavior driven by lattice-polarity is most likely due to the relatively lower growth rate of the (0001̅) plane, which results from the fact that the diffusion barriers of In and N adatoms on the (0001) plane (0.18 and 1.0 eV, respectively) are about 2-fold larger in magnitude than those on the (0001̅) plane (0.07 and 0.52 eV), as calculated by first-principles density functional theory (DFT). The formation of diagonal pyramids for the N-polarity hexagonal NWs affords a novel way to locate quantum dot in the kink position, suggesting a new recipe for the fabrication of dot-based devices. PMID:26694227
Influence of lattice defects on the ferromagnetic resonance behaviour of 2D magnonic crystals
NASA Astrophysics Data System (ADS)
Manzin, Alessandra; Barrera, Gabriele; Celegato, Federica; Coïsson, Marco; Tiberto, Paola
2016-02-01
This paper studies, from a modelling point of view, the influence of randomly distributed lattice defects (non-patterned areas and variable hole size) on the ferromagnetic resonance behaviour and spin wave mode profiles of 2D magnonic crystals based on Ni80Fe20 antidot arrays with hexagonal lattice. A reference sample is first defined via the comparison of experimental and simulated hysteresis loops and magnetoresistive curves of patterned films, prepared by self-assembly of polystyrene nanospheres. Second, a parametric analysis of the dynamic response is performed, investigating how edge, quasi-uniform and localized modes are affected by alterations of the lattice geometry and bias field amplitude. Finally, some results about the possible use of magnetic antidot arrays in frequency-based sensors for magnetic bead detection are presented, highlighting the need for an accurate control of microstructural features.
Influence of lattice defects on the ferromagnetic resonance behaviour of 2D magnonic crystals.
Manzin, Alessandra; Barrera, Gabriele; Celegato, Federica; Coïsson, Marco; Tiberto, Paola
2016-01-01
This paper studies, from a modelling point of view, the influence of randomly distributed lattice defects (non-patterned areas and variable hole size) on the ferromagnetic resonance behaviour and spin wave mode profiles of 2D magnonic crystals based on Ni80Fe20 antidot arrays with hexagonal lattice. A reference sample is first defined via the comparison of experimental and simulated hysteresis loops and magnetoresistive curves of patterned films, prepared by self-assembly of polystyrene nanospheres. Second, a parametric analysis of the dynamic response is performed, investigating how edge, quasi-uniform and localized modes are affected by alterations of the lattice geometry and bias field amplitude. Finally, some results about the possible use of magnetic antidot arrays in frequency-based sensors for magnetic bead detection are presented, highlighting the need for an accurate control of microstructural features. PMID:26911336
Influence of lattice defects on the ferromagnetic resonance behaviour of 2D magnonic crystals
Manzin, Alessandra; Barrera, Gabriele; Celegato, Federica; Coïsson, Marco; Tiberto, Paola
2016-01-01
This paper studies, from a modelling point of view, the influence of randomly distributed lattice defects (non-patterned areas and variable hole size) on the ferromagnetic resonance behaviour and spin wave mode profiles of 2D magnonic crystals based on Ni80Fe20 antidot arrays with hexagonal lattice. A reference sample is first defined via the comparison of experimental and simulated hysteresis loops and magnetoresistive curves of patterned films, prepared by self-assembly of polystyrene nanospheres. Second, a parametric analysis of the dynamic response is performed, investigating how edge, quasi-uniform and localized modes are affected by alterations of the lattice geometry and bias field amplitude. Finally, some results about the possible use of magnetic antidot arrays in frequency-based sensors for magnetic bead detection are presented, highlighting the need for an accurate control of microstructural features. PMID:26911336
Optical properties of GaAs 2D hexagonal and cubic photonic crystal
Arab, F. Assali, A.; Grain, R.; Kanouni, F.
2015-03-30
In this paper we present our theoretical study of 2D hexagonal and cubic rods GaAs in air, with plan wave expansion (PWE) and finite difference time domain (FDTD) by using BandSOLVE and FullWAVE of Rsoft photonic CAD package. In order to investigate the effect of symmetry and radius, we performed calculations of the band structures for both TM and TE polarization, contour and electromagnetic propagation and transmission spectra. Our calculations show that the hexagonal structure gives a largest band gaps compare to cubic one for a same filling factor.
NASA Astrophysics Data System (ADS)
Kim, Hyun-Jung; Li, Chaokai; Feng, Ji; Cho, Jun-Hyung; Zhang, Zhenyu
2016-01-01
The exploration of topological states is of significant fundamental and practical importance in contemporary condensed matter physics, for which the extension to two-dimensional (2D) organometallic systems is particularly attractive. Using first-principles calculations, we show that a 2D hexagonal triphenyl-lead lattice composed of only main group elements is susceptible to a magnetic instability, characterized by a considerably more stable antiferromagnetic (AFM) insulating state rather than the topologically nontrivial quantum spin Hall state proposed recently. Even though this AFM phase is topologically trivial, it possesses an intricate emergent degree of freedom, defined by the product of spin and valley indices, leading to Berry curvature-induced spin and valley currents under electron or hole doping. Furthermore, such a trivial band insulator can be tuned into a topologically nontrivial matter by the application of an out-of-plane electric field, which destroys the AFM order, favoring instead ferrimagnetic spin ordering and a quantum anomalous Hall state with a nonzero topological invariant. These findings further enrich our understanding of 2D hexagonal organometallic lattices for potential applications in spintronics and valleytronics.
Beyond the simple hexagonal Abrikosov vortex lattice in layered superconductors
NASA Astrophysics Data System (ADS)
Feinberg, D.; Ettouhami, A. M.
1993-01-01
In layered superconductors as high-Tc materials but also organic superconductors, chalcogenides and superlattices, the simple concept of an distorted hexagonal lattice of straight vortices becomes unsufficient. Due to anisotropy and short coherence lengths, quite new vortex structures may arise. Some of them, as staircase vortices, simply add a modulation in the direction of vortex lines. This phenomenon is reviewed, together with the resulting lock-in transition, especially when the effects of the layered structure are weak. More exotic structures like a decomposed vortex lattice can also occur in specific situations: they involve two perpendicular sublattices, one parallel and one normal to the layers. We propose that extended defects as twin boundaries or irradiation tracks can trigger such a structure even in moderately anisotropic compounds as Y:123.
Proposal for generating synthetic magnetic fields in hexagonal optical lattices
NASA Astrophysics Data System (ADS)
Tian, Binbin; Endres, Manuel; Pekker, David
2015-05-01
We propose a new approach to generating synthetic magnetic fields in ultra cold atom systems that does not rely on either Raman transitions nor periodic drive. Instead, we consider a hexagonal optical lattice produced by the intersection of three laser beams at 120 degree angles, where the intensity of one or more of the beams is spatially non-uniform. The resulting optical lattice remains hexagonal, but has spatially varying hopping matrix elements. For atoms near the Dirac points, these spatial variations appear as a gauge field, similar to the fictitious gauge field that is induced for for electrons in strained graphene. We suggest that a robust way to generate a gauge field that corresponds to a uniform flux is to aligning three gaussian beams to intersect in an equilateral triangle. Using realistic experimental parameters, we show how the proposed setup can be used to observe cyclotron motion of an atom cloud - the conventional Hall effect and distinct Landau levels - the integer quantum Hall effect.
Discrete solitons and vortices in anisotropic hexagonal and honeycomb lattices
NASA Astrophysics Data System (ADS)
Hoq, Q. E.; Kevrekidis, P. G.; Bishop, A. R.
2016-02-01
In the present work, we consider the self-focusing discrete nonlinear Schrödinger equation on hexagonal and honeycomb lattice geometries. Our emphasis is on the study of the effects of anisotropy, motivated by the tunability afforded in recent optical and atomic physics experiments. We find that multi-soliton and discrete vortex states undergo destabilizing bifurcations as the relevant anisotropy control parameter is varied. We quantify these bifurcations by means of explicit analytical calculations of the solutions, as well as of their spectral linearization eigenvalues. Finally, we corroborate the relevant stability picture through direct numerical computations. In the latter, we observe the prototypical manifestation of these instabilities to be the spontaneous rearrangement of the solution, for larger values of the coupling, into localized waveforms typically centered over fewer sites than the original unstable structure. For weak coupling, the instability appears to result in a robust breathing of the relevant waveforms.
Discrete solitons and vortices in anisotropic hexagonal and honeycomb lattices
Hoq, Q. E.; Kevrekidis, P. G.; Bishop, A. R.
2016-01-14
We consider the self-focusing discrete nonlinear Schrödinger equation on hexagonal and honeycomb lattice geometries. Our emphasis is on the study of the effects of anisotropy, motivated by the tunability afforded in recent optical and atomic physics experiments. We find that multi-soliton and discrete vortex states undergo destabilizing bifurcations as the relevant anisotropy control parameter is varied. Furthermore, we quantify these bifurcations by means of explicit analytical calculations of the solutions, as well as of their spectral linearization eigenvalues. Finally, we corroborate the relevant stability picture through direct numerical computations. In the latter, we observe the prototypical manifestation of these instabilitiesmore » to be the spontaneous rearrangement of the solution, for larger values of the coupling, into localized waveforms typically centered over fewer sites than the original unstable structure. In weak coupling, the instability appears to result in a robust breathing of the relevant waveforms.« less
Giant, Hollow 2D Metalloarchitecture: Stepwise Self-Assembly of a Hexagonal Supramolecular Nut.
Li, Yiming; Jiang, Zhilong; Wang, Ming; Yuan, Jie; Liu, Die; Yang, Xiaoyu; Chen, Mingzhao; Yan, Jun; Li, Xiaopeng; Wang, Pingshan
2016-08-10
A polyterpyridinyl building block-based nutlike hexagonal bismetallo architecture with a central hollow Star of David was assembled by a stepwise strategy. This nanoarchitecture can be viewed as a recursive mathematical form that possesses a supramolecular corner-connected cyclic structure, i.e., a triangle or rhombus at various levels of scale or detail. The key metallo-organic ligand (MOL) with four uncomplexed free terpyridines was obtained by a final Suzuki cross-coupling reaction with a tetrabromoterpyridine Ru dimer. The molecular metallorhombus was prepared by reacting the MOL with a 60° bis-terpyridine and Fe(2+). The giant hollow hexagonal nut with a diameter of more than 11 nm and a molecular weight of ca. 33 kDa was obtained in near-quantitative yield by mixing the two types of multi-terpyridine ligands with Fe(2+). The supramolecular architecture was characterized by NMR ((1)H and (13)C), 2D NMR (COSY and ROESY), and DOSY spectroscopies, high-resolution electrospray ionization mass spectrometry, traveling-wave ion mobility mass spectrometry, and transmission electron microscopy. PMID:27447878
Electronic and geometrical properties of monoatomic and diatomic 2D honeycomb lattices. A DFT study
NASA Astrophysics Data System (ADS)
Rojas, Ángela; Rey, Rafael; Fonseca, Karen; Grupo de Óptica e Información Cuántica Team
Since the discovery of graphene by Geim and Novoselov at 2004, several analogous systems have been theoretically and experimentally studied, due to their technological interest. Both monoatomic lattices, such as silicine and germanene, and diatomic lattices (h-GaAs and h-GaN) have been studied. Using Density Functional Theory we obtain and confirm the chemical stability of these hexagonal 2D systems through the total energy curves as a function of interatomic distance. Unlike graphene, silicine and germanene, gapless materials, h-GaAs and h-GaN exhibit electronic gaps, different from that of the bulk, which could be interesting for the industry. On the other hand, the ab initio band structure calculations for graphene, silicene and germanene show a non-circular cross section around K points, at variance with the prediction of usual Tight-binding models. In fact, we have found that Dirac cones display a dihedral group symmetry. This implies that Fermi speed can change up to 30 % due to the orientation of the wave vector, for both electrons and holes. Traditional analytic studies use the Dirac equation for the electron dynamics at low energies. However, this equation assumes an isotropic, homogeneous and uniform space. Authors would like to thank the División de Investigación Sede Bogotá for their financial support at Universidad Nacional de Colombia. A. M. Rojas-Cuervo would also like to thank the Colciencias, Colombia.
Thermally induced formation of 2D hexagonal BN nanoplates with tunable characteristics
Nersisyan, Hayk; Lee, Tae-Hyuk; Lee, Kap-Ho; Jeong, Seong-Uk; Kang, Kyung-Soo; Bae, Ki-Kwang; Lee, Jong-Hyeon
2015-05-15
We have investigated a thermally induced combustion route for preparing 2D hexagonal BN nanoplates from B{sub 2}O{sub 3}+(3+0.5k)Mg+kNH{sub 4}Cl solid system, for k=1–4 interval. Temperature–time profiles recorded by thermocouples indicated the existence of two sequential exothermic processes in the combustion wave leading to the BN nanoplates formation. The resulting BN nanoplates were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, PL spectrometry, and Brunauer–Emmett–Teller surface area analysis. It was found that B{sub 2}O{sub 3} was converted into BN completely (by XRD) at 1450–1930 °C within tens of seconds in a single-step synthesis process. The BN prepared at a k=1–4 interval comprised well-shaped nanoplates with an average edge length ranging from 50 nm to several micrometer and thickness from 5 to 100 nm. The specific surface area of BN nanoplates was 13.7 g/m{sup 2} for k=2 and 28.4 m{sup 2}/g for k=4. - Graphical abstract: 2D hexagonal BN nanoplates with an average edge length ranging from 50 nm to several micrometer and thickness from 5 to 100 nm were prepared by combustion of B{sub 2}O{sub 3}+(3+0.5k)Mg+kNH{sub 4}Cl solid mixture in nitrogen atmosphere. - Highlights: • Thermally induced combustion route was developed for synthesizing BN nanoplates from B{sub 2}O{sub 3}. • Mg was used as reductive agent and NH{sub 4}Cl as an effective nitrogen source. • Temperature–time profiles and the combustion parameters were recorded and discussed. • BN with an average edge length from 50 nm to several micrometer and thickness from 5 to 100 nm were prepared. • Our study clarifies the formation mechanism of BN in the combustion wave.
Thermal Conductance at the 2D MoS2-hexagonal Boron Nitride Interface
NASA Astrophysics Data System (ADS)
Liu, Yi; Hippalgaonkar, Kedar; Ong, Zhun Yong; Thong, John Tl; Qiu, Chengwei
In recent years, a number of 2D heterostructure devices have emerged, including graphene/hexagonal boron nitride (h-BN), graphene/MoS2 and MoS2/ h-BN. Among them, MoS2/ h-BN field-effect transistors with MoS2 channels and h-BN dielectric have been reported to have higher carrier mobility and reduced hysteresis compared to MoS2 on SiO2. Despite relatively high in-plane thermal conductivity of MoS2 and h-BN, heat dissipation from these 2D devices is mainly limited by heat transfer in the vertical direction. Consequently, their operating temperatures are strongly influenced by the interface thermal conductance. In this work, we demonstrate the measurement of interface thermal conductance between MoS2 and h-BN. This is realized by electrically heating MoS2 and monitoring their temperatures through Raman spectroscopy. The obtained interface thermal conductance between MoS2 and h-BN is 1.77 MW/m2K, smaller than the reported value for the graphene/ h-BN interface, due to the weak coupling of phonon modes between MoS2 and h-BN based on our NEGF calculation. The low interface thermal conductance value suggests this interface is not favorable for heat dissipation, and should be considered carefully for the design of electronic and optoelectronic devices based on MoS2/ h-BN heterostructures.
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.
Thermally induced formation of 2D hexagonal BN nanoplates with tunable characteristics
NASA Astrophysics Data System (ADS)
Nersisyan, Hayk; Lee, Tae-Hyuk; Lee, Kap-Ho; Jeong, Seong-Uk; Kang, Kyung-Soo; Bae, Ki-Kwang; Lee, Jong-Hyeon
2015-05-01
We have investigated a thermally induced combustion route for preparing 2D hexagonal BN nanoplates from B2O3+(3+0.5k)Mg+kNH4Cl solid system, for k=1-4 interval. Temperature-time profiles recorded by thermocouples indicated the existence of two sequential exothermic processes in the combustion wave leading to the BN nanoplates formation. The resulting BN nanoplates were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, PL spectrometry, and Brunauer-Emmett-Teller surface area analysis. It was found that B2O3 was converted into BN completely (by XRD) at 1450-1930 °C within tens of seconds in a single-step synthesis process. The BN prepared at a k=1-4 interval comprised well-shaped nanoplates with an average edge length ranging from 50 nm to several micrometer and thickness from 5 to 100 nm. The specific surface area of BN nanoplates was 13.7 g/m2 for k=2 and 28.4 m2/g for k=4.
Lattice Boltzmann Equation On a 2D Rectangular Grid
NASA Technical Reports Server (NTRS)
Bouzidi, MHamed; DHumieres, Dominique; Lallemand, Pierre; Luo, Li-Shi; Bushnell, Dennis M. (Technical Monitor)
2002-01-01
We construct a multi-relaxation lattice Boltzmann model on a two-dimensional rectangular grid. The model is partly inspired by a previous work of Koelman to construct a lattice BGK model on a two-dimensional rectangular grid. The linearized dispersion equation is analyzed to obtain the constraints on the isotropy of the transport coefficients and Galilean invariance for various wave propagations in the model. The linear stability of the model is also studied. The model is numerically tested for three cases: (a) a vortex moving with a constant velocity on a mesh periodic boundary conditions; (b) Poiseuille flow with an arbitrasy inclined angle with respect to the lattice orientation: and (c) a cylinder &symmetrically placed in a channel. The numerical results of these tests are compared with either analytic solutions or the results obtained by other methods. Satisfactory results are obtained for the numerical simulations.
Mechaly, Ariel E; Bellomio, Augusto; Morante, Koldo; Agirre, Jon; Gil-Cartón, David; Valle, Mikel; González-Mañas, Juan Manuel; Guérin, Diego M A
2012-11-01
The recent high-resolution structure of the toxin FraC derived from the sea anemone Actinia fragacea has provided new insight into the mechanism of pore formation by actinoporins. In this work, we report two new crystal forms of FraC in its oligomeric prepore conformation. Together with the previously reported structure, these two new structures reveal that ring-like nonamers of the toxin assemble into compact two-dimensional hexagonal arrays. This supramolecular organization is maintained in different relative orientations adopted by the oligomers within the crystal layers. Analyses of the aggregation of FraC pores in both planar and curved (vesicles) model membranes show similar 2D hexagonal arrangements. Our observations support a model in which hexagonal pore-packing is a clustering mechanism that maximizes toxin-driven membrane damage in the target cell. PMID:22728830
Lattice Formulation of 2D SQCD with exact supersymmetry
Sugino, Fumihiko
2008-11-23
We construct a lattice model for two-dimensional N = (2,2) supersymmetric QCD (SQCD), with the matter multiplets belonging to the fundamental or anti-fundamental representation of the gauge group U(N) or SU(N). The construction is based on the topological field theory (twisted supercharge) formulation and exactly preserves one supercharge. In order to avoid the species doublers of the matter multiplets, we introduce the Wilson terms and the model is defined for the case of the number of the fundamental matters (n{sub +}) equal to that of the anti-fundamental matters (n{sub -}). If some of the matter multiplets decouple from the theory by sending the corresponding anti-holomorphic twisted masses to the infinity, we can analyze the general n{sub +}{ne}n{sub -} case, although the lattice model is defined for n{sub +} = n{sub -}. By computing the anomaly of the U(1){sub A} R-symmetry in the lattice perturbation, we see that the decoupling is achieved and the anomaly for n{sub +}{ne}n{sub -} is correctly obtained.
2D and 3D Anilato-Based Heterometallic M(I)M(III) Lattices: The Missing Link.
Benmansour, Samia; Vallés-García, Cristina; Gómez-Claramunt, Patricia; Mínguez Espallargas, Guillermo; Gómez-García, Carlos J
2015-06-01
The similar bis-bidentate coordination mode of oxalato and anilato-based ligands is exploited here to create the first examples of 2D and 3D heterometallic lattices based on anilato ligands combining M(I) and a M(III) ions, phases already observed with oxalato but unknown with anilato-type ligands. These lattices are prepared with alkaline metal ions and magnetic chiral tris(anilato)metalate molecular building blocks: [M(III)(C6O4X2)3](3-) (M(III) = Fe and Cr; X = Cl and Br; (C6O4X2)(2-) = dianion of the 3,6-disubstituted derivatives of 2,5-dihydroxy-1,4-benzoquinone, H4C6O4). The new compounds include two very similar 2D lattices formulated as (PBu3Me)2[NaCr(C6O4Br2)3] (1) and (PPh3Et)2[KFe(C6O4Cl2)3](dmf)2 (2), both presenting hexagonal [M(I)M(III)(C6O4X2)3](2-) honeycomb layers with (PBu3Me)(+) in 1 or (PPh3Et)(+) and dmf in 2 inserted between them. Minor modifications in the synthetic conditions yield the novel 3D lattice (NEt3Me)[Na(dmf)][NaFe(C6O4Cl2)3] (3), in which hexagonal layers analogous to 1 and 2 are interconnected through Na(+) cations, and (NBu3Me)2[NaCr(C6O4Br2)3] (4), the first heterometallic 3D lattice based on anilato ligands. This compound presents two interlocked chiral 3D (10,3) lattices with opposite chiralities. Attempts to prepare 4 in larger quantities result in the 2D polymorph of compound 4 (4'). Magnetic properties of compounds 1, 3, and 4' are reported, and in all cases we observe, as expected, paramagnetic behaviors that can be satisfactorily reproduced with simple monomer models including a zero field splitting (ZFS) of the corresponding S = 3/2 for Cr(III) in 1 and 4' or S = 5/2 for Fe(III) in 3. PMID:25965415
Dynamical polarizability of the 2D pseudospin-1 dice lattice
NASA Astrophysics Data System (ADS)
Malcolm, John; Nicol, Elisabeth
The two-dimensional dice lattice is composed of three triangular sublattices whose low-energy excitation spectrum consists of Dirac-Weyl fermions with pseudospin-1. The energy dispersion has two Dirac cones, like the pseudospin-1/2 two-triangular-sublattice graphene, with an additional third band exactly at zero energy. We present theoretical results for the electronic dynamical polarization function in the material. This is a fundamental entity in many-body physics, renormalizing the Coulomb interaction through the dielectric function. From the polarization function we also obtain the Lindhard function, the plasmon branch, and can discuss other screening effects. These are constrasted with those of graphene.
Quantum spin Hall phase in 2D trigonal lattice.
Wang, Z F; Jin, Kyung-Hwan; Liu, Feng
2016-01-01
The quantum spin Hall (QSH) phase is an exotic phenomena in condensed-matter physics. Here we show that a minimal basis of three orbitals (s, px, py) is required to produce a QSH phase via nearest-neighbour hopping in a two-dimensional trigonal lattice. Tight-binding model analyses and calculations show that the QSH phase arises from a spin-orbit coupling (SOC)-induced s-p band inversion or p-p bandgap opening at Brillouin zone centre (Γ point), whose topological phase diagram is mapped out in the parameter space of orbital energy and SOC. Remarkably, based on first-principles calculations, this exact model of QSH phase is shown to be realizable in an experimental system of Au/GaAs(111) surface with an SOC gap of ∼73 meV, facilitating the possible room-temperature measurement. Our results will extend the search for substrate supported QSH materials to new lattice and orbital types. PMID:27599580
Valley Hall Effect in Two-Dimensional Hexagonal Lattices
NASA Astrophysics Data System (ADS)
Yamamoto, Michihisa; Shimazaki, Yuya; Borzenets, Ivan V.; Tarucha, Seigo
2015-12-01
Valley is a quantum number defined for energetically degenerate but nonequivalent structures in energy bands of a crystalline material. Recent discoveries of two-dimensional (2D) layered materials have shed light on the potential use of this degree of freedom for information carriers because the valley can now be potentially manipulated in integrated 2D architectures. The valleys separated by a long distance in a momentum space are robust against external disturbance and the flow of the valley, the valley current, is nondissipative because it carries no net electronic current. Among the various 2D valley materials, graphene has by far the highest crystal quality, leading to an extremely long valley relaxation length in the bulk. In this review, we first describe the theoretical background of the valley Hall effect, which converts an electric field into a valley current. We then describe the first observation of the valley Hall effect in monolayer MoS2. Finally, we describe experiments on the generation and detection of the pure valley current in monolayer and bilayer graphene, achieved recently using the valley Hall effect and inverse valley Hall effect. While we show unambiguous evidence of a pure valley current flowing in graphene, we emphasize that the field of "valleytronics" is still in its infancy and that further theoretical and experimental investigations are necessary.
Computation of neutron fluxes in clusters of fuel pins arranged in hexagonal assemblies (2D and 3D)
Prabha, H.; Marleau, G.
2012-07-01
For computations of fluxes, we have used Carvik's method of collision probabilities. This method requires tracking algorithms. An algorithm to compute tracks (in 2D and 3D) has been developed for seven hexagonal geometries with cluster of fuel pins. This has been implemented in the NXT module of the code DRAGON. The flux distribution in cluster of pins has been computed by using this code. For testing the results, they are compared when possible with the EXCELT module of the code DRAGON. Tracks are plotted in the NXT module by using MATLAB, these plots are also presented here. Results are presented with increasing number of lines to show the convergence of these results. We have numerically computed volumes, surface areas and the percentage errors in these computations. These results show that 2D results converge faster than 3D results. The accuracy on the computation of fluxes up to second decimal is achieved with fewer lines. (authors)
Grid Cell Responses in 1D Environments Assessed as Slices through a 2D Lattice.
Yoon, KiJung; Lewallen, Sam; Kinkhabwala, Amina A; Tank, David W; Fiete, Ila R
2016-03-01
Grid cells, defined by their striking periodic spatial responses in open 2D arenas, appear to respond differently on 1D tracks: the multiple response fields are not periodically arranged, peak amplitudes vary across fields, and the mean spacing between fields is larger than in 2D environments. We ask whether such 1D responses are consistent with the system's 2D dynamics. Combining analytical and numerical methods, we show that the 1D responses of grid cells with stable 1D fields are consistent with a linear slice through a 2D triangular lattice. Further, the 1D responses of comodular cells are well described by parallel slices, and the offsets in the starting points of the 1D slices can predict the measured 2D relative spatial phase between the cells. From these results, we conclude that the 2D dynamics of these cells is preserved in 1D, suggesting a common computation during both types of navigation behavior. PMID:26898777
Manet, Sabine; Schmitt, Julien; Impéror-Clerc, Marianne; Zholobenko, Vladimir; Durand, Dominique; Oliveira, Cristiano L P; Pedersen, Jan Skov; Gervais, Christel; Baccile, Niki; Babonneau, Florence; Grillo, Isabelle; Meneau, Florian; Rochas, Cyrille
2011-10-01
The different steps of the self-assembly in solution of several 2D-hexagonal silica nanostructured SBA-15 materials have been investigated by SAXS and SANS in situ experiments. Unique quantitative information about the shape and size evolution upon time of the micellar aggregates throughout the self-assembly process is obtained using a complete model that describes well the scattering data for the various synthesis conditions. In all cases, before the precipitation of the material, the micelles shape changes from spherical to rod-like, where the structure of the rod-like micelles is linked to the structure of the 2D-hexagonal precipitated material. In addition, the kinetics of hydrolysis of the inorganic precursor (TEOS) has been determined by in situ Raman spectroscopy. More specifically, by comparing synthesis made with different acids (HNO(3), HBr, HCl, H(2)SO(4), and H(3)PO(4)), it is found that materials prepared using the "salting-out" anions (SO(4)(2-) and H(2)PO(4)(-)) are much better ordered than with the "salting-in" anions (NO(3)(-) and Br(-)). PMID:21863844
Interaction of water molecules with hexagonal 2D systems. A DFT study
NASA Astrophysics Data System (ADS)
Rojas, Ángela; Rey, Rafael
Over the years water sources have been contaminated with many chemical agents, becoming issues that affect health of the world population. The advances of the nanoscience and nanotechnology in the development new materials constitute an alternative for design molecular filters with great efficiencies and low cost for water treatment and purification. In the nanoscale, the process of filtration or separation of inorganic and organic pollutants from water requires to study interactions of these atoms or molecules with different nano-materials. Specifically, it is necessary to understand the role of these interactions in physical and chemical properties of the nano-materials. In this work, the main interest is to do a theoretical study of interaction between water molecules and 2D graphene-like systems, such as silicene (h-Si) or germanene (h-Ge). Using Density Functional Theory we calculate total energy curves as function of separation between of water molecules and 2D systems. Different spatial configurations of water molecules relative to 2D systems are considered. Structural relaxation effects and changes of electronic charge density also are reported. Universidad Nacional de Colombia.
Light Trapping Enhancement in a Thin Film with 2D Conformal Periodic Hexagonal Arrays.
Yang, Xi; Zhou, Suqiong; Wang, Dan; He, Jian; Zhou, Jun; Li, Xiaofeng; Gao, Pingqi; Ye, Jichun
2015-12-01
Applying a periodic light trapping array is an effective method to improve the optical properties in thin-film solar cells. In this work, we experimentally and theoretically investigate the light trapping properties of two-dimensional periodic hexagonal arrays in the framework of a conformal amorphous silicon film. Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles. The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss. The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %. The methodology applied in this work can be generalized to rational design of other types of high-performance thin-film photovoltaic devices based on a broad range of materials. PMID:26153124
Light Trapping Enhancement in a Thin Film with 2D Conformal Periodic Hexagonal Arrays
NASA Astrophysics Data System (ADS)
Yang, Xi; Zhou, Suqiong; Wang, Dan; He, Jian; Zhou, Jun; Li, Xiaofeng; Gao, Pingqi; Ye, Jichun
2015-07-01
Applying a periodic light trapping array is an effective method to improve the optical properties in thin-film solar cells. In this work, we experimentally and theoretically investigate the light trapping properties of two-dimensional periodic hexagonal arrays in the framework of a conformal amorphous silicon film. Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles. The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss. The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %. The methodology applied in this work can be generalized to rational design of other types of high-performance thin-film photovoltaic devices based on a broad range of materials.
Effect of large deformation pre-loads on the wave properties of hexagonal lattices
NASA Astrophysics Data System (ADS)
Pal, Raj Kumar; Rimoli, Julian; Ruzzene, Massimo
2016-05-01
We study linear wave propagation in nonlinear hexagonal lattices capable of undergoing large deformations, under different levels of pre-load. The lattices are composed of a set of masses connected by linear axial and angular springs, with the nonlinearity arising solely from geometric effects. By applying different levels of pre-load, the small amplitude linear wave propagation response can be varied from isotropic to highly directional. Analytical expressions for the stiffness of a unit cell in the deformed configuration are derived and they are used to analyze the dispersion surfaces and group velocity variation with pre-load. Numerical simulations on finite lattices demonstrate the validity of our unit cell predictions and illustrate the wave steering potential of our lattice.
Dust Lattice Waves in Two-Dimensional Hexagonal Dust Crystals with an External Magnetic Field
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.
Single pentagon in a hexagonal carbon lattice revealed by scanning tunneling microscopy
An, B.; Fukuyama, S.; Yokogawa, K.; Yoshimura, M.; Egashira, M.; Korai, Y.; Mochida, I.
2001-06-04
The electronic structure of a single pentagon in a hexagonal carbon lattice has been revealed on an atomic scale by scanning tunneling microscopy. The pentagon is located at the apex of the conical protuberance of the graphitic particle. The enhanced charge density localized at each carbon atom in the pentagon is identified, and the ringlike pattern of the ({radical}3{times}{radical}3)R30{degree} superstructure of graphite is clearly observed around the pentagon. {copyright} 2001 American Institute of Physics.
NASA Astrophysics Data System (ADS)
Alexandrov, Y. V.; Batanova, A. A.; Gladkova, E. V.; Dyshlovenko, P. E.; Nagatkin, A. N.; Nizametdinov, A. F.
2016-02-01
Two-dimensional charge-stabilized colloidal crystal with a hexagonal crystal lattice and constant electric potential on the particles is studied numerically. The properties of the crystal are described by the theory based on the Poisson-Boltzmann non-linear differential equation. The force constants and the equilibrium pressure in the crystals are calculated for a broad range of the lattice parameters. The numerical procedures of the force constant and pressure determination is briefly described. Elastic constants of the first and second order are also calculated on the base of the force constants to obtain information about the many-body effective interactions in the system. It was shown that the Cauchy relation between the elastic constants breaks down in the whole range of the lattice parameter especially at higher densities. This can be interpreted as an effect of the many-body effective interaction between the particles in the crystal.
Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application
Arslan Shehzad, Muhammad; Hoang Tien, Dung; Waqas Iqbal, M; Eom, Jonghwa; Park, J. H.; Hwang, Chanyong; Seo, Yongho
2015-01-01
We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment. PMID:26290037
Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application
NASA Astrophysics Data System (ADS)
Arslan Shehzad, Muhammad; Hoang Tien, Dung; Waqas Iqbal, M.; Eom, Jonghwa; Park, J. H.; Hwang, Chanyong; Seo, Yongho
2015-08-01
We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.
Devil's staircase in a quantum dimer model on the hexagonal lattice
NASA Astrophysics Data System (ADS)
Barthel, Thomas; Misguich, Grégoire; Schlittler, Thiago M.; Vidal, Julien; Mosseri, Rémy
Quantum dimer models appear in different contexts when describing dynamics in constrained low-energy manifolds, such as for frustrated Ising models in weak transverse fields. In this talk, I address a particularly interesting case, where a quantum dimer model on the hexagonal lattice, in addition to the standard Rokhsar-Kivelson Hamiltonian, includes a competing potential term, counting dimer-free hexagons. It has a rich zero-temperature phase diagram that comprises a cascade of rapidly changing flux quantum numbers (tilt in the height language). This cascade is partially of fractal nature and the model provides, in particular, a microscopic realization of the ``devil's staircase'' scenario [E. Fradkin et al. Phys. Rev. B 69, 224415 (2004)]. We have studied the system by means of quantum Monte-Carlo simulations and the results can be explained using perturbation theory, RG, and variational arguments.References: arXiv:1507.04643, arXiv:1501.02242.
Phase Diagram of an Extended Quantum Dimer Model on the Hexagonal Lattice
NASA Astrophysics Data System (ADS)
Schlittler, Thiago; Barthel, Thomas; Misguich, Grégoire; Vidal, Julien; Mosseri, Rémy
2015-11-01
We introduce a quantum dimer model on the hexagonal lattice that, in addition to the standard three-dimer kinetic and potential terms, includes a competing potential part counting dimer-free hexagons. The zero-temperature phase diagram is studied by means of quantum Monte Carlo simulations, supplemented by variational arguments. It reveals some new crystalline phases and a cascade of transitions with rapidly changing flux (tilt in the height language). We analyze perturbatively the vicinity of the Rokhsar-Kivelson point, showing that this model has the microscopic ingredients needed for the "devil's staircase" scenario [Eduardo Fradkin et al. Phys. Rev. B 69, 224415 (2004)], and is therefore expected to produce fractal variations of the ground-state flux.
Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application.
Arslan Shehzad, Muhammad; Hoang Tien, Dung; Waqas Iqbal, M; Eom, Jonghwa; Park, J H; Hwang, Chanyong; Seo, Yongho
2015-01-01
We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment. PMID:26290037
Magnetic structure and resonance properties of a hexagonal lattice of antidots
NASA Astrophysics Data System (ADS)
Marchenko, A. I.; Krivoruchko, V. N.
2012-02-01
Static and resonance properties of ferromagnetic films with a hexagonal lattice of antidots (pores in the film) were studied. The description of the system is based on micromagnetic modeling and analytical solutions of the Landau-Lifshitz equation. The dependences of ferromagnetic resonance spectra on the in-plane direction of applied magnetic field and on the lattice parameters were investigated. The nature of the dependences of a dynamic system response on the frequency at fixed magnetic fields and on the field at fixed frequency when the field changes were explored. They cause the static magnetic order to change. It was found that the specific peculiarities of the system dynamics remain unchanged for both of these experimental conditions. Namely, for low damping the resonance spectra contain three quasi-homogeneous modes which are due to the resonance of different regions (domains) of the antidot lattice cell. It is shown that the angular field dependences of each mode are characterized by a twofold symmetry, and the related easy axes are mutually rotated by 60 °. As a result, a hexagonal symmetry of the system's static and dynamic magnetic characteristics is realized. The existence in the resonance spectrum of several quasi-homogeneous modes related to different regions of the unit cell could be fundamental for the function of the working elements of magnonics devices.
Multi-component quantum gases in spin-dependent hexagonal lattices
NASA Astrophysics Data System (ADS)
Soltan-Panahi, P.; Struck, J.; Hauke, P.; Bick, A.; Plenkers, W.; Meineke, G.; Becker, C.; Windpassinger, P.; Lewenstein, M.; Sengstock, K.
2011-05-01
In solid-state materials, the static and dynamic properties as well as the magnetic and electronic characteristics are crucially influenced by the crystal symmetry. Hexagonal structures play a particularly important role and lead to novel physics, such as that of carbon nanotubes or graphene. Here we report on the realization of ultracold atoms in a spin-dependent optical lattice with hexagonal symmetry. We show how the combined effects of the lattice and interactions between atoms lead to a forced antiferromagnetic Néel order when two spin-components localize at different lattice sites. We also demonstrate that the coexistence of two components--one Mott-insulating and the other one superfluid--leads to an interaction-induced modulation of the superfluid density, which is observed spectroscopically. Our studies reveal the vast impact of the interaction-induced modulation on the superfluid-to-Mott-insulator transition. The observations are consistent with theoretical predictions using Gutzwiller mean-field theory.
Conical wave propagation and diffraction in two-dimensional hexagonally packed granular lattices
NASA Astrophysics Data System (ADS)
Chong, C.; Kevrekidis, P. G.; Ablowitz, M. J.; Ma, Yi-Ping
2016-01-01
Linear and nonlinear mechanisms for conical wave propagation in two-dimensional lattices are explored in the realm of phononic crystals. As a prototypical example, a statically compressed granular lattice of spherical particles arranged in a hexagonal packing configuration is analyzed. Upon identifying the dispersion relation of the underlying linear problem, the resulting diffraction properties are considered. Analysis both via a heuristic argument for the linear propagation of a wave packet and via asymptotic analysis leading to the derivation of a Dirac system suggests the occurrence of conical diffraction. This analysis is valid for strong precompression, i.e., near the linear regime. For weak precompression, conical wave propagation is still possible, but the resulting expanding circular wave front is of a nonoscillatory nature, resulting from the complex interplay among the discreteness, nonlinearity, and geometry of the packing. The transition between these two types of propagation is explored.
Conical wave propagation and diffraction in two-dimensional hexagonally packed granular lattices
Chong, C.; Kevrekidis, P. G.; Ablowitz, M. J.; Ma, Yi-Ping
2016-01-25
We explore linear and nonlinear mechanisms for conical wave propagation in two-dimensional lattices in the realm of phononic crystals. As a prototypical example, a statically compressed granular lattice of spherical particles arranged in a hexagonal packing configuration is analyzed. Upon identifying the dispersion relation of the underlying linear problem, the resulting diffraction properties are considered. Analysis both via a heuristic argument for the linear propagation of a wave packet and via asymptotic analysis leading to the derivation of a Dirac system suggests the occurrence of conical diffraction. This analysis is valid for strong precompression, i.e., near the linear regime. Formore » weak precompression, conical wave propagation is still possible, but the resulting expanding circular wave front is of a nonoscillatory nature, resulting from the complex interplay among the discreteness, nonlinearity, and geometry of the packing. Lastly, the transition between these two types of propagation is explored.« less
Hexagonal RMnO3: a model system for two-dimensional triangular lattice antiferromagnets.
Sim, Hasung; Oh, Joosung; Jeong, Jaehong; Le, Manh Duc; Park, Je Geun
2016-02-01
The hexagonal RMnO3(h-RMnO3) are multiferroic materials, which exhibit the coexistence of a magnetic order and ferroelectricity. Their distinction is in their geometry that both results in an unusual mechanism to break inversion symmetry and also produces a two-dimensional triangular lattice of Mn spins, which is subject to geometrical magnetic frustration due to the antiferromagnetic interactions between nearest-neighbor Mn ions. This unique combination makes the h-RMnO3 a model system to test ideas of spin-lattice coupling, particularly when both the improper ferroelectricity and the Mn trimerization that appears to determine the symmetry of the magnetic structure arise from the same structure distortion. In this review we demonstrate how the use of both neutron and X-ray diffraction and inelastic neutron scattering techniques have been essential to paint this comprehensive and coherent picture of h-RMnO3. PMID:26830792
2D photonic crystals on the Archimedean lattices (tribute to Johannes Kepler (1571 1630))
NASA Astrophysics Data System (ADS)
Gajić, R.; class="cross-out">D. Jovanović,
2008-03-01
Results of our research on 2D Archemedean lattice photonic crystals are presented. This involves the calculations of the band structures, band-gap maps, equifrequency contours and FDTD simulations of electromagnetic propagation through the structures as well as an experimental verification of negative refraction at microwaves. The band-gap dependence on dielectric contrast is established both for dielectric rods in air and air-holes in dielectric materials. A special emphasis is placed on possibilities of negative refraction and left-handedness in these structures. Together with the familiar Archimedean lattices like square, triangular, honeycomb and Kagome' ones, we consider also, the less known, (3 2, 4, 3, 4) (ladybug) and (3, 4, 6, 4) (honeycomb-ring) structures.
Light trapping at Dirac point in 2D triangular Archimedean-like lattice photonic crystal.
Mao, Qiuping; Xie, Kang; Hu, Lei; Li, Qian; Zhang, Wei; Jiang, Haiming; Hu, Zhijia; Wang, Erlei
2016-04-20
Optical cavities and waveguides are critical parts of modern optical devices. Traditionally, optical cavities and waveguides rely on photonic bandgaps, or total internal reflection, to achieve light trapping. It has been reported that a novel light trapping, which exists in triangular and honeycomb lattices, is attributed to the so-called Dirac point. Our analysis reveals that 2D triangular Archimedean-like lattice photonic crystals also can support this Dirac mode with similar characteristics. This is a new type of localized mode with a different algebraic field profile at a different specified Dirac frequency, which is also beyond any complete photonic bandgap. The new wave localization has different features and can be applied to the design of new optical devices. PMID:27140119
NASA Astrophysics Data System (ADS)
Zeller, Patrick; Günther, Sebastian
2014-08-01
We present a systematic investigation of two coinciding lattices and their spatial beating frequencies that lead to the formation of moiré patterns. A mathematical model was developed and applied for the case of a hexagonally arranged adsorbate on a hexagonal support lattice. In particular, it describes the moiré patterns observed for graphene grown on a hexagonally arranged transition metal surface, a system that serves as one of the promising synthesis routes for the formation of this highly wanted material. The presented model uses a geometric construction that derives analytic expressions for first and higher order beating frequencies occurring for arbitrarily oriented graphene on the underlying substrate lattice. By solving the corresponding equations, we predict the size and orientation of the resulting moiré pattern. Adding the constraints for commensurability delivers further solvable analytic equations that predict whether or not first or higher order commensurable phases occur. We explicitly treat the case for first, second and third order commensurable phases. The universality of our approach is tested by comparing our data with moiré patterns that are experimentally observed for graphene on Ir(111) and on Pt(111). Our analysis can be applied for graphene, hexagonal boron nitride (h-BN), or other sp2-networks grown on any hexagonally packed support surface predicting the size, orientation and properties of the resulting moiré patterns. In particular, we can determine which commensurate phases are expected for these systems. The derived information can be used to critically discuss the moiré phases reported in the literature.
Exact ground state for the four-electron problem in a 2D finite honeycomb lattice
NASA Astrophysics Data System (ADS)
Trencsényi, Réka; Glukhov, Konstantin; Gulácsi, Zsolt
2014-07-01
Working in a subspace with dimensionality much smaller than the dimension of the full Hilbert space, we deduce exact four-particle ground states in 2D samples containing hexagonal repeat units and described by Hubbard type of models. The procedure identifies first a small subspace ? in which the ground state ? is placed, than deduces ? by exact diagonalization in ?. The small subspace is obtained by the repeated application of the Hamiltonian ? on a carefully chosen starting wave vector describing the most interacting particle configuration, and the wave vectors resulting from the application of ?, till the obtained system of equations closes in itself. The procedure which can be applied in principle at fixed but arbitrary system size and number of particles is interesting on its own since it provides exact information for the numerical approximation techniques which use a similar strategy, but apply non-complete basis for ?. The diagonalization inside ? provides an incomplete image of the low lying part of the excitation spectrum, but provides the exact ?. Once the exact ground state is obtained, its properties can be easily analysed. The ? is found always as a singlet state whose energy, interestingly, saturates in the ? limit. The unapproximated results show that the emergence probabilities of different particle configurations in the ground state presents 'Zittern' (trembling) characteristics which are absent in 2D square Hubbard systems. Consequently, the manifestation of the local Coulomb repulsion in 2D square and honeycomb types of systems presents differences, which can be a real source in the differences in the many-body behaviour.
Analysis of the antiferromagnetic phase transitions of the 2D Kondo lattice
NASA Astrophysics Data System (ADS)
Jones, Barbara
2010-03-01
The Kondo lattice continues to present an interesting and relevant challenge, with its interactions between Kondo, RKKY, and coherent order. We present our study[1] of the antiferromagnetic quantum phase transitions of a 2D Kondo-Heisenberg square lattice. Starting from the nonlinear sigma model as a model of antiferromagnetism, we carry out a renormalization group analysis of the competing Kondo-RKKY interaction to one-loop order in an ɛ-expansion. We find a new quantum critical point (QCP) strongly affected by Kondo fluctuations. Near this QCP, there is a breakdown of hydrodynamic behavior, and the spin waves are logarithmically frozen out. The renormalization group results allow us to propose a new phase diagram near the antiferromagnetic fixed point of this 2D Kondo lattice model. The T=0 phase diagram contains four phases separated by a tetracritical point, the new QCP. For small spin fluctuations, we find a stable local magnetic moment antiferromagnet. For stronger coupling, region II is a metallic quantum disordered paramagnet. We find in region III a paramagnetic phase driven by Kondo interactions, with possible ground states of a heavy fermion liquid or a Kondo driven spin-liquid. The fourth phase is a spiral phase, or a large-Fermi-surface antiferromagnetic phase. We will describe these phases in more detail, including possible experimental confirmation of the spiral phase. The existence of the tetracritical point found here would be expected to affect the phase diagram at finite temperatures as well. In addition, It is hoped that these results, and particularly the Kondo interaction paramagnetic phase, will serve to bridge to solutions starting from the opposite limit, of a Kondo effect leading to a heavy fermion ground state. Work in collaboration with T. Tzen Ong. [4pt] [1] T. Ong and B. A. Jones, Phys. Rev. Lett. 103, 066405 (2009).
Identifying topological edge states in 2D optical lattices using light scattering
NASA Astrophysics Data System (ADS)
Goldman, Nathan; Beugnon, Jérôme; Gerbier, Fabrice
2013-02-01
We recently proposed in a Letter [Phys. Rev. Lett. 108, 255303] a novel scheme to detect topological edge states in an optical lattice, based on a generalization of Bragg spectroscopy. The scope of the present article is to provide a more detailed and pedagogical description of the system - the Hofstadter optical lattice - and probing method. We first show the existence of topological edge states, in an ultra-cold gas trapped in a 2D optical lattice and subjected to a synthetic magnetic field. The remarkable robustness of the edge states is verified for a variety of external confining potentials. Then, we describe a specific laser probe, made from two lasers in Laguerre-Gaussian modes, which captures unambiguous signatures of these edge states. In particular, the resulting Bragg spectra provide the dispersion relation of the edge states, establishing their chiral nature. In order to make the Bragg signal experimentally detectable, we introduce a "shelving method", which simultaneously transfers angular momentum and changes the internal atomic state. This scheme allows to directly visualize the selected edge states on a dark background, offering an instructive view on topological insulating phases, not accessible in solid-state experiments.
Size dependence of bandgaps in a two-dimensional plasmonic crystal with a hexagonal lattice.
Saito, Hikaru; Yamamoto, Naoki
2015-02-01
The optical properties of surface plasmon polaritons (SPPs) are investigated at the Г point in a two-dimensional plasmonic crystal with a hexagonal lattice (Hex-PlC). The cathodoluminescence (CL) technique combined with a scanning transmission electron microscope (STEM) are used to produce spectral images of the SPP standing waves at the Г point and identify the four types of band-edge modes predicted by group theory. The systematic measurement of the band-edge energies employed here is used to determine the characteristic dependence of each band-edge mode on the structure parameters, which provides some criteria for the design of plasmonic devices with Hex-PlCs. PMID:25836118
Chiral phonons at high-symmetry points in monolayer hexagonal lattices.
Zhang, Lifa; Niu, Qian
2015-09-11
In monolayer hexagonal lattices, the intravalley and intervalley scattering of electrons can involve chiral phonons at Brillouin-zone center and corners, respectively. At these high-symmetry points, there is a threefold rotational symmetry endowing phonon eigenmodes with a quantized pseudoangular momentum, which includes orbital and spin parts. Conservation of pseudoangular momentum yields selection rules for intravalley and intervalley scattering of electrons by phonons. Concrete predictions of helicity-resolved optical phenomena are made on monolayer molybdenum disulfide. The chiral phonons at Brillouin-zone corners excited by polarized photons can be detected by a valley phonon Hall effect. The chiral phonons, together with phonon circular polarization, phonon pseudoangular momentum, selection rules, and valley phonon Hall effect will extend the basis for valley-based electronics and phononics applications in the future. PMID:26406841
Chiral Phonons at High-Symmetry Points in Monolayer Hexagonal Lattices
NASA Astrophysics Data System (ADS)
Zhang, Lifa; Niu, Qian
2015-09-01
In monolayer hexagonal lattices, the intravalley and intervalley scattering of electrons can involve chiral phonons at Brillouin-zone center and corners, respectively. At these high-symmetry points, there is a threefold rotational symmetry endowing phonon eigenmodes with a quantized pseudoangular momentum, which includes orbital and spin parts. Conservation of pseudoangular momentum yields selection rules for intravalley and intervalley scattering of electrons by phonons. Concrete predictions of helicity-resolved optical phenomena are made on monolayer molybdenum disulfide. The chiral phonons at Brillouin-zone corners excited by polarized photons can be detected by a valley phonon Hall effect. The chiral phonons, together with phonon circular polarization, phonon pseudoangular momentum, selection rules, and valley phonon Hall effect will extend the basis for valley-based electronics and phononics applications in the future.
Validation of KENO-VI: A comparison with hexagonal lattice light-water-reactor critical experiments
Lichtenwalter, J.J.
1998-06-01
The KENO-VI Monte Carlo code, released with Version 4.3 of the SCALE Code System, provides the capability to model more complex geometries than previously allowed by KENO-V.a. One significant improvement is the simplistic specification of hexprism unit cells and hexagonal arrays, an arduous task to complete in KENO-V.a. This report documents the validation of KENO-VI against 30 critical experiments consisting of low enriched uranium, light water reactor (LWR) fuel rods in hexagonal lattices with no poisons. The reference, enrichment, pitch, cladding, and core identification of the experiments are given. The results indicate that KENO-VI accurately calculates these critical experiments, with a bias of {minus}0.51% for the 238 group cross section library and {minus}0.24% for the 44 group cross section library. If these biases are properly taken into account, the KENO-VI code can be used with confidence for the design and safety analysis of storage and transportation systems of similar LWR type fuels.
NASA Astrophysics Data System (ADS)
Rodrigues, Clóves G.
2016-06-01
In this work we investigate the interatomic correlation moments in two-dimensional model of a weakly anharmonic crystal (i.e., not very high temperatures) with hexagonal lattice, using the Correlative Method of Unsymmetrized Self-Consistent Field (CUSF). The numerical results are obtained (and compared) by using the Morse and Lenard-Jones potentials.
Two-dimensional XXZ -Ising model on a square-hexagon lattice
NASA Astrophysics Data System (ADS)
Valverde, J. S.; Rojas, Onofre; de Souza, S. M.
2009-04-01
We study a two-dimensional XXZ -Ising model on a square-hexagon (denoted for simplicity by 4-6) lattice with spin 1/2. The phase diagram at zero temperature is discussed, where five states are found, two types of ferrimagnetic states, two types of antiferromagnetic states, and one ferromagnetic state. To solve this model, we have mapped onto the eight-vertex model with union Jack interaction term, and it was verified that the model cannot be completely mapped onto eight-vertex model. However, by imposing an exact solution condition, we have found the region where the XXZ -Ising model on 4-6 lattice is exactly soluble with one free parameter, particularly for the case of symmetric eight-vertex model condition. In this manner we have explored the properties of the system and have analyzed the interacting competition parameters which preserve the region where there is an exact solution. Unfortunately the present model does not satisfy the free fermion condition of the eight-vertex model, unless for a trivial solution. Even so, we are able to discuss the critical point region, beyond the region of exact resolvability.
NASA Astrophysics Data System (ADS)
Leconte, Nicolas; Martinez-Gordillo, Rafael; MacDonald, Allan; Jung, Jeil
Clear signatures of the Hofstadter butterfly have been experimentally observed in graphene on hexagonal boron nitride (G/BN), thanks to an appropriate balance between the length scale and the quality of the moiré superlattices. During this talk, I will present a methodology to map the continuum moiré pattern of incommensurable G/BN crystals obtained from ab initio calculations onto supercell lattice tight-binding Hamiltonians. Using efficient Lanczos recursion techniques for simulating large scale systems containing millions of atoms, the density of states and the dc conductivity are obtained as a function of energy or carrier density and magnetic field. The calculated Hofstadter butterflies and Landau fan diagrams show that the site potential variations, the mass, and substantial virtual strain contributions that appear even in the absence of real strains in the band Hamiltonian sensitively affect the electron-hole asymmetry, the gaps at the secondary Dirac points, as well as the tertiary features that appear at high-carrier densities.
Study on hexagonal super-lattice pattern with surface discharges in dielectric barrier discharge
Liu, Ying; Dong, Lifang Niu, Xuejiao; Gao, Yenan; Zhang, Chao
2015-10-15
The hexagonal super-lattice pattern with surface discharges (SDs) in dielectric barrier discharge is investigated by intensified charge-coupled device. The pattern is composed of the bright spot and the dim spot which is located at the centroid of surrounding other three bright spots. The phase diagram of the pattern as a function of the gas pressure and the argon concentration is given. The instantaneous images indicate that the bright spot emerging at the front of the current pulse is formed by the volume discharge (VD), and dim spot occurring at the tail of the current pulse is formed by the SD. The above result shows that the SD is induced by the VD. The simulation of the electric fields of wall charges accumulated by VDs confirms that the dim spot is formed by the confluences of the SDs of surrounding other three bright spots. By using optical emission spectrum method, both the molecule vibration temperature and electron density of the SD are larger than that of the VD.
Study on hexagonal super-lattice pattern with surface discharges in dielectric barrier discharge
NASA Astrophysics Data System (ADS)
Liu, Ying; Dong, Lifang; Niu, Xuejiao; Gao, Yenan; Zhang, Chao
2015-10-01
The hexagonal super-lattice pattern with surface discharges (SDs) in dielectric barrier discharge is investigated by intensified charge-coupled device. The pattern is composed of the bright spot and the dim spot which is located at the centroid of surrounding other three bright spots. The phase diagram of the pattern as a function of the gas pressure and the argon concentration is given. The instantaneous images indicate that the bright spot emerging at the front of the current pulse is formed by the volume discharge (VD), and dim spot occurring at the tail of the current pulse is formed by the SD. The above result shows that the SD is induced by the VD. The simulation of the electric fields of wall charges accumulated by VDs confirms that the dim spot is formed by the confluences of the SDs of surrounding other three bright spots. By using optical emission spectrum method, both the molecule vibration temperature and electron density of the SD are larger than that of the VD.
Lattice Boltzmann simulations of 2D laminar flows past two tandem cylinders
NASA Astrophysics Data System (ADS)
Mussa, Alberto; Asinari, Pietro; Luo, Li-Shi
2009-03-01
We apply the lattice Boltzmann equation (LBE) with multiple-relaxation-time (MRT) collision model to simulate laminar flows in two-dimensions (2D). In order to simulate flows in an unbounded domain with the LBE method, we need to address two issues: stretched non-uniform mesh and inflow and outflow boundary conditions. We use the interpolated grid stretching method to address the need of non-uniform mesh. We demonstrate that various inflow and outflow boundary conditions can be easily and consistently realized with the MRT-LBE. The MRT-LBE with non-uniform stretched grids is first validated with a number of test cases: the Poiseuille flow, the flow past a cylinder asymmetrically placed in a channel, and the flow past a cylinder in an unbounded domain. We use the LBE method to simulate the flow past two tandem cylinders in an unbounded domain with Re = 100. Our results agree well with existing ones. Through this work we demonstrate the effectiveness of the MRT-LBE method with grid stretching.
NASA Astrophysics Data System (ADS)
Villalobos, Gabriel; Linero, Dorian L.; Muñoz, José D.
2011-01-01
A 2D, hexagonal in geometry, statistical model of fracture is proposed. The model is based on the drying fracture process of the bamboo Guadua angustifolia. A network of flexible cells are joined by brittle junctures of fixed Young moduli that break at a certain thresholds in tensile force. The system is solved by means of the Finite Element Method (FEM). The distribution of avalanche breakings exhibits a power law with exponent -2.93(9), in agreement with the random fuse model (Bhattacharyya and Chakrabarti, 2006) [1].
NASA Astrophysics Data System (ADS)
Guillamon, I.; Vieira, S.; Suderow, H.; Cordoba, R.; Sese, J.; de Teresa, J. M.; Ibarra, R.
In two dimensional (2D) systems, theory has proposed that random disorder destroys long range correlations driving a transition to a glassy state. Here, I will discuss new insights into this issue obtained through the direct visualization of the critical behaviour of a 2D superconducting vortex lattice formed in a thin film with a smooth 1D thickness modulation. Using scanning tunneling microscopy at 0.1K, we have tracked the modification in the 2D vortex arrangements induced by the 1D thickness modulation while increasing the vortex density by three orders of magnitude. Upon increasing the field, we observed a two-step order-disorder transition in the 2D vortex lattice mediated by the appearance of dislocations and disclinations and accompanied by an increase in the local vortex density fluctuations. Through a detailed analysis of correlation functions, we find that the transition is driven by the incommensurate 1D thickness modulation. We calculate the critical points and exponents and find that they are well above theoretical expectation for random disorder. Our results show that long range 1D correlations in random potentials enhance the stability range of the ordered phase in a 2D vortex lattice. Work supported by Spanish MINECO, CIG Marie Curie Grant, Axa Research Fund and FBBVA.
