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Sample records for quantum confinement vi

  1. Strong confinement effects in CdTe/MnTe quantum wells: A new strained layer binary II VI heterostructure

    NASA Astrophysics Data System (ADS)

    Fu, Q.; Pelekanos, N.; Nurmikko, A. V.; Durbin, S.; Han, J.; Sungki, O.; Menke, D.; Kobayashi, M.; Gunshor, R. L.

    1990-04-01

    A range of optical studies have been carried out on a series of single quantum wells of CdTe/MnTe. The structures appear to be nearly pseudomorphic and show evidence for robust electron-hole confinement. Exciton states have been characterized in terms of lifetime and coupling to optical phonons.

  2. Spectroscopy of CdTe/MnTe single quantum wells: A strained-layer II-VI heterostructure with strong electronic confinement

    NASA Astrophysics Data System (ADS)

    Pelekanos, N.; Fu, Q.; Ding, J.; Wałecki, W.; Nurmikko, A. V.; Durbin, S. M.; Han, J.; Kobayashi, M.; Gunshor, R. L.

    1990-05-01

    A range of optical studies has been carried out on a series of single quantum wells of CdTe/MnTe. The structures appear to be nearly pseudomorphic and show evidence for very effective electron-hole confinement. For thin quantum-well layers, efficient low-temperature photoluminescence up to yellow-green photon energies has been obtained. Coupling of excitons to longitudinal-optical phonons has also been measured.

  3. Comparison of quantum confinement effects between quantum wires and dots

    SciTech Connect

    Li, Jingbo; Wang, Lin-Wang

    2004-03-30

    Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantum dots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.

  4. Einstein's Photoemission from Quantum Confined Superlattices.

    PubMed

    Debbarma, S; Ghatak, K P

    2016-01-01

    This paper is dedicated to the 83th Birthday of Late Professor B. R. Nag, D.Sc., formerly Head of the Departments of Radio Physics and Electronics and Electronic Science of the University of Calcutta, a firm believer of the concept of theoretical minimum of Landau and an internationally well known semiconductor physicist, to whom the second author remains ever grateful as a student and research worker from 1974-2004. In this paper, an attempt is made to study, the Einstein's photoemission (EP) from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum well heavily doped superlattices (QWHDSLs) with graded interfaces in the presence of quantizing magnetic field on the basis of newly formulated electron dispersion relations within the frame work of k · p formalism. The EP from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum wells of heavily doped effective mass superlattices respectively has been presented under magnetic quantization. Besides the said emissions, from the quantum dots of the aforementioned heavily doped SLs have further investigated for the purpose of comparison and complete investigation in the context of EP from quantum confined superlattices. Using appropriate SLs, it appears that the EP increases with increasing surface electron concentration and decreasing film thickness in spiky manners, which are the characteristic features of such quantized hetero structures. Under magnetic quantization, the EP oscillates with inverse quantizing magnetic field due to Shuvnikov-de Haas effect. The EP increases with increasing photo energy in a step-like manner and the numerical values of EP with all the physical variables are totally band structure dependent for all the cases. The most striking features are that the presence of poles in the dispersion relation of the materials in the absence of band tails create the complex energy spectra in the corresponding HD constituent materials of such quantum confined superlattices and effective electron

  5. A Review of Quantum Confinement

    SciTech Connect

    Connerade, Jean-Patrick

    2009-12-03

    A succinct history of the Confined Atom problem is presented. The hydrogen atom confined to the centre of an impenetrable sphere counts amongst the exactly soluble problems of physics, alongside much more noted exact solutions such as Black Body Radiation and the free Hydrogen atom in absence of any radiation field. It shares with them the disadvantage of being an idealisation, while at the same time encapsulating in a simple way particular aspects of physical reality. The problem was first formulated by Sommerfeld and Welker - henceforth cited as SW - in connection with the behaviour of atoms at very high pressures, and the solution was published on the occasion of Pauli's 60th birthday celebration. At the time, it seemed that there was not much other connection with physical reality beyond a few simple aspects connected to the properties of atoms in solids, for which more appropriate models were soon developed. Thus, confined atoms attracted little attention until the advent of the metallofullerene, which provided the first example of a confined atom with properties quite closely related to those originally considered by SW. Since then, the problem has received much more attention, and many more new features of quantum confinement, quantum compression, the quantum Faraday cage, electronic reorganisation, cavity resonances, etc have been described, which are relevant to real systems. Also, a number of other situations have been uncovered experimentally to which quantum confinement is relevant. Thus, studies of the confined atom are now more numerous, and have been extended both in terms of the models used and the systems to which they can be applied. Connections to thermodynamics are explored through the properties of a confined two-level atom adapted from Einstein's celebrated model, and issues of dynamical screening of electromagnetic radiation by the confining shell are discussed in connection with the Faraday cage produced by a confining conducting shell. The

  6. CORRELATIONS IN CONFINED QUANTUM PLASMAS

    SciTech Connect

    DUFTY J W

    2012-01-11

    This is the final report for the project 'Correlations in Confined Quantum Plasmas', NSF-DOE Partnership Grant DE FG02 07ER54946, 8/1/2007 - 7/30/2010. The research was performed in collaboration with a group at Christian Albrechts University (CAU), Kiel, Germany. That collaboration, almost 15 years old, was formalized during the past four years under this NSF-DOE Partnership Grant to support graduate students at the two institutions and to facilitate frequent exchange visits. The research was focused on exploring the frontiers of charged particle physics evolving from new experimental access to unusual states associated with confinement. Particular attention was paid to combined effects of quantum mechanics and confinement. A suite of analytical and numerical tools tailored to the specific inquiry has been developed and employed

  7. Electronic quantum confinement in cylindrical potential well

    NASA Astrophysics Data System (ADS)

    Baltenkov, Arkadiy S.; Msezane, Alfred Z.

    2016-04-01

    The effects of quantum confinement on the momentum distribution of electrons confined within a cylindrical potential well have been analyzed. The motivation is to understand specific features of the momentum distribution of electrons when the electron behavior is completely controlled by the parameters of a non-isotropic potential cavity. It is shown that studying the solutions of the wave equation for an electron confined in a cylindrical potential well offers the possibility to analyze the confinement behavior of an electron executing one- or two-dimensional motion in the three-dimensional space within the framework of the same mathematical model. Some low-lying electronic states with different symmetries have been considered and the corresponding wave functions have been calculated; the behavior of their nodes and their peak positions with respect to the parameters of the cylindrical well has been analyzed. Additionally, the momentum distributions of electrons in these states have been calculated. The limiting cases of the ratio of the cylinder length H and its radius R0 have been considered; when the cylinder length H significantly exceeds its radius R0 and when the cylinder radius is much greater than its length. The cylindrical quantum confinement effects on the momentum distribution of electrons in these potential wells have been analyzed. The possible application of the results obtained here for the description of the general features in the behavior of electrons in nanowires with metallic type of conductivity (or nanotubes) and ultrathin epitaxial films (or graphene sheets) are discussed. Possible experiments are suggested where the quantum confinement can be manifested. Contribution to the Topical Issue "Atomic Cluster Collisions (7th International Symposium)", edited by Gerardo Delgado Barrio, Andrey Solov'Yov, Pablo Villarreal, Rita Prosmiti.

  8. Confinement of Fractional Quantum Hall States

    NASA Astrophysics Data System (ADS)

    Willett, Robert; Manfra, Michael; West, Ken; Pfeiffer, Loren

    2008-03-01

    Confinement of small-gapped fractional quantum Hall states facilitates quasiparticle manipulation and is an important step towards quasiparticle interference measurements. Demonstrated here is conduction through top gate defined, narrow channels in high density, ultra-high mobility heterostructures. Transport evidence for the persistence of a correlated state at filling fraction 5/3 is shown in channels of 2μm length but gated to near 0.3μm in width. The methods employed to achieve this confinement hold promise for interference devices proposed for studying potential non-Abelian statistics at filling fraction 5/2. R.L. Willett, M.J. Manfra, L.N. Pfeiffer, K.W. West, Appl. Phys. Lett. 91, 052105 (2007).

  9. Using Quantum Confinement to Uniquely Identify Devices.

    PubMed

    Roberts, J; Bagci, I E; Zawawi, M A M; Sexton, J; Hulbert, N; Noori, Y J; Young, M P; Woodhead, C S; Missous, M; Migliorato, M A; Roedig, U; Young, R J

    2015-01-01

    Modern technology unintentionally provides resources that enable the trust of everyday interactions to be undermined. Some authentication schemes address this issue using devices that give a unique output in response to a challenge. These signatures are generated by hard-to-predict physical responses derived from structural characteristics, which lend themselves to two different architectures, known as unique objects (UNOs) and physically unclonable functions (PUFs). The classical design of UNOs and PUFs limits their size and, in some cases, their security. Here we show that quantum confinement lends itself to the provision of unique identities at the nanoscale, by using fluctuations in tunnelling measurements through quantum wells in resonant tunnelling diodes (RTDs). This provides an uncomplicated measurement of identity without conventional resource limitations whilst providing robust security. The confined energy levels are highly sensitive to the specific nanostructure within each RTD, resulting in a distinct tunnelling spectrum for every device, as they contain a unique and unpredictable structure that is presently impossible to clone. This new class of authentication device operates with minimal resources in simple electronic structures above room temperature. PMID:26553435

  10. Using Quantum Confinement to Uniquely Identify Devices

    NASA Astrophysics Data System (ADS)

    Roberts, J.; Bagci, I. E.; Zawawi, M. A. M.; Sexton, J.; Hulbert, N.; Noori, Y. J.; Young, M. P.; Woodhead, C. S.; Missous, M.; Migliorato, M. A.; Roedig, U.; Young, R. J.

    2015-11-01

    Modern technology unintentionally provides resources that enable the trust of everyday interactions to be undermined. Some authentication schemes address this issue using devices that give a unique output in response to a challenge. These signatures are generated by hard-to-predict physical responses derived from structural characteristics, which lend themselves to two different architectures, known as unique objects (UNOs) and physically unclonable functions (PUFs). The classical design of UNOs and PUFs limits their size and, in some cases, their security. Here we show that quantum confinement lends itself to the provision of unique identities at the nanoscale, by using fluctuations in tunnelling measurements through quantum wells in resonant tunnelling diodes (RTDs). This provides an uncomplicated measurement of identity without conventional resource limitations whilst providing robust security. The confined energy levels are highly sensitive to the specific nanostructure within each RTD, resulting in a distinct tunnelling spectrum for every device, as they contain a unique and unpredictable structure that is presently impossible to clone. This new class of authentication device operates with minimal resources in simple electronic structures above room temperature.

  11. Using Quantum Confinement to Uniquely Identify Devices

    PubMed Central

    Roberts, J.; Bagci, I. E.; Zawawi, M. A. M.; Sexton, J.; Hulbert, N.; Noori, Y. J.; Young, M. P.; Woodhead, C. S.; Missous, M.; Migliorato, M. A.; Roedig, U.; Young, R. J.

    2015-01-01

    Modern technology unintentionally provides resources that enable the trust of everyday interactions to be undermined. Some authentication schemes address this issue using devices that give a unique output in response to a challenge. These signatures are generated by hard-to-predict physical responses derived from structural characteristics, which lend themselves to two different architectures, known as unique objects (UNOs) and physically unclonable functions (PUFs). The classical design of UNOs and PUFs limits their size and, in some cases, their security. Here we show that quantum confinement lends itself to the provision of unique identities at the nanoscale, by using fluctuations in tunnelling measurements through quantum wells in resonant tunnelling diodes (RTDs). This provides an uncomplicated measurement of identity without conventional resource limitations whilst providing robust security. The confined energy levels are highly sensitive to the specific nanostructure within each RTD, resulting in a distinct tunnelling spectrum for every device, as they contain a unique and unpredictable structure that is presently impossible to clone. This new class of authentication device operates with minimal resources in simple electronic structures above room temperature. PMID:26553435

  12. Quantum confinement in transition metal oxide quantum wells

    SciTech Connect

    Choi, Miri; Lin, Chungwei; Butcher, Matthew; Posadas, Agham B.; Demkov, Alexander A.; Rodriguez, Cesar; Zollner, Stefan; He, Qian; Borisevich, Albina Y.

    2015-05-11

    We report on the quantum confinement in SrTiO{sub 3} (STO) quantum wells (QWs) grown by molecular beam epitaxy. The QW structure consists of LaAlO{sub 3} (LAO) and STO layers grown on LAO substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized. Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry in the range of 1.0 eV–6.0 eV at room temperature. We observed that the absorption edge was blue-shifted by approximately 0.39 eV as the STO quantum well thickness was reduced to two unit cells. This demonstrates that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material.

  13. Quantum confinement in metal nanofilms: Optical spectra

    NASA Astrophysics Data System (ADS)

    Khmelinskii, Igor; Makarov, Vladimir I.

    2016-05-01

    We report optical absorption and photoluminescence spectra of Au, Fe, Co and Ni polycrystalline nanofilms in the UV-vis-NIR range, featuring discrete bands resulting from transverse quantum confinement. The film thickness ranged from 1.1 to 15.6 nm, depending on the material. The films were deposited on fused silica substrates by sputtering/thermo-evaporation, with Fe, Co and Ni protected by a SiO2 film deposited on top. The results are interpreted within the particle-in-a-box model, with the box width equal to the mass thickness of the nanofilm. The transverse-quantized energy levels and transition energies scale as the inverse square of the film thickness. The calculated values of the effective electron mass are 0.93 (Au), 0.027 (Fe), 0.21 (Co) and 0.16 (Ni), in units of mo - the mass of the free electron, being independent on the film thickness. The uncertainties in the effective mass values are ca. 2.5%, determined by the film thickness calibration. The second calculated model parameter, the quantum number n of the HOMO, was thickness-independent in Au (5.00) and Fe (6.00), and increased with the film thickness in Co (from 7 to 9) and Ni (from 7 to 11). The transitions observed in the absorbance all start at the level n and correspond to Δn=+1, +2, +3, etc. The photoluminescence bands exhibit large Stokes shifts, shifting to higher energies with the increased excitation energy. The photoluminescence quantum yields grow linearly with the excitation energy, showing evidence of multiple exciton generation. A prototype Fe-SnO2 nanofilm photovoltaic cell demonstrated at least 90% quantum yield of photoelectrons at 77 K.

  14. Quantum chromodynamics near the confinement limit

    SciTech Connect

    Quigg, C.

    1985-09-01

    These nine lectures deal at an elementary level with the strong interaction between quarks and its implications for the structure of hadrons. Quarkonium systems are studied as a means for measuring the interquark interaction. This is presumably (part of) the answer a solution to QCD must yield, if it is indeed the correct theory of the strong interactions. Some elements of QCD are reviewed, and metaphors for QCD as a confining theory are introduced. The 1/N expansion is summarized as a way of guessing the consequences of QCD for hadron physics. Lattice gauge theory is developed as a means for going beyond perturbation theory in the solution of QCD. The correspondence between statistical mechanics, quantum mechanics, and field theory is made, and simple spin systems are formulated on the lattice. The lattice analog of local gauge invariance is developed, and analytic methods for solving lattice gauge theory are considered. The strong-coupling expansion indicates the existence of a confining phase, and the renormalization group provides a means for recovering the consequences of continuum field theory. Finally, Monte Carlo simulations of lattice theories give evidence for the phase structure of gauge theories, yield an estimate for the string tension characterizing the interquark force, and provide an approximate description of the quarkonium potential in encouraging good agreement with what is known from experiment.

  15. Quantum-confined CdS nanoparticles on DNA templates

    NASA Astrophysics Data System (ADS)

    Rho, Young Gyu

    As electronic devices became smaller, interest in quantum-confined semiconductor nanostructures increased. Self-assembled mesoscale semiconductor structures of II-VI nanocrystals are an especially exciting subject because of their controllable band gap and unique photophysical properties. Several preparative methods to synthesize and control the sizes of the individual nanocrystallites and the electronic and optical properties have been intensively studied. Fabrication of patterned nanostructures composed of quantum-confined nanoparticles is the next step toward practical applications. We have developed an innovative method to fabricate diverse nanostructures which relies on the size and a shape of a chosen deoxyribonucleic acid (DNA) template. DNA has anionic atoms which can bind transition metal ions such as Znsp{2+},\\ Cdsp{2+} and Hgsp{2+}. The DNA can thus control the location of nanoparticles synthesis. Therefore, diverse nanoscale structures composed of arrays of quantum-confined nanocrystallite can be fabricated on DNA fixed to solid substrates without using complicated and expensive photolithography. Mesoscale arrays of Q-CdS nanostructures were fabricated on pUCLeu4 plasmid and PhiX 174 RF II DNA. These DNAs are double-stranded and in A-form are 2.55 nm in diameter, and 0.85 mum and 1.32 mum long, respectively. The samples were prepared either by dropping Cdsp{2+}/DNA complexes on carbon-coated TEM grids or by floating the grids on Cdsp{2+}/DNA or DNA only solutions. The grids with DNA only were reacted with Cdsp{2+} by dipping the grids into a cadmium solution. The grids were exposed to Hsb2S gas to form the Q-CdS nanoparticle arrays. Various ratios and concentrations of Cdsp{2+}/DNA were examined. Conventional, analytical and high resolution transmission electron microscopy were used to characterize the Q-CdS nanostructures. Absorption and photoluminescence spectroscopies were used for optical characterization. The experimental results demonstrate the

  16. Emergent quantum confinement at topological insulator surfaces

    NASA Astrophysics Data System (ADS)

    Bahramy, M. S.; King, P. D. C.; de la Torre, A.; Chang, J.; Shi, M.; Patthey, L.; Balakrishnan, G.; Hofmann, Ph.; Arita, R.; Nagaosa, N.; Baumberger, F.

    2012-10-01

    Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z2 topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement.

  17. Gate-defined Quantum Confinement in Suspended Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Allen, Monica

    2013-03-01

    Quantum confined devices in carbon-based materials offer unique possibilities for applications ranging from quantum computation to sensing. In particular, nanostructured carbon is a promising candidate for spin-based quantum computation due to the ability to suppress hyperfine coupling to nuclear spins, a dominant source of spin decoherence. Yet graphene lacks an intrinsic bandgap, which poses a serious challenge for the creation of such devices. We present a novel approach to quantum confinement utilizing tunnel barriers defined by local electric fields that break sublattice symmetry in suspended bilayer graphene. This technique electrostatically confines charges via band structure control, thereby eliminating the edge and substrate disorder that hinders on-chip etched nanostructures to date. We report clean single electron tunneling through gate-defined quantum dots in two regimes: at zero magnetic field using the energy gap induced by a perpendicular electric field and at finite magnetic fields using Landau level confinement. The observed Coulomb blockade periodicity agrees with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates quantum confinement with pristine device quality and access to vibrational modes, enabling wide applications from electromechanical sensors to quantum bits. More broadly, the ability to externally tailor the graphene bandgap over nanometer scales opens a new unexplored avenue for creating quantum devices.

  18. Correlation studies in weakly confining quantum dot potentials

    NASA Astrophysics Data System (ADS)

    Kimani, Peter; Jones, Preston; Winkler, Peter

    We investigate the electron correlation in few-electron closed-shell atomic systems and similarly in few-electron quantum dots under weak confinement. As usual we start with restricted Hartree-Fock (HF) calculations and add electron correlation in steps in a series of approximations based on the single particle Green's function approach: (i) second-order Green function (GF); (ii) 2ph-Tamm-Dancoff approximation (TDA); and (iii) an extended version thereof which introduces ground-state correlation into the TDA. Our studies exhibit similarities and differences between weakly confined quantum dots and standard atomic systems. The calculations support the application of HF, GF, and TDA techniques in the modeling of three-dimensional quantum dot systems. The observed differences emphasize the significance of confinement and electronic features unique to quantum dots, such as the increased binding of electrons with higher angular momentum and thus - compared to atomic systems - modified shell-filling sequences.

  19. Si quantum dots in silicon nitride: Quantum confinement and defects

    SciTech Connect

    Goncharova, L. V. Karner, V. L.; D'Ortenzio, R.; Chaudhary, S.; Mokry, C. R.; Simpson, P. J.; Nguyen, P. H.

    2015-12-14

    Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiN{sub x}:H) matrix was examined over a broad range of stoichiometries from Si{sub 3}N{sub 2.08} to Si{sub 3}N{sub 4.14}, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiN{sub x} films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH{sub 4} and NH{sub 3} gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si{sub 3}N{sub x} compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (∼Si{sub 3}N{sub 3.15}) in which the maximum of light emission is observed.

  20. Si quantum dots in silicon nitride: Quantum confinement and defects

    NASA Astrophysics Data System (ADS)

    Goncharova, L. V.; Nguyen, P. H.; Karner, V. L.; D'Ortenzio, R.; Chaudhary, S.; Mokry, C. R.; Simpson, P. J.

    2015-12-01

    Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiNx:H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si3Nx compositions. There is a red-shift of the measured peaks from ˜2.3 eV to ˜1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (˜Si3N3.15) in which the maximum of light emission is observed.

  1. Bench-scale column experiments to study the containment of Cr(VI) in confined aquifers by bio-transformation.

    PubMed

    Shashidhar, T; Philip, Ligy; Murty Bhallamudi, S

    2006-04-17

    Bench-scale soil column experiments were conducted to study the effectiveness of Cr(VI) containment in confined aquifers using in situ bio-transformation. Batch adsorption studies were carried out to estimate the adsorption capacities of two different soils for Cr(VI) and Cr(III). Bio-kinetic parameters were evaluated for the enriched microbial system. The inhibition constant, evaluated using Monod's inhibition model, was found to be 11.46 mg/L of Cr(VI). Transport studies indicated that it would not be possible to contain Cr(VI) by adsorption alone. Transport and bio-transformation studies indicated that the pore velocity and the initial bio-mass concentration significantly affect the containment process. In situ bio-remediation is effective in the case of silty aquifers. Cr(VI) concentration of 25 mg/L was effectively contained within 60 cm of a confined silty aquifer. Cr(VI) containment could be achieved in sandy aquifers when the pore velocity was very low and the initial augmented bio-mass was high. A bio-barrier of approximately one meter width would be able to contain Cr(VI) if the initial Cr(VI) concentration is as much as 25 mg/L. PMID:16263213

  2. Suppression of Quantum Scattering in Strongly Confined Systems

    SciTech Connect

    Kim, J. I.; Melezhik, V. S.; Schmelcher, P.

    2006-11-10

    We demonstrate that scattering of particles strongly interacting in three dimensions (3D) can be suppressed at low energies in a quasi-one-dimensional (1D) confinement. The underlying mechanism is the interference of the s- and p-wave scattering contributions with large s- and p-wave 3D scattering lengths being a necessary prerequisite. This low-dimensional quantum scattering effect might be useful in 'interacting' quasi-1D ultracold atomic gases, guided atom interferometry, and impurity scattering in strongly confined quantum wire-based electronic devices.

  3. Quantum confinement in Si and Ge nanostructures: Theory and experiment

    SciTech Connect

    Barbagiovanni, Eric G.; Lockwood, David J.; Simpson, Peter J.; Goncharova, Lyudmila V.

    2014-03-15

    The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO{sub 2}, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si{sub 3}N{sub 4} and Al{sub 2}O{sub 3}. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

  4. Quantum confinement in Si and Ge nanostructures: Theory and experiment

    NASA Astrophysics Data System (ADS)

    Barbagiovanni, Eric G.; Lockwood, David J.; Simpson, Peter J.; Goncharova, Lyudmila V.

    2014-03-01

    The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

  5. Confinement of Dirac electrons in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Jolie, Wouter; Craes, Fabian; Petrović, Marin; Atodiresei, Nicolae; Caciuc, Vasile; Blügel, Stefan; Kralj, Marko; Michely, Thomas; Busse, Carsten

    2014-04-01

    We observe spatial confinement of Dirac states on epitaxial graphene quantum dots with low-temperature scanning tunneling microscopy after using oxygen as an intercalant to suppress the surface state of Ir(111) and to effectively decouple graphene from its metal substrate. We analyze the confined electronic states with a relativistic particle-in-a-box model and find a linear dispersion relation. The oxygen-intercalated graphene is p doped [ED=0.64±0.07 eV] and has a Fermi velocity close to the one of free-standing graphene [vF=0.96±0.07×106 m/s].

  6. Spectroscopic study of Gd nanostructures quantum confined in Fe corrals

    SciTech Connect

    Cao, R. X.; Sun, L.; Miao, B. F.; Li, Q. L.; Zheng, C.; Wu, D.; You, B.; Zhang, W.; Han, P.; Bader, S. D.; Zhang, W. Y.; Ding, H. F.

    2015-07-10

    Low dimensional nanostructures have attracted attention due to their rich physical properties and potential applications. The essential factor for their functionality is their electronic properties, which can be modified by quantum confinement. Here the electronic states of Gd atom trapped in open Fe corrals on Ag(111) were studied via scanning tunneling spectroscopy. A single spectroscopic peak above the Fermi level is observed after Gd adatoms are trapped inside Fe corrals, while two peaks appear in empty corrals. The single peak position is close to the higher energy peak of the empty corrals. These findings, attributed to quantum confinement of the corrals and Gd structures trapped inside, are supported by tight-binding calculations. As a result, this demonstrates and provides insights into atom trapping in open corrals of various diameters, giving an alternative approach to modify the properties of nano-objects.

  7. Interplay between quantum confinement and dielectric mismatch for ultrashallow dopants

    NASA Astrophysics Data System (ADS)

    Mol, J. A.; Salfi, J.; Miwa, J. A.; Simmons, M. Y.; Rogge, S.

    2013-06-01

    Understanding the electronic properties of dopants near an interface is a critical challenge for nanoscale devices. We have determined the effect of dielectric mismatch and quantum confinement on the ionization energy of individual acceptors beneath a hydrogen passivated silicon (100) surface. While dielectric mismatch between the vacuum and the silicon at the interface results in an image charge which enhances the binding energy of subsurface acceptors, quantum confinement is shown to reduce the binding energy. Using scanning tunneling spectroscopy we measure resonant transport through the localized states of individual acceptors. Thermal broadening of the conductance peaks provides a direct measure for the absolute energy scale. Our data unambiguously demonstrates that these two independent effects compete with the result that the ionization energy is less than 5 meV lower than the bulk value for acceptors less than a Bohr radius from the interface.

  8. Molecular Limits to the Quantum Confinement Model in Diamond Clusters

    SciTech Connect

    Willey, T M; Bostedt, C; van Buuren, T; Dahl, J E; Liu, S E; Carlson, R K; Terminello, L J; Moller, T

    2005-04-07

    The electronic structure of monodisperse, hydrogen-passivated diamond clusters in the gas phase has been studied with x-ray absorption spectroscopy. The data show that the bulk-related unoccupied states do not exhibit any quantum confinement. Additionally, density of states below the bulk absorption edge appears, consisting of features correlated to CH and CH{sub 2} hydrogen surface termination, resulting in an effective red shift of the lowest unoccupied states. The results contradict the commonly used and very successful quantum confinement model for semiconductors which predicts increasing band edge blue shifts with decreasing particle size. Our findings indicate that in the ultimate size limit for nanocrystals a more molecular description is necessary.

  9. Spectroscopic study of Gd nanostructures quantum confined in Fe corrals

    DOE PAGESBeta

    Cao, R. X.; Sun, L.; Miao, B. F.; Li, Q. L.; Zheng, C.; Wu, D.; You, B.; Zhang, W.; Han, P.; Bader, S. D.; et al

    2015-07-10

    Low dimensional nanostructures have attracted attention due to their rich physical properties and potential applications. The essential factor for their functionality is their electronic properties, which can be modified by quantum confinement. Here the electronic states of Gd atom trapped in open Fe corrals on Ag(111) were studied via scanning tunneling spectroscopy. A single spectroscopic peak above the Fermi level is observed after Gd adatoms are trapped inside Fe corrals, while two peaks appear in empty corrals. The single peak position is close to the higher energy peak of the empty corrals. These findings, attributed to quantum confinement of themore » corrals and Gd structures trapped inside, are supported by tight-binding calculations. As a result, this demonstrates and provides insights into atom trapping in open corrals of various diameters, giving an alternative approach to modify the properties of nano-objects.« less

  10. Quantum states of confined hydrogen plasma species: Monte Carlo calculations

    NASA Astrophysics Data System (ADS)

    Micca Longo, G.; Longo, S.; Giordano, D.

    2015-12-01

    The diffusion Monte Carlo method with symmetry-based state selection is used to calculate the quantum energy states of \\text{H}2+ confined into potential barriers of atomic dimensions (a model for these ions in solids). Special solutions are employed, permitting one to obtain satisfactory results with rather simple native code. As a test case, {{}2}{{\\Pi}u} and {{}2}{{\\Pi}g} states of \\text{H}2+ ions under spherical confinement are considered. The results are interpreted using the correlation of \\text{H}2+ states to atomic orbitals of H atoms lying on the confining surface and perturbation calculations. The method is straightforwardly applied to cavities of any shape and different hydrogen plasma species (at least one-electron ones, including H) for future studies with real crystal symmetries.

  11. Quantum Confinement of Surface Electrons by Molecular Nanohoop Corrals.

    PubMed

    Taber, Benjamen N; Gervasi, Christian F; Mills, Jon M; Kislitsyn, Dmitry A; Darzi, Evan R; Crowley, William G; Jasti, Ramesh; Nazin, George V

    2016-08-18

    Quantum confinement of two-dimensional surface electronic states has been explored as a way for controllably modifying the electronic structures of a variety of coinage metal surfaces. In this Letter, we use scanning tunneling microscopy and spectroscopy (STM/STS) to study the electron confinement within individual ring-shaped cycloparaphenylene (CPP) molecules forming self-assembled films on Ag(111) and Au(111) surfaces. STM imaging and STS mapping show the presence of electronic states localized in the interiors of CPP rings, inconsistent with the expected localization of molecular electronic orbitals. Electronic energies of these states show considerable variations correlated with the molecular shape. These observations are explained by the presence of localized states formed due to confinement of surface electrons by the CPP skeletal framework, which thus acts as a molecular electronic "corral". Our experiments suggest an approach to robust large-area modification of the surface electronic structure via quantum confinement within molecules forming self-assembled layers. PMID:27459268

  12. Twinned silicon and germanium nanocrystals: Formation, stability and quantum confinement

    SciTech Connect

    Yu, Ting; Pi, Xiaodong Ni, Zhenyi; Zhang, Hui; Yang, Deren

    2015-03-15

    Although twins are often observed in Si/Ge nanocrystals (NCs), little theoretical investigation has been carried out to understand this type of important planar defects in Si/Ge NCs. We now study the twinning of Si/Ge NCs in the frame work of density functional theory by representatively considering single-twinned and fivefold-twinned Si/Ge NCs. It is found that the formation of twinned Si/Ge NCs is thermodynamically possible. The effect of twinning on the formation of Si NCs is different from that of Ge NCs. For both Si and Ge NCs twinning enhances their stability. The quantum confinement effect is weakened by twinning for Si NCs. Twinning actually enhances the quantum confinement of Ge NCs when they are small (<136 atoms), while weakening the quantum confinement of Ge NCs as their size is large (>136 atoms). The current results help to better understand the experimental work on twinned Si/Ge NCs and guide the tuning of Si/Ge-NC structures for desired properties.

  13. Enhanced confinement in compositionally heterogeneous alloy quantum dots

    NASA Astrophysics Data System (ADS)

    Hossain, Zubaer

    While there is a growing need to increase solar cell efficiencies and reduce the cost per watt, reported efficiencies are still well below the thermodynamic limit of photovoltaic energy conversion. The major factor that affects the efficiency (by more than 40%) is the lack of absorption or thermalization of electrons. To improve absorption, existing approaches, till date, are focused on combining multiple materials in the form of heterostructures. This talk will show the application of a physics-based mechanistic approach to engineer absorption by using alloy quantum dots and exploiting its heterogeneous compositional and deformation fields. Using a multiscale computational framework that combines density functional theory, k.p method and the finite element calculations, the work shows that heterogeneous distribution of composition and strain fields can lead to substantial confinement in alloy quantum dots. Subsequently alloy quantum dots that are much larger (on the order of 50 nm) in size - compared to their single crystalline counterparts (which are on the order of 5 nm) - can still provide significant confinement. The findings uncover new fundamental insights for engineering confinement that are unattainable under conventional homogenization approximations.

  14. Quantum tunneling and vibrational dynamics of ultra-confined water

    NASA Astrophysics Data System (ADS)

    Kolesnikov, Alexander I.; Anovitz, Lawrence M.; Ehlers, Georg; Mamontov, Eugene; Podlesnyak, Andrey; Prisk, Timothy R.; Seel, Andrew; Reiter, George F.

    2015-03-01

    Vibrational dynamics of ultra-confined water in single crystals beryl, the structure of which contains ~ 5 Å diameter channels along the c-axis was studied with inelastic (INS), quasi-elastic (QENS) and deep inelastic (DINS) neutron scattering. The results reveal significantly anisotropic dynamical behavior of confined water, and show that effective potential experienced by water perpendicular to the channels is significantly softer than along them. The observed 7 peaks in the INS spectra (at energies 0.25 to 15 meV), based on their temperature and momentum transfer dependences, are explained by transitions between the split ground states of water in beryl due to water quantum tunneling between the 6-fold equivalent positions across the channels. DINS study of beryl at T=4.3 K shows narrow, anisotropic water proton momentum distribution with corresponding kinetic energy, EK=95 meV, which is much less than was previously observed in bulk water (~150 meV). We believe that the exceptionally small EK in beryl is a result of water quantum tunneling ∖ delocalization in the nanometer size confinement and weak water-cage interaction. The neutron experiment at ORNL was sponsored by the Sci. User Facilities Div., BES, U.S. DOE. This research was sponsored by the Div. Chemical Sci, Geosciences, and Biosciences, BES, U.S. DOE. The STFC RAL is thanked for access to ISIS neutron facilities.

  15. Impurity binding energies in quantum dots with parabolic confinement

    NASA Astrophysics Data System (ADS)

    Abramov, Arnold

    2015-03-01

    We present an effective numerical procedure to calculate the binding energies and wave functions of the hydrogen-like impurity states in a quantum dot (QD) with parabolic confinement. The unknown wave function was expressed as an expansion over one-dimensional harmonic oscillator states, which describes the electron's movement along the defined z-axis. Green's function technique used to obtain the solution of Schredinger equation for electronic states in a transverse plane. Binding energy of impurity states is defined as poles of the wave function. The dependences of the binding energy on the position of an impurity, the size of the QD and the magnetic field strength are presented and discussed.

  16. Confined quantum time of arrival for the vanishing potential

    SciTech Connect

    Galapon, Eric A.; Caballar, Roland F.; Bahague, Ricardo

    2005-12-15

    We give full account of our recent report in E. A. Galapon, R. Caballar, and R. Bahague, Phys. Rev. Lett. 93, 180406 (2004), where it is shown that formulating the free quantum time of arrival problem in a segment of the real line suggests rephrasing the quantum time of arrival problem to finding a complete set of states that evolve to unitarily arrive at a given point at a definite time. For a spatially confined particle, here it is shown explicitly that the problem admits a solution in the form of an eigenvalue problem of a class of compact and self-adjoint time of arrival operators derived by a quantization of the classical time of arrival. The eigenfunctions of these operators are numerically demonstrated to unitarily arrive at the origin at their respective eigenvalues.

  17. Graphene/Si-quantum-dot heterojunction diodes showing high photosensitivity compatible with quantum confinement effect.

    PubMed

    Shin, Dong Hee; Kim, Sung; Kim, Jong Min; Jang, Chan Wook; Kim, Ju Hwan; Lee, Kyeong Won; Kim, Jungkil; Oh, Si Duck; Lee, Dae Hun; Kang, Soo Seok; Kim, Chang Oh; Choi, Suk-Ho; Kim, Kyung Joong

    2015-04-24

    Graphene/Si quantum dot (QD) heterojunction diodes are reported for the first time. The photoresponse, very sensitive to variations in the size of the QDs as well as in the doping concentration of graphene and consistent with the quantum-confinement effect, is remarkably enhanced in the near-ultraviolet range compared to commercially available bulk-Si photodetectors. The photoresponse proves to be dominated by the carriertunneling mechanism. PMID:25776865

  18. The Interplay of Quantum Confinement and Hydrogenation in Amorphous Silicon Quantum Dots.

    PubMed

    Askari, Sadegh; Svrcek, Vladmir; Maguire, Paul; Mariotti, Davide

    2015-12-22

    Hydrogenation in amorphous silicon quantum dots (QDs) has a dramatic impact on the corresponding optical properties and band energy structure, leading to a quantum-confined composite material with unique characteristics. The synthesis of a-Si:H QDs is demonstrated with an atmospheric-pressure plasma process, which allows for accurate control of a highly chemically reactive non-equilibrium environment with temperatures well below the crystallization temperature of Si QDs. PMID:26523743

  19. Excitation-induced Quantum Confined Stark Effect in a Coupled Double Quantum Wells

    NASA Astrophysics Data System (ADS)

    Shin, Y. H.; Park, Y. H.; Kim, Yongmin; Perry, C. H.

    2011-12-01

    We report a photoluminescence detected anticrossing of the energy levels in an undoped asymmetric coupled-double-quantum-well buried in a p-i-n structure. Due to the built-in electric field, the quantum wells are tilted in such a way that the symmetric energy level is higher than that of the antisymmetric one in the conduction band. Keeping the laser excitation energy below the barrier, with increasing laser power, the level anticrossing and the quantum confined Stark effect were observed due to decreasing built-in electric field by the photogenerated electron and hole pairs.

  20. Quantum confinement effects across two-dimensional planes in MoS{sub 2} quantum dots

    SciTech Connect

    Gan, Z. X.; Liu, L. Z.; Wu, H. Y.; Hao, Y. L.; Shan, Y.; Wu, X. L. E-mail: paul.chu@cityu.edu.hk; Chu, Paul K. E-mail: paul.chu@cityu.edu.hk

    2015-06-08

    The low quantum yield (∼10{sup −5}) has restricted practical use of photoluminescence (PL) from MoS{sub 2} composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS{sub 2} quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS{sub 2} QDs is also found to be sensitive to aggregation due to the size effect.

  1. Investigation of quantum confinement behavior of zinc sulphide quantum dots synthesized via various chemical methods

    SciTech Connect

    Jose, Meera Sakthivel, T. Chandran, Hrisheekesh T. Nivea, R. Gunasekaran, V.