Orso, G.; Stringari, S.; Menotti, C.
2006-11-10
We use Bogoliubov theory to calculate the beyond mean field correction to the equation of state of a weakly interacting Bose gas in the presence of a tight 2D optical lattice. We show that the lattice induces a characteristic 3D to 1D crossover in the behavior of quantum fluctuations. Using the hydrodynamic theory of superfluids, we calculate the corresponding shift of the collective frequencies of a harmonically trapped gas. We find that this correction can be of the order of a few percent and hence easily measurable in current experiments. The behavior of the quantum depletion of the condensate is also discussed.
NASA Astrophysics Data System (ADS)
Gräfe, Joachim; Weigand, Markus; Stahl, Claudia; Träger, Nick; Kopp, Michael; Schütz, Gisela; Goering, Eberhard J.; Haering, Felix; Ziemann, Paul; Wiedwald, Ulf
2016-01-01
The magnetization reversal in nanoscaled antidot lattices is widely investigated to understand the tunability of the magnetic anisotropy and the coercive field through nanostructuring of thin films. By investigating highly ordered focused ion beam milled antidot lattices with a combination of first-order reversal curves and magnetic x-ray microscopy, we fully elucidate the magnetization reversal along the distinct orientations of a hexagonal antidot lattice. This combination proves especially powerful as all partial steps of this complex magnetization reversal can be identified and subsequently imaged. Through this approach we discovered several additional steps that were neglected in previous studies. Furthermore, by imaging the microscopic magnetization state during each reversal step, we were able to link the coercive and interaction fields determined by the first-order reversal curve method to true microscopic magnetization configurations and determine their origin.
Jollet, Véronique; Albela, Belén; Sénéchal-David, Katell; Jégou, Pascale; Kolodziej, Emilie; Sainton, Joëlle; Bonneviot, Laurent; Banse, Frédéric
2013-08-28
A mixed amine pyridine polydentate Fe(II) complex was covalently tethered in hexagonal mesoporous silica of the MCM-41 type. Metal site isolation was generated using adsorbed tetramethylammonium cations acting as a patterned silanol protecting mask and trimethylsilylazane as a capping agent. Then, the amine/pyridine ligand bearing a tethering triethoxysilane group was either grafted to such a pretreated silica surface prior to or after complexation to Fe(II). These two synthetic routes, denoted as two-step and one-step, respectively, were also applied to fumed silica for comparison, except that the silanol groups were capped after tethering the metal unit. The coordination of the targeted complex was monitored using UV-visible spectrophotometry and, according to XPS, the best control was achieved inside the channels of the mesoporous silica for the two-step route. For the solid prepared according to the one-step route, tethering of the complex occurred mainly at the entrance of the channel. PMID:23824307
NASA Astrophysics Data System (ADS)
Volčko, Dušan; Quader, Khandker F.
2012-12-01
We consider fermions on a 2D square lattice with a finite-range pairing interaction, and obtain signatures for unconventional pair-symmetry states, dx2-y2 and extended-s (s*), in the Bardeen-Cooper-Schrieffer-Bose-Einstein Condensation crossover region. We find that the fermion momentum distribution function, vk2, the ratio of the Bogoliubov coefficients, vk/uk, and the Fourier transform of vk2 are strikingly different for d and s* symmetries in the crossover region. The chemical potential and the gap functions for both pairing symmetries show several interesting features as a function of interaction. Fermionic atoms in 2D optical lattices may provide a way to test these signatures. We discuss current generation cold atom experiments that may be utilized.
Zharova, Yu. A. Fedulova, G. V.; Astrova, E. V.; Baldycheva, A. V.; Tolmachev, V. A.; Perova, T. S.
2011-08-15
Design and fabrication technology of a microcavity structure based on a double heterojunction in macroporous silicon is suggested. The fabrication process of a strip of a 2D photonic crystal constituted by a finite number of lattice periods and the technique for defect formation by local opening of macropores on the substrate side, followed by filling of these macropores with a nematic liquid crystal, are considered.
NASA Technical Reports Server (NTRS)
Pogorzelski, Ronald J.
2004-01-01
When electronic oscillators are coupled to nearest neighbors to form an array on a hexagonal lattice, the planar phase distributions desired for excitation of a phased array antenna are not steady state solutions of the governing non-linear equations describing the system. Thus the steady state phase distribution deviates from planar. It is shown to be possible to obtain an exact solution for the steady state phase distribution and thus determine the deviation from the desired planar distribution as a function of beam steering angle.
Chemseddine, A. Bloeck, U.
2008-10-15
The structure and structural evolution of tungstic acid solutions, sols and gels are investigated by high-resolution electron microscopy (HRTEM). Acidification of sodium tungstate solutions, through a proton exchange resin, is achieved in a way that ensures homogeneity in size and shape of intermediate polytungstic species. Gelation is shown to involve polycondensation followed by a self-assembling process of polytungstic building blocks leading to sheets with a layered hexagonal structure. Single layers of this new metastable phase are composed of three-, four- and six-membered rings of WO{sub 6} octahedra located in the same plane. This is the first time that a 2D oxide crystal is isolated and observed by direct atomic resolution. Further ageing and structural evolution leading to single sheets of 2D ReO{sub 3}-type structure is directly observed by HRTEM. Based on this atomic level imaging, a model for the formation of the oxide network structure involving a self-assembling process of tritungstic based polymeric chain is proposed. The presence of tritungstic groups and their packing in electrochromic WO{sub 3} films made by different techniques is discussed. - Graphical abstract: From the isopolyanion to the extended bulk tungsten oxide: HRTEM imaging.
Anomalous giant piezoresistance in AlAs 2D electron systems with antidot lattices.
Gunawan, O; Gokmen, T; Shkolnikov, Y P; De Poortere, E P; Shayegan, M
2008-01-25
An AlAs two-dimensional electron system patterned with an antidot lattice exhibits a giant piezoresistance effect at low temperatures, with a sign opposite to the piezoresistance observed in the unpatterned region. We suggest that the origin of this anomalous giant piezoresistance is the nonuniform strain in the antidot lattice and the exclusion of electrons occupying the two conduction-band valleys from different regions of the sample. This is analogous to the well-known giant magnetoresistance effect, with valley playing the role of spin and strain the role of magnetic field. PMID:18233015
NASA Astrophysics Data System (ADS)
Sharma, Basant Lal
2015-12-01
Diffraction problems, associated with waves scattered by a semi-infinite crack and rigid constraint, in a hexagonal (honeycomb) lattice model, with nearest neighbor interactions, are solved exactly using the method of Wiener and Hopf. Asymptotic expressions for the scattered waves in far field are provided for both problems, by application of the method of stationary phase to corresponding diffraction integrals. Additionally, for the crack diffraction problem, bond lengths on the semi-infinite row complementing the crack, as well as the crack opening displacement, are provided in closed form except for the presence of concomitant Fourier coefficients of the Wiener-Hopf kernel. For the rigid constraint diffraction problem, the solution on the semi-infinite row complementing the constrained lattice sites, as well as that adjacent to the constrained row, are provided in similar closed form. The amplitude, as well as phase, of waves in far field is compared, through graphical plots, with that of a numerical solution on finite grid. Also, the analytical solution for few sites near the tip of each defect is compared with numerical solution. Both discrete Sommerfeld diffraction problems and their solutions are also relevant to numerical solution of the two-dimensional Helmholtz equation using a 4-point hexagonal grid, besides having applications inherent to the scattering of waves on a honeycomb structure.
Microscopy of a Quantum Gas in a 2D Optical Lattice
NASA Astrophysics Data System (ADS)
Bakr, Waseem; Peng, Amy; Tai, Ming; Ma, Ruichao; Jotzu, Gregor; Gillen, Jonathon; Foelling, Simon; Greiner, Markus
2010-03-01
Ultracold quantum gases in optical lattices provide a rich experimental toolbox for simulating the physics of condensed matter systems. With atoms in the lattice playing the role of electrons or Cooper pairs in real materials, it is possible to experimentally realize condensed matter Hamiltonians in a controlled way. To realize the full potential of such quantum simulations, we have created a quantum gas microscope (NA = 0.8) which can spatially resolve the atoms in the optical lattice at the single site level, and project arbitrary potential landscapes onto the atoms by combining the high resolution optics with static holographic masks or a spatial light modulator. The high resolution microscope operates with the atoms trapped in a two dimensional optical lattice at a distance of 10 microns from a glass surface that is part of the microscope. We have experimentally verified a resolution of ˜ 600 nm, providing the capability to study the phase diagram of the Bose Hubbard model by measuring occupation number at individual sites.
Lattice Boltzmann methods for some 2-D nonlinear diffusion equations:Computational results
Elton, B.H.; Rodrigue, G.H. . Dept. of Applied Science Lawrence Livermore National Lab., CA ); Levermore, C.D. . Dept. of Mathematics)
1990-01-01
In this paper we examine two lattice Boltzmann methods (that are a derivative of lattice gas methods) for computing solutions to two two-dimensional nonlinear diffusion equations of the form {partial derivative}/{partial derivative}t u = v ({partial derivative}/{partial derivative}x D(u){partial derivative}/{partial derivative}x u + {partial derivative}/{partial derivative}y D(u){partial derivative}/{partial derivative}y u), where u = u({rvec x},t), {rvec x} {element of} R{sup 2}, v is a constant, and D(u) is a nonlinear term that arises from a Chapman-Enskog asymptotic expansion. In particular, we provide computational evidence supporting recent results showing that the methods are second order convergent (in the L{sub 1}-norm), conservative, conditionally monotone finite difference methods. Solutions computed via the lattice Boltzmann methods are compared with those computed by other explicit, second order, conservative, monotone finite difference methods. Results are reported for both the L{sub 1}- and L{sub {infinity}}-norms.
Buckling in 2D periodic, soft and porous structures: effect of pore shape and lattice pattern
NASA Astrophysics Data System (ADS)
Shan, Sicong; Bertoldi, Katia; Shim, Jongmin; Overvelde, Johannes T. B.; Kang, Sung Hoon
2013-03-01
Adaptive structures allowing dramatic shape changes offer unique opportunities for the design of responsive and reconfigurable devices. Traditional morphing and foldable structures with stiff structural members and mechanical joints remains a challenge in manufacturing at small length scales. Soft structures where the folding mechanisms are induced by a mechanical instability represent a new class of novel adaptive materials which can be easily manufactured over a wide range of length scales. More specifically, soft porous structures with deliberately designed patterns can significantly change their architecture in response to diverse stimuli, opening avenues for reconfigurable devices that change their shapes to respond to their environment. While so far only two-dimensional periodic porous structures with circular holes arranged on a square or triangular lattice have been investigated, here we investigate both numerically and experimentally the effects of pore shape and lattice pattern on the macroscopic properties of the structures. Our results show that both the pore shape and lattice pattern can be used to effectively design desired materials and pave the way for the development of a new class of soft, active and reconfigurable devices over a wide range of length scales.
Algebraic rings of integers and some 2D lattice problems in physics
NASA Astrophysics Data System (ADS)
Nanxian, Chen; Zhaodou, Chen; Shaojun, Liu; Yanan, Shen; Xijin, Ge
1996-09-01
This paper develops the Möbius inversion formula for the Gaussian integers and Eisenstein's integers, and gives two applications. The first application is to the two-dimensional arithmetic Fourier transform (AFT), which is suitable for parallel processing. The second application is to two-dimensional inverse lattice problems, and is illustrated with the recovery of interatomic potentials from the cohesive energy for monolayer graphite. The paper demonstrates the potential application in the physical science of integral domains other than the standard integers.
Complex zeros of the 2 d Ising model on dynamical random lattices
NASA Astrophysics Data System (ADS)
Ambjørn, J.; Anagnostopoulos, K. N.; Magnea, U.
1998-04-01
We study the zeros in the complex plane of the partition function for the Ising model coupled to 2 d quantum gravity for complex magnetic field and for complex temperature. We compute the zeros by using the exact solution coming from a two matrix model and by Monte Carlo simulations of Ising spins on dynamical triangulations. We present evidence that the zeros form simple one-dimensional patterns in the complex plane, and that the critical behaviour of the system is governed by the scaling of the distribution of singularities near the critical point.
Hubbard Model study of Off Diagonally Confined fermions in a 2D Optical Lattice
NASA Astrophysics Data System (ADS)
Cone, Dave; Chiesa, Simone; Scalettar, Richard; Batrouni, George
2010-03-01
We report Quantum Monte Carlo simulations of a Hubbard Hamiltonian which incorporates a proposed new method for confining atoms in an optical lattice employing an inhomogeneous array of hopping matrix elements which trap atoms by going to zero at the lattice edges. This has been termed ``Off Diagonal Confinement (ODC)'' [1] to distinguish it from the more conventional use of a parabolic trap coupling to (diagonal) density operators. It has the advantage of producing systems which, while still being inhomogeneous, are entirely in the Mott phase, and allow simulations which are free of the sign problem at low temperatures. We analyze the effects of using ODC traps on the local density, density fluctuation, spin, and pairing correlation functions. Finally, we will discuss the advantages and importance of this new confinement technique for modeling correlated systems. Research supported by the Department of Energy, Office of Science SCIDAC program, DOE-DE-FC0206ER25793. [1] V.G. Rousseau et al., arXiv:0909.3543
Modeling Selective Local Interactions with Memory: Motion on a 2D Lattice.
Weinberg, Daniel; Levy, Doron
2014-06-15
We consider a system of particles that simultaneously move on a two-dimensional periodic lattice at discrete times steps. Particles remember their last direction of movement and may either choose to continue moving in this direction, remain stationary, or move toward one of their neighbors. The form of motion is chosen based on predetermined stationary probabilities. Simulations of this model reveal a connection between these probabilities and the emerging patterns and size of aggregates. In addition, we develop a reaction diffusion master equation from which we derive a system of ODEs describing the dynamics of the particles on the lattice. Simulations demonstrate that solutions of the ODEs may replicate the aggregation patterns produced by the stochastic particle model. We investigate conditions on the parameters that influence the locations at which particles prefer to aggregate. This work is a two-dimensional generalization of [Galante & Levy, Physica D, http://dx.doi.org/10.1016/j.physd.2012.10.010], in which the corresponding one-dimensional problem was studied. PMID:25045193
Yang Xuefeng; Cui Jian; Zhang Yuan; Liu Yue
2012-07-15
The dispersion relations of the externally and thermally (naturally) excited dust lattice modes (both longitudinal and transverse) in two-dimensional Debye-Yukawa complex plasma crystals are investigated. The dispersion relations are calculated numerically by taking the neutral gas damping effects into account and the numerical results are in agreement with the experimental data given by Nunomura et al.[Phys. Rev. E 65, 066402 (2002)]. It is found that for the mode excited by an external disturbance with a real frequency, the dispersion properties are changed at a critical frequency near where the group velocity of the mode goes to zero. Therefore, the high frequency branch with negative dispersion cannot be reached. In contrast, for the thermally excited mode, the dispersion curve can extend all the way to the negative dispersion region, while a 'cut-off' wave number exists at the long wavelength end of the dispersion in the transverse mode.
NASA Astrophysics Data System (ADS)
Dai, Jian-Qing; Zhu, Jian-Hui; Xu, Jie-Wang
2016-07-01
The recently discovered hexagonal ABC-type hyperferroelectrics, in which the polarization persists in the presence of the depolarization filed, exhibit a variety of intriguing and potentially useful properties [Garrity et al., Phys. Rev. Lett. 112, 127601 (2014)]. For the existing prototype of LiBeSb, we present detailed first-principles calculations concerning the lattice dynamics, electronic structure, and optical properties. An unstable longitudinal optic mode in the high-symmetry structure and a large polarization of 0.5 C/m2 in the polar phase are reported, including the remarkable dependence of Born effective charges on structural distortion. Using the HSE06 hybrid functional, we predict that LiBeSb has a small band-gap of 1.5 eV and shows dominant asymmetric covalent bonding character. Importantly, we find that there are remarkable absorptions in the whole visible spectrum. These features, combined with the enhanced carrier mobility, make LiBeSb as well as the whole family of hexagonal ABC-type hyperferroelectrics as promising candidates for ferroelectric photovoltaic materials with large bulk photovoltaic effect in the visible spectrum.
Ordering of Sphere Forming SISO Tetrablock Terpolymers on a Simple Hexagonal Lattice
Zhang, Jingwen; Sides, Scott; Bates, Frank S.
2012-03-15
Hexagonally ordered spherical and cylindrical morphologies (P{sub 6}/mmm and P{sub 6}/mm space group symmetries) have been identified in bulk poly(styrene-b-isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock terpolymers. These materials were synthesized by adding up to 32% by volume O blocks to a parent hydroxy-terminated SIS triblock copolymer containing 40% S by volume, and the resulting morphologies were characterized by small-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry and dynamic mechanical spectroscopy. Disordered, spherical and cylindrical phases were documented with increasing O content, where both ordered states exhibit hexagonal symmetry. Theoretical calculations based on a numerical self-consistent field theory for polymers provide crucial insights into the molecular configurations associated with these morphologies. These results offer a new approach to independently control domain shape and packing in block copolymer melts through manipulation of the magnitude and sequencing of the binary segment-segment interactions ({chi}{sub SI} {<=} {chi}{sub SO} << {chi}{sub IO}), which dictate core segregation and the effective interdomain interactions.
Particulate nature of blood determines macroscopic rheology: a 2-D lattice Boltzmann analysis.
Sun, Chenghai; Munn, Lance L
2005-03-01
Historically, predicting macroscopic blood flow characteristics such as viscosity has been an empirical process due to the difficulty in rigorously including the particulate nature of blood in a mathematical representation of blood rheology. Using a two-dimensional lattice Boltzmann approach, we have simulated the flow of red blood cells in a blood vessel to estimate flow resistance at various hematocrits and vessel diameters. By including white blood cells (WBCs) in the flow, we also calculate the increase in resistance due to white cell rolling and adhesion. The model considers the blood as a suspension of particles in plasma, accounting for cell-cell and cell-wall interactions to predict macroscopic blood rheology. The model is able to reproduce the Fahraeus-Lindqvist effect, i.e., the increase in relative apparent viscosity as tube size increases, and the Fahraeus effect, i.e., tube hematocrit is lower than discharge hematocrit. In addition, the model allows direct assessment of the effect of WBCs on blood flow in the microvasculature, reproducing the dramatic increases in flow resistance as WBCs enter short capillary segments. This powerful and flexible model can be used to predict blood flow properties in any vessel geometry and with any blood composition. PMID:15613630
Adiabatic and Hamiltonian computing on a 2D lattice with simple two-qubit interactions
NASA Astrophysics Data System (ADS)
Lloyd, Seth; Terhal, Barbara M.
2016-02-01
We show how to perform universal Hamiltonian and adiabatic computing using a time-independent Hamiltonian on a 2D grid describing a system of hopping particles which string together and interact to perform the computation. In this construction, the movement of one particle is controlled by the presence or absence of other particles, an effective quantum field effect transistor that allows the construction of controlled-NOT and controlled-rotation gates. The construction translates into a model for universal quantum computation with time-independent two-qubit ZZ and XX+YY interactions on an (almost) planar grid. The effective Hamiltonian is arrived at by a single use of first-order perturbation theory avoiding the use of perturbation gadgets. The dynamics and spectral properties of the effective Hamiltonian can be fully determined as it corresponds to a particular realization of a mapping between a quantum circuit and a Hamiltonian called the space-time circuit-to-Hamiltonian construction. Because of the simple interactions required, and because no higher-order perturbation gadgets are employed, our construction is potentially realizable using superconducting or other solid-state qubits.
A hierarchical lattice spring model to simulate the mechanics of 2-D materials-based composites
NASA Astrophysics Data System (ADS)
Brely, Lucas; Bosia, Federico; Pugno, Nicola
2015-07-01
In the field of engineering materials, strength and toughness are typically two mutually exclusive properties. Structural biological materials such as bone, tendon or dentin have resolved this conflict and show unprecedented damage tolerance, toughness and strength levels. The common feature of these materials is their hierarchical heterogeneous structure, which contributes to increased energy dissipation before failure occurring at different scale levels. These structural properties are the key to exceptional bioinspired material mechanical properties, in particular for nanocomposites. Here, we develop a numerical model in order to simulate the mechanisms involved in damage progression and energy dissipation at different size scales in nano- and macro-composites, which depend both on the heterogeneity of the material and on the type of hierarchical structure. Both these aspects have been incorporated into a 2-dimensional model based on a Lattice Spring Model, accounting for geometrical nonlinearities and including statistically-based fracture phenomena. The model has been validated by comparing numerical results to continuum and fracture mechanics results as well as finite elements simulations, and then employed to study how structural aspects impact on hierarchical composite material properties. Results obtained with the numerical code highlight the dependence of stress distributions on matrix properties and reinforcement dispersion, geometry and properties, and how failure of sacrificial elements is directly involved in the damage tolerance of the material. Thanks to the rapidly developing field of nanocomposite manufacture, it is already possible to artificially create materials with multi-scale hierarchical reinforcements. The developed code could be a valuable support in the design and optimization of these advanced materials, drawing inspiration and going beyond biological materials with exceptional mechanical properties.
Beyond classical nucleation theory: A 2-D lattice-gas automata model
NASA Astrophysics Data System (ADS)
Hickey, Joseph
Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster's size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), eta (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system's concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system's equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, delta, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of eta and Esw (Ess). On the other hand, delta increases as eta increases while E ss and Esw are held constant. The constant surface tension term of the CNT, Sigma0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as eta is increased. Together
ROHATGI,U.S.; JO,J.; CHUNG,B.D.; TAKAHASHI,H.
2002-06-09
Safety analyses of a proliferation resistant, economically competitive, high conversion, boiling water reactor (HCBWR) fueled with fissile plutonium and fertile thorium oxide fuel elements, and with passive safety systems are presented here. The HCBWR developed here is characterized by a very tight lattice with a relatively small water volume fraction in the core which therefore operates with a fast reactor neutron spectrum, and a considerably improved neutron economy compared to the current generation of Light Water Reactors. The tight lattice core has a very narrow flow channels with a hydraulic diameter less than half of the regular BWR core and, thus, presents a special challenge to core cooling, because of reduced water inventory and high friction in the core. The primary safety concern when reducing the moderator to fuel ratio and when using a tightly packed lattice arrangement is to maintain adequate cooling of the core during both normal operation and accident scenarios. In the preliminary HCBWR design, the core has been placed in a vessel with a large chimney section, and the vessel is connected with Isolation Condenser System (ICs). The vessel is placed in containment with Gravity Driven Cooling System (GDCS) and Passive Containment Cooling System (PCCS) in a configuration similar to General Electric's Simplified Boiling Water Reactor (SBWR). The safety systems are similar to SBWR; ICs and PCCS are scaled with power. An internal recirculation pump was placed in the downcomer to augment the buoyancy head provided by the chimney, since the buoyancy provided by the chimney alone could not generate sufficient recirculation in the vessel as the tight lattice configuration resulted in much larger friction in the core than the SBWR. The constitutive relationships for RELAP5 were assessed for narrow channels, and as a result the heat transfer package was modified. The modified RELAP5 was used to simulate and analyze two of the most limiting events for a tight
Ao, L; Pham, A; Xiao, H Y; Zu, X T; Li, S
2016-03-14
We have systematically investigated the effects of different vacancy defects in 2D d(0) materials SnS2 and ZrS2 using first principles calculations. The theoretical results show that the single cation vacancy and the vacancy complex like V-SnS6 can induce large magnetic moments (3-4 μB) in these single layer materials. Other defects, such as V-SnS3, V-S, V-ZrS3 and V-ZrS6, can result in n-type conductivity. In addition, the ab initio studies also reveal that the magnetic and conductive properties from the cation vacancy and the defect complex V-SnS6 can be modified using the compressive/tensile strain of the in-plane lattices. Specifically, the V-Zr doped ZrS2 monolayer can be tuned from a ferromagnetic semiconductor to a metallic/half-metallic material with decreasing/increasing magnetic moments depending on the external compressive/tensile strains. On the other hand, the semiconducting and magnetic properties of V-Sn doped SnS2 is preserved under different lattice compression and tension. For the defect complex like V-SnS6, only the lattice compression can tune the magnetic moments in SnS2. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of SnS2 and ZrS2 can be tuned without the need for chemical doping. PMID:26888010
NASA Astrophysics Data System (ADS)
Woods, Justin; Bhat, Vinayak; Farmer, Barry; Sklenar, Joseph; Teipel, Eric; Ketterson, John; Hastings, J. Todd; de Long, Lance
2015-03-01
Artificial spin ice (ASI) systems are composed of nanoscale ferromagnetic segments whose shape anisotropy dictates they behave as mesoscopic Ising spins. Most ASI have segments patterned on periodic lattices and a single vertex topology. We have continuously distorted 2D honeycomb and square lattices such that the pattern vertex spacings follow a Fibonacci chain sequence along primitive lattice directions. The Fibonacci distortion is related to the aperiodic translational symmetry of 2D artificial quasicrystals1 that cannot be viewed as continuous distortions of periodic lattices due to their forbidden (e.g., fivefold) rotational symmetries. In contrast, Fibonacci distortions of 2D periodic lattices can be ``turned on'' by control of the ratio of two lattice parameters d1 and d2. Distortions alter film segments such that pattern vertices are no longer equivalent and traditional spin ice rules are no longer strictly valid. We have performed OOMMF simulations of magnetization reversal for samples having different levels of distortion, and found the magnetic reversal to be dramatically slowed by small distortions (d1/d2 ~ 1). Research at Kentucky is supported by U.S. DoE Grant DE-FG02-97ER45653 and NSF Grant EPS-0814194.
An exact and efficient first passage time algorithm for reaction–diffusion processes on a 2D-lattice
Bezzola, Andri; Bales, Benjamin B.; Alkire, Richard C.; Petzold, Linda R.
2014-01-01
We present an exact and efficient algorithm for reaction–diffusion–nucleation processes on a 2D-lattice. The algorithm makes use of first passage time (FPT) to replace the computationally intensive simulation of diffusion hops in KMC by larger jumps when particles are far away from step-edges or other particles. Our approach computes exact probability distributions of jump times and target locations in a closed-form formula, based on the eigenvectors and eigenvalues of the corresponding 1D transition matrix, maintaining atomic-scale resolution of resulting shapes of deposit islands. We have applied our method to three different test cases of electrodeposition: pure diffusional aggregation for large ranges of diffusivity rates and for simulation domain sizes of up to 4096×4096 sites, the effect of diffusivity on island shapes and sizes in combination with a KMC edge diffusion, and the calculation of an exclusion zone in front of a step-edge, confirming statistical equivalence to standard KMC simulations. The algorithm achieves significant speedup compared to standard KMC for cases where particles diffuse over long distances before nucleating with other particles or being captured by larger islands.
NASA Astrophysics Data System (ADS)
Velarde, M. G.; Ebeling, W.; Chetverikov, A. P.
2013-01-01
We study the thermal excitation of intrinsic localized modes in the form of solitons in 1d and 2d anharmonic lattices at moderately high temperatures. Such finite-amplitude fluctuations form long-living dynamical structures with life-time in the pico-second range thus surviving a relatively long time in comparison to other thermal fluctuations. Further we discuss the influence of such long-living fluctuations on the dynamics of added excess free electrons. The atomic lattice units are treated as quasi-classical objects interacting by Morse forces and stochastically moving according to Langevin equations. In 2d the atoms are initially organized in a triangular lattice. The electron distributions are in a first estimate represented by equilibrium adiabatic distributions in the actual polarization fields. Computer simulations show that in 2d systems such excitations are moving with supersonic velocities along lattice rows oriented with the cristallographic axes. By following the electron distributions we have also been able to study the excitations of solectron type (electron-soliton dynamic bound states) and estimate their life times.
NASA Astrophysics Data System (ADS)
Akahama, Y.; Ishihara, D.; Yamashita, H.; Fujihisa, H.; Hirao, N.; Ohishi, Y.
2016-08-01
The pressure-temperature (P -T ) phase diagram of N2-O2 mixture with a composition of N2-48 mol % O2 has been investigated using x-ray diffraction and the phase stability of a hexagonal phase (space group: P 6 /mmm), with the kagome lattice examined under high-pressure and low-temperature conditions. While the phase appears as a low-temperature phase of the cubic phase (P m 3 n ) with the structure of γ -O2 or δ -N2 and is stable in a wide range of pressures and temperatures, it transforms to lower symmetry monoclinic or orthorhombic phases at lower temperature, accompanied with a distortion of the kagome lattice. Based on Rietveld refinements, the monoclinic and orthorhombic phases are found to be in the P 21/a and Cmmm space groups, respectively. In magnetization measurements, a magnetic transition is observed with a relatively large drop of magnetization, corresponding to the cubic-to-hexagonal phase transition. This suggests that the hexagonal phase has a certain magnetic ordered state that arises from the molecular magnetic moment of O2.
NASA Astrophysics Data System (ADS)
Sfyris, D.; Koukaras, E. N.; Pugno, N.; Galiotis, C.
2015-08-01
Continuum modeling of free-standing graphene monolayer, viewed as a two dimensional 2-lattice, requires specification of the components of the shift vector that acts as an auxiliary variable. If only in-plane motions are considered, the energy depends on an in-plane strain measure and the shift vector. The assumption of geometrical and material linearity leads to quadratic energy terms with respect to the shift vector, the strain tensor, and their combinations. Graphene's hexagonal symmetry reduces the number of independent moduli then to four. We evaluate these four material parameters using molecular calculations and the adaptive intermolecular reactive empirical bond order potential and compare them with standard linear elastic constitutive modeling. The results of our calculations show that the predicted values are in reasonable agreement with those obtained solely from our molecular calculations as well as those from the literature. To the best of our knowledge, this is the first attempt to measure mechanical properties when graphene is modeled as a hexagonal 2-lattice. This work targets at the continuum scale when the insight measurements come from finer scales using atomistic simulations.
Sfyris, D. E-mail: dsfyris@sfyris.net; Koukaras, E. N.; Pugno, N.; Galiotis, C.
2015-08-21
Continuum modeling of free-standing graphene monolayer, viewed as a two dimensional 2-lattice, requires specification of the components of the shift vector that acts as an auxiliary variable. If only in-plane motions are considered, the energy depends on an in-plane strain measure and the shift vector. The assumption of geometrical and material linearity leads to quadratic energy terms with respect to the shift vector, the strain tensor, and their combinations. Graphene's hexagonal symmetry reduces the number of independent moduli then to four. We evaluate these four material parameters using molecular calculations and the adaptive intermolecular reactive empirical bond order potential and compare them with standard linear elastic constitutive modeling. The results of our calculations show that the predicted values are in reasonable agreement with those obtained solely from our molecular calculations as well as those from the literature. To the best of our knowledge, this is the first attempt to measure mechanical properties when graphene is modeled as a hexagonal 2-lattice. This work targets at the continuum scale when the insight measurements come from finer scales using atomistic simulations.
NaKV4O9·2H2O: a new 2D magnetic compound with a 1/5-depleted square lattice.
Cui, Meiyan; He, Zhangzhen; Wang, Nannan; Tang, Yingying; Guo, Wenbin; Zhang, Suyun; Wang, Lin; Xiang, Hongping
2016-03-15
A new vanadate compound NaKV4O9·2H2O is successfully synthesized by a conventional hydrothermal method. This compound crystallizes in the monoclinic system with the space group C2/c, showing a typical 2D layered structure built from VO5 pyramids, in which the layers are separated by Na(+), K(+), and H2O. The topology structure of magnetic V(4+) ions shows a quite interesting 1/5-depleted square lattice, which is quite similar to that of a famous low-dimensional quantum spin system CaV4O9. A structural and magnetic comparison confirmed that the title compound may exhibit a more pronounced 2D character with a large spin gap. PMID:26892907
NASA Astrophysics Data System (ADS)
Yang, PeiPei; Wen, Zhi; Dou, RuiFeng; Liu, Xunliang
2016-08-01
Flow and heat transfer through a 2D random porous medium are studied by using the lattice Boltzmann method (LBM). For the random porous medium, the influence of disordered cylinder arrangement on permeability and Nusselt number are investigated. Results indicate that the permeability and Nusselt number for different cylinder locations are unequal even with the same number and size of cylinders. New correlations for the permeability and coefficient b‧Den of the Forchheimer equation are proposed for random porous medium composed of Gaussian distributed circular cylinders. Furthermore, a general set of heat transfer correlations is proposed and compared with existing experimental data and empirical correlations. Our results show that the Nu number increases with the increase of the porosity, hence heat transfer is found to be accurate considering the effect of porosity.
Liu, Ying; Dong, Li-fang; Niu, Xue-jiao; Zhang, Chao
2016-02-01
The hexagonal super-lattice pattern composed of the light spot and the dim spot is firstly observed and investigated in the discharge of gas mixture of air and argon by using the dielectric barrier discharge device with double water electrodes. It is found that the dim spot is located at the center of its surrounding three light spots by observing the discharge image. Obviously, the brightness of the light spot and the dim spot are different, which indicates that the plasma states of the light spot and the dim spot may be different. The optical emission spectrum method is used to further study the several plasma parameters of the light spot and the dim spot in different argon content. The emission spectra of the N₂ second positive band (C³IIu --> B³IIg) are measured, from which the molecule vibration temperatures of the light spot and the dim spot are calculated. Based on the relative intensity ratio of the line at 391.4 nm and the N₂ line at 394.1 nm, the average electron energies of the light spot and the dim spot are investigated. The broadening of spectral line 696.57 nm (2P₂-1S₅) is used to study the electron densities of the light spot and the dim spot. The experiment shows that the molecule vibration temperature, average electron energy and the electron density of the dim spot are higher than those of the light spot in the same argon content. The molecule vibration temperature and electron density of the light spot and dim spot increase with the argon content increasing from 70% to 95%, while average electron energies of the light spot and dim spot decrease gradually. The short-exposure image recorded by a high speed video camera shows that the dim spot results from the surface discharges (SDs). The surface discharge induced by the volume discharge (VD) has the decisive effect on the formation of the dim spot. The experiment above plays an important role in studying the formation mechanism of the hexagonal super-lattice pattern with light spot and
Defects in bilayer silica and graphene: common trends in diverse hexagonal two-dimensional systems
Björkman, Torbjörn; Kurasch, Simon; Lehtinen, Ossi; Kotakoski, Jani; Yazyev, Oleg V.; Srivastava, Anchal; Skakalova, Viera; Smet, Jurgen H.; Kaiser, Ute; Krasheninnikov, Arkady V.
2013-01-01
By combining first-principles and classical force field calculations with aberration-corrected high-resolution transmission electron microscopy experiments, we study the morphology and energetics of point and extended defects in hexagonal bilayer silica and make comparison to graphene, another two-dimensional (2D) system with hexagonal symmetry. We show that the motifs of isolated point defects in these 2D structures with otherwise very different properties are similar, and include Stone-Wales-type defects formed by structural unit rotations, flower defects and reconstructed double vacancies. The morphology and energetics of extended defects, such as grain boundaries have much in common as well. As both sp2-hybridised carbon and bilayer silica can also form amorphous structures, our results indicate that the morphology of imperfect 2D honeycomb lattices is largely governed by the underlying symmetry of the lattice. PMID:24336488
NASA Astrophysics Data System (ADS)
Paul, Barnita; Chatterjee, Swastika; Gop, Sumana; Roy, Anushree; Grover, Vinita; Shukla, Rakesh; Tyagi, A. K.
2016-07-01
Rare-earth indates emerge as one of the efficient geometric ferroelectric materials, in which the spontaneous polarization can be tuned by varying their crystal structure along the 4f rare-earth series. We report a systematic study of structural changes in hexagonal REInO3 perovskite (RE = Ho3+, Dy3+, Tb3+, Gd3+, Eu3+, Sm3+) and YInO3 of P63 cm space group by powder x-ray diffraction (XRD) and Raman scattering measurements. The crystal structure of these materials could be investigated by the Rietveld refinement of their XRD patterns. We have calculated density of states of phonons using density functional theory and examined the atomic displacements corresponding to observed Raman modes. The evolution of lattice dynamics of REInO3 has been probed by correlating various Raman modes with the structural distortion of the unit cell and the characteristics of the rare-earth ions. We report the appearance of the coupled mode in the phonon spectra. We have estimated spontaneous polarization from the structural distortion in this system and shown that it can be modulated by varying RE3+ ions in REInO3. We also report the appearance of a ferroelectric soft Raman mode, a unique characteristic of these materials.
Ashley, Carlee E; Dunphy, Darren R; Jiang, Zhang; Carnes, Eric C; Yuan, Zhen; Petsev, Dimiter N; Atanassov, Plamen B; Velev, Orlin D; Sprung, Michael; Wang, Jin; Peabody, David S; Brinker, C Jeffrey
2011-04-18
The rapid assembly of icosohedral virus-like particles (VLPs) into highly ordered (domain size > 600 nm), oriented 2D superlattices directly onto a solid substrate using convective coating is demonstrated. In-situ grazing-incidence small-angle X-ray scattering (GISAXS) is used to follow the self-assembly process in real time to characterize the mechanism of superlattice formation, with the ultimate goal of tailoring film deposition conditions to optimize long-range order. From water, GISAXS data are consistent with a transport-limited assembly process where convective flow directs assembly of VLPs into a lattice oriented with respect to the water drying line. Addition of a nonvolatile solvent (glycerol) modified this assembly pathway, resulting in non-oriented superlattices with improved long-range order. Modification of electrostatic conditions (solution ionic strength, substrate charge) also alters assembly behavior; however, a comparison of in-situ assembly data between VLPs derived from the bacteriophages MS2 and Qβ show that this assembly process is not fully described by a simple Derjaguin-Landau-Verwey-Overbeek model alone. PMID:21425464
Pérez-Montoto, Lázaro G; Santana, Lourdes; González-Díaz, Humberto
2009-11-01
We introduce here a new class of invariants for MD trajectories based on the spectral moments pi(k)(L) of the Markov matrix associated to lattice network-like (LN) graph representations of Molecular Dynamics (MD) trajectories. The procedure embeds the MD energy profiles on a 2D Cartesian coordinates system using simple heuristic rules. At the same time, we associate the LN with a Markov matrix that describes the probabilities of passing from one state to other in the new 2D space. We construct this type of LNs for 422 MD trajectories obtained in DNA-drug docking experiments of 57 furocoumarins. The combined use of psoralens+ultraviolet light (UVA) radiation is known as PUVA therapy. PUVA is effective in the treatment of skin diseases such as psoriasis and mycosis fungoides. PUVA is also useful to treat human platelet (PTL) concentrates in order to eliminate Leishmania spp. and Trypanosoma cruzi. Both are parasites that cause Leishmaniosis (a dangerous skin and visceral disease) and Chagas disease, respectively; and may circulate in blood products collected from infected donors. We included in this study both lineal (psoralens) and angular (angelicins) furocoumarins. In the study, we grouped the LNs on two sets; set1: DNA-drug complex MD trajectories for active compounds and set2: MD trajectories of non-active compounds or no-optimal MD trajectories of active compounds. We calculated the respective pi(k)(L) values for all these LNs and used them as inputs to train a new classifier that discriminate set1 from set2 cases. In training series the model correctly classifies 79 out of 80 (specificity=98.75%) set1 and 226 out of 238 (Sensitivity=94.96%) set2 trajectories. In independent validation series the model correctly classifies 26 out of 26 (specificity=100%) set1 and 75 out of 78 (sensitivity=96.15%) set2 trajectories. We propose this new model as a scoring function to guide DNA-docking studies in the drug design of new coumarins for anticancer or antiparasitic
Dea-Ayuela, María Auxiliadora; Pérez-Castillo, Yunierkis; Meneses-Marcel, Alfredo; Ubeira, Florencio M; Bolas-Fernández, Francisco; Chou, Kuo-Chen; González-Díaz, Humberto
2008-08-15
The toxicity and inefficacy of actual organic drugs against Leishmaniosis justify research projects to find new molecular targets in Leishmania species including Leishmania infantum (L. infantum) and Leishmaniamajor (L. major), both important pathogens. In this sense, quantitative structure-activity relationship (QSAR) methods, which are very useful in Bioorganic and Medicinal Chemistry to discover small-sized drugs, may help to identify not only new drugs but also new drug targets, if we apply them to proteins. Dyneins are important proteins of these parasites governing fundamental processes such as cilia and flagella motion, nuclear migration, organization of the mitotic splinde, and chromosome separation during mitosis. However, despite the interest for them as potential drug targets, so far there has been no report whatsoever on dyneins with QSAR techniques. To the best of our knowledge, we report here the first QSAR for dynein proteins. We used as input the Spectral Moments of a Markov matrix associated to the HP-Lattice Network of the protein sequence. The data contain 411 protein sequences of different species selected by ClustalX to develop a QSAR that correctly discriminates on average between 92.75% and 92.51% of dyneins and other proteins in four different train and cross-validation datasets. We also report a combined experimental and theoretic study of a new dynein sequence in order to illustrate the utility of the model to search for potential drug targets with a practical example. First, we carried out a 2D-electrophoresis analysis of L. infantum biological samples. Next, we excised from 2D-E gels one spot of interest belonging to an unknown protein or protein fragment in the region M<20,200 and pI<4. We used MASCOT search engine to find proteins in the L. major data base with the highest similarity score to the MS of the protein isolated from L. infantum. We used the QSAR model to predict the new sequence as dynein with probability of 99.99% without
NASA Astrophysics Data System (ADS)
Kumar, Ajit; Verma, Sanjay K.; Alvi, P. A.; Jasrotia, Dinesh
2016-04-01
The nanospatial morphological features of [ZnCl]- [C5H4NCH3]+ hybrid derivative depicts 28 nm granular size and 3D spreader shape packing pattern as analyzed by FESEM and single crystal XRD structural studies. The organic moiety connect the inorganic components through N-H+…Cl- hydrogen bond to form a hybrid composite, the replacement of organic derivatives from 2-methylpyridine to 2-Amino-5-choloropyridine results the increase in granular size from 28nm to 60nm and unit cell packing pattern from 3D-2D lattice dimensionality along ac plane. The change in optical energy direct band gap value from 3.01eV for [ZnCl]- [C5H4NCH3]+ (HM1) to 3.42eV for [ZnCl]- [C5H5ClN2]+ (HM2) indicates the role of organic moiety in optical properties of hybrid materials. The photoluminescence emission spectra is observed in the wavelength range of 370 to 600 nm with maximum peak intensity of 9.66a.u. at 438 nm for (HM1) and 370 to 600 nm with max peak intensity of 9.91 a.u. at 442 nm for (HM2), indicating that the emission spectra lies in visible range. PL excitation spectra depicts the maximum excitation intensity [9.8] at 245.5 nm for (HM1) and its value of 9.9 a.u. at 294 nm, specify the excitation spectra lies in UV range. Photoluminescence excitation spectra is observed in the wavelength range of 280 to 350 nm with maximum peak intensity of 9.4 a.u. at 285.5 nm and 9.9 a.u. at 294 and 297 nm, indicating excitation in the UV spectrum. Single crystal growth process and detailed physiochemical characterization such as XRD, FESEM image analysis photoluminescence property reveals the structure stability with non-covalent interactions, lattice dimensionality (3D-2D) correlations interweaving into the design of inorganic-organic hybrid materials.
B{sub 27}{sup −}: Appearance of the smallest planar boron cluster containing a hexagonal vacancy
Li, Wei-Li; Piazza, Zachary A.; Wang, Lai-Sheng; Pal, Rhitankar; Zeng, Xiao Cheng
2015-05-28
Photoelectron spectroscopy and ab initio calculations have been carried out to probe the structures and chemical bonding of the B{sub 27}{sup −} cluster. Comparison between the experimental spectrum and the theoretical results reveals a two-dimensional (2D) global minimum with a triangular lattice containing a tetragonal defect (I) and two low-lying 2D isomers (II and III), each with a hexagonal vacancy. All three 2D isomers have 16 peripheral boron atoms and 11 inner boron atoms. Isomer I is shown to be mainly responsible for the observed photoelectron spectrum with isomers II and III as minor contributors. Chemical bonding analyses of these three isomers show that they all feature 16 localized peripheral B–B σ-bonds. Additionally, isomer I possesses 16 delocalized σ bonds and nine delocalized π bonds, while isomers II and III each contain 17 delocalized σ bonds and eight delocalized π bonds. It is found that the hexagonal vacancy is associated generally with an increase of delocalized σ bonds at the expense of delocalized π bonds in 2D boron clusters. The hexagonal vacancy, characteristic of borophenes, is found to be a general structural feature for mid-sized boron clusters. The current study shows that B{sub 27}{sup −} is the first boron cluster, where a hexagonal vacancy appears among the low-lying isomers accessible experimentally.
NASA Astrophysics Data System (ADS)
Bouchiat, Marie-Anne; Bouchiat, Claude
2012-10-01
We have constructed the geometric phases emerging from the non-trivial topology of a space-dependent magnetic field B(r), interacting with the spin magnetic moment of a neutral particle. Our basic tool, adapted from a previous work on Berry’s phases, is the space-dependent unitary transformation {U}({\\mathbf {r}}), which leads to the identity, {U}({\\mathbf {r}})^{\\dag }\\, {\\mathbf {S}}\\,{\\bm \\cdot}\\, {\\mathbf {B}}({\\mathbf {r}}) \\, {U}({\\mathbf {r}}) = \\vert {\\mathbf {B}}({\\mathbf {r}}) \\vert \\, S_z, at each point r. In the ‘rotated’ Hamiltonian \\widehat{ H}, \\frac{ \\partial }{\\partial {\\mathbf {r}}} is replaced by the non-Abelian covariant derivative \\frac{ \\partial }{\\partial {\\mathbf {r}}}- \\frac{i}{\\hbar } {A}({\\mathbf {r}}) where {A}({\\mathbf {r}}) = i \\hbar \\, {U}^{\\dag }\\,{\\bm\\cdot}\\, \\frac{ \\partial }{\\partial {\\mathbf {r}}} {U} can be written as A1(r)Sx + A2(r)Sy + A3(r)Sz. The Abelian differentials Ak(r)·dr are given in terms of the Euler angles defining the orientation of B(r). The non-Abelian field {A}({\\mathbf {r}}) transforms as a Yang-Mills field; however, its vanishing ‘curvature’ reveals its purely geometric character. We have defined a perturbation scheme based upon the assumption that in \\widehat{ H} the longitudinal field A3(r) dominates the transverse field A1, 2(r) contributions, evaluated to second order. The geometry embedded in both the vector field A3(r) and the geometric magnetic field \\mathbf { B}_3 ({\\mathbf {r}}) = \\frac{ \\partial }{\\partial {\\mathbf {r}}}\\wedge {{\\mathbf {A}}}_3({\\mathbf {r}}) is described by their associated Aharonov-Bohm phase. As an illustration we study the physics of cold 171Yb atoms dressed by overlaying two circularly polarized stationary waves with orthogonal directions, which form a 2D square optical lattice. The frequency is tuned midway between the two hyperfine levels of the (6s6p)3P1 states to protect the optical B(r) field generated by the
Gao, Song; Fan, Rui Qing; Wang, Xin Ming; Wei, Li Guo; Song, Yang; Du, Xi; Xing, Kai; Wang, Ping; Yang, Yu Lin
2016-07-28
In this work, a rare 2D → 3D single-crystal-to-single-crystal transformation (SCSC) is observed in metal-organic coordination complexes, which is triggered by thermal treatment. The 2D two-fold interpenetrating square lattice layer [Cd(IBA)2]n (1) is irreversibly converted into a 3D four-fold interpenetrating diamond framework {[Cd(IBA)2(H2O)]·2.5H2O}n (2) (HIBA = 4-(1H-imidazol-1-yl)benzoic acid). Consideration is given to these two complexes with different interpenetrating structures and dimensionality, and their influence on photovoltaic properties are studied. Encouraged by the UV-visible absorption and HOMO-LUMO energy states matched for sensitizing TiO2, the two complexes are employed in combination with N719 in dye-sensitized solar cells (DSSCs) to compensate absorption in the ultraviolet and blue-violet region, offset competitive visible light absorption of I3(-) and reducing charge the recombination of injected electrons. After co-sensitization with 1 and 2, the device co-sensitized by 1/N719 and 2/N719 to yield overall efficiencies of 7.82% and 8.39%, which are 19.94% and 28.68% higher than that of the device sensitized only by N719 (6.52%). Consequently, high dimensional interpenetrating complexes could serve as excellent co-sensitizers and have application in DSSCs. PMID:27356177
Beam-Plasma Interaction and Instabilities in a 2D Yukawa Plasma
NASA Astrophysics Data System (ADS)
Kyrkos, S.; Kalman, G.; Rosenberg, M.
2008-11-01
In a complex plasma, penetrating charged particle beams may lead to beam-plasma instabilities. When either the plasma, the beam, or both, are strongly interacting [1], the features of the instability are different from those in a weakly coupled plasma. We consider the case when a 2D dusty plasma forms a lattice, and the beam is moving in the lattice plane. Both the grains and the beam particles interact through a Yukawa potential; the beam particles are weakly coupled to each other and to the lattice. The system develops both a longitudinal and a transverse instability. Based on the phonon spectrum of a 2D hexagonal Yukawa lattice [2], we determine and compare the transverse and longitudinal growth rates. As a function of the wavenumber, the growth rates exhibit remarkable gaps, where no instability is excited. The gap locations are governed by the ratio of the lattice and the beam plasma frequencies. The behavior of the growth rates also depends on the direction of the beam and on the relationship between the beam speed and the longitudinal and transverse sound speeds. [1] GJ Kalman, M Rosenberg, JPA 36, 5963 (2003). [2] T Sullivan, GJ Kalman, S Kyrkos, P Bakshi, M Rosenberg, Z Donko, JPA 39, 4607 (2006).
Metrology for graphene and 2D materials
NASA Astrophysics Data System (ADS)
Pollard, Andrew J.
2016-09-01
The application of graphene, a one atom-thick honeycomb lattice of carbon atoms with superlative properties, such as electrical conductivity, thermal conductivity and strength, has already shown that it can be used to benefit metrology itself as a new quantum standard for resistance. However, there are many application areas where graphene and other 2D materials, such as molybdenum disulphide (MoS2) and hexagonal boron nitride (h-BN), may be disruptive, areas such as flexible electronics, nanocomposites, sensing and energy storage. Applying metrology to the area of graphene is now critical to enable the new, emerging global graphene commercial world and bridge the gap between academia and industry. Measurement capabilities and expertise in a wide range of scientific areas are required to address this challenge. The combined and complementary approach of varied characterisation methods for structural, chemical, electrical and other properties, will allow the real-world issues of commercialising graphene and other 2D materials to be addressed. Here, examples of metrology challenges that have been overcome through a multi-technique or new approach are discussed. Firstly, the structural characterisation of defects in both graphene and MoS2 via Raman spectroscopy is described, and how nanoscale mapping of vacancy defects in graphene is also possible using tip-enhanced Raman spectroscopy (TERS). Furthermore, the chemical characterisation and removal of polymer residue on chemical vapour deposition (CVD) grown graphene via secondary ion mass spectrometry (SIMS) is detailed, as well as the chemical characterisation of iron films used to grow large domain single-layer h-BN through CVD growth, revealing how contamination of the substrate itself plays a role in the resulting h-BN layer. In addition, the role of international standardisation in this area is described, outlining the current work ongoing in both the International Organization of Standardization (ISO) and the
Collective spin excitations in 2D paramagnet with dipole interaction
NASA Astrophysics Data System (ADS)
Tsiberkin, Kirill
2016-02-01
The collective spin excitations in the unbounded 2D paramagnetic system with dipole interactions are studied. The model Hamiltonian includes Zeeman energy and dipole interaction energy, while the exchange vanishes. The system is placed into a constant uniform magnetic field which is orthogonal to the lattice plane. It provides the equilibrium state with spin ordering along the field direction, and the saturation is reached at zero temperature. We consider the deviations of spin magnetic moments from its equilibrium position along the external field. The Holstein-Primakoff representation is applied to spin operators in low-temperature approximation. When the interaction between the spin waves is negligible and only two-magnon terms are taken into account, the Hamiltonian diagonalisation is possible. We obtain the dispersion relation for spin waves in the square and hexagonal honeycomb lattice. Bose-Einstein statistics determine the average number of spin deviations, and total system magnetization. The lattice structure does not influence on magnetization at the long-wavelength limit. The dependencies of the relative magnetization and longitudinal susceptibility on temperature and external field intensity are found. The internal energy and specific heat of the Bose gas of spin waves are calculated. The collective spin excitations play a significant role in the properties of the paramagnetic system at low temperature and strong external magnetic field.
Weisemoeller, T.; Bertram, F.; Gevers, S.; Greuling, A.; Deiter, C.; Tobergte, H.; Neumann, M.; Wollschlaeger, J.; Giussani, A.; Schroeder, T.
2009-06-15
Films of hexagonal praseodymium sesquioxide (h-Pr{sub 2}O{sub 3}) were deposited on Si(111) by molecular beam epitaxy and thereafter annealed in 1 atm oxygen at different temperatures, ranging from 100 to 700 deg. C. The films of the samples annealed at 300 deg. C or more were transformed to PrO{sub 2} with B-oriented Fm3m structure, while films annealed at lower temperatures kept the hexagonal structure. The films are composed of PrO{sub 2} and PrO{sub 2-d}elta species, which coexist laterally and are tetragonally distorted due to the interaction at the interface between oxide film and Si substrate. Compared to PrO{sub 2}, PrO{sub 2-d}elta has the same cubic structure but with oxygen vacancies. The oxygen vacancies are partly ordered and increase the vertical lattice constant of the film, whereas the lateral lattice constant is almost identical for both species and on all samples. The latter lattice constant matches the lattice constant of the originally crystallized hexagonal praseodymium sesquioxide. That means that no long range reordering of the praseodymium atoms takes place during the phase transformation.
Triplet Pairing in Electron Systems with Hexagonal Symmetry
NASA Astrophysics Data System (ADS)
Tanaka, Akihiro; Hu, Xiao
2004-03-01
Inspired by the recently discovered superconductor Na_xCoO_2otyH_2O[1], we discuss how a novel triplet pairing state can occur from fermi surface effects/electron correlations in 2d electron systems with hexagonal symmetry[2]. This would serve as a complementary approach to studies based on the RVB picture, which basically concentrate on singlet pairing correlations. Spin and charge transports arising from the nontrivial topology (Chern numbers etc.) in k-space are investigated, and compared with the case of the square lattice. [1] K. Takada et al, Nature vol. 422, 53 (2003). [2] A. Tanaka and X. Hu, Phys. Rev. Lett., in press (cond-mat/0304409).
Effect of binding of lanthanide ions on the bacteriorhodopsin hexagonal structure: An x-ray study
Griffiths, J.A.; El-Sayed, M.A.; Capel, M.
1996-07-18
The effect of the binding of trivalent lanthanide metal cations (Eu{sup 3+}, Ho{sup 3+}, and Dy{sup 3+}) on the hexagonal structure of bacteriorhodopsin (bR) is investigated at different pH using x-Ray diffraction to examine films made by slow evaporation of the corresponding regenerated bR. It is observed that the lanthanide-regenerated bR (at a ratio of 2:1 metal ion to bR) does not form a 2D structure isomorphous to that of native bR or Ca{sup 2+}-regenerated samples at low sample pH. The native bR hexagonal structure is recovered by titration of the sample with sodium hydroxide. The pH at which the hexagonal structure is recovered depends on the charge density of the lanthanide ion used for the regeneration. The higher the charge density of the ion, the higher pH at which an isomorphous lattice is formed. A model is proposed in which at normal or low pH a complex bidentate and monodentate type binding (which disrupts the lattice hexagonal structure) exists between a lanthanide ion, the O{sup -} of PO{sub 2}{sup -} groups, and/or the amino acid residues. At high pH, complexation with OH{sup -} takes place, which converts this binding to a simple monodentate type complex that leads to the recovery of the lattice structure. An equation is derived for the pH at which this conversion takes place. 48 refs., 4 figs.
Wu, Yongfeng; Xiao, Weike
2014-02-01
We introduced a new two-dimensional (2D) hexagon technique for probing the topological structure of the universe in which we mapped regions of the sky with high and low galaxy densities onto a 2D lattice of hexagonal unit cells. We defined filled cells as corresponding to high-density regions and empty cells as corresponding to low-density regions. The numbers of filled cells and empty cells were kept the same by controlling the size of the cells. By analyzing the six sides of each hexagon, we could obtain and compare the statistical topological properties of high-density and low-density regions of the universe in order to have a better understanding of the evolution of the universe. We applied this hexagonal method to Two Micron All Sky Survey data and discovered significant topological differences between the high-density and low-density regions. Both regions had significant (>5σ) topological shifts from both the binomial distribution and the random distribution.
ERIC Educational Resources Information Center
Burkhauser, Beth; Porter, Dave
2010-01-01
This article discusses the international interdependence Hexagon Project for Haiti which invites students, ages five through eighteen, to create an image within a hexagonal template and respond to big questions surrounding a global culture of interdependence. The hexagon is a visual metaphor for interdependence, with its potential to infinitely…
2D electronic materials for army applications
NASA Astrophysics Data System (ADS)
O'Regan, Terrance; Perconti, Philip
2015-05-01
The record electronic properties achieved in monolayer graphene and related 2D materials such as molybdenum disulfide and hexagonal boron nitride show promise for revolutionary high-speed and low-power electronic devices. Heterogeneous 2D-stacked materials may create enabling technology for future communication and computation applications to meet soldier requirements. For instance, transparent, flexible and even wearable systems may become feasible. With soldier and squad level electronic power demands increasing, the Army is committed to developing and harnessing graphene-like 2D materials for compact low size-weight-and-power-cost (SWAP-C) systems. This paper will review developments in 2D electronic materials at the Army Research Laboratory over the last five years and discuss directions for future army applications.