    2014-10-15

    In this work, undoped and Ag-doped ZnS quantum dots were synthesized using various chemical methods. The products were characterized using X-ray diffraction (XRD), UV-visible spectroscopy and Photoluminescence spectroscopy. Our results revealed that the size of the as-prepared samples range from 1–6 nm in diameter and have a cubic zinc-blende structure. Also, we observed the emission of different wavelength of light from different sized quantum dots of the same material due to quantum confinement effect. The results will be presented in detail and ZnS can be a potential candidate for optical device development and applications.

  2. Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots

    SciTech Connect

    Lee, J I; Meulenberg, R W; Hanif, K M; Mattoussi, H; Klepeis, J E; Terminello, L J; van Buuren, T

    2006-12-15

    Recent theoretical descriptions as to the magnitude of effect that quantum confinement has on he conduction band (CB) of CdSe quantum dots (QD) have been conflicting. In this manuscript, we experimentally identify quantum confinement effects in the CB of CdSe QDs for the first time. Using X-ray absorption spectroscopy, we have unambiguously witnessed the CB minimum shift to higher energy with decreasing particle size and have been able to compare these results to recent theories. Our experiments have been able to identify which theories correctly describe the CB states in CdSe QDs. In particular, our experiments suggest that multiple theories describe the shifts in the CB of CdSe QDs and are not mutually exclusive.

  3. Quantum Metrology with Lattice-Confined Ultracold SR Atoms

    NASA Astrophysics Data System (ADS)

    Ludlow, A. D.; Campbell, G. K.; Blatt, S.; Boyd, M. M.; Martin, M. J.; Nicholson, T. L.; Swallows, M.; Thomsen, J. W.; Fortier, T.; Oates, C. W.; Diddams, S. A.; Lemke, N. D.; Barber, Z.; Porsev, S. G.; Ye, Jun

    2009-04-01

    Quantum state engineering of ultracold matter and precise control of optical fields have together allowed accurate measurement of light-matter interactions for applications in precision tests of fundamental physics. State-of-the-art lasers maintain optical phase coherence over one second. Optical frequency combs distribute this optical phase coherence across the entire visible and infrared parts of the electromagnetic spectrum, leading to the direct visualization and measurement of light ripples. At the same time, ultracold atoms confined in an optical lattice with zero differential ac Stark shift between two clock states allow us to minimize quantum decoherence while strengthening the clock signal. For 87Sr, we achieve a resonance quality factor > 2.4 × 1014 on the 1S0 - 3P0 doubly forbidden clock transition at 698 nm [1]. The uncertainty of this new clock has reached 1 × 10-16 and its instability approaches 1 × 10-15 at 1 s [2]. These developments represent a remarkable convergence of ultracold atoms, laser stabilization, and ultrafast science. Further improvements are still tantalizing, with quantum measurement and precision metrology combining forces to explore the next frontier.

  4. Quantum distillation and confinement of vacancies in a doublon sea

    NASA Astrophysics Data System (ADS)

    Xia, Lin; Zundel, Laura A.; Carrasquilla, Juan; Reinhard, Aaron; Wilson, Joshua M.; Rigol, Marcos; Weiss, David S.

    2015-04-01

    Ultracold atomic gases have revolutionized the study of non-equilibrium dynamics in quantum many-body systems. Many counterintuitive non-equilibrium effects have been observed, such as suppressed thermalization in a one-dimensional (1D) gas, the formation of repulsive self-bound dimers, and identical behaviours for attractive and repulsive interactions. Here, we observe the expansion of a bundle of ultracold 1D Bose gases in a flat-bottomed optical lattice potential. By combining in situ measurements with photoassociation, we follow the spatial dynamics of singly, doubly and triply occupied lattice sites. The system sheds interaction energy by dissolving some doublons and triplons. Some singlons quantum distil out of the doublon centre, whereas others remain confined. Our Gutzwiller mean-field model captures these experimental features in a physically clear way. These experiments might be used to study thermalization in systems with particle losses, the evolution of quantum entanglement or, if applied to fermions, to prepare very low entropy states.

  5. Exciton Kinetics in Strained II-Vi Semiconductor Multiple Quantum Wells.

    NASA Astrophysics Data System (ADS)

    Hefetz, Yaron

    1987-09-01

    Two groups of wide gap II-VI semiconductor superlattices based on ZnSe/Zn(,1-x)Mn(,x)Se and CdTe/ZnTe were investigated using CW and time-resolved photoluminescence, excitation, reflectance, and photomodulated reflectance spectroscopy at various temperatures and under an external magnetic field. All these lattice mismatch strained layer structures were grown by MBE technique and exhibit strong excitonic photoluminescence at low temperatures. By studying the dynamics of the exciton recombination processes, the role of strain, quantum confinement and localization effects were revealed. In the CdTe/ZnTc system where the lattice mismatch is (DELTA)a/a (TURNEQ) 6% the inhomogeneously broadened ((TURN)40 mev) luminescence line is governed by excitonic localization in well width fluctuations. Exchange interactions of the carriers with the Mn('++) ions in the dilute magnetic semiconductor Zn(,1-x)Mn(,x)Se in thin film and the barrier of the MQW structures influence their optical behavior in an exernal magnetic field. "Giant" Zeeman splittings of up to (TURN)10 mev/Tesla were measured in samples with moderate Mn concentration (x = .23). Antiferromagnetic interaction reduces these splittings in samples with higher Mn concentrations. In observing the time evolution of the carrier in Zn(,1-x)Mn(,x)Se MQW we found that the capture time of these carriers into the well is on the order of 1 psec but the last stages of thermalization, exciton formations and localization is (TURN)70 ps. The fast capture of electrons and holes into the quantum wells bypass the energy transfer into the Mn internal transition that is responsible to the efficient "yellow" luminescence in ZnMnSe mixed crystals.

  6. Relativistic quantum model of confinement and the current quark masses

    NASA Astrophysics Data System (ADS)

    Soloviev, L. D.

    1998-08-01

    We consider a relativistic quantum model of confined massive spinning quarks and antiquarks which describes the leading Regge trajectories of mesons. The quarks are described by the Dirac equations and the gluon contribution is approximated by the Nambu-Goto straight-line string. The string tension and the current quark masses are the main parameters of the model. Additional parameters are phenomenological constants which approximate nonstring short-range contributions. A comparison of the measured meson masses with the model predictions allows one to determine the current quark masses (in MeV) to be ms=227+/-5, mc=1440+/-10, and mb=4715+/-20. The chiral SU3 model makes it possible to estimate from here the u- and d-quark masses to be mu=6.2+/-0.2 Mev and md=11.1+/-0.4 Mev.

  7. Diamagnetic susceptibility of a confined donor in inhomogeneous quantum dots

    NASA Astrophysics Data System (ADS)

    Rahmani, K.; Zorkani, I.; Jorio, A.

    2011-03-01

    The binding energy and diamagnetic susceptibility χdia are estimated for a shallow donor confined to move in GaAs-GaAlAs inhomogeneous quantum dots. The calculation was performed within the effective mass approximation and using the variational method. The results show that the binding energy and the diamagnetic susceptibility χdia depend strongly on the core radius and the shell radius. We have demonstrated that there is a critical value of the ratio of the inner radius to the outer radius which may be important for nanofabrication techniques. The binding energy Eb shows a minimum for a critical value of this ratio depending on the value of the outer radius and shows a maximum when the donor is placed at the center of the spherical layer. The diamagnetic susceptibility is more sensitive to variations of the radius for a large spherical layer. The binding energy and diamagnetic susceptibility depend strongly on the donor position.

  8. Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstones.

    PubMed

    Gorshunov, Boris P; Zhukova, Elena S; Torgashev, Victor I; Lebedev, Vladimir V; Shakurov, Gil'man S; Kremer, Reinhard K; Pestrjakov, Efim V; Thomas, Victor G; Fursenko, Dimitry A; Dressel, Martin

    2013-06-20

    When water is confined to nanocavities, its quantum mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls. PMID:26283245

  9. Confinement-driven phase separation of quantum liquid mixtures.

    PubMed

    Prisk, T R; Pantalei, C; Kaiser, H; Sokol, P E

    2012-08-17

    We report small-angle neutron scattering studies of liquid helium mixtures confined in Mobil Crystalline Material-41 (MCM-41), a porous silica glass with narrow cylindrical nanopores (d=3.4 nm). MCM-41 is an ideal model adsorbent for fundamental studies of gas sorption in porous media because its monodisperse pores are arranged in a 2D triangular lattice. The small-angle scattering consists of a series of diffraction peaks whose intensities are determined by how the imbibed liquid fills the pores. Pure (4)He adsorbed in the pores show classic, layer-by-layer film growth as a function of pore filling, leaving the long range symmetry of the system intact. In contrast, the adsorption of (3)He-(4)He mixtures produces a structure incommensurate with the pore lattice. Neither capillary condensation nor preferential adsorption of one helium isotope to the pore walls can provide the symmetry-breaking mechanism. The scattering is consistent with the formation of randomly distributed liquid-liquid microdomains ∼2.3 nm in size, providing evidence that confinement in a nanometer scale capillary can drive local phase separation in quantum liquid mixtures. PMID:23006380

  10. Exciton-Phonon Interaction Effects in II-Vi Compound Semiconductor Quantum Wells

    NASA Astrophysics Data System (ADS)

    Pelekanos, Nikolaos Themelis

    1992-01-01

    In this thesis, we report on two specific examples of exciton-LO phonon Frohlich interaction effects, namely, hot carrier relaxation and temperature dependent exciton linewidth broadening. These phenomena are considered in the context of quasi-two dimensional excitons in strongly polar II-VI semiconductor quantum wells. Hot-exciton luminescence phenomena are investigated in a single quantum well of ZnTe/MnTe where tunneling through thin MnTe barrier layers suppresses the formation of thermalized luminescence. For near resonant photoexcitation, the secondary emission spectrum is modulated by distinct LO-phonon peaks, which, for sufficiently high order of scattering ( >=4), behave like hot luminescence (HPL) as opposed to resonant Raman scattering. This is confirmed by time-resolved spectroscopy as well as by steady-state characteristics such as linewidth broadening and lack of polarization memory. Several novel observations are made: (1) The LO-phonon intermediated energy relaxation involves Coulomb-correlated pairs, i.e. hot excitons, as opposed to independently-relaxing free electrons and holes. (2) The additional weak disorder originating from QW thickness fluctuations plays a major role in the details of the HPL spectra. The major contribution to the ground state exciton linewidth at room temperature originates from LO phonon -intermediated exciton scattering to higher exciton states. A measure of the effect is given by the parameter Gamma_{LO} which increases with the polarity of the material and is independent of dimensionality provided that the LO phonon energy is greater than the exciton binding energy. Measurements of Gamma_{LO} are performed in two quantum well systems: CdTe/MnTe and (Zn,Cd)Se/ZnSe. In the latter system, a strong reduction of Gamma _{LO} is observed as the quantum well width becomes comparable to the three-dimensional exciton Bohr radius. This is explained in terms of a model where quasi-2D confinement effects increase the exciton binding

  11. Quantum-confined Stark effect measurements in Ge/SiGe quantum-well structures.

    PubMed

    Chaisakul, Papichaya; Marris-Morini, Delphine; Isella, Giovanni; Chrastina, Daniel; Le Roux, Xavier; Gatti, Eleonora; Edmond, Samson; Osmond, Johann; Cassan, Eric; Vivien, Laurent

    2010-09-01

    We investigate the room-temperature quantum-confined Stark effect in Ge/SiGe multiple quantum wells (MQWs) grown by low-energy plasma-enhanced chemical vapor deposition. The active region is embedded in a p-i-n diode, and absorption spectra at different reverse bias voltages are obtained from optical transmission, photocurrent, and differential transmission measurements. The measurements provide accurate values of the fraction of light absorbed per well of the Ge/SiGe MQWs. Both Stark shift and reduction of exciton absorption peak are observed. Differential transmission indicates that there is no thermal contribution to these effects. PMID:20808367

  12. Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots

    SciTech Connect

    Lee, Jonathan R. I.; Meulenberg, Robert W.; Klepeis, John E.; Terminello, Louis J.; Buuren, Tony van; Hanif, Khalid M.; Mattoussi, Hedi

    2007-04-06

    X-ray absorption spectroscopy has been used to characterize the evolution in the conduction band (CB) density of states of CdSe quantum dots (QDs) as a function of particle size. We have unambiguously witnessed the CdSe QD CB minimum (CBM) shift to higher energy with decreasing particle size, consistent with quantum confinement effects, and have directly compared our results with recent theoretical calculations. At the smallest particle size, evidence for a pinning of the CBM is presented. Our observations can be explained by considering a size-dependent change in the angular-momentum-resolved states at the CBM.

  13. Return of the Quantum Cellular Automata: Episode VI

    NASA Astrophysics Data System (ADS)

    Carr, Lincoln D.; Hillberry, Logan E.; Rall, Patrick; Halpern, Nicole Yunger; Bao, Ning; Montangero, Simone

    2016-05-01

    There are now over 150 quantum simulators or analog quantum computers worldwide. Although exploring quantum phase transitions, many-body localization, and the generalized Gibbs ensemble are exciting and worthwhile endeavors, there are totally untapped directions we have not yet pursued. One of these is quantum cellular automata. In the past a principal goal of quantum cellular automata was to reproduce continuum single particle quantum physics such as the Schrodinger or Dirac equation from simple rule sets. Now that we begin to really understand entanglement and many-body quantum physics at a deeper level, quantum cellular automata present new possibilities. We explore several time evolution schemes on simple spin chains leading to high degrees of quantum complexity and nontrivial quantum dynamics. We explain how the 256 known classical elementary cellular automata reduce to just a few exciting quantum cases. Our analysis tools include mutual information based complex networks as well as more familiar quantifiers like sound speed and diffusion rate. Funded by NSF and AFOSR.

  14. Anomalous Light Emission and Wide Photoluminescence Spectra in Graphene Quantum Dot: Quantum Confinement from Edge Microstructure.

    PubMed

    Huang, Pu; Shi, Jun-Jie; Zhang, Min; Jiang, Xin-He; Zhong, Hong-Xia; Ding, Yi-Min; Cao, Xiong; Wu, Meng; Lu, Jing

    2016-08-01

    The physical origin of the observed anomalous photoluminescence (PL) behavior, that is, the large-size graphene quantum dots (GQDs) exhibiting higher PL energy than the small ones and the broadening PL spectra from deep ultraviolet to near-infrared, has been debated for many years. Obviously, it is in conflict with the well-accepted quantum confinement. Here we shed new light on these two notable debates by state-of-the-art first-principles calculations based on many-body perturbation theory. We find that quantum confinement is significant in GQDs with remarkable size-dependent exciton absorption/emission. The edge environment from alkaline to acidic conditions causes a blue shift of the PL peak. Furthermore, carbon vacancies are inclined to assemble at the GQD edge and form the tiny edge microstructures. The bound excitons, localized inside these edge microstructures, determine the anomalous PL behavior (blue and UV emission) of large-size GQDs. The bound excitons confined in the whole GQD lead to the low-energy transition. PMID:27409980

  15. Confinement of color states in a stochastic vacuum of quantum chromodynamics

    NASA Astrophysics Data System (ADS)

    Kuvshinov, V. I.; Bagashov, E. G.

    2015-09-01

    We show that in the framework of the stochastic vacuum model of quantum chromodynamics, quark confinement can be described as the decoherence of a color state of a particle into a mixed quantum state with equal probabilities for different colors. We evaluate the quantum characteristics of one-particle and multiparticle states: purity, fidelity, and the von Neumann entropy.

  16. Photooxidation and quantum confinement effects in exfoliated black phosphorus

    NASA Astrophysics Data System (ADS)

    Favron, Alexandre; Gaufrès, Etienne; Fossard, Frédéric; Phaneuf-L'Heureux, Anne-Laurence; Tang, Nathalie Y.-W.; Lévesque, Pierre L.; Loiseau, Annick; Leonelli, Richard; Francoeur, Sébastien; Martel, Richard

    2015-08-01

    Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the Ag1/Ag2 intensity ratio for ultrathin layers, a signature of oxidation.

  17. Experimental investigations of quantum confined silicon nanoparticle light emitting devices

    NASA Astrophysics Data System (ADS)

    Ligman, Rebekah Kristine

    2007-12-01

    As the demands on our world's energy resources continue to grow, alternative high efficiency materials such as quantum confined silicon nanoparticles (Si nps) are desirable for their potential low cost application in white light illumination, in optical displays, and in on-chip optical interconnects. Many fabrication and passivation techniques exist that produce Si nps with high photogenerated quantum yield. However, high electrically generated Si np quantum efficiency has eluded our society. Predominantly due to the lack of a stable surface passivation and a device fabrication technique that preserves the Si np optical properties. To amend these deficiencies, the passivation of nonthermal plasma fabricated Si nps with a surface oxide grown under UV exposure was first investigated. Control over the surface oxidized Si np (Si/SiO2) passivation growth was demonstrated and the optical stability of Si/SiO2 nps was suitable for demonstrating Si np electroluminescence (EL). Two approaches for constructing hybrid organic light emitting diode (OLED) devices around nonthermal plasma fabricated Si nps were then investigated. Multilayer devices, composed of a nonthermal plasma fabricated Si np layer embedded within an OLED, were first studied. However, no EL from Si nps was obtained using the multilayer device architecture due to poor control over the Si np film thickness. Single layer polymer(Si/SiO2) hybrid devices, composed of nps randomly dispersed within an extrinsic conductive polymer, were then studied and EL from Si/SiO2 nps was obtained. The hybrid device optical and electrical response was enhanced over the control devices, possibly due to morphology changes induced by the Si/SiO2 nps. The energy transfer (ET) processes in single layer polymer(Si/SiO 2) hybrid devices were then investigated by imposing known spatial separations between the intrinsic conductive polymers and Si/SiO2 nps. No measurable Si/SiO2 np emission was observed from the intrinsic hybrid devices

  18. Quantum confined stark effect in wide parabolic quantum wells: real density matrix approach

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David

    2015-12-01

    We show how to compute the optical functions of wide parabolic quantum wells (WPQWs) exposed to uniform electric F applied in the growth direction, in the excitonic energy region. The effect of the coherence between the electron-hole pair and the electromagnetic field of the propagating wave including the electron-hole screened Coulomb potential is adopted, and the valence band structure is taken into account in the cylindrical approximation. The role of the interaction potential and of the applied electric field, which mix the energy states according to different quantum numbers and create symmetry forbidden transitions, is stressed. We use the real density matrix approach (RDMA) and an effective e-h potential, which enable to derive analytical expressions for the WPQWs electrooptical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a GaAs/GaAlAs WPQWs. We have obtained a red shift of the absorption maxima (quantum confined Stark effect), asymmetric upon the change of the direction of the applied field ( F → - F), parabolic for the ground state and strongly dependent on the confinement parameters (the QWs sizes), changes in the oscillator strengths, and new peaks related to the states with different parity for electron and hole.

  19. XANES: observation of quantum confinement in the conduction band of colloidal PbS quantum dots

    NASA Astrophysics Data System (ADS)

    Demchenko, I. N.; Chernyshova, M.; He, X.; Minikayev, R.; Syryanyy, Y.; Derkachova, A.; Derkachov, G.; Stolte, W. C.; Piskorska-Hommel, E.; Reszka, A.; Liang, H.

    2013-04-01

    The presented investigations aimed at development of inexpensive method for synthesized materials suitable for utilization of solar energy. This important issue was addressed by focusing, mainly, on electronic local structure studies with supporting x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of colloidal galena nano-particles (NPs) and quantum dots (QDs) synthesized using wet chemistry under microwave irradiation. Performed x-ray absorption near edge structure (XANES) analysis revealed an evidence of quantum confinement for the sample with QDs, where the bottom of the conduction band was shifted to higher energy. The QDs were found to be passivated with oxides at the surface. Existence of sulfate/sulfite and thiosulfate species in pure PbS and QDs, respectively, was identified.

  20. Confined optical-phonon-assisted cyclotron resonance in quantum wells via two-photon absorption process

    NASA Astrophysics Data System (ADS)

    Phuc, Huynh Vinh; Hien, Nguyen Dinh; Dinh, Le; Phong, Tran Cong

    2016-06-01

    The effect of confined phonons on the phonon-assisted cyclotron resonance (PACR) via both one and two photon absorption processes in a quantum well is theoretically studied. We consider cases when electrons are scattered by confined optical phonons described by the Fuchs-Kliewer slab, Ridley's guided, and Huang-Zhu models. The analytical expression of the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. It predicts resonant peaks caused by transitions between Landau levels and electric subband accompanied by confined phonons emission in the absorption spectrum. The MOAC and the full-width at half-maximum (FWHM) for the intra- and inter-subband transitions are given as functions of the magnetic field, temperature, and quantum well width. In narrow quantum wells, the phonon confinement becomes more important and should be taken into account in studying FWHM.

  1. Quantum theory of an optical maser. VI - Transient behavior.

    NASA Technical Reports Server (NTRS)

    Wang, Y. K.; Lamb, W. E., Jr.

    1973-01-01

    The transient behavior of a laser is discussed using the quantum theory as did Scully and Lamb. The formal solution of the density-matrix equation is expressed in terms of exponentially decaying eigenmodes. Some of the lower decay constants are obtained numerically. The equations for the moments of the density matrix are then derived and solved by a truncation method. The equations of motion are integrated numerically for the case where the average number of photons in a laser cavity has the realistically large value 1.3 x 100,000. An alternative Fokker-Planck-equation approach is discussed.

  2. Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence.

    PubMed

    Yeh, Te-Fu; Huang, Wei-Lun; Chung, Chung-Jen; Chiang, I-Ting; Chen, Liang-Che; Chang, Hsin-Yu; Su, Wu-Chou; Cheng, Ching; Chen, Shean-Jen; Teng, Hsisheng

    2016-06-01

    Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp(2) domains, which leads to a change in the π*-π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology. PMID:27192445

  3. Confinement and inhomogeneous broadening effects in the quantum oscillatory magnetization of quantum dot ensembles.

    PubMed

    Herzog, F; Heedt, S; Goerke, S; Ibrahim, A; Rupprecht, B; Heyn, Ch; Hardtdegen, H; Schäpers, Th; Wilde, M A; Grundler, D

    2016-02-01

    We report on the magnetization of ensembles of etched quantum dots with a lateral diameter of 460 nm, which we prepared from InGaAs/InP heterostructures. The quantum dots exhibit 1/B-periodic de-Haas-van-Alphen-type oscillations in the magnetization M(B) for external magnetic fields B  >  2 T, measured by torque magnetometry at 0.3 K. We compare the experimental data to model calculations assuming different confinement potentials and including ensemble broadening effects. The comparison shows that a hard wall potential with an edge depletion width of 100 nm explains the magnetic behavior. Beating patterns induced by Rashba spin-orbit interaction (SOI) as measured in unpatterned and nanopatterned InGaAs/InP heterostructures are not observed for the quantum dots. From our model we predict that signatures of SOI in the magnetization could be observed in larger dots in tilted magnetic fields. PMID:26740509

  4. Efficient Multi-Dimensional Simulation of Quantum Confinement Effects in Advanced MOS Devices

    NASA Technical Reports Server (NTRS)

    Biegel, Bryan A.; Ancona, Mario G.; Rafferty, Conor S.; Yu, Zhiping

    2000-01-01

    We investigate the density-gradient (DG) transport model for efficient multi-dimensional simulation of quantum confinement effects in advanced MOS devices. The formulation of the DG model is described as a quantum correction ot the classical drift-diffusion model. Quantum confinement effects are shown to be significant in sub-100nm MOSFETs. In thin-oxide MOS capacitors, quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion of quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion of quantum effects in simulations dramatically improves the match between C-V simulations and measurements for oxide thickness down to 2 nm. Significant quantum corrections also occur in the I-V characteristics of short-channel (30 to 100 nm) n-MOSFETs, with current drive reduced by up to 70%. This effect is shown to result from reduced inversion charge due to quantum confinement of electrons in the channel. Also, subthreshold slope is degraded by 15 to 20 mV/decade with the inclusion of quantum effects via the density-gradient model, and short channel effects (in particular, drain-induced barrier lowering) are noticeably increased.

  5. Optimal quantum control via numerical pulse shape optimization for two exciton qubits confined to semiconductor quantum dots

    NASA Astrophysics Data System (ADS)

    Mathew, Reuble; Shi Yang, Hong Yi; Hall, Kimberley

    2015-03-01

    Optimal quantum control (OQC), which iteratively optimizes the control Hamiltonian to achieve a target quantum state, is a versatile approach for manipulating quantum systems. For optically-active transitions, OQC can be implemented using femtosecond pulse shaping which provides control over the amplitude and/or phase of the electric field. Optical pulse shaping has been employed to optimize physical processes such as nonlinear optical signals, photosynthesis, and has recently been applied to optimizing single-qubit gates in multiple semiconductor quantum dots. In this work, we examine the use of numerical pulse shape optimization for optimal quantum control of multiple qubits confined to quantum dots as a function of their electronic structure parameters. The numerically optimized pulse shapes were found to produce high fidelity quantum gates for a range of transition frequencies, dipole moments, and arbitrary initial and final states. This work enhances the potential for scalability by reducing the laser resources required to control multiple qubits.

  6. Efficient Multi-Dimensional Simulation of Quantum Confinement Effects in Advanced MOS Devices

    NASA Technical Reports Server (NTRS)

    Biegel, Bryan A.; Rafferty, Conor S.; Ancona, Mario G.; Yu, Zhi-Ping

    2000-01-01

    We investigate the density-gradient (DG) transport model for efficient multi-dimensional simulation of quantum confinement effects in advanced MOS devices. The formulation of the DG model is described as a quantum correction to the classical drift-diffusion model. Quantum confinement effects are shown to be significant in sub-100nm MOSFETs. In thin-oxide MOS capacitors, quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion or quantum effects in simulations dramatically improves the match between C-V simulations and measurements for oxide thickness down to 2 nm. Significant quantum corrections also occur in the I-V characteristics of short-channel (30 to 100 nm) n-MOSFETs, with current drive reduced by up to 70%. This effect is shown to result from reduced inversion charge due to quantum confinement of electrons in the channel. Also, subthreshold slope is degraded by 15 to 20 mV/decade with the inclusion of quantum effects via the density-gradient model, and short channel effects (in particular, drain-induced barrier lowering) are noticeably increased.

  7. Relaxation and coherent oscillations in the spin dynamics of II-VI diluted magnetic quantum wells

    NASA Astrophysics Data System (ADS)

    Ungar, F.; Cygorek, M.; Tamborenea, P. I.; Axt, V. M.

    2015-10-01

    We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic quantum wells in the presence of spin-orbit interaction. We extend a recent study where it was shown that the spin-orbit interaction and the exchange sd coupling in bulk and quantum wells can compete resulting in qualitatively new dynamics when they act simultaneously. We concentrate on Hg1-x-yMnxCdyTe quantum wells, which have a highly tunable Rashba spin-orbit coupling. Our calculations use a recently developed formalism which incorporates electronic correlations originating from the exchange sd-coupling. We find that the dependence of electronic spin oscillations on the excess energy changes qualitatively depending on whether or not the spin-orbit interaction dominates or is of comparable strength with the sd interaction.

  8. Spectral properties of a confined nonlinear quantum oscillator in one and three dimensions

    SciTech Connect

    Schulze-Halberg, Axel; Gordon, Christopher R.

    2013-04-15

    We analyze the spectral behaviour of a nonlinear quantum oscillator model under confinement. The underlying potential is given by a harmonic oscillator interaction plus a nonlinear term that can be weakened or strengthened through a parameter. Numerical eigenvalues of the model in one and three dimensions are presented. The asymptotic behaviour of the eigenvalues for confinement relaxation and for vanishing nonlinear term in the potential is investigated. Our findings are compared with existing results.

  9. Electron quasi-confined-optical-phonon interactions in wurtzite GaN/AlN quantum wells

    NASA Astrophysics Data System (ADS)

    Li, L.; Liu, D.; Shi, J.-J.

    2005-04-01

    The equation of motion for the p-polarization field in a wurtzite GaN/AlN multilayer heterostructure is solved for the quasi-confined-optical-phonon modes based on the dielectric-continuum model and Loudon’s uniaxial crystal model. The polarization eigenvector, the dispersion relation of the quasi-confined-optical-phonon modes and the electron-quasi-confined-phonon interaction Fröhlich-like Hamiltonian are derived. The analytical formulas can be directly applied to single/multiple quantum wells (QW’s) and superlattices. The electron-quasi-confined-phonon coupling functions are investigated for a given AlN/GaN/AlN single QW with full account of the strains of the QW structures and the anisotropy effect of wurtzite crystals. We find that there are two kinds of quasi-confined-optical-phonon modes in the GaN/AlN QW’s: the GaN-layer quasi-confined-optical-phonon modes and the AlN-layer quasi-confined-optical-phonon modes. There are infinite quasi-confined-optical-phonon branches, labelled by a quantum number n (n=1,2,...), with definite symmetry with respect to the center of the AlN/GaN/AlN single QW for a given phonon wave number q. The dispersions of the quasi-confined-optical-phonon modes with smaller n are more obvious than the ones with larger n. Moreover, the modes with smaller n are much more important for their electron-quasi-confined-phonon interactions than those with larger n. In most cases, it is enough to consider the modes with n≤ 8 for the electron-quasi-confined-phonon interactions in a single GaN/AlN QW. The higher frequency modes are more significant than the lower ones. The long-wavelength quasi-confined-optical-phonon modes are much more important for the electron-quasi-confined-phonon interactions. The GaN-layer quasi-confined-optical-phonon energies and their electron-quasi-confined-phonon interaction strength are markedly increased due to the strains of the QW structures. The influence of the strains on the the AlN-layer electron-quasi-confined

  10. Quantum confinement of zero-dimensional hybrid organic-inorganic polaritons at room temperature

    SciTech Connect

    Nguyen, H. S.; Lafosse, X.; Amo, A.; Bouchoule, S.; Bloch, J.; Abdel-Baki, K.; Lauret, J.-S.; Deleporte, E.

    2014-02-24

    We report on the quantum confinement of zero-dimensional polaritons in perovskite-based microcavity at room temperature. Photoluminescence of discrete polaritonic states is observed for polaritons localized in symmetric sphere-like defects which are spontaneously nucleated on the top dielectric Bragg mirror. The linewidth of these confined states is found much sharper (almost one order of magnitude) than that of photonic modes in the perovskite planar microcavity. Our results show the possibility to study organic-inorganic cavity polaritons in confined microstructure and suggest a fabrication method to realize integrated polaritonic devices operating at room temperature.

  11. Role of confinements on the melting of Wigner molecules in quantum dots

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Dyuti; Filinov, Alexei V.; Ghosal, Amit; Bonitz, Michael

    2016-03-01

    We explore the stability of a Wigner molecule (WM) formed in confinements with different geometries emulating the role of disorder and analyze the melting (or crossover) of such a system. Building on a recent calculation [D. Bhattacharya, A. Ghosal, Eur. Phys. J. B 86, 499 (2013)] that discussed the effects of irregularities on the thermal crossover in classical systems, we expand our studies in the untested territory by including both the effects of quantum fluctuations and of disorder. Our results, using classical and quantum (path integral) Monte Carlo techniques, unfold complementary mechanisms that drive the quantum and thermal crossovers in a WM and show that the symmetry of the confinement plays no significant role in determining the quantum crossover scale n X . This is because the zero-point motion screens the boundary effects within short distances. The phase diagram as a function of thermal and quantum fluctuations determined from independent criteria is unique, and shows "melting" from the WM to both the classical and quantum "liquids". An intriguing signature of weakening liquidity with increasing temperature, T, is found in the extreme quantum regime. The crossover is associated with production of defects. However, these defects appear to play distinct roles in driving the quantum and thermal "melting". Our analyses carry serious implications for a variety of experiments on many-particle systems - semiconductor heterostructure quantum dots, trapped ions, nanoclusters, colloids and complex plasma.

  12. Crystal-Phase Control by Solution-Solid-Solid Growth of II-VI Quantum Wires.

    PubMed

    Wang, Fudong; Buhro, William E

    2016-02-10

    A simple and potentially general means of eliminating the planar defects and phase alternations that typically accompany the growth of semiconductor nanowires by catalyzed methods is reported. Nearly phase-pure, defect-free wurtzite II-VI semiconductor quantum wires are grown from solid rather than liquid catalyst nanoparticles. The solid-catalyst nanoparticles are morphologically stable during growth, which minimizes the spontaneous fluctuations in nucleation barriers between zinc blende and wurtzite phases that are responsible for the defect formation and phase alternations. Growth of single-phase (in our cases the wurtzite phase) nanowires is thus favored. PMID:26731426

  13. Quantum confinement of excitons in wurtzite InP nanowires

    SciTech Connect

    Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.

    2015-05-21

    Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.

  14. Strongly confined tunnel-coupled one-dimensional electron systems from an asymmetric double quantum well

    NASA Astrophysics Data System (ADS)

    Buchholz, S. S.; Fischer, S. F.; Kunze, U.; Schuh, D.; Abstreiter, G.

    2008-03-01

    Vertically stacked quantum point contacts (QPCs) are prepared by atomic force microscope (AFM) lithography from an asymmetric GaAs/AlGaAs double quantum well (DQW) heterostructure. Top- and back-gate voltages are used to tune the tunnel-coupled QPCs, and back-gate bias cooling is employed to investigate coupled and decoupled one-dimensional (1D) modes. Parity dependent mode coupling is invoked by the particular asymmetry in the vertical DQW confinement.

  15. Quantum-Carnot engine for particle confined to cubic potential

    SciTech Connect

    Sutantyo, Trengginas Eka P. Belfaqih, Idrus H. Prayitno, T. B.

    2015-09-30

    Carnot cycle consists of isothermal and adiabatic processes which are reversible. Using analogy in quantum mechanics, these processes can be well explained by replacing variables in classical process with a quantum system. Quantum system which is shown in this paper is a particle that moves under the influence of a cubic potential which is restricted only to the state of the two energy levels. At the end, the efficiency of the system is shown as a function of the width ratio between the initial conditions and the farthest wall while expanding. Furthermore, the system efficiency will be considered 1D and 2D cases. The providing efficiencies are different due to the influence of the degeneration of energy and the degrees of freedom of the system.

  16. Quantum Dot Channel (QDC) FETs with Wraparound II-VI Gate Insulators: Numerical Simulations

    NASA Astrophysics Data System (ADS)

    Jain, F.; Lingalugari, M.; Kondo, J.; Mirdha, P.; Suarez, E.; Chandy, J.; Heller, E.

    2016-08-01

    This paper presents simulations predicting the feasibility of 9-nm wraparound quantum dot channel (QDC) field-effect transistors (FETs). In particular, II-VI lattice-matched layers which reduce the density of interface states, serving as top (tunnel gate), side, and bottom gate insulators, have been simulated. Quantum simulations show FET operation with voltage swing of ~0.2 V. Incorporation of cladded quantum dots, such as SiO x -Si and GeO x -Ge, under the gate tunnel oxide results in electrical transport in one or more quantum dot layers which form a quantum dot superlattice (QDSL). Long-channel QDC FETs have experimental multistate drain current (I D)-gate voltage (V G) and drain current (I D)-drain voltage (V D) characteristics, which can be attributed to the manifestation of extremely narrow energy minibands formed in the QDSL. An approach for modeling the multistate I D-V G characteristics is reported. The multistate characteristics of QDC FETs permit design of compact two-bit multivalued logic circuits.

  17. Demonstration of quantum entanglement between a single electron spin confined to an InAs quantum dot and a photon.

    PubMed

    Schaibley, J R; Burgers, A P; McCracken, G A; Duan, L-M; Berman, P R; Steel, D G; Bracker, A S; Gammon, D; Sham, L J

    2013-04-19

    The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this Letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to a single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dot's excited state. We obtain a lower bound on the fidelity of entanglement of 0.59±0.04, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement-based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3×10(3) s(-1). This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network. PMID:23679636

  18. Electronic structure and electron correlation in weakly confining spherical quantum dot potentials

    NASA Astrophysics Data System (ADS)

    Kimani, Peter Borgia Ndungu

    The electronic structure and electron correlations in weakly confining spherical quantum dots potentials are investigated. Following a common practice, the investigation starts with the restricted Hartree-Fock (HF) approximation. Then electron correlation is added in steps in a series of approximations based on the single particle Green's function approach: (i) Second-order Green function (GF) (ii) 2ph-Tamm-Dancoff approximation (TDA) and (iii) an extended version thereof (XTDA) which introduces ground-state correlation into the TDA. The study includes as well Hartree-Fock V (N-1) potential approximation in which framework the Hartree-Fock virtual orbitals are calculated in the field of the N-1 electrons as opposed to the regular but unphysical N-electron field Hartree-Fock calculation of virtual orbitals. For contrast and comparison, the same approximation techniques are applied to few-electron closed-shell atoms and few-electron negative ions for which pertinent data is readily available. The results for the weakly confining spherical quantum dot potentials and the standard atomic systems exhibit fundamental similarities as well as significant differences. For the most part the results of these calculations are in favor of application of HF, GF, and TDA techniques in the modeling of three-dimensional weakly confining quantum dot potentials. The observed differences emphasize the significance of confinement and electronic features unique to quantum dots such as the increased binding of electrons with higher angular momentum and the modified shell filling sequences.

  19. Stark Effect of Excitons in a Quantum Nano-rod with Parabolic Confinement

    NASA Astrophysics Data System (ADS)

    Lyo, S. K.

    2013-03-01

    We study the exciton binding energy and the oscillator strength as a function of a DC electric field in a quasi-one-dimensional quantum dot (i.e., nano rod) with parabolic confinements in the conduction and valence bands. The relative importance of the quantum confinement and electron-hole interaction is examined by varying the the linear confinement length (i.e., rod length). We find an abrupt decrease of the oscillator strength, loss of exciton binding energy, and a sudden increase of the root-mean-square average of electron-hole separation as the excitons are dissociated at the threshold field. The field dependence of the effects are also investigated as a function of the rod length and the radius of the nano rod. The numerical results are applied to GaAs and CdSe rods. This work was supported by DOE/BES under Contract No.DE-AC04-94AL85000.

  20. Ground state of the holes localized in II-VI quantum dots with Gaussian potential profiles

    NASA Astrophysics Data System (ADS)

    Semina, M. A.; Golovatenko, A. A.; Rodina, A. V.

    2016-01-01

    We report on a theoretical study of the hole states in II-IV quantum dots of spherical and ellipsoidal shapes, described by smooth potential confinement profiles that can be modeled by Gaussian functions in all three dimensions. The universal dependencies of the hole energy, g factor, and localization length on the quantum dot barrier height, as well as the ratio of effective masses of the light and heavy holes are presented for the spherical quantum dots. The splitting of the fourfold degenerate ground state into two doublets is derived for anisotropic (oblate or prolate) quantum dots. Variational calculations are combined with numerical ones in the framework of the Luttinger Hamiltonian. Constructed trial functions are optimized by comparison with the numerical results. The effective hole g factor is found to be independent of the quantum dot size and barrier height and is approximated by a simple universal expression depending only on the effective mass parameters. The results can be used for interpreting and analyzing experimental spectra measured in various structures with quantum dots of different semiconductor materials.

  1. Vortex anomaly in low-dimensional fermionic condensates: Quantum confinement breaks chirality

    NASA Astrophysics Data System (ADS)

    Chen, Yajiang; Shanenko, A. A.; Peeters, F. M.

    2014-02-01

    Chiral fermions are responsible for low-temperature properties of vortices in fermionic condensates, both superconducting (charged) and superfluid (neutral). One of the most striking consequences of this fact is that the core of a single-quantum vortex collapses at low temperatures, T →0 (i.e., the Kramer-Pesch effect for superconductors), due to the presence of chiral quasiparticles in the vortex-core region. We show that the situation changes drastically for fermionic condensates confined in quasi-one-dimensional and quasi-two-dimensional geometries. Here quantum confinement breaks the chirality of in-core fermions. As a result, instead of the ultimate shrinking, the core of a single-quantum vortex extends at low temperatures, and the condensate profile surprisingly mimics the multiquantum vortex behavior. Our findings are relevant for nanoscale superconductors, such as recent metallic nanoislands on silicon, and also for ultracold superfluid Fermi gases in cigar-shaped and pancake-shaped atomic traps.

  2. Effect of phonon confinement on lattice thermal conductivity of lead Telluride quantum well structure

    SciTech Connect

    Tripathi, Madhvendra Nath

    2014-04-24

    The paper examines the effect of spatial confinement of acoustic phonons on average group velocity and consequently the lattice thermal conductivity of a free-standing PbTe quantum well structure and their temperature dependence. The average group velocity at 100 Å decreases 30% to the bulk value and falls more rapidly on reducing the width of quantum well. Moreover, the lattice thermal conductivity of 100 Å wide PbTe quantum well with value of 0.60 W/mK shows considerable decrease of 70% compared to it’s bulk value. It is observed that the effect of reduction in well width is less pronounce as temperature increases. This appears mainly due to dominance of umklapp processes over the confinement effects.

  3. Electronic and optical properties of exciton, trions and biexciton in II-VI parabolic quantum dot

    NASA Astrophysics Data System (ADS)

    Sujanah, P.; John Peter, A.; Woo Lee, Chang

    2015-08-01

    Binding energies of exciton, trions and biexciton and their interband optical transition energies are studied in a CdTe/ZnTe quantum dot nanostructure taking into consideration the geometrical confinement effect. The radial spread of the wavefunctions, binding energies, optical transition energies, oscillator strength, radiative life time and the absorption coefficients of exciton, positively and negatively charged excitons and biexciton are carried out. It is found that the ratio of the radiative life time of exciton with the trions and biexciton enhances with the reduction of geometrical confinement. The results show that (i) the binding energies of exciton, positive and negative trions and the biexciton have strong influence on the reduction of geometrical confinement effect, (ii) the binding energy is found to decrease from the binding energies of exciton to positive trion through biexciton and negative trion binding energies, (iii) the oscillator strength of trions is found to be lesser than exciton and the biexciton and (iv) the electronic and optical properties of exciton, trions and the biexciton are considerably dependent on the spatial confinement, incident photon energy and the radiative life time. The obtained results are in good agreement with the other existing literature.

  4. Confinement-deconfinement transition due to spontaneous symmetry breaking in quantum Hall bilayers.

    PubMed

    Pikulin, D I; Silvestrov, P G; Hyart, T

    2016-01-01

    Band-inverted electron-hole bilayers support quantum spin Hall insulator and exciton condensate phases. Interest in quantum spin Hall effect in these systems has recently put them in the spotlight. We investigate such a bilayer in an external magnetic field. We show that the interlayer correlations lead to formation of a helical quantum Hall exciton condensate state. Existence of the counterpropagating edge modes in this system results in formation of a ground state spin-texture not supporting gapless single-particle excitations. The charged edge excitations in a sufficiently narrow Hall bar are confined: a charge on one of the edges always gives rise to an opposite charge on the other edge. Magnetic field and gate voltages allow the control of a confinement-deconfinement transition of charged edge excitations, which can be probed with nonlocal conductance. Confinement-deconfinement transitions are of great interest, not least because of their possible significance in shedding light on the confinement problem of quarks. PMID:26804790

  5. Confinement-deconfinement transition due to spontaneous symmetry breaking in quantum Hall bilayers

    PubMed Central

    Pikulin, D. I.; Silvestrov, P. G.; Hyart, T.

    2016-01-01

    Band-inverted electron-hole bilayers support quantum spin Hall insulator and exciton condensate phases. Interest in quantum spin Hall effect in these systems has recently put them in the spotlight. We investigate such a bilayer in an external magnetic field. We show that the interlayer correlations lead to formation of a helical quantum Hall exciton condensate state. Existence of the counterpropagating edge modes in this system results in formation of a ground state spin-texture not supporting gapless single-particle excitations. The charged edge excitations in a sufficiently narrow Hall bar are confined: a charge on one of the edges always gives rise to an opposite charge on the other edge. Magnetic field and gate voltages allow the control of a confinement-deconfinement transition of charged edge excitations, which can be probed with nonlocal conductance. Confinement-deconfinement transitions are of great interest, not least because of their possible significance in shedding light on the confinement problem of quarks. PMID:26804790

  6. Quantum mechanical solver for confined heterostructure tunnel field-effect transistors

    SciTech Connect

    Verreck, Devin Groeseneken, Guido; Van de Put, Maarten; Sorée, Bart; Magnus, Wim; Verhulst, Anne S.; Collaert, Nadine; Thean, Aaron; Vandenberghe, William G.

    2014-02-07

    Heterostructure tunnel field-effect transistors (HTFET) are promising candidates for low-power applications in future technology nodes, as they are predicted to offer high on-currents, combined with a sub-60 mV/dec subthreshold swing. However, the effects of important quantum mechanical phenomena like size confinement at the heterojunction are not well understood, due to the theoretical and computational difficulties in modeling realistic heterostructures. We therefore present a ballistic quantum transport formalism, combining a novel envelope function approach for semiconductor heterostructures with the multiband quantum transmitting boundary method, which we extend to 2D potentials. We demonstrate an implementation of a 2-band version of the formalism and apply it to study confinement in realistic heterostructure diodes and p-n-i-n HTFETs. For the diodes, both transmission probabilities and current densities are found to decrease with stronger confinement. For the p-n-i-n HTFETs, the improved gate control is found to counteract the deterioration due to confinement.

  7. Confinement-deconfinement transition due to spontaneous symmetry breaking in quantum Hall bilayers

    NASA Astrophysics Data System (ADS)

    Pikulin, D. I.; Silvestrov, P. G.; Hyart, T.

    2016-01-01

    Band-inverted electron-hole bilayers support quantum spin Hall insulator and exciton condensate phases. Interest in quantum spin Hall effect in these systems has recently put them in the spotlight. We investigate such a bilayer in an external magnetic field. We show that the interlayer correlations lead to formation of a helical quantum Hall exciton condensate state. Existence of the counterpropagating edge modes in this system results in formation of a ground state spin-texture not supporting gapless single-particle excitations. The charged edge excitations in a sufficiently narrow Hall bar are confined: a charge on one of the edges always gives rise to an opposite charge on the other edge. Magnetic field and gate voltages allow the control of a confinement-deconfinement transition of charged edge excitations, which can be probed with nonlocal conductance. Confinement-deconfinement transitions are of great interest, not least because of their possible significance in shedding light on the confinement problem of quarks.

  8. Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction.

    PubMed

    Lei, Fengcai; Liu, Wei; Sun, Yongfu; Xu, Jiaqi; Liu, Katong; Liang, Liang; Yao, Tao; Pan, Bicai; Wei, Shiqiang; Xie, Yi

    2016-01-01

    Ultrathin metal layers can be highly active carbon dioxide electroreduction catalysts, but may also be prone to oxidation. Here we construct a model of graphene confined ultrathin layers of highly reactive metals, taking the synthetic highly reactive tin quantum sheets confined in graphene as an example. The higher electrochemical active area ensures 9 times larger carbon dioxide adsorption capacity relative to bulk tin, while the highly-conductive graphene favours rate-determining electron transfer from carbon dioxide to its radical anion. The lowered tin-tin coordination numbers, revealed by X-ray absorption fine structure spectroscopy, enable tin quantum sheets confined in graphene to efficiently stabilize the carbon dioxide radical anion, verified by 0.13 volts lowered potential of hydroxyl ion adsorption compared with bulk tin. Hence, the tin quantum sheets confined in graphene show enhanced electrocatalytic activity and stability. This work may provide a promising lead for designing efficient and robust catalysts for electrolytic fuel synthesis. PMID:27585984

  9. First-principles study of orbital-dependent quantum confinement in Si/Ge nanowire superlattices

    NASA Astrophysics Data System (ADS)

    Kim, Min-Kook; Choi, Hyoung Joon

    2012-02-01

    We study electronic structures of H-passivated Si/Ge nanowire superlattices (NWSLs) oriented along [110] direction, using an ab-initio pseudopotential density-functional method with the local density approximation. Obtained electronic structures of the Si/Ge NWSLs show both dispersive and non-dispersive bands in conduction and valence bands due to band-selective quantum confinement: the highest valence band and the lowest conduction band are not confined in either the Si- or Ge-nanowire segment but they are extended throughout the whole NWSLs, while there exist non-dispersive bands confined in either Si- or Ge-nanowire segment below the top of the valence band and above the bottom of the conduction band. This feature originates from strong orbital-dependence of quantum confinement of electronic states, making conventional band-offset diagrams for superlattices invalid in Si/Ge NWSLs. Effects of atomic geometries on the confinement are studied with different diameters and superlattice periodicities. This work was supported by NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2011-C3-05).

  10. Quantum confined electron-phonon interaction in silicon nanocrystals.

    PubMed

    Sagar, D M; Atkin, Joanna M; Palomaki, Peter K B; Neale, Nathan R; Blackburn, Jeffrey L; Johnson, Justin C; Nozik, Arthur J; Raschke, Markus B; Beard, Matthew C

    2015-03-11

    We study the micro-Raman spectra of colloidal silicon nanocrystals as a function of size, excitation wavelength, and excitation intensity. We find that the longitudinal optical (LO) phonon spectrum is asymmetrically broadened toward the low energy side and exhibits a dip or antiresonance on the high-energy side, both characteristics of a Fano line shape. The broadening depends on both nanocrystal size and Raman excitation wavelength. We propose that the Fano line shape results from interference of the optical phonon response with a continuum of electronic states that become populated by intraband photoexcitation of carriers. The asymmetry exhibits progressive enhancement with decreasing particle size and with increasing excitation energy for a given particle size. We compare our observations with those reported for p- and n-doped bulk Si, where Fano interference has also been observed, but we find opposite wavelength dependence of the asymmetry for the bulk and nanocrystalline Si. Our results have important implications for potentially controlling carrier energy relaxation channels in strongly confined Si nanocrystals. PMID:25626139

  11. Lateral carrier confinement in InGaN quantum-well nanorods

    SciTech Connect

    Shi, Chentian; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min

    2015-07-15

    We review our studies on lateral carrier diffusion in micro-fabricated samples of InGaN nanorods and their parent quantum wells. The carrier diffusion is observed to be strongly confined in nanorods, as manifested by the reduction in the delayed-rise component of time-resolved photoluminescence traces. We further argue that the confinement of carrier diffusion can be applied to suppress the efficiency droop related to defect state recombination and to assist in the energy transfer between InGaN nanorods and nanocrystal phosphors for color conversion.

  12. Influence of the nanoparticles agglomeration state in the quantum-confinement effects: Experimental evidences

    SciTech Connect

    Lorite, I.; Romero, J. J.; Fernandez, J. F.

    2015-03-15

    The agglomeration state facilitates particle-particle interaction which produces important effects in the phonon confinement effects at the nanoscale. A partial phonon transmission between close nanoparticles yields a lower momentum conservation relaxation than in a single isolated nanoparticle. It means a larger red shift and broadening of the Raman modes than the expected ones for Raman quantum confinement effects. This particle-particle interaction can drive to error when Raman responses are used to estimate the size of the nanoscaled materials. In this work different corrections are suggested to overtake this source of error.

  13. Atomic layer epitaxy of II-VI quantum wells and superlattices

    NASA Astrophysics Data System (ADS)

    Faschinger, W.

    1993-01-01

    Atomic Layer Epitaxy (ALE) under ultra high vacuum conditions is a variation of MBE which makes use of a self-regulating growth process, leading to digital growth in steps of monolayers or even fractions of monolayers. We report on fundamental aspects of the ALE growth of tellurides and selenides, and give three examples on the physics of ALE-grown structures: (a) Phonon confinement in CdTe/ZnTe superlattices (b) "Spin Sheet" superlattices of cubic MnTe with CdTe and (c) Luminescence tuning in ultra-thin CdSe quantum wells embedded in ZnSe.

  14. Engineering the hole confinement for CdTe-based quantum dot molecules

    NASA Astrophysics Data System (ADS)

    Kłopotowski, Ł.; Wojnar, P.; Kret, S.; Parlińska-Wojtan, M.; Fronc, K.; Wojtowicz, T.; Karczewski, G.

    2015-06-01

    We demonstrate an efficient method to engineer the quantum confinement in a system of two quantum dots grown in a vertical stack. We achieve this by using materials with a different lattice constant for the growth of the outer and inner barriers. We monitor the resulting dot morphology with transmission electron microscopy studies and correlate the results with ensemble quantum dot photoluminescence. Furthermore, we embed the double quantum dots into diode structures and study photoluminescence as a function of bias voltage. We show that in properly engineered structures, it is possible to achieve a resonance of the hole states by tuning the energy levels with electric field. At the resonance, we observe signatures of a formation of a molecular state, hybridized over the two dots.

  15. Engineering the hole confinement for CdTe-based quantum dot molecules

    SciTech Connect

    Kłopotowski, Ł. Wojnar, P.; Kret, S.; Fronc, K.; Wojtowicz, T.; Karczewski, G.

    2015-06-14

    We demonstrate an efficient method to engineer the quantum confinement in a system of two quantum dots grown in a vertical stack. We achieve this by using materials with a different lattice constant for the growth of the outer and inner barriers. We monitor the resulting dot morphology with transmission electron microscopy studies and correlate the results with ensemble quantum dot photoluminescence. Furthermore, we embed the double quantum dots into diode structures and study photoluminescence as a function of bias voltage. We show that in properly engineered structures, it is possible to achieve a resonance of the hole states by tuning the energy levels with electric field. At the resonance, we observe signatures of a formation of a molecular state, hybridized over the two dots.

  16. Fluorescent sensor for Cr(VI) based in functionalized silicon quantum dots with dendrimers.

    PubMed

    Campos, B B; Algarra, M; Alonso, B; Casado, C M; Jiménez-Jiménez, J; Rodríguez-Castellón, E; Esteves da Silva, J C G

    2015-11-01

    Highly luminescent nanoparticles based in Silicon quantum dots, coated by hydroxyl PAMAM dendrimer (PAMAM-OH) of 5th generation, were obtained by one step process by hydrothermal treatment of 3-Aminopropyl)triethoxysilane (APTES) in aqueous solution. Previous to the optimization of the synthesis procedure, different dendritic molecules of 5th generation were tested to obtain the most intense fluorescence signal. The influence of different parameters such ratio APTES/PAMAM-OH, pH and ionic strength on the fluorescence intensity was studied. The fluorescence spectra showed maximum excitation and emission wavelengths at 370 and 446 nm, respectively. The obtained silicon nanoparticles (SiQDs@PAMAM-OH) were characterized by TEM, DLS and XPS, and were found to detect selectively Cr(VI) in aqueous solutions at 2.7 μM level of detection, sensitivity of 0.2 μM with a RSD of 0.16% (n=10). To study the feasibility of the proposed system for Cr(VI) detection, it was tested in real electrochemical solution bath and a tanning effluent obtained from electrochemical industry and with two certified waters, demonstrating promising outcomes as nano-sensor. PMID:26452901

  17. Photoluminescence quantum efficiency of various ternary II VI semiconductor solid solutions

    NASA Astrophysics Data System (ADS)

    Westphäling, R.; Bauer, S.; Klingshirn, C.; Reznitstsky, A.; Verbin, S.

    1998-02-01

    As a result of the spatial localization of excitons in II-VI mixed crystals the external luminescence quantum efficiency η lum is expected to be remarkably higher than in the corresponding binary compounds. To investigate this assumption we built a new experimental setup with a miniature integrating sphere fitted into a cryostat. At low temperatures in the binary systems CdS and CdSe we always found η lum ⩽ 25% in the main luminescence bands (arising from bound excitons (D 0X, A 0X) and donor—acceptor pair recombination). For the free-exciton luminescence η lum was more than two orders of magnitude less. In contrast, CdS 1- xSe x mixed crystals show η lum up to 70% in the luminescence from localized states, indicating that the nonradiative recombination is strongly suppressed for localized excitons. Other II-VI alloys (ZnSe 1- xTe x Zn 1- xCd xS and Zn 1- xCd xSe) show partly considerably lower values for η lum. The temperature dependence of η lum gives information about various activation processes to nonradiative recombination channels.

  18. Self-screening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers

    SciTech Connect

    Zhang, Zi-Hui; Liu, Wei; Ju, Zhengang; Tiam Tan, Swee; Ji, Yun; Kyaw, Zabu; Zhang, Xueliang; Wang, Liancheng; Wei Sun, Xiao E-mail: volkan@stanfordalumni.org; Volkan Demir, Hilmi E-mail: volkan@stanfordalumni.org

    2014-06-16

    InGaN/GaN light-emitting diodes (LEDs) grown along the polar orientations significantly suffer from the quantum confined Stark effect (QCSE) caused by the strong polarization induced electric field in the quantum wells, which is a fundamental problem intrinsic to the III-nitrides. Here, we show that the QCSE is self-screened by the polarization induced bulk charges enabled by designing quantum barriers. The InN composition of the InGaN quantum barrier graded along the growth orientation opportunely generates the polarization induced bulk charges in the quantum barrier, which well compensate the polarization induced interface charges, thus avoiding the electric field in the quantum wells. Consequently, the optical output power and the external quantum efficiency are substantially improved for the LEDs. The ability to self-screen the QCSE using polarization induced bulk charges opens up new possibilities for device engineering of III-nitrides not only in LEDs but also in other optoelectronic devices.

  19. Self-screening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers

    NASA Astrophysics Data System (ADS)

    Zhang, Zi-Hui; Liu, Wei; Ju, Zhengang; Tiam Tan, Swee; Ji, Yun; Kyaw, Zabu; Zhang, Xueliang; Wang, Liancheng; Wei Sun, Xiao; Volkan Demir, Hilmi

    2014-06-01

    InGaN/GaN light-emitting diodes (LEDs) grown along the polar orientations significantly suffer from the quantum confined Stark effect (QCSE) caused by the strong polarization induced electric field in the quantum wells, which is a fundamental problem intrinsic to the III-nitrides. Here, we show that the QCSE is self-screened by the polarization induced bulk charges enabled by designing quantum barriers. The InN composition of the InGaN quantum barrier graded along the growth orientation opportunely generates the polarization induced bulk charges in the quantum barrier, which well compensate the polarization induced interface charges, thus avoiding the electric field in the quantum wells. Consequently, the optical output power and the external quantum efficiency are substantially improved for the LEDs. The ability to self-screen the QCSE using polarization induced bulk charges opens up new possibilities for device engineering of III-nitrides not only in LEDs but also in other optoelectronic devices.

  20. Excitons in artificial quantum dots in the weak spatial confinement regime

    SciTech Connect

    Zaitsev, S. V. Welsch, M. K.; Forchel, A.; Bacher, G.

    2007-12-15

    The exciton states in individual quantum dots prepared by the selective interdiffusion method in CdTe/CdMgTe quantum wells are studied by the methods of steady-state optical spectroscopy. The annealing-induced diffusion of Mg atoms inward to the bulk of the quantum well, which is significantly enhanced under the SiO{sub 2} mask, leads to a modulation of the bandgap width in the plane of the well, with the minima of the potential being located in the mask aperture areas. A lateral potential that arises, whose height is in the range 30-270 meV and characteristic scale is about 100 nm, efficiently localizes carriers, which form quasi-zero-dimensional excitons in the weak spatial confinement regime. Detailed magnetooptical studies show that Coulomb correlations play a significant role in the formation of exciton states under such a regime, which, in particular, manifests itself in the localization of the wavefunction of carriers on scales that are considerably smaller than the scale of the lateral potential. The particular features of the interlevel splitting, of the biexciton binding energy, and of the diamagnetic shift are discussed. A strong dependence of the interlevel relaxation on the interlevel splitting (the phonon neck) indicates that alternative relaxation mechanisms in the quantum dots studied are weak. The excited states are populated according to the Pauli principle, which indicates that it is possible to apply the shell model of many-exciton states to quantum dots under the weak spatial confinement conditions.

  1. Photoluminescence in quantum-confined SnO2 nanocrystals: Evidence of free exciton decay

    NASA Astrophysics Data System (ADS)

    Lee, E. J. H.; Ribeiro, C.; Giraldi, T. R.; Longo, E.; Leite, E. R.; Varela, J. A.

    2004-03-01

    Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2. The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process.

  2. Quantum confinement effect in cheese like silicon nano structure fabricated by metal induced etching

    SciTech Connect

    Saxena, Shailendra K. Sahu, Gayatri; Sagdeo, Pankaj R.; Kumar, Rajesh

    2015-08-28

    Quantum confinement effect has been studied in cheese like silicon nano-structures (Ch-SiNS) fabricated by metal induced chemical etching using different etching times. Scanning electron microscopy is used for the morphological study of these Ch-SiNS. A visible photoluminescence (PL) emission is observed from the samples under UV excitation at room temperature due to quantum confinement effect. The average size of Silicon Nanostructures (SiNS) present in the samples has been estimated by bond polarizability model using Raman Spectroscopy from the red-shift observed from SiNSs as compared to its bulk counterpart. The sizes of SiNS present in the samples decreases as etching time increase from 45 to 75 mintunes.

  3. Dielectric confinement influenced screened Coulomb potential for a semiconductor quantum wire

    NASA Astrophysics Data System (ADS)

    Aharonyan, K. H.; Margaryan, N. B.

    2016-01-01

    A formalism of the Thomas-Fermi method has been applied for studying the screening effect due to quasi-one-dimensional electron gas in a semiconductor cylindrical quantum wire embedded in the barrier environment. With taking into account of strongly low dielectric properties of the barrier material, an applicability of the quantum wire effective interaction potential of the confined charge carriers has been revealed. Both screened quasi- one-dimensional interaction potential and effective screening length analytical expressions are derived in the first time. It is shown that in the long wavelength moderate limit dielectric confinement effect enhances strength of the screening potential depending on the both radius of the wire and effective screening length, whereas in the long wavelength strong limit the screening potential solely is determined by barrier environment dielectric properties.

  4. Quantum confinement effect in cheese like silicon nano structure fabricated by metal induced etching

    NASA Astrophysics Data System (ADS)

    Saxena, Shailendra K.; Sahu, Gayatri; Sagdeo, Pankaj R.; Kumar, Rajesh

    2015-08-01

    Quantum confinement effect has been studied in cheese like silicon nano-structures (Ch-SiNS) fabricated by metal induced chemical etching using different etching times. Scanning electron microscopy is used for the morphological study of these Ch-SiNS. A visible photoluminescence (PL) emission is observed from the samples under UV excitation at room temperature due to quantum confinement effect. The average size of Silicon Nanostructures (SiNS) present in the samples has been estimated by bond polarizability model using Raman Spectroscopy from the red-shift observed from SiNSs as compared to its bulk counterpart. The sizes of SiNS present in the samples decreases as etching time increase from 45 to 75 mintunes.

  5. Confined acoustic and optical plasmons in double-layered quantum-wire arrays with strong tunneling

    NASA Astrophysics Data System (ADS)

    Dethlefsen, A. F.; Heyn, Ch.; Heitmann, D.; Schüller, C.

    2006-05-01

    We investigate electronic excitations in GaAs-AlxGa1-xAs double-layered quantum wire arrays with strong tunneling coupling by resonant inelastic light scattering. By applying an external electric field, we can change the one-dimensional (1D) electron density and the symmetry of the double quantum-well (DQW) structure at the same time. We identify confined optical 1D intersubband plasmons (COP) and confined acoustic 1D intersubband plasmons (CAP). Due to the tunneling coupling, the energies of the CAP exhibit a minimum for a symmetric DQW potential, whereas the energies of the COP are dominated by the total carrier density, and are nearly insensitive to the symmetry of the potential.

  6. Quantum-confined emission and fluorescence blinking of individual exciton complexes in CdSe nanowires.

    PubMed

    Franz, Dennis; Reich, Aina; Strelow, Christian; Wang, Zhe; Kornowski, Andreas; Kipp, Tobias; Mews, Alf

    2014-11-12

    One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with high-resolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure-property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra. PMID:25343231

  7. Self-Induced Oscillation for Electron-Hole Pair Confined in Quantum Dot

    SciTech Connect

    Tagawa, Tomoki; Tsubaki, Atsushi; Ishizuki, Masamu; Takeda, Kyozaburo

    2011-12-23

    We study the time-dependent (TD) phenomena of the electron-hole or electron-electron pair confined in the square quantum dot (SQD) system by computationally solving TD Schroedinger equation under the unrestricted Hartree-Fock (UHF) approach. A typical vacillation is found both in the electron and hole when the charged pair is strongly confined in the SQD while the charged particles have initially the same orbital symmetry. The FFT analysis elucidates that the transition matrix element due to the coulomb interaction involves the eigen frequency {omega} being equal to the excitation energy when the resonative vacillation appears. Thus, Coulomb potential has a potential to cause the self-induced ''Rabi'' oscillation when the charged-particle pair is confined only in the QD.

  8. Energies and densities of electrons confined in elliptical and ellipsoidal quantum dots.

    PubMed

    Halder, Avik; Kresin, Vitaly V

    2016-10-01

    We consider a droplet of electrons confined within an external harmonic potential well of elliptical or ellipsoidal shape, a geometry commonly encountered in work with semiconductor quantum dots and other nanoscale or mesoscale structures. For droplet sizes exceeding the effective Bohr radius, the dominant contribution to average system parameters in the Thomas-Fermi approximation comes from the potential energy terms, which allows us to derive expressions describing the electron droplet's shape and dimensions, its density, total and capacitive energy, and chemical potential. The analytical results are in very good agreement with experimental data and numerical calculations, and make it possible to follow the dependence of the properties of the system on its parameters (the total number of electrons, the axial ratios and curvatures of the confinement potential, and the dielectric constant of the material). An interesting feature is that the eccentricity of the electron droplet is not the same as that of its confining potential well. PMID:27502044

  9. Size control and quantum confinement in Cu2ZnSnS4 nanocrystals.

    PubMed

    Khare, Ankur; Wills, Andrew W; Ammerman, Lauren M; Norris, David J; Aydil, Eray S

    2011-11-14

    Starting with metal dithiocarbamate complexes, we synthesize colloidal Cu(2)ZnSnS(4) (CZTS) nanocrystals with diameters ranging from 2 to 7 nm. Structural and Raman scattering data confirm that CZTS is obtained rather than other possible material phases. The optical absorption spectra of nanocrystals with diameters less than 3 nm show a shift to higher energy due to quantum confinement. PMID:21952415

  10. Quantum confinement and quasiparticle corrections in α-HgS from first principles

    NASA Astrophysics Data System (ADS)

    Lanzillo, Nicholas A.; Roy, Sujit; Nayak, Saroj K.

    2015-06-01

    Using a combination of density functional theory and many-body GW corrections, we calculate the quasiparticle band gap of bulk α-HgS and investigate the effect of quantum confinement on the geometric, electronic and optical structures. The basic structural unit of α-HgS is a one-dimensional helical chain consisting of covalently bound Hg and S atoms. When isolated to just a single helix or to a few-helix configuration, we find that α-HgS becomes a wide-gap semiconductor with a quasiparticle band gap as large as 7.0 eV, in contrast to the bulk structure with a direct quasiparticle band gap of 2.8 eV and an indirect gap of 2.14 eV. This dramatic increase in the band gap is attributed to quantum confinement effects on the geometry and intra-helix bonding. Shifts in the band gaps are also reflected as shifts in the low-energy optical absorption spectra calculated via density functional theory. As more helical chains are added, the band gap decreases sharply while the geometry becomes more bulk-like. This work illustrates the strong effects of quantum confinement in low-dimensional α-HgS nanostructures.

  11. Third generation photovoltaics based on multiple exciton generation in quantum confined semiconductors.

    PubMed

    Beard, Matthew C; Luther, Joseph M; Semonin, Octavi E; Nozik, Arthur J

    2013-06-18

    Improving the primary photoconversion process in a photovoltaiccell by utilizing the excess energy that is otherwise lost as heat can lead to an increase in the overall power conversion efficiency (PCE). Semiconductor nanocrystals (NCs) with at least one dimension small enough to produce quantum confinement effects provide new ways of controlling energy flow not achievable in thin film or bulk semiconductors. Researchers have developed various strategies to incorporate these novel structures into suitable solar conversion systems. Some of these methods could increase the PCE past the Shockley-Queisser (SQ) limit of ∼33%, making them viable "third generation photovoltaic" (TGPV) cell architectures. Surpassing the SQ limit for single junction solar cells presents both a scientific and a technological challenge, and the use of semiconductor NCs to enhance the primary photoconversion process offers a promising potential solution. The NCs are synthesized via solution phase chemical reactions producing stable colloidal solutions, where the reaction conditions can be modified to produce a variety of shapes, compositions, and structures. The confinement of the semiconductor NC in one dimension produces quantum films, wells, or discs. Two-dimensional confinement leads to quantum wires or rods (QRs), and quantum dots (QDs) are three-dimensionally confined NCs. The process of multiple exciton generation (MEG) converts a high-energy photon into multiple electron-hole pairs. Although many studies have demonstrated that MEG is enhanced in QDs compared with bulk semiconductors, these studies have either used ultrafast spectroscopy to measure the photon-to-exciton quantum yields (QYs) or theoretical calculations. Implementing MEG in a working solar cell has been an ongoing challenge. In this Account, we discuss the status of MEG research and strategies towards implementing MEG in working solar cells. Recently we showed an external quantum efficiency for photocurrent of greater

  12. Ligand-Mediated Control of the Confinement Potential in Semiconductor Quantum Dots

    NASA Astrophysics Data System (ADS)

    Amin, Victor

    This thesis describes the mechanisms by which organic surfactants, particularly thiophenols and phenyldithiocarbamates, reduce the confinement potential experienced by the exciton of semiconductor quantum dots (QDs). The reduction of the confinement potential is enabled by the creation of interfacial electronic states near the band edge of the QD upon ligand adsorption. In the case of thiophenols, we find that this ligand adsorbs in two distinct binding modes, (i) a tightly bound mode capable of exciton delocalization, and (ii) a more weakly bound mode that has no discernable effect on exciton confinement. Both the adsorption constant and reduction in confinement potential are tunable by para substitution and are generally anticorrelated. For tightly bound thiophenols and other moderately delocalizing ligands, the degree of delocalization induced in the QD is approximately linearly proportional to the fractional surface area occupied by the ligand for all sizes of QDs. In the case of phenyldithiocarbamates, the reduction in the confinement potential is much greater, and ligand adjacency must be accounted for to model exciton delocalization. We find that at high surface coverages, exciton delocalization by phenyldithiocarbamates and other highly delocalizing ligands is dominated by ligand packing effects. Finally, we construct a database of electronic structure calculations on organic molecules and propose an algorithm that combines experimental and computational screening to find novel delocalizing ligands.

  13. Anisotropic Quantum Confinement Effect and Electric Control of Surface States in Dirac Semimetal Nanostructures

    PubMed Central

    Xiao, Xianbo; Yang, Shengyuan A.; Liu, Zhengfang; Li, Huili; Zhou, Guanghui

    2015-01-01

    The recent discovery of Dirac semimetals represents a new achievement in our fundamental understanding of topological states of matter. Due to their topological surface states, high mobility, and exotic properties associated with bulk Dirac points, these new materials have attracted significant attention and are believed to hold great promise for fabricating novel topological devices. For nanoscale device applications, effects from finite size usually play an important role. In this report, we theoretically investigate the electronic properties of Dirac semimetal nanostructures. Quantum confinement generally opens a bulk band gap at the Dirac points. We find that confinement along different directions shows strong anisotropic effects. In particular, the gap due to confinement along vertical c-axis shows a periodic modulation, which is absent for confinement along horizontal directions. We demonstrate that the topological surface states could be controlled by lateral electrostatic gating. It is possible to generate Rashba-like spin splitting for the surface states and to shift them relative to the confinement-induced bulk gap. These results will not only facilitate our fundamental understanding of Dirac semimetal nanostructures, but also provide useful guidance for designing all-electrical topological spintronics devices. PMID:25600392

  14. Heterostructure quantum confined Stark effect electrooptic modulators operating at 938 nm

    NASA Astrophysics Data System (ADS)

    Hayduk, Michael J.; Krol, Mark F.; Boncek, Raymond K.

    1993-12-01

    Electro-optics modulators are a necessary component of emerging optical fiber based local area interconnects. One type of modulator, suitable for use in optical interconnects, is an asymmetric Fabry-Perot reflection modulator (ARM). This type of an intensity modulator uses an electro-optic material as the spacer material to balance the normally unequal front and back mirror reflectances. The quantum confined Franz-Keldysh and Stark effects shift the absorption edge of semiconductor multiple quantum well (MQW) materials to longer wavelengths in the presence of an external electric field applied perpendicular to the MQW layers, thereby changing the reflectance of the etalon. The combined coherence effects of the etalon coupled with the quantum effects of the MQW materials result in a large modulation depth and a low insertion loss. P-I-N diode structures using an In Ha As/GaAs MQW structure as the intrinsic region were fabricated for the purpose of characterizing the electro-absorption associated with different applied electric fields. Quantum confined Franz-Keldysh and Stark shifts were observed for applied electric fields as large as 6.58 x 10000 V/cm. The resulting change in the absorption coefficient was found to be -3.7 x 1000 cm to the minus 1st power which is sufficient to design a high-speed ARM with a large modulation depth and a low insertion loss.

  15. The surface termination effect on the quantum confinement and electron affinities of 3C-SiC quantum dots: a first-principles study

    NASA Astrophysics Data System (ADS)

    Zhang, Zhenkui; Dai, Ying; Yu, Lin; Guo, Meng; Huang, Baibiao; Whangbo, Myung-Hwan

    2012-02-01

    In light of the established differences between the quantum confinement effect and the electron affinities between hydrogen-passivated C and Si quantum dots, we carried out theoretical investigations on SiC quantum dots, with surfaces uniformly terminated by C-H or Si-H bonds, to explore the role of surface terminations on these two aspects. Surprisingly, it was found that the quantum confinement effect is present (or absent) in the highest occupied (or lowest unoccupied) molecular orbital of the SiC quantum dots regardless of their surface terminations. Thus, the quantum confinement effect related to the energy gap observed experimentally (Phys. Rev. Lett., 2005, 94, 026102) is contributed to by the size-dependence of the highest occupied states; the absence of quantum confinement in the lowest unoccupied states is in contrary to the usual belief based on hydrogen-passivated C quantum dots. However, the cause of the absence of the quantum confinement in C nanodots is not transferable to SiC. We propose a model that provides a clear explanation for all findings on the basis of the nearest-neighbor and next-nearest-neighbor interactions between the valence atomic p-orbital in the frontier occupied/unoccupied states. We also found that the electron affinities of the SiC quantum dots, which closely depend on the surface environments, are negative for the C-H termination and positive for the Si-H termination. The prediction of negative electron affinities in SiC quantum dots by simple C-H termination indicates a promising application for these materials in electron-emitter devices. Our model predicts that GeC quantum dots with hydrogen passivation exhibit similar features to SiC quantum dots and our study confirms the crucial role that the surface environment plays in these nanoscale systems.In light of the established differences between the quantum confinement effect and the electron affinities between hydrogen-passivated C and Si quantum dots, we carried out

  16. The role of the interface in germanium quantum dots: when not only size matters for quantum confinement effects.

    PubMed

    Cosentino, S; Mio, A M; Barbagiovanni, E G; Raciti, R; Bahariqushchi, R; Miritello, M; Nicotra, G; Aydinli, A; Spinella, C; Terrasi, A; Mirabella, S

    2015-07-14

    Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects. PMID:26077313

  17. Mapping the spatial distribution of charge carriers in quantum-confined heterostructures

    NASA Astrophysics Data System (ADS)

    Smith, Andrew M.; Lane, Lucas A.; Nie, Shuming

    2014-07-01

    Quantum-confined nanostructures are considered ‘artificial atoms’ because the wavefunctions of their charge carriers resemble those of atomic orbitals. For multiple-domain heterostructures, however, carrier wavefunctions are more complex and still not well understood. We have prepared a unique series of cation-exchanged HgxCd1-xTe quantum dots (QDs) and seven epitaxial core-shell QDs and measured their first and second exciton peak oscillator strengths as a function of size and chemical composition. A major finding is that carrier locations can be quantitatively mapped and visualized during shell growth or cation exchange simply using absorption transition strengths. These results reveal that a broad range of quantum heterostructures with different internal structures and band alignments exhibit distinct carrier localization patterns that can be used to further improve the performance of optoelectronic devices and enhance the brightness of QD probes for bioimaging.