Wang, Pengfei; Gaitanaros, Stavros; Lee, Seungwoo; Bathe, Mark; Shih, William M; Ke, Yonggang
2016-06-22
Scaffolded DNA origami has proven to be a versatile method for generating functional nanostructures with prescribed sub-100 nm shapes. Programming DNA-origami tiles to form large-scale 2D lattices that span hundreds of nanometers to the micrometer scale could provide an enabling platform for diverse applications ranging from metamaterials to surface-based biophysical assays. Toward this end, here we design a family of hexagonal DNA-origami tiles using computer-aided design and demonstrate successful self-assembly of micrometer-scale 2D honeycomb lattices and tubes by controlling their geometric and mechanical properties including their interconnecting strands. Our results offer insight into programmed self-assembly of low-defect supra-molecular DNA-origami 2D lattices and tubes. In addition, we demonstrate that these DNA-origami hexagon tiles and honeycomb lattices are versatile platforms for assembling optical metamaterials via programmable spatial arrangement of gold nanoparticles (AuNPs) into cluster and superlattice geometries. PMID:27224641
Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice
NASA Astrophysics Data System (ADS)
Luo, Wei; Xiang, Hongjun
Two-dimensional (2D) topological insulators (TIs), are excellent candidates for coherent spin transport related applications Currently, most known 2D TIs are based on a hexagonal lattice. Here, we propose that there exists the quantum spin Hall effect (QSHE) in a new tight-binding (TB) model for a two-orbital system with the buckled square lattices. We show that the band inversion is due to the hybridization between thepx andpyorbitals, while the spin-orbit coupling (SOC) induced nearest-neighbor effective hopping is responsible for a band gap opening at the Dirac cone. Through performing global structure optimization, we predict a new three-layer quasi-2D (Q2D) structure which has the lowest energy among all structures with the thickness less than 6.0 Å for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69 eV). The electronic states of the Q2D BiF system near the Fermi level are mainly contributed by the middle Bi square lattice, which are sandwiched by two inert BiF2 layers. This is beneficial since the interaction between a substrate and the Q2D material may not change the topological properties of the system, as we demonstrate in the case of the NaF substrate. Our analysis shows that the low-energy physics of the Q2D BiF system can be qualitatively described by our newly proposed two-orbital TB model. Our study not only predicts a Q2D QSH insulator for realistic room temperature (RT) applications, but also provides a new lattice system for engineering topological states such as quantum anomalous Hall effect.
Hexagonal OsB2 reduction upon heating in H2 containing environment
Xie, Zhilin; Blair, Richard G.; Orlovskaya, Nina; Payzant, E. Andrew
2014-10-23
The stability of hexagonal ReB2 type OsB2 powder upon heating under reforming gas was investigated. Pure Os metal particles were detected by powder X-ray diffraction starting at 375⁰ C and complete transformation of OsB2 to metallic Os was observed at 725⁰ C. The mechanisms of precipitation of metallic Os is proposed and changes in the lattice parameters of OsB2 upon heating are analysed in terms of the presence of oxygen or water vapour in the heating chamber. Previous studies suggested that Os atoms possess (0) valence, while B atoms possess both (+3) and ( 3) valences in the alternating boron/osmiummore » sheet structure of hexagonal (P63/mmc, No. 194) OsB2; if controllable method for Os removal from the lattice could be found, the opportunity would arise to form two-dimensional (2D) layers consisting of pure B atoms.« less
Duality Between Spin Networks and the 2D Ising Model
NASA Astrophysics Data System (ADS)
Bonzom, Valentin; Costantino, Francesco; Livine, Etera R.
2016-06-01
The goal of this paper is to exhibit a deep relation between the partition function of the Ising model on a planar trivalent graph and the generating series of the spin network evaluations on the same graph. We provide respectively a fermionic and a bosonic Gaussian integral formulation for each of these functions and we show that they are the inverse of each other (up to some explicit constants) by exhibiting a supersymmetry relating the two formulations. We investigate three aspects and applications of this duality. First, we propose higher order supersymmetric theories that couple the geometry of the spin networks to the Ising model and for which supersymmetric localization still holds. Secondly, after interpreting the generating function of spin network evaluations as the projection of a coherent state of loop quantum gravity onto the flat connection state, we find the probability distribution induced by that coherent state on the edge spins and study its stationary phase approximation. It is found that the stationary points correspond to the critical values of the couplings of the 2D Ising model, at least for isoradial graphs. Third, we analyze the mapping of the correlations of the Ising model to spin network observables, and describe the phase transition on those observables on the hexagonal lattice. This opens the door to many new possibilities, especially for the study of the coarse-graining and continuum limit of spin networks in the context of quantum gravity.
Competing structures in two dimensions: Square-to-hexagonal transition
NASA Astrophysics Data System (ADS)
Gränz, Barbara; Korshunov, Sergey E.; Geshkenbein, Vadim B.; Blatter, Gianni
2016-08-01
We study a system of particles in two dimensions interacting via a dipolar long-range potential D /r3 and subject to a square-lattice substrate potential V (r ) with amplitude V and lattice constant b . The isotropic interaction favors a hexagonal arrangement of the particles with lattice constant a , which competes against the square symmetry of the underlying substrate lattice. We determine the minimal-energy states at fixed external pressure p generating the commensurate density n =1 /b2=(4/3 ) 1 /2/a2 in the absence of thermal and quantum fluctuations, using both analytical techniques based on the harmonic and continuum elastic approximations as well as numerical relaxation of particle configurations. At large substrate amplitude V >0.2 eD, with eD=D /b3 the dipolar energy scale, the particles reside in the substrate minima and hence arrange in a square lattice. Upon decreasing V , the square lattice turns unstable with respect to a zone-boundary shear mode and deforms into a period-doubled zigzag lattice. Analytic and numerical results show that this period-doubled phase in turn becomes unstable at V ≈0.074 eD towards a nonuniform phase developing an array of domain walls or solitons; as the density of solitons increases, the particle arrangement approaches that of a rhombic (or isosceles triangular) lattice. At a yet smaller substrate value estimated as V ≈0.046 eD, a further solitonic transition establishes a second nonuniform phase which smoothly approaches the hexagonal (or equilateral triangular) lattice phase with vanishing amplitude V . At small but finite amplitude V , the hexagonal phase is distorted and hexatically locked at an angle of φ ≈3 .8∘ with respect to the substrate lattice. The square-to-hexagonal transformation in this two-dimensional commensurate-incommensurate system thus involves a complex pathway with various nontrivial lattice- and modulated phases.
Kasala, Kailash; Saravanamuttu, Kalaichelvi
2013-01-29
We report that incandescent beams patterned with amplitude depressions (dips) suffer instability in a photopolymerizable system and organize into lattices of black and bright self-trapped beams propagating respectively, through self-induced black and bright waveguides. Such optochemically organized lattices emerge when beams embedded with a hexagonal or square array of dips initiate free-radical polymerization and corresponding changes in refractive index (Δn) along their propagation paths. Under these nonlinear conditions, the dips evolve into a hexagonal or square lattice of black beams, while their bright interstitial regions become unstable and divide spontaneously into multiple filaments of light. These filaments have a characteristic diameter (d(f)) and organize into a variety of geometries, which are determined by the shape and dimensions of the bright interstices. At interstitial widths > 2d(f), filaments are randomly positioned in space, whereas at widths < 2d(f), the interstices are occupied by a single file of filaments encircling each dark channel. When the interstitial width ≈ d(f), the filaments organize into lattices with long-range hexagonal or square symmetry. By employing anisotropic interstices such as rectangles, filamentation can be selectively elicited along the long axis, leading to a lattice of filament doublets. This work demonstrates the versatility and significant potential of optochemical organization to generate complex, optically functional polymer lattices, which cannot be constructed through conventional lithography or self-assembly. Specifically, the study introduces a new generation of waveguide lattices, in which light propagation is co-operatively managed by black and bright waveguides; the former suppress local light propagation and, in this way, enhance light confinement and guidance in proximal bright waveguides. PMID:23252718
Rubin, Irwin
1978-01-01
A solar energy panel comprises a support upon which silicon cells are arrayed. The cells are wafer thin and of two geometrical types, both of the same area and electrical rating, namely hexagon cells and hourglass cells. The hourglass cells are composites of half hexagons. A near perfect nesting relationship of the cells achieves a high density packing whereby optimum energy production per panel area is achieved.
Optimized geometries for future generation optical lattice clocks
NASA Astrophysics Data System (ADS)
Krämer, S.; Ostermann, L.; Ritsch, H.
2016-04-01
Atoms deeply trapped in magic wavelength optical lattices provide a Doppler- and collision-free dense ensemble of quantum emitters ideal for high-precision spectroscopy and they are the basis of some of the best optical atomic clocks to date. However, despite their minute optical dipole moments the inherent long-range dipole-dipole interactions in such lattices still generate line shifts, dephasing and modified decay. We show that in a perfectly filled lattice line shifts and decay are resonantly enhanced depending on the lattice constant and geometry. Potentially, this yields clock shifts of many atomic linewidths and reduces the measurement by optimizing the lattice geometry. Such collective effects can be tailored to yield zero effective shifts and prolong dipole lifetimes beyond the single-atom decay. In particular, we identify dense 2D hexagonal or square lattices as the most promising configurations for an accuracy and precision well below the independent ensemble limit. This geometry should also be an ideal basis for related applications such as superradiant lasers, precision magnetometry or long-lived quantum memories.
2D Crystal heterostructures properties and growth by molecular beam epitaxy
NASA Astrophysics Data System (ADS)
Xing, Grace Huili
Two-dimensional (2D) crystals such as transition metal dichalcogenides (TMDs) along with other families of layered materials including graphene, SnSe2, GaSe, BN etc, has attracted intense attention from the scientific community. One monolayer of such materials represent the thinnest ``quantum wells''. These layered materials typically possess an in-plane hexagonal crystal structure, and can be stacked together by interlayer van der Waals interactions. Therefore, it is possible to create novel heterostructures by stacking materials with large lattice mismatches and different properties, for instance, superconductors (NbSe2) , metals, semi-metals (graphene), semiconductors (MoS2) and insulators (BN). Numerous novel material properties and device concepts have been discovered, proposed and demonstrated lately. However, the low internal photoluminescence efficiency (IPE, <1%) and low carrier mobility observed in the 2D semiconductors suggest strongly that the materials under investigation today most likely suffer from a high concentration of defects. In this talk, I will share our progress and the challenges we face in terms of preparing, characterizing these 2D crystals as well as pursuing their applications. This work has been supported in part by NSF, AFOSR and LEAST, one of the STARnet centers.
Domain wall kinetics of lithium niobate single crystals near the hexagonal corner
Choi, Ju Won; Ko, Do-Kyeong; Yu, Nan Ei E-mail: jhro@pnu.edu; Kitamura, Kenji; Ro, Jung Hoon E-mail: jhro@pnu.edu
2015-03-09
A mesospheric approach based on a simple microscopic 2D Ising model in a hexagonal lattice plane is proposed to explain macroscopic “asymmetric in-out domain wall motion” observation in the (0001) plane of MgO-doped stoichiometric lithium niobate. Under application of an electric field that was higher than the conventional coercive field (E{sub c}) to the ferroelectric crystal, a natural hexagonal domain was obtained with walls that were parallel to the Y-axis of the crystal. When a fraction of the coercive field of around 0.1E{sub c} is applied in the reverse direction, this hexagonal domain is shrunk (moved inward) from the corner site into a shape with a corner angle of around 150° and 15° wall slopes to the Y-axis. A flipped electric field of 0.15E{sub c} is then applied to recover the natural hexagonal shape, and the 150° corner shape changes into a flat wall with 30° slope (moved outward). The differences in corner domain shapes between inward and outward domain motion were analyzed theoretically in terms of corner and wall site energies, which are described using the domain corner angle and wall slope with respect to the crystal Y-axis, respectively. In the inward domain wall motion case, the energy levels of the evolving 150° domain corner and 15° slope walls are most competitive, and could co-exist. In the outward case, the energy levels of corners with angles >180° are highly stable when compared with the possible domain walls; only a flat wall with 30° slope to the Y-axis is possible during outward motion.
Hofmann, Martin; Anderssohn, Robert; Bahr, Hans-Achim; Weiß, Hans-Jürgen; Nellesen, Jens
2015-10-01
Basalt columns with their preferably hexagonal cross sections are a fascinating example of pattern formation by crack propagation. Junctions of three propagating crack faces rearrange such that the initial right angles between them tend to approach 120°, which enables the cracks to form a pattern of regular hexagons. To promote understanding of the path on which the ideal configuration can be reached, two periodically repeatable models are presented here involving linear elastic fracture mechanics and applying the principle of maximum energy release rate. They describe the evolution of the crack pattern as a transition from rectangular start configuration to the hexagonal pattern. This is done analytically and by means of three-dimensional finite element simulation. The latter technique reproduces the curved crack path involved in this transition. PMID:26550724
2005-07-01
Aniso2d is a two-dimensional seismic forward modeling code. The earth is parameterized by an X-Z plane in which the seismic properties Can have monoclinic with x-z plane symmetry. The program uses a user define time-domain wavelet to produce synthetic seismograms anrwhere within the two-dimensional media.
Critical Surface of the Hexagonal Polygon Model
NASA Astrophysics Data System (ADS)
Grimmett, Geoffrey R.; Li, Zhongyang
2016-05-01
The hexagonal polygon model arises in a natural way via a transformation of the 1-2 model on the hexagonal lattice, and it is related to the high temperature expansion of the Ising model. There are three types of edge, and three corresponding parameters α ,β ,γ >0. By studying the long-range order of a certain two-edge correlation function, it is shown that the parameter space (0,∞)^3 may be divided into subcritical and supercritical regions, separated by critical surfaces satisfying an explicitly known formula. This result complements earlier work on the Ising model and the 1-2 model. The proof uses the Pfaffian representation of Fisher, Kasteleyn, and Temperley for the counts of dimers on planar graphs.
NASA Astrophysics Data System (ADS)
Kim, Hyun-Jung; Li, Chaokai; Feng, Ji; Zhang, Zhenyu; Cho, Jun-Hyung
The tuning of topological states is of significant fundamental and practical importance in contemporary condensed matter physics, for which the extension to two-dimensional (2D) organometallic systems is particularly attractive. Using first-principles calculations, we find that a 2D hexagonal triphenyl-lead lattice composed of only main group elements is susceptible to a magnetic instability, characterized by a antiferromagnetic (AFM) insulating state with a renormalized valley gaps with gap difference of 24 meV due to the spin and valley coupling. This AFM state will be subject to a anomalous valley Hall effect under the action of Berry curvature-induced spin and valley currents via, for example, injection of circularly polarized light. Furthermore, such a AFM band insulator can be tuned into a topologically nontrivial quantum anomalous Hall state with a Chern number of one by the application of an out-of-plane electric field. These findings further enrich our understanding of 2D hexagonal organometallic lattices for potential applications in spintronics and valleytronics.
Greg Flach, Frank Smith
2011-12-31
Mesh2d is a Fortran90 program designed to generate two-dimensional structured grids of the form [x(i),y(i,j)] where [x,y] are grid coordinates identified by indices (i,j). The x(i) coordinates alone can be used to specify a one-dimensional grid. Because the x-coordinates vary only with the i index, a two-dimensional grid is composed in part of straight vertical lines. However, the nominally horizontal y(i,j0) coordinates along index i are permitted to undulate or otherwise vary. Mesh2d also assigns an integer material type to each grid cell, mtyp(i,j), in a user-specified manner. The complete grid is specified through three separate input files defining the x(i), y(i,j), and mtyp(i,j) variations.
2011-12-31
Mesh2d is a Fortran90 program designed to generate two-dimensional structured grids of the form [x(i),y(i,j)] where [x,y] are grid coordinates identified by indices (i,j). The x(i) coordinates alone can be used to specify a one-dimensional grid. Because the x-coordinates vary only with the i index, a two-dimensional grid is composed in part of straight vertical lines. However, the nominally horizontal y(i,j0) coordinates along index i are permitted to undulate or otherwise vary. Mesh2d also assignsmore » an integer material type to each grid cell, mtyp(i,j), in a user-specified manner. The complete grid is specified through three separate input files defining the x(i), y(i,j), and mtyp(i,j) variations.« less
NASA Astrophysics Data System (ADS)
Lotsch, Bettina V.
2015-07-01
Graphene's legacy has become an integral part of today's condensed matter science and has equipped a whole generation of scientists with an armory of concepts and techniques that open up new perspectives for the postgraphene area. In particular, the judicious combination of 2D building blocks into vertical heterostructures has recently been identified as a promising route to rationally engineer complex multilayer systems and artificial solids with intriguing properties. The present review highlights recent developments in the rapidly emerging field of 2D nanoarchitectonics from a materials chemistry perspective, with a focus on the types of heterostructures available, their assembly strategies, and their emerging properties. This overview is intended to bridge the gap between two major—yet largely disjunct—developments in 2D heterostructures, which are firmly rooted in solid-state chemistry or physics. Although the underlying types of heterostructures differ with respect to their dimensions, layer alignment, and interfacial quality, there is common ground, and future synergies between the various assembly strategies are to be expected.
Hexagonal OsB_{2} reduction upon heating in H_{2} containing environment
Xie, Zhilin; Blair, Richard G.; Orlovskaya, Nina; Payzant, E. Andrew
2014-10-23
The stability of hexagonal ReB_{2} type OsB_{2} powder upon heating under reforming gas was investigated. Pure Os metal particles were detected by powder X-ray diffraction starting at 375⁰ C and complete transformation of OsB_{2} to metallic Os was observed at 725⁰ C. The mechanisms of precipitation of metallic Os is proposed and changes in the lattice parameters of OsB_{2} upon heating are analysed in terms of the presence of oxygen or water vapour in the heating chamber. Previous studies suggested that Os atoms possess (0) valence, while B atoms possess both (+3) and ( 3) valences in the alternating boron/osmium sheet structure of hexagonal (P63/mmc, No. 194) OsB_{2}; if controllable method for Os removal from the lattice could be found, the opportunity would arise to form two-dimensional (2D) layers consisting of pure B atoms.
Peters, Roswell D. M.
1982-01-01
A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively .+-.60.degree. away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency.
Turing patterns beyond hexagons and stripes.
Yang, Lingfa; Dolnik, Milos; Zhabotinsky, Anatol M; Epstein, Irving R
2006-09-01
The best known Turing patterns are composed of stripes or simple hexagonal arrangements of spots. Until recently, Turing patterns with other geometries have been observed only rarely. Here we present experimental studies and mathematical modeling of the formation and stability of hexagonal and square Turing superlattice patterns in a photosensitive reaction-diffusion system. The superlattices develop from initial conditions created by illuminating the system through a mask consisting of a simple hexagonal or square lattice with a wavelength close to a multiple of the intrinsic Turing pattern's wavelength. We show that interaction of the photochemical periodic forcing with the Turing instability generates multiple spatial harmonics of the forcing patterns. The harmonics situated within the Turing instability band survive after the illumination is switched off and form superlattices. The square superlattices are the first examples of time-independent square Turing patterns. We also demonstrate that in a system where the Turing band is slightly below criticality, spatially uniform internal or external oscillations can create oscillating square patterns. PMID:17014248
Hexagonal uniformly redundant arrays for coded-aperture imaging
NASA Technical Reports Server (NTRS)
Finger, M. H.; Prince, T. A.
1985-01-01
Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images without mirrors or lenses. The URAs constructed on hexagonal lattices, are outlined. Details are presented for the construction of a special class of URAs, the skew-Hadamard URAs, which have the following properties: (1) nearly half open and half closed (2) antisymmetric upon rotation by 180 deg except for the central cell and its repetitions. Some of the skew-Hadamard URAs constructed on a hexagonal lattice have additional symmetries. These special URAs that have a hexagonal unit pattern, and are antisymmetric upon rotation by 60 deg, called hexagonal uniformly redundant arrays (HURAs). The HURAs are particularly suited to gamma-ray imaging in high background situations. In a high background situation the best sensitivity is obtained with a half open and half closed mask. The hexagonal symmetry of an HURA is more appropriate for a round position-sensitive detector or a closed-packed array of detectors than a rectangular symmetry.
Peters, R.D.M.
1982-11-02
A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively [+-]60[degree] away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency. 3 figs.
Room-Temperature Multiferroic Hexagonal LuFeO3
Cheng, Xuemei; Balke, Nina; Chi, Miaofang; Gai, Zheng; Keavney, David; Lee, Ho Nyung; Shen, Jian; Snijders, Paul C; Wang, Wenbin; Ward, Thomas Z; Xu, Xiaoshan; Yi, Jieyu; Zhu, Leyi; Christen, Hans M; Zhao, Jun
2013-01-01
We observed the coexistence of ferroelectricity and weak ferromagnetism at room temperature in the hexagonal phase of LuFeO3 stabilized by epitaxial thin film growth. While the ferroelectricity in hexagonal LuFeO3 can be understood as arising from its polar structure, the observation of weak ferromagnetism at room temperature is remarkable considering the frustrated triangular spin structure. An explanation of the room temperature weak ferromagnetism is proposed in terms of a subtle lattice distortion revealed by the structural characterization. The combination of ferroelectricity and weak ferromagnetism in epitaxial films at room temperature offers great potential for the application of this novel multiferroic material in next generation devices.
Hexagonal boron nitride: Ubiquitous layered dielectric for two-dimensional electronics
NASA Astrophysics Data System (ADS)
Jain, Nikhil
Hexagonal boron nitride (h-BN), a layer-structured dielectric with very similar crystalline lattice to that of graphene, has been studied as a ubiquitous dielectric for two-dimensional electronics. While 2D materials may lead to future platform for electronics, traditional thin-film dielectrics (e.g., various oxides) make highly invasive interface with graphene. Multiple key roles of h-BN in graphene electronics are explored in this thesis. 2D graphene/h-BN heterostructures are designed and implemented in diverse configurations in which h-BN is evaluated as a supporting substrate, a gate dielectric, a passivation layer, or an interposing barrier in "3D graphene" superlattice. First, CVD-grown graphene on h-BN substrate shows improved conductivity and resilience to thermally induced breakdown, as compared with graphene on SiO2, potentially useful for high-speed graphene devices and on-chip interconnects. h-BN is also explored as a gate dielectric for graphene field-effect transistor with 2D heterostructure design. The dielectric strength and tunneling behavior of h-BN are investigated, confirming its robust nature. Next, h-BN is studied as a passivation layer for graphene electronics. In addition to significant improvement in current density and breakdown threshold, fully encapsulated graphene exhibits minimal environmental sensitivity, a key benefit to 2D materials which have only surfaces. Lastly, reduction in interlayer carrier scattering is observed in a double-layered graphene setup with ultrathin h-BN multilayer as an interposing layer. The DFT simulation and Raman spectral analysis indicate reduction in interlayer scattering. The decoupling of the two graphene monolayers is further confirmed by electrical characterization, as compared with other referencing mono- and multilayer configurations. The heterostructure serves as the building element in "3D graphene", a versatile platform for future electronics.
Localized surface plasmon effects of two dimensional lattice of metal nanoislands
NASA Astrophysics Data System (ADS)
Oda, Yukari; Shimada, Ryoko; Japan Women's University Team
2015-03-01
Localized surface plasmon (LSP) of metal nanoparticles results from non-propagating excitation of their conduction electrons coupled to the electromagnetic field. LSP localizes the electric field and enhances light emission from fluorescent materials. In this study, a two dimensional (2D) lattice of silver (Ag) nanoislands was fabricated by nanosphere lithography (NSL) method utilizing self-assembled, close-packed hexagonal structures of polystyrene spheres as the etching mask. This 2D lattice was subjected to the electric field for investigating a role of the periodicity of metal islands in the LSP effect. 9,10-di(2-naphthyl) anthracene (ADN), a well-known blue-emitting material in the field of electroluminescence, was used for the study of the enhancement of emission due to the LSP effect. Hybrid thin films of poly(methyl methacrylate) containing ADN were prepared with spin-casting onto the 2D lattice of Ag nanoislands. Transmission and photoluminescence measurements were conducted for these hybrid thin films at room temperature. Detailed results will be presented on site.
Microwave Assisted 2D Materials Exfoliation
NASA Astrophysics Data System (ADS)
Wang, Yanbin
Two-dimensional materials have emerged as extremely important materials with applications ranging from energy and environmental science to electronics and biology. Here we report our discovery of a universal, ultrafast, green, solvo-thermal technology for producing excellent-quality, few-layered nanosheets in liquid phase from well-known 2D materials such as such hexagonal boron nitride (h-BN), graphite, and MoS2. We start by mixing the uniform bulk-layered material with a common organic solvent that matches its surface energy to reduce the van der Waals attractive interactions between the layers; next, the solutions are heated in a commercial microwave oven to overcome the energy barrier between bulk and few-layers states. We discovered the minutes-long rapid exfoliation process is highly temperature dependent, which requires precise thermal management to obtain high-quality inks. We hypothesize a possible mechanism of this proposed solvo-thermal process; our theory confirms the basis of this novel technique for exfoliation of high-quality, layered 2D materials by using an as yet unknown role of the solvent.
Ab initio engineering of materials with stacked hexagonal tin frameworks.
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N
2016-01-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator. PMID:27387140
Ab initio engineering of materials with stacked hexagonal tin frameworks
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N.
2016-01-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator. PMID:27387140
Ab initio engineering of materials with stacked hexagonal tin frameworks
NASA Astrophysics Data System (ADS)
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N.
2016-07-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator.
NASA Astrophysics Data System (ADS)
Wang, Jin; Ma, Jianyong; Zhou, Changhe
2014-11-01
A 3×3 high divergent 2D-grating with period of 3.842μm at wavelength of 850nm under normal incidence is designed and fabricated in this paper. This high divergent 2D-grating is designed by the vector theory. The Rigorous Coupled Wave Analysis (RCWA) in association with the simulated annealing (SA) is adopted to calculate and optimize this 2D-grating.The properties of this grating are also investigated by the RCWA. The diffraction angles are more than 10 degrees in the whole wavelength band, which are bigger than the traditional 2D-grating. In addition, the small period of grating increases the difficulties of fabrication. So we fabricate the 2D-gratings by direct laser writing (DLW) instead of traditional manufacturing method. Then the method of ICP etching is used to obtain the high divergent 2D-grating.
On the perfect hexagonal packing of rods
NASA Astrophysics Data System (ADS)
Starostin, E. L.
2006-04-01
In most cases the hexagonal packing of fibrous structures or rods extremizes the energy of interaction between strands. If the strands are not straight, then it is still possible to form a perfect hexatic bundle. Conditions under which the perfect hexagonal packing of curved tubular structures may exist are formulated. Particular attention is given to closed or cycled arrangements of the rods like in the DNA toroids and spools. The closure or return constraints of the bundle result in an allowable group of automorphisms of the cross-sectional hexagonal lattice. The structure of this group is explored. Examples of open helical-like and closed toroidal-like bundles are presented. An expression for the elastic energy of a perfectly packed bundle of thin elastic rods is derived. The energy accounts for both the bending and torsional stiffnesses of the rods. It is shown that equilibria of the bundle correspond to solutions of a variational problem formulated for the curve representing the axis of the bundle. The functional involves a function of the squared curvature under the constraints on the total torsion and the length. The Euler-Lagrange equations are obtained in terms of curvature and torsion and due to the existence of the first integrals the problem is reduced to the quadrature. The three-dimensional shape of the bundle may be readily reconstructed by integration of the Ilyukhin-type equations in special cylindrical coordinates. The results are of universal nature and are applicable to various fibrous structures, in particular, to intramolecular liquid crystals formed by DNA condensed in toroids or packed inside the viral capsids. International Workshop on Biopolymers: Thermodynamics, Kinetics and Mechanics of DNA, RNA and Proteins, 30.05.2005-3.06.2005, The Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy.
NASA Astrophysics Data System (ADS)
Sánchez-Cano, R.; Porras-Montenegro, N.
2016-04-01
We present numerical predictions for the photonic TE-like band gap ratio and the quality factors of symmetric localized defect as a function of the thickness slab and temperature by the use of plane wave expansion and the finite-difference time-domain methods. The photonic-crystal hole slab is composed of a 2D hexagonal array with identical air holes and a circular cross section, embedded in a non-dispersive III-V semiconductor quaternary alloy slab, which has a high value of dielectric function in the near-infrared region, and the symmetric defect is formed by increasing the radius of a single hole in the 2D hexagonal lattice. We show that the band gap ratio depends linearly on the temperature in the range 150-400 K. Our results show a strong temperature dependence of the quality factor Q, the maximum (Q = 7000) is reached at T = 350 hbox {K}, but if the temperature continues to increase, the efficiency drops sharply. Furthermore, we present numerical predictions for the electromagnetic field distribution at T = 350 hbox {K}.
Structural properties of hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Ooi, N.; Rajan, V.; Gottlieb, J.; Catherine, Y.; Adams, J. B.
2006-04-01
The electronic and structural properties of hexagonal boron nitride (BN) were studied using density functional theory calculations. Three different approximations for the exchange—correlation energy (the local density and two forms of the generalized gradient)—were used to calculate properties such as the bulk modulus, cohesive energy and lattice constants to determine their relative predictive abilities for this system. In general, calculations using the local density approximation produced properties slightly closer to experimental values than calculations with either generalized gradient approximations. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine the equilibrium stacking(s) and it was found that the different stackings have similar cohesive energies and bulk moduli. Energy versus volume curves were calculated for each stacking using two different methods to determine their relative efficacy. Bulk moduli values obtained assuming no pressure dependence were closer to experimental values than those obtained from three common equations of state. Comparisons between the cohesive energies of hexagonal BN and cubic BN show that the cubic phase is more stable. The pressure/volume dependence of the band structure was studied for several different stackings and all showed similar behaviour, specifically a 3-4.5 eV band gap that was nearly independent of pressure in the -500 to +500 kb regime. These calculated results of the pressure/volume dependence of the band structure are the first reports for this system.
2D materials for photon conversion and nanophotonics
NASA Astrophysics Data System (ADS)
Tahersima, Mohammad H.; Sorger, Volker J.
2015-09-01
The field of two-dimensional (2D) materials has the potential to enable unique applications across a wide range of the electromagnetic spectrum. While 2D-layered materials hold promise for next-generation photon-conversion intrinsic limitations and challenges exist that shall be overcome. Here we discuss the intrinsic limitations as well as application opportunities of this new class of materials, and is sponsored by the NSF program Designing Materials to Revolutionize and Engineer our Future (DMREF) program, which links to the President's Materials Genome Initiative. We present general material-related details for photon conversion, and show that taking advantage of the mechanical flexibility of 2D materials by rolling MoS2/graphene/hexagonal boron nitride stack to a spiral solar cell allows for solar absorption up to 90%.
The directed cooperative assembly of proteorhodopsin into 2D and 3D polarized arrays
Liang, Hongjun; Whited, Gregg; Nguyen, Chi; Stucky, Galen D.
2007-01-01
Proteorhodopsin is the membrane protein used by marine bacterioplankton as a light-driven proton pump. Here, we describe a rapid cooperative assembly process directed by universal electrostatic interactions that spontaneously organizes proteorhodopsin molecules into ordered arrays with well defined orientation and packing density. We demonstrate the charge density-matching mechanism that selectively controls the assembly process. The interactions among different components in the system are tuned by varying their charge densities to yield different organized transmembrane protein arrays: (i) a bacteriorhodopsin purple membrane-like structure where proteorhodopsin molecules are cooperatively arranged with charged lipids into a 2D hexagonal lattice; (ii) selected liquid-crystalline states in which crystalline lamellae made up of the coassembled proteorhodopsin and charged lipid molecules are coupled three-dimensionally with polarized proteorhodopsin orientation persisting through the macroscopic scale. Understanding this rapid electrostatically driven assembly process sheds light on organizing membrane proteins in general, which is a prerequisite for membrane protein structural and mechanistic studies as well as in vitro applications. PMID:17488827
Lattice-resolution imaging of the sapphire (0 0 0 1) surface in air by AFM
NASA Astrophysics Data System (ADS)
Gan, Yang; Wanless, Erica J.; Franks, George V.
2007-02-01
Lattice-resolution images of single-crystal α-alumina (sapphire) (0 0 0 1) surfaces have been obtained using contact-mode AFM under ambient conditions. It was found that the hexagonal surface lattice has a periodicity of 0.47 ± 0.11 nm, which is identical to that reported previously when the same surface was imaged in water. Large lattice corrugations (as high as 1 nm) were observed, but were concluded to be imaging artifacts because of the strong friction which causes additional deflection of the cantilever. The additional deflection of the cantilever is registered by the detector of the optical beam-deflection AFM resulting in an overestimation of the height at each lattice point. Abrupt changes were also resolved in the topography including honeycomb patterns and a transition from 2D lattices to 1D parallel stripes, with scanning direction. These phenomena can be explained by the commensurate sliding between the tip and sapphire surface due to the strong contact force.
NASA Technical Reports Server (NTRS)
Park, Yeonjoon (Inventor); Choi, Sang H. (Inventor); King, Glen C. (Inventor)
2011-01-01
Hetero-epitaxial semiconductor materials comprising cubic crystalline semiconductor alloys grown on the basal plane of trigonal and hexagonal substrates, in which misfit dislocations are reduced by approximate lattice matching of the cubic crystal structure to underlying trigonal or hexagonal substrate structure, enabling the development of alloyed semiconductor layers of greater thickness, resulting in a new class of semiconductor materials and corresponding devices, including improved hetero-bipolar and high-electron mobility transistors, and high-mobility thermoelectric devices.
Kang, Joongoo; Zhang, Lijun; Wei, Su-Huai
2016-02-18
Many important layered semiconductors, such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), are derived from a hexagonal lattice. A single layer of such hexagonal semiconductors generally has a direct bandgap at the high-symmetry point K, whereas it becomes an indirect, optically inactive semiconductor as the number of layers increases to two or more. Here, taking hBN and MoS2 as examples, we reveal the microscopic origin of the direct-to-indirect bandgap transition of hexagonal layered materials. Our symmetry analysis and first-principles calculations show that the bandgap transition arises from the lack of the interlayer orbital couplings for the band-edge states at K, which are inherently weak because of the crystal symmetries of hexagonal layered materials. Therefore, it is necessary to judiciously break the underlying crystal symmetries to design more optically active, multilayered semiconductors from hBN or TMDs. PMID:26800573
An Explanation for Saturn's Hexagon
NASA Astrophysics Data System (ADS)
Kohler, Susanna
2015-08-01
For over three decades, weve been gathering observations of the mysterious hexagonal cloud pattern encircling Saturns north pole. Now, researchers believe they have a model that can better explain its formation.Fascinating GeometrySaturns northern Hexagon is a cloud band circling Saturns north pole at 78 N, first observed by the Voyager flybys in 198081. This remarkable pattern has now persisted for more than a Saturn year (29.5 Earth years).Eight frames demonstrating the motion within Saturns Hexagon. Click to watch the animation! The view is from a reference frame rotating with Saturn. [NASA/JPL-Caltech/SSI/Hampton University]Observations by Voyager and, more recently, Cassini have helped to identify many key characteristics of this bizarre structure. Two interesting things weve learned are:The Hexagon is associated with an eastward zonal jet moving at more than 200 mph.The cause of the Hexagon is believed to be a jet stream, similar to the ones that we experience on Earth. The path of the jet itself appears to follow the hexagons outline.The Hexagon rotates at roughly the same rate as Saturns overall rotation.While we observe individual storms and cloud patterns moving at different speeds within the Hexagon, the vertices of the Hexagon move at almost exactly the same rotational speed as that of Saturn itself.Attempts to model the formation of the Hexagon with a jet stream have yet to fully reproduce all of the observed features and behavior. But now, a team led by Ral Morales-Juberas of the New Mexico Institute of Mining and Technology believes they have created a model that better matches what we see.Simulating a Meandering JetThe team ran a series of simulations of an eastward, Gaussian-profile jet around Saturns pole. They introduced small perturbations to the jet and demonstrated that, as a result of the perturbations, the jet can meander into a hexagonal shape. With the initial conditions of the teams model, the meandering jet is able to settle into a
Thermal stability of hexagonal OsB2
Xie, Zhilin; Blair, Richard G.; Orlovskaya, Nina; Cullen, David A; Payzant, E Andrew
2014-01-01
The synthesis of novel hexagonal ReB2-type OsB2 ceramic powder was performed by high energy ball milling of elemental Os and B powders. Two different sources of B powder have been used for this mechanochemical synthesis. One B powder consisted of a mixture of amorphous and crystalline phases and a mixture of 10B and 11B isotopes with a fine particle size, while another B powder was a purely crystalline (rhombohedral) material consisting of enriched 11B isotope with coarse particle size. The same Os powder was used for the synthesis in both cases. It was established that, in the first case, the hexagonal OsB2 phase was the main product of synthesis with a small quantity of Os2B3 phase present after synthesis as an intermediate product. In the second case, where coarse crystalline 11B powder was used as a raw material, only Os2B3 boride was synthesized mechanochemically. The thermal stability of hexagonal OsB2 powder was studied by heating under argon up to 876 C and cooling in vacuo down to 225 C. During the heating, the sacrificial reaction 2OsB2+3O2 2Os+2B2O3 took place due to presence of O2/water vapor molecules in the heating chamber, resulting in the oxidation of B atoms and formation of B2O3 and precipitation of Os metal out of the OsB2 lattice. As a result of such phase changes during heating, the lattice parameters of hexagonal OsB2 changed significantly. The shrinkage of the a lattice parameter was recorded in 276 426 C temperature range upon heating, which was attributed to the removal of B atoms from the OsB2 lattice due to oxidation followed by the precipitation of Os atoms and formation of Os metal. While significant structural changes occurred upon heating due to presence of O2, the hexagonal OsB2 ceramic demonstrated good phase stability upon cooling in vacuo with linear shrinkage of the lattice parameters and no phase changes detected during cooling.
2004-08-01
AnisWave2D is a 2D finite-difference code for a simulating seismic wave propagation in fully anisotropic materials. The code is implemented to run in parallel over multiple processors and is fully portable. A mesh refinement algorithm has been utilized to allow the grid-spacing to be tailored to the velocity model, avoiding the over-sampling of high-velocity materials that usually occurs in fixed-grid schemes.
Phase Engineering of 2D Tin Sulfides.
Mutlu, Zafer; Wu, Ryan J; Wickramaratne, Darshana; Shahrezaei, Sina; Liu, Chueh; Temiz, Selcuk; Patalano, Andrew; Ozkan, Mihrimah; Lake, Roger K; Mkhoyan, K A; Ozkan, Cengiz S
2016-06-01
Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase-selective growth of both hexagonal tin (IV) sulfide SnS2 and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO2 substrates through atmospheric pressure vapor-phase method in a conventional horizontal quartz tube furnace with SnO2 and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations. PMID:27099950
Uehara, Kazuhiro; Oishi, Takamichi; Hirose, Takayuki; Mizuno, Noritaka
2013-10-01
Structural control among hexagonal (trimer), rhomboidal (dimer), and infinite-chain supramolecular complexes with three different supporting ligands of ethylenediamine (en), N,N,N',N'-tetramethylethylenediamine (en*), and 1,2-bis(diphenyl)phosphinoethane (dppe) [(en)Pd(L)]3(OTf)6 1t·OTf, [(en*)Pd(L)]2(PF6)4 2d·PF6, and [(dppe)Pd(L)(OTf)2]∞ 3·OTf (OTf = trifluoromethane sulfonate; L = 1,3-bis(4-pyridylethynyl)benzene) in the solid and solution states was investigated. The encapsulation of a large Keggin-type polyoxometalate [α-PW12O40](3-) by these complexes was also examined. As the steric bulkiness of the supporting ligands increased in the order of en < en* < dppe, the hexagonal, rhomboidal, and infinite-chain structures were obtained, as confirmed by X-ray crystallography. In solution, equilibrium between the molecular hexagon (1t·OTf/2t·PF6) and the molecular rhomboid (1d·OTf/2d·PF6) was observed in the en/en* ligand systems, whereas 3·OTf with the dppe ligand did not exhibit equilibrium and instead existed as a single species. These phenomena were established by cold-spray ionization mass spectroscopy (CSI-MS) and (1)H diffusion ordered NMR spectroscopy (DOSY). The addition of the highly negatively charged Keggin-type phosphododecatungstate [α-PW12O40](3-) to a solution of 2t/2d·PF6 resulted in the encapsulation of the tungstate species in the cavity of the molecular hexagon to form {[(en*)Pd(L)]3[⊃α-PW12O40]}(PF6)3 2t·[α-PW12O40](3-), as confirmed by a combination of (1)H and (31)P DOSY and CSI-MS spectral data. PMID:24050509
Structural analysis of Li-intercalated hexagonal boron nitride
Sumiyoshi, A.; Hyodo, H.; Kimura, K.
2012-03-15
A structural investigation of Li-intercalated hexagonal boron nitride (Li-h-BNIC) was performed by synchrotron powder X-ray diffraction analysis and transmission electron microscopy. The host BN framework of Li-h-BNIC was expanded by Li-intercalation. The intralayer B-N bond length was increased by 2.48(1)% and the interlayer distance was expanded by 12.86(1)%. No superlattice structure of intercalated Li was observed. - Graphical abstract: XRD pattern fitting of the sample and schematic view of host h-BN lattice. Highlights: Black-Right-Pointing-Pointer Li-intercalated h-BN was investigated by synchrotron radiation powder XRD. Black-Right-Pointing-Pointer Lattice parameter of host h-BN lattice was increased by intercalation. Black-Right-Pointing-Pointer Increase ratio of B-N bond length was considerably larger than those of Li GICs.
EBSD study of substrate-mediated growth of hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Dias, J.; Kidambi, P. R.; Hofmann, S.; Ducati, C.
2014-06-01
Hexagonal Boron Nitride (h-BN) is a promising insulating material to complement and enable graphene electronics. Given the good lattice match to graphite, graphene/h-BN heterostructures may be grown with negligible amounts of strain and defect states, resulting in high carrier mobilities approaching values for suspended graphene. Chemical vapour deposition (CVD) has emerged as one of the preferred routes for the synthesis of 2D materials for electronic applications. Here we report on the growth of h-BN by low pressure CVD, using borazine as a precursor. Electron backscattered diffraction (EBSD) in conjunction with topographic imaging in the scanning electron microscope are used to investigate the change in crystal structure and orientation of three metallic catalyst substrates: Co, Ni and Cu, by high temperature processing and the growth of nanoscale h-BN domains. The behaviour of the metal foils is interpreted in light of the prevalent growth models. EBSD and imaging conditions are optimized to allow efficient acquisitions for these composite and nanostructured specimens.
Liquid phase deposition synthesis of hexagonal molybdenum trioxide thin films
Deki, Shigehito; Beleke, Alexis Bienvenu; Kotani, Yuki; Mizuhata, Minoru
2009-09-15
Hexagonal molybdenum trioxide thin films with good crystallinity and high purity have been fabricated by the liquid phase deposition (LPD) technique using molybdic acid (H{sub 2}MoO{sub 4}) dissolved in 2.82% hydrofluoric acid (HF) and H{sub 3}BO{sub 3} as precursors. The crystal was found to belong to a hexagonal hydrate system MoO{sub 3}.nH{sub 2}O (napprox0.56). The unit cell lattice parameters are a=10.651 A, c=3.725 A and V=365.997 A{sup 3}. Scanning electron microscope (SEM) images of the as-deposited samples showed well-shaped hexagonal rods nuclei that grew and where the amount increased with increase in reaction time. X-ray photon electron spectroscopy (XPS) spectra showed a Gaussian shape of the doublet of Mo 3d core level, indicating the presence of Mo{sup 6+} oxidation state in the deposited films. The deposited films exhibited an electrochromic behavior by lithium intercalation and deintercalation, which resulted in coloration and bleaching of the film. Upon dehydration at about 450 deg. C, the hexagonal MoO{sub 3}.nH{sub 2}O was transformed into the thermodynamically stable orthorhombic phase. - Abstract: SEM photograph of typical h-MoO{sub 3}.nH{sub 2}O thin film nuclei obtained after 36 h at 40 deg. C by the LPD method. Display Omitted
Stability of skyrmion lattices and symmetries of Dzyaloshinskii-Moriya magnets
NASA Astrophysics Data System (ADS)
Güngördü, Utkan; Nepal, Rabindra; Tretiakov, Oleg A.; Belashchenko, Kirill; Kovalev, Alexey A.
Recently, there has been substantial interest in realizations of skyrmions, in particular in 2D systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultra-dense magnetic memories. Here, we use the most general form of the 2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices with four-fold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental rea lizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyro-dynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. DOE Early Career Award DE-SC0014189, NSF Grants Nos. Phy-1415600, PHY11-25915, DMR-1420645, and DMR-1308751; Grants-in-Aid from MEXT and SpinNet (Nos. 25800184, 25247056, and 15H01009).
Stability of skyrmion lattices and symmetries of quasi-two-dimensional chiral magnets
Gungordu, Utkan; Nepal, Rabindra; Tretiakov, Oleg A.; Belashchenko, Kirill; Kovalev, Alexey A.
2016-02-24
Recently there has been substantial interest in realizations of skyrmions, in particular in quasi-two-dimensional (2D) systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultradense magnetic memories. Here we use the most general form of the quasi-2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that the skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices withmore » fourfold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental realizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyrodynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. As a result, we find that under certain conditions, isolated skyrmions can move along the current without a side motion which can have implications for realizations of magnetic memories.« less
Stability of skyrmion lattices and symmetries of quasi-two-dimensional chiral magnets
NASA Astrophysics Data System (ADS)
Güngördü, Utkan; Nepal, Rabindra; Tretiakov, Oleg A.; Belashchenko, Kirill; Kovalev, Alexey A.
2016-02-01
Recently there has been substantial interest in realizations of skyrmions, in particular in quasi-two-dimensional (2D) systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultradense magnetic memories. Here we use the most general form of the quasi-2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that the skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices with fourfold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental realizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyrodynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. We find that under certain conditions, isolated skyrmions can move along the current without a side motion which can have implications for realizations of magnetic memories.
2D ice from first principles: structures and phase transitions
NASA Astrophysics Data System (ADS)
Chen, Ji; Schusteritsch, Georg; Pickard, Chris J.; Salzmann, Christoph G.; Michaelides, Angelos
Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to ca. 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.
Hexagonal liquid crystal lens array for 3D endoscopy.
Hassanfiroozi, Amir; Huang, Yi-Pai; Javidi, Bahram; Shieh, Han-Ping D
2015-01-26
A liquid crystal lens array with a hexagonal arrangement is investigated experimentally. The uniqueness of this study exists in the fact that using convex-ring electrode provides a smooth and controllable applied potential profile across the aperture to manage the phase profile. We observed considerable differences between flat electrode and convex-ring electrode; in particular the lens focal length is variable in a wider range from 2.5cm to infinity. This study presents several noteworthy characteristics such as low driving voltage; 30 μm cell gap and the lens is electrically switchable between 2D/3D modes. We demonstrate a hexagonal LC-lens array for capturing 3D images by using single sensor using integral imaging. PMID:25835856
NASA Astrophysics Data System (ADS)
Mayor, Louise
2016-05-01
Graphene might be the most famous example, but there are other 2D materials and compounds too. Louise Mayor explains how these atomically thin sheets can be layered together to create flexible “van der Waals heterostructures”, which could lead to a range of novel applications.
Thermal conductivity measurements in a 2D Yukawa system
NASA Astrophysics Data System (ADS)
Nosenko, V.; Ivlev, A.; Zhdanov, S.; Morfill, G.; Goree, J.; Piel, A.
2007-03-01
Thermal conductivity was measured for a 2D Yukawa system. First, we formed a monolayer suspension of microspheres in a plasma, i.e., a dusty plasma, which is like a colloidal suspension, but with an extremely low volume fraction and a partially-ionized rarefied gas instead of solvent. In the absence of manipulation, the suspension forms a 2D triangular lattice. To melt this lattice and form a liquid, we used a laser-heating method. Two focused laser beams were moved rapidly around in the monolayer. The kinetic temperature of the particles increased with the laser power applied, and above a threshold a melting transition occurred. We used digital video microscopy for direct imaging and particle tracking. The spatial profiles of the particle kinetic temperature were calculated. Using the heat transport equation with an additional term to account for the energy dissipation due to the gas drag, we analyzed the temperature distribution to derive the thermal conductivity.
Thermal stability of hexagonal OsB{sub 2}
Xie, Zhilin; Blair, Richard G.; Orlovskaya, Nina; Cullen, David A.; Andrew Payzant, E.
2014-11-15
The synthesis of novel hexagonal ReB{sub 2}-type OsB{sub 2} ceramic powder was performed by high energy ball milling of elemental Os and B powders. Two different sources of B powder have been used for this mechanochemical synthesis. One B powder consisted of a mixture of amorphous and crystalline phases and a mixture of {sup 10}B and {sup 11}B isotopes with a fine particle size, while another B powder was a purely crystalline (rhombohedral) material consisting of enriched {sup 11}B isotope with coarse particle size. The same Os powder was used for the synthesis in both cases. It was established that, in the first case, the hexagonal OsB{sub 2} phase was the main product of synthesis with a small quantity of Os{sub 2}B{sub 3} phase present after synthesis as an intermediate product. In the second case, where coarse crystalline {sup 11}B powder was used as a raw material, only Os{sub 2}B{sub 3} boride was synthesized mechanochemically. The thermal stability of hexagonal OsB{sub 2} powder was studied by heating under argon up to 876 °C and cooling in vacuo down to −225 °C. During the heating, the sacrificial reaction 2OsB{sub 2}+3O{sub 2}→2Os+2B{sub 2}O{sub 3} took place due to presence of O{sub 2}/water vapor molecules in the heating chamber, resulting in the oxidation of B atoms and formation of B{sub 2}O{sub 3} and precipitation of Os metal out of the OsB{sub 2} lattice. As a result of such phase changes during heating, the lattice parameters of hexagonal OsB{sub 2} changed significantly. The shrinkage of the a lattice parameter was recorded in 276–426 °C temperature range upon heating, which was attributed to the removal of B atoms from the OsB{sub 2} lattice due to oxidation followed by the precipitation of Os atoms and formation of Os metal. While significant structural changes occurred upon heating due to presence of O{sub 2}, the hexagonal OsB{sub 2} ceramic demonstrated good phase stability upon cooling in vacuo with linear shrinkage of the lattice
The interface between ferroelectric and 2D material for a Ferroelectric Field-Effect Transistor
NASA Astrophysics Data System (ADS)
Park, Nahee; Kang, Haeyong; Lee, Sang-Goo; Lee, Young Hee; Suh, Dongseok
We have studied electrical property of ferroelectric field-effect transistor which consists of graphene on hexagonal Boron-Nitride (h-BN) gated by a ferroelectric, PMN-PT (i.e. (1-x)Pb(Mg1/3Nb2/3) O3-xPbTiO3) single-crystal substrate. The PMN-PT was expected to have an effect on polarization field into the graphene channel and to induce a giant amount of surface charge. The hexagonal Boron-Nitride (h-BN) flake was directly exfoliated on the PMN-PT substrate for preventing graphene from directly contacting on the PMN-PT substrate. It can make us to observe the effect of the interface between ferroelectric and 2D material on the device operation. Monolayer graphene as 2D channel material, which was confirmed by Raman spectroscopy, was transferred on top of the hexagonal Boron-Nitride (h-BN) by using the conventional dry-transfer method. Here, we can demonstrate that the structure of graphene/hexagonal-BN/ferroelectric field-effect transistor makes us to clearly understand the device operation as well as the interface between ferroelectric and 2D materials by inserting h-BN between them. The phenomena such as anti-hysteresis, current saturation behavior, and hump-like increase of channel current, will be discussed by in terms of ferroelectric switching, polarization-assisted charge trapping.
Hexagonal diamonds in meteorites: implications.
Hanneman, R E; Strong, H M; Bundy, F P
1967-02-24
A new polymorph of carbon, hexagonal diamond, has been discovered in the Canyon Diablo and Goalpara meteorites. This phase had been synthesized recently under specific high-pressure conditions in the laboratory. Our results: provide strong evidence that diamonds found in these meteorites were produced by intense shock pressures acting on crystalline graphite inclusions present within the meteorite before impact, rather than by disintegration of larger, statically grown diamonds, as some theories propose. PMID:17830485
2001-01-31
This software reduces the data from two-dimensional kSA MOS program, k-Space Associates, Ann Arbor, MI. Initial MOS data is recorded without headers in 38 columns, with one row of data per acquisition per lase beam tracked. The final MOSS 2d data file is reduced, graphed, and saved in a tab-delimited column format with headers that can be plotted in any graphing software.
Phase transformation of strontium hexagonal ferrite
NASA Astrophysics Data System (ADS)
Bilovol, V.; Martínez-García, R.
2015-11-01
The phase transformation of strontium hexagonal ferrite (SrFe12O19) to magnetite (Fe3O4) as main phase and strontium carbonate (SrCO3) as secondary phase is reported here. SrFe12O19 powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe12O19 ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe12O19 due to a low synthesis temperature, the reduction of Fe3+ to Fe2+ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe12O19 and Fe3O4. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe12O19 and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe57 Mössbauer spectroscopy provided evidences of the reduction of Fe3+ to Fe2+ in the SrFe12O19 crystal.
Fluorescent Defects in Hexagonal Boron Nitride
NASA Astrophysics Data System (ADS)
Exarhos, Annemarie L.; Oser, Kameron; Hopper, David A.; Grote, Richard R.; Bassett, Lee C.
Mono- and few-layer hexagonal boron nitride (h-BN) can host defects whose electronic states lie deep within the bandgap, similar to the nitrogen-vacancy color center in bulk diamond. Here, we study defect creation in h-BN through irradiation and thermal annealing. We employ confocal photoluminescence (PL) imaging and spectroscopy under various excitation energies on both supported and suspended h-BN to identify and characterize the emission of isolated defect centers. Polarization- and temperature-dependent measurements of the observed PL are used to map out the electronic structure of the defects, enabling optical control of fluorescent defects in h-BN. This knowledge, coupled with the spatial confinement to 2D and the unique electrical, optical, and mechanical properties of h-BN, will enable the use of these defects for quantum sensing and other applications in quantum information processing. Work supported by the ARO (W911NF-15-1-0589) and NSF MRSEC (DMR-1120901).
A hexagonal orthogonal-oriented pyramid as a model of image representation in visual cortex
NASA Technical Reports Server (NTRS)
Watson, Andrew B.; Ahumada, Albert J., Jr.
1989-01-01
Retinal ganglion cells represent the visual image with a spatial code, in which each cell conveys information about a small region in the image. In contrast, cells of the primary visual cortex use a hybrid space-frequency code in which each cell conveys information about a region that is local in space, spatial frequency, and orientation. A mathematical model for this transformation is described. The hexagonal orthogonal-oriented quadrature pyramid (HOP) transform, which operates on a hexagonal input lattice, uses basis functions that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The basis functions, which are generated from seven basic types through a recursive process, form an image code of the pyramid type. The seven basis functions, six bandpass and one low-pass, occupy a point and a hexagon of six nearest neighbors on a hexagonal lattice. The six bandpass basis functions consist of three with even symmetry, and three with odd symmetry. At the lowest level, the inputs are image samples. At each higher level, the input lattice is provided by the low-pass coefficients computed at the previous level. At each level, the output is subsampled in such a way as to yield a new hexagonal lattice with a spacing square root of 7 larger than the previous level, so that the number of coefficients is reduced by a factor of seven at each level. In the biological model, the input lattice is the retinal ganglion cell array. The resulting scheme provides a compact, efficient code of the image and generates receptive fields that resemble those of the primary visual cortex.
Nanoimprint lithography: 2D or not 2D? A review
NASA Astrophysics Data System (ADS)
Schift, Helmut
2015-11-01
Nanoimprint lithography (NIL) is more than a planar high-end technology for the patterning of wafer-like substrates. It is essentially a 3D process, because it replicates various stamp topographies by 3D displacement of material and takes advantage of the bending of stamps while the mold cavities are filled. But at the same time, it keeps all assets of a 2D technique being able to pattern thin masking layers like in photon- and electron-based traditional lithography. This review reports about 20 years of development of replication techniques at Paul Scherrer Institut, with a focus on 3D aspects of molding, which enable NIL to stay 2D, but at the same time enable 3D applications which are "more than Moore." As an example, the manufacturing of a demonstrator for backlighting applications based on thermally activated selective topography equilibration will be presented. This technique allows generating almost arbitrary sloped, convex and concave profiles in the same polymer film with dimensions in micro- and nanometer scale.
Madelung energy of Yukawa lattices.
Pereira, P C N; Apolinario, S W S
2012-10-01
We propose a method to obtain an approximate closed form expression for the Madelung energy (ME) of Yukawa lattices. Such a method is applied for lattices of different topologies and dimensions. The obtained Madelung energies have a satisfactory accuracy for all ranges of the screening parameter κ of the Yukawa potential, and it becomes exact in the asymptotic limits κ→0 and κ→+∞. For instance, for the triangular lattice, the maximum relative error of the ME given by the method is about 0.0047. Also, satisfactory results are obtained for the one-component plasma limit. The Madelung constants of the two-dimensional hexagonal BN and square NaCl and the three-dimensional cubic NaCl crystals are estimated with a relative error of 0.004, 0.006, and 0.03, respectively. Finally, different ways to improve the method are presented and discussed. PMID:23214705
Mechanics of advanced fiber reinforced lattice composites
NASA Astrophysics Data System (ADS)
Fan, Hua-Lin; Zeng, Tao; Fang, Dai-Ning; Yang, Wei
2010-12-01
Fiber reinforced lattice composites are light-weight attractive due to their high specific strength and specific stiffness. In the past 10 years, researchers developed three-dimensional (3D) lattice trusses and two-dimensional (2D) lattice grids by various methods including interlacing, weaving, interlocking, filament winding and molding hot-press. The lattice composites have been applied in the fields of radar cross-section reduction, explosive absorption and heat-resistance. In this paper, topologies of the lattice composites, their manufacturing routes, as well as their mechanical and multifunctional applications, were surveyed.
Resolving 2D Amorphous Materials with Scanning Probe Microscopy
NASA Astrophysics Data System (ADS)
Burson, Kristen M.; Buechner, Christin; Lewandowski, Adrian; Heyde, Markus; Freund, Hans-Joachim
Novel two-dimensional (2D) materials have garnered significant scientific interest due to their potential technological applications. Alongside the emphasis on crystalline materials, such as graphene and hexagonal BN, a new class of 2D amorphous materials must be pursued. For amorphous materials, a detailed understanding of the complex structure is necessary. Here we present a structural study of 2D bilayer silica on Ru(0001), an insulating material which is weakly coupled to the substrate. Atomic structure has been determined with a dual mode atomic force microscopy (AFM) and scanning tunneling microscopy (STM) sensor in ultra-high vacuum (UHV) at low temperatures, revealing a network of different ring sizes. Liquid AFM measurements with sub-nanometer resolution bridge the gap between clean UHV conditions and the environments that many material applications demand. Samples are grown and characterized in vacuum and subsequently transferred to the liquid AFM. Notably, the key structural features observed, namely nanoscale ring networks and larger holes to the substrate, show strong quantitative agreement between the liquid and UHV microscopy measurements. This provides direct evidence for the structural stability of these silica films for nanoelectronics and other applications. KMB acknowledges support from the Alexander von Humboldt Foundation.