  18. Mapping the spatial distribution of charge carriers in quantum-confined heterostructures

    PubMed Central

    Smith, Andrew M.; Lane, Lucas A.; Nie, Shuming

    2014-01-01

    Quantum-confined nanostructures are considered ‘artificial atoms’ because the wavefunctions of their charge carriers resemble those of atomic orbitals. For multiple-domain heterostructures, however, carrier wavefunctions are more complex and still not well understood. We have prepared a unique series of cation-exchanged HgxCd1−xTe quantum dots (QDs) and seven epitaxial core–shell QDs and measured their first and second exciton peak oscillator strengths as a function of size and chemical composition. A major finding is that carrier locations can be quantitatively mapped and visualized during shell growth or cation exchange simply using absorption transition strengths. These results reveal that a broad range of quantum heterostructures with different internal structures and band alignments exhibit distinct carrier localization patterns that can be used to further improve the performance of optoelectronic devices and enhance the brightness of QD probes for bioimaging. PMID:25080298

  19. First-principle study of quantum confinement effect on small sized silicon quantum dots using density-functional theory

    SciTech Connect

    Anas, M. M.; Othman, A. P.; Gopir, G.

    2014-09-03

    Density functional theory (DFT), as a first-principle approach has successfully been implemented to study nanoscale material. Here, DFT by numerical basis-set was used to study the quantum confinement effect as well as electronic properties of silicon quantum dots (Si-QDs) in ground state condition. Selection of quantum dot models were studied intensively before choosing the right structure for simulation. Next, the computational result were used to examine and deduce the electronic properties and its density of state (DOS) for 14 spherical Si-QDs ranging in size up to ∼ 2 nm in diameter. The energy gap was also deduced from the HOMO-LUMO results. The atomistic model of each silicon QDs was constructed by repeating its crystal unit cell of face-centered cubic (FCC) structure, and reconstructed until the spherical shape obtained. The core structure shows tetrahedral (T{sub d}) symmetry structure. It was found that the model need to be passivated, and hence it was noticed that the confinement effect was more pronounced. The model was optimized using Quasi-Newton method for each size of Si-QDs to get relaxed structure before it was simulated. In this model the exchange-correlation potential (V{sub xc}) of the electrons was treated by Local Density Approximation (LDA) functional and Perdew-Zunger (PZ) functional.

  20. Fractional Quantum Hall Effect at ν = 1 / 2 in Hole Systems Confined to GaAs Wide Quantum Wells

    NASA Astrophysics Data System (ADS)

    Hasdemir, Sukret; Liu, Yang; Graninger, Aurelius; Shayegan, Mansour; Pfeiffer, Loren; West, Ken; Baldwin, Kirk; Winkler, Roland

    2014-03-01

    We observe fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor ν = 1 / 2 in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayer-like charge distributions. The ν = 1 / 2 FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole ν = 1 / 2 FQHE to be consistent with a two-component, Halperin-Laughlin (Ψ331) state. We acknowledge support through the DOE BES (DE-FG02-00-ER45841) for measurements, and the Gordon and Betty Moore Foundation (Grant GBMF2719), Keck Foundation, and the NSF (DMR-0904117, DMR-1305691 and MRSEC DMR-0819860) for sample fabrication. Work at Arg.

  1. Quantum confinement of correlated eg^1 electrons in rare earth nickelate heterostructures

    NASA Astrophysics Data System (ADS)

    Liu, Jian; van Veenendaal, M.; Okamoto, S.; Kareev, M.; Gray, B.; Ryan, P.; Freeland, J. W.; Chakhalian, J.

    2012-02-01

    Complex oxide heterostrutures have emerged as a new playground for controlling the mutually coupled charge, spin, orbital and lattice degrees of freedom, and a promising route to stabilize unusual phases not existing in the bulk. In particular, quantum well structures have recently attracted attention due to the potential in creating novel two-dimensional systems with confined correlated electrons. To this end, we have studied the eg^1 system based on the 3d^7 low-spin state in perovskite rare earth nickelates which are artificially confined by wide-gap dielectrics LaAlO3. The combination of transport measurements and dynamical-mean-field calculations indicate that, a Mott-type metal-insulator transition can be induced by confinement via dimensionality-control. X-ray absorption spectroscopy reveals that the electronic modification in proximity to the confining interfaces is caused by modulated covalency, which is in good agreement with cluster calculations. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808.

  2. Multi-band Bloch equations and gain spectra of highly excited II-VI semiconductor quantum wells

    SciTech Connect

    Girndt, A.; Jahnke, F.; Knorr, A.; Koch, S.W.; Chow, W.W.

    1997-04-21

    Quasi-equilibrium excitation dependent optical probe spectra of II-VI semiconductor quantum wells at room temperature are investigated within the framework of multi-band semiconductor Bloch equations. The calculations include correlation effects beyond the Hartree-Fock level which describe dephasing, interband Coulomb correlations and band-gap renormalization in second Born approximation. In addition to the carrier-Coulomb interaction, the influence of carrier-phonon scattering and inhomogeneous broadening is considered. The explicit calculation of single particle properties like band structure and dipole matrix elements using k {center_dot} p theory makes it possible to investigate various II-VI material combinations. Numerical results are presented for CdZnSe/ZnSe and CdZnSe/MnZnSSe semiconductor quantum-well systems.

  3. Bernoulli's formula and Poisson's equations for a confined quantum gas: Effects due to a moving piston

    NASA Astrophysics Data System (ADS)

    Nakamura, Katsuhiro; Sobirov, Zarifboy A.; Matrasulov, Davron U.; Avazbaev, Sanat K.

    2012-12-01

    We study a nonequilibrium equation of states of an ideal quantum gas confined in the cavity under a moving piston with a small but finite velocity in the case in which the cavity wall suddenly begins to move at the time origin. Confining ourselves to the thermally isolated process, the quantum nonadiabatic (QNA) contribution to Poisson's adiabatic equations and to Bernoulli's formula which bridges the pressure and internal energy is elucidated. We carry out a statistical mean of the nonadiabatic (time-reversal-symmetric) force operator found in our preceding paper [Nakamura , Phys. Rev. EPLEEE81539-375510.1103/PhysRevE.83.041133 83, 041133 (2011)] in both the low-temperature quantum-mechanical and high-temperature quasiclassical regimes. The QNA contribution, which is proportional to the square of the piston's velocity and to the inverse of the longitudinal size of the cavity, has a coefficient that is dependent on the temperature, gas density, and dimensionality of the cavity. The investigation is done for a unidirectionally expanding three-dimensional (3D) rectangular parallelepiped cavity as well as its 1D version. Its relevance in a realistic nanoscale heat engine is discussed.

  4. Quantum criticality and confinement effects in an Ising chain in transverse field

    NASA Astrophysics Data System (ADS)

    Coldea, Radu

    2011-03-01

    The Ising chain in transverse field is one of the key paradigms for the theory of continuous zero-temperature quantum phase transitions. We have recently realized this system experimentally by applying strong magnetic fields to the quasi- 1D, low-exchange Ising ferromagnet CoNb2O6 to drive it to its quantum critical point where the spontaneous long-range magnetic order is suppressed by magnetic field. Using high-resolution single-crystal neutron scattering we have probed how the spin dynamics evolves with the applied field and have observed a dramatic change in the character of spin excitations at the quantum critical point, from pairs of domain-wall (kink) quasiparticles in the magnetically-ordered phase, to sharp spin- flip quasiparticles in the paramagnetic phase. The weak, but finite couplings between the chains significantly enrich the physics by stabilizing a complex structure of two-kink bound states due to mean-field confinement effects. In zero field the rich spectrum of bound states can be quantitatitively understood following McCoy and Wu's analytic theory of weak confinement. Just below the critical field the energies of the two lowest bound states approach the ``golden ratio'' as predicted by Zamolodchikov's E8 scaling limit solution of the off-critical Ising model in a weak longitudinal field.

  5. Computer simulation of liquid-vapor coexistence of confined quantum fluids

    SciTech Connect

    Trejos, Víctor M.; Gil-Villegas, Alejandro Martinez, Alejandro

    2013-11-14

    The liquid-vapor coexistence (LV) of bulk and confined quantum fluids has been studied by Monte Carlo computer simulation for particles interacting via a semiclassical effective pair potential V{sub eff}(r) = V{sub LJ} + V{sub Q}, where V{sub LJ} is the Lennard-Jones 12-6 potential (LJ) and V{sub Q} is the first-order Wigner-Kirkwood (WK-1) quantum potential, that depends on β = 1/kT and de Boer's quantumness parameter Λ=h/σ√(mε), where k and h are the Boltzmann's and Planck's constants, respectively, m is the particle's mass, T is the temperature of the system, and σ and ε are the LJ potential parameters. The non-conformal properties of the system of particles interacting via the effective pair potential V{sub eff}(r) are due to Λ, since the LV phase diagram is modified by varying Λ. We found that the WK-1 system gives an accurate description of the LV coexistence for bulk phases of several quantum fluids, obtained by the Gibbs Ensemble Monte Carlo method (GEMC). Confinement effects were introduced using the Canonical Ensemble (NVT) to simulate quantum fluids contained within parallel hard walls separated by a distance L{sub p}, within the range 2σ ⩽ L{sub p} ⩽ 6σ. The critical temperature of the system is reduced by decreasing L{sub p} and increasing Λ, and the liquid-vapor transition is not longer observed for L{sub p}/σ < 2, in contrast to what has been observed for the classical system.

  6. Spin blockade and exchange in Coulomb-confined silicon double quantum dots.

    PubMed

    Weber, Bent; Tan, Y H Matthias; Mahapatra, Suddhasatta; Watson, Thomas F; Ryu, Hoon; Rahman, Rajib; Hollenberg, Lloyd C L; Klimeck, Gerhard; Simmons, Michelle Y

    2014-06-01

    Electron spins confined to phosphorus donors in silicon are promising candidates as qubits because of their long coherence times, exceeding seconds in isotopically purified bulk silicon. With the recent demonstrations of initialization, readout and coherent manipulation of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling in two tunnel-coupled phosphorus donors. Spin-to-charge conversion via Pauli spin blockade, an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (∼45 meV), requiring large electric fields (>1 MV m(-1)) to transfer both electron spins onto the same donor. Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems. PMID:24727686

  7. Spectroscopy and dynamics of charge transfer excitons in type-II band aligned quantum confined heterostructures

    NASA Astrophysics Data System (ADS)

    Kushavah, Dushyant; Mohapatra, P. K.; Rustagi, K. C.; Bahadur, D.; Vasa, P.; Singh, B. P.

    2015-05-01

    We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ˜5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ˜597 to ˜746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ˜51 ns as compared to ˜6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons.

  8. Spectroscopy and dynamics of charge transfer excitons in type-II band aligned quantum confined heterostructures

    SciTech Connect

    Kushavah, Dushyant; Mohapatra, P. K.; Vasa, P.; Singh, B. P.; Rustagi, K. C.; Bahadur, D.

    2015-05-15

    We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ∼5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ∼597 to ∼746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ∼51 ns as compared to ∼6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons.

  9. Quantum-confined bandgap narrowing of TiO2 nanoparticles by graphene quantum dots for visible-light-driven applications.

    PubMed

    Wang, Shujun; Cole, Ivan S; Li, Qin

    2016-07-28

    We for the first time report a quantum-confined bandgap narrowing mechanism through which the absorption of two UV absorbers, namely the graphene quantum dots (GQDs) and TiO2 nanoparticles, can be easily extended into the visible light range in a controllable manner. Such a mechanism may be of great importance for light harvesting, photocatalysis and optoelectronics. PMID:27297746

  10. Quantum-Carnot engine for particle confined to 2D symmetric potential well

    SciTech Connect

    Belfaqih, Idrus Husin Sutantyo, Trengginas Eka Putra Prayitno, T. B.; Sulaksono, Anto

    2015-09-30

    Carnot model of heat engine is the most efficient cycle consisting of isothermal and adiabatic processes which are reversible. Although ideal gas usually used as a working fluid in the Carnot engine, Bender used quantum particle confined in 1D potential well as a working fluid. In this paper, by following Bender we generalize the situation to 2D symmetric potential well. The efficiency is express as the ratio of the initial length of the system to the final length of the compressed system. The result then is shown that for the same ratio, 2D potential well is more efficient than 1D potential well.

  11. Impurity with two electrons in the spherical quantum dot with Unite confinement potential

    NASA Astrophysics Data System (ADS)

    Baghdasaryan, D. A.; Ghaltaghchyan, H. Ts; Kazaryan, E. M.; Sarkisyan, H. A.

    2016-01-01

    Two-electron states in a spherical QD with the hydrogenic impurity located in the center and with a finite height confinement potential barrier are investigated. The effective mass mismatch have been taken into account. The dependence of ground state energy and Coulomb electron-electron interaction energy correction on the QD size is studied. The problem of the state exchange time control in QD is discussed, taking into account the spins of the electrons in the Russell-Saunders approximation. The effect of quantum emission has been shown.

  12. Wide-gap II-VI heterostructures

    NASA Astrophysics Data System (ADS)

    Gunshor, R. L.; Kolodziejski, L. A.; Kobayashi, M.; Otsuka, N.; Nurmikko, A. V.

    1990-04-01

    Recent advances in the growth of II-VI/II-VI and II-VI/III-V heterostructures based on the widegap II-VI semiconductors CdTe and ZnTe are discussed. The potentially important pseudomorphic epilayer/epilayer heterojunction consisting of ZnTe on AlSb has been grown by MBE and characterized. Both microstructural and optical evaluation indicate a high degree of structural quality and the potential for future development of novel light-emitting device structures. Metastable zincblende MnTe, for which TEM and X-ray evaluation reveal the presence of only zincblende phases, has been grown by MBE. Single quantum well structures using zincblende MnTe for the barrier layers have been fabricated and found to show strong carrier confinement, further confirming the predicted zincblende MnTe bandgap at 3.2 eV.

  13. Fluorescent carbon nano dots from lignite: unveiling the impeccable evidence for quantum confinement.

    PubMed

    Kumar Thiyagarajan, Senthil; Raghupathy, Suresh; Palanivel, Dharmalingam; Raji, Kaviyarasan; Ramamurthy, Perumal

    2016-04-28

    Synthesizing nano carbon from its bulk precursors is of recent research interest. In this report, luminescent carbon nanoparticles (CNPs) with tunable particle size and surface functionality are fabricated from lignite using ethylenediamine as the reactive solvent and surface passivating agent via different experimental methods. From the steady-state and time-resolved photophysical studies of these differently sized CNPs, it is unveiled that the energy of the excitons generated after photoexcitation is quantum confined, and it influences the observed photophysical behaviour significantly only when the particle size is less than 10 nm. A larger size of the CNPs and less surface functionalization lead to aggregation, and quenching of the fluorescence. But by dispersing smaller size CNPs in sodium sulfate matrix exhibits fluorescence in the solid state with an absolute fluorescence quantum yield of ∼34%. The prospective application of this hybrid material in sensing and removal of moisture in the atmosphere is illustrated. PMID:27067247

  14. Carrier dynamics in highly quantum-confined, colloidal indium antimonide nanocrystals.

    PubMed

    Chang, Angela Y; Liu, Wenyong; Talapin, Dmitri V; Schaller, Richard D

    2014-08-26

    Nanometer-sized particles of indium antimonide (InSb) offer opportunities in areas such as solar energy conversion and single photon sources. Here, we measure electron-hole pair dynamics, spectra, and absorption cross sections of strongly quantum-confined colloidal InSb nanocrystal quantum dots using femtosecond transient absorption. For all samples, we observe a bleach feature that develops on ultrafast time scales, which notably moves to lower energy during the first several picoseconds following excitation. We associate this unusual red shift, which becomes larger for larger particles and more distinct at lower sample temperatures, with hot exciton cooling through states that we suggest arise from energetically proximal conduction band levels. From controlled optical excitation intensities, we determine biexciton lifetimes, which range from 2 to 20 ps for the studied 3-6 nm diameter particle sizes. PMID:25106893

  15. Enhanced Quantum Confined Stark Effect in a mesoporous hybrid multifunctional system

    NASA Astrophysics Data System (ADS)

    Gogoi, M.; Deb, P.; Sen, D.; Mazumder, S.; Kostka, A.

    2014-06-01

    Quantum Confined Stark Effect in hybrid of CdTe quantum dot with superparamagnetic iron oxide nanoparticles in both nonporous and mesoporous silica matrix has been realized. The observed QCSE is due to the local electric field induced by charge dispersion at SiO2/polar solvent interface. Enhanced Stark shift of 89.5 meV is observed in case of mesoporous hybrid structure and the corresponding local electric field has been evaluated as 4.38×104 V/cm. The enhancement is assumed to be caused by greater density of charge in the mesoporous hybrid. The conjugation of superparamagnetic nanoparticles in this tailored hybrid microstructure has not imparted any alteration to the Stark shift, but has added multifunctional attribute. The present study on the local electric field induced enhanced QCSE with wavelength modulation towards red end paves the way of developing magneto-fluorescent hybrid systems for biomedical imaging application.

  16. Strongly confining bare core CdTe quantum dots in polymeric microdisk resonators

    SciTech Connect

    Flatae, Assegid Grossmann, Tobias; Beck, Torsten; Wiegele, Sarah; Kalt, Heinz

    2014-01-01

    We report on a simple route to the efficient coupling of optical emission from strongly confining bare core CdTe quantum dots (QDs) to the eigenmodes of a micro-resonator. The quantum emitters are embedded into QD/polymer sandwich microdisk cavities. This prevents photo-oxidation and yields the high dot concentration necessary to overcome Auger enhanced surface trapping of carriers. In combination with the very high cavity Q-factors, interaction of the QDs with the cavity modes in the weak coupling regime is readily observed. Under nanosecond pulsed excitation the CdTe QDs in the microdisks show lasing with a threshold energy as low as 0.33 μJ.

  17. Probing the excited subband dispersion of holes confined to GaAs wide quantum wells

    NASA Astrophysics Data System (ADS)

    Jo, Insun; Liu, Yang; Deng, H.; Shayegan, M.; Pfeiffer, L. N.; West, K. W.; Baldwin, K. W.; Winkler, R.

    Owing to the strong spin-orbit coupling and their large effective mass, the two-dimensional (2D) holes in modulation-doped GaAs quantum wells provide a fertile test bed to study the rich physics of low-dimensional systems. In a wide quantum well, even at moderate 2D densities, the holes start to occupy the excited subband, a subband whose dispersion is very unusual and has a non-monotonic dependence on the wave vector. Here, we study a 2D hole system confined to a 40-nm-thick (001) GaAs quantum well and demonstrate that, via the application of both front and back gates, the density can be tuned in a wide range, between ~1 and 2 ×1011 cm-2. Using Fourier analysis of the low-field Shubnikov-de Haas oscillations, we investigate the population of holes and the spin-orbit interaction induced spin-splitting in different subbands. We discuss the results in light of self-consistent quantum calculations of magneto-oscillations. Work support by the DOE BES (DE-FG02-00-ER45841), the NSF (Grants DMR-1305691 and MRSEC DMR-1420541), the Gordon and Betty Moore Foundation (Grant GBMF4420), and Keck Foundation for experiments, and the NSF Grant DMR-1310199 for calculations.

  18. Interplay between quantum confinement and Fulde-Ferrell-Larkin-Ovchinnikov phase in superconducting nanofilms

    NASA Astrophysics Data System (ADS)

    Wójcik, P.; Zegrodnik, M.

    2016-09-01

    In superconducting nanofilms the energy quantization induced by the confinement in the direction perpendicular to the film splits the band of single-electron states into series of subbands. The quantum size effect leads to the experimentally observed oscillations of the critical magnetic field with increasing nanofilm thickness. Here, we study the influence of the quantum confinement on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in superconducting nanofilms. We show that the range of the magnetic fields for which the FFLO phase is stable oscillates as a function of the film thickness with the phase shift equal to one half of the period corresponding to the critical magnetic field oscillations. Due to the multiband character of the system a division of the FFLO phase stability region appears leading to a phase diagram which is qualitatively different than the one corresponding to a single-band situation. The number of subregions created in such manner depends on the number of bands participating in the formation of the paired state.

  19. Decoupling the effects of confinement and passivation on semiconductor quantum dots.

    PubMed

    Rudd, Roya; Hall, Colin; Murphy, Peter J; Reece, Peter J; Charrault, Eric; Evans, Drew

    2016-07-20

    Semiconductor (SC) quantum dots (QDs) have recently been fabricated by both chemical and plasma techniques for specific absorption and emission of light. Their optical properties are governed by the size of the QD and the chemistry of any passivation at their surface. Here, we decouple the effects of confinement and passivation by utilising DC magnetron sputtering to fabricate SC QDs in a perfluorinated polyether oil. Very high band gaps are observed for fluorinated QDs with increasing levels of quantum confinement (from 4.2 to 4.6 eV for Si, and 2.5 to 3 eV for Ge), with a shift down to 3.4 eV for Si when oxygen is introduced to the passivation layer. In contrast, the fluorinated Si QDs display a constant UV photoluminescence (3.8 eV) irrespective of size. This ability to tune the size and passivation independently opens a new opportunity to extending the use of simple semiconductor QDs. PMID:27385513

  20. Elementary framework for cold field emission from quantum-confined, non-planar emitters

    NASA Astrophysics Data System (ADS)

    Patterson, A. A.; Akinwande, A. I.

    2015-05-01

    For suitably small field emitters, the effects of quantum confinement at the emitter tip may have a significant impact on the emitter performance and total emitted current density (ECD). Since the geometry of a quantum system uniquely determines the magnitude and distribution of its energy levels, a framework for deriving ECD equations from cold field electron emitters of arbitrary geometry and dimensionality is developed. In the interest of obtaining semi-analytical ECD equations, the framework is recast in terms of plane wave solutions to the Schrödinger equation via the use of the Jeffreys-Wentzel-Kramers-Brillouin approximation. To demonstrate the framework's consistency with our previous work and its capabilities in treating emitters with non-planar geometries, ECD equations were derived for the normally unconfined cylindrical nanowire (CNW) and normally confined (NC) CNW emitter geometries. As a function of the emitter radius, the NC CNW emitter ECD profile displayed a strong dependence on the Fermi energy and had an average ECD that exceeded the Fowler-Nordheim equation for typical values of the Fermi energy due to closely spaced, singly degenerate energy levels (excluding electron spin), comparatively large electron supply values, and the lack of a transverse, zero-point energy. Such characteristics suggest that emitters with non-planar geometries may be ideal for emission from both an electron supply and electrostatics perspective.

  1. Elementary framework for cold field emission from quantum-confined, non-planar emitters

    SciTech Connect

    Patterson, A. A. Akinwande, A. I.

    2015-05-07

    For suitably small field emitters, the effects of quantum confinement at the emitter tip may have a significant impact on the emitter performance and total emitted current density (ECD). Since the geometry of a quantum system uniquely determines the magnitude and distribution of its energy levels, a framework for deriving ECD equations from cold field electron emitters of arbitrary geometry and dimensionality is developed. In the interest of obtaining semi-analytical ECD equations, the framework is recast in terms of plane wave solutions to the Schrödinger equation via the use of the Jeffreys-Wentzel-Kramers-Brillouin approximation. To demonstrate the framework's consistency with our previous work and its capabilities in treating emitters with non-planar geometries, ECD equations were derived for the normally unconfined cylindrical nanowire (CNW) and normally confined (NC) CNW emitter geometries. As a function of the emitter radius, the NC CNW emitter ECD profile displayed a strong dependence on the Fermi energy and had an average ECD that exceeded the Fowler-Nordheim equation for typical values of the Fermi energy due to closely spaced, singly degenerate energy levels (excluding electron spin), comparatively large electron supply values, and the lack of a transverse, zero-point energy. Such characteristics suggest that emitters with non-planar geometries may be ideal for emission from both an electron supply and electrostatics perspective.

  2. Real-Time Reciprocal Space Mapping of Nano-Islands Induced by Quantum Confinement

    NASA Astrophysics Data System (ADS)

    Hong, Hawoong; Gray, Aaron; Chiang, T.-C.

    2011-01-01

    The effects of quantum confinement have been observed pronouncedly in the island morphology of Pb thin films. The evolution of these nano-islands on Si (111)-(7 × 7) and sapphire (001) surfaces has been studied with a new X-ray diffraction method. A charge-coupled device (CCD) camera was used to collect two- and three-dimensional (2-D and 3-D, respectively) maps of the surface X-ray diffraction in real time. Large ranges of the reflectivity curves, with rocking curves at every point on the reflectivity curves, could be measured continuously in a relatively short amount of time. The abundance of information from 2-D k-space maps reveals clear changes in the growth modes of these thin Pb films. With the 3-D extension of this method, it was possible to observe the ordering of the islands. The islands maintain a nearly uniform interisland distance but lack any angular correlation. The interisland ordering is correlated well with the development of "magic" island heights caused by quantum confinement.

  3. Real time reciprocal space mapping of nano-islands induced by quantum confinment.

    SciTech Connect

    Hong, H.; Gray, A.; Chiang, T. C.

    2011-01-01

    The effects of quantum confinement have been observed pronouncedly in the island morphology of Pb thin films. The evolution of these nano-islands on Si (111)-(7 x 7) and sapphire (001) surfaces has been studied with a new X-ray diffraction method. A charge-coupled device (CCD) camera was used to collect two- and three-dimensional (2-D and 3-D, respectively) maps of the surface X-ray diffraction in real time. Large ranges of the reflectivity curves, with rocking curves at every point on the reflectivity curves, could be measured continuously in a relatively short amount of time. The abundance of information from 2-D k-space maps reveals clear changes in the growth modes of these thin Pb films. With the 3-D extension of this method, it was possible to observe the ordering of the islands. The islands maintain a nearly uniform interisland distance but lack any angular correlation. The interisland ordering is correlated well with the development of 'magic' island heights caused by quantum confinement.

  4. Maximal Wavelength of Confined Quarks and Gluons and Properties of Quantum Chromodynamics

    SciTech Connect

    Brodsky, Stanley J.; Shrock, Robert; /YITP, Stony Brook

    2008-08-01

    Because quarks and gluons are confined within hadrons, they have a maximum wavelength of order the confinement scale. Propagators, normally calculated for free quarks and gluons using Dyson-Schwinger equations, are modified by bound-state effects in close analogy to the calculation of the Lamb shift in atomic physics. Because of confinement, the effective quantum chromodynamic coupling stays finite in the infrared. The quark condensate which arises from spontaneous chiral symmetry breaking in the bound state Dyson-Schwinger equation is the expectation value of the operator {bar q}q evaluated in the background of the fields of the other hadronic constituents, in contrast to a true vacuum expectation value. Thus quark and gluon condensates reside within hadrons. The effects of instantons are also modified. We discuss the implications of the maximum quark and gluon wavelength for phenomena such as deep inelastic scattering and annihilation, the decay of heavy quarkonia, jets, and dimensional counting rules for exclusive reactions. We also discuss implications for the zero-temperature phase structure of a vectorial SU(N) gauge theory with a variable number N{sub f} of massless fermions.

  5. Magneto-optical absorption in semiconducting spherical quantum dots: Influence of the dot-size, confining potential, and magnetic field

    SciTech Connect

    Kushwaha, Manvir S.

    2014-12-15

    Semiconducting quantum dots – more fancifully dubbed artificial atoms – are quasi-zero dimensional, tiny, man-made systems with charge carriers completely confined in all three dimensions. The scientific quest behind the synthesis of quantum dots is to create and control future electronic and optical nanostructures engineered through tailoring size, shape, and composition. The complete confinement – or the lack of any degree of freedom for the electrons (and/or holes) – in quantum dots limits the exploration of spatially localized elementary excitations such as plasmons to direct rather than reciprocal space. Here we embark on a thorough investigation of the magneto-optical absorption in semiconducting spherical quantum dots characterized by a confining harmonic potential and an applied magnetic field in the symmetric gauge. This is done within the framework of Bohm-Pines’ random-phase approximation that enables us to derive and discuss the full Dyson equation that takes proper account of the Coulomb interactions. As an application of our theoretical strategy, we compute various single-particle and many-particle phenomena such as the Fock-Darwin spectrum; Fermi energy; magneto-optical transitions; probability distribution; and the magneto-optical absorption in the quantum dots. It is observed that the role of an applied magnetic field on the absorption spectrum is comparable to that of a confining potential. Increasing (decreasing) the strength of the magnetic field or the confining potential is found to be analogous to shrinking (expanding) the size of the quantum dots: resulting into a blue (red) shift in the absorption spectrum. The Fermi energy diminishes with both increasing magnetic-field and dot-size; and exhibits saw-tooth-like oscillations at large values of field or dot-size. Unlike laterally confined quantum dots, both (upper and lower) magneto-optical transitions survive even in the extreme instances. However, the intra-Landau level

  6. Magneto-optical absorption in semiconducting spherical quantum dots: Influence of the dot-size, confining potential, and magnetic field

    NASA Astrophysics Data System (ADS)

    Kushwaha, Manvir S.

    2014-12-01

    Semiconducting quantum dots - more fancifully dubbed artificial atoms - are quasi-zero dimensional, tiny, man-made systems with charge carriers completely confined in all three dimensions. The scientific quest behind the synthesis of quantum dots is to create and control future electronic and optical nanostructures engineered through tailoring size, shape, and composition. The complete confinement - or the lack of any degree of freedom for the electrons (and/or holes) - in quantum dots limits the exploration of spatially localized elementary excitations such as plasmons to direct rather than reciprocal space. Here we embark on a thorough investigation of the magneto-optical absorption in semiconducting spherical quantum dots characterized by a confining harmonic potential and an applied magnetic field in the symmetric gauge. This is done within the framework of Bohm-Pines' random-phase approximation that enables us to derive and discuss the full Dyson equation that takes proper account of the Coulomb interactions. As an application of our theoretical strategy, we compute various single-particle and many-particle phenomena such as the Fock-Darwin spectrum; Fermi energy; magneto-optical transitions; probability distribution; and the magneto-optical absorption in the quantum dots. It is observed that the role of an applied magnetic field on the absorption spectrum is comparable to that of a confining potential. Increasing (decreasing) the strength of the magnetic field or the confining potential is found to be analogous to shrinking (expanding) the size of the quantum dots: resulting into a blue (red) shift in the absorption spectrum. The Fermi energy diminishes with both increasing magnetic-field and dot-size; and exhibits saw-tooth-like oscillations at large values of field or dot-size. Unlike laterally confined quantum dots, both (upper and lower) magneto-optical transitions survive even in the extreme instances. However, the intra-Landau level transitions are seen

  7. Charge Carrier Dynamics of Quantum Confined Semiconductor Nanoparticles Analyzed via Transient Absorption Spectroscopy

    NASA Astrophysics Data System (ADS)

    Thibert, Arthur Joseph, III

    Semiconductor nanoparticles are tiny crystalline structures (typically range from 1 - 100 nm) whose shape in many cases can be dictated through tailored chemical synthesis with atomic scale precision. The small size of these nanoparticles often results in quantum confinement (spatial confinement of wave functions), which imparts the ability to manipulate band-gap energies thus allowing them to be optimally engineered for different applications (i.e., photovoltaics, photocatalysis, imaging). However, charge carriers excited within these nanoparticles are often involved in many different processes: trapping, trap migration, Auger recombination, non-radiative relaxation, radiative relaxation, oxidation / reduction, or multiple exciton generation. Broadband ultrafast transient absorption laser spectroscopy is used to spectrally resolve the fate of excited charge carriers in both wavelength and time, providing insight as to what synthetic developments or operating conditions will be necessary to optimize their efficiency for certain applications. This thesis outlines the effort of resolving the dynamics of excited charge carriers for several Cd and Si based nanoparticle systems using this experimental technique. The thesis is organized into five chapters and two appendices as indicated below. Chapter 1 provides a brief introduction to the photophysics of semiconductor nanoparticles. It begins by defining what nanoparticles, semiconductors, charge carriers, and quantum confinement are. From there it details how the study of charge carrier dynamics within nanoparticles can lead to increased efficiency in applications such as photocatalysis. Finally, the experimental methodology associated with ultrafast transient absorption spectroscopy is introduced and its power in mapping charge carrier dynamics is established. Chapter 2 (JPCC, 19647, 2011) introduces the first of the studied samples: water-solubilized 2D CdSe nanoribbons (NRs), which were synthesized in the Osterloh

  8. Quantum confinement in semiconductor nanofilms: Optical spectra and multiple exciton generation

    NASA Astrophysics Data System (ADS)

    Khmelinskii, Igor; Makarov, Vladimir I.

    2016-04-01

    We report optical absorption and photoluminescence (PL) spectra of Si and SnO2 nanocrystalline films in the UV-vis-NIR range, featuring discrete bands resulting from transverse quantum confinement, observed in the optical spectra of nanofilms for the first time ever. The film thickness ranged from 3.9 to 12.2 nm, depending on the material. The results are interpreted within the particle-in-a-box model, with infinite walls. The calculated values of the effective electron mass are independent on the film thickness and equal to 0.17mo (Si) and 0.21mo (SnO2), with mo the mass of the free electron. The second calculated model parameter, the quantum number n of the HOMO (valence band), was also thickness-independent: 8.00 (Si) and 7.00 (SnO2). The transitions observed in absorption all start at the level n and correspond to Δn = 1, 2, 3, …. The photoluminescence bands exhibit large Stokes shifts, shifting to higher energies with increased excitation energy. In effect, nanolayers of Si, an indirect-gap semiconductor, behave as a direct-gap semiconductor, as regards the transverse-quantized level system. A prototype Si-SnO2 nanofilm photovoltaic cell demonstrated photoelectron quantum yields achieving 2.5, showing clear evidence of multiple exciton generation, for the first time ever in a working nanofilm device.

  9. Narrow divergence, single quantum well, separate confinement, AlGaAs laser

    SciTech Connect

    Haw, T.E.; Williams, J.E.; Wober, M.A.

    1991-01-29

    This patent describes a improvement in a structure for a narrow divergence, single quantum well, separate confinement, laser. It comprises: an n-AlGaAs cladding epitaxial layer, a first AlGaAs waveguide epitaxial layer, a GaAs quantum well active epitaxial layer, a second AlGaAs waveguide epitaxial layer, a p-AlGaAs cladding epitaxial layer, and a GaAs cap epitaxial layer, all sequentially grown with respect to each other. The improvement comprises: the n-AlGaAs cladding layer dimensioned to a thickness which is greater than 2 microns and doped to a density less than 5 {times} 10{sup 18}/cm{sup 3}; the first AlGaAs waveguide layer dimensioned to a thickness in a range between 400 and 700 Angstroms; the GaAs quantum well layer dimensioned to a thickness in a range between 50 and 200 Angstroms; the second AlGaAs waveguide layer dimensioned to a thickness in a range between 400 and 700 Angstroms; and the p-AlGaAs cladding layer dimensioned to a thickness which is greater than 2.0 microns and doped to a density less than 5 {times} 10{sup 18}/cm{sup 3}.

  10. Probing quantum confinement at the atomic scale with optically detected nuclear magnetic resonance

    NASA Astrophysics Data System (ADS)

    Kempf, James G.

    2001-09-01

    Near-band-gap circularly polarized excitation in III-V semiconductors provides spin-polarized electrons that transfer spin order to lattice nuclei via fluctuations in the contact hyperfine interaction. This process of optical nuclear polarization and the complementary technique of optical detection of nuclear magnetic resonance (NMR) provide extreme sensitivity enhancement and spatial selectivity in structured samples, enabling collection of NMR spectra from samples such as single quantum wells or dots containing as few as ˜105 nuclei. Combining these advances with novel techniques for high spectral resolution, we have probed quantum-confined electronic states near the interface of a single epitaxially grown Al1-x As/GaAs (x = 0.36) heterojunction. Using a novel strategy that we refer to as POWER (p&barbelow;erturbations o&barbelow;bserved w&barbelow;ith e&barbelow;nhanced ṟesolution) NMR, multiple-pulse time suspension is synchronized with bandgap optical irradiation to reveal spectra of effective spin Hamiltonians that are differences between those of the occupied and unoccupied photoexcited electronic state. The underlying NMR linewidth is reduced by three orders of magnitude in these experiments, enabling resolution of an asymmetric line shape due to light-induced hyperfine interactions. The results are successfully fit with the coherent nuclear spin evolution and relaxation theoretically expected for sites distributed over the volume of an electronic excitation weakly localized at a point defect. This analysis establishes a one-to-one relationship, which can be used to follow nuclear spin diffusion, between optical Knight shift and the radial position of lattice nuclei. We have also introduced POWER NMR techniques to characterize the change in electric field associated with cycling from light-on to light-off states via a linear quadrupole Stark effect (LQSE) of the nuclear spins. Simulations of these NMR spectra in terms of the radial electric fields of

  11. Dominant luminescence is not due to quantum confinement in molecular-sized silicon carbide nanocrystals.

    PubMed

    Beke, David; Szekrényes, Zsolt; Czigány, Zsolt; Kamarás, Katalin; Gali, Ádám

    2015-07-01

    Molecular-sized colloid silicon carbide (SiC) nanoparticles are very promising candidates to realize bioinert non-perturbative fluorescent nanoparticles for in vivo bioimaging. Furthermore, SiC nanoparticles with engineered vacancy-related emission centres may realize magneto-optical probes operating at nanoscale resolution. Understanding the nature of molecular-sized SiC nanoparticle emission is essential for further applications. Here we report an efficient and simple method to produce a relatively narrow size distribution of water soluble molecular-sized SiC nanoparticles. The tight control of their size distribution makes it possible to demonstrate a switching mechanism in the luminescence correlated with particle size. We show that molecular-sized SiC nanoparticles of 1-3 nm show a relatively strong and broad surface related luminescence whilst the larger ones exhibit a relatively weak band edge and structural defect luminescence with no evidence of quantum confinement effect. PMID:26055555

  12. Macroscopic Excitations in Confined Bose-Einstein Condensates, Searching for Quantum Turbulence

    NASA Astrophysics Data System (ADS)

    Zamora-Zamora, R.; Adame-Arana, O.; Romero-Rochin, V.

    2015-07-01

    We present a survey of macroscopic excitations of harmonically confined Bose-Einstein condensates (BEC), described by Gross-Pitaevskii (GP) equation, in search of routes to develop quantum turbulence. These excitations can all be created by phase-imprinting techniques on an otherwise equilibrium BEC. We analyze two crossed vortices, two parallel anti-vortices, a vortex ring, a vortex with topological charge , and a tangle of four vortices. Since GP equation is time-reversal invariant, we are careful to distinguish time intervals in which this symmetry is preserved and those in which rounding errors play a role. We find that the system tends to reach stationary states that may be widely classified as having either an array of vortices with collective excitations at different length scales or an agitated state composed mainly of Bogoliubov phonons.

  13. Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al, Ga) As lasers

    NASA Technical Reports Server (NTRS)

    Derry, P. L.; Chen, H. Z.; Morkoc, H.; Yariv, A.; Lau, K. Y.

    1988-01-01

    Broad area graded-index separate-confinement heterostructure single quantum well lasers grown by molecular-beam epitaxy (MBE) with threshold current density as low as 93 A/sq cm (520 microns long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A CW threshold current of 0.55 mA was obtained for a laser with facet reflectivities of about 80 percent, a cavity length of 120 micron, and an active region stripe width of 1 micron. These devices driven directly with logic level signals have switch-on delays less than 50 ps without any current prebias. Such lasers permit fully on-off switching while at the same time obviating the need for bias monitoring and feedback control.

  14. Time-Dependent Configuration Interaction Approach for Multielectron System Confined in Two-Dimensional Quantum Dot

    NASA Astrophysics Data System (ADS)

    Okunishi, Takuma; Clark, Richard; Takeda, Kyozaburo; Kusakabe, Koichi; Tomita, Norikazu

    2013-02-01

    We extend the static multireference description (resonant unrestricted Hartree-Fock) to a dynamical system in order to include the correlation effect dynamically. The resulting time-dependent (TD) Schrödinger equation is simplified into the time-developed rate equation (TD-CI), where the TD external field \\hatH‧(t) is taken into account directly in the Hamiltonian without any approximations. This TD-CI approach also has an advantage in that it takes into account the electron correlation by narrowing down the number of employed Slater determinants. We apply our TD-CI approach to the case of two electrons confined in the square quantum dot (QD) having the spin singlet multiplicity, and study theoretically the spatial and temporal fluctuation of the two-electron ground state under photon injection and pulse field application.

  15. Quantum confinement, core level shifts, and dopant segregation in P-doped Si⟨110⟩ nanowires

    NASA Astrophysics Data System (ADS)

    Han, Jiaxin; Chan, Tzu-Liang; Chelikowsky, James R.

    2010-10-01

    We examine P-doped Si⟨110⟩ nanowires by employing a real-space pseudopotential method. We find the defect wave function becomes more localized along the nanowire axis and the donor ionization energy increases, owing to quantum confinement. It is more difficult to dope a P atom into a Si⟨110⟩ nanowire than to dope Si bulk because the formation energy increases with decreasing size. By comparing the formation energy for different P positions within a nanowire, we find that if a P atom at the nanowire surface can overcome the energy barrier close to the surface, there is a tendency for the dopant to reside within the nanowire core. We calculate P core levels shift as P changes position within the nanowire and provide a means for x-ray photoelectron spectroscopy experiments to determine the location of P atoms within a Si nanowire.

  16. Study of photoluminescence and electroluminescence mechanisms in quantum-confined InSb/InAs heterostructures

    SciTech Connect

    Terent'ev, Ya. V. Mukhin, M. S.; Solov'ev, V. A.; Semenov, A. N.; Meltser, B. Ya.; Usikova, A. A.; Ivanov, S. V.

    2010-08-15

    Photoluminescence and electroluminescence in InSb/InAs heterostructures with ultrathin InSb insertions grown by molecular-beam epitaxy have been systematically studied. Measurements were made in the temperature range from 2 to 300 K on a large set of samples of various designs, with both the InAs matrix and ultrathin InSb insertions grown by different methods. The primary goal of the study was to identify the main radiative recombination channels in these heterostructures. It is shown that optical transitions associated with acceptor impurity centers in the InAs matrix represent an important mechanism diminishing the efficiency of luminescence from InSb insertions at room temperature. The results obtained are important for development of optimal growth modes and design of the active region of light-emitting devices based on quantum-confined InSb/InAs structures emitting in the range 3-5 {mu}m.

  17. Enhancement of thermoelectric performance in InAs nanotubes by tuning quantum confinement effect

    SciTech Connect

    Zhou, Wu-Xing; Tan, Shihua; Chen, Ke-Qiu; Hu, Wenping

    2014-03-28

    By using the nonequilibrium Green's function method, we study the thermoelectric properties of InAs nanotubes. The results show that InAs nanotube with a certain internal diameter has much higher ZT value than nanowire due to the enhancement of quantum confinement effect leading to the increase of the power factor S{sup 2}G. The ZT value of InAs nanotube can reach 1.74, which is about three times greater than that of nanowires. Moreover, it is found that the ZT values of InAs nanotubes decrease rapidly with the increase of internal diameter, which results from the rapid increase of phonons thermal conductance due to the “red shift” of low-frequency optical phonon modes.

  18. Quantum propagation and confinement in 1D systems using the transfer-matrix method

    NASA Astrophysics Data System (ADS)

    Pujol, Olivier; Carles, Robert; Pérez, José-Philippe

    2014-05-01

    The aim of this article is to provide some Matlab scripts to the teaching community in quantum physics. The scripts are based on the transfer-matrix formalism and offer a very efficient and versatile tool to solve problems of a physical object (electron, proton, neutron, etc) with one-dimensional (1D) stationary potential energy. Resonant tunnelling through a multiple-barrier or confinement in wells of various shapes is particularly analysed. The results are quantitatively discussed with semiconductor heterostructures, harmonic and anharmonic molecular vibrations, or neutrons in a gravity field. Scripts and other examples (hydrogen-like ions and transmission by a smooth variation of potential energy) are available freely at http://www-loa.univ-lille1.fr/˜pujol in three languages: English, French and Spanish.

  19. Construction of novel quantum-confined structure with lead halide/gemini surfactant hybrids

    NASA Astrophysics Data System (ADS)

    Takeoka, Yuko; Kawahara, Mitsuyasu; Rikukawa, Masahiro

    2012-11-01

    Novel organic-inorganic hybrid compounds, C16H44N4Pb3I10 and C14H34N2Pb2I6, were synthesized by solvent diffusion recrystallization from the dimethylsulfoxide solution containing PbI2 and gemini ammonium surfactants with different head groups. The results of single crystal X-ray structural analysis showed that the inorganic region of C16H44N4Pb3I10 has quasi one-dimensional chains of [Pb3I10]4- units, whereas that of C12H30N2Pb2I6 has one-dimensional chains of face-sharing PbI6 octahedra. The absorption and fluorescence spectra of these compounds also indicate the formation of one-dimensional inorganic chains and quantum-confined structures.

  20. Confinement and Lattice Quantum-Electrodynamic Electric Flux Tubes Simulated with Ultracold Atoms

    SciTech Connect

    Zohar, Erez; Reznik, Benni

    2011-12-30

    We propose a method for simulating (2+1)D compact lattice quantum-electrodynamics, using ultracold atoms in optical lattices. In our model local Bose-Einstein condensates' (BECs) phases correspond to the electromagnetic vector potential, and the local number operators represent the conjugate electric field. The well-known gauge-invariant Kogut-Susskind Hamiltonian is obtained as an effective low-energy theory. The field is then coupled to external static charges. We show that in the strong coupling limit this gives rise to ''electric flux tubes'' and to confinement. This can be observed by measuring the local density deviations of the BECs, and is expected to hold even, to some extent, outside the perturbative calculable regime.

  1. Confinement in Maxwell-Chern-Simons planar quantum electrodynamics and the 1/N approximation

    SciTech Connect

    Hofmann, Christoph P.; Raya, Alfredo; Madrigal, Saul Sanchez

    2010-11-01

    We study the analytical structure of the fermion propagator in planar quantum electrodynamics coupled to a Chern-Simons term within a four-component spinor formalism. The dynamical generation of parity-preserving and parity-violating fermion mass terms is considered, through the solution of the corresponding Schwinger-Dyson equation for the fermion propagator at leading order of the 1/N approximation in Landau gauge. The theory undergoes a first-order phase transition toward chiral symmetry restoration when the Chern-Simons coefficient {theta} reaches a critical value which depends upon the number of fermion families considered. Parity-violating masses, however, are generated for arbitrarily large values of the said coefficient. On the confinement scenario, complete charge screening - characteristic of the 1/N approximation - is observed in the entire (N,{theta})-plane through the local and global properties of the vector part of the fermion propagator.

  2. Plasmon response of a quantum-confined electron gas probed by core-level photoemission

    SciTech Connect

    Ozer, Mustafa M; Moon, Eun Ju; Eguiluz, Adolfo G; Weitering, Harm H

    2011-01-01

    We demonstrate the existence of quantized 'bulk' plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d{sup 2} all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.

  3. Quantum confined colloidal nanorod heterostructures for solar-to-fuel conversion.

    PubMed

    Wu, Kaifeng; Lian, Tianquan

    2016-07-11

    Solar energy conversion, particularly solar-driven chemical fuel formation, has been intensely studied in the past decades as a potential approach for renewable energy generation. Efficient solar-to-fuel conversion requires artificial photosynthetic systems with strong light absorption, long-lived charge separation and efficient catalysis. Colloidal quantum confined nanoheterostructures have emerged as promising materials for this application because of the ability to tailor their properties through size, shape and composition. In particular, colloidal one-dimensional (1D) semiconductor nanorods (NRs) offer the opportunity to simultaneously maintain quantum confinement in radial dimensions for tunable light absorptions and bulk like carrier transport in the axial direction for long-distance charge separations. In addition, the versatile chemistry of colloidal NRs enables the formation of semiconductor heterojunctions (such as CdSe/CdS dot-in-rod NRs) to separate photogenerated electron-hole pairs and deposition of metallic domains to accept charges and catalyze redox reactions. In this review, we summarize research progress on colloidal NR heterostructures and their applications for solar energy conversion, emphasizing mechanistic insights into the working principle of these systems gained from spectroscopic studies. Following a brief overview of synthesis of various NRs and heterostructures, we introduce their electronic structures and dynamics of exciton and carrier transport and interfacial transfer. We discuss how these exciton and carrier dynamics are controlled by their structures and provide key mechanistic understanding on their photocatalytic performance, including the photo-reduction of a redox mediator (methyl viologen) and light driven H2 generation. We discuss the solar-driven H2 generation mechanism, key efficiency limiting steps, and potential approaches for rational improvement in semiconductor NR/metal heterostructures (such as Pt tipped Cd

  4. Photoemission Studies of Quantum Confinement in Nonmagnetic/Magnetic Film and Wedge Structures

    NASA Astrophysics Data System (ADS)

    Li, Dongqi

    1996-03-01

    The field of giant magnetoresistance (GMR) has generated much excitement. Photoemission provides a powerful tool to address some of the outstanding, yet fundamental issues in this field. It probes the spin-polarized metallic quantum well (QW) states in GMR materials, which underlie the oscillatory exchange coupling.(J. E. Ortega and F. J. Himpsel, Phys. Rev. Lett. 69, 844 (1992).) Angle- and spin-resolved photoemission experiments were performed at NSLS undulator beamline U5. For the Cu/Co system, both sp- and d-band derived minority-spin QW states of Cu overlayers on Co(100) grown on a Cu(100) substrate are identified. Also, the degree of confinement of these states is quantified by inserting a Co wedge to form a barrier between the Cu overlayer and substrate.(Dongqi Li, et al., Phys. Rev. B 51, 7195 (1995).) The characteristic length scale for the quantum confinement coincides with that known to influence the GMR. This provides a glimpse into understanding the importance of spin-dependent interfacial scattering. Another basic issue still under debate is the origin of the "long-period" oscillation in systems such as Fe/Cr. The QW picture is recognized as equivalent to the RKKY description of the coupling periodicity. There are three regions of the Cr Fermi surface whose spanning vectors can explain the periodicity, and thus, where the characteristic QW states might emerge: (i) the nested region (due to aliasing); (ii) the N-centered ellipse; or (iii) the d-derived "lens". Angle- resolved photoemission provides a novel methodology to search k-space for the features responsible for the coupling. Work done in collaboration with S. D. Bader, D.-J. Huang, P. D. Johnson, J. E. Mattson, J. Pearson, E. Vescovo. * Supported by DOE BES-MS under #W-31-109-ENG-38 and ONR under #N-00014- 94-F-0085.

  5. Surface confined quantum well state in MoS{sub 2}(0001) thin film

    SciTech Connect

    Sun, Jia-Tao Song, S. R.; Meng, S.; Du, S. X.; Gao, H. J.; Liu, F.

    2015-10-19

    Surface confined quantum well state (scQWS) is a QWS confined around the surface of a thin film whose electronic energy is smaller than the work function of the film. The scQWS is rather rare in most thin films. Here, we show the existence of scQWS in thin films of transition metal dichalcogenides, MoS{sub 2}. Signatures of scQWS are identified as the overall downward band dispersion in the bulk gap of 2 H-MoS{sub 2} thin film at larger binding energy range. These scQWSs are also characterized with a Shockley-type surface state having an inverse parabolic decay into the film and a symmetric (asymmetric) distribution of projected charge density at the two surfaces of odd-layer (even-layer) films. Our findings of scQWS in MoS{sub 2} shed some light on understanding the electronic properties of 2D materials with implications in future 2D electronic devices.

  6. Control of physical and optical properties of II-VI quantum dots

    NASA Astrophysics Data System (ADS)

    Sooklal, Kelly Sonja

    This thesis primarily concentrates on two semiconductors, CdS and ZnS, both of which have been widely used in the fabrication of electrical devices. Nanoparticles of CdS and ZnS have both been prepared using a variety of synthetic methods. These "quantum confined" particles exhibit a wide range of size dependent properties which can be modified by either altering their size and/or surface chemistry. In one set of experiments, it was found that the location of Mn 2+ profoundly affects the photophysics of ZnS nanoclusters. Mn 2+ substituted for Zn2+ in the ZnS lattice produced orange emission with lifetimes that were intermediate between those found for micron clusters and smaller nanoclusters. The addition of Mn2+ to the outside of the preformed ZnS nanoclusters showed near-band gap emission in the ultraviolet with even shorter lifetimes. We have also used these Mn2+ doped nanoclusters to fabricate electroluminescent devices. In another set of experiments, the effects of different ions on the photophysics of ZnS nanoclusters was investigated. Depending on the cation, we have been able to produce ZnS nanoclusters that emit in the blue, green, yellow and orange regions of the visible spectrum by incorporating Cu2+, Pb2+ and Mn2+. Quantum dots of CdS have also been prepared using several different stabilizing agents. CdS nanoparticles that have been synthesized using dendrimers as hosts exhibit striking optical and electronic features. Intense blue-green emission is observed when the CdS-dendrimer nanocomposites are formed in methanol and/or acidified methanol solutions. Bright yellow emission is observed when the semiconductor-dendrimer nanocomposites are prepared in water and/or basic methanol solutions. One additional experiment was performed using capping groups to modify the photophysics of CdS. Nanometer-sized CdS were prepared using a series of 4-substituted thiophenols as capping agents. The 4-substituents included both electron-donating and electron

  7. Confinement transition of Z2 gauge theory coupled to fermions. A sign problem free quantum Monte Carlo study

    NASA Astrophysics Data System (ADS)

    Gazit, Snir; Randeria, Mohit; Vishwanath, Ashvin

    In two space dimensions, the Z2 lattice gauge theory is known to undergo a zero temperature confinement to de-confinment quantum phase transition . In this work, we study how this transition is modified in the presence of lattice fermions which are minimally coupled to the Z2 gauge field. This may be viewed as an extreme version of the BEC-BCS transition where fermions are confined in the strong coupling phase. We investigate both a square lattice model with a large fermi surface and Dirac fermions realized on a π flux and honeycomb lattices. The models are found to be free of the numerical sign problem for all fermion density. In addition, we introduce a numerical method to stochastically incorporate the Gauss law constraint in a quantum Monte Carlo (QMC) simulation. The phase diagram as a function of the model parameters, chemical potential and temperature is determined by means of a large scale determinant QMC.

  8. Assessment of field-induced quantum confinement in heterogate germanium electron–hole bilayer tunnel field-effect transistor

    SciTech Connect

    Padilla, J. L. Alper, C.; Ionescu, A. M.; Gámiz, F.

    2014-08-25

    The analysis of quantum mechanical confinement in recent germanium electron–hole bilayer tunnel field-effect transistors has been shown to substantially affect the band-to-band tunneling (BTBT) mechanism between electron and hole inversion layers that constitutes the operating principle of these devices. The vertical electric field that appears across the intrinsic semiconductor to give rise to the bilayer configuration makes the formerly continuous conduction and valence bands become a discrete set of energy subbands, therefore increasing the effective bandgap close to the gates and reducing the BTBT probabilities. In this letter, we present a simulation approach that shows how the inclusion of quantum confinement and the subsequent modification of the band profile results in the appearance of lateral tunneling to the underlap regions that greatly degrades the subthreshold swing of these devices. To overcome this drawback imposed by confinement, we propose an heterogate configuration that proves to suppress this parasitic tunneling and enhances the device performance.

  9. Improvement of the quantum confined Stark effect characteristics by means of energy band profile modulation: The case of Gaussian quantum wells

    NASA Astrophysics Data System (ADS)

    Ramírez-Morales, A.; Martínez-Orozco, J. C.; Rodríguez-Vargas, I.

    2011-11-01

    We study the quantum confined stark effect (QCSE) characteristics in Gaussian quantum wells (GQW). This special energy band profile is built varying the aluminum concentration of the AlGaAs ternary alloy in Gaussian fashion. The semi-empirical sp3s* tight-binding model including spin is used to obtain the energy Stark shifts (ESS) and the wave-function Gaussian spatial overlap (GSO) between electrons and holes for different electric field strengths, quantum well widths and aluminum concentrations. We find that both the ESS and the GSO depend parabolically with respect to the electric field strength and the quantum well width. These QCSE characteristics show an asymmetry for the electric field in the forward and reverse directions, related directly to the different band-offset of electrons and holes, being the negative electric fields (reverse direction) more suitable to reach greater ESS. Two important features are presented by this special energy band profile: (1) reductions of the ESS and (2) enhancements of the GSO of tents to hundreds with respect to parabolic and rectangular quantum wells. Even more, tailoring the quantum well width it is possible to reach GSO of thousands with respect to rectangular quantum wells. Finally, it is important to mention that similar results could be obtained in other quantum well heterostructures of materials such as nitrides, oxides (ZnO), and SiGe whenever the confinement band profiles are modulated in Gaussian form.

  10. Bias activated dielectric response of excitons and excitonic Mott transition in quantum confined lasers structures.

    NASA Astrophysics Data System (ADS)

    Bhunia, Amit; Bansal, Kanika; Datta, Shouvik; Alshammari, Marzook S.; Henini, Mohamed

    In contrast to the widely reported optical techniques, there are hardly any investigations on corresponding electrical signatures of condensed matter physics of excitonic phenomena. We studied small signal steady state capacitance response in III-V materials based multi quantum well (AlGaInP) and MBE grown quantum dot (InGaAs) laser diodes to identify signatures of excitonic presence. Conductance activation by forward bias was probed using frequency dependent differential capacitance response (fdC/df), which changes characteristically with the onset of light emission indicating the occurrence of negative activation energy. Our analysis shows that it is connected with a steady state population of exciton like bound states. Calculated average energy of this bound state matches well with the binding energy of weakly confined excitons in this type of structures. Further increase in charge injection decreases the differential capacitive response in AlGaInP based diodes, indicating a gradual Mott transition of excitonic states into electron hole plasma. This electrical description of excitonic Mott transition is fully supplemented by standard optical spectroscopic signatures of band gap renormalization and phase space filling effects.

  11. Phosphorene confined systems in magnetic field, quantum transport, and superradiance in the quasiflat band

    NASA Astrophysics Data System (ADS)

    Ostahie, B.; Aldea, A.

    2016-02-01

    Spectral and transport properties of electrons in confined phosphorene systems are investigated in a five hopping parameter tight-binding model, using analytical and numerical techniques. The main emphasis is on the properties of the topological edge states accommodated by the quasiflat band that characterizes the phosphorene energy spectrum. We show, in the particular case of phosphorene, how the breaking of the bipartite lattice structure gives rise to the electron-hole asymmetry of the energy spectrum. The properties of the topological edge states in the zigzag nanoribbons are analyzed under different aspects: degeneracy, localization, extension in the Brillouin zone, dispersion of the quasiflat band in magnetic field. The finite-size phosphorene plaquette exhibits a Hofstadter-type spectrum made up of two unequal butterflies separated by a gap, where a quasiflat band composed of zigzag edge states is located. The transport properties are investigated by simulating a four-lead Hall device (importantly, all leads are attached on the same zigzag side), and using the Landauer-Büttiker formalism. We find out that the chiral edge states due to the magnetic field yield quantum Hall plateaus, but the topological edge states in the gap do not support the quantum Hall effect and prove a dissipative behavior. By calculating the complex eigenenergies of the non-Hermitian effective Hamiltonian that describes the open system (plaquette+leads), we prove the superradiance effect in the energy range of the quasiflat band, with consequences for the density of states and electron transmission properties.

  12. Nuclear Quantum Effects in H(+) and OH(-) Diffusion along Confined Water Wires.

    PubMed

    Rossi, Mariana; Ceriotti, Michele; Manolopoulos, David E

    2016-08-01

    The diffusion of protons and hydroxide ions along water wires provides an efficient mechanism for charge transport that is exploited by biological membrane channels and shows promise for technological applications such as fuel cells. However, what is lacking for a better control and design of these systems is a thorough theoretical understanding of the diffusion process at the atomic scale. Here we focus on two aspects of this process that are often disregarded because of their high computational cost: the use of first-principles potential energy surfaces and the treatment of the nuclei as quantum particles. We consider proton and hydroxide ions in finite water wires using density functional theory augmented with an apolar cylindrical confining potential. We employ machine learning techniques to identify the charged species, thus obtaining an agnostic definition that takes explicitly into account the delocalization of the charge in the Grotthus-like mechanism. We include nuclear quantum effects (NQEs) through the thermostated ring polymer molecular dynamics method and model finite system size effects by considering Langevin dynamics on the potential of mean force of the charged species, allowing us to extract the same "universal" diffusion coefficient from simulations with different wire sizes. In the classical case, diffusion coefficients depend significantly on the potential energy surface, in particular on how dispersion forces modulate water-water distances. NQEs, however, make the diffusion less sensitive to the underlying potential and geometry of the wire. PMID:27440483

  13. Effects of thermolysis and ferrocyanide quenching on quantum-confined CdS stabilized by polynucleotides

    SciTech Connect

    Bigham, S.R.; Coffer, J.L.

    1993-12-31

    Cadmium sulfide semiconductor clusters in the quantum confined size regime (Q-CdS) may be successfully stabilized by double-stranded deoxyribonucleic acid (DNA) from calf thymus and E. Coli as well as by single-stranded ribonucleic acids (RNA) in the forms of Poly[A], Poly[C], Poly[G] and Poly [U]. These Q-CdS/ploynucleotide clusters exhibit broad trap emission characteristic of both cadmium and sulfur related defect sites at the semiconductor surface. Here the paper discusses differences in the nature of the stabilizer-cluster interaction between single-stranded and double-stranded polynucleotides, as probed by monitoring changes in photoluminescence after thermolysis or ferrocyanide addition. Thermolysis of Q-CdS/polynucleotide samples affects the interfacial interaction between cluster and stabilizer as demonstrated by a shift in the emission maximum and a change in quantum yield. Stern-Volmer analysis of photoluminescence quenching with ferrocyanide anions exhibits nonlinear behavior. Ferrocyanide anions quench the photoluminescene of Q-CdS/DNA approximately 38% more efficiently (in terms of integrated intensity) than Q-CdS/RNA after 0.17 mN addition. Such behavior suggests that single-stranded polynucleotides are better than double-stranded polynucleotides in terms of protecting the semiconductor surface from the highly negatively charged ferrocyanide anion.

  14. Polarization of emission from non-polar III-nitride quantum wells: the influence of confinement

    NASA Astrophysics Data System (ADS)

    Arora, Ashish; Ghosh, Sandip

    2014-01-01

    Taking M-plane oriented GaN quantum wells (QW) as an example, it is shown that the finite out-of-plane crystal momentum arising from quantum confinement modifies valence band mixing in a way that can significantly alter the emission polarization properties of strained non-polar oriented wurtzite group III-nitride QWs. For certain values of strain, the emission polarization direction can rotate by 90° either within the QW plane, or from being out-of-plane to being in-plane which is desirable for light emission applications. The study based on a k · p type perturbation theory simultaneously accounts for the influence of anisotropic in-plane strain which arises in such QWs and also affects the optical polarization properties. An important practical implication of these results is that M-plane oriented AlxGa1-xN QWs under anisotropic in-plane tensile strain can work as efficient ultra-violet light emitters, unlike bulk AlxGa1-xN films with identical composition and strain. After including the influence of the out-of-plane crystal momentum, the emission polarization criterion allows for larger concentration of Al in such QW active layers if the well width is kept sufficiently small. These results are also applicable to A-plane oriented QWs.

  15. Direct Measurement of Quantum Confinement and Environmental Pinning Effects on Metal/Nanostructure Schottky Contacts

    NASA Astrophysics Data System (ADS)

    Tivarus, Cristian

    2005-03-01

    I will discuss direct nm-resolution measurements of metal/quantum well (QW) Schottky contacts made using Cross- sectional Ballistic Electron Emission Microscopy (XBEEM), in order to quantify the influence of small-size effects on hot- carrier injection into semiconductor nanostructures. Molecular Beam Epitaxy was used to grow a sequence of GaAs QWs with width varying from 1nm to 15 nm, separated by thick Al0.3Ga0.7As barrier layers. The samples were cleaved ex-situ and polycrystalline Au contacts were electron-beam evaporated on the cleaved edge using shadow mask or photo-lithography. Samples were studied in ultra-high vacuum using Scanning Tunneling Microscopy and XBEEM. The Schottky barrier height over the QWs was found to systematically increase with decreasing QW width, by up to ˜140 meV for the 1 nm QW. This is mostly due to a large quantum-confinement increase ( up to ˜200 meV) of the QW conduction band minimum (CBM), as estimated by a simple 1D QW model. We also did finite element electrostatic modeling to estimate the ``environmental" effects of the surrounding metal/Al0.3Ga0.7As interface on the QW CBM. Excellent quantitative agreement over the full QW width range is obtained when both quantum confinement and electrostatic effects are considered.I will also discuss on-going measurements to use the metal/QW nanocontacts as unique ``nano-apertures" to directly image and quantify the lateral hot-electron spreading profile in the metal film. This profile is surprisingly large, with a FWHM of ˜15nm (˜21nm) for a 4nm (7nm) thick Au film. XBEEM images directly show that hot-electron spreading is strongly modified by the grain structure in the metal film. In collaboration with J.P. Pelz, M.K. Hudait, and S.A. Ringel. Work supported by NSF and ONR

  16. Theory of the vortex-clustering transition in a confined two-dimensional quantum fluid

    NASA Astrophysics Data System (ADS)

    Yu, Xiaoquan; Billam, Thomas P.; Nian, Jun; Reeves, Matthew T.; Bradley, Ashton S.

    2016-08-01

    Clustering of like-sign vortices in a planar bounded domain is known to occur at negative temperature, a phenomenon that Onsager demonstrated to be a consequence of bounded phase space. In a confined superfluid, quantized vortices can support such an ordered phase, provided they evolve as an almost isolated subsystem containing sufficient energy. A detailed theoretical understanding of the statistical mechanics of such states thus requires a microcanonical approach. Here we develop an analytical theory of the vortex clustering transition in a neutral system of quantum vortices confined to a two-dimensional disk geometry, within the microcanonical ensemble. The choice of ensemble is essential for identifying the correct thermodynamic limit of the system, enabling a rigorous description of clustering in the language of critical phenomena. As the system energy increases above a critical value, the system develops global order via the emergence of a macroscopic dipole structure from the homogeneous phase of vortices, spontaneously breaking the Z2 symmetry associated with invariance under vortex circulation exchange, and the rotational SO (2 ) symmetry due to the disk geometry. The dipole structure emerges characterized by the continuous growth of the macroscopic dipole moment which serves as a global order parameter, resembling a continuous phase transition. The critical temperature of the transition, and the critical exponent associated with the dipole moment, are obtained exactly within mean-field theory. The clustering transition is shown to be distinct from the final state reached at high energy, known as supercondensation. The dipole moment develops via two macroscopic vortex clusters and the cluster locations are found analytically, both near the clustering transition and in the supercondensation limit. The microcanonical theory shows excellent agreement with Monte Carlo simulations, and signatures of the transition are apparent even for a modest system of 100

  17. Effect of thermal annealing on the emission properties of heterostructures containing a quantum-confined GaAsSb layer

    SciTech Connect

    Dikareva, N. V. Vikhrova, O. V.; Zvonkov, B. N.; Malekhonova, N. V.; Nekorkin, S. M.; Pirogov, A. V.; Pavlov, D. A.

    2015-01-15

    Heterostructures containing single GaAsSb/GaAs quantum wells and bilayer GaAsSb/InGaAs quantum wells are produced by metal-organic vapor-phase epitaxy at atmospheric pressure. The growth temperature of the quantum-confined layers is 500–570°C. The structural quality of the samples and the quality of heterointerfaces of the quantum wells are studied by the high-resolution transmission electron microscopy of cross sections. The emission properties of the heterostructures are studied by photoluminescence measurements. The structures are subjected to thermal annealing under conditions chosen in accordance with the temperature and time of growth of the upper cladding p-InGaP layer during the formation of GaAs/InGaP laser structures with an active region containing quantum-confined GaAsSb layers. It is found that such heat treatment can have a profound effect on the emission properties of the active region, only if a bilayer GaAsSb/InGaAs quantum well is formed.

  18. Cyclotron transition line-width due to interactions with the flexural wave of a phonon confined in a quantum well

    NASA Astrophysics Data System (ADS)

    Kang, Nam Lyong

    2016-03-01

    The cyclotron transition line-width for a system of electrons interacting with the flexural wave of phonons confined in a quantum well structure of silicon was calculated using the optical conductivity formula derived by the projection-reduction method. Only a few confined phonons with low energy make a significant contribution to the line-width, which increases with increasing temperature. The well width and magnetic field dependence of the line-width are complicated and the flexural mode contributes to the line-width more strongly than the dilatational mode at low magnetic fields and for small well widths.

  19. Collectively Induced Quantum-Confined Stark Effect in Monolayers of Molecules Consisting of Polar Repeating Units

    PubMed Central

    2011-01-01

    The electronic structure of terpyrimidinethiols is investigated by means of density-functional theory calculations for isolated molecules and monolayers. In the transition from molecule to self-assembled monolayer (SAM), we observe that the band gap is substantially reduced, frontier states increasingly localize on opposite sides of the SAM, and this polarization in several instances is in the direction opposite to the polarization of the overall charge density. This behavior can be analyzed by analogy to inorganic semiconductor quantum-wells, which, as the SAMs studied here, can be regarded as semiperiodic systems. There, similar observations are made under the influence of a, typically external, electric field and are known as the quantum-confined Stark effect. Without any external perturbation, in oligopyrimidine SAMs one encounters an energy gradient that is generated by the dipole moments of the pyrimidine repeat units. It is particularly strong, reaching values of about 1.6 eV/nm, which corresponds to a substantial electric field of 1.6 × 107 V/cm. Close-lying σ- and π-states turn out to be a particular complication for a reliable description of the present systems, as their order is influenced not only by the docking groups and bonding to the metal, but also by the chosen computational approach. In the latter context we demonstrate that deliberately picking a hybrid functional allows avoiding pitfalls due to the infamous self-interaction error. Our results show that when aiming to build a monolayer with a specific electronic structure one can not only resort to the traditional technique of modifying the molecular structure of the constituents, but also try to exploit collective electronic effects. PMID:21955058

  20. Investigation of quantum confinement within the tunneling-percolation transition for ultrathin bismuth films

    NASA Astrophysics Data System (ADS)

    Oller, Declan; Fernandes, Gustavo E.; Kim, Jin Ho; Xu, Jimmy

    2015-10-01

    We investigate conduction phenomena in ultrathin bismuth (Bi) films that are thermally evaporated onto flat quartz. Critical points in the conductance as a function of deposition time are identified and used to scale the data from time dependence to coverage dependence. The resulting nonlinear coverage scaling equation is verified independently via analysis done on transmission electron microscope images of the evaporated films. The scaled data yields critical exponents in very good agreement with classical percolation theory, and clearly shows the transition from the tunneling regime into percolation. Surprisingly, no noticeable signatures of size-quantization effects in the nucleation sites as a function of deposition time is observed in either regime. We discuss our findings in light of Boltzmann transport modeling of 1D conduction as an approximation to the narrow percolative paths that form at the onset of percolation. Our results suggest that lack of a preferred crystallite orientation in the nucleation process may indeed cause quantum-confinement to be too smeared out to be observable in the tunneling to percolation transition.

  1. Energy transfer in finite-size exciton-phonon systems: Confinement-enhanced quantum decoherence

    NASA Astrophysics Data System (ADS)

    Pouthier, Vincent

    2012-09-01

    Based on the operatorial formulation of the perturbation theory, the exciton-phonon problem is revisited for investigating exciton-mediated energy flow in a finite-size lattice. Within this method, the exciton-phonon entanglement is taken into account through a dual dressing mechanism so that exciton and phonons are treated on an equal footing. In a marked contrast with what happens in an infinite lattice, it is shown that the dynamics of the exciton density is governed by several time scales. The density evolves coherently in the short-time limit, whereas a relaxation mechanism occurs over intermediated time scales. Consequently, in the long-time limit, the density converges toward a nearly uniform distributed equilibrium distribution. Such a behavior results from quantum decoherence that originates in the fact that the phonons evolve differently depending on the path followed by the exciton to tunnel along the lattice. Although the relaxation rate increases with the temperature and with the coupling, it decreases with the lattice size, suggesting that the decoherence is inherent to the confinement.

  2. Quantum confinement induced band gaps in MgB2 nanosheets

    NASA Astrophysics Data System (ADS)

    Xu, Bo Z.; Beckman, Scott P.

    2016-09-01

    The discovery of two-dimensional semiconducting materials, a decade ago, spawned an entire sub-field within solid-state physics that is focused on the development of nanoelectronics. Here we present a new class of semiconducting two-dimensional material based on hexagonal MgB2. Although MgB2 is a semimetal, similar to the other well-studied transition metal diborides, we demonstrate that, unlike the transition metal diborides, thinning MgB2, to create nanosheets, opens a band gap in the density of states. We predict that a 7 Å thick MgB2 nanosheet will have a band gap of 0.51 eV. MgB2 nanosheets differ from other two-dimensional semiconductors in that the band gap is introduced by (001) surfaces and is opened by the quantum confinement effect. The implications of these findings are that nanostructured MgB2 is not merely a new composition, but also has intrinsic mechanisms for tuning its electronic properties, which may facilitate the development of nanoelectronics.

  3. Role of quantum confinement and hyperfine splitting in lithium-doped ZnO nanocrystals

    NASA Astrophysics Data System (ADS)

    Kwak, Hyunwook; Tiago, Murilo L.; Chan, Tzu-Liang; Chelikowsky, James R.

    2008-11-01

    The role of quantum confinement on the electronic properties of Li interstitial impurities in ZnO nanocrystals was examined using a real-space pseudopotential-density-functional method. The Li impurity was found to be partially ionized resulting in a significant charge transfer around the impurity site. To calculate the hyperfine interaction for this system using pseudopotentials, we modified Van de Walle and Blöchl’s method to include explicitly the off-site contribution of the Li impurity wave function. Our modifications dramatically enhanced the agreement between the calculated and the measured isotropic hyperfine splitting constants. Our analysis with an effective-mass model demonstrates that the partial ionization of the impurity atom plays an important role both in the binding energy and in the shape of its wave function. Comparison between calculations using the local-density approximation (LDA) with LDA+U indicates that the local Coulomb correlation does not play a significant role in altering the impurity electronic states of interstitial Li-doped ZnO nanocrystals.

  4. Acid-free sol-gel fabrication of glass thin films embedded with II-VI colloidal quantum dots

    NASA Astrophysics Data System (ADS)

    Jani, Hemang; Duan, Lingze

    2015-01-01

    II-VI colloidal quantum dots (QDs) are ideal for optical sensors thanks to their high fluorescent brightness and good size uniformity. However, embedding colloidal QDs into a glass matrix with the standard sol-gel process leads to the QDs being damaged by the acid catalyst. Here, we report an acid-free sol-gel technique, which proves to be both simple and effective in fabricating silica glass thin films embedded with commercial II-VI colloidal QDs. Octadecylamine ligands are used as a bifunctional aid to not only stabilize the QDs in solution, but also assist the formation of the SiO2 gel. We demonstrate that high-quality QD-embedded glass thin films can be developed with this technique, and our fluorescent tests indicate that, except for a small blueshift in the emission spectrum, the QDs are very well preserved through the sol-gel process. This method offers a fast and low-cost path towards thin-film QD sensors with good mechanical and thermal stabilities, which are desirable for applications involving highly focused laser beams, such as ultrafast nanophotonics.

  5. Features of the electroluminescence spectra of quantum-confined silicon p{sup +}-n heterojunctions in the infrared spectral region

    SciTech Connect

    Bagraev, N. T.; Klyachkin, L. E.; Kuzmin, R. V. Malyarenko, A. M.; Mashkov, V. A.

    2013-11-15

    The results of studying the characteristics of optical emission in various regions of quantum-confined silicon p{sup +}-n heterojunctions heavily doped with boron are analyzed. The results obtained allow one to conclude that near-infrared electroluminescence arises near the heterointerface between the nanostructured wide-gap silicon p{sup +}-barrier heavily doped with boron and n-type silicon (100), the formation of which included the active involvement of boron dipole centers.

  6. Wet chemical synthesis of quantum confined nanostructured tin oxide thin films by successive ionic layer adsorption and reaction technique

    SciTech Connect

    Murali, K.V.; Ragina, A.J.; Preetha, K.C.; Deepa, K.; Remadevi, T.L.