Role of defects in frictional properties of 2-D materials
NASA Astrophysics Data System (ADS)
Kavalur, Aditya; Kim, Woo Kyun
Graphene and other 2-D materials have provided a promising prospect to improve the tribological properties of small length scale devices such as MEMS/NEMS due to their low friction coefficient and excellent wear resistance. Several recent research efforts have been devoted to unveiling the physical origin of the superior tribological properties of these 2-D materials from both experimental and theoretical standpoints, however, many of them still remain far from clearly understood. Recently, it was shown that lamellar materials do not conform to the predictions of the Prandtl-Tomlinson model due to additional friction mechanisms of delamination and visco-elastic ploughing. These mechanisms are critical as they explain the low and negative coefficients of friction observed in recent AFM experiments. However, thus far, most simulation and theoretical studies about these novel friction mechanisms have focused on only pristine graphene whereas real graphene sheets prepared by CVD and other conventional techniques possess various forms of defects such as vacancies and non-hexagonal rings. In this study we examine the role of these defects in frictional properties of 2-D materials in relation to delamination and visco-elastic ploughing.
Bornyakov, V.G.
2005-06-01
Possibilities that are provided by a lattice regularization of QCD for studying nonperturbative properties of QCD are discussed. A review of some recent results obtained from computer calculations in lattice QCD is given. In particular, the results for the QCD vacuum structure, the hadron mass spectrum, and the strong coupling constant are considered.
Lattice-gas automata for the Navier-Stokes equation
NASA Astrophysics Data System (ADS)
Frisch, U.; Hasslacher, B.; Pomeau, Y.
1986-04-01
It is shown that a class of deterministic lattice gases with discrete Boolean elements simulates the Navier-Stokes equations, and can be used to design simple, massively parallel computing machines. A hexagonal lattice gas (HLG) model consisting of a triangular lattice with hexagonal symmetry is developed, and is shown to lead to the two-dimensional Navier-Stokes equations. The three-dimensional formulation is obtained by a splitting method in which the nonlinear term in the three-dimensional Navier-Stokes equation is recasts as the sum of two terms, each containing spurious elements and each realizable on a different lattice. Freed slip and rigid boundary conditions are easily implemented. It is noted that lattice-gas models must be run at moderate Mach numbers to remain incompressible, and to avoid spurious high-order nonlinear terms. The model gives a concrete hydrodynamical example of how cellular automata can be used to simulate classical nonlinear fields.
NASA Astrophysics Data System (ADS)
Singh, Kevin; Geiger, Zachary; Senaratne, Ruwan; Rajagopal, Shankari; Fujiwara, Kurt; Weld, David; Weld Group Team
2015-05-01
Quasiperiodicity is intimately involved in quantum phenomena from localization to the quantum Hall effect. Recent experimental investigation of quasiperiodic quantum effects in photonic and electronic systems have revealed intriguing connections to topological phenomena. However, such experiments have been limited by the absence of techniques for creating tunable quasiperiodic structures. We propose a new type of quasiperiodic optical lattice, constructed by intersecting a Gaussian beam with a 2D square lattice at an angle with an irrational tangent. The resulting potential, a generalization of the Fibonacci lattice, is a physical realization of the mathematical ``cut-and-project'' construction which underlies all quasiperiodic structures. Calculation of the energies and wavefunctions of atoms loaded into the proposed quasiperiodic lattice demonstrate a fractal energy spectrum and the existence of edge states. We acknowledge support from the ONR (award N00014-14-1-0805), the ARO and the PECASE program (award W911NF-14-1-0154), the AFOSR (award FA9550-12-1-0305), and the Alfred P. Sloan foundation (grant BR2013-110).
Luther, Pradeep K.; Squire, John M.
2014-01-01
Myosin filaments in vertebrate striated muscle have a long roughly cylindrical backbone with cross-bridge projections on the surfaces of both halves except for a short central bare zone. In the middle of this central region the filaments are cross-linked by the M-band which holds them in a well-defined hexagonal lattice in the muscle A-band. During muscular contraction the M-band-defined rotation of the myosin filaments around their long axes influences the interactions that the cross-bridges can make with the neighbouring actin filaments. We can visualise this filament rotation by electron microscopy of thin cross-sections in the bare-region immediately adjacent to the M-band where the filament profiles are distinctly triangular. In the muscles of teleost fishes, the thick filament triangular profiles have a single orientation giving what we call the simple lattice. In other vertebrates, for example all the tetrapods, the thick filaments have one of two orientations where the triangles point in opposite directions (they are rotated by 60° or 180°) according to set rules. Such a distribution cannot be developed in an ordered fashion across a large 2D lattice, but there are small domains of superlattice such that the next-nearest neighbouring thick filaments often have the same orientation. We believe that this difference in the lattice forms can lead to different contractile behaviours. Here we provide a historical review, and when appropriate cite recent work related to the emergence of the simple and superlattice forms by examining the muscles of several species ranging back to primitive vertebrates and we discuss the functional differences that the two lattice forms may have. PMID:25478994
Lattice Green's functions in all dimensions
NASA Astrophysics Data System (ADS)
Guttmann, Anthony J.
2010-07-01
We give a systematic treatment of lattice Green's functions (LGF) on the d-dimensional diamond, simple cubic, body-centred cubic and face-centred cubic lattices for arbitrary dimensionality d >= 2 for the first three lattices, and for 2 <= d <= 5 for the hyper-fcc lattice. We show that there is a close connection between the LGF of the d-dimensional hyper-cubic lattice and that of the (d - 1)-dimensional diamond lattice. We give constant-term formulations of LGFs for each of these lattices in all dimensions. Through a still under-developed connection with Mahler measures, we point out an unexpected connection between the coefficients of the sc, bcc and diamond LGFs and some Ramanujan-type formulae for 1/π.
Lattice kinetic simulation of nonisothermal magnetohydrodynamics.
Chatterjee, Dipankar; Amiroudine, Sakir
2010-06-01
In this paper, a lattice kinetic algorithm is presented to simulate nonisothermal magnetohydrodynamics in the low-Mach number incompressible limit. The flow and thermal fields are described by two separate distribution functions through respective scalar kinetic equations and the magnetic field is governed by a vector distribution function through a vector kinetic equation. The distribution functions are only coupled via the macroscopic density, momentum, magnetic field, and temperature computed at the lattice points. The novelty of the work is the computation of the thermal field in conjunction with the hydromagnetic fields in the lattice Boltzmann framework. A 9-bit two-dimensional (2D) lattice scheme is used for the numerical computation of the hydrodynamic and thermal fields, whereas the magnetic field is simulated in a 5-bit 2D lattice. Simulation of Hartmann flow in a channel provides excellent agreement with corresponding analytical results. PMID:20866540
The Laplacian-Energy-Like Invariants of Three Types of Lattices
Chu, Zheng-Qing; Liu, Jia-Bao; Li, Xiao-Xin
2016-01-01
This paper mainly studies the Laplacian-energy-like invariants of the modified hexagonal lattice, modified Union Jack lattice, and honeycomb lattice. By utilizing the tensor product of matrices and the diagonalization of block circulant matrices, we derive closed-form formulas expressing the Laplacian-energy-like invariants of these lattices. In addition, we obtain explicit asymptotic values of these invariants with software-aided computations of some integrals. PMID:27190675
Electrodeposited Silver Nanoparticles Patterned Hexagonally for SERS
Gu, Geun Hoi; Lee, Sue Yeone; Suh, Jung Sang
2010-08-06
We have fabricated hexagonally patterned silver nanoparticles for surface-enhanced Raman scattering (SERS) by electrodepositing silver on the surface of an aluminum plate prepared by completely removing the oxide from anodic aluminum oxide (AAO) templates. Even after completely removing the oxide, well-ordered hexagonal patterns, similar to the shape of graphene, remained on the surface of the aluminum plate. The borders of the hexagonal pattern protruded up to form sorts of nano-mountains at both the sides and apexes of the hexagon, with the apexes protruding even more significantly than the sides. The aluminum plate prepared by completely removing the oxide has been used in the preparation of SERS substrates by sputter-coating of gold or silver on it. Instead of sputter-coating, here we have electro-deposited silver on the aluminum plate. When silver was electro-deposited on the plate, silver nanoparticles were made along the hexagonal margins.
ERIC Educational Resources Information Center
Parris, Richard
2011-01-01
Given a segment that joins two lattice points in R[superscript 3], when is it possible to form a lattice cube that uses this segment as one of its twelve edges? A necessary and sufficient condition is that the length of the segment be an integer. This paper presents an algorithm for finding such a cube when the prime factors of the length are…
NKG2D ligands as therapeutic targets
Spear, Paul; Wu, Ming-Ru; Sentman, Marie-Louise; Sentman, Charles L.
2013-01-01
The Natural Killer Group 2D (NKG2D) receptor plays an important role in protecting the host from infections and cancer. By recognizing ligands induced on infected or tumor cells, NKG2D modulates lymphocyte activation and promotes immunity to eliminate ligand-expressing cells. Because these ligands are not widely expressed on healthy adult tissue, NKG2D ligands may present a useful target for immunotherapeutic approaches in cancer. Novel therapies targeting NKG2D ligands for the treatment of cancer have shown preclinical success and are poised to enter into clinical trials. In this review, the NKG2D receptor and its ligands are discussed in the context of cancer, infection, and autoimmunity. In addition, therapies targeting NKG2D ligands in cancer are also reviewed. PMID:23833565
NASA Astrophysics Data System (ADS)
Prentice, Joseph C. A.; Coldea, Amalia I.
2016-06-01
By solving the Boltzmann transport equation we investigate theoretically the general form of oscillations in the resistivity caused by varying the direction of an applied magnetic field for the case of quasi-two-dimensional systems on hexagonal lattices. The presence of the angular magnetoresistance oscillations can be used to map out the topology of the Fermi surface and we study how this effect varies as a function of the degree of interplane warping as well as a function of the degree of isotropic scattering. We find that the angular-dependent effect due to in-plane rotation follows the symmetry imposed by the lattice whereas for interplane rotation the degree of warping dictates the dominant features observed in simulations. Our calculations make predictions for specific angle-dependent magnetotransport signatures in magnetic fields expected for quasi-two-dimensional hexagonal compounds similar to PdCoO2 and PtCoO2.
Flatbands in 2D boroxine-linked covalent organic frameworks.
Wang, Rui-Ning; Zhang, Xin-Ran; Wang, Shu-Fang; Fu, Guang-Sheng; Wang, Jiang-Long
2016-01-14
Density functional calculations have been performed to analyze the electronic and mechanical properties of a number of 2D boroxine-linked covalent organic frameworks (COFs), which are experimentally fabricated from di-borate aromatic molecules. Furthermore, the band structures are surprising and show flat-band characteristics which are mainly attributed to the delocalized π-conjugated electrons around the phenyl rings and can be better understood within aromaticity theories. Next, the effects of branch sizes and hydrostatic strains on their band structures are systematically considered within generalized gradient approximations. It is found that their band gaps will start to saturate when the branch size reaches 9. For boroxine-linked COFs with only one benzene ring in the branch, the band gap is robust under compressive strain while it decreases with the tensile strain increasing. When the branch size is equal or greater than 2, their band gaps will monotonously increase with the strain increasing in the range of [-1.0, 2.0] Å. All boroxine-linked COFs are semiconductors with controllable band gaps, depending on the branch length and the applied strain. In comparison with other 2D materials, such as graphene, hexagonal boron nitride, and even γ-graphyne, all boroxine-linked COFs are much softer and even more stable. That is, they can maintain the planar features under a larger compressive strain, which means that they are good candidates in flexible electronics. PMID:26662215
Subwavelength Lattice Optics by Evolutionary Design
2015-01-01
This paper describes a new class of structured optical materials—lattice opto-materials—that can manipulate the flow of visible light into a wide range of three-dimensional profiles using evolutionary design principles. Lattice opto-materials are based on the discretization of a surface into a two-dimensional (2D) subwavelength lattice whose individual lattice sites can be controlled to achieve a programmed optical response. To access a desired optical property, we designed a lattice evolutionary algorithm that includes and optimizes contributions from every element in the lattice. Lattice opto-materials can exhibit simple properties, such as on- and off-axis focusing, and can also concentrate light into multiple, discrete spots. We expanded the unit cell shapes of the lattice to achieve distinct, polarization-dependent optical responses from the same 2D patterned substrate. Finally, these lattice opto-materials can also be combined into architectures that resemble a new type of compound flat lens. PMID:25380062
Subwavelength lattice optics by evolutionary design.
Huntington, Mark D; Lauhon, Lincoln J; Odom, Teri W
2014-12-10
This paper describes a new class of structured optical materials--lattice opto-materials--that can manipulate the flow of visible light into a wide range of three-dimensional profiles using evolutionary design principles. Lattice opto-materials are based on the discretization of a surface into a two-dimensional (2D) subwavelength lattice whose individual lattice sites can be controlled to achieve a programmed optical response. To access a desired optical property, we designed a lattice evolutionary algorithm that includes and optimizes contributions from every element in the lattice. Lattice opto-materials can exhibit simple properties, such as on- and off-axis focusing, and can also concentrate light into multiple, discrete spots. We expanded the unit cell shapes of the lattice to achieve distinct, polarization-dependent optical responses from the same 2D patterned substrate. Finally, these lattice opto-materials can also be combined into architectures that resemble a new type of compound flat lens. PMID:25380062
Ballistic Transport in Graphene Antidot Lattices.
Sandner, Andreas; Preis, Tobias; Schell, Christian; Giudici, Paula; Watanabe, Kenji; Taniguchi, Takashi; Weiss, Dieter; Eroms, Jonathan
2015-12-01
The bulk carrier mobility in graphene was shown to be enhanced in graphene-boron nitride heterostructures. However, nanopatterning graphene can add extra damage and drastically degrade the intrinsic properties by edge disorder. Here we show that graphene embedded into a heterostructure with hexagonal boron nitride (hBN) on both sides is protected during a nanopatterning step. In this way, we can prepare graphene-based antidot lattices where the high mobility is preserved. We report magnetotransport experiments in those antidot lattices with lattice periods down to 50 nm. We observe pronounced commensurability features stemming from ballistic orbits around one or several antidots. Due to the short lattice period in our samples, we can also explore the boundary between the classical and the quantum transport regime, as the Fermi wavelength of the electrons approaches the smallest length scale of the artificial potential. PMID:26598218
Increasing skyrmion lattice stability: theory and experiment
NASA Astrophysics Data System (ADS)
Kruchkov, Alex; White, Jonathan; Ronnow, Henrik; Zivkovic, Ivica
Magnetic skyrmions are vortices of spins, considered to be topologically protected against perturbations, and envisaged as very possible next-generation information carriers due to their nanoscale size. In chiral ferromagnets they form a two-dimensional hexagonal array - the skyrmion lattice. A key challenge is that bulk skyrmions have been restricted so far to a tiny region in the temperature-field phase diagram. In this work we address theoretically the stability of the skyrmion lattice. We demonstrate that tuning anisotropy can lead to dramatic (20 times) enhancement of the skyrmion phase volume, which has been recently revealed in our experiment.
Lattice Gas Model with Nonlocal Interactions
NASA Astrophysics Data System (ADS)
Das, Shankar P.
We analyze the nature of the hydrodynamic modes in a Lattice Gas Automata (LGA) model defined on a hexagonal lattice and having nonlocal interactions of attractive and repulsive type simultaneously. The model is similar in spirit to the liquid gas model of Appert and Zaleski [Phys. Rev. Lett. 64, 1 (1990)]. The phase diagram for the model is computed using the kinetic pressure. The dynamics is studied with a mean field type approach in the Boltzmann approximation ignoring effects of correlated collisions. We compute the transport coefficients and the speed of sound propagation. The presence of attractive interactions show increase in the transport coefficients at intermediate densities.
Gluing hexagons at three loops
NASA Astrophysics Data System (ADS)
Basso, Benjamin; Goncalves, Vasco; Komatsu, Shota; Vieira, Pedro
2016-06-01
We perform extensive three-loop tests of the hexagon bootstrap approach for structure constants in planar N = 4 SYM theory. We focus on correlators involving two BPS operators and one non-BPS operator in the so-called SL (2) sector. At three loops, such correlators receive wrapping corrections from mirror excitations flowing in either the adjacent or the opposing channel. Amusingly, we find that the first type of correction coincides exactly with the leading wrapping correction for the spectrum (divided by the one-loop anomalous dimension). We develop an efficient method for computing the second type of correction for operators with any spin. The results are in perfect agreement with the recently obtained three-loop perturbative data by Chicherin, Drummond, Heslop, Sokatchev [2] and by Eden [3]. We also derive the integrand for general multi-particle wrapping corrections, which turns out to take a remarkably simple form. As an application we estimate the loop order at which various new physical effects are expected to kick-in.
Fermionic pentagons and NMHV hexagon
NASA Astrophysics Data System (ADS)
Belitsky, A. V.
2015-05-01
We analyze the near-collinear limit of the null polygonal hexagon super Wilson loop in the planar N = 4 super-Yang-Mills theory. We focus on its Grassmann components which are dual to next-to-maximal helicity-violating (NMHV) scattering amplitudes. The kinematics in question is studied within a framework of the operator product expansion that encodes propagation of excitations on the background of the color flux tube stretched between the sides of Wilson loop contour. While their dispersion relation is known to all orders in 't Hooft coupling from previous studies, we find their form factor couplings to the Wilson loop. This is done making use of a particular tessellation of the loop where pentagon transitions play a fundamental role. Being interested in NMHV amplitudes, the corresponding building blocks carry a nontrivial charge under the SU(4) R-symmetry group. Restricting the current consideration to twist-two accuracy, we analyze two-particle contributions with a fermion as one of the constituents in the pair. We demonstrate that these nonsinglet pentagons obey bootstrap equations that possess consistent solutions for any value of the coupling constant. To confirm the correctness of these predictions, we calculate their contribution to the super Wilson loop demonstrating agreement with recent results to four-loop order in 't Hooft coupling.
Ultracold quantum gases in triangular optical lattices
NASA Astrophysics Data System (ADS)
Becker, C.; Soltan-Panahi, P.; Kronjäger, J.; Dörscher, S.; Bongs, K.; Sengstock, K.
2010-06-01
Over recent years, exciting developments in the field of ultracold atoms confined in optical lattices have led to numerous theoretical proposals devoted to the quantum simulation of problems e.g. known from condensed matter physics. Many of those ideas demand experimental environments with non-cubic lattice geometries. In this paper, we report on the implementation of a versatile three-beam lattice allowing for the generation of triangular as well as hexagonal optical lattices. As an important step, the superfluid-Mott insulator (SF-MI) quantum phase transition has been observed and investigated in detail in this lattice geometry for the first time. In addition to this, we study the physics of spinor Bose-Einstein condensates (BEC) in the presence of the triangular optical lattice potential, especially spin changing dynamics across the SF-MI transition. Our results suggest that, below the SF-MI phase transition, a well-established mean-field model describes the observed data when renormalizing the spin-dependent interaction. Interestingly, this opens up new perspectives for a lattice-driven tuning of a spin dynamics resonance occurring through the interplay of the quadratic Zeeman effect and spin-dependent interaction. Finally, we discuss further lattice configurations that can be realized with our setup.
Alignment of paired molecules of C60 within a hexagonal platform networked through hydrogen-bonds.
Hisaki, Ichiro; Nakagawa, Shoichi; Sato, Hiroyasu; Tohnai, Norimitsu
2016-07-28
We demonstrate, for the first time, that a hydrogen-bonded low-density organic framework can be applied as a platform to achieve periodic alignment of paired molecules of C60, which is the smallest example of a finite-numbered cluster of C60. The framework is a layered assembly of a hydrogen-bonded 2D hexagonal network (LA-H-HexNet) composed of dodecadehydrotribenzo[18]annulene derivatives. PMID:27417325
Columnar epitaxy of hexagonal and orthorhombic silicides on Si(111)
NASA Technical Reports Server (NTRS)
Fathauer, R. W.; Nieh, C. W.; Xiao, Q. F.; Hashimoto, Shin
1990-01-01
Columnar grains of PtSi and CrSi2 surrounded by high-quality epitaxial silicon are obtained by ultrahigh vacuum codeposition of Si and metal in an approximately 10:1 ratio on Si(111) substrates heated to 610-840 C. This result is similar to that found previously for CoSi2 (a nearly-lattice-matched cubic-fluorite crystal) on Si(111), in spite of the respective orthorhombic and hexagonal structures of PtSi and CrSi2. The PtSi grains are epitaxial and have one of three variants of the relation defined by PtSi(010)/Si(111), with PtSi 001 line/Si 110 line type.
Activated chemoreceptor arrays remain intact and hexagonally packed
Briegel, Ariane; Beeby, Morgan; Thanbichler, Martin; Jensen, Grant J.
2013-01-01
Summary Bacterial chemoreceptors cluster into exquisitively sensitive, tunable, highly ordered, polar arrays. While these arrays serve as paradigms of cell signalling in general, it remains unclear what conformational changes transduce signals from the periplasmic tips, where attractants and repellents bind, to the cytoplasmic signalling domains. Conflicting reports support and contest the hypothesis that activation causes large changes in the packing arrangement of the arrays, up to and including their complete disassembly. Using electron cryotomography, here we show that in Caulobacter crescentus, chemoreceptor arrays in cells grown in different media and immediately after exposure to the attractant galactose all exhibit the same 12 nm hexagonal packing arrangement, array size and other structural parameters. ΔcheB and ΔcheR mutants mimicking attractant- or repellent-bound states prior to adaptation also show the same lattice structure. We conclude that signal transduction and amplification must be accomplished through only small, nanoscale conformational changes. PMID:21992450
Predicting Sizes of Hexagonal and Gyroid Metal Nanostructures from Liquid Crystal Templating.
Asghar, Kaleem A; Rowlands, Daniel A; Elliott, Joanne M; Squires, Adam M
2015-11-24
We describe a method to predict and control the lattice parameters of hexagonal and gyroid mesoporous materials formed by liquid crystal templating. In the first part, we describe a geometric model with which the lattice parameters of different liquid crystal mesophases can be predicted as a function of their water/surfactant/oil volume fractions, based on certain geometric parameters relating to the constituent surfactant molecules. We demonstrate the application of this model to the lamellar (Lα), hexagonal (H1), and gyroid bicontinuous cubic (V1) mesophases formed by the binary Brij-56 (C16EO10)/water system and the ternary Brij-56/hexadecane/water system. In this way, we demonstrate predictable and independent control over the size of the cylinders (with hexadecane) and their spacing (with water). In the second part, we produce mesoporous platinum using as templates hexagonal and gyroid phases with different compositions and show that in each case the symmetry and lattice parameter of the metal nanostructure faithfully replicate those of the liquid crystal template, which is itself in agreement with the model. This demonstrates a rational control over the geometry, size, and spacing of pores in a mesoporous metal. PMID:26493862
Elastic interaction of point defects in cubic and hexagonal crystals
NASA Astrophysics Data System (ADS)
Kukushkin, S. A.; Osipov, A. V.; Telyatnik, R. S.
2016-05-01
The elastic interaction of two point defects in cubic and hexagonal structures has been considered. On the basis of the exact expression for the tensor Green's function of the elastic field obtained by the Lifschitz-Rozentsveig for a hexagonal medium, an exact formula for the interaction energy of two point defects has been obtained. The solution is represented as a function of the angle of their relative position on the example of semiconductors such as III-nitrides and α-SiC. For the cubic medium, the solution is found on the basis of the Lifschitz-Rozentsveig Green's tensors corrected by Ostapchuk, in the weak-anisotropy approximation. It is proven that the calculation of the interaction energy by the original Lifschitz-Rozentsveig Green's tensor leads to the opposite sign of the energy. On the example of the silicon crystal, the approximate solution is compared with the numerical solution, which is represented as an approximation by a series of spherical harmonics. The range of applicability of the continual approach is estimated by the quantum mechanical calculation of the lattice Green's function.
Parametric analysis of 2D guided-wave photonic band gap structures
NASA Astrophysics Data System (ADS)
Ciminelli, C.; Peluso, F.; Armenise, M. N.
2005-11-01
The parametric analysis of the electromagnetic properties of 2D guided wave photonic band gap structures is reported with the aim of providing a valid tool for the optimal design. The modelling approach is based on the Bloch-Floquet method. Different lattice configurations and geometrical parameters are considered. An optimum value for the ratio between the hole (or rod) radius and the lattice constant does exist and the calculation demonstrated that it is almost independent from the etching depth, only depending on the lattice type. The results are suitable for the design optimisation of photonic crystal reflectors to be used in integrated optical devices.
Parametric analysis of 2D guided-wave photonic band gap structures.
Ciminelli, C; Peluso, F; Armenise, M
2005-11-28
The parametric analysis of the electromagnetic properties of 2D guided wave photonic band gap structures is reported with the aim of providing a valid tool for the optimal design. The modelling approach is based on the Bloch-Floquet method. Different lattice configurations and geometrical parameters are considered. An optimum value for the ratio between the hole (or rod) radius and the lattice constant does exist and the calculation demonstrated that it is almost independent from the etching depth, only depending on the lattice type. The results are suitable for the design optimisation of photonic crystal reflectors to be used in integrated optical devices. PMID:19503180
Novel high pressure hexagonal OsB{sub 2} by mechanochemistry
Xie, Zhilin; Graule, Moritz; Orlovskaya, Nina; Andrew Payzant, E.; Cullen, David A.; Blair, Richard G.
2014-07-01
Hexagonal OsB{sub 2}, a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction indicated that formation of hexagonal OsB{sub 2} begins after 2.5 h of milling, and the reaction reaches equilibrium after 18 h of milling. Rietveld refinement of the powder data indicated that hexagonal OsB{sub 2} crystallizes in the P63/mmc space group (No. 194) with lattice parameters of a=2.916 Å and c=7.376 Å. Transmission electron microscopy confirmed the appearance of the hexagonal OsB{sub 2} phase after high energy ball milling. in situ X-ray diffraction experiments showed that the phase is stable from −225 °C to 1050 °C. The hexagonal OsB{sub 2} powder was annealed at 1050 °C for 6 days in vacuo to improve crystallinity and remove strain induced during the mechanochemical synthesis. The structure partially converted to the orthorhombic phase (20 wt%) after fast current assisted sintering of hexagonal OsB{sub 2} at 1500 °C for 5 min. Mechanochemical approaches to the synthesis of hard boride materials allow new phases to be produced that cannot be prepared using conventional methods. - Graphical abstract: High resolution transmission electron micrograph of hexagonal OsB{sub 2} nanocrystallite with corresponding fast Fourier transform and simulated diffraction pattern. - Highlights: • Hexagonal OsB{sub 2} has been synthesized for the first time by mechanochemical method. • Hexagonal OsB{sub 2} crystallizes in P63/mmc space group (No. 194), a=2.916 Å and c=7.376 Å. • The hexagonal structure was confirmed by a transmission electron microscope. • No phase transformation was observed after being annealed at 1050 °C for 6 days. • 20 wt% of h-OsB{sub 2} was transformed to o-OsB{sub 2} after being sintered at 1500 °C for 5 min.
Hexagonal photonic crystal waveguide based on barium titanate thin films
NASA Astrophysics Data System (ADS)
Li, Jianheng; Liu, Zhifu; Wessels, Bruce W.; Tu, Yongming; Ho, Seng-Tiong; Joshi-Imre, Alexandra; Ocola, Leonidas E.
2011-03-01
The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in epitaxial BaTiO3 were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB bandwidth of 4 THz.
Perspectives for spintronics in 2D materials
NASA Astrophysics Data System (ADS)
Han, Wei
2016-03-01
The past decade has been especially creative for spintronics since the (re)discovery of various two dimensional (2D) materials. Due to the unusual physical characteristics, 2D materials have provided new platforms to probe the spin interaction with other degrees of freedom for electrons, as well as to be used for novel spintronics applications. This review briefly presents the most important recent and ongoing research for spintronics in 2D materials.
NASA Astrophysics Data System (ADS)
Li, Shuai; Qiu, Wen-Xuan; Gao, Jin-Hua
2016-06-01
Recently, a new kind of artificial two dimensional (2D) electron lattice on the nanoscale, i.e. molecular graphene, has drawn a lot of interest, where the metal surface electrons are transformed into a honeycomb lattice via absorbing a molecular lattice on the metal surface [Gomes et al., Nature, 2012, 438, 306; Wang et al., Phys. Rev. Lett., 2014, 113, 196803]. In this work, we theoretically demonstrate that this technique can be readily used to build other complex 2D electron lattices on a metal surface, which are of high interest in the field of condensed matter physics. The main challenge to build a complex 2D electron lattice is that this is a quantum antidot system, where the absorbed molecule normally exerts a repulsive potential on the surface electrons. Thus, there is no straightforward corresponding relation between the molecular lattice pattern and the desired 2D lattice of surface electrons. Here, we give an interesting example about the Kagome lattice, which has exotic correlated electronic states. We design a special molecular pattern and show that this molecular lattice can transform the surface electrons into a Kagome-like lattice. The numerical simulation is conducted using a Cu(111) surface and CO molecules. We first estimate the effective parameters of the Cu/CO system by fitting experimental data of the molecular graphene. Then, we calculate the corresponding energy bands and LDOS of the surface electrons in the presence of the proposed molecular lattice. Finally, we interpret the numerical results by the tight binding model of the Kagome lattice. We hope that our work can stimulate further theoretical and experimental interest in this novel artificial 2D electron lattice system.
Li, Shuai; Qiu, Wen-Xuan; Gao, Jin-Hua
2016-07-01
Recently, a new kind of artificial two dimensional (2D) electron lattice on the nanoscale, i.e. molecular graphene, has drawn a lot of interest, where the metal surface electrons are transformed into a honeycomb lattice via absorbing a molecular lattice on the metal surface [Gomes et al., Nature, 2012, 438, 306; Wang et al., Phys. Rev. Lett., 2014, 113, 196803]. In this work, we theoretically demonstrate that this technique can be readily used to build other complex 2D electron lattices on a metal surface, which are of high interest in the field of condensed matter physics. The main challenge to build a complex 2D electron lattice is that this is a quantum antidot system, where the absorbed molecule normally exerts a repulsive potential on the surface electrons. Thus, there is no straightforward corresponding relation between the molecular lattice pattern and the desired 2D lattice of surface electrons. Here, we give an interesting example about the Kagome lattice, which has exotic correlated electronic states. We design a special molecular pattern and show that this molecular lattice can transform the surface electrons into a Kagome-like lattice. The numerical simulation is conducted using a Cu(111) surface and CO molecules. We first estimate the effective parameters of the Cu/CO system by fitting experimental data of the molecular graphene. Then, we calculate the corresponding energy bands and LDOS of the surface electrons in the presence of the proposed molecular lattice. Finally, we interpret the numerical results by the tight binding model of the Kagome lattice. We hope that our work can stimulate further theoretical and experimental interest in this novel artificial 2D electron lattice system. PMID:27279292
Benchmarking of calculation schemes in APOLLO2 and COBAYA3 for WER lattices
Zheleva, N.; Ivanov, P.; Todorova, G.; Kolev, N.; Herrero, J. J.
2013-07-01
This paper presents solutions of the NURISP WER lattice benchmark using APOLLO2, TRIPOLI4 and COBAYA3 pin-by-pin. The main objective is to validate MOC based calculation schemes for pin-by-pin cross-section generation with APOLLO2 against TRIPOLI4 reference results. A specific objective is to test the APOLLO2 generated cross-sections and interface discontinuity factors in COBAYA3 pin-by-pin calculations with unstructured mesh. The VVER-1000 core consists of large hexagonal assemblies with 2 mm inter-assembly water gaps which require the use of unstructured meshes in the pin-by-pin core simulators. The considered 2D benchmark problems include 19-pin clusters, fuel assemblies and 7-assembly clusters. APOLLO2 calculation schemes with the step characteristic method (MOC) and the higher-order Linear Surface MOC have been tested. The comparison of APOLLO2 vs. TRIPOLI4 results shows a very close agreement. The 3D lattice solver in COBAYA3 uses transport corrected multi-group diffusion approximation with interface discontinuity factors of Generalized Equivalence Theory (GET) or Black Box Homogenization (BBH) type. The COBAYA3 pin-by-pin results in 2, 4 and 8 energy groups are close to the reference solutions when using side-dependent interface discontinuity factors. (authors)
Annotated Bibliography of EDGE2D Use
J.D. Strachan and G. Corrigan
2005-06-24
This annotated bibliography is intended to help EDGE2D users, and particularly new users, find existing published literature that has used EDGE2D. Our idea is that a person can find existing studies which may relate to his intended use, as well as gain ideas about other possible applications by scanning the attached tables.
Staring 2-D hadamard transform spectral imager
Gentry, Stephen M.; Wehlburg, Christine M.; Wehlburg, Joseph C.; Smith, Mark W.; Smith, Jody L.
2006-02-07
A staring imaging system inputs a 2D spatial image containing multi-frequency spectral information. This image is encoded in one dimension of the image with a cyclic Hadamarid S-matrix. The resulting image is detecting with a spatial 2D detector; and a computer applies a Hadamard transform to recover the encoded image.
NASA Astrophysics Data System (ADS)
Buchheim, Jakob; Wyss, Roman M.; Shorubalko, Ivan; Park, Hyung Gyu
2016-04-01
We report experimentally and theoretically the behavior of freestanding graphene subjected to bombardment of energetic ions, investigating the capability of large-scale patterning of freestanding graphene with nanometer sized features by focused ion beam technology. A precise control over the He+ and Ga+ irradiation offered by focused ion beam techniques enables investigating the interaction of the energetic particles and graphene suspended with no support and allows determining sputter yields of the 2D lattice. We found a strong dependency of the 2D sputter yield on the species and kinetic energy of the incident ion beams. Freestanding graphene shows material semi-transparency to He+ at high energies (10-30 keV) allowing the passage of >97% He+ particles without creating destructive lattice vacancy. Large Ga+ ions (5-30 keV), in contrast, collide far more often with the graphene lattice to impart a significantly higher sputter yield of ~50%. Binary collision theory applied to monolayer and few-layer graphene can successfully elucidate this collision mechanism, in great agreement with experiments. Raman spectroscopy analysis corroborates the passage of a large fraction of He+ ions across graphene without much damaging the lattice whereas several colliding ions create single vacancy defects. Physical understanding of the interaction between energetic particles and suspended graphene can practically lead to reproducible and efficient pattern generation of unprecedentedly small features on 2D materials by design, manifested by our perforation of sub-5 nm pore arrays. This capability of nanometer-scale precision patterning of freestanding 2D lattices shows the practical applicability of focused ion beam technology to 2D material processing for device fabrication and integration.We report experimentally and theoretically the behavior of freestanding graphene subjected to bombardment of energetic ions, investigating the capability of large-scale patterning of
Mirror effects and optical meta-surfaces in 2d atomic arrays
NASA Astrophysics Data System (ADS)
Shahmoon, Ephraim; Wild, Dominik; Lukin, Mikhail; Yelin, Susanne
2016-05-01
Strong optical response of natural and artificial (meta-) materials typically relies on the fact that the lattice constant that separates their constituent particles (atoms or electromagnetic resonators, respectively) is much smaller than the optical wavelength. Here we consider a single layer of a 2d atom array with a lattice constant on the order of an optical wavelength, which can be thought of as a highly dilute 2d metamaterial (meta-surface). Our theoretical analysis shows how strong scattering of resonant incoming light off the array can be controlled by choosing its lattice constant, e.g. allowing the array to operate as a perfect mirror or a retro-reflector for most incident angles of the incoming light. We discuss the prospects for quantum metasurfaces, i.e. the ability to shape the output quantum state of light by controlling the atomic states, and the possible generality of our results as a universal wave phenomena.
Duality analysis on random planar lattices.
Ohzeki, Masayuki; Fujii, Keisuke
2012-11-01
The conventional duality analysis is employed to identify a location of a critical point on a uniform lattice without any disorder in its structure. In the present study, we deal with the random planar lattice, which consists of the randomized structure based on the square lattice. We introduce the uniformly random modification by the bond dilution and contraction on a part of the unit square. The random planar lattice includes the triangular and hexagonal lattices in extreme cases of a parameter to control the structure. A modern duality analysis fashion with real-space renormalization is found to be available for estimating the location of the critical points with a wide range of the randomness parameter. As a simple test bed, we demonstrate that our method indeed gives several critical points for the cases of the Ising and Potts models and the bond-percolation thresholds on the random planar lattice. Our method leads to not only such an extension of the duality analyses on the classical statistical mechanics but also a fascinating result associated with optimal error thresholds for a class of quantum error correction code, the surface code on the random planar lattice, which is known as a skillful technique to protect the quantum state. PMID:23214752
Duality analysis on random planar lattices
NASA Astrophysics Data System (ADS)
Ohzeki, Masayuki; Fujii, Keisuke
2012-11-01
The conventional duality analysis is employed to identify a location of a critical point on a uniform lattice without any disorder in its structure. In the present study, we deal with the random planar lattice, which consists of the randomized structure based on the square lattice. We introduce the uniformly random modification by the bond dilution and contraction on a part of the unit square. The random planar lattice includes the triangular and hexagonal lattices in extreme cases of a parameter to control the structure. A modern duality analysis fashion with real-space renormalization is found to be available for estimating the location of the critical points with a wide range of the randomness parameter. As a simple test bed, we demonstrate that our method indeed gives several critical points for the cases of the Ising and Potts models and the bond-percolation thresholds on the random planar lattice. Our method leads to not only such an extension of the duality analyses on the classical statistical mechanics but also a fascinating result associated with optimal error thresholds for a class of quantum error correction code, the surface code on the random planar lattice, which is known as a skillful technique to protect the quantum state.
Hydrothermal synthesis of hexagonal magnesium hydroxide nanoflakes
Wang, Qiang; Li, Chunhong; Guo, Ming; Sun, Lingna; Hu, Changwen
2014-03-01
Graphical abstract: Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method in the presence of PEG-20,000. Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake yielded different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. - Highlights: • Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method. • PEG-20,000 plays an important role in the formation of hexagonal nanostructure. • Mg(OH){sub 2} nanoflakes show different crystalline structures at different positions. • The probable formation mechanism of hexagonal Mg(OH){sub 2} nanoflakes was reported. - Abstract: Hexagonal magnesium hydroxide (Mg(OH){sub 2}) nanoflakes were successfully synthesized via hydrothermal method in the presence of the surfactant polyethylene glycol 20,000 (PEG-20,000). Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The composition, morphologies and structure of the Mg(OH){sub 2} nanoflakes were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake show different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. Brunauer–Emmett–Teller (BET) analysis were performed to investigate the porous structure and surface area of the as-obtained nanoflakes.
Buchheim, Jakob; Wyss, Roman M; Shorubalko, Ivan; Park, Hyung Gyu
2016-04-21
We report experimentally and theoretically the behavior of freestanding graphene subjected to bombardment of energetic ions, investigating the capability of large-scale patterning of freestanding graphene with nanometer sized features by focused ion beam technology. A precise control over the He(+) and Ga(+) irradiation offered by focused ion beam techniques enables investigating the interaction of the energetic particles and graphene suspended with no support and allows determining sputter yields of the 2D lattice. We found a strong dependency of the 2D sputter yield on the species and kinetic energy of the incident ion beams. Freestanding graphene shows material semi-transparency to He(+) at high energies (10-30 keV) allowing the passage of >97% He(+) particles without creating destructive lattice vacancy. Large Ga(+) ions (5-30 keV), in contrast, collide far more often with the graphene lattice to impart a significantly higher sputter yield of ∼50%. Binary collision theory applied to monolayer and few-layer graphene can successfully elucidate this collision mechanism, in great agreement with experiments. Raman spectroscopy analysis corroborates the passage of a large fraction of He(+) ions across graphene without much damaging the lattice whereas several colliding ions create single vacancy defects. Physical understanding of the interaction between energetic particles and suspended graphene can practically lead to reproducible and efficient pattern generation of unprecedentedly small features on 2D materials by design, manifested by our perforation of sub-5 nm pore arrays. This capability of nanometer-scale precision patterning of freestanding 2D lattices shows the practical applicability of focused ion beam technology to 2D material processing for device fabrication and integration. PMID:27043304
Cold atoms in a rotating optical lattice
NASA Astrophysics Data System (ADS)
Foot, Christopher J.
2009-05-01
We have demonstrated a novel experimental arrangement which can rotate a two-dimensional optical lattice at frequencies up to several kilohertz. Our arrangement also allows the periodicity of the optical lattice to be varied dynamically, producing a 2D ``accordion lattice'' [1]. The angles of the laser beams are controlled by acousto-optic deflectors and this allows smooth changes with little heating of the trapped cold (rubidium) atoms. We have loaded a BEC into lattices with periodicities ranging from 1.8μm to 18μm, observing the collapse and revival of the diffraction orders of the condensate over a large range of lattice parameters as recently reported by a group in NIST [2]. We have also imaged atoms in situ in a 2D lattice over a range of lattice periodicities. Ultracold atoms in a rotating lattice can be used for the direct quantum simulation of strongly correlated systems under large effective magnetic fields, i.e. the Hamiltonian of the atoms in the rotating frame resembles that of a charged particle in a strong magnetic field. In the future, we plan to use this to investigate a range of phenomena such as the analogue of the fractional quantum Hall effect. [4pt] [1] R. A. Williams, J. D. Pillet, S. Al-Assam, B. Fletcher, M. Shotter, and C. J. Foot, ``Dynamic optical lattices: two-dimensional rotating and accordion lattices for ultracold atoms,'' Opt. Express 16, 16977-16983 (2008) [0pt] [2] J. H. Huckans, I. B. Spielman, B. Laburthe Tolra, W. D. Phillips, and J. V. Porto, Quantum and Classical Dynamics of a BEC in a Large-Period Optical Lattice, arXiv:0901.1386v1
Light field morphing using 2D features.
Wang, Lifeng; Lin, Stephen; Lee, Seungyong; Guo, Baining; Shum, Heung-Yeung
2005-01-01
We present a 2D feature-based technique for morphing 3D objects represented by light fields. Existing light field morphing methods require the user to specify corresponding 3D feature elements to guide morph computation. Since slight errors in 3D specification can lead to significant morphing artifacts, we propose a scheme based on 2D feature elements that is less sensitive to imprecise marking of features. First, 2D features are specified by the user in a number of key views in the source and target light fields. Then the two light fields are warped view by view as guided by the corresponding 2D features. Finally, the two warped light fields are blended together to yield the desired light field morph. Two key issues in light field morphing are feature specification and warping of light field rays. For feature specification, we introduce a user interface for delineating 2D features in key views of a light field, which are automatically interpolated to other views. For ray warping, we describe a 2D technique that accounts for visibility changes and present a comparison to the ideal morphing of light fields. Light field morphing based on 2D features makes it simple to incorporate previous image morphing techniques such as nonuniform blending, as well as to morph between an image and a light field. PMID:15631126
2D materials for nanophotonic devices
NASA Astrophysics Data System (ADS)
Xu, Renjing; Yang, Jiong; Zhang, Shuang; Pei, Jiajie; Lu, Yuerui
2015-12-01
Two-dimensional (2D) materials have become very important building blocks for electronic, photonic, and phononic devices. The 2D material family has four key members, including the metallic graphene, transition metal dichalcogenide (TMD) layered semiconductors, semiconducting black phosphorous, and the insulating h-BN. Owing to the strong quantum confinements and defect-free surfaces, these atomically thin layers have offered us perfect platforms to investigate the interactions among photons, electrons and phonons. The unique interactions in these 2D materials are very important for both scientific research and application engineering. In this talk, I would like to briefly summarize and highlight the key findings, opportunities and challenges in this field. Next, I will introduce/highlight our recent achievements. We demonstrated atomically thin micro-lens and gratings using 2D MoS2, which is the thinnest optical component around the world. These devices are based on our discovery that the elastic light-matter interactions in highindex 2D materials is very strong. Also, I would like to introduce a new two-dimensional material phosphorene. Phosphorene has strongly anisotropic optical response, which creates 1D excitons in a 2D system. The strong confinement in phosphorene also enables the ultra-high trion (charged exciton) binding energies, which have been successfully measured in our experiments. Finally, I will briefly talk about the potential applications of 2D materials in energy harvesting.
Inertial solvation in femtosecond 2D spectra
NASA Astrophysics Data System (ADS)
Hybl, John; Albrecht Ferro, Allison; Farrow, Darcie; Jonas, David
2001-03-01
We have used 2D Fourier transform spectroscopy to investigate polar solvation. 2D spectroscopy can reveal molecular lineshapes beneath ensemble averaged spectra and freeze molecular motions to give an undistorted picture of the microscopic dynamics of polar solvation. The transition from "inhomogeneous" to "homogeneous" 2D spectra is governed by both vibrational relaxation and solvent motion. Therefore, the time dependence of the 2D spectrum directly reflects the total response of the solvent-solute system. IR144, a cyanine dye with a dipole moment change upon electronic excitation, was used to probe inertial solvation in methanol and propylene carbonate. Since the static Stokes' shift of IR144 in each of these solvents is similar, differences in the 2D spectra result from solvation dynamics. Initial results indicate that the larger propylene carbonate responds more slowly than methanol, but appear to be inconsistent with rotational estimates of the inertial response. To disentangle intra-molecular vibrations from solvent motion, the 2D spectra of IR144 will be compared to the time-dependent 2D spectra of the structurally related nonpolar cyanine dye HDITCP.
Internal Photoemission Spectroscopy of 2-D Materials
NASA Astrophysics Data System (ADS)
Nguyen, Nhan; Li, Mingda; Vishwanath, Suresh; Yan, Rusen; Xiao, Shudong; Xing, Huili; Cheng, Guangjun; Hight Walker, Angela; Zhang, Qin
Recent research has shown the great benefits of using 2-D materials in the tunnel field-effect transistor (TFET), which is considered a promising candidate for the beyond-CMOS technology. The on-state current of TFET can be enhanced by engineering the band alignment of different 2D-2D or 2D-3D heterostructures. Here we present the internal photoemission spectroscopy (IPE) approach to determine the band alignments of various 2-D materials, in particular SnSe2 and WSe2, which have been proposed for new TFET designs. The metal-oxide-2-D semiconductor test structures are fabricated and characterized by IPE, where the band offsets from the 2-D semiconductor to the oxide conduction band minimum are determined by the threshold of the cube root of IPE yields as a function of photon energy. In particular, we find that SnSe2 has a larger electron affinity than most semiconductors and can be combined with other semiconductors to form near broken-gap heterojunctions with low barrier heights which can produce a higher on-state current. The details of data analysis of IPE and the results from Raman spectroscopy and spectroscopic ellipsometry measurements will also be presented and discussed.
Blue-noise halftoning for hexagonal grids.
Lau, Daniel L; Ulichney, Robert
2006-05-01
In this paper, we closely scrutinize the spatial and spectral properties of aperiodic halftoning schemes on rectangular and hexagonal sampling grids. Traditionally, hexagonal sampling grids have been shunned due to their inability to preserve the high-frequency components of blue-noise dither patterns at gray-levels near one-half, but as will be shown, only through the introduction of diagonal correlations between dots can even rectangular sampling grids preserve these frequencies. And by allowing the sampling grid to constrain the placement of dots, a particular algorithm may introduce visual artifacts just as disturbing as excess energy below the principal frequency. If, instead, the algorithm maintains radial symmetry by introducing a minimum degree of clustering, then that algorithm can maintain its grid defiance illusion fundamental to the spirit of the blue-noise model. As such, this paper shows that hexagonal grids are preferrable because they can support gray-levels near one-half with less required clustering of minority pixels and a higher principal frequency. Along with a thorough Fourier analysis of blue-noise dither patterns on both rectangular and hexagonal sampling grids, this paper also demonstrates the construction of a blue-noise dither array for hexagonal grids. PMID:16671307
Expansion of the hexagonal phase-forming region of Lu1-xScxFeO3 by containerless processing.
Masuno, Atsunobu; Ishimoto, Atsushi; Moriyoshi, Chikako; Kawaji, Hitoshi; Kuroiwa, Yoshihiro; Inoue, Hiroyuki
2015-10-01
Hexagonal Lu1-xScxFeO3 (0 ≤ x ≤ 0.8) was directly solidified from an undercooled melt by containerless processing with an aerodynamic levitation furnace. The hexagonal phase-forming region was considerably extended compared to that of the conventional solid-state reaction (x ∼ 0.5). Synchrotron X-ray diffraction measurements revealed that the crystal structure of the hexagonal phase was isomorphous to hexagonal ferroelectric RMnO3 (R = a rare earth ion) with a polar space group of P63cm. As x increased, the a-axis lattice constant decreased linearly, strengthening the antiferromagnetic interaction between the Fe(3+) ions on the a-b plane. Accordingly, the weak ferromagnetic transition temperature increased from 150 K for x = 0 to 175 K for x = 0.7. These transition temperatures were much higher than those of hexagonal Lu1-xScxMnO3. The results indicate that hexagonal Lu1-xScxFeO3 is a suitable alternative magnetic dielectric for use at higher temperatures. PMID:26376708
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
NASA Astrophysics Data System (ADS)
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-02-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics.
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2.
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
Arbitrary lattice symmetries via block copolymer nanomeshes
Majewski, Pawel W.; Rahman, Atikur; Black, Charles T.; Yager, Kevin G.
2015-01-01
Self-assembly of block copolymers is a powerful motif for spontaneously forming well-defined nanostructures over macroscopic areas. Yet, the inherent energy minimization criteria of self-assembly give rise to a limited library of structures; diblock copolymers naturally form spheres on a cubic lattice, hexagonally packed cylinders and alternating lamellae. Here, we demonstrate multicomponent nanomeshes with any desired lattice symmetry. We exploit photothermal annealing to rapidly order and align block copolymer phases over macroscopic areas, combined with conversion of the self-assembled organic phase into inorganic replicas. Repeated photothermal processing independently aligns successive layers, providing full control of the size, symmetry and composition of the nanoscale unit cell. We construct a variety of symmetries, most of which are not natively formed by block copolymers, including squares, rhombuses, rectangles and triangles. In fact, we demonstrate all possible two-dimensional Bravais lattices. Finally, we elucidate the influence of nanostructure on the electrical and optical properties of nanomeshes. PMID:26100566
Lattice Strain Defects in a Ceria Nanolayer
2016-01-01
An ultrathin two-dimensional CeO2 (ceria) phase on a Cu(110) surface has been fabricated and fully characterized by high-resolution scanning tunneling microscopy, photoelectron spectroscopy, and density functional theory. The atomic lattice structure of the ceria/Cu(110) system is revealed as a hexagonal CeO2(111)-type monolayer separated from the Cu(110) surface by a partly disordered Cu–O intercalated buffer layer. The epitaxial coupling of the two-dimensional ceria overlayer to the Cu(110)-O surface leads to a nanoscopic stripe pattern, which creates defect regions of quasi-periodic lattice distortions. The symmetry and lattice mismatch at the interface is clarified to be responsible for the topographic stripe geometry and the related anisotropic strain defect regions at the ceria surface. This ceria monolayer is in a fully oxidized and thermodynamically stable state. PMID:26988695
Micellar hexagonal phases in lyotropic liquid crystals
NASA Astrophysics Data System (ADS)
Amaral, L. Q.; Gulik, A.; Itri, R.; Mariani, P.
1992-09-01
The hexagonal cell parameter a of the system sodium dodecyl lauryl sulfate and water as a function of volume concentration cv in phase Hα shows the functional behavior expected for micelles of finite length: a~c-1/3v. The interpretation of x-ray data based on finite micelles leads to an alternative description of the hexagonal phase Hα: spherocylindrical micelles of constant radius with length that may grow along the range of the Hα phase. Results are compared with recent statistical-mechanical calculations for the isotropic I-Hα transition. The absence of diffraction in the direction perpendicular to the hexagonal plane is ascribed to polydispersity of micellar length, which also is a necessary condition for the occurrence of direct I-Hα transitions.
Brittle damage models in DYNA2D
Faux, D.R.
1997-09-01
DYNA2D is an explicit Lagrangian finite element code used to model dynamic events where stress wave interactions influence the overall response of the system. DYNA2D is often used to model penetration problems involving ductile-to-ductile impacts; however, with the advent of the use of ceramics in the armor-anti-armor community and the need to model damage to laser optics components, good brittle damage models are now needed in DYNA2D. This report will detail the implementation of four brittle damage models in DYNA2D, three scalar damage models and one tensor damage model. These new brittle damage models are then used to predict experimental results from three distinctly different glass damage problems.
Ginsparg, P.
1991-01-01
These are introductory lectures for a general audience that give an overview of the subject of matrix models and their application to random surfaces, 2d gravity, and string theory. They are intentionally 1.5 years out of date.
Ginsparg, P.
1991-12-31
These are introductory lectures for a general audience that give an overview of the subject of matrix models and their application to random surfaces, 2d gravity, and string theory. They are intentionally 1.5 years out of date.
2-d Finite Element Code Postprocessor
1996-07-15
ORION is an interactive program that serves as a postprocessor for the analysis programs NIKE2D, DYNA2D, TOPAZ2D, and CHEMICAL TOPAZ2D. ORION reads binary plot files generated by the two-dimensional finite element codes currently used by the Methods Development Group at LLNL. Contour and color fringe plots of a large number of quantities may be displayed on meshes consisting of triangular and quadrilateral elements. ORION can compute strain measures, interface pressures along slide lines, reaction forcesmore » along constrained boundaries, and momentum. ORION has been applied to study the response of two-dimensional solids and structures undergoing finite deformations under a wide variety of large deformation transient dynamic and static problems and heat transfer analyses.« less
Chemical Approaches to 2D Materials.
Samorì, Paolo; Palermo, Vincenzo; Feng, Xinliang
2016-08-01
Chemistry plays an ever-increasing role in the production, functionalization, processing and applications of graphene and other 2D materials. This special issue highlights a selection of enlightening chemical approaches to 2D materials, which nicely reflect the breadth of the field and convey the excitement of the individuals involved in it, who are trying to translate graphene and related materials from the laboratory into a real, high-impact technology. PMID:27478083
Thermally induced microstrain broadening in hexagonal zinc
Lawson, Andrew C; Valdez, James A; Roberts, Joyce A; Leineweber, Andreas; Mittemeijer, E J; Kreher, W
2008-01-01
Neutron powder-diffraction experiments on polycrystalline hexagonal zinc show considerable temperature-dependent line broadening. Whereas as-received zinc at 300 K exhibits narrow reflections, during cooling to a minimum temperature of 10K considerable line-broadening appears, which largely disappears again during reheating. The line broadening may be ascribed to microstrains induced by thermal microstresses due to the anisotropy of the thermal expansion (shrinkage) of hexagonal zinc. Differences between the thermal microstrains and theoretical predictions considering elastic deformation of the grains can be explained by plastic deformation and surface effects.
Intervalley scattering in hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Cassabois, G.; Valvin, P.; Gil, B.
2016-01-01
We report photoluminescence experiments bringing the evidence for intervalley scattering in bulk hexagonal boron nitride. From a quantitative analysis of the defect-related emission band, we demonstrate that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states. Time-resolved experiments highlight the different recombination dynamics of the phonon replicas implying either virtual excitonic states or real electronic states in the structural defects.
Extended 2D generalized dilaton gravity theories
NASA Astrophysics Data System (ADS)
de Mello, R. O.
2008-09-01
We show that an anomaly-free description of matter in (1+1) dimensions requires a deformation of the 2D relativity principle, which introduces a non-trivial centre in the 2D Poincaré algebra. Then we work out the reduced phase space of the anomaly-free 2D relativistic particle, in order to show that it lives in a noncommutative 2D Minkowski space. Moreover, we build a Gaussian wave packet to show that a Planck length is well defined in two dimensions. In order to provide a gravitational interpretation for this noncommutativity, we propose to extend the usual 2D generalized dilaton gravity models by a specific Maxwell component, which guages the extra symmetry associated with the centre of the 2D Poincaré algebra. In addition, we show that this extension is a high energy correction to the unextended dilaton theories that can affect the topology of spacetime. Further, we couple a test particle to the general extended dilaton models with the purpose of showing that they predict a noncommutativity in curved spacetime, which is locally described by a Moyal star product in the low energy limit. We also conjecture a probable generalization of this result, which provides strong evidence that the noncommutativity is described by a certain star product which is not of the Moyal type at high energies. Finally, we prove that the extended dilaton theories can be formulated as Poisson Sigma models based on a nonlinear deformation of the extended Poincaré algebra.
Formation of hexagonal and cubic ice during low-temperature growth
Thürmer, Konrad; Nie, Shu
2013-01-01
From our daily life we are familiar with hexagonal ice, but at very low temperature ice can exist in a different structure––that of cubic ice. Seeking to unravel the enigmatic relationship between these two low-pressure phases, we examined their formation on a Pt(111) substrate at low temperatures with scanning tunneling microscopy and atomic force microscopy. After completion of the one-molecule-thick wetting layer, 3D clusters of hexagonal ice grow via layer nucleation. The coalescence of these clusters creates a rich scenario of domain-boundary and screw-dislocation formation. We discovered that during subsequent growth, domain boundaries are replaced by growth spirals around screw dislocations, and that the nature of these spirals determines whether ice adopts the cubic or the hexagonal structure. Initially, most of these spirals are single, i.e., they host a screw dislocation with a Burgers vector connecting neighboring molecular planes, and produce cubic ice. Films thicker than ∼20 nm, however, are dominated by double spirals. Their abundance is surprising because they require a Burgers vector spanning two molecular-layer spacings, distorting the crystal lattice to a larger extent. We propose that these double spirals grow at the expense of the initially more common single spirals for an energetic reason: they produce hexagonal ice. PMID:23818592
Vorticity Generation by Rough Walls in 2D Decaying Turbulence
NASA Astrophysics Data System (ADS)
Tóth, Gábor; Jánosi, Imre M.
2015-12-01
In this work we present Lattice Boltzmann simulations of a decaying vortex array in a 2D rectangular domain, which is bounded by a random rough wall from one side. In order to separate the effects of the collisions with the rough wall, the opposite (smooth) rigid wall is placed at a larger distance from the center of the vortex array. Periodic boundary condition is imposed in the perpendicular direction. Well defined random roughness is generated by the widely studied Wolf-Villain surface growth algorithm. The main finding is that collisions with a rough wall generate excess vorticity compared with a smooth boundary, while the kinetic energy decreases monotonously. A proper measure is the integrated excess enstrophy, which exhibits an apparent maximum at an "optimal" roughness range. Numerical values of the excess enstrophy are very sensitive to a particular configuration (wall shape and vortex lattice randomization), however the "optimal" roughness exhibits surface features of similar characteristic sizes than that of the decaying vortices.