    2013-09-01

    Graphical abstract: - Highlights: • Quantum confined SnO{sub 2} thin films were synthesized at 80 °C by SILAR technique. • Film formation mechanism is discussed. • Films with snow like crystallite morphology offer high specific surface area. • The blue-shifted value of band gap confirmed the quantum confinement effect. • Present synthesis has advantages – low cost, low temperature and green friendly. - Abstract: Quantum confined nanostructured SnO{sub 2} thin films were synthesized at 353 K using ammonium chloride (NH{sub 4}Cl) and other chemicals by successive ionic layer adsorption and reaction technique. Film formation mechanism is discussed. Structural, morphological, optical and electrical properties were investigated and compared with the as-grown and annealed films fabricated without NH{sub 4}Cl solution. SnO{sub 2} films were polycrystalline with crystallites of tetragonal structure with grain sizes lie in the 5–8 nm range. Films with snow like crystallite morphology offer high specific surface area. The blue-shifted value of band gap of as-grown films confirmed the quantum confinement effect of grains. Refractive index of the films lies in the 2.1–2.3 range. Films prepared with NH{sub 4}Cl exhibit relatively lower resistivity of the order of 10{sup 0}–10{sup −1} Ω cm. The present synthesis has advantages such as low cost, low temperature and green friendly, which yields small particle size, large surface–volume ratio, and high crystallinity SnO{sub 2} films.

  7. Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces

    NASA Astrophysics Data System (ADS)

    Kepčija, N.; Huang, T.-J.; Klappenberger, F.; Barth, J. V.

    2015-03-01

    Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green's functions for finite geometries and electron plane wave expansion for periodic systems. The "particle in a box" picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

  8. Diffusion of H2 and D2 Confined in Single-Walled Carbon Nanotubes: Quantum Dynamics and Confinement Effects.

    PubMed

    Mondelo-Martell, Manel; Huarte-Larrañaga, Fermín

    2016-08-25

    We present quantum dynamics calculations of the diffusion constant of H2 and D2 along a single-walled carbon nanotube at temperatures between 50 and 150 K. We calculate the respective diffusion rates in the low-pressure limit by adapting well-known approaches and methods from the chemical dynamics field using two different potential energy surfaces to model the C-H interaction. Our results predict a usual kinetic isotope effect, with H2 diffusing faster than D2 in the higher temperature range but a reverse trend at temperatures below 50-70 K. These findings are consistent with experimental observation in similar systems and can be explained by the different effective size of both isotopes resulting from their different zero-point energy. PMID:27459476

  9. Structural Metastability and Quantum Confinement in Zn1-xCoxO Nanoparticles.

    PubMed

    Almonacid, G; Martín-Rodríguez, R; Renero-Lecuna, C; Pellicer-Porres, J; Agouram, S; Valiente, R; González, J; Rodríguez, F; Nataf, L; Gamelin, D R; Segura, A

    2016-08-10

    This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1-xCoxO nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption, and transmission electron microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W-NPs. In contrast to the abrupt phase transition in W-Zn1-xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co(2+) are observed in NPs from ambient pressure to about 17 GPa. Above this pressure, several changes in the absorption spectrum reveal a gradual and irreversible W-to-RS phase transition: (i) the fundamental band-to-band edge shifts to higher photon energies; (ii) the charge-transfer absorption band virtually disappears (or overlaps the fundamental edge); and (iii) the intensity of the crystal-field absorption peaks of Co(2+) around 2 eV decreases by an order of magnitude and shifts to 2.5 eV. After incomplete phase transition pressure cycles, the absorption edge of nontransformed W-NPs at ambient pressure exhibits a blue shift of 0.22 eV. This extra shift is interpreted in terms of quantum confinement effects. The observed gradual phase transition and metastability are related to the NP size distribution: the larger the NP, the lower the W-to-RS transition pressure. PMID:27390839

  10. Quantum chemistry study of uranium(VI), neptunium(V), and plutonium(IV,VI) complexes with preorganized tetradentate phenanthrolineamide ligands.

    PubMed

    Xiao, Cheng-Liang; Wu, Qun-Yan; Wang, Cong-Zhi; Zhao, Yu-Liang; Chai, Zhi-Fang; Shi, Wei-Qun

    2014-10-20

    The preorganized tetradentate 2,9-diamido-1,10-phenanthroline ligand with hard-soft donors combined in the same molecule has been found to possess high selectivity toward actinides in an acidic aqueous solution. In this work, density functional theory (DFT) coupled with the quasi-relativistic small-core pseudopotential method was used to investigate the structures, bonding nature, and thermodynamic behavior of uranium(VI), neptunium(V), and plutonium(IV,VI) with phenanthrolineamides. Theoretical optimization shows that Et-Tol-DAPhen and Et-Et-DAPhen ligands are both coordinated with actinides in a tetradentate chelating mode through two N donors of the phenanthroline moiety and two O donors of the amide moieties. It is found that [AnO2L(NO3)](n+) (An = U(VI), Np(V), Pu(VI); n = 0, 1) and PuL(NO3)4 are the main 1:1 complexes. With respect to 1:2 complexes, the reaction [Pu(H2O)9](4+)(aq) + 2L(org) + 2NO3(-)(aq) → [PuL2(NO3)2](2+)(org) + 9H2O(aq) might be another probable extraction mechanism for Pu(IV). From the viewpoint of energy, the phenanthrolineamides extract actinides in the order of Pu(IV) > U(VI) > Pu(VI) > Np(V), which agrees well with the experimental results. Additionally, all of the thermodynamic reactions are more energetically favorable for the Et-Tol-DAPhen ligand than the Et-Et-DAPhen ligand, indicating that substitution of one ethyl group with one tolyl group can enhance the complexation abilities toward actinide cations (anomalous aryl strengthening). PMID:25268674

  11. Quantum-confinement effect in individual Ge1-xSnx quantum dots on Si(111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy

    NASA Astrophysics Data System (ADS)

    Nakamura, Yoshiaki; Masada, Akiko; Ichikawa, Masakazu

    2007-07-01

    The authors observed a quantum-confinement effect in individual Ge1-xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ˜1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening.

  12. Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces

    SciTech Connect

    Kepčija, N.; Huang, T.-J.; Klappenberger, F. Barth, J. V.

    2015-03-14

    Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green’s functions for finite geometries and electron plane wave expansion for periodic systems. The “particle in a box” picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

  13. Theory of confined states of positronium in spherical and circular quantum dots with Kane’s dispersion law

    PubMed Central

    2013-01-01

    Confined states of a positronium (Ps) in the spherical and circular quantum dots (QDs) are theoretically investigated in two size quantization regimes: strong and weak. Two-band approximation of Kane’s dispersion law and parabolic dispersion law of charge carriers are considered. It is shown that electron-positron pair instability is a consequence of dimensionality reduction, not of the size quantization. The binding energies for the Ps in circular and spherical QDs are calculated. The Ps formation dependence on the QD radius is studied. PMID:23826867

  14. Size control, quantum confinement, and oxidation kinetics of silicon nanocrystals synthesized at a high rate by expanding thermal plasma

    SciTech Connect

    Han, Lihao E-mail: A.H.M.Smets@tudelft.nl; Zeman, Miro; Smets, Arno H. M. E-mail: A.H.M.Smets@tudelft.nl

    2015-05-25

    The growth mechanism of silicon nanocrystals (Si NCs) synthesized at a high rate by means of expanding thermal plasma chemical vapor deposition technique are studied in this letter. A bimodal Gaussian size distribution is revealed from the high-resolution transmission electron microscopy images, and routes to reduce the unwanted large Si NCs are discussed. Photoluminescence and Raman spectroscopies are employed to study the size-dependent quantum confinement effect, from which the average diameters of the small Si NCs are determined. The surface oxidation kinetics of Si NCs are studied using Fourier transform infrared spectroscopy and the importance of post-deposition passivation treatments of hydrogenated crystalline silicon surfaces are demonstrated.

  15. Long-lived nanosecond spin coherence in high-mobility 2DEGs confined in double and triple quantum wells

    NASA Astrophysics Data System (ADS)

    Ullah, S.; Gusev, G. M.; Bakarov, A. K.; Hernandez, F. G. G.

    2016-06-01

    We investigated the spin coherence of high-mobility two-dimensional electron gases confined in multilayer GaAs quantum wells. The dynamics of the spin polarization was optically studied using pump-probe techniques: time-resolved Kerr rotation and resonant spin amplification. For double and triple quantum wells doped beyond the metal-to-insulator transition, the spin-orbit interaction was tailored by the sample parameters of structural symmetry (Rashba constant), width, and electron density (Dresselhaus linear and cubic constants) which allow us to attain long dephasing times in the nanoseconds range. The determination of the scales, namely, transport scattering time, single-electron scattering time, electron-electron scattering time, and spin polarization decay time further supports the possibility of using n-doped multilayer systems for developing spintronic devices.

  16. Engineering Electronic Structure of a Two-Dimensional Topological Insulator Bi(111) Bilayer on Sb Nanofilms by Quantum Confinement Effect.

    PubMed

    Bian, Guang; Wang, Zhengfei; Wang, Xiao-Xiong; Xu, Caizhi; Xu, SuYang; Miller, Thomas; Hasan, M Zahid; Liu, Feng; Chiang, Tai-Chang

    2016-03-22

    We report on the fabrication of a two-dimensional topological insulator Bi(111) bilayer on Sb nanofilms via a sequential molecular beam epitaxy growth technique. Our angle-resolved photoemission measurements demonstrate the evolution of the electronic band structure of the heterostructure as a function of the film thickness and reveal the existence of a two-dimensional spinful massless electron gas within the top Bi bilayer. Interestingly, our first-principles calculation extrapolating the observed band structure shows that, by tuning down the thickness of the supporting Sb films into the quantum dimension regime, a pair of isolated topological edge states emerges in a partial energy gap at 0.32 eV above the Fermi level as a consequence of quantum confinement effect. Our results and methodology of fabricating nanoscale heterostructures establish the Bi bilayer/Sb heterostructure as a platform of great potential for both ultra-low-energy-cost electronics and surface-based spintronics. PMID:26932368

  17. Quantum well intermixing technique using proton implantation for carrier confinement of vertical-cavity surface-emitting lasers

    NASA Astrophysics Data System (ADS)

    Moriwaki, Shouhei; Saitou, Minoru; Miyamoto, Tomoyuki

    2016-08-01

    We investigated quantum well intermixing (QWI) using proton implantation to form the carrier confinement structure in the active layer of a vertical-cavity surface-emitting laser (VCSEL). The required potential barrier height is discussed referring to the result of numerical analysis. The bandgap change due to the QWI was investigated experimentally for various quantum well structures, proton dose densities, and thermal annealing conditions. A potential barrier height of 30 meV was observed using a high-indium and thin-well structure. High crystalline quality was confirmed by photoluminescence intensity measurement, even after the QWI process, and the lasing of the fabricated QWI-VCSEL was observed without any deterioration. The proposed technique would be effective in improving the device performance in a simple fabrication process.

  18. Spectroscopy in CdTe/MnTe and ZnTe/MnTe single quantum wells; new binary wide gap II VI heterostructures

    NASA Astrophysics Data System (ADS)

    Pelekanos, N.; Fu, Q.; Nurmikko, A. V.; Durbin, S.; Han, J.; Sungki, O.; Menke, D.; Kobayashi, M.; Gunshor, R. L.

    1990-04-01

    With the incorporation of cubic zincblende MnTe, a range of optical studies have been carried out on single quantum wells of ZnTe/MnTe and CdTe/MnTe. By using thin MnTe barrier layers the structures appear to be nearly pseudomorphic and show evidence for good electron-hole confinement.

  19. Non-resonant elastic scattering of low-energy photons by atomic sodium confined in quantum plasmas

    SciTech Connect

    Ghosh, Avijit Ray, Debasis

    2015-03-15

    The non-resonant elastic scattering of low-energy photons by the bound valence electron in the ground state 3s of atomic sodium confined in quantum plasmas is investigated theoretically. The incident photon energy is assumed to be much smaller than the 3s-3p excitation energy. The alkali atom sodium is first formulated as an effective one-electron problem in which the attractive interaction between the valence electron and the atomic ion core is simulated by a spherically symmetric model potential. The Shukla-Eliasson oscillatory exponential cosine screened-Coulomb potential model is then used to mimic the effective two-body (valence-core) interaction within quantum plasmas. Non-relativistic calculations performed within the electric dipole approximation indicate that the non-resonant elastic photon scattering cross-section undergoes a dramatic growth by several orders of magnitude as the quantum wave number increases. A qualitative explanation of this phenomenon is presented. In the absence of the oscillatory cosine screening term, a similar growth is observed at larger values of the quantum wave number. Our computed relevant atomic data are in very good agreement with the experimental as well as the previous theoretical data for the zero-screening (free atom) case, and with the very limited, accurate theoretical results available for the case of exponential screened-Coulomb two-body interaction, without the cosine screening term.

  20. Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous

    PubMed Central

    Dolui, Kapildeb; Quek, Su Ying

    2015-01-01

    Two-dimensional (2D) materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, Eext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, Ec. Increasing Eext beyond Ec can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semimetal transition. PMID:26129645

  1. First-principles study of quantum confinement and surface effects on the electronic properties of InAs nanowires

    SciTech Connect

    Ning, Feng; Tang, Li-Ming Zhang, Yong; Chen, Ke-Qiu

    2013-12-14

    We have used first principles methods to systematically investigate the quantum confinement effect on the electronic properties of zinc-blende (ZB) and wurtzite (WZ) InAs nanowires (NWs) with different orientations and diameters, and compared their electronic properties before and after pseudo-hydrogen passivation. The results show that the calculated carrier effective masses are dependent on the NW diameter, except for [110] ZB NWs, and the hole effective masses of [111] ZB NWs are larger than the electron effective masses when the NW diameter is ≥26 Å. The band alignments of [111] ZB and [0001] WZ NWs reveal that the effect of quantum confinement on the conduction bands is greater than on the valence bands, and the position of the valence band maximum level changes little with increasing NW diameter. The pseudo-hydrogen passivated NWs have larger band gaps than the corresponding unpassivated NWs. The carrier effective masses and mobilities can be adjusted by passivating the surface dangling bonds.

  2. Quantum confinement, carrier dynamics and interfacial processes in nanostructured direct/indirect-gap semiconductor-glass composites

    SciTech Connect

    Joseph H. Simmons

    2002-08-13

    The behavior of semiconductor clusters precipitated in an insulated matrix was investigated. Semiconductor compositions of CdTe, Si and Ge were studies and the insulating matrix was amorphous SiO2. As a function of size, quantum confinement effects were observed in all three composite systems. However significant differences were observed between the direct-gap column 2-6 semiconductors and the indirect-gap column 4 semiconductors. As observed by others, the direct-gap 2-6 semiconductors showed a distinct saturation in the energy-gap blue shift with decreasing size. Theoretical studies using a 20-band k dot p calculation of the electronic and valence bands for a 3-dimensionally confined CdTe semiconductor showed that mixing of the conduction band states leads to a flattening of the central valley. This increases the electron mass drastically and saturates the size dependent blue shift in the bandgap. In contrast, the blue shift in the Si and Ge nanocrystals showed no sign of saturation and increased drastically with decreasing size. In fact, Si and Ge crystals were formed with blue shift values that moved the bandgap to the near UV region. We examined the absorption curves to determine whether the bandgap was direct or indirect in the quantum dots. The results are that the absorption shows an indirect gap for all but the smallest Si crystals and an indirect gap for all Ge crystals. Raman studies showed negligible size dependence due to a lack of phonon confinement in the matrix embedded clusters. Exciton saturation and recovery times were found to be very short (of the order of 400fs) and are the fastest reported for any quantum dot system. Work to examine the type of confinement obtained in a matrix that consists of a transparent conductor is under way. Studies of the photoinduced absorption change in GeSe glasses showed a significant effect of photodarkening, regardless of composition. The photodarkening effect appears to be composed of permanent and transient

  3. Power loss of a single electron charge distribution confined in a quantum plasma

    SciTech Connect

    Mehramiz, A.; Mahmoodi, J.; Sobhanian, S.

    2011-05-15

    The dielectric tensor for a quantum plasma is derived by using a linearized quantum hydrodynamic theory. The wave functions for a nanostructure bound system have been investigated. Finally, the power loss for an oscillating charge distribution of a mixed state will be calculated, using the dielectric function formalism.

  4. Influence of quantum-confined Stark effect on optical properties within trench defects in InGaN quantum wells with different indium content

    SciTech Connect

    Vaitkevičius, A. Mickevičius, J.; Dobrovolskas, D.; Tamulaitis, G.; Tuna, Ö.; Giesen, C.; Heuken, M.

    2014-06-07

    The trench defects in InGaN/GaN multiple quantum well structures are studied using confocal photoluminescence (PL) spectroscopy and atomic force microscopy. A strong blueshift (up to ∼280 meV) and an intensity increase (by up to a factor of 700) of the emission are demonstrated for regions enclosed by trench loops. The influence of the difference in the well width inside and outside the trench loops observed by transmission electron microscopy, the compositional pulling effect, the strain relaxation inside the loop, and corresponding reduction in the built-in field on the PL band peak position and intensity were estimated. The competition of these effects is mainly governed by the width of the quantum wells in the structure. It is shown that the PL band blueshift observed within the trench defect loops in the InGaN structures with wide quantum wells is mainly caused by the reduction in efficiency of the quantum-confined Stark effect due to strain relaxation.

  5. High detectivity short-wavelength II-VI quantum cascade detector

    SciTech Connect

    Ravikumar, Arvind P. Gmachl, Claire F.; Garcia, Thor A.; Tamargo, Maria C.; Jesus, Joel De

    2014-08-11

    We report on the experimental demonstration of a ZnCdSe/ZnCdMgSe-based short-wavelength photovoltaic Quantum Cascade Detector (QCD). The QCD operates in two spectral bands centered around 2.6 μm and 3.6 μm. Calibrated blackbody measurements yield a peak responsivity of 0.1 mA/W or 2400 V/W at 80 K, and a corresponding 300 K background radiation limited infrared performance detectivity (BLIP) of ∼2.5 × 10{sup 10 }cm √Hz/W. Comparison of background illuminated and dark current-voltage measurements demonstrates a BLIP temperature of 200 K. The device differential resistance-area product, decreases from about 10{sup 6} Ω cm{sup 2} at 80 K to about 8000 Ω cm{sup 2} at 300 K, indicative of the ultra-low Johnson noise in the detectors.

  6. Impact of field-induced quantum confinement on the onset of tunneling field-effect transistors: Experimental verification

    SciTech Connect

    Smets, Quentin Verreck, Devin; Heyns, Marc M.; Verhulst, Anne S.; Martens, Koen; Lin, Han Chung; Kazzi, Salim El; Simoen, Eddy; Collaert, Nadine; Thean, Aaron; Raskin, Jean-Pierre

    2014-11-17

    The Tunneling Field-Effect Transistor (TFET) is a promising device for future low-power logic. Its performance is often predicted using semiclassical simulations, but there is usually a large discrepancy with experimental results. An important reason is that Field-Induced Quantum Confinement (FIQC) is neglected. Quantum mechanical simulations show FIQC delays the onset of Band-To-Band Tunneling (BTBT) with hundreds of millivolts in the promising line-TFET configuration. In this letter, we provide experimental verification of this delayed onset. We accomplish this by developing a method where line-TFET are modeled using highly doped MOS capacitors (MOS-CAP). Using capacitance-voltage measurements, we demonstrate AC inversion by BTBT, which was so far unobserved in MOS-CAP. Good agreement is shown between the experimentally obtained BTBT onset and quantum mechanical predictions, proving the need to include FIQC in all TFET simulations. Finally, we show that highly doped MOS-CAP is promising for characterization of traps deep into the conduction band.

  7. Spectral broadening and electron-photon coupling in III-V infrared detectors of low dimensional quantum confined system

    NASA Astrophysics Data System (ADS)

    Joy, Soumitra R.; Mohammedy, Farseem M.

    2016-05-01

    Present work explores the mid-IR photodetection mechanism in III-V quantum confined system in twofold ways. Firstly, it models the extent of spectral linewidth broadening of photo-detector. Secondly, it investigates whether a strong perturbation of light can modulate the electronic bandstructure. Photo-absorption mechanism in the detector correlated to reduced carrier lifetime in ground state leading to homogeneous spectral widening is calculated. Besides, contribution of non-uniform size and composition of quantum dots towards spectral broadening is modeled in order to get the envelop of inhomogeneously broadened photocurrent spectrum. Our model generates photocurrent spectrum with 1.4 μm broadening centered at 3.5 μm at 77 K for a DWELL-IP, which agrees with the experimental result. The calculated photocurrent spectral width of 1.3 μm for GaAs/AlGaAs Quantum Well (QW) centered at 8.31 μm at 77 K also supports experimental data. In addition, our calculation reveals the emergence of a broad resonant peak in the spectrum of QW-IP in far infrared region (20-50 μm) as the photon volume density increases up to 0.1% of carrier density inside the active region. We introduce a hybrid density-of-states for strongly coupled electron-photon system to explain both mid and far IR peak.

  8. Study of the confined states in Al{sub x}Ga{sub 1−x}As/GaAs/vacuum surface quantum well

    SciTech Connect

    Xiong, D.-Y. Wang, J.-Q.

    2014-04-14

    In this paper, we investigate the optical properties of confined electronic states in ultra-thin Al{sub 0.27}Ga{sub 0.73}As/GaAs/vacuum surface quantum wells by using photoreflectance spectroscopy at room temperature. Well-resolved doublet structures were found in the spectra. The energy of the features increases with decreasing well width in agreement with the predictions of a model of the transition energy between confined electron and hole states in a surface quantum well. Both the transition broadening and intensity behaviors are also well explained by the effective mass approximation theory. The offset between the un-perturbed theoretical transition energy and the experimental data has been explained by surface-state interaction effects. Moreover, the fact that the light hole ground state in the surface quantum well can be pushed out from the surface quantum well has been directly observed experimentally.

  9. Quantum criticality, kink confinement, and emergent symmetries in coupled Ising chains and ladders

    NASA Astrophysics Data System (ADS)

    Tennant, Alan

    2011-03-01

    In this talk I cover the physics in three of the central quantum phase transitions in 1D. First, the transverse Ising model which is realized in CoNb2O6. While this is perhaps the simplest textbook case of a quantum phase transition, a remarkable emergence of E8 symmetry arises close to the quantum critical point. This manifests itself in an octave of bound states. We observe these experimentally and in particular the interval of the first two resonances on this octave which are found to match the golden ratio 1.618... - just as predicted for the emergence of this extraordinary symmetry. I then plan to show with the example of the Heisenberg chain how we can probe the quantum critical volume experimentally and show the characteristic scaling behaviour in space and time. The third example is of a spin ladder CaCu2O3 which is near the long sought after Wess-Zumino-Novikov-Witten quantum critical point.

  10. Towards enhancing two-dimensional electron gas quantum confinement effects in perovskite oxide heterostructures

    SciTech Connect

    Nazir, Safdar; Behtash, Maziar; Yang, Kesong

    2015-03-21

    We explore the possibility of achieving highly confined two-dimensional electron gas (2DEG) within one single atomic layer through a comprehensive comparison study on three prototypical perovskite heterostructures, LaAlO{sub 3}/ATiO{sub 3} (A = Ca, Sr, and Ba), using first-principles electronic structure calculations. We predict that the heterostructure LaAlO{sub 3}/BaTiO{sub 3} has a highly confined 2DEG within a single atomic layer of the substrate BaTiO{sub 3}, and exhibits relatively higher interfacial charge carrier density and larger magnetic moments than the well-known LaAlO{sub 3}/SrTiO{sub 3} system. The long Ti-O bond length in the ab-plane of the LaAlO{sub 3}/BaTiO{sub 3} heterostructure is responsible for the superior charge confinement. We propose BaTiO{sub 3} as an exceptional substrate material for 2DEG systems with potentially superior properties.

  11. Trap-size scaling in confined-particle systems at quantum transitions

    SciTech Connect

    Campostrini, Massimo; Vicari, Ettore

    2010-02-15

    We develop a trap-size scaling theory for trapped particle systems at quantum transitions. As a theoretical laboratory, we consider a quantum XY chain in an external transverse field acting as a trap for the spinless fermions of its quadratic Hamiltonian representation. We discuss trap-size scaling at the Mott insulator to superfluid transition in the Bose-Hubbard model. We present exact and accurate numerical results for the XY chain and for the low-density Mott transition in the hard-core limit of the one-dimensional Bose-Hubbard model. Our results are relevant for systems of cold atomic gases in optical lattices.

  12. Quantum Confinement Induced Oscillatory Electric Field on a Stepped Pb(111) Film and Its Influence on Surface Reactivity

    SciTech Connect

    Liu, Xiaojie; Wang, Cai-Zhuang; Hupalo, Myron; Lin, Hai-Quing; Ho, Kai-Ming; Tringides, Michael C.

    2014-01-06

    When the thickness of ultrathin metal films approaches the nanometer scale comparable to the coherence length of the electrons, significant effects on the structure stability and the electronic properties of the metal films emerge due to electron confinement and quantization of the allowed electronic states in the direction perpendicular to the film. Using first-principles calculations, we showed that such quantum size effects can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even- or odd-layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment.

  13. Quantum confinement effect in Bi anti-dot thin films with tailored pore wall widths and thicknesses

    SciTech Connect

    Park, Y.; Hirose, Y.; Fukumura, T.; Hasegawa, T.; Nakao, S.; Xu, J.

    2014-01-13

    We investigated quantum confinement effects in Bi anti-dot thin films grown on anodized aluminium oxide templates. The pore wall widths (w{sub Bi}) and thickness (t) of the films were tailored to have values longer or shorter than Fermi wavelength of Bi (λ{sub F} = ∼40 nm). Magnetoresistance measurements revealed a well-defined weak antilocalization effect below 10 K. Coherence lengths (L{sub ϕ}) as functions of temperature were derived from the magnetoresistance vs field curves by assuming the Hikami-Larkin-Nagaoka model. The anti-dot thin film with w{sub Bi} and t smaller than λ{sub F} showed low dimensional electronic behavior at low temperatures where L{sub ϕ}(T) exceed w{sub Bi} or t.

  14. Effect of a lateral electric field on an off-center single dopant confined in a thin quantum disk

    NASA Astrophysics Data System (ADS)

    Dujardin, F.; Oukerroum, A.; Feddi, E.; Bosch Bailach, J.; Martínez-Pastor, J.; Zazi, M.

    2012-02-01

    The effect of a lateral electric field on a donor impurity confined in a thin quantum disk is studied theoretically in the framework of mass approximation and using the Ritz variational approach. We show that the binding energy depends on several parameters: the dot size, the position of the donor impurity, the lateral field strength, and its orientation relative to the axis containing the impurity. When the impurity is located at one edge and the electric field is oriented in the opposite direction, the binding energy is considerably reinforced due to the simultaneous additive effects of coulombic potential and electrostatic force. The competition between these effects modifies considerably the probability densities and allows a better comprehension of the binding energy variations. This interesting behavior can contribute to an better understanding of the experimental optical response.

  15. Quantum-confined Stark effect on photoluminescence and electroluminescence characteristics of InGaN-based light-emitting diodes

    NASA Astrophysics Data System (ADS)

    Masui, Hisashi; Sonoda, Junichi; Pfaff, Nathan; Koslow, Ingrid; Nakamura, Shuji; Den Baars, Steven P.

    2008-08-01

    The quantum-confined Stark effect (QCSE) on InGaN-based light-emitting diodes (LEDs) was investigated as a part of the continuing study of exploring differences between photoluminescence (PL) and electroluminescence (EL) characteristics. The luminescence characteristics were related to electrical characteristics of green and amber LEDs by employing the electrical-bias-applied PL technique. By inspecting the band diagram, it has been found that the separation of quasi-Fermi levels, which strongly affects the QCSE, can be quantified and related to the luminescence. In order to compare PL and EL characteristics, attention was paid to the QCSE during the PL and EL measurements. Despite the control of the QCSE, differences were still confirmed between PL and EL characteristics, which have led us to the conclusion to that there are other unrevealed origins for the differences.

  16. Density matrix for an electron confined in quantum dots under uniform magnetic field and static electrical field

    NASA Astrophysics Data System (ADS)

    Pang, Qian-Jun

    2007-01-01

    Using unitary transformations, this paper obtains the eigenvalues and the common eigenvector of Hamiltonian and a new-defined generalized angular momentum (Lz) for an electron confined in quantum dots under a uniform magnetic field (UMF) and a static electric field (SEF). It finds that the eigenvalue of Lz just stands for the expectation value of a usual angular momentum lz in the eigen-state. It first obtains the matrix density for this system via directly calculating a transfer matrix element of operator exp(-βH) in some representations with the technique of integral within an ordered products (IWOP) of operators, rather than via solving a Bloch equation. Because the quadratic homogeneity of potential energy is broken due to the existence of SEF, the virial theorem in statistical physics is not satisfactory for this system, which is confirmed through the calculation of thermal averages of physical quantities.

  17. Strong quantum confinement effects in kesterite Cu2ZnSnS4 nanospheres for organic optoelectronic cells.

    PubMed

    Arul, Narayanasamy Sabari; Yun, Dong Yeol; Lee, Dea Uk; Kim, Tae Whan

    2013-12-01

    X-ray photoelectron spectra, X-ray diffraction patterns, scanning electron microscopy images, and high-resolution transmission electron microscopy images showed that the as-prepared samples were Cu2ZnSnS4 (CZTS) nanospheres with a kesterite phase. Ultraviolet-visible absorption spectra for the CZTS nanospheres with an average crystallite size of 3.26 nm showed that the absorption edge corresponding to the energy gap shifted to the higher energy side due to the quantum confinement within the CZTS nanoparticles. Current-density measurements showed that the power conversion efficiency (0.952%) of the organic photovoltaic cells with CZTS nanospheres was much higher than that (0.120%) of the cells without CZTS nanospheres. PMID:24129972

  18. Space-charge waves in magnetized and collisional quantum plasma columns confined in carbon nanotubes

    SciTech Connect

    Bagheri, Mehran; Abdikian, Alireza

    2014-04-15

    We study the dispersion relation of electrostatic waves propagating in a column of quantum magnetized collisional plasma embraced completely by a metallic single-walled carbon nanotubes. The analysis is based on the quantum linearized hydrodynamic formalism of collective excitations within the quasi-static approximation. It is shown when the electronic de Broglie's wavelength of the plasma is comparable in the order of magnitude to the radius of the nanotube, the quantum effects are quite meaningful and our model anticipates one acoustical and two optical space-charge waves which are positioned into three propagating bands. With increasing the nanotube radius, the features of the acoustical branch remain unchanged, yet two distinct optical branches are degenerated and the classical behavior is recovered. This study might provide a platform to create new finite transverse cross section quantum magnetized plasmas and to devise nanometer dusty plasmas based on the metallic carbon nanotubes in the absence of either a drift or a thermal electronic velocity and their existence could be experimentally examined.

  19. Quantum-mechanical engines working with an ideal gas with a finite number of particles confined in a power-law trap

    NASA Astrophysics Data System (ADS)

    Wang, Jianhui; Ma, Yongli; He, Jizhou

    2015-07-01

    Based on quantum thermodynamic processes, we make a quantum-mechanical (QM) extension of the typical heat engine cycles, such as the Carnot, Brayton, Otto, Diesel cycles, etc., with no introduction of the concept of temperature. When these QM engine cycles are implemented by an ideal gas confined in an arbitrary power-law trap, a relation between the quantum adiabatic exponent and trap exponent is found. The differences and similarities between the efficiency of a given QM engine cycle and its classical counterpart are revealed and discussed.

  20. Quantum dynamics of a hydrogen molecule confined in a cylindrical potential

    NASA Astrophysics Data System (ADS)

    Yildirim, Taner; Harris, A. B.

    2003-06-01

    We study the coupled rotation-vibration levels of a hydrogen molecule in a confining potential with cylindrical symmetry. We include the coupling between rotations and translations and show how this interaction is essential to obtain the correct degeneracies of the energy level scheme. We applied our formalism to study the dynamics of H2 molecules inside a “smooth” carbon nanotube as a function of tube radius. The results are obtained both by numerical solution of the (2J+1)-component radial Schrödinger equation and by developing an effective Hamiltonian to describe the splitting of a manifold of states of fixed angular momentum J and number of phonons N. For nanotube radius smaller than ≈3.5 Å, the confining potential has a parabolic shape and the results can be understood in terms of a simple toy model. For larger radius, the potential has the “Mexican hat” shape and therefore the H2 molecule is off centered, yielding radial and tangential translational dynamics in addition to rotational dynamics of H2 molecule which we also describe by a simple model. Finally, we make several predictions for the the neutron scattering observation of various transitions between these levels.

  1. Wavelength-tunable visible to near-infrared photoluminescence of carbon dots: the role of quantum confinement and surface states

    NASA Astrophysics Data System (ADS)

    Ghamsari, Morteza Sasani; Bidzard, Ashkan Momeni; Han, Wooje; Park, Hyung-Ho

    2016-04-01

    Carbon quantum dots (C-QDs) with different size distributions and surface characteristics can exhibit good emission properties in the visible and near-infrared (NIR) regions, which can be applicable in optoelectronic devices as well as biomedical applications. Optical properties of colloidal C-QDs in distilled water at different concentrations produced using a method of alkali-assisted surfactant-free oxidation of cellulose acetate is presented. The structural and optical properties of colloidal C-QDs at different concentrations were investigated, with the aim of clarifying the main mechanisms of photoluminescence emissions. We observed a wide range of tunable visible to NIR emissions with good stability from the C-QD colloids at different applied excitation wavelengths. The colloids show dual emissions with maxima at ˜420 and 775 nm (blue and NIR emissions) when excited at the wavelength range near the energy gaps of the C-QDs. Moreover, by increasing the excitation wavelength, tunable visible emissions at the spectral range of 475 to 550 nm are observed. A detailed analysis of the results shows that the blue and NIR luminescence of colloidal C-QDs originate from the oxide-related surface effects whereas quantum confinement is the responsible mechanism for tunable visible emissions of the C-QD colloid.

  2. Quantum-size effects in the energy loss of charged particles interacting with a confined two-dimensional electron gas

    SciTech Connect

    Borisov, A. G.; Juaristi, J. I.

    2006-01-15

    Time-dependent density-functional theory is used to calculate quantum-size effects in the energy loss of antiprotons interacting with a confined two-dimensional electron gas. The antiprotons follow a trajectory normal to jellium circular clusters of variable size, crossing every cluster at its geometrical center. Analysis of the characteristic time scales that define the process is made. For high-enough velocities, the interaction time between the projectile and the target electrons is shorter than the time needed for the density excitation to travel along the cluster. The finite-size object then behaves as an infinite system, and no quantum-size effects appear in the energy loss. For small velocities, the discretization of levels in the cluster plays a role and the energy loss does depend on the system size. A comparison to results obtained using linear theory of screening is made, and the relative contributions of electron-hole pair and plasmon excitations to the total energy loss are analyzed. This comparison also allows us to show the importance of a nonlinear treatment of the screening in the interaction process.

  3. An Infinite Order Discrete Variable Representation of an Effective Mass Hamiltonian: Application to Exciton Wave Functions in Quantum Confined Nanostructures.

    PubMed

    Kaledin, Alexey L; Lian, Tianquan; Hill, Craig L; Musaev, Djamaladdin G

    2014-08-12

    We describe an extension of the conventional Fourier grid discrete variable representation (DVR) to the bound state problem of a particle with a position-dependent mass. An infinite order DVR, derived for a variable mass kinetic energy operator, coupled with an efficient grid contraction scheme yields essentially exact eigenvalues for a chosen grid spacing. Implementation of the method is shown to be very practical due to the fact that in a DVR no integral evaluation is necessary and that the resultant kinetic energy matrix is sparse. Numerical calculations are presented for exciton states of spherical, cylindrical, and toric Type I (CdSe/ZnS) core-shell quantum dots. In these examples, electron-hole interaction is treated explicitly by solving a self-consistent Schrödinger-Poisson equation on a contracted DVR grid. Prospective applications of the developed approach to calculating electron transfer rates between adsorbed molecular acceptors and quantum confined nanocrystals of generic shape, dimensionality, and composition are also discussed. PMID:26588309

  4. Quantum kinetic theory of a Bose-Einstein gas confined in a lattice

    NASA Astrophysics Data System (ADS)

    Rey, Ana Maria; Hu, B. L.; Calzetta, Esteban; Clark, Charles W.

    2005-08-01

    We extend our earlier work on the nonequilibrium dynamics of a Bose-Einstein condensate initially loaded into a one-dimensional optical lattice. From the two-particle-irreducible (2PI) closed-time-path (CTP) effective action for the Bose-Hubbard Hamiltonian we derive causal equations of motion that treat mean-field effects and quantum fluctuations on an equal footing. We demonstrate that these equations reproduce well-known limits when simplifying approximations are introduced. For example, when the system dynamics admits two-time separation, we obtain the Kadanoff-Baym equations of quantum kinetic theory, and in the weakly interacting limit, we show that the local equilibrium solutions of our equations reproduce the second-order corrections to the self-energy of the type originally derived by Beliaev. The derivation of quantum kinetic equations from the 2PI-CTP effective action not only checks the viability of the formalism but also shows it to be a tractable framework for going beyond standard Boltzmann equations of motion.

  5. Quantum kinetic theory of a Bose-Einstein gas confined in a lattice

    SciTech Connect

    Rey, Ana Maria; Hu, B.L.; Calzetta, Esteban; Clark, Charles W.

    2005-08-15

    We extend our earlier work on the nonequilibrium dynamics of a Bose-Einstein condensate initially loaded into a one-dimensional optical lattice. From the two-particle-irreducible (2PI) closed-time-path (CTP) effective action for the Bose-Hubbard Hamiltonian we derive causal equations of motion that treat mean-field effects and quantum fluctuations on an equal footing. We demonstrate that these equations reproduce well-known limits when simplifying approximations are introduced. For example, when the system dynamics admits two-time separation, we obtain the Kadanoff-Baym equations of quantum kinetic theory, and in the weakly interacting limit, we show that the local equilibrium solutions of our equations reproduce the second-order corrections to the self-energy of the type originally derived by Beliaev. The derivation of quantum kinetic equations from the 2PI-CTP effective action not only checks the viability of the formalism but also shows it to be a tractable framework for going beyond standard Boltzmann equations of motion.

  6. Effect of the 2DEG confinement potential on the magnetotransport properties of an open quantum dot under a tilted magnetic field

    NASA Astrophysics Data System (ADS)

    Gustin, C.; Faniel, S.; Hackens, B.; Bayot, V.; de Poortere, E.; Shayegan, M.

    2002-03-01

    We report on the low temperature magnetoresistance of various quantum billiards, each with a different shape of the 2DEG (two-dimensional electron gas) confinement potential. The structures are patterned by electron beam lithography on three different high mobility GaAs/AlGaAs samples, namely a single heterojunction and two quantum wells with widths of 150 and 450Årespectively. By means of electrostatic gates, both the electron density and the shape of the billiard can be controlled, as well as the finite thickness of the 2DEG in the case of the wide quantum well. We discuss the results of low temperature magnetotransport measurements with the open dots subject to an phin-situ tilted magnetic field. More specifically we investigate the influence of the symmetry-asymmetry of the 2DEG confinement potential on the statistics of the universal conductance fluctuations (UCF).

  7. Impact of artificial lateral quantum confinement on exciton-spin relaxation in a two-dimensional GaAs electronic system

    SciTech Connect

    Kiba, Takayuki Murayama, Akihiro; Tanaka, Toru; Tamura, Yosuke; Higo, Akio; Thomas, Cedric; Samukawa, Seiji

    2014-10-15

    We demonstrate the effect of artificial lateral quantum confinement on exciton-spin relaxation in a GaAs electronic system. GaAs nanodisks (NDs) were fabricated from a quantum well (QW) by top-down nanotechnology using neutral-beam etching aided by protein-engineered bio-nano-templates. The exciton-spin relaxation time was 1.4 ns due to ND formation, significantly extended compared to 0.44 ns for the original QW, which is attributed to weakening of the hole-state mixing in addition to freezing of the carrier momentum. The temperature dependence of the spin-relaxation time depends on the ND thickness, reflecting the degree of quantum confinement.

  8. Synthesis of 2-Mercaptonicotinic Acid-Capped CdSe Quantum Dots and its Application to Spectrofluorometric Determination of Cr(VI) in Water Samples.

    PubMed

    Hosseini, Mohammad Saeid; Khorashahi, Somayeh; Hosseini, Navid

    2016-05-01

    The CdSe quantum dots (QDs) capped with 2-mercaptonicotinic acid (H2MN) were prepared through a controllable process at 80 °C. The prepared QDs were characterized by XRD, TEM, IR, UV-Vis and fluorescence (FL) techniques. It was found that the QDs were nearly mono-disperse with the diameters in the range of 8-10 nm. These QDs are capable to exhibit strong FL even in concentrated acidic media. They exhibit an enhanced fluorescence in the presence of Cr(VI), which was used for the determination of Cr(VI) in water samples. The linear range was found to be 1 × 10(-7)-6.0 × 10(-6) M with the RSD and DL of 0.92 % and 5 × 10(-8) M, respectively. Except that Ca(2+) and Fe(3+) which can be eliminated through a simple precipitation process, the other co-existent ions present in natural water were not interfered. The recoveries obtained for the added amounts of Cr(VI) were in the range of 96.9-103.2 %, which denote on application of the method, satisfactorily. PMID:26825078

  9. Tunneling into a quantum confinement created by a single-step nanolithography of conducting oxide interfaces

    NASA Astrophysics Data System (ADS)

    Maniv, E.; Ron, A.; Goldstein, M.; Palevski, A.; Dagan, Y.

    2016-07-01

    A unique nanolithography technique compatible with conducting oxide interfaces, which requires a single lithographic step with no additional amorphous deposition or etching, is presented. It is demonstrated on a SrTiO3/LaAlO3 interface where a constriction is patterned in the electron liquid. We find that an additional backgating can further confine the electron liquid into an isolated island. Conductance and differential conductance measurements show resonant tunneling through the island. The data at various temperatures and magnetic fields are analyzed and the effective island size is found to be of the order of 10 nm. The magnetic field dependence suggests the absence of spin degeneracy in the island. Our method is suitable for creating superconducting and oxide-interface-based electronic devices.

  10. Dispersive measurement of electron spin states in Coulomb-confined silicon double quantum dots

    NASA Astrophysics Data System (ADS)

    House, Matthew; Kobayashi, Takashi; Weber, Bent; Hile, Sam; Rogge, Sven; Simmons, Michelle

    2015-03-01

    We use radio frequency reflectometry with a resonant circuit to investigate a double quantum dot device patterned by the placement of phosphorus donors in silicon with scanning tunnelling microscope lithography. The circuit responds to electron tunnelling to and from the quantum dots, the complex admittance of which provides information about the tunnel coupling between the dots and the leads. With four electrons on two dots, the Pauli Exclusion Principle makes tunnelling of one electron between the two dots spin dependent, which we exploit to measure the electronic spin state. We map the ground state transition between singlet and triplet states as a function of electric and magnetic fields, which shows that the exchange energy can be tuned over an order of magnitude (about 10 to 100 μeV) or more in this device. We apply high frequency pulses to induce an excited spin state and observe that the dispersive measurement can detect the excited spin state in addition to the ground state.

  11. Modeling direct band-to-band tunneling: From bulk to quantum-confined semiconductor devices

    SciTech Connect

    Carrillo-Nuñez, H.; Ziegler, A.; Luisier, M.; Schenk, A.

    2015-06-21

    A rigorous framework to study direct band-to-band tunneling (BTBT) in homo- and hetero-junction semiconductor nanodevices is introduced. An interaction Hamiltonian coupling conduction and valence bands (CVBs) is derived using a multiband envelope method. A general form of the BTBT probability is then obtained from the linear response to the “CVBs interaction” that drives the system out of equilibrium. Simple expressions in terms of the one-electron spectral function are developed to compute the BTBT current in two- and three-dimensional semiconductor structures. Additionally, a two-band envelope equation based on the Flietner model of imaginary dispersion is proposed for the same purpose. In order to characterize their accuracy and differences, both approaches are compared with full-band, atomistic quantum transport simulations of Ge, InAs, and InAs-Si Esaki diodes. As another numerical application, the BTBT current in InAs-Si nanowire tunnel field-effect transistors is computed. It is found that both approaches agree with high accuracy. The first one is considerably easier to conceive and could be implemented straightforwardly in existing quantum transport tools based on the effective mass approximation to account for BTBT in nanodevices.

  12. Solution-processed, barrier-confined, and 1D nanostructure supported quasi-quantum well with large photoluminescence enhancement.

    PubMed

    Yan, Keyou; Zhang, Lixia; Kuang, Qin; Wei, Zhanhua; Yi, Ya; Wang, Jiannong; Yang, Shihe

    2014-04-22

    Planar substrate supported semiconductor quantum well (QW) structures are not amenable to manipulation in miniature devices, while free-standing QW nanostructures, e.g., ultrathin nanosheets and nanoribbons, suffer from mechanical and environmental instability. Therefore, it is tempting to fashion high-quality QW structures on anisotropic and mechanically robust supporting nanostructures such as nanowires and nanoplates. Herein, we report a solution quasi-heteroepitaxial route for growing a barrier-confined quasi-QW structure (ZnSe/CdSe/ZnSe) on the supporting arms of ZnO nanotetrapods, which have a 1D nanowire structure, through the combination of ion exchange and successive deposition assembly. This resulted in highly crystalline and highly oriented quasi-QWs along the whole axial direction of the arms of the nanotetrapod because a transition buffer layer (Zn(x)Cd(1-x)Se) was formed and in turn reduced the lattice mismatch and surface defects. Significantly, such a barrier-confined QW emits excitonic light ∼17 times stronger than the heterojunction (HJ)-type structure (ZnSe/CdSe, HJ) at the single-particle level. Time-resolved photoluminescence from ensemble QWs exhibits a lifetime of 10 ns, contrasting sharply with ∼300 ps for the control HJ sample. Single-particle PL and Raman spectra suggest that the barrier layer of QW has completely removed the surface trap states on the HJ and restored or upgraded the photoelectric properties of the semiconductor layer. Therefore, this deliberate heteroepitaxial growth protocol on the supporting nanotetrapod has realized a several micrometer long QW structure with high mechanical robustness and high photoelectric quality. We envision that such QWs integrated on 1D nanostructures will largely improve the performance of solar cells and bioprobes, among others. PMID:24580094

  13. Quantum confinement effects in lithographic sub-5 nm Silicon nanowire fets and integration of si nanograting fet biosensors

    NASA Astrophysics Data System (ADS)

    Trivedi, Krutarth B.

    In recent years, widespread accessibility to reliable nanofabrication techniques such as high resolution electron beam lithography as well as development of innovative techniques such as nanoimprint lithography and chemically grown nano-materials like carbon nanotubes and graphene have spurred a boom in many fields of research involving nanoscale features and devices. The breadth of fields in which nanoscale features represent a new paradigm is staggering. Scaling down device dimensions to nanoscale enables non-classical quantum behavior and allows for interaction with similarly sized natural materials, like proteins and DNA, as never before, affording an unprecedented level of performance and control and fostering a seemingly boundless array of unique applications. Much of the research effort has been directed toward understanding such interactions to leverage the potential of nanoscale devices to enhance electronic and medical technology. In keeping with the spirit of application based research, my graduate research career has spanned the development of nanoimprint techniques and devices for novel applications, demonstration and study of sub-5 nm Si nanowire FETs exhibiting tangible performance enhancement over conventional MOSFETs, and development of an integrated Si nanograting FET based biosensor and related framework. The following dissertation details my work in fabrication of sub-5 nm Si nanowire FETs and characterization of quantum confinement effects in charge transport of FETs with 2D and 1D channel geometry, fabrication and characterization of schottky contact Si nanograting FET sensors, integration of miniaturized Si nanograting FET biosensors into Chip-in-Strip(c) packaging, development of an automated microfluidic sensing system, and investigation of electrochemical considerations in the Si nanograting FET biosensor gate stack followed by development of a novel patent-pending strategy for a lithographically patterned on-chip gate electrode.

  14. Sandwiched confinement of quantum dots in graphene matrix for efficient electron transfer and photocurrent production

    PubMed Central

    Zhu, Nan; Zheng, Kaibo; Karki, Khadga J.; Abdellah, Mohamed; Zhu, Qiushi; Carlson, Stefan; Haase, Dörthe; Žídek, Karel; Ulstrup, Jens; Canton, Sophie E.; Pullerits, Tõnu; Chi, Qijin

    2015-01-01

    Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 109 s−1. Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications. PMID:25996307

  15. Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites.

    PubMed

    Sapori, Daniel; Kepenekian, Mikaël; Pedesseau, Laurent; Katan, Claudine; Even, Jacky

    2016-03-28

    Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled sizes of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach the atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed. PMID:26705549

  16. Interplay between strain, quantum confinement, and ferromagnetism in strained ferromagnetic semiconductor (In,Fe)As thin films

    SciTech Connect

    Sasaki, Daisuke; Anh, Le Duc; Nam Hai, Pham; Tanaka, Masaaki

    2014-04-07

    We systematically investigated the influence of strain on the electronic structure and ferromagnetism of (In,Fe)As thin films. It is found that while the shift of the critical point energies of compressive-strained (In,Fe)As layers grown on (In{sub 1−y},Ga{sub y})As (y = 0.05, 0.1) buffer layers can be explained by the hydrostatic deformation effect (HDE) alone, those of tensile-strained (In,Fe)As layers grown on (Ga{sub 1−z},Al{sub z})Sb (z = 0, 0.5, 1) buffer layers can be explained by the combination of HDE and the quantum confinement effect (QCE). The Curie temperature T{sub C} of the (In,Fe)As layers strongly depends on the strain, and shows a maximum for the (In,Fe)As layer grown on a GaSb buffer layer. The strain dependence of T{sub C} can be explained by the s-d exchange mechanism taking into account HDE and QCE.

  17. Quantum confinement effect in multilayer structure of alternate CdSe and SiOx insulator matrix thinfilms

    NASA Astrophysics Data System (ADS)

    Melvin David Kumar, M.; Devadason, Suganthi

    2013-06-01

    Multilayer (ML) structure of layer-by-layer deposited CdSe/SiOx thin films and their monolayers were prepared using sequential thermal evaporation technique. X-ray diffraction study confirmed the (002) plane of CdSe with wurtzite structure. It is noticed that the microstrain, developed in ML thin films, increased with decreasing particle size. Experimentally measured band gap energies confirmed the splitting of valence band energy levels which rise due to hole confinement in CdSe. Crystallite sizes (5-7 nm) were calculated using the effective mass approximation model (i.e., Brus model) which shows that the diameter of crystallites was smaller than the Bohr exciton diameter (11.2 nm) of CdSe. The main band in the emission spectra of ML samples gradually shifted to longer wavelength side when particle size was increased from 5 to 7 nm. This is characteristic of quantum size effect. It is inferred that disorderliness in CdSe/SiOx ML thin films would increase when the thickness of CdSe sublayer is greater than that of SiOx matrix layer.

  18. Interplay between strain, quantum confinement, and ferromagnetism in strained ferromagnetic semiconductor (In,Fe)As thin films

    NASA Astrophysics Data System (ADS)

    Sasaki, Daisuke; Anh, Le Duc; Nam Hai, Pham; Tanaka, Masaaki

    2014-04-01

    We systematically investigated the influence of strain on the electronic structure and ferromagnetism of (In,Fe)As thin films. It is found that while the shift of the critical point energies of compressive-strained (In,Fe)As layers grown on (In1-y,Gay)As (y = 0.05, 0.1) buffer layers can be explained by the hydrostatic deformation effect (HDE) alone, those of tensile-strained (In,Fe)As layers grown on (Ga1-z,Alz)Sb (z = 0, 0.5, 1) buffer layers can be explained by the combination of HDE and the quantum confinement effect (QCE). The Curie temperature TC of the (In,Fe)As layers strongly depends on the strain, and shows a maximum for the (In,Fe)As layer grown on a GaSb buffer layer. The strain dependence of TC can be explained by the s-d exchange mechanism taking into account HDE and QCE.

  19. Properties of nanocones formed on a surface of semiconductors by laser radiation: quantum confinement effect of electrons, phonons, and excitons.

    PubMed

    Medvid, Artur; Onufrijevs, Pavels; Mychko, Alexander

    2011-01-01

    On the basis of the analysis of experimental results, a two-stage mechanism of nanocones formation on the irradiated surface of semiconductors by Nd:YAG laser is proposed for elementary semiconductors and solid solutions, such as Si, Ge, SiGe, and CdZnTe. Properties observed are explained in the frame of quantum confinement effect. The first stage of the mechanism is characterized by the formation of a thin strained top layer, due to redistribution of point defects in temperature-gradient field induced by laser radiation. The second stage is characterized by mechanical plastic deformation of the stained top layer leading to arising of nanocones, due to selective laser absorption of the top layer. The nanocones formed on the irradiated surface of semiconductors by Nd:YAG laser possessing the properties of 1D graded bandgap have been found for Si, Ge, and SiGe as well, however QD structure in CdTe was observed. The model is confirmed by "blue shift" of bands in photoluminescence spectrum, "red shift" of longitudinal optical line in Raman back scattering spectrum of Ge crystal, appearance of Ge phase in SiGe solid solution after irradiation by the laser at intensity 20 MW/cm2, and non-monotonous dependence of Si crystal micro-hardness as function of the laser intensity. PMID:22060172

  20. Quantum confinement and photoresponsivity of β-In2Se3 nanosheets grown by physical vapour transport

    NASA Astrophysics Data System (ADS)

    Balakrishnan, Nilanthy; Staddon, Christopher R.; Smith, Emily F.; Stec, Jakub; Gay, Dean; Mudd, Garry W.; Makarovsky, Oleg; Kudrynskyi, Zakhar R.; Kovalyuk, Zakhar D.; Eaves, Laurence; Patanè, Amalia; Beton, Peter H.

    2016-06-01

    We demonstrate that β-In2Se3 layers with thickness ranging from 2.8 to 100 nm can be grown on SiO2/Si, mica and graphite using a physical vapour transport method. The β-In2Se3 layers are chemically stable at room temperature and exhibit a blue-shift of the photoluminescence emission when the layer thickness is reduced, due to strong quantum confinement of carriers by the physical boundaries of the material. The layers are characterised using Raman spectroscopy and x-ray diffraction from which we confirm lattice constants c = 28.31 ± 0.05 Å and a = 3.99 ± 0.02 Å. In addition, these layers show high photoresponsivity of up to ∼2 × 103 A W‑1 at λ = 633 nm, with rise and decay times of τ r = 0.6 ms and τ d = 2.5 ms, respectively, confirming the potential of the as-grown layers for high sensitivity photodetectors.

  1. Light-front holography and superconformal quantum mechanics: A new approach to hadron structure and color confinement

    NASA Astrophysics Data System (ADS)

    Brodsky, Stanley J.; Deur, Alexandre; de Téramond, Guy F.; Dosch, Hans Günter

    2015-11-01

    A primary question in hadron physics is how the mass scale for hadrons consisting of light quarks, such as the proton, emerges from the QCD Lagrangian even in the limit of zero quark mass. If one requires the effective action which underlies the QCD Lagrangian to remain conformally invariant and extends the formalism of de Alfaro, Fubini and Furlan to light-front Hamiltonian theory, then a unique, color-confining potential with a mass parameter κ emerges. The actual value of the parameter κ is not set by the model - only ratios of hadron masses and other hadronic mass scales are predicted. The result is a nonperturbative, relativistic light-front quantum mechanical wave equation, the Light-Front Schrödinger Equation which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics, including a massless pion for zero quark mass and linear Regge trajectories with the identical slope in the radial quantum number n and orbital angular momentum L. The same light-front equations for mesons with spin J also can be derived from the holographic mapping to QCD (3+1) at fixed light-front time from the soft-wall model modification of AdS5 space with a specific dilaton profile. Light-front holography thus provides a precise relation between the bound-state amplitudes in the fifth dimension of AdS space and the boost-invariant light-front wavefunctions describing the internal structure of hadrons in physical space-time. One can also extend the analysis to baryons using superconformal algebra - 2 × 2 supersymmetric representations of the conformal group. The resulting fermionic LF bound-state equations predict striking similarities between the meson and baryon spectra. In fact, the holographic QCD light-front Hamiltonians for the states on the meson and baryon trajectories are identical if one shifts the internal angular momenta of the meson (LM) and baryon (LB) by one unit: LM = LB + 1. We also show how the mass scale κ

  2. Nanometer-scale monitoring of quantum-confined Stark effect and emission efficiency droop in multiple GaN/AlN quantum disks in nanowires

    NASA Astrophysics Data System (ADS)

    Zagonel, L. F.; Tizei, L. H. G.; Vitiello, G. Z.; Jacopin, G.; Rigutti, L.; Tchernycheva, M.; Julien, F. H.; Songmuang, R.; Ostasevicius, T.; de la Peña, F.; Ducati, C.; Midgley, P. A.; Kociak, M.

    2016-05-01

    We report on a detailed study of the intensity dependent optical properties of individual GaN/AlN quantum disks (QDisks) embedded into GaN nanowires (NW). The structural and optical properties of the QDisks were probed by high spatial resolution cathodoluminescence (CL) in a scanning transmission electron microscope (STEM). By exciting the QDisks with a nanometric electron beam at currents spanning over three orders of magnitude, strong nonlinearities (energy shifts) in the light emission are observed. In particular, we find that the amount of energy shift depends on the emission rate and on the QDisk morphology (size, position along the NW and shell thickness). For thick QDisks (>4 nm), the QDisk emission energy is observed to blueshift with the increase of the emission intensity. This is interpreted as a consequence of the increase of carriers density excited by the incident electron beam inside the QDisks, which screens the internal electric field and thus reduces the quantum confined Stark effect (QCSE) present in these QDisks. For thinner QDisks (<3 nm ), the blueshift is almost absent in agreement with the negligible QCSE at such sizes. For QDisks of intermediate sizes there exists a current threshold above which the energy shifts, marking the transition from unscreened to partially screened QCSE. From the threshold value we estimate the lifetime in the unscreened regime. These observations suggest that, counterintuitively, electrons of high energy can behave ultimately as single electron-hole pair generators. In addition, when we increase the current from 1 to 10 pA the light emission efficiency drops by more than one order of magnitude. This reduction of the emission efficiency is a manifestation of the "efficiency droop" as observed in nitride-based 2D light emitting diodes, a phenomenon tentatively attributed to the Auger effect.

  3. Single photon emission at 1.55 μm from charged and neutral exciton confined in a single quantum dash

    SciTech Connect

    Dusanowski, Ł. Syperek, M.; Mrowiński, P.; Rudno-Rudziński, W.; Misiewicz, J.; Sęk, G.; Somers, A.; Kamp, M.; Höfling, S.; Reithmaier, J. P.

    2014-07-14

    We investigate charged and neutral exciton complexes confined in a single self-assembled InAs/InGaAlAs/InP quantum dash emitting at 1.55 μm. The emission characteristics have been probed by measuring high-spatial-resolution polarization-resolved photoluminescence and cross-correlations of photon emission statistics at T = 5 K. The photon auto-correlation histogram of the emission from both the neutral and charged exciton indicates a clear antibunching dip with as-measured g{sup (2)}(0) values of 0.18 and 0.31, respectively. It proves that these exciton complexes confined in single quantum dashes of InP-based material system can act as true single photon emitters being compatible with standard long-distance fiber communication technology.

  4. Thickness-Induced Metal-Insulator Transition in Sb-doped SnO2 Ultrathin Films: The Role of Quantum Confinement

    NASA Astrophysics Data System (ADS)

    Ke, Chang; Zhu, Weiguang; Zhang, Zheng; Soon Tok, Eng; Ling, Bo; Pan, Jisheng

    2015-11-01

    A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition. Both electrical and spectroscopic studies provide clear evidence of a critical thickness for the metallic conductivity in SnO2:Sb thin films and the oxidation state transition of the impurity element Sb. With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect. Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system.

  5. Thickness-Induced Metal-Insulator Transition in Sb-doped SnO2 Ultrathin Films: The Role of Quantum Confinement

    PubMed Central

    Ke, Chang; Zhu, Weiguang; Zhang, Zheng; Soon Tok, Eng; Ling, Bo; Pan, Jisheng

    2015-01-01

    A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition. Both electrical and spectroscopic studies provide clear evidence of a critical thickness for the metallic conductivity in SnO2:Sb thin films and the oxidation state transition of the impurity element Sb. With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect. Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system. PMID:26616286

  6. Thickness-Induced Metal-Insulator Transition in Sb-doped SnO2 Ultrathin Films: The Role of Quantum Confinement.

    PubMed

    Ke, Chang; Zhu, Weiguang; Zhang, Zheng; Tok, Eng Soon; Ling, Bo; Pan, Jisheng

    2015-01-01

    A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition. Both electrical and spectroscopic studies provide clear evidence of a critical thickness for the metallic conductivity in SnO2:Sb thin films and the oxidation state transition of the impurity element Sb. With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect. Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system. PMID:26616286

  7. Evolution of exciton states near the percolation threshold in two-phase systems with II-VI semiconductor quantum dots

    SciTech Connect

    Bondar, N. V. Brodyn, M. S.

    2010-07-15

    From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, 'a dielectric trap' is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.

  8. Gold Quantum Boxes: On the Periodicities and the Quantum Confinement in the Au₂₈, Au₃₆, Au₄₄ , and Au₅₂ Magic Series.

    PubMed

    Zeng, Chenjie; Chen, Yuxiang; Iida, Kenji; Nobusada, Katsuyuki; Kirschbaum, Kristin; Lambright, Kelly J; Jin, Rongchao

    2016-03-30

    Revealing the size-dependent periodicities (including formula, growth pattern, and property evolution) is an important task in metal nanocluster research. However, investigation on this major issue has been complicated, as the size change is often accompanied by a structural change. Herein, with the successful determination of the Au44(TBBT)28 structure, where TBBT = 4-tert-butylbenzenethiolate, the missing size in the family of Au28(TBBT)20, Au36(TBBT)24, and Au52(TBBT)32 nanoclusters is filled, and a neat "magic series" with a unified formula of Au8n+4(TBBT)4n+8 (n = 3-6) is identified. Such a periodicity in magic numbers is a reflection of the uniform anisotropic growth patterns in this magic series, and the n value is correlated with the number of (001) layers in the face-centered cubic lattice. The size-dependent quantum confinement nature of this magic series is further understood by empirical scaling law, classical "particle in a box" model, and the density functional theory calculations. PMID:26934618

  9. Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites

    NASA Astrophysics Data System (ADS)

    Sapori, Daniel; Kepenekian, Mikaël; Pedesseau, Laurent; Katan, Claudine; Even, Jacky

    2016-03-01

    Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled sizes of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach the atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed.Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled

  10. Hole Confinement and 1/ f Noise Characteristics of SiGe Double-Quantum-Well p-Type Metal-Oxide-Semiconductor Field-Effect Transistors

    NASA Astrophysics Data System (ADS)

    Lin, Yu Min; Wu, San Lein; Chang, Shoou Jinn; Chen, Pang Shiu; Liu, Chee Wee

    2006-05-01

    A working p-type SiGe double-quantum-well metal-oxide-semiconductor field effect transistor (DQW-pMOSFETs) has been fabricated and characterized. The upper quantum well with 15%-Ge acts as an induced-carrier buffer to slow holes into the Si surface channel and increases the number of high-mobility holes in the 30%-Ge well at the bottom under high gate voltage by improving carrier confinement. DQW devices with a thinner Si-spacer layer between the two SiGe quantum wells exhibit an improved effective hole mobility and wider gate voltage swings but also reduced 1/ f noise levels than Si-controlled pMOSFETs. The DQW has an enhanced carrier confinement compared to a single quantum-well (SQW) device; however, the degradation of mobility and transconductance observed in a sample DQW indicates that this poor transport mechanism may result from an additional hole scattering effect at the Si/SiGe interface.

  11. Nonequilibrium-Plasma-Synthesized ZnO Nanocrystals with Plasmon Resonance Tunable via Al Doping and Quantum Confinement.

    PubMed

    Greenberg, Benjamin L; Ganguly, Shreyashi; Held, Jacob T; Kramer, Nicolaas J; Mkhoyan, K Andre; Aydil, Eray S; Kortshagen, Uwe R

    2015-12-01

    Metal oxide semiconductor nanocrystals (NCs) exhibit localized surface plasmon resonances (LSPRs) tunable within the infrared (IR) region of the electromagnetic spectrum by vacancy or impurity doping. Although a variety of these NCs have been produced using colloidal synthesis methods, incorporation and activation of dopants in the liquid phase has often been challenging. Herein, using Al-doped ZnO (AZO) NCs as an example, we demonstrate the potential of nonthermal plasma synthesis as an alternative strategy for the production of doped metal oxide NCs. Exploiting unique, thoroughly nonequilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the NC surfaces and local doping levels are high near the NC centers. Thus, we achieve overall doping levels as high as 2 × 10(20) cm(-3) in NCs with diameters ranging from 12.6 to 3.6 nm, and for the first time experimentally demonstrate a clear quantum confinement blue shift of the LSPR energy in vacancy- and impurity-doped semiconductor NCs. We propose that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atoms-which hinder dopant incorporation in colloidal synthesis-are irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces. We explore how the distinctive nucleation and growth kinetics occurring in the plasma influences dopant distribution and activation, defect structure, and impurity phase formation. PMID:26551232

  12. The influence of electron confinement, quantum size effects, and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

    NASA Astrophysics Data System (ADS)

    Politano, Antonio; Chiarello, Gennaro

    2015-05-01

    Plasmons are collective longitudinal modes of charge fluctuation in metal samples excited by an external electric field. Surface plasmons (SPs) are waves that propagate along the surface of a conductor. SPs find applications in magneto-optic data storage, optics, microscopy, and catalysis. The investigation of SPs in silver and gold is relevant as these materials are extensively used in plasmonics. The theoretical approach for calculating plasmon modes in noble metals is complicated by the existence of localized d electrons near the Fermi level. Nevertheless, recent calculations based on linear response theory and time-dependent local density approximation adequately describe the dispersion and damping of SPs in noble metals. Furthermore, in thin films the electronic response is influenced by electron quantum confinement. Confined electrons modify the dynamical screening processes at the film/substrate interface by introducing novel properties with potential applications. The presence of quantum well states in the Ag and Au overlayer affects both the dispersion relation of SP frequency and the damping processes of the SP. Recent calculations indicate the emergence of acoustic surface plasmons (ASP) in Ag thin films exhibiting quantum well states. The slope of the dispersion of ASP decreases with film thickness. High-resolution electron energy loss spectroscopy (HREELS) is the main experimental technique for investigating collective electronic excitations, with adequate resolution in both the energy and momentum domains to investigate surface modes. Herein we review on recent progress of research on collective electronic excitations in Ag and Au films deposited on single-crystal substrates.

  13. Analyzing total optical absorption coefficient of impurity doped quantum dots in presence of noise with special emphasis on electric field, magnetic field and confinement potential

    NASA Astrophysics Data System (ADS)

    Mandal, Arkajit; Sarkar, Sucharita; Ghosh, Arghya Pratim; Ghosh, Manas

    2015-12-01

    We make an extensive investigation of total optical absorption coefficient (TOAC) of impurity doped quantum dots (QDs) in presence and absence of Gaussian white noise. The TOAC profiles have been monitored against incident photon energy with special emphasis on the roles played by the electric field, magnetic field, and the dot confinement potential. Presence of impurity also influences the TOAC profile. In general, presence of noise causes enhancement of TOAC over that of noise-free condition. However, the interplay between the noise and the quantities like electric field, magnetic field, confinement potential and impurity potential bring about rich subtleties in the TOAC profiles. The said subtleties are often manifested by the alterations in TOAC peak intensity, extent of TOAC peak bleaching, and value of saturation intensity. The findings reveal some technologically relevant aspects of TOAC for the doped QD systems, specially in presence of noise.

  14. Molecular beam epitaxial growth and characterization of Bi{sub 2}Se{sub 3}/II-VI semiconductor heterostructures

    SciTech Connect

    Chen, Zhiyi Zhao, Lukas; Krusin-Elbaum, Lia; Garcia, Thor Axtmann; Tamargo, Maria C.; Hernandez-Mainet, Luis C.; Deng, Haiming

    2014-12-15

    Surfaces of three-dimensional topological insulators (TIs) have been proposed to host quantum phases at the interfaces with other types of materials, provided that the topological properties of interfacial regions remain unperturbed. Here, we report on the molecular beam epitaxy growth of II-VI semiconductor–TI heterostructures using c-plane sapphire substrates. Our studies demonstrate that Zn{sub 0.49}Cd{sub 0.51}Se and Zn{sub 0.23}Cd{sub 0.25}Mg{sub 0.52}Se layers have improved quality relative to ZnSe. The structures exhibit a large relative upward shift of the TI bulk quantum levels when the TI layers are very thin (∼6nm), consistent with quantum confinement imposed by the wide bandgap II-VI layers. Our transport measurements show that the characteristic topological signatures of the Bi{sub 2}Se{sub 3} layers are preserved.

  15. Influence of quantum confinement and strain on orbital polarization of four-layer LaNiO3 superlattices: A DFT+DMFT study

    NASA Astrophysics Data System (ADS)

    Park, Hyowon; Millis, Andrew J.; Marianetti, Chris A.

    2016-06-01

    Atomically precise superlattices involving transition-metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substrate) and quantum confinement (controlled by layer thickness). Here we use the combination of density-functional theory and dynamical mean-field theory (DFT+DMFT) to study Ni Egd -orbital polarization in strained LaNiO3/LaAlO3 superlattices consisting of four layers of nominally metallic NiO2 and four layers of insulating AlO2 separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. The effect of strain is determined from the dependence of the results on the Ni-O bond-length ratio and the octahedral rotation angles, quantum confinement is studied by comparison to bulk calculations with similar degrees of strain, and correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent x-ray-absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO3 and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position, and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO3.

  16. Charge transfer, lattice distortion, and quantum confinement effects in Pd, Cu, and Pd-Cu nanoparticles; size and alloying induced modifications in binding energy

    SciTech Connect

    Sengar, Saurabh K.; Mehta, B. R.; Gupta, Govind

    2011-05-09

    In this letter, effect of size and alloying on the core and valence band shifts of Pd, Cu, and Pd-Cu alloy nanoparticles has been studied. It has been shown that the sign and magnitude of the binding energy shifts is determined by the contributions of different effects; with quantum confinement and lattice distortion effects overlapping for size induced shifts in case of core levels and lattice distortion and charge transfer effects overlapping for alloying induced shifts at smaller sizes. These results are important for understanding gas molecule-solid surface interaction in metal and alloy nanoparticles in terms of valance band positions.

  17. Enhanced single photon emission from positioned InP/GaInP quantum dots coupled to a confined Tamm-plasmon mode

    SciTech Connect

    Braun, T.; Baumann, V.; Iff, O.; Schneider, C.; Kamp, M.; Höfling, S.

    2015-01-26

    We report on the enhancement of the spontaneous emission in the visible red spectral range from site-controlled InP/GaInP quantum dots by resonant coupling to Tamm-plasmon modes confined beneath gold disks in a hybrid metal/semiconductor structure. The enhancement of the emission intensity is confirmed by spatially resolved micro-photoluminescence area scans and temperature dependent measurements. Single photon emission from our coupled system is verified via second order autocorrelation measurements. We observe bright single quantum dot emission of up to ∼173 000 detected photons per second at a repetition rate of the excitation source of 82 MHz, and calculate an extraction efficiency of our device as high as 7%.

  18. Giant piezoresistance of p-type nano-thick silicon induced by interface electron trapping instead of 2D quantum confinement.

    PubMed

    Yang, Yongliang; Li, Xinxin

    2011-01-01

    The p-type silicon giant piezoresistive coefficient is measured in top-down fabricated nano-thickness single-crystalline-silicon strain-gauge resistors with a macro-cantilever bending experiment. For relatively thicker samples, the variation of piezoresistive coefficient in terms of silicon thickness obeys the reported 2D quantum confinement effect. For ultra-thin samples, however, the variation deviates from the quantum-effect prediction but increases the value by at least one order of magnitude (compared to the conventional piezoresistance of bulk silicon) and the value can change its sign (e.g. from positive to negative). A stress-enhanced Si/SiO(2) interface electron-trapping effect model is proposed to explain the 'abnormal' giant piezoresistance that should be originated from the carrier-concentration change effect instead of the conventional equivalent mobility change effect for bulk silicon piezoresistors. An interface state modification experiment gives preliminary proof of our analysis. PMID:21135460

  19. Determination of a quantum efficiency of A^II B^VI compounds on the basis of photoacoustic spectra

    NASA Astrophysics Data System (ADS)

    Maliński, M.; Bychto, L.; Zakrzewski, J.

    2005-10-01

    The method of determination of the quantum efficiency η of irradiative relaxation processes in semiconductors is introduced and discussed. The correlation of the amplitude photoacoustic spectra in the high absorption region with the annealing process is used to estimate the values of η. The values of η, both for as grown and annealed crystals were determined and have been discussed. The results are presented for microphone and piezoelectric detection methods.

  20. Field-induced confined states in graphene

    SciTech Connect

    Moriyama, Satoshi; Morita, Yoshifumi; Watanabe, Eiichiro; Tsuya, Daiju

    2014-02-03

    We report an approach to confine the carriers in single-layer graphene, which leads to quantum devices with field-induced quantum confinement. We demonstrated that the Coulomb-blockade effect evolves under a uniform magnetic field perpendicular to the graphene device. Our experimental results show that field-induced quantum dots are realized in graphene, and a quantum confinement-deconfinement transition is switched by the magnetic field.

  1. Dynamical magnetic and nuclear polarization in complex spin systems: semi-magnetic II-VI quantum dots

    NASA Astrophysics Data System (ADS)

    Abolfath, Ramin M.; Trojnar, Anna; Roostaei, Bahman; Brabec, Thomas; Hawrylak, Pawel

    2013-06-01

    Dynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in the presence of a finite number of nuclear spins. The exciton is described in terms of electron and heavy-hole spins interacting via exchange interaction with magnetic impurity, via hyperfine interaction with a finite number of nuclear spins and via dipole interaction with photons. The time evolution of the exciton, magnetic impurity and nuclear spins is calculated exactly between quantum jumps corresponding to exciton radiative recombination. The collapse of the wavefunction and the refilling of the quantum dot with a new spin-polarized exciton is shown to lead to the build up of magnetization of the magnetic impurity as well as nuclear spin polarization. The competition between electron spin transfer to magnetic impurity and to nuclear spins simultaneous with the creation of dark excitons is elucidated. The technique presented here opens up the possibility of studying optically induced dynamical magnetic and nuclear polarization in complex spin systems.

  2. Effect of elemental composition and size on electron confinement in self-assembled SiC quantum dots: A combinatorial approach

    SciTech Connect

    Das Arulsamy, A.; Rider, A. E.; Cheng, Q. J.; Ostrikov, K.; Xu, S.

    2009-05-01

    A high level of control over quantum dot (QD) properties such as size and composition during fabrication is required to precisely tune the eventual electronic properties of the QD. Nanoscale synthesis efforts and theoretical studies of electronic properties are traditionally treated quite separately. In this paper, a combinatorial approach has been taken to relate the process synthesis parameters and the electron confinement properties of the QDs. First, hybrid numerical calculations with different influx parameters for Si{sub 1-x}C{sub x} QDs were carried out to simulate the changes in carbon content x and size. Second, the ionization energy theory was applied to understand the electronic properties of Si{sub 1-x}C{sub x} QDs. Third, stoichiometric (x=0.5) silicon carbide QDs were grown by means of inductively coupled plasma-assisted rf magnetron sputtering. Finally, the effect of QD size and elemental composition were then incorporated in the ionization energy theory to explain the evolution of the Si{sub 1-x}C{sub x} photoluminescence spectra. These results are important for the development of deterministic synthesis approaches of self-assembled nanoscale quantum confinement structures.

  3. Effects of quantum confinement on excited state properties of SrTiO3 from ab initio many-body perturbation theory

    NASA Astrophysics Data System (ADS)

    Reyes-Lillo, Sebastian E.; Rangel, Tonatiuh; Bruneval, Fabien; Neaton, Jeffrey B.