Intrinsic ferromagnetism in hexagonal boron nitride nanosheets
Si, M. S.; Gao, Daqiang E-mail: xueds@lzu.edu.cn; Yang, Dezheng; Peng, Yong; Zhang, Z. Y.; Xue, Desheng E-mail: xueds@lzu.edu.cn; Liu, Yushen; Deng, Xiaohui; Zhang, G. P.
2014-05-28
Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.
Hexagonal and Pentagonal Fractal Multiband Antennas
NASA Technical Reports Server (NTRS)
Tang, Philip W.; Wahid, Parveen
2005-01-01
Multiband dipole antennas based on hexagonal and pentagonal fractals have been analyzed by computational simulations and functionally demonstrated in experiments on prototypes. These antennas are capable of multiband or wide-band operation because they are subdivided into progressively smaller substructures that resonate at progressively higher frequencies by virtue of their smaller dimensions. The novelty of the present antennas lies in their specific hexagonal and pentagonal fractal configurations and the resonant frequencies associated with them. These antennas are potentially applicable to a variety of multiband and wide-band commercial wireless-communication products operating at different frequencies, including personal digital assistants, cellular telephones, pagers, satellite radios, Global Positioning System receivers, and products that combine two or more of the aforementioned functions. Perhaps the best-known prior multiband antenna based on fractal geometry is the Sierpinski triangle antenna (also known as the Sierpinski gasket), shown in the top part of the figure. In this antenna, the scale length at each iteration of the fractal is half the scale length of the preceding iteration, yielding successive resonant frequencies related by a ratio of about 2. The middle and bottom parts of the figure depict the first three iterations of the hexagonal and pentagonal fractals along with typical dipole-antenna configuration based on the second iteration. Successive resonant frequencies of the hexagonal fractal antenna have been found to be related by a ratio of about 3, and those of the pentagonal fractal antenna by a ratio of about 2.59.
Topological Properties of Electrons in Honeycomb Lattice with Detuned Hopping Energy
NASA Astrophysics Data System (ADS)
Wu, Long-Hua; Hu, Xiao
2016-04-01
Honeycomb lattice can support electronic states exhibiting Dirac energy dispersion, with graphene as the icon. We propose to derive nontrivial topology by grouping six neighboring sites of honeycomb lattice into hexagons and enhancing the inter-hexagon hopping energies over the intra-hexagon ones. We reveal that this manipulation opens a gap in the energy dispersion and drives the system into a topological state. The nontrivial topology is characterized by the index associated with a pseudo time-reversal symmetry emerging from the C6 symmetry of the hopping texture, where the angular momentum of orbitals accommodated on the hexagonal “artificial atoms” behaves as the pseudospin. The size of topological gap is proportional to the hopping-energy difference, which can be larger than typical spin-orbit couplings by orders of magnitude and potentially renders topological electronic transports available at high temperatures.
Topological Properties of Electrons in Honeycomb Lattice with Detuned Hopping Energy
Wu, Long-Hua; Hu, Xiao
2016-01-01
Honeycomb lattice can support electronic states exhibiting Dirac energy dispersion, with graphene as the icon. We propose to derive nontrivial topology by grouping six neighboring sites of honeycomb lattice into hexagons and enhancing the inter-hexagon hopping energies over the intra-hexagon ones. We reveal that this manipulation opens a gap in the energy dispersion and drives the system into a topological state. The nontrivial topology is characterized by the index associated with a pseudo time-reversal symmetry emerging from the C6 symmetry of the hopping texture, where the angular momentum of orbitals accommodated on the hexagonal “artificial atoms” behaves as the pseudospin. The size of topological gap is proportional to the hopping-energy difference, which can be larger than typical spin-orbit couplings by orders of magnitude and potentially renders topological electronic transports available at high temperatures. PMID:27076196
Defect properties of cobalt-doped hexagonal barium titanate ceramics
NASA Astrophysics Data System (ADS)
Langhammer, H. T.; Böttcher, R.; Müller, T.; Walther, T.; Ebbinghaus, S. G.
2015-07-01
X-ray diffraction (XRD) patterns, electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) and the magnetic susceptibility of BaTiO3 + 0.04 BaO + x/2 Co2O3 (0.001 ⩽ x ⩽ 0.02) ceramics were studied to investigate the incorporation of Co ions in the BaTiO3 lattice and their valence states as well as the development of the hexagonal phase (6H modification) in dependence on doping level x and sintering temperature Ts. At Ts = 1400 °C the 6H modification begins to occur at a nominal Co concentration x of about 0.001 and for x > 0.005 the samples are completely hexagonal at room temperature. Two different EPR spectra were observed in the 6H modification of BaTiO3, which were both assigned to paramagnetic Co2+ ions located at the two crystallographically non-equivalent Ti sites in 6H-BaTiO3. The EPR g tensor values as well as the molar paramagnetic susceptibility, measured in the temperature range 5 K-300 K at a magnetic field of 9 T, were analyzed in the framework of the ligand field theory using the program CONCORD. The combination of EPR and magnetic measurements reveals that in air-sintered 6H BaTiO3, the incorporated Co occurs as a mixture of paramagnetic Co2+ and diamagnetic Co3+ ions, whereas in samples annealed in reducing atmosphere the majority of Co is in the divalent state. The occurrence of Co4+ can be excluded for all investigated samples. The sample color caused by Co2+ and Co3+ ions is beige/light yellow and dark grey/black, respectively. The majority of the Co2+ ions substitutes Ti in the exclusively corner-sharing oxygen octahedra possessing nearly cubic symmetry. The corresponding ligand field parameter B04(3) amounts to about -28 000 cm-1 (Wybourne notation, 10Dq ≈ 20 000 cm-1). In the reduced samples nearly 5% of the detected Co2+ ions occupy the Ti site in the face-sharing oxygen octahedra, which are significantly trigonally distorted. The negative sign of the obtained ligand field parameter B02 ≈ -7300 cm-1
Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes
Liao, Yunlong; Tu, Kaixiong; Han, Xiaogang; Hu, Liangbing; Connell, John W.; Chen, Zhongfang; Lin, Yi
2015-01-01
Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole- and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics. PMID:26416484
Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes.
Liao, Yunlong; Tu, Kaixiong; Han, Xiaogang; Hu, Liangbing; Connell, John W; Chen, Zhongfang; Lin, Yi
2015-01-01
Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole- and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics. PMID:26416484
Lattice gas and lattice Boltzmann computational physics
Chen, S.
1993-05-01
Recent developments of the lattice gas automata method and its extension to the lattice Boltzmann method have provided new computational schemes for solving a variety of partial differential equations and modeling different physics systems. The lattice gas method, regarded as the simplest microscopic and kinetic approach which generates meaningful macroscopic dynamics, is fully parallel and can be easily programmed on parallel machines. In this talk, the author will review basic principles of the lattice gas and lattice Boltzmann method, its mathematical foundation and its numerical implementation. A detailed comparison of the lattice Boltzmann method with the lattice gas technique and other traditional numerical schemes, including the finite-difference scheme and the pseudo-spectral method, for solving the Navier-Stokes hydrodynamic fluid flows, will be discussed. Recent achievements of the lattice gas and the the lattice Boltzmann method and their applications in surface phenomena, spinodal decomposition and pattern formation in chemical reaction-diffusion systems will be presented.
Reconfigurable lattice mesh designs for programmable photonic processors.
Pérez, Daniel; Gasulla, Ivana; Capmany, José; Soref, Richard A
2016-05-30
We propose and analyse two novel mesh design geometries for the implementation of tunable optical cores in programmable photonic processors. These geometries are the hexagonal and the triangular lattice. They are compared here to a previously proposed square mesh topology in terms of a series of figures of merit that account for metrics that are relevant to on-chip integration of the mesh. We find that that the hexagonal mesh is the most suitable option of the three considered for the implementation of the reconfigurable optical core in the programmable processor. PMID:27410130
Helimagnons in the skyrmion lattice of MnSi
NASA Astrophysics Data System (ADS)
Janoschek, M.; Jonietz, F.; Link, P.; Pfleiderer, C.; Böni, P.
2010-01-01
In MnSi the application of a small magnetic field destabilizes the helimagnetic order in a narrow temperature interval just below the helimagnetic ordering temperature and stabilizes the formation of a hexagonal lattice of skyrmions, i.e., a lattice composed of a type of magnetic vortex lines. We have studied the skyrmion lattice in MnSi using a cold triple-axis spectrometer. Our data suggests that the skyrmion lattice represents a three-dimensional spin structure. The collective spin excitations of the skyrmion lattice are strongly reminiscent of the rich spectrum of helimagnon bands, recently shown to be a universal property of the helimagnetic state of MnSi in zero magnetic field.
Optical modulators with 2D layered materials
NASA Astrophysics Data System (ADS)
Sun, Zhipei; Martinez, Amos; Wang, Feng
2016-04-01
Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.
Large Area Synthesis of 2D Materials
NASA Astrophysics Data System (ADS)
Vogel, Eric
Transition metal dichalcogenides (TMDs) have generated significant interest for numerous applications including sensors, flexible electronics, heterostructures and optoelectronics due to their interesting, thickness-dependent properties. Despite recent progress, the synthesis of high-quality and highly uniform TMDs on a large scale is still a challenge. In this talk, synthesis routes for WSe2 and MoS2 that achieve monolayer thickness uniformity across large area substrates with electrical properties equivalent to geological crystals will be described. Controlled doping of 2D semiconductors is also critically required. However, methods established for conventional semiconductors, such as ion implantation, are not easily applicable to 2D materials because of their atomically thin structure. Redox-active molecular dopants will be demonstrated which provide large changes in carrier density and workfunction through the choice of dopant, treatment time, and the solution concentration. Finally, several applications of these large-area, uniform 2D materials will be described including heterostructures, biosensors and strain sensors.
2D microwave imaging reflectometer electronics
Spear, A. G.; Domier, C. W. Hu, X.; Muscatello, C. M.; Ren, X.; Luhmann, N. C.; Tobias, B. J.
2014-11-15
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program.
2D microwave imaging reflectometer electronics
NASA Astrophysics Data System (ADS)
Spear, A. G.; Domier, C. W.; Hu, X.; Muscatello, C. M.; Ren, X.; Tobias, B. J.; Luhmann, N. C.
2014-11-01
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program.
2D microwave imaging reflectometer electronics.
Spear, A G; Domier, C W; Hu, X; Muscatello, C M; Ren, X; Tobias, B J; Luhmann, N C
2014-11-01
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program. PMID:25430247
2D-Crystal-Based Functional Inks.
Bonaccorso, Francesco; Bartolotta, Antonino; Coleman, Jonathan N; Backes, Claudia
2016-08-01
The possibility to produce and process graphene, related 2D crystals, and heterostructures in the liquid phase makes them promising materials for an ever-growing class of applications as composite materials, sensors, in flexible optoelectronics, and energy storage and conversion. In particular, the ability to formulate functional inks with on-demand rheological and morphological properties, i.e., lateral size and thickness of the dispersed 2D crystals, is a step forward toward the development of industrial-scale, reliable, inexpensive printing/coating processes, a boost for the full exploitation of such nanomaterials. Here, the exfoliation strategies of graphite and other layered crystals are reviewed, along with the advances in the sorting of lateral size and thickness of the exfoliated sheets together with the formulation of functional inks and the current development of printing/coating processes of interest for the realization of 2D-crystal-based devices. PMID:27273554
ERIC Educational Resources Information Center
Scott, Paul
2006-01-01
A lattice is a (rectangular) grid of points, usually pictured as occurring at the intersections of two orthogonal sets of parallel, equally spaced lines. Polygons that have lattice points as vertices are called lattice polygons. It is clear that lattice polygons come in various shapes and sizes. A very small lattice triangle may cover just 3…
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
Magnetic charges and magnetoelectricity in hexagonal rare-earth manganites and ferrites
NASA Astrophysics Data System (ADS)
Ye, Meng; Vanderbilt, David
2015-07-01
Magnetoelectric (ME) materials are of fundamental interest and show broad potential for technological applications. The common dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge Ze and its analog, the dynamical magnetic charge Zm. Our previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites R MnO3 and ferrites R FeO3 (R =Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic, and structural degrees of freedom. The transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a 120∘ noncollinear spin arrangement. In this paper, we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites. We clarify the conditions under which exchange striction leads to enhanced Zm values and anomalously large in-plane spin-lattice ME effects.
The 2D lingual appliance system.
Cacciafesta, Vittorio
2013-09-01
The two-dimensional (2D) lingual bracket system represents a valuable treatment option for adult patients seeking a completely invisible orthodontic appliance. The ease of direct or simplified indirect bonding of 2D lingual brackets in combination with low friction mechanics makes it possible to achieve a good functional and aesthetic occlusion, even in the presence of a severe malocclusion. The use of a self-ligating bracket significantly reduces chair-side time for the orthodontist, and the low-profile bracket design greatly improves patient comfort. PMID:24005953
Inkjet printing of 2D layered materials.
Li, Jiantong; Lemme, Max C; Östling, Mikael
2014-11-10
Inkjet printing of 2D layered materials, such as graphene and MoS2, has attracted great interests for emerging electronics. However, incompatible rheology, low concentration, severe aggregation and toxicity of solvents constitute critical challenges which hamper the manufacturing efficiency and product quality. Here, we introduce a simple and general technology concept (distillation-assisted solvent exchange) to efficiently overcome these challenges. By implementing the concept, we have demonstrated excellent jetting performance, ideal printing patterns and a variety of promising applications for inkjet printing of 2D layered materials. PMID:25169938
Measurement of 2D birefringence distribution
NASA Astrophysics Data System (ADS)
Noguchi, Masato; Ishikawa, Tsuyoshi; Ohno, Masahiro; Tachihara, Satoru
1992-10-01
A new measuring method of 2-D birefringence distribution has been developed. It has not been an easy job to get a birefringence distribution in an optical element with conventional ellipsometry because of its lack of scanning means. Finding an analogy between the rotating analyzer method in ellipsometry and the phase-shifting method in recently developed digital interferometry, we have applied the phase-shifting algorithm to ellipsometry, and have developed a new method that makes the measurement of 2-D birefringence distribution easy and possible. The system contains few moving parts, assuring reliability, and measures a large area of a sample at one time, making the measuring time very short.
Knight shift and spin relaxation in the single band 2D Hubbard model
NASA Astrophysics Data System (ADS)
Leblanc, James; Chen, Xi; Gull, Emanuel
We study in detail the roles of spin and charge fluctuations in the single band 2D Hubbard model. Using dynamical mean field theory and cluster extensions such as the dynamical cluster approximation (DCA), we compute the full two particle susceptibilities in the spin and charge representations. By performing analytic continuations we obtain the temperature and doping dependence of the spin-lattice relaxation (T1- 1) and knight shift in the 2D Hubbard model relevant to NMR results on doped cuprates and connect these to RPA results in weak coupling limits.
Probing electric properties at the boundary of planar 2D heterostructure
NASA Astrophysics Data System (ADS)
Park, Jewook
The quest for novel two-dimensional (2D) materials has led to the discovery of hybridized 2D atomic crystals. Especially, planar 2D heterostructure provides opportunities to explore fascinating electric properties at abrupt one-dimensional (1D) boundaries reminiscent to those seen in the 2D interfaces of complex oxides. By implementing the concept of epitaxy to 2D space, we developed a new growth technique to epitaxially grow hexagonal boron nitride (hBN) from the edges of graphene, forming a coherent planar heterostructure. At the interface of hBN and graphene, a polar-on-nonpolar 1D boundary can be formed which is expected to possess peculiar electronic states associated with the polarity of hBN and edge states of graphene Scanning tunneling microscopy and spectroscopy (STM/S) measurements revealed an abrupt 1D zigzag oriented boundary, with boundary states about 0.6 eV below or above the Fermi level depending on the termination of the hBN at the boundary. The boundary states are extended along the boundary and exponentially decay into the bulk of graphene and hBN. Combined STM/S and first-principles theory study not only disclose spatial and energetic distribution of interfacial state but also reveal the origin of boundary states and the effect of the polarity discontinuity at the interface By probing electric properties at the boundary in the atomic scale, planar 2D heterostructure is demonstrated as a promising platform for discovering emergent phenomena at the 1D interface in 2D materials. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
Two-dimensional hexagonal smectic structure formed by topological defects
NASA Astrophysics Data System (ADS)
Dolganov, P. V.; Shuravin, N. S.; Fukuda, Atsuo
2016-03-01
A two-dimensional hexagonal smectic structure formed by point topological defects and intersecting defect walls was discovered. This unique structure was predicted theoretically about 30 years ago but not observed. For a long time the hexagonal structure was a challenge for experimentalists. A different type of self-organization in smectic films was found and used to form the hexagonal structure. Methods applied for building the hexagonal phase can be used for the formation of complicated liquid-crystal structures.
Critical behavior of the q = 3 , 4-Potts model on quasiperiodic decagonal lattices
NASA Astrophysics Data System (ADS)
Ferraz, Carlos Handrey Araujo
2015-12-01
In this study, we performed Monte Carlo simulations of the q = 3 , 4-Potts model on quasiperiodic decagonal lattices (QDL) to assess the critical behavior of these systems. Using the single histogram technique in conjunction with the finite-size scaling analysis, we estimate the infinite lattice critical temperatures and the leading critical exponents for q = 3 and q = 4 states. Our estimates for the critical exponents on QDL are in good agreement with the exact values on 2D periodic lattices, supporting the claim that both the q = 3 and q = 4 Potts model on quasiperiodic lattices belong to the same universality class as those on 2D periodic lattices.
X , Y , and Z waves: extended structures in nonlinear lattices.
Kevrekidis, P G; Gagnon, J; Frantzeskakis, D J; Malomed, B A
2007-01-01
We propose a new type of waveforms in two-dimensional (2D) and three-dimensional (3D) discrete media-multilegged extended nonlinear structures (ENSs), built as arrays of lattice solitons (tiles and stones, in the 2D and 3D cases, respectively). We study the stability of the tiles and stones analytically, and then extend them numerically to complete ENS forms for both 2D and 3D lattices, aiming to single out stable ENSs. The predicted patterns can be realized in Bose-Einstein condensates trapped in deep optical lattices, crystals built of microresonators, and 2D photonic crystals. In the latter case, the patterns provide for a technique for writing reconfigurable virtual partitions in multipurpose photonic devices. PMID:17358275
X , Y , and Z waves: Extended structures in nonlinear lattices
NASA Astrophysics Data System (ADS)
Kevrekidis, P. G.; Gagnon, J.; Frantzeskakis, D. J.; Malomed, B. A.
2007-01-01
We propose a new type of waveforms in two-dimensional (2D) and three-dimensional (3D) discrete media-multilegged extended nonlinear structures (ENSs), built as arrays of lattice solitons (tiles and stones, in the 2D and 3D cases, respectively). We study the stability of the tiles and stones analytically, and then extend them numerically to complete ENS forms for both 2D and 3D lattices, aiming to single out stable ENSs. The predicted patterns can be realized in Bose-Einstein condensates trapped in deep optical lattices, crystals built of microresonators, and 2D photonic crystals. In the latter case, the patterns provide for a technique for writing reconfigurable virtual partitions in multipurpose photonic devices.
Complex magnetism of Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy
NASA Astrophysics Data System (ADS)
Lužnik, J.; Koželj, P.; Vrtnik, S.; Jelen, A.; Jagličić, Z.; Meden, A.; Feuerbacher, M.; Dolinšek, J.
2015-12-01
Rare earth based equimolar Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy (HEA) is a prototype of an ideal HEA, stabilized by the entropy of mixing at any temperature with random mixing of elements on the hexagonal close-packed lattice. In order to determine intrinsic properties of an ideal HEA characterized by the enormous chemical (substitutional) disorder on a weakly distorted simple lattice, we have performed measurements of its magnetic and electrical response and the specific heat. The results show that the Ho-Dy-Y-Gd-Tb hexagonal HEA exhibits a rich and complex magnetic field-temperature (H ,T ) phase diagram, as a result of competition among the periodic potential arising from the electronic band structure that favors periodic magnetic ordering, the disorder-induced local random potential that favors spin glass-type spin freezing in random directions, the Zeeman interaction with the external field that favors spin alignment along the field direction, and the thermal agitation that opposes any spin ordering. Three characteristic temperature regions were identified in the (H ,T ) phase diagram between room temperature and 2 K. Within the upper temperature region I (roughly between 300 and 75 K), thermal fluctuations average out the effect of local random pinning potential and the spin system behaves as a pure system of compositionally averaged spins, undergoing a thermodynamic phase transition to a long-range ordered helical antiferromagnetic state at the Néel temperature TN=180 K that is a compositional average of the Néel temperatures of pure Tb, Dy, and Ho metals. Region II (between 75 and 20 K) is an intermediate region where the long-range periodic spin order "melts" and the random ordering of spins in the local random potential starts to prevail. Within the low-temperature region III (below 20 K), the spins gradually freeze in a spin glass configuration. The spin glass phase appears to be specific to the rare earths containing hexagonal HEAs, sharing
Lysozyme entrapped within reverse hexagonal mesophases: physical properties and structural behavior.
Mishraki, Tehila; Libster, Dima; Aserin, Abraham; Garti, Nissim
2010-01-01
A model protein (lysozyme) was incorporated into monoolein-based reverse hexagonal (H(II)) mesophase and its structure effects were characterized by small angle X-ray scattering, ATR-FTIR spectroscopy, and rheological measurements. Modifications in molecular organization of the H(II) mesophases as well as the conformational stability of lysozyme (LSZ) as a function of pH and denaturating agent (urea) were clarified. Up to 3 wt.% LSZ can be solubilized into the H(II). The vibration FTIR analysis revealed that LSZ interacted with OH groups of glycerol monooleate (GMO) in the outer interface region, resulting in strong hydrogen bonding between the surfactant and its environment. Simultaneously, the decrease in the hydrogen-bonded carbonyl population of GMO was monitored, indicating dehydration of the monoolein carbonyls. These molecular interactions yielded a minor decrease in the lattice parameter of the systems, as detected by small angle X-ray scattering. Furthermore, LSZ was crystallized within the medium of the hexagonal structures in a single crystal form. The alpha-helix conformation of lysozyme was stabilized at high pH conditions, demonstrating greater helical structure content, compared to D(2)O solution. Moreover, the hexagonal phase decreased the unfavorable alpha-->beta transition in lysozyme, thereby increasing the stability of the protein under chemical denaturation. The rheological behavior of the hexagonal structures varied with the incorporation of LSZ, reflected in stronger elastic properties and pronounced solid-like response of the systems. The hydrogen bonding enhancement in the interface region of the structures was most likely responsible for these phenomena. The results of this study provided valuable information on the use of hexagonal systems as a carrier for incorporation and stabilization of proteins for various applications. PMID:19748240
Stability analysis of lattice Boltzmann methods
Sterling, J.D.; Chen, Shiyi
1996-01-01
The lattice Boltzmann equation describes the evolution of the velocity distribution function on a lattice in a manner that macroscopic fluid dynamical behavior is recovered. Although the equation is a derivative of lattice gas automata, it may be interpreted as a Lagrangian finite-difference method for the numerical simulation of the discrete-velocity Boltzmann equation that makes use of a BGK collision operator. As a result, it is not surprising that numericaI instability of lattice Boltzmann methods have been frequently encountered by researchers. We present an analysis of the stability of perturbations of the particle populations linearized about equilibrium values corresponding to a constant-density uniform mean flow. The linear stability depends on the following parameters: the distribution of the mass at a site between the different discrete speeds, the BGK relaxation time, the mean velocity, and the wave-number of the perturbations. This parameter space is too large to compute the complete stability characteristics. We report some stability results for a subset of the parameter space for a 7-velocity hexagonal lattice, a 9-velocity square lattice, and a 15-velocity cubic lattice. Results common to all three lattices are (1) the BGK relaxation time {tau} must be greater than 1/2 corresponding to positive shear viscosity, (2) there exists a maximum stable mean velocity for fixed values of theother parameters, and (3) as {tau} is increased from 1/2 the maximum stable velocity increases monotonically until some fixed velocity is reached which does not change for larger {tau}.
Microparticle manipulation in optical lattices
NASA Astrophysics Data System (ADS)
Mu, Weiqiang
With the interference of several coherent beams, a periodical potential is produced for the particles trapped inside. The theoretical calculations show that the optical force applied on the particle in such optical lattice is in sinusoidal form. The force amplitudes vary greatly depending on the ratio of the particle size to the spacing of the optical lattice. A setup is constructed to demonstrate this dependence with two different methods: equipartition theorem and hydrodynamic-drag method. Based on this size dependence we develop an approach that allows tunable, size-dependent force selection of a subset of particles from an ensemble containing mixed particles. Combining a universal constant force with the sinusoidal optical force, a tilted washboard potential can be formed for the trapped particle. The diffusion of a particle over the barrier in this tilted washboard potential is briefly discussed. When the washboard potential oscillates, some interesting phenomena will happen: at high oscillation frequency, the particle's movement depends only on the oscillating amplitude; at low frequency, there are some combinations of the oscillation frequency and amplitude that induce the enhanced movement of the particle. This enhancement is first experimentally demonstrated with our setup. By implanting a single laser tweezers into the interferometric optical tweezers, we succeed in dynamically assembling designer colloidal lattices on the background of the interferometric optical tweezers. This new technique provides a flexible tool to design 2-d colloidal lattices.
NASA Astrophysics Data System (ADS)
Tay, Roland Yingjie; Park, Hyo Ju; Ryu, Gyeong Hee; Tan, Dunlin; Tsang, Siu Hon; Li, Hongling; Liu, Wenwen; Teo, Edwin Hang Tong; Lee, Zonghoon; Lifshitz, Yeshayahu; Ruoff, Rodney S.
2016-01-01
Atomically smooth hexagonal boron nitride (h-BN) films are considered as a nearly ideal dielectric interface for two-dimensional (2D) heterostructure devices. Reported mono- to few-layer 2D h-BN films, however, are mostly small grain-sized, polycrystalline and randomly oriented. Here we report the growth of centimetre-sized atomically thin h-BN films composed of aligned domains on resolidified Cu. The films consist of monolayer single crystalline triangular and hexagonal domains with size of up to ~10 μm. The domains converge to symmetrical multifaceted shapes such as ``butterfly'' and ``6-apex-star'' and exhibit ~75% grain alignment for over millimetre distances as verified through transmission electron microscopy. Scanning electron microscopy images reveal that these domains are aligned for over centimetre distances. Defect lines are generated along the grain boundaries of mirroring h-BN domains due to the two different polarities (BN and NB) and edges with the same termination. The observed triangular domains with truncated edges and alternatively hexagonal domains are in accordance with Wulff shapes that have minimum edge energy. This work provides an extensive study on the aligned growth of h-BN single crystals over large distances and highlights the obstacles that are needed to be overcome for a 2D material with a binary configuration.Atomically smooth hexagonal boron nitride (h-BN) films are considered as a nearly ideal dielectric interface for two-dimensional (2D) heterostructure devices. Reported mono- to few-layer 2D h-BN films, however, are mostly small grain-sized, polycrystalline and randomly oriented. Here we report the growth of centimetre-sized atomically thin h-BN films composed of aligned domains on resolidified Cu. The films consist of monolayer single crystalline triangular and hexagonal domains with size of up to ~10 μm. The domains converge to symmetrical multifaceted shapes such as ``butterfly'' and ``6-apex-star'' and exhibit ~75% grain
Parallel stitching of 2D materials
Ling, Xi; Wu, Lijun; Lin, Yuxuan; Ma, Qiong; Wang, Ziqiang; Song, Yi; Yu, Lili; Huang, Shengxi; Fang, Wenjing; Zhang, Xu; et al
2016-01-27
Diverse parallel stitched 2D heterostructures, including metal–semiconductor, semiconductor–semiconductor, and insulator–semiconductor, are synthesized directly through selective “sowing” of aromatic molecules as the seeds in the chemical vapor deposition (CVD) method. Lastly, the methodology enables the large-scale fabrication of lateral heterostructures, which offers tremendous potential for its application in integrated circuits.
Parallel Stitching of 2D Materials.
Ling, Xi; Lin, Yuxuan; Ma, Qiong; Wang, Ziqiang; Song, Yi; Yu, Lili; Huang, Shengxi; Fang, Wenjing; Zhang, Xu; Hsu, Allen L; Bie, Yaqing; Lee, Yi-Hsien; Zhu, Yimei; Wu, Lijun; Li, Ju; Jarillo-Herrero, Pablo; Dresselhaus, Mildred; Palacios, Tomás; Kong, Jing
2016-03-01
Diverse parallel stitched 2D heterostructures, including metal-semiconductor, semiconductor-semiconductor, and insulator-semiconductor, are synthesized directly through selective "sowing" of aromatic molecules as the seeds in the chemical vapor deposition (CVD) method. The methodology enables the large-scale fabrication of lateral heterostructures, which offers tremendous potential for its application in integrated circuits. PMID:26813882
Baby universes in 2d quantum gravity
NASA Astrophysics Data System (ADS)
Ambjørn, Jan; Jain, Sanjay; Thorleifsson, Gudmar
1993-06-01
We investigate the fractal structure of 2d quantum gravity, both for pure gravity and for gravity coupled to multiple gaussian fields and for gravity coupled to Ising spins. The roughness of the surfaces is described in terms of baby universes and using numerical simulations we measure their distribution which is related to the string susceptibility exponent γstring.
Nodal equivalence theory for hexagonal geometry, thermal reactor analysis
Zika, M.; Downar, T. )
1992-01-01
An important aspect of advanced nodal methods is the determination of equivalent few-group parameters for the relatively large homogenized regions used in the nodal flux solution. The theoretical foundation for light water reactor (LWR) assembly homogenization methods has been clearly established, and during the last several years, its successes have secured its position in the stable of dependable LWR analysis methods. Groupwise discontinuity factors that correct for assembly homogenization errors are routinely generated along with the group constants during lattice physics analysis. During the last several years, there has been interest in applying equivalence theory to other reactor types and other geometries. A notable effort has been the work at Argonne National Laboratory to incorporate nodal equivalence theory (NET) for hexagonal lattices into the nodal diffusion option of the DIF3D code. This work was originally intended to improve the neutronics methods used for the analysis of the Experimental Breeder Reactor II (EBR-II), and Ref. 4 discusses the success of that application. More recently, however, attempts were made to apply NET to advanced, thermal reactor designs such as the modular high-temperature gas reactor (MHTGR) and the new production heavy water reactor (NPR/HWR). The same methods that were successful for EBR-II have encountered problems for these reactors. Our preliminary analysis indicates that the sharp global flux gradients in these cores requires large discontinuity factors (greater than 4 or 5) to reproduce the reference solution. This disrupts the convergence of the iterative methods used to solve for the node-wise flux moments and partial currents. Several attempts to remedy the problem have been made over the last few years, including bounding the discontinuity factors and providing improved initial guesses for the flux solution, but nothing has been satisfactory.
Time-resolved photoluminescence study of excitons in hexagonal GaN layers grown on sapphire
NASA Astrophysics Data System (ADS)
Pau, S.; Liu, Z. X.; Kuhl, J.; Ringling, J.; Grahn, H. T.; Khan, M. A.; Sun, C. J.; Ambacher, O.; Stutzmann, M.
1998-03-01
We performed time-resolved and continuous wave photoluminescence on two samples of hexagonal GaN, one with free exciton emission and the other without. For the sample with free exciton emission, very different decay dynamics are observed between the front and backside emission. We find that the strain caused by the lattice mismatch between the sapphire substrate and the GaN film has a large influence on the population decay of the sample with free exciton emission and a minor influence on the decay properties of the sample dominated by bound exciton emission. A polariton picture is used to describe the observed behavior.
Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators
NASA Astrophysics Data System (ADS)
Gaskell, J.; Eaves, L.; Novoselov, K. S.; Mishchenko, A.; Geim, A. K.; Fromhold, T. M.; Greenaway, M. T.
2015-09-01
We assess the potential of two-terminal graphene-hexagonal boron nitride-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.
Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators
Gaskell, J.; Fromhold, T. M.; Greenaway, M. T.; Eaves, L.; Novoselov, K. S.; Mishchenko, A.; Geim, A. K.
2015-09-07
We assess the potential of two-terminal graphene-hexagonal boron nitride-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.
On vortex shedding from a hexagonal cylinder
NASA Astrophysics Data System (ADS)
Khaledi, Hatef A.; Andersson, Helge I.
2011-10-01
The unsteady wake behind a hexagonal cylinder in cross-flow is investigated numerically. The time-dependent three-dimensional Navier-Stokes equations are solved for three different Reynolds numbers Re and for two different cylinder orientations. The topology of the vortex shedding depends on the orientation and the Strouhal frequency is generally higher in the wake of a face-oriented cylinder than behind a corner-oriented cylinder. For both orientations a higher Strouhal number St is observed when Re is increased from 100 to 500 whereas St is unaffected by a further increase up to Re=1000. The distinct variation of St with the orientation of the hexagonal cylinder relative to the oncoming flow is opposite of earlier findings for square cylinder wakes which exhibited a higher St with corner orientation than with face orientation.
Multilayer hexagonal silicon forming in slit nanopore
He, Yezeng; Li, Hui; Sui, Yanwei; Qi, Jiqiu; Wang, Yanqing; Chen, Zheng; Dong, Jichen; Li, Xiongying
2015-01-01
The solidification of two-dimensional liquid silicon confined to a slit nanopore has been studied using molecular dynamics simulations. The results clearly show that the system undergoes an obvious transition from liquid to multilayer hexagonal film with the decrease of temperature, accompanied by dramatic change in potential energy, atomic volume, coordination number and lateral radial distribution function. During the cooling process, some hexagonal islands randomly appear in the liquid first, then grow up to grain nuclei, and finally connect together to form a complete polycrystalline film. Moreover, it is found that the quenching rate and slit size are of vital importance to the freezing structure of silicon film. The results also indicate that the slit nanopore induces the layering of liquid silicon, which further induces the slit size dependent solidification behavior of silicon film with different electrical properties. PMID:26435518
Multilayer hexagonal silicon forming in slit nanopore.
He, Yezeng; Li, Hui; Sui, Yanwei; Qi, Jiqiu; Wang, Yanqing; Chen, Zheng; Dong, Jichen; Li, Xiongying
2015-01-01
The solidification of two-dimensional liquid silicon confined to a slit nanopore has been studied using molecular dynamics simulations. The results clearly show that the system undergoes an obvious transition from liquid to multilayer hexagonal film with the decrease of temperature, accompanied by dramatic change in potential energy, atomic volume, coordination number and lateral radial distribution function. During the cooling process, some hexagonal islands randomly appear in the liquid first, then grow up to grain nuclei, and finally connect together to form a complete polycrystalline film. Moreover, it is found that the quenching rate and slit size are of vital importance to the freezing structure of silicon film. The results also indicate that the slit nanopore induces the layering of liquid silicon, which further induces the slit size dependent solidification behavior of silicon film with different electrical properties. PMID:26435518
Wang, Ming; Wang, Kun; Wang, Chao; Huang, Mingjun; Hao, Xin-Qi; Shen, Ming-Zhan; Shi, Guo-Qing; Zhang, Zhe; Song, Bo; Cisneros, Alejandro; Song, Mao-Ping; Xu, Bingqian; Li, Xiaopeng
2016-07-27
In an effort to exert more precise control over structural features of supramolecules, a series of giant concentric hexagons were assembled as discrete structures using tetratopic terpyridine (tpy) ligands. In preparation of tetratopic ligand, pyrylium and pyridinium salts chemistry significantly facilitated synthesis. The key compounds were obtained by condensation reactions of pyrylium salts with corresponding primary amine derivatives in good yields. These discrete metallo-supramolecular concentric hexagons were fully characterized by NMR, ESI-MS, TWIM-MS, and TEM, establishing their hexagon-in-hexagon architectures. The combination of different tetratopic ligands also assembled hybrid concentric hexagons with increasing diversity and complexity. Furthermore, these concentric hexagon supramolecules with precisely controlled shapes and sizes were utilized as building blocks to hierarchically self-assemble supramolecular metal-organic nanoribbons (SMON) at solid-liquid interfaces. Ambient STM imaging showed the formation of long 1D SMON rather than 2D assembly on the basal plane of highly oriented pyrolytic graphite (HOPG) surface after simple dropcasting of the solution of preassembled concentric hexagons onto a freshly cleaved surface of HOPG. This wet chemical method based on self-assembly may offer simple, economical, and scalable routes to deliver complex materials. PMID:27379457
Control of normal chirality at hexagonal interfaces
Haraldsen, Jason T; Fishman, Randy Scott
2010-01-01
We study the net chirality created by the Dzyaloshinkii-Moriya interaction (DMI) at the boundary between hexagonal layers of magnetic and non-magnetic materials. It is shown that another mechanism besides elastic torsion is required to understand the change in chirality observed in Dy/Y multilayers during field-cooling. The paper shows that due to the overlap between magnetic and non-magnetic atoms, interfacial steps may produce a DMI normal to the interface in magnetic heterostructures.
Application of 2D Non-Graphene Materials and 2D Oxide Nanostructures for Biosensing Technology
Shavanova, Kateryna; Bakakina, Yulia; Burkova, Inna; Shtepliuk, Ivan; Viter, Roman; Ubelis, Arnolds; Beni, Valerio; Starodub, Nickolaj; Yakimova, Rositsa; Khranovskyy, Volodymyr
2016-01-01
The discovery of graphene and its unique properties has inspired researchers to try to invent other two-dimensional (2D) materials. After considerable research effort, a distinct “beyond graphene” domain has been established, comprising the library of non-graphene 2D materials. It is significant that some 2D non-graphene materials possess solid advantages over their predecessor, such as having a direct band gap, and therefore are highly promising for a number of applications. These applications are not limited to nano- and opto-electronics, but have a strong potential in biosensing technologies, as one example. However, since most of the 2D non-graphene materials have been newly discovered, most of the research efforts are concentrated on material synthesis and the investigation of the properties of the material. Applications of 2D non-graphene materials are still at the embryonic stage, and the integration of 2D non-graphene materials into devices is scarcely reported. However, in recent years, numerous reports have blossomed about 2D material-based biosensors, evidencing the growing potential of 2D non-graphene materials for biosensing applications. This review highlights the recent progress in research on the potential of using 2D non-graphene materials and similar oxide nanostructures for different types of biosensors (optical and electrochemical). A wide range of biological targets, such as glucose, dopamine, cortisol, DNA, IgG, bisphenol, ascorbic acid, cytochrome and estradiol, has been reported to be successfully detected by biosensors with transducers made of 2D non-graphene materials. PMID:26861346
Application of 2D Non-Graphene Materials and 2D Oxide Nanostructures for Biosensing Technology.
Shavanova, Kateryna; Bakakina, Yulia; Burkova, Inna; Shtepliuk, Ivan; Viter, Roman; Ubelis, Arnolds; Beni, Valerio; Starodub, Nickolaj; Yakimova, Rositsa; Khranovskyy, Volodymyr
2016-01-01
The discovery of graphene and its unique properties has inspired researchers to try to invent other two-dimensional (2D) materials. After considerable research effort, a distinct "beyond graphene" domain has been established, comprising the library of non-graphene 2D materials. It is significant that some 2D non-graphene materials possess solid advantages over their predecessor, such as having a direct band gap, and therefore are highly promising for a number of applications. These applications are not limited to nano- and opto-electronics, but have a strong potential in biosensing technologies, as one example. However, since most of the 2D non-graphene materials have been newly discovered, most of the research efforts are concentrated on material synthesis and the investigation of the properties of the material. Applications of 2D non-graphene materials are still at the embryonic stage, and the integration of 2D non-graphene materials into devices is scarcely reported. However, in recent years, numerous reports have blossomed about 2D material-based biosensors, evidencing the growing potential of 2D non-graphene materials for biosensing applications. This review highlights the recent progress in research on the potential of using 2D non-graphene materials and similar oxide nanostructures for different types of biosensors (optical and electrochemical). A wide range of biological targets, such as glucose, dopamine, cortisol, DNA, IgG, bisphenol, ascorbic acid, cytochrome and estradiol, has been reported to be successfully detected by biosensors with transducers made of 2D non-graphene materials. PMID:26861346
Saturn's North Polar Hexagon Numerical Modeling Results
NASA Astrophysics Data System (ADS)
Morales-Juberias, R.; Sayanagi, K. M.; Dowling, T. E.
2008-12-01
In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic latitude in the northern hemisphere of Saturn. It was notable for having a dominant planetary wavenumber-six zonal mode, and for being stationary with respect to Saturn's Kilometric Radiation rotation rate measured by Voyager. The center of this hexagonal feature was coincident with the center of a sharp eastward jet with a peak speed of 100 ms-1 and it had a meridional width of about 4 degrees. This hexagonal feature was confirmed in 1991 through ground-based observations, and it was observed again in 2006 with the Cassini VIMS instrument. The latest observations highlight the longevity of the hexagon and suggest that it extends at least several bars deep into the atmosphere. We use the Explicit Planetary Isentropic Code (EPIC) to perform high-resolution numerical simulations of this unique feature. We show that a wavenumber six instability mode arises naturally from initially barotropic jets when seeded with weak random turbulence. We also discuss the properties of the wave activity on the background vertical stability, zonal wind, planetary rotation rate and adjacent vortices. Computational resources were provided by the New Mexico Computing Applications Center and New Mexico Institute of Mining and Technology and the Comparative Planetology Laboratory at the University of Louisville.
Exfoliation of Hexagonal Boron Nitride via Ferric Chloride Intercalation
NASA Technical Reports Server (NTRS)
Hung, Ching-cheh; Hurst, Janet; Santiago, Diana; Rogers, Richard B.
2014-01-01
Sodium fluoride (NaF) was used as an activation agent to successfully intercalate ferric chloride (FeCl3) into hexagonal boron nitride (hBN). This reaction caused the hBN mass to increase by approx.100 percent, the lattice parameter c to decrease from 6.6585 to between 6.6565 and 6.6569 ?, the x-ray diffraction (XRD) (002) peak to widen from 0.01deg to 0.05deg of the full width half maximum value, the Fourier transform infrared (FTIR) spectrum's broad band (1277/cm peak) to change shape, and new FTIR bands to emerge at 3700 to 2700 and 1600/cm. This indicates hBN's structural and chemical properties are significantly changed. The intercalated product was hygroscopic and interacted with moisture in the air to cause further structural and chemical changes (from XRD and FTIR). During a 24-h hold at room temperature in air with 100 percent relative humidity, the mass increased another 141 percent. The intercalated product, hydrated or not, can be heated to 750 C in air to cause exfoliation. Exfoliation becomes significant after two intercalation-air heating cycles, when 20-nm nanosheets are commonly found. Structural and chemical changes indicated by XRD and FTIR data were nearly reversed after the product was placed in hydrochloric acid (HCl), resulting in purified, exfoliated, thin hBN products.
Hexagonal boron nitride film substrate for fabrication of nanostructures
NASA Astrophysics Data System (ADS)
Lee, K. S.; Kim, Y. S.; Tosa, M.; Kasahara, A.; Yosihara, K.
2001-01-01
The fabrication of material with an atomic scale manipulation requires the suitable advanced substrate for epitaxial growth without the effect by the substrate lattice structure. Hexagonal boron nitride (h-BN) can be the advanced substrate for atomic manipulation due to van der Waals' gap with little attractive force along to c axis. We have successfully synthesized h-BN layer on the co-deposited Cu/BN film by surface segregation phenomena using helicon wave plasma enhanced radio frequency (rf) magnetron sputtering system. Auger electron spectroscopy (AES) and X-ray photon spectroscopy (XPS) analysis showed that the h-BN composite segregated on the surface of Cu/BN film covered over 95% of the film annealed at 900 K for 30 min. Atomic forces microscopy (AFM) and scanning tunneling microscopy (STM) analysis showed that attractive force on the film surface is uniformly distributed to an extent of 2nN and that the h-BN surface can be a good electric insulator like sintered h-BN plate.
2D to 3D transition of polymeric carbon nitride nanosheets
Chamorro-Posada, Pedro; Vázquez-Cabo, José; Martín-Ramos, Pablo; Martín-Gil, Jesús; Navas-Gracia, Luis M.; Dante, Roberto C.
2014-11-15
The transition from a prevalent turbostratic arrangement with low planar interactions (2D) to an array of polymeric carbon nitride nanosheets with stronger interplanar interactions (3D), occurring for samples treated above 650 °C, was detected by terahertz-time domain spectroscopy (THz-TDS). The simulated 3D material made of stacks of shifted quasi planar sheets composed of zigzagged polymer ribbons, delivered a XRD simulated pattern in relatively good agreement with the experimental one. The 2D to 3D transition was also supported by the simulation of THz-TDS spectra obtained from quantum chemistry calculations, in which the same broad bands around 2 THz and 1.5 THz were found for 2D and 3D arrays, respectively. This transition was also in accordance with the tightening of the interplanar distance probably due to an interplanar π bond contribution, as evidenced also by a broad absorption around 2.6 eV in the UV–vis spectrum, which appeared in the sample treated at 650 °C, and increased in the sample treated at 700 °C. The band gap was calculated for 1D and 2D cases. The value of 3.374 eV for the 2D case is, within the model accuracy and precision, in a relative good agreement with the value of 3.055 eV obtained from the experimental results. - Graphical abstract: 2D lattice mode vibrations and structural changes correlated with the so called “2D to 3D transition”. - Highlights: • A 2D to 3D transition has been detected for polymeric carbon nitride. • THz-TDS allowed us to discover and detect the 2D to 3D transition of polymeric carbon nitride. • We propose a structure for polymeric carbon nitride confirming it with THz-TDS.
Synthesis and applications of two-dimensional hexagonal boron nitride in electronics manufacturing
NASA Astrophysics Data System (ADS)
Bao, Jie; Jeppson, Kjell; Edwards, Michael; Fu, Yifeng; Ye, Lilei; Lu, Xiuzhen; Liu, Johan
2016-01-01
In similarity to graphene, two-dimensional (2D) hexagonal boron nitride (hBN) has some remarkable properties, such as mechanical robustness and high thermal conductivity. In addition, hBN has superb chemical stability and it is electrically insulating. 2D hBN has been considered a promising material for many applications in electronics, including 2D hBN based substrates, gate dielectrics for graphene transistors and interconnects, and electronic packaging insulators. This paper reviews the synthesis, transfer and fabrication of 2D hBN films, hBN based composites and hBN-based van der Waals heterostructures. In particular, this review focuses on applications in manufacturing electronic devices where the insulating and thermal properties of hBN can potentially be exploited. 2D hBN and related composite systems are emerging as new and industrially important materials, which could address many challenges in future complex electronics devices and systems. [Figure not available: see fulltext.
Static & Dynamic Response of 2D Solids
1996-07-15
NIKE2D is an implicit finite-element code for analyzing the finite deformation, static and dynamic response of two-dimensional, axisymmetric, plane strain, and plane stress solids. The code is fully vectorized and available on several computing platforms. A number of material models are incorporated to simulate a wide range of material behavior including elasto-placicity, anisotropy, creep, thermal effects, and rate dependence. Slideline algorithms model gaps and sliding along material interfaces, including interface friction, penetration and single surfacemore » contact. Interactive-graphics and rezoning is included for analyses with large mesh distortions. In addition to quasi-Newton and arc-length procedures, adaptive algorithms can be defined to solve the implicit equations using the solution language ISLAND. Each of these capabilities and more make NIKE2D a robust analysis tool.« less
Stochastic Inversion of 2D Magnetotelluric Data
2010-07-01
The algorithm is developed to invert 2D magnetotelluric (MT) data based on sharp boundary parametrization using a Bayesian framework. Within the algorithm, we consider the locations and the resistivity of regions formed by the interfaces are as unknowns. We use a parallel, adaptive finite-element algorithm to forward simulate frequency-domain MT responses of 2D conductivity structure. Those unknown parameters are spatially correlated and are described by a geostatistical model. The joint posterior probability distribution function ismore » explored by Markov Chain Monte Carlo (MCMC) sampling methods. The developed stochastic model is effective for estimating the interface locations and resistivity. Most importantly, it provides details uncertainty information on each unknown parameter. Hardware requirements: PC, Supercomputer, Multi-platform, Workstation; Software requirements C and Fortan; Operation Systems/version is Linux/Unix or Windows« less
Stochastic Inversion of 2D Magnetotelluric Data
Chen, Jinsong
2010-07-01
The algorithm is developed to invert 2D magnetotelluric (MT) data based on sharp boundary parametrization using a Bayesian framework. Within the algorithm, we consider the locations and the resistivity of regions formed by the interfaces are as unknowns. We use a parallel, adaptive finite-element algorithm to forward simulate frequency-domain MT responses of 2D conductivity structure. Those unknown parameters are spatially correlated and are described by a geostatistical model. The joint posterior probability distribution function is explored by Markov Chain Monte Carlo (MCMC) sampling methods. The developed stochastic model is effective for estimating the interface locations and resistivity. Most importantly, it provides details uncertainty information on each unknown parameter. Hardware requirements: PC, Supercomputer, Multi-platform, Workstation; Software requirements C and Fortan; Operation Systems/version is Linux/Unix or Windows
Explicit 2-D Hydrodynamic FEM Program
1996-08-07
DYNA2D* is a vectorized, explicit, two-dimensional, axisymmetric and plane strain finite element program for analyzing the large deformation dynamic and hydrodynamic response of inelastic solids. DYNA2D* contains 13 material models and 9 equations of state (EOS) to cover a wide range of material behavior. The material models implemented in all machine versions are: elastic, orthotropic elastic, kinematic/isotropic elastic plasticity, thermoelastoplastic, soil and crushable foam, linear viscoelastic, rubber, high explosive burn, isotropic elastic-plastic, temperature-dependent elastic-plastic. Themore » isotropic and temperature-dependent elastic-plastic models determine only the deviatoric stresses. Pressure is determined by one of 9 equations of state including linear polynomial, JWL high explosive, Sack Tuesday high explosive, Gruneisen, ratio of polynomials, linear polynomial with energy deposition, ignition and growth of reaction in HE, tabulated compaction, and tabulated.« less
Schottky diodes from 2D germanane
NASA Astrophysics Data System (ADS)
Sahoo, Nanda Gopal; Esteves, Richard J.; Punetha, Vinay Deep; Pestov, Dmitry; Arachchige, Indika U.; McLeskey, James T.
2016-07-01
We report on the fabrication and characterization of a Schottky diode made using 2D germanane (hydrogenated germanene). When compared to germanium, the 2D structure has higher electron mobility, an optimal band-gap, and exceptional stability making germanane an outstanding candidate for a variety of opto-electronic devices. One-atom-thick sheets of hydrogenated puckered germanium atoms have been synthesized from a CaGe2 framework via intercalation and characterized by XRD, Raman, and FTIR techniques. The material was then used to fabricate Schottky diodes by suspending the germanane in benzonitrile and drop-casting it onto interdigitated metal electrodes. The devices demonstrate significant rectifying behavior and the outstanding potential of this material.
Layer Engineering of 2D Semiconductor Junctions.
He, Yongmin; Sobhani, Ali; Lei, Sidong; Zhang, Zhuhua; Gong, Yongji; Jin, Zehua; Zhou, Wu; Yang, Yingchao; Zhang, Yuan; Wang, Xifan; Yakobson, Boris; Vajtai, Robert; Halas, Naomi J; Li, Bo; Xie, Erqing; Ajayan, Pulickel
2016-07-01
A new concept for junction fabrication by connecting multiple regions with varying layer thicknesses, based on the thickness dependence, is demonstrated. This type of junction is only possible in super-thin-layered 2D materials, and exhibits similar characteristics as p-n junctions. Rectification and photovoltaic effects are observed in chemically homogeneous MoSe2 junctions between domains of different thicknesses. PMID:27136275
NASA Astrophysics Data System (ADS)
Smith, Greg; Lankshear, Allan
1998-07-01
2dF is a multi-object instrument mounted at prime focus at the AAT capable of spectroscopic analysis of 400 objects in a single 2 degree field. It also prepares a second 2 degree 400 object field while the first field is being observed. At its heart is a high precision robotic positioner that places individual fiber end magnetic buttons on one of two field plates. The button gripper is carried on orthogonal gantries powered by linear synchronous motors and contains a TV camera which precisely locates backlit buttons to allow placement in user defined locations to 10 (mu) accuracy. Fiducial points on both plates can also be observed by the camera to allow repeated checks on positioning accuracy. Field plates rotate to follow apparent sky rotation. The spectrographs both analyze light from the 200 observing fibers each and back- illuminate the 400 fibers being re-positioned during the observing run. The 2dF fiber position and spectrograph system is a large and complex instrument located at the prime focus of the Anglo Australian Telescope. The mechanical design has departed somewhat from the earlier concepts of Gray et al, but still reflects the audacity of those first ideas. The positioner is capable of positioning 400 fibers on a field plate while another 400 fibers on another plate are observing at the focus of the telescope and feeding the twin spectrographs. When first proposed it must have seemed like ingenuity unfettered by caution. Yet now it works, and works wonderfully well. 2dF is a system which functions as the result of the combined and coordinated efforts of the astronomers, the mechanical designers and tradespeople, the electronic designers, the programmers, the support staff at the telescope, and the manufacturing subcontractors. The mechanical design of the 2dF positioner and spectrographs was carried out by the mechanical engineering staff of the AAO and the majority of the manufacture was carried out in the AAO workshops.
Realistic and efficient 2D crack simulation
NASA Astrophysics Data System (ADS)
Yadegar, Jacob; Liu, Xiaoqing; Singh, Abhishek
2010-04-01
Although numerical algorithms for 2D crack simulation have been studied in Modeling and Simulation (M&S) and computer graphics for decades, realism and computational efficiency are still major challenges. In this paper, we introduce a high-fidelity, scalable, adaptive and efficient/runtime 2D crack/fracture simulation system by applying the mathematically elegant Peano-Cesaro triangular meshing/remeshing technique to model the generation of shards/fragments. The recursive fractal sweep associated with the Peano-Cesaro triangulation provides efficient local multi-resolution refinement to any level-of-detail. The generated binary decomposition tree also provides efficient neighbor retrieval mechanism used for mesh element splitting and merging with minimal memory requirements essential for realistic 2D fragment formation. Upon load impact/contact/penetration, a number of factors including impact angle, impact energy, and material properties are all taken into account to produce the criteria of crack initialization, propagation, and termination leading to realistic fractal-like rubble/fragments formation. The aforementioned parameters are used as variables of probabilistic models of cracks/shards formation, making the proposed solution highly adaptive by allowing machine learning mechanisms learn the optimal values for the variables/parameters based on prior benchmark data generated by off-line physics based simulation solutions that produce accurate fractures/shards though at highly non-real time paste. Crack/fracture simulation has been conducted on various load impacts with different initial locations at various impulse scales. The simulation results demonstrate that the proposed system has the capability to realistically and efficiently simulate 2D crack phenomena (such as window shattering and shards generation) with diverse potentials in military and civil M&S applications such as training and mission planning.
Compact 2-D graphical representation of DNA
NASA Astrophysics Data System (ADS)
Randić, Milan; Vračko, Marjan; Zupan, Jure; Novič, Marjana
2003-05-01
We present a novel 2-D graphical representation for DNA sequences which has an important advantage over the existing graphical representations of DNA in being very compact. It is based on: (1) use of binary labels for the four nucleic acid bases, and (2) use of the 'worm' curve as template on which binary codes are placed. The approach is illustrated on DNA sequences of the first exon of human β-globin and gorilla β-globin.
2D materials: Graphene and others
NASA Astrophysics Data System (ADS)
Bansal, Suneev Anil; Singh, Amrinder Pal; Kumar, Suresh
2016-05-01
Present report reviews the recent advancements in new atomically thick 2D materials. Materials covered in this review are Graphene, Silicene, Germanene, Boron Nitride (BN) and Transition metal chalcogenides (TMC). These materials show extraordinary mechanical, electronic and optical properties which make them suitable candidates for future applications. Apart from unique properties, tune-ability of highly desirable properties of these materials is also an important area to be emphasized on.
Mason, W.E.
1983-03-01
A set of finite element codes for the solution of nonlinear, two-dimensional (TACO2D) and three-dimensional (TACO3D) heat transfer problems. Performs linear and nonlinear analyses of both transient and steady state heat transfer problems. Has the capability to handle time or temperature dependent material properties. Materials may be either isotropic or orthotropic. A variety of time and temperature dependent boundary conditions and loadings are available including temperature, flux, convection, radiation, and internal heat generation.
NASA Astrophysics Data System (ADS)
Jeromin, A.; Schaffarczyk, A. P.; Puczylowski, J.; Peinke, J.; Hölling, M.
2014-12-01
For the investigation of atmospheric turbulent flows on small scales a new anemometer was developed, the so-called 2d-Atmospheric Laser Cantilever Anemometer (2d-ALCA). It performs highly resolved measurements with a spatial resolution in millimeter range and temporal resolution in kHz range, thus detecting very small turbulent structures. The anemometer is a redesign of the successfully operating 2d-LCA for laboratory application. The new device was designed to withstand hostile operating environments (rain and saline, humid air). In February 2012, the 2d-ALCA was used for the first time in a test field. The device was mounted in about 53 m above ground level on a lattice tower near the German North Sea coast. Wind speed was measured by the 2d-ALCA at 10 kHz sampling rate and by cup anemometers at 1 Hz. The instantaneous wind speed ranged from 8 m/s to 19 m/s at an average turbulence level of about 7 %. Wind field characteristics were analyzed based on cup anemometer as well as 2d-ALCA. The combination of both devices allowed the study of atmospheric turbulence over several magnitudes in turbulent scales.
Tomosynthesis imaging with 2D scanning trajectories
NASA Astrophysics Data System (ADS)
Khare, Kedar; Claus, Bernhard E. H.; Eberhard, Jeffrey W.
2011-03-01
Tomosynthesis imaging in chest radiography provides volumetric information with the potential for improved diagnostic value when compared to the standard AP or LAT projections. In this paper we explore the image quality benefits of 2D scanning trajectories when coupled with advanced image reconstruction approaches. It is intuitively clear that 2D trajectories provide projection data that is more complete in terms of Radon space filling, when compared with conventional tomosynthesis using a linearly scanned source. Incorporating this additional information for obtaining improved image quality is, however, not a straightforward problem. The typical tomosynthesis reconstruction algorithms are based on direct inversion methods e.g. Filtered Backprojection (FBP) or iterative algorithms that are variants of the Algebraic Reconstruction Technique (ART). The FBP approach is fast and provides high frequency details in the image but at the same time introduces streaking artifacts degrading the image quality. The iterative methods can reduce the image artifacts by using image priors but suffer from a slow convergence rate, thereby producing images lacking high frequency details. In this paper we propose using a fast converging optimal gradient iterative scheme that has advantages of both the FBP and iterative methods in that it produces images with high frequency details while reducing the image artifacts. We show that using favorable 2D scanning trajectories along with the proposed reconstruction method has the advantage of providing improved depth information for structures such as the spine and potentially producing images with more isotropic resolution.
MAGNUM-2D computer code: user's guide
England, R.L.; Kline, N.W.; Ekblad, K.J.; Baca, R.G.
1985-01-01
Information relevant to the general use of the MAGNUM-2D computer code is presented. This computer code was developed for the purpose of modeling (i.e., simulating) the thermal and hydraulic conditions in the vicinity of a waste package emplaced in a deep geologic repository. The MAGNUM-2D computer computes (1) the temperature field surrounding the waste package as a function of the heat generation rate of the nuclear waste and thermal properties of the basalt and (2) the hydraulic head distribution and associated groundwater flow fields as a function of the temperature gradients and hydraulic properties of the basalt. MAGNUM-2D is a two-dimensional numerical model for transient or steady-state analysis of coupled heat transfer and groundwater flow in a fractured porous medium. The governing equations consist of a set of coupled, quasi-linear partial differential equations that are solved using a Galerkin finite-element technique. A Newton-Raphson algorithm is embedded in the Galerkin functional to formulate the problem in terms of the incremental changes in the dependent variables. Both triangular and quadrilateral finite elements are used to represent the continuum portions of the spatial domain. Line elements may be used to represent discrete conduits. 18 refs., 4 figs., 1 tab.
Engineering light outcoupling in 2D materials.
Lien, Der-Hsien; Kang, Jeong Seuk; Amani, Matin; Chen, Kevin; Tosun, Mahmut; Wang, Hsin-Ping; Roy, Tania; Eggleston, Michael S; Wu, Ming C; Dubey, Madan; Lee, Si-Chen; He, Jr-Hau; Javey, Ali
2015-02-11
When light is incident on 2D transition metal dichalcogenides (TMDCs), it engages in multiple reflections within underlying substrates, producing interferences that lead to enhancement or attenuation of the incoming and outgoing strength of light. Here, we report a simple method to engineer the light outcoupling in semiconducting TMDCs by modulating their dielectric surroundings. We show that by modulating the thicknesses of underlying substrates and capping layers, the interference caused by substrate can significantly enhance the light absorption and emission of WSe2, resulting in a ∼11 times increase in Raman signal and a ∼30 times increase in the photoluminescence (PL) intensity of WSe2. On the basis of the interference model, we also propose a strategy to control the photonic and optoelectronic properties of thin-layer WSe2. This work demonstrates the utilization of outcoupling engineering in 2D materials and offers a new route toward the realization of novel optoelectronic devices, such as 2D LEDs and solar cells. PMID:25602462
NMR-based diffusion lattice imaging.
Laun, Frederik Bernd; Müller, Lars; Kuder, Tristan Anselm
2016-03-01
Nuclear magnetic resonance (NMR) diffusion experiments are widely employed as they yield information about structures hindering the diffusion process, e.g., about cell membranes. While it has been shown in recent articles that these experiments can be used to determine the shape of closed pores averaged over a volume of interest, it is still an open question how much information can be gained in open well-connected systems. In this theoretical work, it is shown that the full structure information of connected periodic systems is accessible. To this end, the so-called "SEquential Rephasing by Pulsed field-gradient Encoding N Time intervals" (SERPENT) sequence is used, which employs several diffusion encoding gradient pulses with different amplitudes. Two two-dimensional solid matrices that are surrounded by an NMR-visible medium are considered: a hexagonal lattice of cylinders and a rectangular lattice of isosceles triangles. PMID:27078384
NMR-based diffusion lattice imaging
NASA Astrophysics Data System (ADS)
Laun, Frederik Bernd; Müller, Lars; Kuder, Tristan Anselm
2016-03-01
Nuclear magnetic resonance (NMR) diffusion experiments are widely employed as they yield information about structures hindering the diffusion process, e.g., about cell membranes. While it has been shown in recent articles that these experiments can be used to determine the shape of closed pores averaged over a volume of interest, it is still an open question how much information can be gained in open well-connected systems. In this theoretical work, it is shown that the full structure information of connected periodic systems is accessible. To this end, the so-called "SEquential Rephasing by Pulsed field-gradient Encoding N Time intervals" (SERPENT) sequence is used, which employs several diffusion encoding gradient pulses with different amplitudes. Two two-dimensional solid matrices that are surrounded by an NMR-visible medium are considered: a hexagonal lattice of cylinders and a rectangular lattice of isosceles triangles.
Wang, Y.; Wang, X.; Rybczynski, J.; Wang, D.Z.; Kempa, K.; Ren, Z.F.
2005-04-11
Self-assembly of polystyrene microspheres has been utilized in a two-step masking technique to prepare triangular lattices of catalytic nanodots at low cost. Subsequent triangular lattices of aligned carbon nanotubes on a silicon substrate are achieved by plasma-enhanced chemical vapor deposition. Nickel is used both in the nanodots and in the secondary mask. The triangular lattices of carbon nanotube arrays as two-dimensional photonic crystals show higher geometrical symmetry than the hexagonal lattices previously reported, enabling broader applications including negative index of refraction and subwavelength lensing effect.
Hart, W.E.; Istrail, S.
1996-08-09
This paper considers the protein structure prediction problem for lattice and off-lattice protein folding models that explicitly represent side chains. Lattice models of proteins have proven extremely useful tools for reasoning about protein folding in unrestricted continuous space through analogy. This paper provides the first illustration of how rigorous algorithmic analyses of lattice models can lead to rigorous algorithmic analyses of off-lattice models. The authors consider two side chain models: a lattice model that generalizes the HP model (Dill 85) to explicitly represent side chains on the cubic lattice, and a new off-lattice model, the HP Tangent Spheres Side Chain model (HP-TSSC), that generalizes this model further by representing the backbone and side chains of proteins with tangent spheres. They describe algorithms for both of these models with mathematically guaranteed error bounds. In particular, the authors describe a linear time performance guaranteed approximation algorithm for the HP side chain model that constructs conformations whose energy is better than 865 of optimal in a face centered cubic lattice, and they demonstrate how this provides a 70% performance guarantee for the HP-TSSC model. This is the first algorithm in the literature for off-lattice protein structure prediction that has a rigorous performance guarantee. The analysis of the HP-TSSC model builds off of the work of Dancik and Hannenhalli who have developed a 16/30 approximation algorithm for the HP model on the hexagonal close packed lattice. Further, the analysis provides a mathematical methodology for transferring performance guarantees on lattices to off-lattice models. These results partially answer the open question of Karplus et al. concerning the complexity of protein folding models that include side chains.
Strain-displacement relations for strain engineering in single-layer 2d materials
NASA Astrophysics Data System (ADS)
Midtvedt, Daniel; Lewenkopf, Caio H.; Croy, Alexander
2016-03-01
We investigate the electromechanical coupling in single-layer 2d materials. For non-Bravais lattices, we find important corrections to the standard macroscopic strain-microscopic atomic-displacement theory. We put forward a general and systematic approach to calculate strain-displacement relations for several classes of 2d materials. We apply our findings to graphene as a study case, by combining a tight binding and a valence force-field model to calculate electronic and mechanical properties of graphene nanoribbons under strain. The results show good agreement with the predictions of the Dirac equation coupled to continuum mechanics. For this long wave-limit effective theory, we find that the strain-displacement relations lead to a renormalization correction to the strain-induced pseudo-magnetic fields. A similar renormalization is found for the strain-induced band-gap of black phosphorous. Implications for nanomechanical properties and electromechanical coupling in 2d materials are discussed.
Quantum Diffusion on Molecular Tubes: Universal Scaling of the 1D to 2D Transition
NASA Astrophysics Data System (ADS)
Chuang, Chern; Lee, Chee Kong; Moix, Jeremy M.; Knoester, Jasper; Cao, Jianshu
2016-05-01
The transport properties of disordered systems are known to depend critically on dimensionality. We study the diffusion coefficient of a quantum particle confined to a lattice on the surface of a tube, where it scales between the 1D and 2D limits. It is found that the scaling relation is universal and independent of the temperature, disorder, and noise parameters, and the essential order parameter is the ratio between the localization length in 2D and the circumference of the tube. Phenomenological and quantitative expressions for transport properties as functions of disorder and noise are obtained and applied to real systems: In the natural chlorosomes found in light-harvesting bacteria the exciton transfer dynamics is predicted to be in the 2D limit, whereas a family of synthetic molecular aggregates is found to be in the homogeneous limit and is independent of dimensionality.
Quantum Diffusion on Molecular Tubes: Universal Scaling of the 1D to 2D Transition.
Chuang, Chern; Lee, Chee Kong; Moix, Jeremy M; Knoester, Jasper; Cao, Jianshu
2016-05-13
The transport properties of disordered systems are known to depend critically on dimensionality. We study the diffusion coefficient of a quantum particle confined to a lattice on the surface of a tube, where it scales between the 1D and 2D limits. It is found that the scaling relation is universal and independent of the temperature, disorder, and noise parameters, and the essential order parameter is the ratio between the localization length in 2D and the circumference of the tube. Phenomenological and quantitative expressions for transport properties as functions of disorder and noise are obtained and applied to real systems: In the natural chlorosomes found in light-harvesting bacteria the exciton transfer dynamics is predicted to be in the 2D limit, whereas a family of synthetic molecular aggregates is found to be in the homogeneous limit and is independent of dimensionality. PMID:27232033
Dual Element Intercalation into 2D Layered Bi₂Se₃ Nanoribbons.
Chen, Karen P; Chung, Frank R; Wang, Mengjing; Koski, Kristie J
2015-04-29
We demonstrate the intercalation of multiple zero-valent atomic species into two-dimensional (2D) layered Bi2Se3 nanoribbons. Intercalation is performed chemically through a stepwise combination of disproportionation redox reactions, hydrazine reduction, or carbonyl decomposition. Traditional intercalation is electrochemical thus limiting intercalant guests to a single atomic species. We show that multiple zero-valent atoms can be intercalated through this chemical route into the host lattice of a 2D crystal. Intermetallic species exhibit unique structural ordering demonstrated in a variety of superlattice diffraction patterns. We believe this method is general and can be used to achieve a wide variety of new 2D materials previously inaccessible. PMID:25851420
2D superconductivity by ionic gating
NASA Astrophysics Data System (ADS)
Iwasa, Yoshi
2D superconductivity is attracting a renewed interest due to the discoveries of new highly crystalline 2D superconductors in the past decade. Superconductivity at the oxide interfaces triggered by LaAlO3/SrTiO3 has become one of the promising routes for creation of new 2D superconductors. Also, the MBE grown metallic monolayers including FeSe are also offering a new platform of 2D superconductors. In the last two years, there appear a variety of monolayer/bilayer superconductors fabricated by CVD or mechanical exfoliation. Among these, electric field induced superconductivity by electric double layer transistor (EDLT) is a unique platform of 2D superconductivity, because of its ability of high density charge accumulation, and also because of the versatility in terms of materials, stemming from oxides to organics and layered chalcogenides. In this presentation, the following issues of electric filed induced superconductivity will be addressed; (1) Tunable carrier density, (2) Weak pinning, (3) Absence of inversion symmetry. (1) Since the sheet carrier density is quasi-continuously tunable from 0 to the order of 1014 cm-2, one is able to establish an electronic phase diagram of superconductivity, which will be compared with that of bulk superconductors. (2) The thickness of superconductivity can be estimated as 2 - 10 nm, dependent on materials, and is much smaller than the in-plane coherence length. Such a thin but low resistance at normal state results in extremely weak pinning beyond the dirty Boson model in the amorphous metallic films. (3) Due to the electric filed, the inversion symmetry is inherently broken in EDLT. This feature appears in the enhancement of Pauli limit of the upper critical field for the in-plane magnetic fields. In transition metal dichalcogenide with a substantial spin-orbit interactions, we were able to confirm the stabilization of Cooper pair due to its spin-valley locking. This work has been supported by Grant-in-Aid for Specially
Thermodynamic properties of the magnetized Coulomb crystal lattices
NASA Astrophysics Data System (ADS)
Kozhberov, A. A.
2016-08-01
It is thought that Coulomb crystals of ions with hexagonal close-packed lattice may form in the crust of strongly-magnetized neutron stars (magnetars). In this work we are trying to verify this prediction assuming that the direction of the magnetic field corresponds to the minimum of the zero-point energy. We also continue a detailed study of vibration modes and thermodynamic properties of magnetized Coulomb crystals in a wide range of temperatures and magnetic fields. It is demonstrated that the total Helmholtz free energy of the body-centered cubic Coulomb crystal is always lower than that of the Coulomb crystal with hexagonal close-packed or face-centered cubic lattice, which casts doubt on the hypothesis above.
In situ imaging of field-induced hexagonal columns in magnetite ferrofluids.
Klokkenburg, Mark; Erné, Ben H; Meeldijk, Johannes D; Wiedenmann, Albrecht; Petukhov, Andrei V; Dullens, Roel P A; Philipse, Albert P
2006-11-01
Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition. PMID:17155554
NASA Astrophysics Data System (ADS)
McKee, William C.; Meunier, Vincent; Xu, Ye
2015-12-01
Monolayer hexagonal boron nitride on Rh(111) and graphene on Ru(0001) illustrate a trend of divergence between the density functional theory (DFT) calculated geometric corrugation, and scanning tunneling microscope (STM) measured apparent corrugation, of metal-supported 2D films that feature chemically distinct regions. Notably, the geometric and apparent corrugations differ by up to 2 Å for boron nitride/Rh(111), whereas both the DFT-simulated and the experimentally observed STM images agree in the apparent corrugation over a wide range of bias voltages. The disparity is due to unequal contributions of the low/high-lying atoms to the local density of states in the vicinity of the Fermi level. This phenomenon has important implications for the structural characterization of certain supported 2D films, which are being explored for novel electronic and material applications.
Statistical Transmutation in Floquet Driven Optical Lattices
NASA Astrophysics Data System (ADS)
Sedrakyan, Tigran A.; Galitski, Victor M.; Kamenev, Alex
2015-11-01
We show that interacting bosons in a periodically driven two dimensional (2D) optical lattice may effectively exhibit fermionic statistics. The phenomenon is similar to the celebrated Tonks-Girardeau regime in 1D. The Floquet band of a driven lattice develops the moat shape, i.e., a minimum along a closed contour in the Brillouin zone. Such degeneracy of the kinetic energy favors fermionic quasiparticles. The statistical transmutation is achieved by the Chern-Simons flux attachment similar to the fractional quantum Hall case. We show that the velocity distribution of the released bosons is a sensitive probe of the fermionic nature of their stationary Floquet state.
Statistical Transmutation in Floquet Driven Optical Lattices.
Sedrakyan, Tigran A; Galitski, Victor M; Kamenev, Alex
2015-11-01
We show that interacting bosons in a periodically driven two dimensional (2D) optical lattice may effectively exhibit fermionic statistics. The phenomenon is similar to the celebrated Tonks-Girardeau regime in 1D. The Floquet band of a driven lattice develops the moat shape, i.e., a minimum along a closed contour in the Brillouin zone. Such degeneracy of the kinetic energy favors fermionic quasiparticles. The statistical transmutation is achieved by the Chern-Simons flux attachment similar to the fractional quantum Hall case. We show that the velocity distribution of the released bosons is a sensitive probe of the fermionic nature of their stationary Floquet state. PMID:26588392
Theory of thermal transport in multilayer hexagonal boron nitride and nanotubes
NASA Astrophysics Data System (ADS)
Lindsay, L.; Broido, D. A.
2012-01-01
We present a theory for the lattice thermal conductivity κL of single-walled boron nitride nanotubes (BNNTs) and multilayer hexagonal boron nitride (MLBN), which is based on an exact numerical solution of the phonon Boltzmann equation. Coupling between layers in MLBN and nanotube curvature in BNNTs each break a phonon scattering selection rule found in single-layer hexagonal boron nitride (SLBN), which reduces κL in these systems. We show that out-of-plane flexural phonons in MLBN and out-of-tube phonons in BNNTs provide large contributions to κL, qualitatively similar to multilayer graphene (MLG) and single-walled carbon nanotubes (SWCNTs). However, we find that the κL's in BNNTs and MLBN are considerably smaller compared to similar SWCNTs and MLG structures because of stronger anharmonic phonon scattering in the former. A large and strongly temperature-dependent isotope effect is found reflecting the interplay between anharmonic and isotope scattering phonons. Finally, we also demonstrate convergence of BNNTs into SLBN for large-diameter nanotubes and MLBN to bulk hexagonal boron nitride within a few layers.
Recursive, in-place algorithm for the hexagonal orthogonal oriented quadrature image pyramid
NASA Technical Reports Server (NTRS)
Watson, Andrew B.
1989-01-01
Pyramid image transforms have proven useful in image coding and pattern recognition. The hexagonal orthogonal oriented quadrature image pyramid (HOP), transforms an image into a set of orthogonal, oriented, odd and even bandpass subimages. It operates on a hexagonal input lattice and employs seven kernels, each of which occupies a neighborhood consisting of a point and a hexagon of six nearest neighbors. The kernels consist of one lowpass and six bandpass kernels that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The kernels are first applied to the image samples to create the first level of the pyramid, then to the lowpass coefficients to create the next level. The resulting pyramid is a compact, efficient image code. Here, a recursive, in-place algorithm for computation of the HOP transform is described. The transform may be regarded as a depth-first traversal of a tree structure. It is shown that the algorithm requires a number of operations that is on the order of the number of pixels.
Hydrothermal hexagonal SrFe12O19 ferrite powders: Phase composition, microstructure and acid washing
NASA Astrophysics Data System (ADS)
Xia, Ailin; Hu, Xuzhao; Li, Diankai; Chen, Lu; Jin, Chuangui; Zuo, Conghua; Su, Shubing
2014-03-01
A series of hexagonal m-type SrFe12O19 ferrite powders were hydrothermally synthesized, and their phase composition, microstructure and magnetic properties before/after acid washing were studied. In the synthesis of these specimens, the atomic ratio of Fe/Sr ( R F/S ) in starting materials was set to 4, 5 and 12, respectively. When R F/S = 12, the specimen has morphology of round flat cakes, not typical hexagonal plate-like structure. The results of SEM images and XRD patterns indicate that the specimen with R F/S = 12 was mostly composed of Fe2O3. When R F/S = 4 or 5, the hexagonal plate-like SrFe12O19 ferrite powders were successfully synthesized with only a small quantity of Fe2O3 and SrCO3 impurities. It is also found that acid washing can eliminate the impurities in as-synthesized specimens effectively, and also change their topography, which enhances the saturation magnetization. However, the coercivity changed irregularly after acid washing, which is ascribed to the combination of the changed morphology, introduced stress and lattice defects.
Structural and magnetic properties of Ca-substituted barium W-type hexagonal hexaferrites
NASA Astrophysics Data System (ADS)
Huang, Kai; Liu, Xiansong; Feng, Shuangjiu; Zhang, Zhanjun; Yu, Jiangying; Niu, Xiaofei; Lv, Farui; Huang, Xing
2015-04-01
A series of W-type hexagonal ferrites with the composition Ba1-xCaxCo2Fe16O27 (x=0, 0.1, 0.3, 0.4 and 0.5) were synthesized using a sol-gel method. The effects of doping on structural and magnetic properties are studied by X-ray diffraction, thermal analyzer, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer, respectively. The X-ray diffraction analysis shows that the samples belong to the W-type hexagonal ferrite. The lattice constants a and c decreases as Ca contents increases. The grains exhibit well defined hexagonal shape. The saturation magnetization and the intrinsic coercive force increases with the increase of the Ca substitution amount. The real part of complex permittivity (ε‧) and imaginary part (ε″) increase with more addition of Ca2+ amount. The imaginary part of complex permittivity (μ‧) increases and the real part (μ‧‧) goes down after Ca2+ is doped. Furthermore, the Ca2+ ions doped in the ferrite improved microwave absorbency.
Choi, S. Y.; Lee, B.; Carew, D. B.; Mamak, M; Peiris, F. C.; Speakman, S.; Chopra, N.; Ozin, G. A.; X-Ray Science Division; Univ. of Toronto; ORNL; Xerox Research Centre of Canada
2006-01-01
A straightforward and reproducible synthesis of crack-free large-area thin films of 3D hexagonal (R-3m) mesostructured nanocrystalline titania (meso-nc-TiO{sub 2}) using a Pluronic triblock copolymer (P123)/1-butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high-resolution scanning electron microscopy and scanning transmission electron microscopy), grazing-incidence small-angle X-ray scattering, in situ high-temperature X-ray diffraction, and variable-angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 C. The accessible surface area of 3D hexagonal meso-nc-TiO{sub 2} estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso-nc-TiO{sub 2} at the same annealing temperature.
Choi, S Y; Lee, B; Carew, D B; Peiris, F C; Mamak, M; Speakman, Scott A; Chopra, N; Ozin, G A
2006-01-01
A straightforward and reproducible synthesis of crack-free large-area thin films of 3D hexagonal (R-3m) mesostructured nanocrystalline titania (meso-nc-TiO{sub 2}) using a Pluronic triblock copolymer (P123)/1-butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high-resolution scanning electron microscopy and scanning transmission electron microscopy), grazing-incidence small-angle X-ray scattering, in situ high-temperature X-ray diffraction, and variable-angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 C. The accessible surface area of 3D hexagonal meso-nc-TiO{sub 2} estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso-nc-TiO{sub 2} at the same annealing temperature.
Neutron noise calculations in a hexagonal geometry and comparison with analytical solutions
Tran, H. N.; Demaziere, C.
2012-07-01
This paper presents the development of a neutronic and kinetic solver for hexagonal geometries. The tool is developed based on the diffusion theory with multi-energy groups and multi-groups of delayed neutron precursors allowing the solutions of forward and adjoint problems of static and dynamic states, and is applicable to both thermal and fast systems with hexagonal geometries. In the dynamic problems, the small stationary fluctuations of macroscopic cross sections are considered as noise sources, and then the induced first order noise is calculated fully in the frequency domain. Numerical algorithms for solving the static and noise equations are implemented with a spatial discretization based on finite differences and a power iterative solution. A coarse mesh finite difference method has been adopted for speeding up the convergence. Since no other numerical tool could calculate frequency-dependent noise in hexagonal geometry, validation calculations have been performed and benchmarked to analytical solutions based on a 2-D homogeneous system with two-energy groups and one-group of delayed neutron precursor, in which point-like perturbations of thermal absorption cross section at central and non-central positions are considered as noise sources. (authors)
GBL-2D Version 1.0: a 2D geometry boolean library.
McBride, Cory L. (Elemental Technologies, American Fort, UT); Schmidt, Rodney Cannon; Yarberry, Victor R.; Meyers, Ray J.
2006-11-01
This report describes version 1.0 of GBL-2D, a geometric Boolean library for 2D objects. The library is written in C++ and consists of a set of classes and routines. The classes primarily represent geometric data and relationships. Classes are provided for 2D points, lines, arcs, edge uses, loops, surfaces and mask sets. The routines contain algorithms for geometric Boolean operations and utility functions. Routines are provided that incorporate the Boolean operations: Union(OR), XOR, Intersection and Difference. A variety of additional analytical geometry routines and routines for importing and exporting the data in various file formats are also provided. The GBL-2D library was originally developed as a geometric modeling engine for use with a separate software tool, called SummitView [1], that manipulates the 2D mask sets created by designers of Micro-Electro-Mechanical Systems (MEMS). However, many other practical applications for this type of software can be envisioned because the need to perform 2D Boolean operations can arise in many contexts.
Wargaming in Both Rectilinear and Hexagonal Spaces
NASA Technical Reports Server (NTRS)
Hoover, Alex
2012-01-01
There are two main approaches to managing wargame entity interactions (movement, line of sight, area of effect, etc) freespace and gridded In the freespace approach, the units exist as entities in a continuous volume of (usually) Cartesian 3D space. They move in any direction (based on interaction with "terrain" that occupies the same space) and interact with each other based on references and displacements from their position in that space. In the gridded approach, space is broken up into (usually regular) shaped pieces. Units are considered to occupy the entire volume of one of these pieces, movement, line of sight, and other interactions are based on the relationships among the spaces rather than the absolute positions of the units themselves. Both approaches have advantages and drawbacks. The general issue that this discussion has addressed is that there is no "perfect" approach to implementing a wargaming battlespace. Each of them (and this extends to others not discussed) has different sets of advantages and disadvantages. Nothing will change that basic nature of the various approaches, nor would it be desirable to do so. Along with the advantages, the challenges define the feel of the game and focus the thinking of the players on certain aspects and away from others. The proposed approach to combining square and hexagonal approaches, which we will call the rhombus interface, leverages rhombuses constructed from equilateral triangles into which the hexagon can be decomposed to bridge the gap between the approaches, maintain relative consistency between the two as much as possible, and provide most of the feel of the hexagonal approach.
Solubilization of nutraceuticals into reverse hexagonal mesophases.
Amar-Yuli, Idit; Aserin, Abraham; Garti, Nissim
2008-08-21
The solubilization of four bioactive molecules with different polarities, in three reverse hexagonal (HII) systems has been investigated. The three HII systems were a typical reverse hexagonal composed of glycerol monooleate (GMO)/tricaprylin/water and two fluid hexagonal systems containing either 2.75 wt % Transcutol or ethanol as a fourth component. The phase behavior of the liquid crystalline phases in the presence of ascorbic acid, ascorbyl palmitate, D-alpha-tocopherol and D-alpha-tocopherol acetate were determined by small-angle X-ray scattering (SAXS) and optical microscopy. Differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) techniques were utilized to follow modifications in the thermal behavior and in the vibrations of different functional groups upon solubilizing the bioactive molecules. The nature of each guest molecule (in both geometry and polarity) together with the different HII structures (typical and fluids) determined the corresponding phase behavior, swelling or structural transformations and its location in the HII structures. Ascorbic acid was found to act as a chaotropic guest molecule, localized in the water-rich core and at the interface. The AP was also a chaotropic guest molecule with its head located in the vicinity of the GMO headgroup while its tail embedded close to the surfactant tail. D-alpha-tocopherol and D-alpha-tocopherol acetate were incorporated between the GMO tails; however, the D-alpha-tocopherol was located closer to the interface. Once Transcutol or ethanol was present and upon guest molecule incorporation, partial migration was detected. PMID:18665631
Quasiparticles near domain walls in hexagonal superconductors
NASA Astrophysics Data System (ADS)
Mukherjee, S. P.; Samokhin, K. V.
2016-02-01
We calculate the energy spectrum of quasiparticles trapped by a domain wall separating different time-reversal symmetry-breaking ground states in a hexagonal superconductor, such as UPt3. The bound-state energy is found to be strongly dependent on the gap symmetry, the domain-wall orientation, the quasiparticle's direction of semiclassical propagation, and the phase difference between the domains. We calculate the corresponding density of states and show how one can use its prominent features, in particular, the zero-energy singularity, to distinguish between different pairing symmetries.
Quasiparticles near domain walls in hexagonal superconductors
NASA Astrophysics Data System (ADS)
Mukherjee, Soumya; Samokhin, Kirill
We calculate the energy spectrum of quasiparticles trapped by a domain wall separating different time reversal symmetry-breaking ground states in a hexagonal superconductor, such as UPt3. The bound state energy is found to be strongly dependent on the gap symmetry, the domain wall orientation, the quasiparticle's direction of semiclassical propagation, and the phase difference between the domains. We calculate the corresponding density of states and show how one can use its prominent features, in particular, the zero-energy singularity, to distinguish between different pairing symmetries. Discovery Grant from the Natural Sciences and Engineering Research Council of Canada.
Method for exfoliation of hexagonal boron nitride
NASA Technical Reports Server (NTRS)
Lin, Yi (Inventor); Connell, John W. (Inventor)
2012-01-01
A new method is disclosed for the exfoliation of hexagonal boron nitride into mono- and few-layered nanosheets (or nanoplatelets, nanomesh, nanoribbons). The method does not necessarily require high temperature or vacuum, but uses commercially available h-BN powders (or those derived from these materials, bulk crystals) and only requires wet chemical processing. The method is facile, cost efficient, and scalable. The resultant exfoliated h-BN is dispersible in an organic solvent or water thus amenable for solution processing for unique microelectronic or composite applications.
Diagonal form factors and hexagon form factors
NASA Astrophysics Data System (ADS)
Jiang, Yunfeng; Petrovskii, Andrei
2016-07-01
We study the heavy-heavy-light (HHL) three-point functions in the planar {N} = 4 super-Yang-Mills theory using the recently proposed hexagon bootstrap program [1]. We prove the conjecture of Bajnok, Janik and Wereszczynski [2] on the polynomial L-dependence of HHL structure constant up to the leading finite-size corrections, where L is the length of the heavy operators. The proof is presented for a specific set-up but the method can be applied to more general situations.
Diamagnetic response in zigzag hexagonal silicene rings
NASA Astrophysics Data System (ADS)
Xu, Ning; Chen, Qiao; Tian, Hongyu; Ding, Jianwen; Liu, Junfeng
2016-09-01
Hexagonal silicene rings with unusually large diamagnetic moments have been found in a theoretical study of the electronic and magnetic properties. In the presence of effective spin-orbit coupling, the magnetic-field-driven spin-up electrons flow anticlockwise exhibiting colossal diamagnetic moments, while the spin-down electrons flow clockwise exhibiting colossal paramagnetic moments along the rings. The large diamagnetic moment is thus the result of competition of spin-up and spin-down electrons, which can be modulated by spin-orbit coupling strength and exchange field.
Interparticle Attraction in 2D Complex Plasmas
NASA Astrophysics Data System (ADS)
Kompaneets, Roman; Morfill, Gregor E.; Ivlev, Alexei V.
2016-03-01
Complex (dusty) plasmas allow experimental studies of various physical processes occurring in classical liquids and solids by directly observing individual microparticles. A major problem is that the interaction between microparticles is generally not molecularlike. In this Letter, we propose how to achieve a molecularlike interaction potential in laboratory 2D complex plasmas. We argue that this principal aim can be achieved by using relatively small microparticles and properly adjusting discharge parameters. If experimentally confirmed, this will make it possible to employ complex plasmas as a model system with an interaction potential resembling that of conventional liquids.
Periodically sheared 2D Yukawa systems
Kovács, Anikó Zsuzsa; Hartmann, Peter; Donkó, Zoltán
2015-10-15
We present non-equilibrium molecular dynamics simulation studies on the dynamic (complex) shear viscosity of a 2D Yukawa system. We have identified a non-monotonic frequency dependence of the viscosity at high frequencies and shear rates, an energy absorption maximum (local resonance) at the Einstein frequency of the system at medium shear rates, an enhanced collective wave activity, when the excitation is near the plateau frequency of the longitudinal wave dispersion, and the emergence of significant configurational anisotropy at small frequencies and high shear rates.
ENERGY LANDSCAPE OF 2D FLUID FORMS
Y. JIANG; ET AL
2000-04-01
The equilibrium states of 2D non-coarsening fluid foams, which consist of bubbles with fixed areas, correspond to local minima of the total perimeter. (1) The authors find an approximate value of the global minimum, and determine directly from an image how far a foam is from its ground state. (2) For (small) area disorder, small bubbles tend to sort inwards and large bubbles outwards. (3) Topological charges of the same sign repel while charges of opposite sign attract. (4) They discuss boundary conditions and the uniqueness of the pattern for fixed topology.
A scalable 2-D parallel sparse solver
Kothari, S.C.; Mitra, S.
1995-12-01
Scalability beyond a small number of processors, typically 32 or less, is known to be a problem for existing parallel general sparse (PGS) direct solvers. This paper presents a parallel general sparse PGS direct solver for general sparse linear systems on distributed memory machines. The algorithm is based on the well-known sequential sparse algorithm Y12M. To achieve efficient parallelization, a 2-D scattered decomposition of the sparse matrix is used. The proposed algorithm is more scalable than existing parallel sparse direct solvers. Its scalability is evaluated on a 256 processor nCUBE2s machine using Boeing/Harwell benchmark matrices.
2D stepping drive for hyperspectral systems
NASA Astrophysics Data System (ADS)
Endrödy, Csaba; Mehner, Hannes; Grewe, Adrian; Sinzinger, Stefan; Hoffmann, Martin
2015-07-01
We present the design, fabrication and characterization of a compact 2D stepping microdrive for pinhole array positioning. The miniaturized solution enables a highly integrated compact hyperspectral imaging system. Based on the geometry of the pinhole array, an inch-worm drive with electrostatic actuators was designed resulting in a compact (1 cm2) positioning system featuring a step size of about 15 µm in a 170 µm displacement range. The high payload (20 mg) as required for the pinhole array and the compact system design exceed the known electrostatic inch-worm-based microdrives.
NASA Astrophysics Data System (ADS)
Sommer, W.
2013-03-01
The basic experimental setup of a Fabry-Perot etalon between a collimating and a focusing lens is modified by introducing 2D rectangular lattices between the etalon and the collimating lens. Consequently, the irradiance of the interference fringes on a screen in the focal plane of the focusing lens changes and is modified by the diffraction pattern of the 2D lattice. The constructive interference directions resulting from both the etalon and the diffraction by the 2D lattice have to correlate in order to obtain maximum irradiance. Considering this experiment in a didactical context and analysing how a 2D rectangular lattice is seen through the etalon, the investigation provides us with the concept of an optical space containing a row of virtual 2D lattices. Due to the partially reflecting plane surfaces of the etalon, different virtual images of the 2D lattice form a 3D lattice with a tetragonal or orthorhombic structure. As an optical interface, the simple setup with a 2D lattice and an etalon models a 3D lattice. Using a laser, the diffraction pattern of a 2D lattice and etalon can be used to optically simulate 3D x-ray diffraction. The experiments can be included wherever undergraduate or graduate students have to follow up Laue's formulation of x-ray diffraction.
Superfluid density through 2D superconductor junctions
NASA Astrophysics Data System (ADS)
Nam, Hyoungdo; Shih, Chih-Kang
As S. Qin et al. reported, two monolayer (2 ML) lead film on a silicon (111) substrate has one of two different atomic structures on the silicon substrate: the unstrained 1x1 and the psedumorphically strained √3x √3 (i.e. the same lattice constant as the Si √3x √3 lattice). Most interestingly, although these two different regions show the same quantum well state features, they have different Tc's (5 K and 4 K). These two different regions of 2 ML film naturally form superconductor-superconductor (SS or SS') junctions along silicon step edges. Physical connection of the junction is only 1 ML thickness because of the step height difference of substrate. We will present this study of SS (or SS') junction system using scanning tunneling microscopy/spectroscopy and in-situ double-coil mutual inductance measurement. The transition of superconducting gaps across either SS or SS' junctions should show how to locally affect each other. Double coil measurement show a global Tc close to the lower Tc region with sizable superfluid density. We will discuss the phase rigidity and its relationship to the superfluid density in this ultra-thin Pb film that is only 2 ML thick.
Cooperative dynamics in ultrasoft 2D crystals
NASA Astrophysics Data System (ADS)
Sprakel, Joris; van der Meer, Berend; Dijkstra, Marjolein; van der Gucht, Jasper
2015-03-01
The creation, annihilation, and diffusion of defects in crystal lattices play an important role during crystal melting and deformation. Although it is well understood how defects form and react when crystals are subjected to external stresses, it remains unclear how crystals cope with internal stresses. We report a study in which we create a highly localized internal stress, by means of optical tweezing, in a crystal formed from micrometer-sized colloidal spheres and directly observe how the solid reacts using microscopy. We find that, even though the excitation is highly localized, a collective dance of colloidal particles results; these collective modes take the form of closed rings or open-ended strings, depending on the sequence of events which nucleate the rearrangements. Surprisingly, we find from Brownian Dynamics simulations that these cooperative dynamics are thermally-activated modes inherent to the crystal, and can even occur through a single, sufficiently large thermal fluctuation, resulting in the irreversible displacement of 100s of particles from their lattice sites.
Quantum Simulation with 2D Arrays of Trapped Ions
NASA Astrophysics Data System (ADS)
Richerme, Philip
2016-05-01
The computational difficulty of solving fully quantum many-body spin problems is a significant obstacle to understanding the behavior of strongly correlated quantum matter. This work proposes the design and construction of a 2D quantum spin simulator to investigate the physics of frustrated materials, highly entangled states, mechanisms potentially underpinning high-temperature superconductivity, and other topics inaccessible to current 1D systems. The effective quantum spins will be encoded within the well-isolated electronic levels of trapped ions, confined in a two-dimensional planar geometry, and made to interact using phonon-mediated optical dipole forces. The system will be scalable to 100+ quantum particles, far beyond the realm of classical intractability, while maintaining individual-ion control, long quantum coherence times, and site-resolved projective spin measurements. Once constructed, the two-dimensional quantum simulator will implement a broad range of spin models on a variety of reconfigurable lattices and characterize their behavior through measurements of spin-spin correlations and entanglement. This versatile tool will serve as an important experimental resource for exploring difficult quantum many-body problems in a regime where classical methods fail.
NASA Astrophysics Data System (ADS)
Cokelaer, T.
2007-11-01
Matched filtering is used to search for gravitational waves emitted by inspiralling compact binaries in data from the ground-based interferometers. One of the key aspects of the detection process is the design of a template bank that covers the astrophysically pertinent parameter space. In an earlier paper, we described a template bank that is based on a square lattice. Although robust, we showed that the square placement is overefficient, with the implication that it is computationally more demanding than required. In this paper, we present a template bank based on an hexagonal lattice, which size is reduced by 40% with respect to the proposed square placement. We describe the practical aspects of the hexagonal template bank implementation, its size, and computational cost. We have also performed exhaustive simulations to characterize its efficiency and safeness. We show that the bank is adequate to search for a wide variety of binary systems (primordial black holes, neutron stars, and stellar-mass black holes) and in data from both current detectors (initial LIGO, Virgo and GEO600) as well as future detectors (advanced LIGO and EGO). Remarkably, although our template bank placement uses a metric arising from a particular template family, namely, stationary phase approximation, we show that it can be used successfully with other template families (e.g., Padé resummation and effective one-body approximation). This quality of being effective for different template families makes the proposed bank suitable for a search that would use several of them in parallel (e.g., in a binary black hole search). The hexagonal template bank described in this paper is currently used to search for nonspinning inspiralling compact binaries in data from the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Cokelaer, T.
2007-11-15
Matched filtering is used to search for gravitational waves emitted by inspiralling compact binaries in data from the ground-based interferometers. One of the key aspects of the detection process is the design of a template bank that covers the astrophysically pertinent parameter space. In an earlier paper, we described a template bank that is based on a square lattice. Although robust, we showed that the square placement is overefficient, with the implication that it is computationally more demanding than required. In this paper, we present a template bank based on an hexagonal lattice, which size is reduced by 40% with respect to the proposed square placement. We describe the practical aspects of the hexagonal template bank implementation, its size, and computational cost. We have also performed exhaustive simulations to characterize its efficiency and safeness. We show that the bank is adequate to search for a wide variety of binary systems (primordial black holes, neutron stars, and stellar-mass black holes) and in data from both current detectors (initial LIGO, Virgo and GEO600) as well as future detectors (advanced LIGO and EGO). Remarkably, although our template bank placement uses a metric arising from a particular template family, namely, stationary phase approximation, we show that it can be used successfully with other template families (e.g., Pade resummation and effective one-body approximation). This quality of being effective for different template families makes the proposed bank suitable for a search that would use several of them in parallel (e.g., in a binary black hole search). The hexagonal template bank described in this paper is currently used to search for nonspinning inspiralling compact binaries in data from the Laser Interferometer Gravitational-Wave Observatory (LIGO)
Quiver gauge theories and integrable lattice models
NASA Astrophysics Data System (ADS)
Yagi, Junya
2015-10-01
We discuss connections between certain classes of supersymmetric quiver gauge theories and integrable lattice models from the point of view of topological quantum field theories (TQFTs). The relevant classes include 4d N=1 theories known as brane box and brane tilling models, 3d N=2 and 2d N=(2,2) theories obtained from them by compactification, and 2d N=(0,2) theories closely related to these theories. We argue that their supersymmetric indices carry structures of TQFTs equipped with line operators, and as a consequence, are equal to the partition functions of lattice models. The integrability of these models follows from the existence of extra dimension in the TQFTs, which emerges after the theories are embedded in M-theory. The Yang-Baxter equation expresses the invariance of supersymmetric indices under Seiberg duality and its lower-dimensional analogs.
WFR-2D: an analytical model for PWAS-generated 2D ultrasonic guided wave propagation
NASA Astrophysics Data System (ADS)
Shen, Yanfeng; Giurgiutiu, Victor
2014-03-01
This paper presents WaveFormRevealer 2-D (WFR-2D), an analytical predictive tool for the simulation of 2-D ultrasonic guided wave propagation and interaction with damage. The design of structural health monitoring (SHM) systems and self-aware smart structures requires the exploration of a wide range of parameters to achieve best detection and quantification of certain types of damage. Such need for parameter exploration on sensor dimension, location, guided wave characteristics (mode type, frequency, wavelength, etc.) can be best satisfied with analytical models which are fast and efficient. The analytical model was constructed based on the exact 2-D Lamb wave solution using Bessel and Hankel functions. Damage effects were inserted in the model by considering the damage as a secondary wave source with complex-valued directivity scattering coefficients containing both amplitude and phase information from wave-damage interaction. The analytical procedure was coded with MATLAB, and a predictive simulation tool called WaveFormRevealer 2-D was developed. The wave-damage interaction coefficients (WDICs) were extracted from harmonic analysis of local finite element model (FEM) with artificial non-reflective boundaries (NRB). The WFR-2D analytical simulation results were compared and verified with full scale multiphysics finite element models and experiments with scanning laser vibrometer. First, Lamb wave propagation in a pristine aluminum plate was simulated with WFR-2D, compared with finite element results, and verified by experiments. Then, an inhomogeneity was machined into the plate to represent damage. Analytical modeling was carried out, and verified by finite element simulation and experiments. This paper finishes with conclusions and suggestions for future work.
Optical properties of Tb3+ doped KLaF4 in cubic and hexagonal symmetries
NASA Astrophysics Data System (ADS)
Ahmad, Shahzad; Das, Subrata; Nagarajan, R.; Vijaya Prakash, G.
2013-12-01
Tb3+ doped cubic and hexagonally ordered KLaF4 nano-sized crystals have been synthesized by solution based method with the simultaneous evaluation of their optical and magnetic properties. While higher amounts of Tb3+ (10%) can be doped in cubic KLaF4 lattice, only 3% of Tb3+ can be doped in hexagonal KLaF4 by this method. Cubic KLaF4:Tb3+ samples show a very strong green emission centered at 545 nm (5D4 → 7F5) on excitation with 337 nm laser, the intensity increases monotonically with Tb3+ content. The relative intensity of the excited state 5D3 violet emission is weaker than that of 5D4 emissions even at the low Tb3+ amounts. It is proposed that the 5D3 → 5D4 cross-relaxation may be responsible for the decrease in the 5D3 decay rate with increasing Tb3+ concentrations. The average emission decay lifetime of the green emission (545 nm; 5D4 → 7F5) of Tb3+ doped cubic KLaF4 samples are in the order of 1-3 ms. The absence of the characteristic emissions from 5D3 level in hexagonal KLaF4:Tb3+ phosphor suggests the pronounced non-radiative cross relaxation between the 5D3 to 5D4 energy levels. Further, these cubic KLaF4:Tb3+ samples show intense green upconversion emission when co-doped with moderate concentrations of sensitizer Yb3+ ions. Both cubic and hexagonal KLaF4:Tb3+ samples show paramagnetic behavior at room temperature with χg value ranging from 1.627 × 10-6 to 1.356 × 10-5 emu/g.
Photocurrent spectroscopy of 2D materials
NASA Astrophysics Data System (ADS)
Cobden, David
Confocal photocurrent measurements provide a powerful means of studying many aspects of the optoelectronic and electrical properties of a 2D device or material. At a diffraction-limited point they can provide a detailed absorption spectrum, and they can probe local symmetry, ultrafast relaxation rates and processes, electron-electron interaction strengths, and transport coefficients. We illustrate this with several examples, once being the photo-Nernst effect. In gapless 2D materials, such as graphene, in a perpendicular magnetic field a photocurrent antisymmetric in the field is generated near to the free edges, with opposite sign at opposite edges. Its origin is the transverse thermoelectric current associated with the laser-induced electron temperature gradient. This effect provides an unambiguous demonstration of the Shockley-Ramo nature of long-range photocurrent generation in gapless materials. It also provides a means of investigating quasiparticle properties. For example, in the case of graphene on hBN, it can be used to probe the Lifshitz transition that occurs due to the minibands formed by the Moire superlattice. We also observe and discuss photocurrent generated in other semimetallic (WTe2) and semiconducting (WSe2) monolayers. Work supported by DoE BES and NSF EFRI grants.
Multienzyme Inkjet Printed 2D Arrays.
Gdor, Efrat; Shemesh, Shay; Magdassi, Shlomo; Mandler, Daniel
2015-08-19
The use of printing to produce 2D arrays is well established, and should be relatively facile to adapt for the purpose of printing biomaterials; however, very few studies have been published using enzyme solutions as inks. Among the printing technologies, inkjet printing is highly suitable for printing biomaterials and specifically enzymes, as it offers many advantages. Formulation of the inkjet inks is relatively simple and can be adjusted to a variety of biomaterials, while providing nonharmful environment to the enzymes. Here we demonstrate the applicability of inkjet printing for patterning multiple enzymes in a predefined array in a very straightforward, noncontact method. Specifically, various arrays of the enzymes glucose oxidase (GOx), invertase (INV) and horseradish peroxidase (HP) were printed on aminated glass surfaces, followed by immobilization using glutardialdehyde after printing. Scanning electrochemical microscopy (SECM) was used for imaging the printed patterns and to ascertain the enzyme activity. The successful formation of 2D arrays consisting of enzymes was explored as a means of developing the first surface confined enzyme based logic gates. Principally, XOR and AND gates, each consisting of two enzymes as the Boolean operators, were assembled, and their operation was studied by SECM. PMID:26214072
Lattice Location Determination of Ge in SiC by ALCHEMI
NASA Astrophysics Data System (ADS)
Kups, T.; Voelskow, M.; Skorupa, W.; Soueidan, M.; Ferro, G.; Pezoldt, J.
The incorporation of Ge into cubic and hexagonal silicon carbide is compared for three different doping methods: ion-implantation, molecular beam epitaxy and liquid phase epitaxy. The lattice location of Ge was determined by the transmission electron microscopy based method ALCHEMI (atomic location by channelling enhanced microanalysis). Depending on the method used for incorporation, Ge is able to occupy different silicon carbide lattice locations.
Dielectric-barrier discharges in two-dimensional lattice potentials.
Sinclair, J; Walhout, M
2012-01-20
We use a pin-grid electrode to introduce a corrugated electrical potential into a planar dielectric-barrier discharge (DBD) system, so that the amplitude of the applied electric field has the profile of a two-dimensional square lattice. The lattice potential provides a template for the spatial distribution of plasma filaments in the system and has pronounced effects on the patterns that can form. The positions at which filaments become localized within the lattice unit cell vary with the width of the discharge gap. The patterns that appear when filaments either overfill or underfill the lattice are reminiscent of those observed in other physical systems involving 2D lattices. We suggest that the connection between lattice-driven DBDs and other areas of physics may benefit from the further development of models that treat plasma filaments as interacting particles. PMID:22400753
Nylon flocked swab severely reduces Hexagon Obti sensibility.
Frippiat, Christophe; De Roy, Gilbert; Fontaine, Louis-Marie; Dognaux, Sophie; Noel, Fabrice; Heudt, Laeticia; Lepot, Laurent
2015-02-01
Hexagon Obti immunological blood test and flocked swab are widely used in forensic laboratories. Nevertheless, up to now, no compatibility tests have been published between sampling with the ethylene oxide treated flocked swab and the Hexagon Obti blood detection strip. In this study, we investigated this compatibility. Our work shows that sampling with ethylene oxide treated flocked swab reduces by a factor of at least 100 the detection threshold of blood using the Hexagon Obti immunological test. PMID:25575014
Raman spectra and structural properties of hexagonal Yb1-xDyxMnO3 (x = 0, 0.05 and 0.1)
NASA Astrophysics Data System (ADS)
Sattibabu, Bhumireddi; Bhatnagar, Anil K.; Das, D.
2016-05-01
Single phase Yb1-xDyxMnO3 (x= 0, 0.05 and 0.1) samples are prepared by a solid state reaction method. X-ray powder diffraction shows that all samples crystallize in the hexagonal phase with P63cm space group. The structural analysis shows there is increase in lattice parameter c and cell volume of the hexagonal unit cell with Dy substitution and the average bond length between Mn-O increases. Raman spectra show that the phonon peaks of Yb1-xDyxMnO3 slightly shift to lower frequencies with doping.
Solution of an associating lattice-gas model with density anomaly on a Husimi lattice
NASA Astrophysics Data System (ADS)
Oliveira, Tiago J.; Stilck, Jürgen F.; Barbosa, Marco Aurélio A.
2010-11-01
We study a model of a lattice gas with orientational degrees of freedom which resemble the formation of hydrogen bonds between the molecules. In this model, which is the simplified version of the Henriques-Barbosa model, no distinction is made between donors and acceptors in the bonding arms. We solve the model in the grand-canonical ensemble on a Husimi lattice built with hexagonal plaquettes with a central site. The ground state of the model, which was originally defined on the triangular lattice, is exactly reproduced by the solution on this Husimi lattice. In the phase diagram, one gas and two liquid [high density liquid (HDL) and low density liquid (LDL)] phases are present. All phase transitions (GAS-LDL, GAS-HDL, and LDL-HDL) are discontinuous, and the three phases coexist at a triple point. A line of temperatures of maximum density in the isobars is found in the metastable GAS phase, as well as another line of temperatures of minimum density appears in the LDL phase, part of it in the stable region and another in the metastable region of this phase. These findings are at variance with simulational results for the same model on the triangular lattice, which suggested a phase diagram with two critical points. However, our results show very good quantitative agreement with the simulations, both for the coexistence loci and the densities of particles and of hydrogen bonds. We discuss the comparison of the simulations with our results.
2-D or not 2-D, that is the question: A Northern California test
Mayeda, K; Malagnini, L; Phillips, W S; Walter, W R; Dreger, D
2005-06-06
Reliable estimates of the seismic source spectrum are necessary for accurate magnitude, yield, and energy estimation. In particular, how seismic radiated energy scales with increasing earthquake size has been the focus of recent debate within the community and has direct implications on earthquake source physics studies as well as hazard mitigation. The 1-D coda methodology of Mayeda et al. has provided the lowest variance estimate of the source spectrum when compared against traditional approaches that use direct S-waves, thus making it ideal for networks that have sparse station distribution. The 1-D coda methodology has been mostly confined to regions of approximately uniform complexity. For larger, more geophysically complicated regions, 2-D path corrections may be required. The complicated tectonics of the northern California region coupled with high quality broadband seismic data provides for an ideal ''apples-to-apples'' test of 1-D and 2-D path assumptions on direct waves and their coda. Using the same station and event distribution, we compared 1-D and 2-D path corrections and observed the following results: (1) 1-D coda results reduced the amplitude variance relative to direct S-waves by roughly a factor of 8 (800%); (2) Applying a 2-D correction to the coda resulted in up to 40% variance reduction from the 1-D coda results; (3) 2-D direct S-wave results, though better than 1-D direct waves, were significantly worse than the 1-D coda. We found that coda-based moment-rate source spectra derived from the 2-D approach were essentially identical to those from the 1-D approach for frequencies less than {approx}0.7-Hz, however for the high frequencies (0.7{le} f {le} 8.0-Hz), the 2-D approach resulted in inter-station scatter that was generally 10-30% smaller. For complex regions where data are plentiful, a 2-D approach can significantly improve upon the simple 1-D assumption. In regions where only 1-D coda correction is available it is still preferable over 2
Feigelson, Boris N; Bermudez, Victor M; Hite, Jennifer K; Robinson, Zachary R; Wheeler, Virginia D; Sridhara, Karthik; Hernández, Sandra C
2015-02-28
Atomically thin two dimensional hexagonal boron nitride (2D h-BN) is one of the key materials in the development of new van der Waals heterostructures due to its outstanding properties including an atomically smooth surface, high thermal conductivity, high mechanical strength, chemical inertness and high electrical resistance. The development of 2D h-BN growth is still in the early stages and largely depends on rapid and accurate characterization of the grown monolayer or few layers h-BN films. This paper demonstrates a new approach to characterizing monolayer h-BN films directly on metal substrates by grazing-incidence infrared reflection absorption spectroscopy (IRRAS). Using h-BN films grown by atmospheric-pressure chemical vapor deposition on Cu and Ni substrates, two new sub-bands are found for the A2u out-of-plane stretching mode. It is shown, using both experimental and computational methods, that the lower-energy sub-band is related to 2D h-BN coupled with substrate, while the higher energy sub-band is related to decoupled (or free-standing) 2D h-BN. It is further shown that this newly-observed fine structure in the A2u mode can be used to assess, quickly and easily, the homogeneity of the h-BN-metal interface and the effects of metal surface contamination on adhesion of the layer. PMID:25640166
NASA Astrophysics Data System (ADS)
Feigelson, Boris N.; Bermudez, Victor M.; Hite, Jennifer K.; Robinson, Zachary R.; Wheeler, Virginia D.; Sridhara, Karthik; Hernández, Sandra C.
2015-02-01
Atomically thin two dimensional hexagonal boron nitride (2D h-BN) is one of the key materials in the development of new van der Waals heterostructures due to its outstanding properties including an atomically smooth surface, high thermal conductivity, high mechanical strength, chemical inertness and high electrical resistance. The development of 2D h-BN growth is still in the early stages and largely depends on rapid and accurate characterization of the grown monolayer or few layers h-BN films. This paper demonstrates a new approach to characterizing monolayer h-BN films directly on metal substrates by grazing-incidence infrared reflection absorption spectroscopy (IRRAS). Using h-BN films grown by atmospheric-pressure chemical vapor deposition on Cu and Ni substrates, two new sub-bands are found for the A2u out-of-plane stretching mode. It is shown, using both experimental and computational methods, that the lower-energy sub-band is related to 2D h-BN coupled with substrate, while the higher energy sub-band is related to decoupled (or free-standing) 2D h-BN. It is further shown that this newly-observed fine structure in the A2u mode can be used to assess, quickly and easily, the homogeneity of the h-BN-metal interface and the effects of metal surface contamination on adhesion of the layer.
Numerical Evaluation of 2D Ground States
NASA Astrophysics Data System (ADS)
Kolkovska, Natalia
2016-02-01
A ground state is defined as the positive radial solution of the multidimensional nonlinear problem
Canard configured aircraft with 2-D nozzle
NASA Technical Reports Server (NTRS)
Child, R. D.; Henderson, W. P.
1978-01-01
A closely-coupled canard fighter with vectorable two-dimensional nozzle was designed for enhanced transonic maneuvering. The HiMAT maneuver goal of a sustained 8g turn at a free-stream Mach number of 0.9 and 30,000 feet was the primary design consideration. The aerodynamic design process was initiated with a linear theory optimization minimizing the zero percent suction drag including jet effects and refined with three-dimensional nonlinear potential flow techniques. Allowances were made for mutual interference and viscous effects. The design process to arrive at the resultant configuration is described, and the design of a powered 2-D nozzle model to be tested in the LRC 16-foot Propulsion Wind Tunnel is shown.
2D Electrostatic Actuation of Microshutter Arrays
NASA Technical Reports Server (NTRS)
Burns, Devin E.; Oh, Lance H.; Li, Mary J.; Jones, Justin S.; Kelly, Daniel P.; Zheng, Yun; Kutyrev, Alexander S.; Moseley, Samuel H.
2015-01-01
An electrostatically actuated microshutter array consisting of rotational microshutters (shutters that rotate about a torsion bar) were designed and fabricated through the use of models and experiments. Design iterations focused on minimizing the torsional stiffness of the microshutters, while maintaining their structural integrity. Mechanical and electromechanical test systems were constructed to measure the static and dynamic behavior of the microshutters. The torsional stiffness was reduced by a factor of four over initial designs without sacrificing durability. Analysis of the resonant behavior of the microshutter arrays demonstrates that the first resonant mode is a torsional mode occurring around 3000 Hz. At low vacuum pressures, this resonant mode can be used to significantly reduce the drive voltage necessary for actuation requiring as little as 25V. 2D electrostatic latching and addressing was demonstrated using both a resonant and pulsed addressing scheme.
2D Electrostatic Actuation of Microshutter Arrays
NASA Technical Reports Server (NTRS)
Burns, Devin E.; Oh, Lance H.; Li, Mary J.; Kelly, Daniel P.; Kutyrev, Alexander S.; Moseley, Samuel H.
2015-01-01
Electrostatically actuated microshutter arrays consisting of rotational microshutters (shutters that rotate about a torsion bar) were designed and fabricated through the use of models and experiments. Design iterations focused on minimizing the torsional stiffness of the microshutters, while maintaining their structural integrity. Mechanical and electromechanical test systems were constructed to measure the static and dynamic behavior of the microshutters. The torsional stiffness was reduced by a factor of four over initial designs without sacrificing durability. Analysis of the resonant behavior of the microshutters demonstrates that the first resonant mode is a torsional mode occurring around 3000 Hz. At low vacuum pressures, this resonant mode can be used to significantly reduce the drive voltage necessary for actuation requiring as little as 25V. 2D electrostatic latching and addressing was demonstrated using both a resonant and pulsed addressing scheme.
Graphene suspensions for 2D printing
NASA Astrophysics Data System (ADS)
Soots, R. A.; Yakimchuk, E. A.; Nebogatikova, N. A.; Kotin, I. A.; Antonova, I. V.
2016-04-01
It is shown that, by processing a graphite suspension in ethanol or water by ultrasound and centrifuging, it is possible to obtain particles with thicknesses within 1-6 nm and, in the most interesting cases, 1-1.5 nm. Analogous treatment of a graphite suspension in organic solvent yields eventually thicker particles (up to 6-10 nm thick) even upon long-term treatment. Using the proposed ink based on graphene and aqueous ethanol with ethylcellulose and terpineol additives for 2D printing, thin (~5 nm thick) films with sheet resistance upon annealing ~30 MΩ/□ were obtained. With the ink based on aqueous graphene suspension, the sheet resistance was ~5-12 kΩ/□ for 6- to 15-nm-thick layers with a carrier mobility of ~30-50 cm2/(V s).
Hirobe, Tomohisa; Ito, Shosuke; Wakamatsu, Kazumasa
2013-09-01
The novel mutation named ru2(d) /Hps5(ru2-d) , characterized by light-colored coats and ruby-eyes, prohibits differentiation of melanocytes by inhibiting tyrosinase (Tyr) activity, expression of Tyr, Tyr-related protein 1 (Tyrp1), Tyrp2, and Kit. However, it is not known whether the ru2(d) allele affects pheomelanin synthesis in recessive yellow (e/Mc1r(e) ) or in pheomelanic stage in agouti (A) mice. In this study, effects of the ru2(d) allele on pheomelanin synthesis were investigated by chemical analysis of melanin present in dorsal hairs of 5-week-old mice from F2 generation between C57BL/10JHir (B10)-co-isogenic ruby-eye 2(d) and B10-congenic recessive yellow or agouti. Eumelanin content was decreased in ruby-eye 2(d) and ruby-eye 2(d) agouti mice, whereas pheomelanin content in ruby-eye 2(d) recessive yellow and ruby-eye 2(d) agouti mice did not differ from the corresponding Ru2(d) /- mice, suggesting that the ru2(d) allele inhibits eumelanin but not pheomelanin synthesis. PMID:23672590
Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial
NASA Astrophysics Data System (ADS)
Dai, S.; Ma, Q.; Liu, M. K.; Andersen, T.; Fei, Z.; Goldflam, M. D.; Wagner, M.; Watanabe, K.; Taniguchi, T.; Thiemens, M.; Keilmann, F.; Janssen, G. C. A. M.; Zhu, S.-E.; Jarillo-Herrero, P.; Fogler, M. M.; Basov, D. N.
2015-08-01
Hexagonal boron nitride (h-BN) is a natural hyperbolic material, in which the dielectric constants are the same in the basal plane (ɛt ≡ ɛx = ɛy) but have opposite signs (ɛtɛz < 0) in the normal plane (ɛz). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons—collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN, so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon-phonon polaritons. The hyperbolic plasmon-phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5-2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon-phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone.
Mechanical properties of lattice grid composites
NASA Astrophysics Data System (ADS)
Fan, Hualin; Fang, Daining; Jin, Fengnian
2008-08-01
An equivalent continuum method only considering the stretching deformation of struts was used to study the in-plane stiffness and strength of planar lattice grid composite materials. The initial yield equations of lattices were deduced. Initial yield surfaces were depicted separately in different 3D and 2D stress spaces. The failure envelope is a polyhedron in 3D spaces and a polygon in 2D spaces. Each plane or line of the failure envelope is corresponding to the yield or buckling of a typical bar row. For lattices with more than three bar rows, subsequent yield of the other bar row after initial yield made the lattice achieve greater limit strength. The importance of the buckling strength of the grids was strengthened while the grids were relative sparse. The integration model of the method was used to study the nonlinear mechanical properties of strain hardening grids. It was shown that the integration equation could accurately model the complete stress-strain curves of the grids within small deformations.
Charge carrier transport properties in layer structured hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Doan, T. C.; Li, J.; Lin, J. Y.; Jiang, H. X.
2014-10-01
Due to its large in-plane thermal conductivity, high temperature and chemical stability, large energy band gap (˜ 6.4 eV), hexagonal boron nitride (hBN) has emerged as an important material for applications in deep ultraviolet photonic devices. Among the members of the III-nitride material system, hBN is the least studied and understood. The study of the electrical transport properties of hBN is of utmost importance with a view to realizing practical device applications. Wafer-scale hBN epilayers have been successfully synthesized by metal organic chemical deposition and their electrical transport properties have been probed by variable temperature Hall effect measurements. The results demonstrate that undoped hBN is a semiconductor exhibiting weak p-type at high temperatures (> 700 °K). The measured acceptor energy level is about 0.68 eV above the valence band. In contrast to the electrical transport properties of traditional III-nitride wide bandgap semiconductors, the temperature dependence of the hole mobility in hBN can be described by the form of μ ∝ (T/T0)-α with α = 3.02, satisfying the two-dimensional (2D) carrier transport limit dominated by the polar optical phonon scattering. This behavior is a direct consequence of the fact that hBN is a layer structured material. The optical phonon energy deduced from the temperature dependence of the hole mobility is ħω = 192 meV (or 1546 cm-1), which is consistent with values previously obtained using other techniques. The present results extend our understanding of the charge carrier transport properties beyond the traditional III-nitride semiconductors.
Anisotropic Dielectric Breakdown of Hexagonal Boron Nitride Film
NASA Astrophysics Data System (ADS)
Hattori, Yoshiaki; Taniguchi, Takashi; Watanabe, Kenji; Nagashio, Kosuke
Hexagonal boron nitride (h-BN) is considered as ideal substrate for 2D material devises. However, the reliability of insulating properties of h-BN itself has not been clarified yet. In this study, the anisotropic dielectric breakdown of h-BN is studied. We have found that the dielectric breakdown in c axis direction using a conductive atomic force microscope proceeded in the layer-by-layer manner. The obtained dielectric field strength was ~12 MV/cm, which is comparable to the conventional SiO2. On the other hand, to estimate the dielectric field strength in a direction perpendicular to c axis, voltage is applied to a relatively thick h-BN (10-60 nm) through Cr/Au electrodes fabricated on the h-BN. We realized that the absorbed water on h-BN significantly affect the IV characters and the breakdown voltage. After the adsorbed water was removed by the heating in vacuum, the dielectric field strength was determined to be ~3 MV/cm, which is the same order as that in c axis direction. This value could be increased when we consider the effect of electric field concentration around the metal electrode. Although the large difference in dielectric filed strength for two directions was initially expected due to the highly-anisotropic layered structure with the van der Waals bonding, it was not the case because the sp2 bonding should be broken for dielectric breakdown regardless of its direction. This research was supported by Grants-in-Aid for Scientific Research on Innovative Areas and for Research Activity Start-up by MEXT, Japan.
Eskildsen, M.; Andersen, N.; Mortensen, K.; Bolle, C.; Lieber, C.; Oxx, S.; Sridhar, S.; Canfield, P.
1997-03-01
Small-angle neutron scattering and magnetic decoration both demonstrate a topological transition in the flux line lattice (FLL) in ErNi{sub 2}B{sub 2}C. The high-field square lattice slowly transforms into a hexagonal lattice via an area preserving [100] rhombohedral distortion below roughly 500Oe. The square FLL is aligned with the [110] direction of the tetragonal crystal, while the two domains of the hexagonal FLL are aligned with [100] and [010]. The differences in pinning for the two FLL topologies are reflected in the rf kinetic inductance. {copyright} {ital 1997} {ital The American Physical Society}
Palmer, R.B.
1987-05-01
This paper looks at, and compares three types of damping ring lattices: conventional, wiggler lattice with finite ..cap alpha.., wiggler lattice with ..cap alpha.. = 0, and observes the attainable equilibrium emittances for the three cases assuming a constraint on the attainable longitudinal impedance of 0.2 ohms. The emittance obtained are roughly in the ratio 4:2:1 for these cases.
NASA Astrophysics Data System (ADS)
Venderbos, J. W. F.
2016-03-01
We study hexagonal spin-channel ("triplet") density waves with commensurate M -point propagation vectors. We first show that the three Q =M components of the singlet charge density and charge-current density waves can be mapped to multicomponent Q =0 nonzero angular momentum order in three dimensions (3D) with cubic crystal symmetry. This one-to-one correspondence is exploited to define a symmetry classification for triplet M -point density waves using the standard classification of spin-orbit coupled electronic liquid crystal phases of a cubic crystal. Through this classification we naturally identify a set of noncoplanar spin density and spin-current density waves: the chiral spin density wave and its time-reversal invariant analog. These can be thought of as 3 DL =2 and 4 spin-orbit coupled isotropic β -phase orders. In contrast, uniaxial spin density waves are shown to correspond to α phases. The noncoplanar triple-M spin-current density wave realizes a novel 2 D semimetal state with three flavors of four-component spin-momentum locked Dirac cones, protected by a crystal symmetry akin to nonsymmorphic symmetry, and sits at the boundary between a trivial and topological insulator. In addition, we point out that a special class of classical spin states, defined as classical spin states respecting all lattice symmetries up to global spin rotation, are naturally obtained from the symmetry classification of electronic triplet density waves. These symmetric classical spin states are the classical long-range ordered limits of chiral spin liquids.
Self-alignment of silver nanoparticles in highly ordered 2D arrays
NASA Astrophysics Data System (ADS)
Rodríguez-León, Ericka; Íñiguez-Palomares, Ramón; Urrutia-Bañuelos, Efraín; Herrera-Urbina, Ronaldo; Tánori, Judith; Maldonado, Amir
2015-03-01
We have synthesized silver nanoparticles in the non-polar phase of non-aqueous microemulsions. The nanocrystals have been grown by reducing silver ions in the microemulsion cylindrical micelles formed by the reducing agent (ethylene glycol). By a careful deposit of the microemulsion phase on a substrate, the micelles align in a hexagonal geometry, thus forming a 2D array of parallel strings of individual silver nanoparticles on the substrate. The microemulsions are the ternary system of anionic surfactant, non-polar solvent (isooctane), and solvent polar (ethylene glycol); the size of synthesized nanoparticles is about 7 nm and they are monodisperse. The study of the microstructure was realized by transmission electron microscopy, high-resolution technique transmission electron microscopy (HR-TEM), and Fourier processing using the software Digital Micrograph for the determination of the crystalline structure of the HR-TEM images of the nanocrystals; chemical composition was determined using the energy-dispersive X-ray spectroscopy. Addition technique polarizing light microscopy allowed the observation of the hexagonal phase of the system. This method of synthesis and self-alignment could be useful for the preparation of patterned materials at the nanometer scale.
Self-alignment of silver nanoparticles in highly ordered 2D arrays.
Rodríguez-León, Ericka; Íñiguez-Palomares, Ramón; Urrutia-Bañuelos, Efraín; Herrera-Urbina, Ronaldo; Tánori, Judith; Maldonado, Amir
2015-01-01
We have synthesized silver nanoparticles in the non-polar phase of non-aqueous microemulsions. The nanocrystals have been grown by reducing silver ions in the microemulsion cylindrical micelles formed by the reducing agent (ethylene glycol). By a careful deposit of the microemulsion phase on a substrate, the micelles align in a hexagonal geometry, thus forming a 2D array of parallel strings of individual silver nanoparticles on the substrate. The microemulsions are the ternary system of anionic surfactant, non-polar solvent (isooctane), and solvent polar (ethylene glycol); the size of synthesized nanoparticles is about 7 nm and they are monodisperse. The study of the microstructure was realized by transmission electron microscopy, high-resolution technique transmission electron microscopy (HR-TEM), and Fourier processing using the software Digital Micrograph for the determination of the crystalline structure of the HR-TEM images of the nanocrystals; chemical composition was determined using the energy-dispersive X-ray spectroscopy. Addition technique polarizing light microscopy allowed the observation of the hexagonal phase of the system. This method of synthesis and self-alignment could be useful for the preparation of patterned materials at the nanometer scale. PMID:25883540
NASA Astrophysics Data System (ADS)
Syah Putra, Rudy
2016-02-01
Agar matrix was artificially contaminated with caesium and subjected to rapid assessment of electrokinetic treatment on the basis of the 2D electrode configuration. The effect of caesium concentration on the process was investigated using different electrode configuration (i.e. rectangular, hexagonal and triangular). During treatment the in situ pH distribution, the current flow, and the potential distribution were monitored. At the end of the treatment, the caesium concentration distribution was measured. The results of these experiments showed that for caesium contamination, pH control is essential in order to create a suitable environment throughout the agar matrix to enable contaminant removal. It was found that the type of electrode configuration used to control the pH affected the rate of caesium accumulation. All of the electrode configurations tested was effective, but the highest caesium extraction was achieved when the hexagonal pattern was used to control the pH. After 72 h of treatment at 50 mA, the concentration of caesium decreased gradually from the second and first layer of agar matrix throughout the cell, suggesting that most of the caesium was concentrated on the cathode part.
Transport studies in 2D transition metal dichalcogenides and black phosphorus.
Du, Yuchen; Neal, Adam T; Zhou, Hong; Ye, Peide D
2016-07-01
Two-dimensional (2D) materials are a new family of materials with interesting physical properties, ranging from insulating hexagonal boron nitride, semiconducting or semi-metallic transition metal dichalcogenides, to gapless metallic graphene. In this review, we provide a brief discussion of transport studies in transition metal dichalcogenides, including both semiconducting and semi-metallic phases, as well as a discussion of the newly emerged narrow bandgap layered material, black phosphorus, in terms of its electrical and quantum transport properties at room and cryogenic temperatures. Ultra-thin layered channel materials with atomic layer thickness in the cross-plane direction, together with relatively high carrier mobility with appropriate passivation techniques, provide the promise for new scientific discoveries and broad device applications. PMID:27187790
A 2D zinc-organic network being easily exfoliated into isolated sheets
NASA Astrophysics Data System (ADS)
Yu, Guihong; Li, Ruiqing; Leng, Zhihua; Gan, Shucai
2016-08-01
A metal-organic aggregate, namely {Zn2Cl2(BBC)}n (BBC = 4,4‧,4‧‧-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoate) was obtained by solvothermal synthesis. Its structure is featured with the Zn2(COO)3 paddle-wheels with two chloride anions on axial positions and hexagonal pores in the layers. The exclusion of water in the precursor and the solvent plays a crucial role in the formation of target compound. This compound can be easily dissolved in alkaline solution and exfoliated into isolated sheets, which shows a novel way for the preparation of 2D materials.
Topological Odd-Parity Superconductivity Close to Type-II 2D Van Hove Singularities
NASA Astrophysics Data System (ADS)
Yao, Hong; Yang, Fan
2014-03-01
We study unconventional superconductivity induced by weak repulsive interactions in 2D electronic systems at Van Hove singularity (VHS) where electronic density of states is logarithmically divergent. We define two types of VH saddle points. For type-I VH systems, weak repulsive interactions generically induce unconventional singlet pairing. However and more interestingly, for type-II VH systems renormalization group treatment shows that weak repulsive interactions favor triplet pairing (e.g. p-wave) when the Fermi surface has no good nesting. When such type-II VH systems respecting tetragonal or hexagonal point group symmetry, topological superconductivity (chiral p +ip or time reversal invariant Z2 p +ip pairing) will generally occur. We shall also discuss implications of this study to recently discovered BiS2-based superconductors and other superconducting materials that host type-II VH singularities in their Fermi surfaces.
Transport studies in 2D transition metal dichalcogenides and black phosphorus
NASA Astrophysics Data System (ADS)
Du, Yuchen; Neal, Adam T.; Zhou, Hong; Ye, Peide D.
2016-07-01
Two-dimensional (2D) materials are a new family of materials with interesting physical properties, ranging from insulating hexagonal boron nitride, semiconducting or semi-metallic transition metal dichalcogenides, to gapless metallic graphene. In this review, we provide a brief discussion of transport studies in transition metal dichalcogenides, including both semiconducting and semi-metallic phases, as well as a discussion of the newly emerged narrow bandgap layered material, black phosphorus, in terms of its electrical and quantum transport properties at room and cryogenic temperatures. Ultra-thin layered channel materials with atomic layer thickness in the cross-plane direction, together with relatively high carrier mobility with appropriate passivation techniques, provide the promise for new scientific discoveries and broad device applications.
Quantum emission from hexagonal boron nitride monolayers.
Tran, Toan Trong; Bray, Kerem; Ford, Michael J; Toth, Milos; Aharonovich, Igor
2016-01-01
Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Single-photon emission from two-dimensional materials has been reported, but only at cryogenic temperatures. Here, we demonstrate room-temperature, polarized and ultrabright single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing. PMID:26501751
Quantum emission from hexagonal boron nitride monolayers
NASA Astrophysics Data System (ADS)
Aharonovich, Igor; Tran, Toantrong; Bray, Kerem; Ford, Michael J.; Toth, Milos; MTEE Collaboration
Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Here, we demonstrate room-temperature, polarized single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. The emitters emit at the red and the near infrared spectral range and exhibit narrowband ultra bright emission (~full width at half maximum of below 10 nm with more than three million counts/s). Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing.
Quantum emission from hexagonal boron nitride monolayers
NASA Astrophysics Data System (ADS)
Tran, Toan Trong; Bray, Kerem; Ford, Michael J.; Toth, Milos; Aharonovich, Igor
2016-01-01
Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Single-photon emission from two-dimensional materials has been reported, but only at cryogenic temperatures. Here, we demonstrate room-temperature, polarized and ultrabright single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing.
Hexagonal boron nitride and water interaction parameters
NASA Astrophysics Data System (ADS)
Wu, Yanbin; Wagner, Lucas K.; Aluru, Narayana R.
2016-04-01
The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems.
Hexagonal boron nitride and water interaction parameters.
Wu, Yanbin; Wagner, Lucas K; Aluru, Narayana R
2016-04-28
The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems. PMID:27131542
The hexagon hypothesis: Six disruptive scenarios.
Burtles, Jim
2015-01-01
This paper aims to bring a simple but effective and comprehensive approach to the development, delivery and monitoring of business continuity solutions. To ensure that the arguments and principles apply across the board, the paper sticks to basic underlying concepts rather than sophisticated interpretations. First, the paper explores what exactly people are defending themselves against. Secondly, the paper looks at how defences should be set up. Disruptive events tend to unfold in phases, each of which invites a particular style of protection, ranging from risk management through to business continuity to insurance cover. Their impact upon any business operation will fall into one of six basic scenarios. The hexagon hypothesis suggests that everyone should be prepared to deal with each of these six disruptive scenarios and it provides them with a useful benchmark for business continuity. PMID:26420396
Elasto-static micropolar behavior of a chiral auxetic lattice
NASA Astrophysics Data System (ADS)
Spadoni, A.; Ruzzene, M.
2012-01-01
Auxetic materials expand when stretched, and shrink when compressed. This is the result of a negative Poisson's ratio ν. Isotropic configurations with ν≈-1 have been designed and are expected to provide increased shear stiffness G. This assumes that Young's modulus and ν can be engineered independently. In this article, a micropolar-continuum model is employed to describe the behavior of a representative auxetic structural network, the chiral lattice, in an attempt to remove the indeterminacy in its constitutive law resulting from ν=-1. While this indeterminacy is successfully removed, it is found that the shear modulus is an independent parameter and, for certain configurations, it is equal to that of the triangular lattice. This is remarkable as the chiral lattice is subject to bending deformation of its internal members, and thus is more compliant than the triangular lattice which is stretch dominated. The derived micropolar model also indicates that this unique lattice has the highest characteristic length scale lc of all known lattice topologies, as well as a negative first Lamé constant without violating bounds required for thermodynamic stability. We also find that hexagonal arrangements of deformable rings have a coupling number N=1. This is the first lattice reported in the literature for which couple-stress or Mindlin theory is necessary rather than being adopted a priori.
A new inversion method for (T2, D) 2D NMR logging and fluid typing
NASA Astrophysics Data System (ADS)
Tan, Maojin; Zou, Youlong; Zhou, Cancan
2013-02-01
One-dimensional nuclear magnetic resonance (1D NMR) logging technology has some significant limitations in fluid typing. However, not only can two-dimensional nuclear magnetic resonance (2D NMR) provide some accurate porosity parameters, but it can also identify fluids more accurately than 1D NMR. In this paper, based on the relaxation mechanism of (T2, D) 2D NMR in a gradient magnetic field, a hybrid inversion method that combines least-squares-based QR decomposition (LSQR) and truncated singular value decomposition (TSVD) is examined in the 2D NMR inversion of various fluid models. The forward modeling and inversion tests are performed in detail with different acquisition parameters, such as magnetic field gradients (G) and echo spacing (TE) groups. The simulated results are discussed and described in detail, the influence of the above-mentioned observation parameters on the inversion accuracy is investigated and analyzed, and the observation parameters in multi-TE activation are optimized. Furthermore, the hybrid inversion can be applied to quantitatively determine the fluid saturation. To study the effects of noise level on the hybrid method and inversion results, the numerical simulation experiments are performed using different signal-to-noise-ratios (SNRs), and the effect of different SNRs on fluid typing using three fluid models are discussed and analyzed in detail.
Structural domain walls in polar hexagonal manganites
NASA Astrophysics Data System (ADS)
Kumagai, Yu
2014-03-01
The domain structure in the multiferroic hexagonal manganites is currently intensely investigated, motivated by the observation of intriguing sixfold topological defects at their meeting points [Choi, T. et al,. Nature Mater. 9, 253 (2010).] and nanoscale electrical conductivity at the domain walls [Wu, W. et al., Phys. Rev. Lett. 108, 077203 (2012).; Meier, D. et al., Nature Mater. 11, 284 (2012).], as well as reports of coupling between ferroelectricity, magnetism and structural antiphase domains [Geng, Y. et al., Nano Lett. 12, 6055 (2012).]. The detailed structure of the domain walls, as well as the origin of such couplings, however, was previously not fully understood. In the present study, we have used first-principles density functional theory to calculate the structure and properties of the low-energy structural domain walls in the hexagonal manganites [Kumagai, Y. and Spaldin, N. A., Nature Commun. 4, 1540 (2013).]. We find that the lowest energy domain walls are atomically sharp, with {210}orientation, explaining the orientation of recently observed stripe domains and suggesting their topological protection [Chae, S. C. et al., Phys. Rev. Lett. 108, 167603 (2012).]. We also explain why ferroelectric domain walls are always simultaneously antiphase walls, propose a mechanism for ferroelectric switching through domain-wall motion, and suggest an atomistic structure for the cores of the sixfold topological defects. This work was supported by ETH Zurich, the European Research Council FP7 Advanced Grants program me (grant number 291151), the JSPS Postdoctoral Fellowships for Research Abroad, and the MEXT Elements Strategy Initiative to Form Core Research Center TIES.
Electron Momentum Distribution Mapping of Trans-Stilbene Projected to [101] by Positron 2D-ACAR
NASA Astrophysics Data System (ADS)
Selvakumar, S.; Sivaji, K.; Smith, S. V.
Electron momentum distribution (EMD) on trans-stilbene single crystal projected along [101] direction has been studied by using positron two dimensional -angular correlation of annihilation radiation (2D-ACAR). The projected EMD is explained with respect to the molecular arrangement in the plane. The EMD features reflected the delocalized electronic states in [101] direction. The results of EMD mapping did not show a characteristic ellipsoidal distribution at lower momentum region (LMR) as observed in trans-stilbene projected to [010] direction at room temperature. The LMR region exhibits a hexagonal contour projected to [101] direction.
Two-dimensional B-C-O alloys: a promising class of 2D materials for electronic devices.
Zhou, Si; Zhao, Jijun
2016-04-21
Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp(2) honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor forming alternate patterns into the chain- or ring-like structures embedded in the pristine graphene regions. These B-C-O hybrid sheets can be either metals or semiconductors depending on the B : O ratio. The semiconducting (B2O)nCm and (B6O3)nCm phases exist under the B- and O-rich conditions, and possess a tunable band gap of 1.0-3.8 eV and high carrier mobility, retaining ∼1000 cm(2) V(-1) s(-1) even for half coverage of B and O atoms. These B-C-O alloys form a new class of 2D materials that are promising candidates for high-speed electronic devices. PMID:27072060
NASA Astrophysics Data System (ADS)
Cheng, Chingyun; Kangara, Jayampathi; Arakelyan, Ilya; Thomas, John
2016-05-01
We tune the dimensionality of a strongly interacting degenerate 6 Li Fermi gas from 2D to quasi-2D, by adjusting the radial confinement of pancake-shaped clouds to control the radial chemical potential. In the 2D regime with weak radial confinement, the measured pair binding energies are in agreement with 2D-BCS mean field theory, which predicts dimer pairing energies in the many-body regime. In the qausi-2D regime obtained with increased radial confinement, the measured pairing energy deviates significantly from 2D-BCS theory. In contrast to the pairing energy, the measured radii of the cloud profiles are not fit by 2D-BCS theory in either the 2D or quasi-2D regimes, but are fit in both regimes by a beyond mean field polaron-model of the free energy. Supported by DOE, ARO, NSF, and AFOSR.
Competing coexisting phases in 2D water
Zanotti, Jean-Marc; Judeinstein, Patrick; Dalla-Bernardina, Simona; Creff, Gaëlle; Brubach, Jean-Blaise; Roy, Pascale; Bonetti, Marco; Ollivier, Jacques; Sakellariou, Dimitrios; Bellissent-Funel, Marie-Claire
2016-01-01
The properties of bulk water come from a delicate balance of interactions on length scales encompassing several orders of magnitudes: i) the Hydrogen Bond (HBond) at the molecular scale and ii) the extension of this HBond network up to the macroscopic level. Here, we address the physics of water when the three dimensional extension of the HBond network is frustrated, so that the water molecules are forced to organize in only two dimensions. We account for the large scale fluctuating HBond network by an analytical mean-field percolation model. This approach provides a coherent interpretation of the different events experimentally (calorimetry, neutron, NMR, near and far infra-red spectroscopies) detected in interfacial water at 160, 220 and 250 K. Starting from an amorphous state of water at low temperature, these transitions are respectively interpreted as the onset of creation of transient low density patches of 4-HBonded molecules at 160 K, the percolation of these domains at 220 K and finally the total invasion of the surface by them at 250 K. The source of this surprising behaviour in 2D is the frustration of the natural bulk tetrahedral local geometry and the underlying very significant increase in entropy of the interfacial water molecules. PMID:27185018
2D Radiative Processes Near Cloud Edges
NASA Technical Reports Server (NTRS)
Varnai, T.
2012-01-01
Because of the importance and complexity of dynamical, microphysical, and radiative processes taking place near cloud edges, the transition zone between clouds and cloud free air has been the subject of intense research both in the ASR program and in the wider community. One challenge in this research is that the one-dimensional (1D) radiative models widely used in both remote sensing and dynamical simulations become less accurate near cloud edges: The large horizontal gradients in particle concentrations imply that accurate radiative calculations need to consider multi-dimensional radiative interactions among areas that have widely different optical properties. This study examines the way the importance of multidimensional shortwave radiative interactions changes as we approach cloud edges. For this, the study relies on radiative simulations performed for a multiyear dataset of clouds observed over the NSA, SGP, and TWP sites. This dataset is based on Microbase cloud profiles as well as wind measurements and ARM cloud classification products. The study analyzes the way the difference between 1D and 2D simulation results increases near cloud edges. It considers both monochromatic radiances and broadband radiative heating, and it also examines the influence of factors such as cloud type and height, and solar elevation. The results provide insights into the workings of radiative processes and may help better interpret radiance measurements and better estimate the radiative impacts of this critical region.
Simulation of Yeast Cooperation in 2D.
Wang, M; Huang, Y; Wu, Z
2016-03-01
Evolution of cooperation has been an active research area in evolutionary biology in decades. An important type of cooperation is developed from group selection, when individuals form spatial groups to prevent them from foreign invasions. In this paper, we study the evolution of cooperation in a mixed population of cooperating and cheating yeast strains in 2D with the interactions among the yeast cells restricted to their small neighborhoods. We conduct a computer simulation based on a game theoretic model and show that cooperation is increased when the interactions are spatially restricted, whether the game is of a prisoner's dilemma, snow drifting, or mutual benefit type. We study the evolution of homogeneous groups of cooperators or cheaters and describe the conditions for them to sustain or expand in an opponent population. We show that under certain spatial restrictions, cooperator groups are able to sustain and expand as group sizes become large, while cheater groups fail to expand and keep them from collapse. PMID:26988702
Ion Transport in 2-D Graphene Nanochannels
NASA Astrophysics Data System (ADS)
Xie, Quan; Foo, Elbert; Duan, Chuanhua
2015-11-01
Graphene membranes have recently attracted wide attention due to its great potential in water desalination and selective molecular sieving. Further developments of these membranes, including enhancing their mass transport rate and/or molecular selectivity, rely on the understanding of fundamental transport mechanisms through graphene membranes, which has not been studied experimentally before due to fabrication and measurement difficulties. Herein we report the fabrication of the basic constituent of graphene membranes, i.e. 2-D single graphene nanochannels (GNCs) and the study of ion transport in these channels. A modified bonding technique was developed to form GNCs with well-defined geometry and uniform channel height. Ion transport in such GNCs was studied using DC conductance measurement. Our preliminary results showed that the ion transport in GNCs is still governed by surface charge at low concentrations (10-6M to 10-4M). However, GNCs exhibits much higher ionic conductances than silica nanochannels with the same geometries in the surface-charge-governed regime. This conductance enhancement can be attributed to the pre-accumulation of charges on graphene surfaces. The work is supported by the Faculty Startup Fund (Boston University, USA).
Parallel map analysis on 2-D grids
Berry, M.; Comiskey, J.; Minser, K.
1993-12-31
In landscape ecology, computer modeling is used to assess habitat fragmentation and its ecological iMPLications. Specifically, maps (2-D grids) of habitat clusters must be analyzed to determine number, sizes and geometry of clusters. Models prior to this study relied upon sequential Fortran-77 programs which limited the sizes of maps and densities of clusters which could be analyzed. In this paper, we present more efficient computer models which can exploit recursion or parallelism. Significant improvements over the original Fortran-77 programs have been achieved using both recursive and nonrecursive C implementations on a variety of workstations such as the Sun Sparc 2, IBM RS/6000-350, and HP 9000-750. Parallel implementations on a 4096-processor MasPar MP-1 and a 32-processor CM-5 are also studied. Preliminary experiments suggest that speed improvements for the parallel model on the MasPar MP-1 (written in MPL) and on the CM-5 (written in C using CMMD) can be as much as 39 and 34 times faster, respectively, than the most efficient sequential C program on a Sun Sparc 2 for a 512 map. An important goal in this research effort is to produce a scalable map analysis algorithm for the identification and characterization of clusters for relatively large maps on massively-parallel computers.
2D Turbulence with Complicated Boundaries
NASA Astrophysics Data System (ADS)
Roullet, G.; McWilliams, J. C.
2014-12-01
We examine the consequences of lateral viscous boundary layers on the 2D turbulence that arises in domains with complicated boundaries (headlands, bays etc). The study is carried out numerically with LES. The numerics are carefully designed to ensure all global conservation laws, proper boundary conditions and a minimal range of dissipation scales. The turbulence dramatically differs from the classical bi-periodic case. Boundary layer separations lead to creation of many small vortices and act as a continuing energy source exciting the inverse cascade of energy throughout the domain. The detachments are very intermittent in time. In free decay, the final state depends on the effective numerical resolution: laminar with a single dominant vortex for low Re and turbulent with many vortices for large enough Re. After very long time, the turbulent end-state exhibits a striking tendency for the emergence of shielded vortices which then interact almost elastically. In the forced case, the boundary layers allow the turbulence to reach a statistical steady state without any artificial hypo-viscosity or other large-scale dissipation. Implications are discussed for the oceanic mesoscale and submesoscale turbulence.
Competing coexisting phases in 2D water
NASA Astrophysics Data System (ADS)
Zanotti, Jean-Marc; Judeinstein, Patrick; Dalla-Bernardina, Simona; Creff, Gaëlle; Brubach, Jean-Blaise; Roy, Pascale; Bonetti, Marco; Ollivier, Jacques; Sakellariou, Dimitrios; Bellissent-Funel, Marie-Claire
2016-05-01
The properties of bulk water come from a delicate balance of interactions on length scales encompassing several orders of magnitudes: i) the Hydrogen Bond (HBond) at the molecular scale and ii) the extension of this HBond network up to the macroscopic level. Here, we address the physics of water when the three dimensional extension of the HBond network is frustrated, so that the water molecules are forced to organize in only two dimensions. We account for the large scale fluctuating HBond network by an analytical mean-field percolation model. This approach provides a coherent interpretation of the different events experimentally (calorimetry, neutron, NMR, near and far infra-red spectroscopies) detected in interfacial water at 160, 220 and 250 K. Starting from an amorphous state of water at low temperature, these transitions are respectively interpreted as the onset of creation of transient low density patches of 4-HBonded molecules at 160 K, the percolation of these domains at 220 K and finally the total invasion of the surface by them at 250 K. The source of this surprising behaviour in 2D is the frustration of the natural bulk tetrahedral local geometry and the underlying very significant increase in entropy of the interfacial water molecules.
Competing coexisting phases in 2D water.
Zanotti, Jean-Marc; Judeinstein, Patrick; Dalla-Bernardina, Simona; Creff, Gaëlle; Brubach, Jean-Blaise; Roy, Pascale; Bonetti, Marco; Ollivier, Jacques; Sakellariou, Dimitrios; Bellissent-Funel, Marie-Claire
2016-01-01
The properties of bulk water come from a delicate balance of interactions on length scales encompassing several orders of magnitudes: i) the Hydrogen Bond (HBond) at the molecular scale and ii) the extension of this HBond network up to the macroscopic level. Here, we address the physics of water when the three dimensional extension of the HBond network is frustrated, so that the water molecules are forced to organize in only two dimensions. We account for the large scale fluctuating HBond network by an analytical mean-field percolation model. This approach provides a coherent interpretation of the different events experimentally (calorimetry, neutron, NMR, near and far infra-red spectroscopies) detected in interfacial water at 160, 220 and 250 K. Starting from an amorphous state of water at low temperature, these transitions are respectively interpreted as the onset of creation of transient low density patches of 4-HBonded molecules at 160 K, the percolation of these domains at 220 K and finally the total invasion of the surface by them at 250 K. The source of this surprising behaviour in 2D is the frustration of the natural bulk tetrahedral local geometry and the underlying very significant increase in entropy of the interfacial water molecules. PMID:27185018
2-D wavelet with position controlled resolution
NASA Astrophysics Data System (ADS)
Walczak, Andrzej; Puzio, Leszek
2005-09-01
Wavelet transformation localizes all irregularities in the scene. It is most effective in the case when intensities in the scene have no sharp details. It is the case often present in a medical imaging. To identify the shape one has to extract it from the scene as typical irregularity. When the scene does not contain sharp changes then common differential filters are not efficient tool for a shape extraction. The new 2-D wavelet for such task has been proposed. Described wavelet transform is axially symmetric and has varied scale in dependence on the distance from the centre of the wavelet symmetry. The analytical form of the wavelet has been presented as well as its application for details extraction in the scene. Most important feature of the wavelet transform is that it gives a multi-scale transformation, and if zoom is on the wavelet selectivity varies proportionally to the zoom step. As a result, the extracted shape does not change during zoom operation. What is more the wavelet selectivity can be fit to the local intensity gradient properly to obtain best extraction of the irregularities.
Some properties of correlations of quantum lattice systems in thermal equilibrium
Fröhlich, Jürg; Ueltschi, Daniel
2015-05-15
Simple proofs of uniqueness of the thermodynamic limit of KMS states and of the decay of equilibrium correlations are presented for a large class of quantum lattice systems at high temperatures. New quantum correlation inequalities for general Heisenberg models are described. Finally, a simplified derivation of a general result on power-law decay of correlations in 2D quantum lattice systems with continuous symmetries is given, extending results of McBryan and Spencer for the 2D classical XY model.
NASA Astrophysics Data System (ADS)
Nakata, Yosuke; Urade, Yoshiro; Nakanishi, Toshihiro; Miyamaru, Fumiaki; Takeda, Mitsuo Wada; Kitano, Masao
2016-04-01
We investigate the supersymmetry (SUSY) structures for inductor-capacitor circuit networks on a simple regular graph and its line graph. We show that their eigenspectra must coincide (except, possibly, for the highest eigenfrequency) due to SUSY, which is derived from the topological nature of the circuits. To observe this spectra correspondence in the high-frequency range, we study spoof plasmons on metallic hexagonal and kagomé lattices. The band correspondence between them is predicted by a simulation. Using terahertz time-domain spectroscopy, we demonstrate the band correspondence of fabricated metallic hexagonal and kagomé lattices.
Wang, An; Stout, C. David; Zhang, Qinghai; Johnson, Eric F.
2015-01-01
P450 2D6 contributes significantly to the metabolism of >15% of the 200 most marketed drugs. Open and closed crystal structures of P450 2D6 thioridazine complexes were obtained using different crystallization conditions. The protonated piperidine moiety of thioridazine forms a charge-stabilized hydrogen bond with Asp-301 in the active sites of both complexes. The more open conformation exhibits a second molecule of thioridazine bound in an expanded substrate access channel antechamber with its piperidine moiety forming a charge-stabilized hydrogen bond with Glu-222. Incubation of the crystalline open thioridazine complex with alternative ligands, prinomastat, quinidine, quinine, or ajmalicine, displaced both thioridazines. Quinine and ajmalicine formed charge-stabilized hydrogen bonds with Glu-216, whereas the protonated nitrogen of quinidine is equidistant from Asp-301 and Glu-216 with protonated nitrogen H-bonded to a water molecule in the access channel. Prinomastat is not ionized. Adaptations of active site side-chain rotamers and polypeptide conformations were evident between the complexes, with the binding of ajmalicine eliciting a closure of the open structure reflecting in part the inward movement of Glu-216 to form a hydrogen bond with ajmalicine as well as sparse lattice restraints that would hinder adaptations. These results indicate that P450 2D6 exhibits sufficient elasticity within the crystal lattice to allow the passage of compounds between the active site and bulk solvent and to adopt a more closed form that adapts for binding alternative ligands with different degrees of closure. These crystals provide a means to characterize substrate and inhibitor binding to the enzyme after replacement of thioridazine with alternative compounds. PMID:25555909
2-D Animation's Not Just for Mickey Mouse.
ERIC Educational Resources Information Center
Weinman, Lynda
1995-01-01
Discusses characteristics of two-dimensional (2-D) animation; highlights include character animation, painting issues, and motion graphics. Sidebars present Silicon Graphics animations tools and 2-D animation programs for the desktop computer. (DGM)
Holographic method for site-resolved detection of a 2D array of ultracold atoms
NASA Astrophysics Data System (ADS)
Hoffmann, Daniel Kai; Deissler, Benjamin; Limmer, Wolfgang; Hecker Denschlag, Johannes
2016-08-01
We propose a novel approach to site-resolved detection of a 2D gas of ultracold atoms in an optical lattice. A near-resonant laser beam is coherently scattered by the atomic array, and after passing a lens its interference pattern is holographically recorded by superimposing it with a reference laser beam on a CCD chip. Fourier transformation of the recorded intensity pattern reconstructs the atomic distribution in the lattice with single-site resolution. The holographic detection method requires only about two hundred scattered photons per atom in order to achieve a high reconstruction fidelity of 99.9 %. Therefore, additional cooling during detection might not be necessary even for light atomic elements such as lithium. Furthermore, first investigations suggest that small aberrations of the lens can be post-corrected in imaging processing.
MAZE96. Generates 2D Input for DYNA NIKE & TOPAZ
Sanford, L.; Hallquist, J.O.
1992-02-24
MAZE is an interactive program that serves as an input and two-dimensional mesh generator for DYNA2D, NIKE2D, TOPAZ2D, and CHEMICAL TOPAZ2D. MAZE also generates a basic template for ISLAND input. MAZE has been applied to the generation of input data to study the response of two-dimensional solids and structures undergoing finite deformations under a wide variety of large deformation transient dynamic and static problems and heat transfer analyses.
On 2D graphical representation of DNA sequence of nondegeneracy
NASA Astrophysics Data System (ADS)
Zhang, Yusen; Liao, Bo; Ding, Kequan
2005-08-01
Some two-dimensional (2D) graphical representations of DNA sequences have been given by Gates, Nandy, Leong and Mogenthaler, Randić, and Liao et al., which give visual characterizations of DNA sequences. In this Letter, we introduce a nondegeneracy 2D graphical representation of DNA sequence, which is different from Randić's novel 2D representation and Liao's 2D representation. We also present the nondegeneracy forms corresponding to the representations of Gates, Nandy, Leong and Mogenthaler.
Generates 2D Input for DYNA NIKE & TOPAZ
1996-07-15
MAZE is an interactive program that serves as an input and two-dimensional mesh generator for DYNA2D, NIKE2D, TOPAZ2D, and CHEMICAL TOPAZ2D. MAZE also generates a basic template for ISLAND input. MAZE has been applied to the generation of input data to study the response of two-dimensional solids and structures undergoing finite deformations under a wide variety of large deformation transient dynamic and static problems and heat transfer analyses.
Yao, Xiaoyan; Dong, Shuai
2016-01-01
The expanded classical Kitaev-Heisenberg model on a honeycomb lattice is investigated with the next-nearest-neighboring Heisenberg interaction considered. The simulation shows a rich phase diagram with periodic behavior in a wide parameter range. Beside the double 120° ordered phase, an inhomogeneous phase is uncovered to exhibit a topological triple-vortex lattice, corresponding to the hexagonal domain structure of vector chirality, which is stabilized by the mixed frustration of two sources: the geometrical frustration arising from the lattice structure as well as the frustration from the Kitaev couplings. PMID:27229486
NASA Astrophysics Data System (ADS)
Yao, Xiaoyan; Dong, Shuai
2016-05-01
The expanded classical Kitaev-Heisenberg model on a honeycomb lattice is investigated with the next-nearest-neighboring Heisenberg interaction considered. The simulation shows a rich phase diagram with periodic behavior in a wide parameter range. Beside the double 120° ordered phase, an inhomogeneous phase is uncovered to exhibit a topological triple-vortex lattice, corresponding to the hexagonal domain structure of vector chirality, which is stabilized by the mixed frustration of two sources: the geometrical frustration arising from the lattice structure as well as the frustration from the Kitaev couplings.
Introducing lattice strain to graphene encapsulated in hBN
NASA Astrophysics Data System (ADS)
Tomori, Hikari; Hiraide, Rineka; Ootuka, Youiti; Watanabe, Kenji; Taniguchi, Takashi; Kanda, Akinobu
Due to the characteristic lattice structure, lattice strain in graphene produces an effective gauge field. Theories tell that by controlling spatial variation of lattice strain, one can tailor the electronic state and transport properties of graphene. For example, under uniaxial local strain, graphene exhibits a transport gap at low energies, which is attractive for a graphene application to field effect devices. Here, we develop a method for encapsulating a strained graphene film in hexagonal boron-nitride (hBN). It is known that the graphene carrier mobility is significantly improved by the encapsulation of graphene in hBN, which has never been applied to strained graphene. We encapsulate graphene in hBN using the van der Waals assembly method. Strain is induced by sandwiching a graphene film between patterned hBN sheets. Spatial variation of strain is confirmed with micro Raman spectroscopy. Transport measurement of encapsulated strained graphene is in progress.
Vehicular motion in 2D city traffic network with signals controlled by phase shift
NASA Astrophysics Data System (ADS)
Komada, Kazuhito; Kojima, Kengo; Nagatani, Takashi
2011-03-01
We study the dynamic behavior of vehicular traffic through the series of traffic lights controlled by phase shift in two-dimensional (2D) city traffic network. The nonlinear-map model is presented for the vehicular traffic. The city traffic network is made of one-way perpendicular streets arranged in a square lattice with traffic signals where vertical streets are oriented upwards and horizontal streets are oriented rightwards. There are two traffic lights for the movement to north or that to east at each crossing. The traffic lights are controlled by the cycle time, split, and phase shift. The vehicle moves through the series of signals on a path selected by the driver. The city traffic with a heterogeneous density distribution is also studied. The dependence of the arrival time on cycle time, split, phase shift, selected path, and density is clarified for 2D city traffic. It is shown that the vehicular traffic is efficiently controlled by the phase shift.
Self-Assembly of Shaped Nanoparticles into Free-Standing 2D and 3D Superlattices.
Li, Weikun; Wang, Ke; Zhang, Peng; He, Jie; Xu, Shaoyi; Liao, Yonggui; Zhu, Jintao; Xie, Xiaolin; Nie, Zhihong
2016-01-27
This article describes a novel supramolecular assembly-mediated strategy for the organization of Au nanoparticles (NPs) with different shapes (e.g., spheres, rods, and cubes) into large-area, free-standing 2D and 3D superlattices. This robust approach involves two major steps: (i) the organization of polymer-tethered NPs within the assemblies of supramolecular comblike block copolymers (CBCPs), and (ii) the disassembly of the assembled CBCP structures to produce free-standing NP superlattices. It is demonstrated that the crystal structures and lattice constants of the superlattices can be readily tailored by varying the molecular weight of tethered polymers, the volume fraction of NPs, and the matrix of CBCPs. This template-free approach may open a new avenue for the assembly of NPs into 2D and 3D structures with a wide range of potential applications. PMID:26649814
Electrochemical fabrication of 2D and 3D nickel nanowires using porous anodic alumina templates
NASA Astrophysics Data System (ADS)
Mebed, A. M.; Abd-Elnaiem, Alaa M.; Al-Hosiny, Najm M.
2016-06-01
Mechanically stable nickel (Ni) nanowires array and nanowires network were synthesized by pulse electrochemical deposition using 2D and 3D porous anodic alumina (PAA) templates. The structures and morphologies of as-prepared films were characterized by X-ray diffraction and scanning electron microscopy, respectively. The grown Ni nanowire using 3D PAA revealed more strength and larger surface area than has grown Ni use 2D PAA template. The prepared nanowires have a face-centered cubic crystal structure with average grain size 15 nm, and the preferred orientation of the nucleation of the nanowires is (111). The diameter of the nanowires is about 50-70 nm with length 3 µm. The resulting 3D Ni nanowire lattice, which provides enhanced mechanical stability and an increased surface area, benefits energy storage and many other applications which utilize the large surface area.
2d PDE Linear Symmetric Matrix Solver
1983-10-01
ICCG2 (Incomplete Cholesky factorized Conjugate Gradient algorithm for 2d symmetric problems) was developed to solve a linear symmetric matrix system arising from a 9-point discretization of two-dimensional elliptic and parabolic partial differential equations found in plasma physics applications, such as resistive MHD, spatial diffusive transport, and phase space transport (Fokker-Planck equation) problems. These problems share the common feature of being stiff and requiring implicit solution techniques. When these parabolic or elliptic PDE''s are discretized withmore » finite-difference or finite-element methods,the resulting matrix system is frequently of block-tridiagonal form. To use ICCG2, the discretization of the two-dimensional partial differential equation and its boundary conditions must result in a block-tridiagonal supermatrix composed of elementary tridiagonal matrices. The incomplete Cholesky conjugate gradient algorithm is used to solve the linear symmetric matrix equation. Loops are arranged to vectorize on the Cray1 with the CFT compiler, wherever possible. Recursive loops, which cannot be vectorized, are written for optimum scalar speed. For matrices lacking symmetry, ILUCG2 should be used. Similar methods in three dimensions are available in ICCG3 and ILUCG3. A general source containing extensions and macros, which must be processed by a pre-compiler to obtain the standard FORTRAN source, is provided along with the standard FORTRAN source because it is believed to be more readable. The pre-compiler is not included, but pre-compilation may be performed by a text editor as described in the UCRL-88746 Preprint.« less
2d PDE Linear Asymmetric Matrix Solver
1983-10-01
ILUCG2 (Incomplete LU factorized Conjugate Gradient algorithm for 2d problems) was developed to solve a linear asymmetric matrix system arising from a 9-point discretization of two-dimensional elliptic and parabolic partial differential equations found in plasma physics applications, such as plasma diffusion, equilibria, and phase space transport (Fokker-Planck equation) problems. These equations share the common feature of being stiff and requiring implicit solution techniques. When these parabolic or elliptic PDE''s are discretized with finite-difference or finite-elementmore » methods, the resulting matrix system is frequently of block-tridiagonal form. To use ILUCG2, the discretization of the two-dimensional partial differential equation and its boundary conditions must result in a block-tridiagonal supermatrix composed of elementary tridiagonal matrices. A generalization of the incomplete Cholesky conjugate gradient algorithm is used to solve the matrix equation. Loops are arranged to vectorize on the Cray1 with the CFT compiler, wherever possible. Recursive loops, which cannot be vectorized, are written for optimum scalar speed. For problems having a symmetric matrix ICCG2 should be used since it runs up to four times faster and uses approximately 30% less storage. Similar methods in three dimensions are available in ICCG3 and ILUCG3. A general source, containing extensions and macros, which must be processed by a pre-compiler to obtain the standard FORTRAN source, is provided along with the standard FORTRAN source because it is believed to be more readable. The pre-compiler is not included, but pre-compilation may be performed by a text editor as described in the UCRL-88746 Preprint.« less
Ultrasonic 2D matrix PVDF transducer
NASA Astrophysics Data System (ADS)
Ptchelintsev, A.; Maev, R. Gr.
2000-05-01
During the past decade a substantial amount of work has been done in the area of ultrasonic imaging technology using 2D arrays. The main problems arising for the two-dimensional matrix transducers at megahertz frequencies are small size and huge count of the elements, high electrical impedance, low sensitivity, bad SNR and slower data acquisition rate. The major technological difficulty remains the high density of the interconnect. To solve these problems numerous approaches have been suggested. In the present work, a 24×24 elements (24 transmit+24 receive) matrix and a switching board were developed. The transducer consists of two 52 μm PVDF layers each representing a linear array of 24 elements placed one on the top of the other. Electrodes in these two layers are perpendicular and form the grid of 0.5×0.5 mm pitch. The layers are bonded together with the ground electrode being monolithic and located between the layers. The matrix is backed from the rear surface with an epoxy composition. During the emission, a linear element from the emitting layer generates a longitudinal wave pulse propagating inside the test object. Reflected pulses are picked-up by the receiving layer. During one transmit-receive cycle one transmit element and one receive element are selected by corresponding multiplexers. These crossed elements emulate a small element formed by their intersection. The present design presents the following advantages: minimizes number of active channels and density of the interconnect; reduces the electrical impedance of the element improving electrical matching; enables the transmit-receive mode; due to the efficient backing provides bandwidth and good time resolution; and, significantly reduces the electronics complexity. The matrix can not be used for the beam steering and focusing. Owing to this impossibility of focusing, the penetration depth is limited as well by the diffraction phenomena.
Detecting the BCS pairing amplitude via a sudden lattice ramp in a honeycomb lattice
NASA Astrophysics Data System (ADS)
Tiesinga, Eite; Nuske, Marlon; Mathey, Ludwig
2016-05-01
We determine the exact time evolution of an initial Bardeen-Cooper-Schrieffer (BCS) state of ultra-cold atoms in a hexagonal optical lattice. The dynamical evolution is triggered by ramping the lattice potential up, such that the interaction strength Uf is much larger than the hopping amplitude Jf. The quench initiates collective oscillations with frequency | Uf | /(2 π) in the momentum occupation numbers and imprints an oscillating phase with the same frequency on the order parameter Δ. The latter is not reproduced by treating the time evolution in mean-field theory. The momentum density-density or noise correlation functions oscillate at frequency | Uf | /(2 π) as well as its second harmonic. For a very deep lattice, with negligible tunneling energy, the oscillations of momentum occupation numbers are undamped. Non-zero tunneling after the quench leads to dephasing of the different momentum modes and a subsequent damping of the oscillations. This occurs even for a finite-temperature initial BCS state, but not for a non-interacting Fermi gas. We therefore propose to use this dephasing to detect a BCS state. Finally, we predict that the noise correlation functions in a honeycomb lattice will develop strong anti-correlations near the Dirac point. We acknowledge funding from the National Science Foundation.
NASA Astrophysics Data System (ADS)
Ciminelli, C.; Armenise, M. N.
2007-07-01
In this paper report on the design of a 2D PBG filter in polymeric material. The filter is a Fabry-Perot cavity having a self-sustained membrane configuration. A deep parametric analysis has been carried out for improving the performance, taking also into account the fabrication tolerances Best performance in terms of lateral confinement have been obtained in case of square lattice. As for materials, polystyrene shown best in terms of refractive index value, length of the photonic crystal structure and attenuation value in the band gap. The filter can be used either in sensing applications or in telecommunication field.
Canonical vs. micro-canonical sampling methods in a 2D Ising model
Kepner, J.
1990-12-01
Canonical and micro-canonical Monte Carlo algorithms were implemented on a 2D Ising model. Expressions for the internal energy, U, inverse temperature, Z, and specific heat, C, are given. These quantities were calculated over a range of temperature, lattice sizes, and time steps. Both algorithms accurately simulate the Ising model. To obtain greater than three decimal accuracy from the micro-canonical method requires that the more complicated expression for Z be used. The overall difference between the algorithms is small. The physics of the problem under study should be the deciding factor in determining which algorithm to use. 13 refs., 6 figs., 2 tabs.
Zhang, Zi-Xuan; Ding, Ni-Ni; Zhang, Wen-Hua; Chen, Jin-Xiang; Young, David J; Hor, T S Andy
2014-04-25
A 2D coordination polymer prepared with bulky diethylformamide solvates exhibits channels which allow dipyridyl bridging ligands to diffuse into the crystal lattice. The absorbed dipyridyls thread through the pores of one layer and substitute the surface diethylformamide molecules on the neighboring layers to stitch alternate layers to form flexible interpenetrated metal-orgaic frameworks. The threading process also results in exchange of the bulky diethylformamide solvates for aqua to minimize congestion and, more strikingly, forces the slippage of two-dimensional layers, while still maintaining crystallinity. PMID:24692130
Incommensurate lattice modulations in Potassium Vanadate
NASA Astrophysics Data System (ADS)
Chakoumakos, Bryan; Banerjee, Arnab; Mark, Lumsden; Cao, Huibo; Kim, Jong-Woo; Hoffman, Christina; Wang, Xiaoping
Potassium Vanadate (K2V3O8) is an S = 1/2 2D square lattice antiferromagnet that shows spin reorientation indicating a strong coupling between the magnetism and its dielectric properties with a promise of rich physics that promises multiferroicity. These tangible physical properties are strongly tied through a spin-lattice coupling to the underlying lattice and superlattice behavior. It has a superlattice (SL) onsetting below Tc = 115 K with an approximate [3 x 3 x 2] modulation. Here we present our recent experiments at TOPAZ beamline at SNS which for the first time proves conclusively that the lattice modulations are incommensurate, with an in-plane Q of 0.315. We will also show our attempts to refine the data using JANA which requires a redefinition of the lattice, as well as the temperature and Q dependence of the superlattice modulation measured using neutrons at HFIR and synchrotron x-rays at APS. Our results are not only relevant for the ongoing search of multifunctional behavior in K2V3O8 but also generally for the superlattice modulations observed in a large family of fresnoites. Work performed at ORNL and ANL is supported by U.S. Dept. of Energy, Office of Basic Energy Sciences and Office of User Facilities Division.
NASA Astrophysics Data System (ADS)
Joe, Andrew; Jauregui, Luis; High, Alex; Dibos, Alan; Gulpinar, Elgin; Pistunova, Kateryna; Park, Hongkun; Kim, Philip
, Luis A. Jauregui, Alex A. High, Alan Dibos, Elgin Gulpinar, Kateryna Pistunova, Hongkun Park, Philip Kim Harvard University, Physics Department -abstract- Single layer transition metal dichalcogenides (TMDC) are 2-dimensional (2D) semiconductors van der Waals (vdW) characterized by a direct optical bandgap in the visible wavelength (~2 eV). Characterization of the band alignment between TMDC and the barrier is important for the fabrication of tunneling devices. Here, we fabricate coupled quantum well (CQW) heterostructures made of 2D TMDCs with hexagonal Boron nitride (hBN) as an atomically thin barrier and gate dielectric and with top and bottom metal (or graphite) as gate electrodes. We observe a clear dependence of the photo-generated current with varying hBN thickness, electrode workfunctions, electric field, laser excitation power, excitation wavelength, and temperature. We will discuss the implication of photocurrent in relation to quantum transport process across the vdW interfaces.
Electrostatic 2D assembly of bionanoparticles on a cationic lipid monolayer.
NASA Astrophysics Data System (ADS)
Kewalramani, Sumit; Wang, Suntao; Fukuto, Masafumi; Yang, Lin; Niu, Zhongwei; Nguyen, Giang; Wang, Qian
2010-03-01
We present a grazing-incidence small-angle X-ray scattering (GISAXS) study on 2D assembly of cowpea mosaic virus (CPMV) under a mixed cationic-zwitterionic (DMTAP^+-DMPC) lipid monolayer at the air-water interface. The inter-particle and particle-lipid electrostatic interactions were varied by controlling the subphase pH and the membrane charge density. GISAXS data show that 2D crystals of CPMV are formed above a threshold membrane charge density and only in a narrow pH range just above CPMV's isoelectric point, where the charge on CPMV is expected to be weakly negative. The particle density for the 2D crystals is similar to that for the densest lattice plane in the 3D crystals of CPMV. The results show that the 2D crystallization is achieved in the part of the phase space where the electrostatic interactions are expected to maximize the adsorption of CPMV onto the lipid membrane. This electrostatics-based strategy for controlling interfacial nanoscale assembly should be generally applicable to other nanoparticles.
A Planar Quantum Transistor Based on 2D-2D Tunneling in Double Quantum Well Heterostructures
Baca, W.E.; Blount, M.A.; Hafich, M.J.; Lyo, S.K.; Moon, J.S.; Reno, J.L.; Simmons, J.A.; Wendt, J.R.
1998-12-14
We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate-control of two-dimensional -- two-dimensional (2D-2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally-defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch (EBASE) flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 microns can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I-V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets ({approximately} 21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain, and high-speed.
A lattice-Boltzmann scheme of the Navier-Stokes equations on a 3D cuboid lattice
NASA Astrophysics Data System (ADS)
Min, Haoda; Peng, Cheng; Wang, Lian-Ping
2015-11-01
The standard lattice-Boltzmann method (LBM) for fluid flow simulation is based on a square (in 2D) or cubic (in 3D) lattice grids. Recently, two new lattice Boltzmann schemes have been developed on a 2D rectangular grid using the MRT (multiple-relaxation-time) collision model, by adding a free parameter in the definition of moments or by extending the equilibrium moments. Here we developed a lattice Boltzmann model on 3D cuboid lattice, namely, a lattice grid with different grid lengths in different spatial directions. We designed our MRT-LBM model by matching the moment equations from the Chapman-Enskog expansion with the Navier-Stokes equations. The model guarantees correct hydrodynamics. A second-order term is added to the equilibrium moments in order to restore the isotropy of viscosity on a cuboid lattice. The form and the coefficients of the extended equilibrium moments are determined through an inverse design process. An additional benefit of the model is that the viscosity can be adjusted independent of the stress-moment relaxation parameter, thus improving the numerical stability of the model. The resulting cuboid MRT-LBM model is then validated through benchmark simulations using laminar channel flow, turbulent channel flow, and the 3D Taylor-Green vortex flow.
Enhanced Tunnel Spin Injection into Graphene using Chemical Vapor Deposited Hexagonal Boron Nitride
Kamalakar, M. Venkata; Dankert, André; Bergsten, Johan; Ive, Tommy; Dash, Saroj P.
2014-01-01
The van der Waals heterostructures of two-dimensional (2D) atomic crystals constitute a new paradigm in nanoscience. Hybrid devices of graphene with insulating 2D hexagonal boron nitride (h-BN) have emerged as promising nanoelectronic architectures through demonstrations of ultrahigh electron mobilities and charge-based tunnel transistors. Here, we expand the functional horizon of such 2D materials demonstrating the quantum tunneling of spin polarized electrons through atomic planes of CVD grown h-BN. We report excellent tunneling behavior of h-BN layers together with tunnel spin injection and transport in graphene using ferromagnet/h-BN contacts. Employing h-BN tunnel contacts, we observe enhancements in both spin signal amplitude and lifetime by an order of magnitude. We demonstrate spin transport and precession over micrometer-scale distances with spin lifetime up to 0.46 nanosecond. Our results and complementary magnetoresistance calculations illustrate that CVD h-BN tunnel barrier provides a reliable, reproducible and alternative approach to address the conductivity mismatch problem for spin injection into graphene. PMID:25156685
Enhanced tunnel spin injection into graphene using chemical vapor deposited hexagonal boron nitride.
Kamalakar, M Venkata; Dankert, André; Bergsten, Johan; Ive, Tommy; Dash, Saroj P
2014-01-01
The van der Waals heterostructures of two-dimensional (2D) atomic crystals constitute a new paradigm in nanoscience. Hybrid devices of graphene with insulating 2D hexagonal boron nitride (h-BN) have emerged as promising nanoelectronic architectures through demonstrations of ultrahigh electron mobilities and charge-based tunnel transistors. Here, we expand the functional horizon of such 2D materials demonstrating the quantum tunneling of spin polarized electrons through atomic planes of CVD grown h-BN. We report excellent tunneling behavior of h-BN layers together with tunnel spin injection and transport in graphene using ferromagnet/h-BN contacts. Employing h-BN tunnel contacts, we observe enhancements in both spin signal amplitude and lifetime by an order of magnitude. We demonstrate spin transport and precession over micrometer-scale distances with spin lifetime up to 0.46 nanosecond. Our results and complementary magnetoresistance calculations illustrate that CVD h-BN tunnel barrier provides a reliable, reproducible and alternative approach to address the conductivity mismatch problem for spin injection into graphene. PMID:25156685
Li, An-Ping; Park, Jewook; Lee, Jaekwang; Liu, Lei; Baddorf, Arthur P.; Gu, Gong; Yoon, Mina; Park, Changwon; Durand, Corentin P.; Sumpter, Bobby G.
2014-01-01
Two-dimensional (2D) interfaces between crystalline materials have been shown to generate unusual interfacial electronic states in complex oxides1-4. Recently, a onedimensional (1D) polar-on-nonpolar interface has been realized in hexagonal boron nitride (hBN) and graphene heterostructures 5-10, where a coherent 1D boundary is expected to possess peculiar electronic states dictated by edge states of graphene and the polarity of hBN 11-13. Here we present a combined scanning tunneling microscopy (STM) and firstprinciples theory study of the graphene-hBN boundary to provide a rare glimpse into the spatial and energetic distributions of the 1D boundary states in real-space. The interfaces studied here aremore » crystallographically coherent with sharp transitions from graphene zigzag edges to B (or N) terminated hBN atomic layers on a Cu foil substrate5. The revealed boundary states are about 0.6 eV below or above the Fermi energy depending on the termination of the hBN at the boundary, and are extended along but localized at the boundary with a lateral thickness of 2-3nm. These results suggest that unconventional physical effects similar to those observed at 2D interfaces can also exist in lower dimensions, opening a route for tuning of electronic properties at interfaces in 2D heterostructures.« less
Li, An-Ping; Park, Jewook; Lee, Jaekwang; Liu, Lei; Baddorf, Arthur P.; Gu, Gong; Yoon, Mina; Park, Changwon; Durand, Corentin P.; Sumpter, Bobby G.
2014-01-01
Two-dimensional (2D) interfaces between crystalline materials have been shown to generate unusual interfacial electronic states in complex oxides1-4. Recently, a onedimensional (1D) polar-on-nonpolar interface has been realized in hexagonal boron nitride (hBN) and graphene heterostructures 5-10, where a coherent 1D boundary is expected to possess peculiar electronic states dictated by edge states of graphene and the polarity of hBN 11-13. Here we present a combined scanning tunneling microscopy (STM) and firstprinciples theory study of the graphene-hBN boundary to provide a rare glimpse into the spatial and energetic distributions of the 1D boundary states in real-space. The interfaces studied here are crystallographically coherent with sharp transitions from graphene zigzag edges to B (or N) terminated hBN atomic layers on a Cu foil substrate5. The revealed boundary states are about 0.6 eV below or above the Fermi energy depending on the termination of the hBN at the boundary, and are extended along but localized at the boundary with a lateral thickness of 2-3nm. These results suggest that unconventional physical effects similar to those observed at 2D interfaces can also exist in lower dimensions, opening a route for tuning of electronic properties at interfaces in 2D heterostructures.
Correlated Electron Phenomena in 2D Materials
NASA Astrophysics Data System (ADS)
Lambert, Joseph G.
In this thesis, I present experimental results on coherent electron phenomena in layered two-dimensional materials: single layer graphene and van der Waals coupled 2D TiSe2. Graphene is a two-dimensional single-atom thick sheet of carbon atoms first derived from bulk graphite by the mechanical exfoliation technique in 2004. Low-energy charge carriers in graphene behave like massless Dirac fermions, and their density can be easily tuned between electron-rich and hole-rich quasiparticles with electrostatic gating techniques. The sharp interfaces between regions of different carrier densities form barriers with selective transmission, making them behave as partially reflecting mirrors. When two of these interfaces are set at a separation distance within the phase coherence length of the carriers, they form an electronic version of a Fabry-Perot cavity. I present measurements and analysis of multiple Fabry-Perot modes in graphene with parallel electrodes spaced a few hundred nanometers apart. Transition metal dichalcogenide (TMD) TiSe2 is part of the family of materials that coined the term "materials beyond graphene". It contains van der Waals coupled trilayer stacks of Se-Ti-Se. Many TMD materials exhibit a host of interesting correlated electronic phases. In particular, TiSe2 exhibits chiral charge density waves (CDW) below TCDW ˜ 200 K. Upon doping with copper, the CDW state gets suppressed with Cu concentration, and CuxTiSe2 becomes superconducting with critical temperature of T c = 4.15 K. There is still much debate over the mechanisms governing the coexistence of the two correlated electronic phases---CDW and superconductivity. I will present some of the first conductance spectroscopy measurements of proximity coupled superconductor-CDW systems. Measurements reveal a proximity-induced critical current at the Nb-TiSe2 interfaces, suggesting pair correlations in the pure TiSe2. The results indicate that superconducting order is present concurrently with CDW in
Bootstrapping the Three-Loop Hexagon
Dixon, Lance J.; Drummond, James M.; Henn, Johannes M.; /Humboldt U., Berlin /Santa Barbara, KITP
2011-11-08
We consider the hexagonal Wilson loop dual to the six-point MHV amplitude in planar N = 4 super Yang-Mills theory. We apply constraints from the operator product expansion in the near-collinear limit to the symbol of the remainder function at three loops. Using these constraints, and assuming a natural ansatz for the symbol's entries, we determine the symbol up to just two undetermined constants. In the multi-Regge limit, both constants drop out from the symbol, enabling us to make a non-trivial confirmation of the BFKL prediction for the leading-log approximation. This result provides a strong consistency check of both our ansatz for the symbol and the duality between Wilson loops and MHV amplitudes. Furthermore, we predict the form of the full three-loop remainder function in the multi-Regge limit, beyond the leading-log approximation, up to a few constants representing terms not detected by the symbol. Our results confirm an all-loop prediction for the real part of the remainder function in multi-Regge 3 {yields} 3 scattering. In the multi-Regge limit, our result for the remainder function can be expressed entirely in terms of classical polylogarithms. For generic six-point kinematics other functions are required.
Hyperbolic phonon polaritons in hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Dai, Siyuan
2015-03-01
Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [Science, 343, 1125-1129 (2014)]. Additionally, we carried out the modification of hyperbolic response in heterostructures comprised of a mononlayer graphene deposited on hBN. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the ``hyperlens'' for subdiffractional imaging and focusing using a slab of hBN.
Hexagonal Antiprismatic Metallacarborane Clusters for Hydrogen Storage
NASA Astrophysics Data System (ADS)
Berkdemir, Cüneyt; Lin, Ping; Sofo, Jorge
2011-03-01
We investigated the adsorption properties of molecular hydrogen attached to hexagonal antiprismatic metallacarborane clusters, RuNi C2 B10 H12 and Ru 2 C2 B10 H12 , using density functional theory. These clusters have been recently synthesized using the reduction-metallation (RedMet) approach and their structures have been resolved. The hydrogen molecules are sequentially attached to these clusters until the H2 binding energies fall below 0.2 eV, which is the minimum value of ideal H2 binding energy in the range of 0.2-0.4 eV/H2 for the practical vehicle applications. We included the van der Waals interactions between metallacarborane clusters and molecular hydrogens. We also evaluated the contribution of zero point vibrational energies to the H2 binding energy. The kinetic stability of these clusters before and after hydrogen adsorption is discussed by analyzing the energy gap. The results show that RuNi C2 B10 H12 and Ru 2 C2 B10 H12 clusters can bind up to 8.5 wt % and 9.8 wt % molecular hydrogen, respectively. These results suggest that these metallacarborane clusters are potential hydrogen storage materials to meet the targets of DOE for 2015.
Janse Van Rensburg, E.J.
1996-12-31
The geometry of polygonal knots in the cubic lattice may be used to define some knot invariants. One such invariant is the minimal edge number, which is the minimum number of edges necessary (and sufficient) to construct a lattice knot of given type. In addition, one may also define the minimal (unfolded) surface number, and the minimal (unfolded) boundary number; these are the minimum number of 2-cells necessary to construct an unfolded lattice Seifert surface of a given knot type in the lattice, and the minimum number of edges necessary in a lattice knot to guarantee the existence of an unfolded lattice Seifert surface. In addition, I derive some relations amongst these invariants. 8 refs., 5 figs., 2 tabs.
Gold nanorods grown on microgels leading to hexagonal nanostructures.
Kumar, V R Rajeev; Samal, A K; Sreeprasad, T S; Pradeep, T
2007-08-14
Hexagonal patterns of gold nanorods were made by assembling gold nanorod-coated poly(N-isopropyl acrylamide) microgels. The required population of nanorods on the microgels was achieved by attaching nanoparticle seeds on the latter and growing them to nanorods. The various materials prepared were characterized by UV-vis spectroscopy and transmission electron microscopy. Similar experiments with nanoparticle-coated or prefabricated nanorod-coated microgels did not give such hexagonal patterns. We suggest that the interlocking of nanorods leads to these regular structures. This is the first report of a solution phase method for assembling nanorods into a hexagonal pattern. PMID:17637011
Defect Chaos of Oscillating Hexagons in Rotating Convection
Echebarria, Blas; Riecke, Hermann
2000-05-22
Using coupled Ginzburg-Landau equations, the dynamics of hexagonal patterns with broken chiral symmetry are investigated, as they appear in rotating non-Boussinesq or surface-tension-driven convection. We find that close to the secondary Hopf bifurcation to oscillating hexagons the dynamics are well described by a single complex Ginzburg-Landau equation (CGLE) coupled to the phases of the hexagonal pattern. At the band center these equations reduce to the usual CGLE and the system exhibits defect chaos. Away from the band center a transition to a frozen vortex state is found. (c) 2000 The American Physical Society.
CYP2D7 Sequence Variation Interferes with TaqMan CYP2D6*15 and *35 Genotyping
Riffel, Amanda K.; Dehghani, Mehdi; Hartshorne, Toinette; Floyd, Kristen C.; Leeder, J. Steven; Rosenblatt, Kevin P.; Gaedigk, Andrea
2016-01-01
TaqMan™ genotyping assays are widely used to genotype CYP2D6, which encodes a major drug metabolizing enzyme. Assay design for CYP2D6 can be challenging owing to the presence of two pseudogenes, CYP2D7 and CYP2D8, structural and copy number variation and numerous single nucleotide polymorphisms (SNPs) some of which reflect the wild-type sequence of the CYP2D7 pseudogene. The aim of this study was to identify the mechanism causing false-positive CYP2D6*15 calls and remediate those by redesigning and validating alternative TaqMan genotype assays. Among 13,866 DNA samples genotyped by the CompanionDx® lab on the OpenArray platform, 70 samples were identified as heterozygotes for 137Tins, the key SNP of CYP2D6*15. However, only 15 samples were confirmed when tested with the Luminex xTAG CYP2D6 Kit and sequencing of CYP2D6-specific long range (XL)-PCR products. Genotype and gene resequencing of CYP2D6 and CYP2D7-specific XL-PCR products revealed a CC>GT dinucleotide SNP in exon 1 of CYP2D7 that reverts the sequence to CYP2D6 and allows a TaqMan assay PCR primer to bind. Because CYP2D7 also carries a Tins, a false-positive mutation signal is generated. This CYP2D7 SNP was also responsible for generating false-positive signals for rs769258 (CYP2D6*35) which is also located in exon 1. Although alternative CYP2D6*15 and *35 assays resolved the issue, we discovered a novel CYP2D6*15 subvariant in one sample that carries additional SNPs preventing detection with the alternate assay. The frequency of CYP2D6*15 was 0.1% in this ethnically diverse U.S. population sample. In addition, we also discovered linkage between the CYP2D7 CC>GT dinucleotide SNP and the 77G>A (rs28371696) SNP of CYP2D6*43. The frequency of this tentatively functional allele was 0.2%. Taken together, these findings emphasize that regardless of how careful genotyping assays are designed and evaluated before being commercially marketed, rare or unknown SNPs underneath primer and/or probe regions can impact
CYP2D7 Sequence Variation Interferes with TaqMan CYP2D6 (*) 15 and (*) 35 Genotyping.
Riffel, Amanda K; Dehghani, Mehdi; Hartshorne, Toinette; Floyd, Kristen C; Leeder, J Steven; Rosenblatt, Kevin P; Gaedigk, Andrea
2015-01-01
TaqMan™ genotyping assays are widely used to genotype CYP2D6, which encodes a major drug metabolizing enzyme. Assay design for CYP2D6 can be challenging owing to the presence of two pseudogenes, CYP2D7 and CYP2D8, structural and copy number variation and numerous single nucleotide polymorphisms (SNPs) some of which reflect the wild-type sequence of the CYP2D7 pseudogene. The aim of this study was to identify the mechanism causing false-positive CYP2D6 (*) 15 calls and remediate those by redesigning and validating alternative TaqMan genotype assays. Among 13,866 DNA samples genotyped by the CompanionDx® lab on the OpenArray platform, 70 samples were identified as heterozygotes for 137Tins, the key SNP of CYP2D6 (*) 15. However, only 15 samples were confirmed when tested with the Luminex xTAG CYP2D6 Kit and sequencing of CYP2D6-specific long range (XL)-PCR products. Genotype and gene resequencing of CYP2D6 and CYP2D7-specific XL-PCR products revealed a CC>GT dinucleotide SNP in exon 1 of CYP2D7 that reverts the sequence to CYP2D6 and allows a TaqMan assay PCR primer to bind. Because CYP2D7 also carries a Tins, a false-positive mutation signal is generated. This CYP2D7 SNP was also responsible for generating false-positive signals for rs769258 (CYP2D6 (*) 35) which is also located in exon 1. Although alternative CYP2D6 (*) 15 and (*) 35 assays resolved the issue, we discovered a novel CYP2D6 (*) 15 subvariant in one sample that carries additional SNPs preventing detection with the alternate assay. The frequency of CYP2D6 (*) 15 was 0.1% in this ethnically diverse U.S. population sample. In addition, we also discovered linkage between the CYP2D7 CC>GT dinucleotide SNP and the 77G>A (rs28371696) SNP of CYP2D6 (*) 43. The frequency of this tentatively functional allele was 0.2%. Taken together, these findings emphasize that regardless of how careful genotyping assays are designed and evaluated before being commercially marketed, rare or unknown SNPs underneath primer
Hexagonal plaquette spin-spin interactions and quantum magnetism in a two-dimensional ion crystal
NASA Astrophysics Data System (ADS)
Nath, R.; Dalmonte, M.; Glaetzle, A. W.; Zoller, P.; Schmidt-Kaler, F.; Gerritsma, R.
2015-06-01
We propose a trapped ion scheme en route to realize spin Hamiltonians on a Kagome lattice which, at low energies, are described by emergent {{{Z}}}2 gauge fields, and support a topological quantum spin liquid ground state. The enabling element in our scheme is the hexagonal plaquette spin-spin interactions in a two-dimensional ion crystal. For this, the phonon-mode spectrum of the crystal is engineered by standing-wave optical potentials or by using Rydberg excited ions, thus generating localized phonon-modes around a hexagon of ions selected out of the entire two-dimensional crystal. These tailored modes can mediate spin-spin interactions between ion-qubits on a hexagonal plaquette when subject to state-dependent optical dipole forces. We discuss how these interactions can be employed to emulate a generalized Balents-Fisher-Girvin model in minimal instances of one and two plaquettes. This model is an archetypical Hamiltonian in which gauge fields are the emergent degrees of freedom on top of the classical ground state manifold. Under realistic situations, we show the emergence of a discrete Gauss’s law as well as the dynamics of a deconfined charge excitation on a gauge-invariant background using the two-plaquettes trapped ions spin-system. The proposed scheme in principle allows further scaling in a future trapped ion quantum simulator, and we conclude that our work will pave the way towards the simulation of emergent gauge theories and quantum spin liquids in trapped ion systems.
Growth and characterization of Cl-doped ZnO hexagonal nanodisks
Yousefi, Ramin; Zak, A.K.; Mahmoudian, M.R.
2011-10-15
Cl-doped ZnO nanodisks were grown on a Si(111) substrate using a thermal evaporation method. The prepared nanodisks exhibited a hexagonal shape with an average thickness of 50 nm and average diagonal of 270 nm. In addition, undoped ZnO disks with hexagonal shape were grown under the same conditions, but the sizes of these undoped ZnO disks were on the micrometer order. A possible mechanism was proposed for the growth of the Cl-doped ZnO nanodisks, and it was shown that the Cl{sup 1-} anions play a crucial role in controlling the size. X-ray diffraction and Raman spectroscopy clearly showed an extension in the crystal lattice of ZnO because of the presence of chlorine. In addition, these nanodisks produced a strong photoluminescence emission peak in the ultraviolet (UV) region and a weak peak in the green region of the electromagnetic spectrum. Furthermore, the UV peak of the Cl-doped ZnO nanodisks was blueshifted with respect to that of the undoped ZnO disks. - Graphical abstract: Cl-doped ZnO nanodisks and undoped ZnO microdisks have been grown using a thermal evaporation method. Highlights: > Cl-doped ZnO nanodisks with hexagonal shape are grown using a thermal evaporation method. > It is shown that the Cl{sup 1-} anions play a crucial role in controlling the size of the nanodisks. > XRD and Raman results showed that the Cl-doped ZnO nanodisks structure is under a biaxial stress. > UV peak of the PL spectrum is blueshifted for the Cl-doped ZnO nanodisks.
Synthesis and oxygen content dependent properties of hexagonal DyMnO{sub 3 + sub delta}.
Remsen, S.; Dabrowski, B.; Chmaissem, O.; Mais, J.; Szewczyk, A.
2011-07-01
Oxygen deficient polycrystalline samples of hexagonal P6{sub 3}cm (space group No.185) DyMnO{sub 3+{delta}} ({delta} < 0) were synthesized in Ar by intentional decomposition of its perovskite phase obtained in air. The relative stability of these phases is in accord with our previous studies of the temperature and oxygen vacancy dependent tolerance factor. Thermogravimetric measurements have shown that hexagonal samples of DyMnO{sub 3+{delta}} (0 {<=} {delta} {<=} 0.4) exhibit unusually large excess oxygen content, which readily incorporates on heating near 300 C in various partial-pressures of oxygen atmospheres. Neutron and synchrotron diffraction data show the presence of two new structural phases at {delta} {approx} 0.25 (Hex{sub 2}) and {delta} {approx} 0.40 (Hex{sub 3}). Rietveld refinements of the Hex{sub 2} phase strongly suggest it is well modeled by the R3 space group (No.146). These phases were observed to transform back to P6{sub 3}cm above {approx}350 C when material becomes stoichiometric in oxygen content ({delta} = 0). Chemical expansion of the crystal lattice corresponding to these large changes of oxygen was found to be 3.48 x 10{sup -2} mol{sup -1}. Thermal expansion of stoichiometric phases were determined to be 11.6 x 10{sup -6} and 2.1 x 10{sup -6} K{sup -1} for the P6{sub 3}cm and Hex{sub 2} phases, respectively. Our measurements also indicate that the oxygen non-stoichiometry of hexagonal RMnO{sub 3+{delta}} materials may have important influence on their multiferroic properties.
Synthesis and oxygen content dependent properties of hexagonal DyMnO[subscript 3+delta
Remsen, S.; Dabrowski, B.; Chmaissem, O.; Mais, J.; Szewczyk, A.
2011-10-28
Oxygen deficient polycrystalline samples of hexagonal P6{sub 3}cm (space group No.185) DyMnO{sub 3+{delta}} ({delta} < 0) were synthesized in Ar by intentional decomposition of its perovskite phase obtained in air. The relative stability of these phases is in accord with our previous studies of the temperature and oxygen vacancy dependent tolerance factor. Thermogravimetric measurements have shown that hexagonal samples of DyMnO{sub 3+{delta}} (0 {le} {delta} {le} 0.4) exhibit unusually large excess oxygen content, which readily incorporates on heating near 300 C in various partial-pressures of oxygen atmospheres. Neutron and synchrotron diffraction data show the presence of two new structural phases at {delta} {approx} 0.25 (Hex{sub 2}) and {delta} {approx} 0.40 (Hex{sub 3}). Rietveld refinements of the Hex{sub 2} phase strongly suggest it is well modeled by the R3 space group (No.146). These phases were observed to transform back to P6{sub 3}cm above {approx} 350 C when material becomes stoichiometric in oxygen content ({delta} = 0). Chemical expansion of the crystal lattice corresponding to these large changes of oxygen was found to be 3.48 x 10{sup -2} mol{sup -1}. Thermal expansion of stoichiometric phases were determined to be 11.6 x 10{sup -6} and 2.1 x 10{sup -6} K{sup -1} for the P6{sub 3}cm and Hex{sub 2} phases, respectively. Our measurements also indicate that the oxygen non-stoichiometry of hexagonal RMnO{sub 3+{delta}} materials may have important influence on their multiferroic properties.
Loading an Rb-87 MOT directly into a variable-period accordion lattice
NASA Astrophysics Data System (ADS)
Huckans, John
2015-05-01
We report on our progress toward loading an Rb-87 three-dimensional magneto-optical trap (3D MOT) directly into a two-dimensional variable-period optical lattice (2D accordion lattice). Preliminary calculations suggest the feasibility of achieving an approximate 102 increase in phase space density by combining gray-molasses-type cooling techniques with spatial density compression of a 3D MOT with an accordion lattice.
Structure and decompression melting of a novel, high-pressure nanoconfined 2-D ice.
Wang, Jianwei; Kalinichev, Andrey G; Kirkpatrick, R James
2005-08-01
Molecular dynamics (MD) simulations of water confined in nanospaces between layers of talc (system composition Mg(3)Si(4)O(10)(OH)(2) + 2H(2)O) at 300 K and pressures of approximately 0.45 GPa show the presence of a novel 2-D ice structure, and the simulation results at lower pressures provide insight into the mechanisms of its decompression melting. Talc is hydrophobic at ambient pressure and temperature, but weak hydrogen bonding between the talc surface and the water molecules plays an important role in stabilizing the hydrated structure at high pressure. The simulation results suggest that experimentally accessible elevated pressures may cause formation of a wide range of previously unknown water structures in nanoconfinement. In the talc 2-D ice, each water molecule is coordinated by six O(b) atoms of one basal siloxane sheet and three water molecules. The water molecules are arranged in a buckled hexagonal array in the a-b crystallographic plane with two sublayers along [001]. Each H(2)O molecule has four H-bonds, accepting one from the talc OH group and one from another water molecule and donating one to an O(b) and one to another water molecule. In plan view, the molecules are arranged in six-member rings reflecting the substrate talc structure. Decompression melting occurs by migration of water molecules to interstitial sites in the centers of six-member rings and eventual formation of separate empty and water-filled regions. PMID:16852798
Reconstruction of vortex lattices at low inductions
Sudbro, A.; Brandt, E.H. Max-Planck-Institut fuer Metallforschung, Institut fuer Physik, Heisenbergstrasse 1, D-7000 Stuttgart 80 )
1992-03-16
The nonlocal elasticity and the normal modes of an unpinned flux-line lattice (FLL) in a uniaxially anisotropic supeconductor are considered in the low-induction regime {ital b}={ital B}/{ital B}{sub {ital c}2}{much lt}1 at oblique angles {theta} between {bold B} and the {bold {cflx c}} axis. A novel feature of anisotropic superconductors is {ital strongly} {ital dispersive} {ital shear} {ital moduli} when {theta}{gt}0. Thus, the normal modes of the FLL may become soft away from the zone center, signaling a {bold k}{ne}{bold 0} {ital structural} instability of the distorted hexagonal FLL. Vortex structures of a novel type are thus possible in anisotropic superconductors.
Landau Levels in Strained Optical Lattices
NASA Astrophysics Data System (ADS)
Tian, Binbin; Endres, Manuel; Pekker, David
2015-12-01
We propose a hexagonal optical lattice system with spatial variations in the hopping matrix elements. Just like in the valley Hall effect in strained graphene, for atoms near the Dirac points the variations in the hopping matrix elements can be described by a pseudomagnetic field and result in the formation of Landau levels. We show that the pseudomagnetic field leads to measurable experimental signatures in momentum resolved Bragg spectroscopy, Bloch oscillations, cyclotron motion, and quantization of in situ densities. Our proposal can be realized by a slight modification of existing experiments. In contrast to previous methods, pseudomagnetic fields are realized in a completely static system avoiding common heating effects and therefore opening the door to studying interaction effects in Landau levels with cold atoms.
Reported and predicted structures of Ba(Co,Nb)(1-δ)O₃ hexagonal perovskite phases.
Bradley, Kathryn A; Collins, Christopher; Dyer, Matthew S; Claridge, John B; Darling, George R; Rosseinsky, Matthew J
2014-10-21
The Extended Module Materials Assembly computational method for structure solution and prediction has been implemented for close-packed lattices. Exploring the family of B-site deficient materials in hexagonal perovskite barium cobalt niobates, it is found that the EMMA procedure returns the experimental structures as the most stable for the known compositions of Ba3CoNb2O9, Ba5Nb4O15 and Ba8CoNb6O24. The unknown compositions Ba11Co2Nb8O33 and Ba13CoNb10O39, having longer stacking sequences, are predicted to form as intergrowths of Ba3CoNb2O9 and Ba5Nb4O15, and are found to have similar stability to pure Ba3CoNb2O9 and Ba5Nb4O15, indicating that it is likely they can be synthesised. PMID:24871400
Monte Carlo study of the hetero-polytypical growth of cubic on hexagonal silicon carbide polytypes
NASA Astrophysics Data System (ADS)
Camarda, Massimo
2012-08-01
In this article we use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on misoriented hexagonal (4H and 6H) substrates. We analyze the quality of the 3C-SiC film varying the polytype, the miscut angle and the initial surface morphology of the substrate. We find that the use of 6H misoriented (4°-10° off) substrates, with step bunched surfaces, can strongly improve the quality of the cubic epitaxial film whereas the 3C/4H growth is affected by the generation of dislocations, due to the incommensurable periodicity of the 3C (3) and the 4H (4) polytypes. For these reasons, a proper pre-growth treatment of 6H misoriented substrates can be the key for the growth of high quality, twin free, 3C-SiC films.
Metal organic vapor phase epitaxy of hexagonal Ge-Sb-Te (GST)
NASA Astrophysics Data System (ADS)
Schuck, Martin; Rieß, Sally; Schreiber, Marcel; Mussler, Gregor; Grützmacher, Detlev; Hardtdegen, Hilde
2015-06-01
Epitaxial, hexagonal Ge-Sb-Te was grown on Si(111) substrates by Metal Organic Vapor Phase Epitaxy (MOVPE) using the precursor digermane. The effect of reactor pressure, growth temperature and in situ pre-treatment on morphology and Ge-Sb-Te composition was studied. The composition is sensitive to reactor pressure and growth temperature. Compositional control is achieved at a reactor pressure of 50 hPa. Substrate pre-treatment affects film coalescence. The use of hydrogen and a suitable precursor pre-treatment leads to enhanced surface coverage. X-ray diffraction reveals a trigonal structure with lattice parameters close to that reported for Ge1Sb2Te4 crystallizing in the R 3 bar m phase. The composition was confirmed by energy-dispersive X-ray spectroscopy.
Preparation and characterization of hexagonal MnTe and ZnO layers
NASA Astrophysics Data System (ADS)
Przedziecka, E.; Kamiska, E.; Dynowska, E.; Butkut, R.; Dobrowolski, W.; Kpa, H.; Jakiela, R.; Aleszkiewicz, M.; Usakowska, E.; Janik, E.; Kossut, J.
2005-02-01
MnTe layers of high crystalline quality were grown on Al2O3 substrates (0001)-oriented by molecular beam epitaxy (MBE). Characterization of MnTe by X-ray diffraction (XRD) revealed a hexagonal structure of NiAs-type with lattice parameters a = 4.166 Å and c = 6.694 Å. The energy gap of MnTe, evaluated from the optical transmission spectra measured at 10 K, was found to be 1.7 eV. The Néel temperature obtained from neutron diffraction (ND) measurements was 284.1 K. ZnO:N layers were fabricated by thermal oxidation of metallic Zn and ZnTe grown by MBE on different substrates. In order to achieve p-type conductivity, the starting materials were doped by nitrogen. The XRD spectra of the oxidized samples showed peaks related to hexagonal ZnO. The Hall measurements demonstrated p-type conductivity with the hole concentration of 2.2 × 1019 cm-3 and 6.7 × 1017 cm^{-3} for ZnO:N deposited on GaAs and ZnTe, respectively.
Structural, electronic and optical properties of hexagonal TaN compound
NASA Astrophysics Data System (ADS)
Chen, Zhongjun; Yan, Jungan; Kuang, Zhong; Chen, Taihong; Li, Dehua
2016-01-01
Structural and electronic properties of hexagonal Tantalum nitride (TaN) in CoSn and WC structures are studied using the first-principle calculations. Lattice constants and electronic band structures are in an excellent agreement with the available experimental and other theoretical values. TaN in both structures studied has a metallic nature and a strong hybridization of Ta 5d and N 2p are found from the spin density of states (DOS). Meanwhile, our LSDA+U calculations predicted a strong ferromagnetic state for CoSn-type structure and an obvious paramagnetic nature for WC-type structure. No phase transition are observed within cubic and hexagonal CoSn and WC structures under high pressures. Our results show WC-type TaN is the calculated ground-state structure among the three crystallographic structures studied under 120 GPa. Optical properties show that TaN in CoSn-type structure is a better dielectric material.
Broadband optical amplification with water-free hexagonal double-clad Bi doped silica fiber
NASA Astrophysics Data System (ADS)
Kobayashi, S.; Takahashi, M.; Ohara, M.; Kondo, I.; Fujii, Yusuke
2016-03-01
1.3 - 1.55 micron optical amplifiers for the long distance up-stream and down-stream networks for a future increase of fiber access networks in telecommunications are attractive. A bismuth-doped silica glass has a potential of the broadband spectrum as lasers and amplifier applications at 1.3 -1.55 micron. The bismuth-doped fiber lasers and amplifiers were discussed by the MOCVD method. In this report optical amplification characteristics at 1.3 - 1.55 micron are presented with the water free hexagonal double-clad bismuth-doped silica fiber (HDC-BDF) made by the vertical axial vapor-phase deposition (VAD) method. The bismuth and aluminum ions were vapor-phase doped into the silicon and germanium oxide. Pumping into the HDC-BDF was performed by using the tilt-polished fiber from the hexagonal surface with the multimode fiber pigtail of the pumping LD. 2 dB amplified gain was obtained with less than -40 dBm CW input signal power at 1310nm.
Tunneling measurements in graphene-hexagonal boron nitride-based heterostructures
NASA Astrophysics Data System (ADS)
Chandni, U.; Watanabe, K.; Taniguchi, T.; Eisenstein, J. P.
2015-03-01
Van der Waals heterostructures is an emerging field involving the study of layered materials consisting of various crystalline atomic planes exfoliated from bulk crystals and then stacked, often by hand, in custom-made patterns. Vertical tunneling structures made out of such quasi-2D crystals are potentially very interesting and may provide a new playground to observe electron-electron interaction effects in graphene and related materials. In the present work, we report the fabrication and study of several such tunnel junctions, including metal-hexagonal boron nitride (hBN)-metal, metal-hBN-graphite and metal-hBN-graphene devices. Tunneling measurements done at low temperatures and high magnetic fields reveal interesting and distinct features in each of these designs. We acknowledge funding provided by the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through Grant GBMF1250.
Synthesis of Low-Density, Carbon-Doped, Porous Hexagonal Boron Nitride Solids.
Gautam, Chandkiram; Tiwary, Chandra Sekhar; Jose, Sujin; Brunetto, Gustavo; Ozden, Sehmus; Vinod, Soumya; Raghavan, Prasanth; Biradar, Santoshkumar; Galvao, Douglas Soares; Ajayan, Pulickel M
2015-12-22
Here, we report the scalable synthesis and characterization of low-density, porous, three-dimensional (3D) solids consisting of two-dimensional (2D) hexagonal boron nitride (h-BN) sheets. The structures are synthesized using bottom-up, low-temperature (∼300 °C), solid-state reaction of melamine and boric acid giving rise to porous and mechanically stable interconnected h-BN layers. A layered 3D structure forms due to the formation of h-BN, and significant improvements in the mechanical properties were observed over a range of temperatures, compared to graphene oxide or reduced graphene oxide foams. A theoretical model based on Density Functional Theory (DFT) is proposed for the formation of h-BN architectures. The material shows excellent, recyclable absorption capacity for oils and organic solvents. PMID:26580810
Controlling Catalyst Bulk Reservoir Effects for Monolayer Hexagonal Boron Nitride CVD
2016-01-01
Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This “pre-filling” of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process, which is also relevant to other 2D materials and their heterostructures. PMID:26756610
Hwan Lee, Seung; Lee, Jia; Ho Ra, Chang; Liu, Xiaochi; Hwang, Euyheon; Sup Choi, Min; Hee Choi, Jun; Zhong, Jianqiang; Chen, Wei; Jong Yoo, Won
2014-02-03
A tunneling rectifier prepared from vertically stacked two-dimensional (2D) materials composed of chemically doped graphene electrodes and hexagonal boron nitride (h-BN) tunneling barrier was demonstrated. The asymmetric chemical doping to graphene with linear dispersion property induces rectifying behavior effectively, by facilitating Fowler-Nordheim tunneling at high forward biases. It results in excellent diode performances of a hetero-structured graphene/h-BN/graphene tunneling diode, with an asymmetric factor exceeding 1000, a nonlinearity of ∼40, and a peak sensitivity of ∼12 V{sup −1}, which are superior to contending metal-insulator-metal diodes, showing great potential for future flexible and transparent electronic devices.
Controlling Catalyst Bulk Reservoir Effects for Monolayer Hexagonal Boron Nitride CVD.
Caneva, Sabina; Weatherup, Robert S; Bayer, Bernhard C; Blume, Raoul; Cabrero-Vilatela, Andrea; Braeuninger-Weimer, Philipp; Martin, Marie-Blandine; Wang, Ruizhi; Baehtz, Carsten; Schloegl, Robert; Meyer, Jannik C; Hofmann, Stephan
2016-02-10
Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This "pre-filling" of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process, which is also relevant to other 2D materials and their heterostructures. PMID:26756610
NASA Astrophysics Data System (ADS)
Salmaoui, Samiha; Sediri, Faouzi; Gharbi, Néji; Perruchot, Christian; Aeiyach, Salah; Rutkowska, Iwona A.; Kulesza, Pawel J.; Jouini, Mohamed
2011-07-01
Tungsten trioxide, unhydrated with hexagonal structure (h-WO 3), has been prepared by hydrothermal method at a temperature of 180 °C in acidified sodium tungstate solution. Thus prepared h-WO 3 has been characterized by X-ray diffraction (XRD) method and using electrochemical techniques. The morphology has been examined by scanning and transmission electron microscopies (SEM and TEM) and it is consistent with existence of nanorods of 50-70 nm diameter and up to 5 μm length. Cyclic voltammetric characterization of thin films of h-WO 3 nanorods has revealed reversible redox behaviour with charge-discharge cycling corresponding to the reversible lithium intercalation/deintercalation into the crystal lattice of the h-WO 3 nanorods. In propylene carbonate containing LiClO 4, two successive redox processes of hexagonal WO 3 nanorods are observed at the scan rate of 50 mV/s. Such behaviour shall be attributed to the presence of at least two W atoms of different surroundings in the lattice structure of h-WO 3 nanorods. On the other hand, in aqueous LiClO 4 solution, only one redox process is observed at the scan rate of 10 mV/s. The above observations can be explained in terms of differences in the diffusion of ions inside two types of channel cavities existing in the structure of the h-WO 3 nanorods. Moreover, the material can be applied as active support for the catalytic bi-metallic Pt-Ru nanoparticles during electrooxidation of ethanol in acid medium (0.5 mol dm -3 H 2SO 4).
Mandal, R.; Barman, S.; Saha, S.; Barman, A.; Otani, Y.
2015-08-07
Ferromagnetic antidot lattices are important systems for magnetic data storage and magnonic devices, and understanding their magnetization dynamics by varying their structural parameters is an important problems in magnetism. Here, we investigate the variation in spin wave spectrum in two-dimensional nanoscale Ni{sub 80}Fe{sub 20} antidot lattices with lattice symmetry. By varying the bias magnetic field values in a broadband ferromagnetic resonance spectrometer, we observed a stark variation in the spin wave spectrum with the variation of lattice symmetry. The simulated mode profiles showed further difference in the spatial nature of the modes between different lattices. While for square and rectangular lattices extended modes are observed in addition to standing spin wave modes, all modes in the hexagonal, honeycomb, and octagonal lattices are either localized or standing waves. In addition, the honeycomb and octagonal lattices showed two different types of modes confined within the honeycomb (octagonal) units and between two such consecutive units. Simulated internal magnetic fields confirm the origin of such a wide variation in the frequency and spatial nature of the spin wave modes. The tunability of spin waves with the variation of lattice symmetry is important for the design of future magnetic data storage and magnonic devices.
Stability of hexagonal patterns in Benard-Marangoni convection
Echebarria, B.; Perez-Garcia, C.
2001-06-01
Hexagonal patterns in Benard-Marangoni (BM) convection are studied within the framework of amplitude equations. Near threshold they can be described with Ginzburg-Landau equations that include spatial quadratic terms. The planform selection problem between hexagons and rolls is investigated by explicitly calculating the coefficients of the Ginzburg-Landau equations in terms of the parameters of the fluid. The results are compared with previous studies and with recent experiments. In particular, steady hexagons that arise near onset can become unstable as a result of long-wave instabilities. Within weakly nonlinear theory, a two-dimensional phase equation for long-wave perturbations is derived. This equation allows us to find stability regions for hexagon patterns in BM convection.
Differential CYP 2D6 Metabolism Alters Primaquine Pharmacokinetics
Potter, Brittney M. J.; Xie, Lisa H.; Vuong, Chau; Zhang, Jing; Zhang, Ping; Duan, Dehui; Luong, Thu-Lan T.; Bandara Herath, H. M. T.; Dhammika Nanayakkara, N. P.; Tekwani, Babu L.; Walker, Larry A.; Nolan, Christina K.; Sciotti, Richard J.; Zottig, Victor E.; Smith, Philip L.; Paris, Robert M.; Read, Lisa T.; Li, Qigui; Pybus, Brandon S.; Sousa, Jason C.; Reichard, Gregory A.
2015-01-01
Primaquine (PQ) metabolism by the cytochrome P450 (CYP) 2D family of enzymes is required for antimalarial activity in both humans (2D6) and mice (2D). Human CYP 2D6 is highly polymorphic, and decreased CYP 2D6 enzyme activity has been linked to decreased PQ antimalarial activity. Despite the importance of CYP 2D metabolism in PQ efficacy, the exact role that these enzymes play in PQ metabolism and pharmacokinetics has not been extensively studied in vivo. In this study, a series of PQ pharmacokinetic experiments were conducted in mice with differential CYP 2D metabolism characteristics, including wild-type (WT), CYP 2D knockout (KO), and humanized CYP 2D6 (KO/knock-in [KO/KI]) mice. Plasma and liver pharmacokinetic profiles from a single PQ dose (20 mg/kg of body weight) differed significantly among the strains for PQ and carboxy-PQ. Additionally, due to the suspected role of phenolic metabolites in PQ efficacy, these were probed using reference standards. Levels of phenolic metabolites were highest in mice capable of metabolizing CYP 2D6 substrates (WT and KO/KI 2D6 mice). PQ phenolic metabolites were present in different quantities in the two strains, illustrating species-specific differences in PQ metabolism between the human and mouse enzymes. Taking the data together, this report furthers understanding of PQ pharmacokinetics in the context of differential CYP 2D metabolism and has important implications for PQ administration in humans with different levels of CYP 2D6 enzyme activity. PMID:25645856
Optoelectronic properties of hexagonal boron nitride epilayers
NASA Astrophysics Data System (ADS)
Cao, X. K.; Majety, S.; Li, J.; Lin, J. Y.; Jiang, H. X.
2013-01-01
This paper summarizes recent progress primarily achieved in authors' laboratory on synthesizing hexagonal boron nitride (hBN) epilayers by metal organic chemical vapor deposition (MCVD) and studies of their structural and optoelectronic properties. The structural and optical properties of hBN epilayers have been characterized by x-ray diffraction (XRD) and photoluminescence (PL) studies and compared to the better understood wurtzite AIN epilayers with a comparable energy bandgap. These MOCVD grown hBN epilayers exhibit highly efficient band-edge PL emission lines centered at around 5.5 eVat room temperature. The band-edge emission of hBN is two orders of magnitude higher than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hEN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (Eemi⊥c) is about 1.7 times stronger than the component along the c-axis (Eemillc). This is in contrast to AIN, in which the band edge emission is known to be polarized along the c-axis, (Eemillc). Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7x105 cm-1, which is more than 3 times higher than the value for AlN (~2x105 cm-1 . The hBN epilayer based photodetectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge PL emission peak and virtually no responses in the long wavelengths. The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.5 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate.
NASA Astrophysics Data System (ADS)
Wen, Yao; Shang, Xunzhong; Dong, Ji; Xu, Kai; He, Jun; Jiang, Chao
2015-07-01
Atomically thin hexagonal boron nitride (h-BN) has been demonstrated to be an excellent dielectric layer as well as an ideal van der Waals epitaxial substrate for fabrication of two-dimensional (2D) atomic layers and their vertical heterostructures. Although many groups have obtained large-scale monolayer h-BN through low pressure chemical vapor deposition (LPCVD), it is still a challenge to grow clean monolayers without the reduction of domain size. Here we report the synthesis of large-area (4 × 2 cm2) high quality monolayer h-BN with an ultraclean and unbroken surface on copper foil by using LPCVD. A detailed investigation of the key factors affecting growth and transfer of the monolayer was carried out in order to eliminate the adverse effects of impurity particles. Furthermore, an optimized transfer approach allowed the nondestructive and clean transfer of the monolayer from copper foil onto an arbitrary substrate, including a flexible substrate, under mild conditions. Atomic force microscopy indicated that the root-mean-square (RMS) roughness of the monolayer h-BN on SiO2 was less than 0.269 nm for areas with fewer wrinkles. Selective area electron diffraction analysis of the h-BN revealed a pattern of hexagonal diffraction spots, which unambiguously demonstrated its highly crystalline character. Our work paves the way toward the use of ultraclean and large-area monolayer h-BN as the dielectric layer in the fabrication of high performance electronic and optoelectronic devices for novel 2D atomic layer materials.
2D to 3D to 2D Dimensionality Crossovers in Thin BSCCO Films
NASA Astrophysics Data System (ADS)
Williams, Gary A.
2003-03-01
With increasing temperature the superfluid fraction in very thin BSCCO films undergoes a series of dimensionality crossovers. At low temperatures the strong anisotropy causes the thermal excitations to be 2D pancake-antipancake pairs in uncoupled layers. At higher temperatures where the c-axis correlation length becomes larger than a layer there is a crossover to 3D vortex loops. These are initially elliptical, but as the 3D Tc is approached they become more circular as the anisotropy scales away, as modeled by Shenoy and Chattopadhyay [1]. Close to Tc when the correlation length becomes comparable to the film thickness there is a further crossover to a 2D Kosterlitz-Thouless transition, with a drop of the superfluid fraction to zero at T_KT which can be of the order of 1 K below T_c. Good agreement with this model is found for experiments on thin BSCCO 2212 films [2]. 1. S. R. Shenoy and B. Chattopadhyay, Phys. Rev. B 51, 9129 (1995). 2. K. Osborn et al., cond-mat/0204417.
Mechanical characterization of 2D, 2D stitched, and 3D braided/RTM materials
NASA Technical Reports Server (NTRS)
Deaton, Jerry W.; Kullerd, Susan M.; Portanova, Marc A.
1993-01-01
Braided composite materials have potential for application in aircraft structures. Fuselage frames, floor beams, wing spars, and stiffeners are examples where braided composites could find application if cost effective processing and damage tolerance requirements are met. Another important consideration for braided composites relates to their mechanical properties and how they compare to the properties of composites produced by other textile composite processes being proposed for these applications. Unfortunately, mechanical property data for braided composites do not appear extensively in the literature. Data are presented in this paper on the mechanical characterization of 2D triaxial braid, 2D triaxial braid plus stitching, and 3D (through-the-thickness) braid composite materials. The braided preforms all had the same graphite tow size and the same nominal braid architectures, (+/- 30 deg/0 deg), and were resin transfer molded (RTM) using the same mold for each of two different resin systems. Static data are presented for notched and unnotched tension, notched and unnotched compression, and compression after impact strengths at room temperature. In addition, some static results, after environmental conditioning, are included. Baseline tension and compression fatigue results are also presented, but only for the 3D braided composite material with one of the resin systems.
Dirac plasmons in bipartite lattices of metallic nanoparticles
NASA Astrophysics Data System (ADS)
Jebb Sturges, Thomas; Woollacott, Claire; Weick, Guillaume; Mariani, Eros
2015-03-01
We study theoretically ‘graphene-like’ plasmonic metamaterials constituted by two-dimensional arrays of metallic nanoparticles, including perfect honeycomb structures with and without inversion symmetry, as well as generic bipartite lattices. The dipolar interactions between localized surface plasmons (LSPs) in different nanoparticles gives rise to collective plasmons (CPs) that extend over the whole lattice. We study the band structure of CPs and unveil its tunability with the orientation of the dipole moments associated with the LSPs. Depending on the dipole orientation, we identify a phase diagram of gapless or gapped phases in the CP dispersion. We show that the gapless phases in the phase diagram are characterized by CPs behaving as massless chiral Dirac particles, in analogy with electrons in graphene. When the inversion symmetry of the honeycomb structure is broken, CPs are described as gapped chiral Dirac modes with an energy-dependent Berry phase. We further relax the geometric symmetry of the honeycomb structure by analysing generic bipartite hexagonal lattices. In this case we study the evolution of the phase diagram and unveil the emergence of a sequence of topological phase transitions when one hexagonal sublattice is progressively shifted with respect to the other.
Intercalation of hexagonal boron nitride with potassium
Doll, G.L.; Speck, J.S.; Dresselhaus, G.; Dresselhaus, M.S. ); Nakamura, K.; Tanuma, S.
1989-09-15
We have performed photoluminescence, photoexcitation, and transmission electron microscopy measurements on boron nitride films grown by chemical vapor deposition and later reacted with potassium. After reaction, the potassium atoms were found to intercalate the BN host and to form a (2{times}2){ital R}0{degree} in-plane structure which is commensurate with the pristine BN lattice. Optical transitions with {similar to}2.7 eV onsets were found to occur within the {similar to}5-eV BN band gap and have been interpreted as {Gamma}-point transitions between the K(4{ital s}) band and the BN(2{ital p}) bands. The absence of an appreciable shift in the {ital E}{sub 2{ital g}{sub 2}} phonon frequency of the pristine and reacted films suggests that the charge transfer between the K and BN bands is very small.
NASA Astrophysics Data System (ADS)
Bergner, Georg; Catterall, Simon
2016-08-01
We discuss the motivations, difficulties and progress in the study of supersymmetric lattice gauge theories focusing in particular on 𝒩 = 1 and 𝒩 = 4 super-Yang-Mills in four dimensions. Brief reviews of the corresponding lattice formalisms are given and current results are presented and discussed. We conclude with a summary of the main aspects of current work and prospects for the future.
Flat Band Quastiperiodic Lattices
NASA Astrophysics Data System (ADS)
Bodyfelt, Joshua; Flach, Sergej; Danieli, Carlo
2014-03-01
Translationally invariant lattices with flat bands (FB) in their band structure possess irreducible compact localized flat band states, which can be understood through local rotation to a Fano structure. We present extension of these quasi-1D FB structures under incommensurate lattices, reporting on the FB effects to the Metal-Insulator Transition.
Laterally closed lattice homomorphisms
NASA Astrophysics Data System (ADS)
Toumi, Mohamed Ali; Toumi, Nedra
2006-12-01
Let A and B be two Archimedean vector lattices and let be a lattice homomorphism. We call that T is laterally closed if T(D) is a maximal orthogonal system in the band generated by T(A) in B, for each maximal orthogonal system D of A. In this paper we prove that any laterally closed lattice homomorphism T of an Archimedean vector lattice A with universal completion Au into a universally complete vector lattice B can be extended to a lattice homomorphism of Au into B, which is an improvement of a result of M. Duhoux and M. Meyer [M. Duhoux and M. Meyer, Extended orthomorphisms and lateral completion of Archimedean Riesz spaces, Ann. Soc. Sci. Bruxelles 98 (1984) 3-18], who established it for the order continuous lattice homomorphism case. Moreover, if in addition Au and B are with point separating order duals (Au)' and B' respectively, then the laterally closedness property becomes a necessary and sufficient condition for any lattice homomorphism to have a similar extension to the whole Au. As an application, we give a new representation theorem for laterally closed d-algebras from which we infer the existence of d-algebra multiplications on the universal completions of d-algebras.
Björner, Anders
1987-01-01
A continuous analogue to the partition lattices is presented. This is the metric completion of the direct limit of a system of embeddings of the finite partition lattices. The construction is analogous to von Neumann's construction of a continuous geometry over a field F from the finite-dimensional projective geometries over F. PMID:16593874
Honeycomb lattices with defects
NASA Astrophysics Data System (ADS)
Spencer, Meryl A.; Ziff, Robert M.
2016-04-01
In this paper, we introduce a variant of the honeycomb lattice in which we create defects by randomly exchanging adjacent bonds, producing a random tiling with a distribution of polygon edges. We study the percolation properties on these lattices as a function of the number of exchanged bonds using an alternative computational method. We find the site and bond percolation thresholds are consistent with other three-coordinated lattices with the same standard deviation in the degree distribution of the dual; here we can produce a continuum of lattices with a range of standard deviations in the distribution. These lattices should be useful for modeling other properties of random systems as well as percolation.
Adaptive bimaterial lattices to mitigate thermal expansion mismatch stresses in satellite structures
NASA Astrophysics Data System (ADS)
Toropova, Marina M.; Steeves, Craig A.
2015-08-01
Earth-orbiting satellites regularly pass from sunlight to shade and back; these transitions are typically accompanied by significant temperature changes. When adjoining parts of a satellite that are made of different materials are subjected to large temperature changes, thermal mismatch stresses arise that are a function of the temperature change and the difference in coefficients of thermal expansion (CTEs) between the two materials. These thermal stresses are linked to undesirable deformation and, through long-term cycling, fatigue and failure of the structure. This paper describes a type of anisotropic lattice that can serve as a stress-free adaptor between two materials, eliminating thermal mismatch stresses and their concomitant consequences. The lattices consist of planar nonidentical anisotropic bimaterial cells, each designed based on a virtual triangle. Physically the cells consist of a triangle made of material with higher CTE surrounded by a hexagon made of material with lower CTE. Different skew angles of the hexagon make a particular cell and the whole lattice anisotropic. The cells can be designed and combined in a lattice in such a way that one edge of the lattice has CTE that coincides with the CTE of the first part of the structure (substrate 1), while the other edge of the lattice has CTE equal to the CTE of the second part of the structure (substrate 2). If all joints between the parts of each cell, neighbouring cells, and the lattice and the substrates are pinned, the whole structure will be free of thermal stresses. This paper will discuss the fundamental principles governing such lattices, their refinement for special circumstances, and opportunities for improving the structural performance of the lattices. This will be presented coupled to a rational strategy for lattice design.
Differential Cytochrome P450 2D Metabolism Alters Tafenoquine Pharmacokinetics
Vuong, Chau; Xie, Lisa H.; Potter, Brittney M. J.; Zhang, Jing; Zhang, Ping; Duan, Dehui; Nolan, Christina K.; Sciotti, Richard J.; Zottig, Victor E.; Nanayakkara, N. P. Dhammika; Tekwani, Babu L.; Walker, Larry A.; Smith, Philip L.; Paris, Robert M.; Read, Lisa T.; Li, Qigui; Pybus, Brandon S.; Sousa, Jason C.; Reichard, Gregory A.; Smith, Bryan
2015-01-01
Cytochrome P450 (CYP) 2D metabolism is required for the liver-stage antimalarial efficacy of the 8-aminoquinoline molecule tafenoquine in mice. This could be problematic for Plasmodium vivax radical cure, as the human CYP 2D ortholog (2D6) is highly polymorphic. Diminished CYP 2D6 enzyme activity, as in the poor-metabolizer phenotype, could compromise radical curative efficacy in humans. Despite the importance of CYP 2D metabolism for tafenoquine liver-stage efficacy, the exact role that CYP 2D metabolism plays in the metabolism and pharmacokinetics of tafenoquine and other 8-aminoquinoline molecules has not been extensively studied. In this study, a series of tafenoquine pharmacokinetic experiments were conducted in mice with different CYP 2D metabolism statuses, including wild-type (WT) (reflecting extensive metabolizers for CYP 2D6 substrates) and CYPmouse 2D knockout (KO) (reflecting poor metabolizers for CYP 2D6 substrates) mice. Plasma and liver pharmacokinetic profiles from a single 20-mg/kg of body weight dose of tafenoquine differed between the strains; however, the differences were less striking than previous results obtained for primaquine in the same model. Additionally, the presence of a 5,6-ortho-quinone tafenoquine metabolite was examined in both mouse strains. The 5,6-ortho-quinone species of tafenoquine was observed, and concentrations of the metabolite were highest in the WT extensive-metabolizer phenotype. Altogether, this study indicates that CYP 2D metabolism in mice affects tafenoquine pharmacokinetics and could have implications for human tafenoquine pharmacokinetics in polymorphic CYP 2D6 human populations. PMID:25870069
Low lattice thermal conductivity of stanene
Peng, Bo; Zhang, Hao; Shao, Hezhu; Xu, Yuchen; Zhang, Xiangchao; Zhu, Heyuan
2016-01-01
A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures. PMID:26838731
Low lattice thermal conductivity of stanene.
Peng, Bo; Zhang, Hao; Shao, Hezhu; Xu, Yuchen; Zhang, Xiangchao; Zhu, Heyuan
2016-01-01
A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures. PMID:26838731
Statistical Transmutation in Periodically Driven Optical Lattices
NASA Astrophysics Data System (ADS)
Sedrakyan, Tigran; Galitski, Victor; Kamenev, Alex
We show that interacting bosons in a periodically driven two dimensional (2D) optical lattice may effectively exhibit fermionic statistics. The phenomenon is similar to the celebrated Tonks-Girardeau regime in 1D. The Floquet band of a driven lattice develops the moat shape, i.e., a minimum along a closed contour in the Brillouin zone. Such degeneracy of the kinetic energy favors fermionic quasiparticles. The statistical transmutation is achieved by the Chern-Simons flux attachment similar to the fractional quantum Hall case. We show that the velocity distribution of the released bosons is a sensitive probe of the fermionic nature of their stationary Floquet state. This work was supported by the PFC-JQI (T.S.), USARO and Simons Foundation (V.G.), and DOE Contract DE-FG02-08ER46482 (A.K.).
A Geometric Boolean Library for 2D Objects
2006-01-05
The 2D Boolean Library is a collection of C++ classes -- which primarily represent 2D geometric data and relationships, and routines -- which contain algorithms for 2D geometric Boolean operations and utility functions. Classes are provided for 2D points, lines, arcs, edgeuses, loops, surfaces and mask sets. Routines are provided that incorporate the Boolean operations Union(OR), XOR, Intersection and Difference. Various analytical geometry routines and routines for importing and exporting the data in various filemore » formats, are also provided in the library.« less
A Geometric Boolean Library for 2D Objects
McBride, Corey L.; Yarberry, Victor; Jorgensen, Craig
2006-01-05
The 2D Boolean Library is a collection of C++ classes -- which primarily represent 2D geometric data and relationships, and routines -- which contain algorithms for 2D geometric Boolean operations and utility functions. Classes are provided for 2D points, lines, arcs, edgeuses, loops, surfaces and mask sets. Routines are provided that incorporate the Boolean operations Union(OR), XOR, Intersection and Difference. Various analytical geometry routines and routines for importing and exporting the data in various file formats, are also provided in the library.