    2016-07-01

    The Ruddlesden-Popper (RP) homologous series Srn +1TinO3 n +1 provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO3. We use ab initio many-body perturbation theory within the G W approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectra of Srn +1TinO3 n +1 for n =1 -5 and ∞ . Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small n and reach a value of 330 meV for n =1 , a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr2TiO4 (n =1 ) localizes in the two-dimensional plane defined by the TiO2 layer, and we explain the origin of its localization.

  4. Influence of quantum confinement and strain on orbital polarization of strained four-layer LaNiO3 superlattices: a DFT+DMFT study

    NASA Astrophysics Data System (ADS)

    Park, Hyowon; Millis, Andrew; Marianetti, Chris

    Here we use the combination of density functional theory and dynamical mean field theory to study Ni d orbital polarization in strained LaNiO3/LaAlO3 superlattices consisting of four layers of nominally metallic NiO2 and four layers of insulating AlO2 separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analysed in terms of structural, quantum confinement, and correlation effects. The overall dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent X-ray absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found depending on the sign of strain. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO3 and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO3. The quantum confinement effect is as important as the strain effect and more stronger for tensile strain. This work is supported by DOE ER-046169 and FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

  5. 1D versus 3D quantum confinement in 1-5 nm ZnO nanoparticle agglomerations for application in charge-trapping memory devices.

    PubMed

    El-Atab, Nazek; Nayfeh, Ammar

    2016-07-01

    ZnO nanoparticles (NPs) have attracted considerable interest from industry and researchers due to their excellent properties with applications in optoelectronic devices, sunscreens, photocatalysts, sensors, biomedical sciences, etc. However, the agglomeration of NPs is considered to be a limiting factor since it can affect the desirable physical and electronic properties of the NPs. In this work, 1-5 nm ZnO NPs deposited by spin- and dip-coating techniques are studied. The electronic and physical properties of the resulting agglomerations of NPs are studied using UV-vis-NIR spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM), and their application in metal-oxide-semiconductor (MOS) memory devices is analyzed. The results show that both dip- and spin-coating techniques lead to agglomerations of the NPs mostly in the horizontal direction. However, the width of the ZnO clusters is larger with dip-coating which leads to 1D quantum confinement, while the smaller ZnO clusters obtained by spin-coating enable 3D quantum confinement in ZnO. The ZnO NPs are used as the charge-trapping layer of a MOS-memory structure and the analysis of the high-frequency C-V measurements allow further understanding of the electronic properties of the ZnO agglomerations. A large memory window is achieved in both devices which confirms that ZnO NPs provide large charge-trapping density. In addition, ZnO confined in 3D allows for a larger memory window at lower operating voltages due to the Poole-Frenkel charge-emission mechanism. PMID:27232717

  6. 1D versus 3D quantum confinement in 1–5 nm ZnO nanoparticle agglomerations for application in charge-trapping memory devices

    NASA Astrophysics Data System (ADS)

    El-Atab, Nazek; Nayfeh, Ammar

    2016-07-01

    ZnO nanoparticles (NPs) have attracted considerable interest from industry and researchers due to their excellent properties with applications in optoelectronic devices, sunscreens, photocatalysts, sensors, biomedical sciences, etc. However, the agglomeration of NPs is considered to be a limiting factor since it can affect the desirable physical and electronic properties of the NPs. In this work, 1–5 nm ZnO NPs deposited by spin- and dip-coating techniques are studied. The electronic and physical properties of the resulting agglomerations of NPs are studied using UV–vis–NIR spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM), and their application in metal-oxide-semiconductor (MOS) memory devices is analyzed. The results show that both dip- and spin-coating techniques lead to agglomerations of the NPs mostly in the horizontal direction. However, the width of the ZnO clusters is larger with dip-coating which leads to 1D quantum confinement, while the smaller ZnO clusters obtained by spin-coating enable 3D quantum confinement in ZnO. The ZnO NPs are used as the charge-trapping layer of a MOS-memory structure and the analysis of the high-frequency C–V measurements allow further understanding of the electronic properties of the ZnO agglomerations. A large memory window is achieved in both devices which confirms that ZnO NPs provide large charge-trapping density. In addition, ZnO confined in 3D allows for a larger memory window at lower operating voltages due to the Poole–Frenkel charge-emission mechanism.

  7. Lorentz-covariant quantum 4-potential and orbital angular momentum for the transverse confinement of matter waves

    NASA Astrophysics Data System (ADS)

    Ducharme, R.; da Paz, I. G.

    2016-08-01

    In two recent papers exact Hermite-Gaussian solutions to relativistic wave equations were obtained for both electromagnetic and particle beams. The solutions for particle beams correspond to those of the Schrödinger equation in the nonrelativistic limit. Here, it will be shown that each beam particle has additional 4-momentum resulting from transverse localization compared to a free particle traveling in the same direction as the beam with the same speed. This will be referred to as the quantum 4-potential term since it will be shown to play an analogous role in relativistic Hamiltonian quantum mechanics as the Bohm potential in the nonrelativistic quantum Hamilton-Jacobi equation. Low-order localization effects include orbital angular momentum, Gouy phase, and beam spreading. Toward a more systematic approach for calculating localization effects at all orders, it will be shown that both the electromagnetic and quantum 4-potentials couple into the canonical 4-momentum of a particle in a similar way. This offers the prospect that traditional methods used to calculate the affect of an electromagnetic field on a particle can now be adapted to take localization effects into account. The prospects for measuring higher order quantum 4-potential related effects experimentally are also discussed alongside some questions to challenge the quantum information and quantum field theorists.

  8. High-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well lasers on Si substrates

    NASA Technical Reports Server (NTRS)

    Kim, Jae-Hoon; Lang, Robert J.; Radhakrishnan, Gouri; Katz, Joseph; Narayanan, Authi A.

    1989-01-01

    A high-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well laser on Si substrates has been demonstrated for the first time by a hybrid growth of migration-enhanced molecular beam epitaxy followed by metalorganic vapor phase epitaxy. The quantum well laser showed an output power of more than 400 mW per facet under pulsed conditions. A room-temperature threshold current of 300 mA was obtained with a differential quantum efficiency of 40 percent without facet coating. The threshold current density was 550 A/sq cm for a cavity length of 500 microns. These results show the highest peak power reported to date for low-threshold lasers on Si substrates. The full width at half maximum of the far-field pattern parallel to the junction was 6 deg. Threshold current densities as low as 250 A/sq cm were obtained for lasers on GaAs substrates.

  9. Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations

    DOE PAGESBeta

    Wood, R. M.; Saha, D.; McCarthy, L. A.; Tokarski, III, J. T.; Sanders, G. D.; Kuhns, P. L.; McGill, S. A.; Reyes, A. P.; Reno, J. L.; Stanton, C. J.; et al

    2014-10-29

    A combined experimental-theoretical study of optically pumped NMR (OPNMR) has been performed in a GaAs/Al0.1Ga0.9As quantum well film with thermally induced biaxial strain. The photon energy dependence of the Ga-71 OPNMR signal was recorded at magnetic fields of 4.9 and 9.4 T at a temperature of 4.8-5.4 K. The data were compared to the nuclear spin polarization calculated from differential absorption to spin-up and spin-down states of the conduction band using a modified Pidgeon Brown model. Reasonable agreement between theory and experiment is obtained, facilitating assignment of features in the OPNMR energy dependence to specific interband transitions. Despite the approximationsmore » made in the quantum-mechanical model and the inexact correspondence between the experimental and calculated observables, the results provide insight into how effects of strain and quantum confinement are manifested in OPNMR signals« less

  10. Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations

    SciTech Connect

    Wood, R. M.; Saha, D.; McCarthy, L. A.; Tokarski, III, J. T.; Sanders, G. D.; Kuhns, P. L.; McGill, S. A.; Reyes, A. P.; Reno, J. L.; Stanton, C. J.; Bowers, C. R.

    2014-10-29

    A combined experimental-theoretical study of optically pumped NMR (OPNMR) has been performed in a GaAs/Al0.1Ga0.9As quantum well film with thermally induced biaxial strain. The photon energy dependence of the Ga-71 OPNMR signal was recorded at magnetic fields of 4.9 and 9.4 T at a temperature of 4.8-5.4 K. The data were compared to the nuclear spin polarization calculated from differential absorption to spin-up and spin-down states of the conduction band using a modified Pidgeon Brown model. Reasonable agreement between theory and experiment is obtained, facilitating assignment of features in the OPNMR energy dependence to specific interband transitions. Despite the approximations made in the quantum-mechanical model and the inexact correspondence between the experimental and calculated observables, the results provide insight into how effects of strain and quantum confinement are manifested in OPNMR signals

  11. Optical phonons in nanostructured thin films composed by zincblende zinc selenide quantum dots in strong size-quantization regime: Competition between phonon confinement and strain-related effects

    SciTech Connect

    Pejova, Biljana

    2014-05-01

    Raman scattering in combination with optical spectroscopy and structural studies by X-ray diffraction was employed to investigate the phonon confinement and strain-induced effects in 3D assemblies of variable-size zincblende ZnSe quantum dots close packed in thin film form. Nanostructured thin films were synthesized by colloidal chemical approach, while tuning of the nanocrystal size was enabled by post-deposition thermal annealing treatment. In-depth insights into the factors governing the observed trends of the position and half-width of the 1LO band as a function of the average QD size were gained. The overall shifts in the position of 1LO band were found to result from an intricate compromise between the influence of phonon confinement and lattice strain-induced effects. Both contributions were quantitatively and exactly modeled. Accurate assignments of the bands due to surface optical (SO) modes as well as of the theoretically forbidden transverse optical (TO) modes were provided, on the basis of reliable physical models (such as the dielectric continuum model of Ruppin and Englman). The size-dependence of the ratio of intensities of the TO and LO modes was studied and discussed as well. Relaxation time characterizing the phonon decay processes in as-deposited samples was found to be approximately 0.38 ps, while upon post-deposition annealing already at 200 °C it increases to about 0.50 ps. Both of these values are, however, significantly smaller than those characteristic for a macrocrystalline ZnSe sample. - Graphical abstract: Optical phonons in nanostructured thin films composed by zincblende zinc selenide quantum dots in strong size-quantization regime: competition between phonon confinement and strain-related effects. - Highlights: • Phonon confinement vs. strain-induced effects in ZnSe 3D QD assemblies were studied. • Shifts of the 1LO band result from an intricate compromise between the two effects. • SO and theoretically forbidden TO modes were

  12. GaInAsP/InP Distributed Reflector Lasers and Integration of Front Power Monitor by Using Lateral Quantum Confinement Effect

    NASA Astrophysics Data System (ADS)

    Ullah, Saeed Mahmud; Lee, SeungHun; Suemitsu, Ryo; Nishiyama, Nobuhiko; Arai, Shigehisa

    2008-06-01

    By using lateral quantum confinement effect, a sub mA threshold current operation of a long wavelength GaInAsP/InP distributed-reflector (DR) laser, consisting of wide and narrow wirelike active regions, was demonstrated for the first time under room-temperature continuous-wave (RT-CW) condition. Eye-patterns have been studied for back-to-back as well as 10-km optical fiber transmission line. Even though devices had broad contact electrodes, clear eye opening was observed up to 2 Gbps pseudo random bit sequence (PRBS; 231 - 1) data. A monolithic integration of a power monitor (PM) at the front side of the DR laser was also demonstrated for the first time. Almost linear photocurrent characteristic with very high isolation resistance of 60 MΩ was obtained by separating the PM section by a narrow (500 nm) and deep groove filled with benzocyclobutene (BCB).

  13. Colloidal Synthesis of Quantum Confined Single Crystal CsPbBr3 Nanosheets with Lateral Size Control up to the Micrometer Range

    PubMed Central

    2016-01-01

    We report the nontemplated colloidal synthesis of single crystal CsPbBr3 perovskite nanosheets with lateral sizes up to a few micrometers and with thickness of just a few unit cells (i.e., below 5 nm), hence in the strong quantum confinement regime, by introducing short ligands (octanoic acid and octylamine) in the synthesis together with longer ones (oleic acid and oleylamine). The lateral size is tunable by varying the ratio of shorter ligands over longer ligands, while the thickness is mainly unaffected by this parameter and stays practically constant at 3 nm in all the syntheses conducted at short-to-long ligands volumetric ratio below 0.67. Beyond this ratio, control over the thickness is lost and a multimodal thickness distribution is observed. PMID:27228475

  14. Enhanced optical output and reduction of the quantum-confined Stark effect in surface plasmon-enhanced green light-emitting diodes with gold nanoparticles.

    PubMed

    Cho, Chu-Young; Park, Seong-Ju

    2016-04-01

    We report the optical properties of localized surface plasmon (LSP)-enhanced green light-emitting diodes (LEDs) containing gold (Au) nanoparticles embedded in a p-GaN layer. The photoluminescence (PL) and electroluminescence (EL) intensities of a green LED with Au nanoparticles were enhanced by the coupling between excitons and LSPs. Excitation power-dependent PL and injection current-dependent EL measurements revealed that the blue-shift of PL and EL peaks with increasing carrier density was smaller for the LSP-enhanced LED compared with that for a conventional LED. The increased optical output power and decrease in blue-shift of the LED with Au nanoparticles were attributed to the increased radiative recombination efficiency of carriers induced by the LSP-coupling process and the compensation of the polarization-induced electric fields with LSP-enhanced local fields, both of which suppressed the quantum-confined Stark effect. PMID:27137038

  15. Effect of Rashba spin-orbit interaction on the ground state energy of a D0 centre in a GaAs quantum dot with Gaussian confinement

    NASA Astrophysics Data System (ADS)

    Kumar, D. Sanjeev; Boda, Aalu; Mukhopadhyay, Soma; Chatterjee, Ashok

    2015-12-01

    The ground state energy of a neutral hydrogenic donor impurity (D0) trapped in a three-dimensional GaAs quantum dot with Gaussian confinement is calculated in the presence of Rashba spin-orbit interaction. The effect of the spin-orbit interaction is incorporated by performing a unitary transformation and retaining terms up to quadratic in the spin-orbit interaction coefficient. For the resulting Hamiltonian, the Rayleigh-Ritz variational method is employed with a simple wave function within the framework of effective-mass envelope function theory to determine the ground state energy and the binding energy of the donor complex. The results show that the Rashba spin-orbit interaction reduces the total GS energy of the donor impurity.

  16. Colloidal Synthesis of Quantum Confined Single Crystal CsPbBr3 Nanosheets with Lateral Size Control up to the Micrometer Range.

    PubMed

    Shamsi, Javad; Dang, Zhiya; Bianchini, Paolo; Canale, Claudio; Stasio, Francesco Di; Brescia, Rosaria; Prato, Mirko; Manna, Liberato

    2016-06-15

    We report the nontemplated colloidal synthesis of single crystal CsPbBr3 perovskite nanosheets with lateral sizes up to a few micrometers and with thickness of just a few unit cells (i.e., below 5 nm), hence in the strong quantum confinement regime, by introducing short ligands (octanoic acid and octylamine) in the synthesis together with longer ones (oleic acid and oleylamine). The lateral size is tunable by varying the ratio of shorter ligands over longer ligands, while the thickness is mainly unaffected by this parameter and stays practically constant at 3 nm in all the syntheses conducted at short-to-long ligands volumetric ratio below 0.67. Beyond this ratio, control over the thickness is lost and a multimodal thickness distribution is observed. PMID:27228475

  17. Absence of quantum confinement effects in the photoluminescence of Si{sub 3}N{sub 4}–embedded Si nanocrystals

    SciTech Connect

    Hiller, D. Zelenina, A.; Gutsch, S.; Zacharias, M.; Dyakov, S. A.; López-Conesa, L.; López-Vidrier, J.; Peiró, F.; Garrido, B.; Estradé, S.; Schnabel, M.; Weiss, C.; Janz, S.

    2014-05-28

    Superlattices of Si-rich silicon nitride and Si{sub 3}N{sub 4} are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si{sub 3}N{sub 4}. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 10{sup 5} times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si{sub 3}N{sub 4} reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si{sub 3}N{sub 4} band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si{sub 3}N{sub 4} interface defect literature. In addition, the role of Si{sub 3}N{sub 4} valence band tail states as potential hole traps is discussed. Most strikingly, the PL peak blueshift with decreasing NC size, which is often observed in literature and typically attributed to quantum confinement (QC), is identified as optical artifact by transfer matrix method simulations of the PL spectra. Finally, criteria for a critical examination of a potential QC-related origin of the PL from Si{sub 3}N{sub 4}-embedded Si NCs are suggested.

  18. Phase Coherent Photorefractive Effect in II-VI Semiconductor Quantum Wells and its Application for Optical Coherence Imaging

    NASA Astrophysics Data System (ADS)

    Kabir, Amin

    The phase coherent photorefractive (PCP) effect in different ZnSe quantum well structures and its dependence on various extrinsic and intrinsic parameters have been investigated using 90 fs laser pulse in a two-beam four-wave-mixing (FWM) configuration. At low excitation intensities the signal is dominated by the PCP effect (which is attributed to a long living electron grating formed in the QW due to coherent QW excitons) and pulse overlap (PO) effect while at high excitation intensities it is governed by chi(3) FWM processes and the PO effect. With increasing excitation intensity the signal dip at pulse overlap (tau ≈ 0 ) which is characteristic for the destructive interference between the PO and PCP effect shifts to positive delay times tau > 0. The higher PCP diffraction efficiency value of ˜1.5 x10-3 in QW B (Zn0.92Mg0.08Se/ZnSe) as compared to the value of ˜3.5 x 10-4 in QW A (Zn0.94Mg 0.06Se/ZnSe) at 55 K is attributed to an increased Mg concentration in the barrier of QW B leading to a higher captured equilibrium electron density ne. Repetition rate dependent measurements on QW B show a drop of the diffraction efficiency for repetition times larger than 1.25 mus which is attributed to the reduction of the electron grating amplitude due to thermally activated electron tunneling. FWM experiments on two 10 nm ZnSe QWs with different barrier thicknesses of 20 (QW1) and 50 nm (QW2) between the QW and substrate show a redshift of the exciton line and an increased exciton dephasing rate due to increasing E-field induced tilt of the QW structure indicating an increased density of captured electrons ne. At temperatures below 35 K and laser excitation close to the exciton energy the creation of trions significantly compensates the formation of the spatially modulated electron density grating. At lower excitation energies increasing space-charge-fields significantly tilt the QW which reduces the trion binding energy leading to an enhanced thermal ionization of

  19. The Quantum Mechanics of Nano-Confined Water: New Cooperative Effects Revealed with Neutron and X-Ray Compton Scattering

    NASA Astrophysics Data System (ADS)

    Reiter, G. F.; Deb, Aniruddha

    2014-12-01

    Neutron Compton scattering(NCS) measurements of the momentum distribution of light ions using the Vesuvio instrument at ISIS provide a sensitive local probe of the environment of those ions. NCS measurements of the proton momentum distribution in bulk water show only small deviations from the usual picture of water as a collection of molecules, with the protons covalently bonded to an oxygen and interacting weakly, primarily electrostatically, with nearby molecules. However, a series of measurements of the proton momentum distribution in carbon nanotubes, xerogel, and Nafion show that the proton delocalizes over distances of 0.2-0.3Å when water is confined on the scale of 20Å. This delocalization must be the result of changes in the Born-Oppenheimer surface for the protons, which would imply that there are large deviations in the electron distribution from that of a collection of weakly interacting molecules. This has been observed at Spring-8 using x-ray Compton scattering. The observed deviation in the valence electron momentum distribution from that of bulk water is more than an order of magnitude larger than the change observed in bulk water as the water is heated from just above melting to just below boiling. We conclude that the protons and electrons in nano-confined water are in a qualitatively different ground state from that of bulk water. Since the properties of this state persist at room temperature, and the confinement distance necessary to observe it is comparable to the distance between the elements of biological cells, this state presumably plays a role in the functioning of those cells.

  20. On the weak confinement of kinks in the one-dimensional quantum ferromagnet CoNb2O6

    NASA Astrophysics Data System (ADS)

    Rutkevich, S. B.

    2010-07-01

    In a recent paper Coldea et al (2010 Science 327 177) report observations of the weak confinement of kinks in the Ising spin chain ferromagnet CoNb2O6 at low temperatures. To interpret the entire spectra of magnetic excitations measured via neutron scattering, they introduce a phenomenological model, which takes into account only the two-kink configurations of the spin chain. We present the exact solution of this model. The explicit expressions for the two-kink bound-state energy spectra and for the relative intensities of neutron scattering on these magnetic modes are obtained in terms of the Bessel function.

  1. An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. II. Application—Effect of quantum confinement and homogeneous strain on Cu conductance

    SciTech Connect

    Hegde, Ganesh Povolotskyi, Michael; Kubis, Tillmann; Charles, James; Klimeck, Gerhard

    2014-03-28

    The Semi-Empirical tight binding model developed in Part I Hegde et al. [J. Appl. Phys. 115, 123703 (2014)] is applied to metal transport problems of current relevance in Part II. A systematic study of the effect of quantum confinement, transport orientation, and homogeneous strain on electronic transport properties of Cu is carried out. It is found that quantum confinement from bulk to nanowire boundary conditions leads to significant anisotropy in conductance of Cu along different transport orientations. Compressive homogeneous strain is found to reduce resistivity by increasing the density of conducting modes in Cu. The [110] transport orientation in Cu nanowires is found to be the most favorable for mitigating conductivity degradation since it shows least reduction in conductance with confinement and responds most favorably to compressive strain.

  2. Direct observation of induced ferromagnetism in SrTiO3 quantum wells confined in GdTiO3 heterostructures

    NASA Astrophysics Data System (ADS)

    Need, Ryan; Isaac, Brandon; Kirby, Brian; Borchers, Julie; Stemmer, Susanne; Wilson, Stephen

    Complex oxide thin film heterostructures with charge transfer at the interface provide a uniquely tunable environment in which to study the physics of highly-correlated and quantum-confined electron systems. Previous studies using magnetoresistance measurements had suggested that thin SrTiO3 layers become ferromagnetic when grown between thicker GdTiO3 layers. Here we report the direct observation of induced ferromagnetism in SrTiO3 quantum wells using polarized neutron reflectometry (PNR). Four GdTiO3/SrTiO3 superlattice structures with varying SrTiO3 layer thickness were grown epitaxially on LSAT (001) substrates by hybrid molecular beam epitaxy. Chemical and magnetic depth-profiles were refined using a combination of x-ray and polarized neutron reflectivity measurements taken at temperatures ranging from 4-300K. We observed a critical thickness below which the SrTiO3 layer have non-zero magnetism in the center of the well. These results are in excellent agreement with the previous magnetoresistance measurements and provide the first direct observation of induced-magnetism in this system.

  3. Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions.

    PubMed

    Masuda, Hidetoshi; Sakai, Hideaki; Tokunaga, Masashi; Yamasaki, Yuichi; Miyake, Atsushi; Shiogai, Junichi; Nakamura, Shintaro; Awaji, Satoshi; Tsukazaki, Atsushi; Nakao, Hironori; Murakami, Youichi; Arima, Taka-hisa; Tokura, Yoshinori; Ishiwata, Shintaro

    2016-01-01

    For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm(2)/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials. PMID:27152326

  4. Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions

    PubMed Central

    Masuda, Hidetoshi; Sakai, Hideaki; Tokunaga, Masashi; Yamasaki, Yuichi; Miyake, Atsushi; Shiogai, Junichi; Nakamura, Shintaro; Awaji, Satoshi; Tsukazaki, Atsushi; Nakao, Hironori; Murakami, Youichi; Arima, Taka-hisa; Tokura, Yoshinori; Ishiwata, Shintaro

    2016-01-01

    For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm2/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials. PMID:27152326

  5. Confined acoustic phonon-mediated spin relaxation in a twodimensional quantum dot in the presence of perpendicular magnetic field

    NASA Astrophysics Data System (ADS)

    Vardanyan, K. A.; Vartanian, A. L.; Stepanyan, A. G.; Kirakosyan, A. A.

    2015-10-01

    The spin-relaxation time due to the electron-acoustic phonon scattering in GaAs quantum dots is studied after the exact diagonalization of the electron Hamiltonian with the spin-orbit coupling. It has been shown that in comparison with flexural phonons, the electron coupling with the dilatational phonons causes 3 orders faster spin relaxation. We have found that the relaxation rate of the spin-flip is an order of magnitude smaller than that of the spin- conserving.

  6. Resonant enhancement of third-order nonlinear optical susceptibilities of Cd-free chalcopyrite nanocrystals within quantum confinement regime

    NASA Astrophysics Data System (ADS)

    Hamanaka, Yasushi; Ogawa, Tetsuya; Tsuzuki, Masakazu; Kuzuya, Toshihiro; Sumiyama, Kenji

    2013-07-01

    Third-order nonlinear optical susceptibilities (χ(3)) have been investigated for chalcopyrite CuInS2 and AgInS2 nanocrystals within a strong confinement regime. The imaginary part of χ(3) (Imχ(3)) of 2.0- and 4.9-nm-sized CuInS2 nanocrystals and 2.6- and 4.3-nm-sized AgInS2 nanocrystals are negative and exhibit resonant enhancement around the absorption between the highest quantized levels of valence band and the lowest conduction band due to the state-filling effect. Figure of merit of |Imχ(3)| ranges 10-20-10-19 m3/V2, which is comparable to those of CdSSe nanocrystals.

  7. Control of orbital reconstruction in (LaAlO3)M/(SrTiO3)N(001) quantum wells by strain and confinement

    PubMed Central

    Doennig, David; Pentcheva, Rossitza

    2015-01-01

    The diverse functionality emerging at oxide interfaces calls for a fundamental understanding of the mechanisms and control parameters of electronic reconstructions. Here, we explore the evolution of electronic phases in (LaAlO3)M/(SrTiO3)N (001) superlattices as a function of strain and confinement of the SrTiO3 quantum well. Density functional theory calculations including a Hubbard U term reveal a charge ordered Ti3+ and Ti4+ state for N = 2 with an unanticipated orbital reconstruction, displaying alternating dxz and dyz character at the Ti3+ sites, unlike the previously reported dxy state, obtained only for reduced c-parameter at aSTO. At aLAO c-compression leads to a Dimer-Mott insulator with alternating dxz, dyz sites and an almost zero band gap. Beyond a critical thickness of N = 3 (aSTO) and N = 4 (aLAO) an insulator-to-metal transition takes place, where the extra e/2 electron at the interface is redistributed throughout the STO slab with a dxy interface orbital occupation and a mixed dxz + dyz occupation in the inner layers. Chemical variation of the SrTiO3 counterpart (LaAlO3 vs. NdGaO3) proves that the significant octahedral tilts and distortions in the SrTiO3 quantum well are induced primarily by the electrostatic doping at the polar interface and not by variation of the SrTiO3 counterpart. PMID:25601648

  8. A quantum-mechanical description of ion motion within the confining potentials of voltage-gated ion channels.

    PubMed

    Summhammer, Johann; Salari, Vahid; Bernroider, Gustav

    2012-06-01

    Voltage-gated channel proteins cooperate in the transmission of membrane potentials between nerve cells. With the recent progress in atomic-scaled biological chemistry, it has now become established that these channel proteins provide highly correlated atomic environments that may maintain electronic coherences even at warm temperatures. Here we demonstrate solutions of the Schrödinger equation that represent the interaction of a single potassium ion within the surrounding carbonyl dipoles in the Berneche-Roux model of the bacterial KcsA model channel. We show that, depending on the surrounding carbonyl-derived potentials, alkali ions can become highly delocalized in the filter region of proteins at warm temperatures. We provide estimations on the temporal evolution of the kinetic energy of ions depending on their interaction with other ions, their location within the oxygen cage of the proteins filter region, and depending on different oscillation frequencies of the surrounding carbonyl groups. Our results provide the first evidence that quantum mechanical properties are needed to explain a fundamental biological property such as ion selectivity in transmembrane ion currents and the effect on gating kinetics and shaping of classical conductances in electrically excitable cells. PMID:22744820

  9. Quantum confinement-tunable ultrafast charge transfer at the PbS quantum dot and phenyl-C₆₁-butyric acid methyl ester interface.

    PubMed

    El-Ballouli, Ala'a O; Alarousu, Erkki; Bernardi, Marco; Aly, Shawkat M; Lagrow, Alec P; Bakr, Osman M; Mohammed, Omar F

    2014-05-14

    Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion. PMID:24521255

  10. Nanocrystalline-Si-dot multi-layers fabrication by chemical vapor deposition with H-plasma surface treatment and evaluation of structure and quantum confinement effects

    SciTech Connect

    Kosemura, Daisuke Mizukami, Yuki; Takei, Munehisa; Numasawa, Yohichiroh; Ogura, Atsushi; Ohshita, Yoshio

    2014-01-15

    100-nm-thick nanocrystalline silicon (nano-Si)-dot multi-layers on a Si substrate were fabricated by the sequential repetition of H-plasma surface treatment, chemical vapor deposition, and surface oxidation, for over 120 times. The diameter of the nano-Si dots was 5–6 nm, as confirmed by both the transmission electron microscopy and X-ray diffraction analysis. The annealing process was important to improve the crystallinity of the nano-Si dot. We investigated quantum confinement effects by Raman spectroscopy and photoluminescence (PL) measurements. Based on the experimental results, we simulated the Raman spectrum using a phenomenological model. Consequently, the strain induced in the nano-Si dots was estimated by comparing the experimental and simulated results. Taking the estimated strain value into consideration, the band gap modulation was measured, and the diameter of the nano-Si dots was calculated to be 5.6 nm by using PL. The relaxation of the q ∼ 0 selection rule model for the nano-Si dots is believed to be important to explain both the phenomena of peak broadening on the low-wavenumber side observed in Raman spectra and the blue shift observed in PL measurements.

  11. Rheology of water ices V and VI

    USGS Publications Warehouse

    Durham, W.B.; Stern, L.A.; Kirby, S.H.

    1996-01-01

    We have measured the mechanical strength (??) of pure water ices V and VI under steady state deformation conditions. Constant displacement rate compressional tests were conducted in a gas apparatus at confining pressures from 400 250 K. Ices V and VI are thus Theologically distinct but by coincidence have approximately the same strength under the conditions chosen for these experiments. To avoid misidentification, these tests are therefore accompanied by careful observations of the occurrences and characteristics of phase changes. One sample each of ice V and VI was quenched at pressure to metastably retain the high-pressure phase and the acquired deformation microstructures; X ray diffraction analysis of these samples confirmed the phase identification. Surface replicas of the deformed and quenched samples suggest that ice V probably deforms largely by dislocation creep, while ice VI deforms by a more complicated process involving substantial grain size reduction through recrystallization.

  12. Confinement from spontaneous breaking of scale symmetry

    NASA Astrophysics Data System (ADS)

    Gaete, Patricio; Guendelman, Eduardo

    2006-09-01

    We show that one can obtain naturally the confinement of static charges from the spontaneous symmetry breaking of scale invariance in a gauge theory. At the classical level a confining force is obtained and at the quantum level, using a gauge invariant but path-dependent variables formalism, the Cornell confining potential is explicitly obtained. Our procedure answers completely to the requirements by 't Hooft for "perturbative confinement".

  13. Reducing Blinking in Small Core-Multishell Quantum Dots by Carefully Balancing Confinement Potential and Induced Lattice Strain: The "Goldilocks" Effect.

    PubMed

    Omogo, Benard; Gao, Feng; Bajwa, Pooja; Kaneko, Mizuho; Heyes, Colin D

    2016-04-26

    Currently, the most common way to reduce blinking in quantum dots (QDs) is accomplished by using very thick and/or perfectly crystalline CdS shells on CdSe cores. Ideally, a nontoxic material such as ZnS is preferred to be the outer material in order to reduce environmental and cytotoxic effects. Blinking suppression with multishell configurations of CdS and ZnS has been reported only for "giant" QDs of 15 nm or more. One of the main reasons for the limited progress is that the role that interfacial trap states play in blinking in these systems is not very well understood. Here, we show a "Goldilocks" effect to reduce blinking in small (∼7 nm) QDs by carefully controlling the thicknesses of the shells in multishell QDs. Furthermore, by correlating the fluorescence lifetime components with the fraction of time that a QD spends in the on-state, both with and without applying a threshold, we found evidence for two types of blinking that separately affect the average fluorescence lifetime of a single QD. A thorough characterization of the time-resolved fluorescence at the ensemble and single-particle level allowed us to propose a detailed physical model involving both short-lived interfacial trap states and long-lived surface trap states that are coupled. This model highlights a strategy of reducing QD blinking in small QDs by balancing the magnitude of the induced lattice strain, which results in the formation of interfacial trap states between the inner shell and the outer shell, and the confinement potential that determines how accessible the interfacial trap states are. The combination of reducing blinking while maintaining a small overall QD size and using a Cd-free outer shell of ZnS will be useful in a wide array of applications, particularly for advanced bioimaging. PMID:27058120

  14. Cranial mononeuropathy VI

    MedlinePlus

    ... Abducens palsy; Lateral rectus palsy; Vith nerve palsy; Cranial nerve VI palsy ... mononeuropathy VI is damage to the sixth cranial (skull) nerve. This nerve, also called the abducens nerve, helps ...

  15. Cranial mononeuropathy VI

    MedlinePlus

    Abducens paralysis; Abducens palsy; Lateral rectus palsy; Vith nerve palsy; Cranial nerve VI palsy ... VI is damage to the sixth cranial (skull) nerve. This nerve, also called the abducens nerve, helps ...

  16. Evolution of excitonic states in two-phase systems with quantum dots of II-VI semiconductors near the percolation threshold

    NASA Astrophysics Data System (ADS)

    Bondar, N. V.; Brodyn, M. S.

    2010-03-01

    In two-phase disordered media composed of borosilicate glass with ZnSe or CdS quantum dots, the formation of a phase percolation transition of carriers for near-threshold concentrations that are manifested in optical spectra has been observed. Microscopic fluctuations of the quantum-dot density near the percolation threshold were found that resembled the phenomenon of critical opalescence, where similar fluctuations of the density of a pure substance appear near to a phase transition. It is proposed that the dielectric mismatch between a matrix and ZnSe or CdS quantum dots plays a significant role in the carrier (exciton) delocalization, resulting in the appearance of a “dielectric Coulomb trap” beyond the QD border and the formation of surface states of excitons. The spatial overlapping of excitonic states at the critical density of quantum dots results in a tunneling of carriers and the formation of a phase percolation transition in such media.

  17. Limiting Spectra from Confining Potentials.

    ERIC Educational Resources Information Center

    Nieto, Michael Martin; Simmons, L. M., Jr.

    1979-01-01

    The author explains that, for confining potentials and large quantum numbers, the bound-state energies rise more rapidly as a function of n the more rapidly the potential rises with distance. However, the spectrum can rise no faster than n squared in the nonrelativistic case, or n in the relativistic case. (Author/GA)

  18. A facile integration of zero- (I-III-VI quantum dots) and one- (single SnO2 nanowire) dimensional nanomaterials: fabrication of a nanocomposite photodetector with ultrahigh gain and wide spectral response.

    PubMed

    Lu, Meng-Lin; Lai, Chih-Wei; Pan, Hsing-Ju; Chen, Chung-Tse; Chou, Pi-Tai; Chen, Yang-Fang

    2013-05-01

    Via the integration of nanocomposites comprising I-III-VI semiconductor quantum dots (QDs) decorated onto a single SnO2 nanowire (NW), we successfully fabricate an ultrahigh-sensitivity and wide spectral-response photodetector. Under the illumination of He-Cd laser (325 nm) with the photon energy larger than the band gap of SnO2 nanowire, remarkably, an ultrahigh photocurrent gain up to 2.5 × 10(5) has been achieved, and an enhancement factor can reach up to 700% (cf. bare SnO2 NW) as light illumination onto the wire with an excitation intensity of 15 W/m(2). Also, a high gain value up to 1.3 × 10(5) is attained with the excited photon energy (488 nm) smaller than the band gap of SnO2 nanowire. Several key factors contribute to ultrahigh photocurrent gain and wide spectral response. First, the decorated quantum dot processes an inherent nature of a large absorption coefficient above its band gap. Furthermore, the single SnO2 nanowire provides an excellent conduction path for the photogenerated carriers as well as bears a large surface-to-volume ratio so that the coupling strength with quantum dots can be greatly enhanced. Most importantly, the spatial separation of photogenerated electrons and holes can be easily achieved due to the charge transfer arising from a type II band alignment between QDs and SnO2 NW. This work thus demonstrates a new approach in which by selectively decorating suitable QDs the photocurrent gain of SnO2 NWs can be greatly enhanced and extended to a wide spectral range of photoresponse previously inaccessible, providing a very useful guideline to create cheap, nontoxic, and highly efficient photodetectors. PMID:23574534

  19. Surfactant ligand removal and rational fabrication of inorganically connected quantum dots.

    PubMed

    Zhang, Haitao; Hu, Bo; Sun, Liangfeng; Hovden, Robert; Wise, Frank W; Muller, David A; Robinson, Richard D

    2011-12-14

    A novel method is reported to create inorganically connected nanocrystal (NC) assemblies for both II-VI and IV-VI semiconductors by removing surfactant ligands using (NH4)2S. This surface modification process differs from ligand exchange methods in that no new surfactant ligands are introduced and the post-treated NC surfaces are nearly bare. The detailed mechanism study shows that the high reactivity between (NH4)2S and metal-surfactant ligand complexes enables the complete removal of surfactant ligands in seconds and converts the NC metal-rich shells into metal sulfides. The post-treated NCs are connected through metal-sulfide bonding and form a larger NCs film assembly, while still maintaining quantum confinement. Such "connected but confined" NC assemblies are promising new materials for electronic and optoelectronic devices. PMID:22011091

  20. Causal signal transmission by quantum fields. VI: The Lorentz condition and Maxwell’s equations for fluctuations of the electromagnetic field

    SciTech Connect

    Plimak, L.I.; Stenholm, S.

    2013-11-15

    The general structure of electromagnetic interactions in the so-called response representation of quantum electrodynamics (QED) is analysed. A formal solution to the general quantum problem of the electromagnetic field interacting with matter is found. Independently, a formal solution to the corresponding problem in classical stochastic electrodynamics (CSED) is constructed. CSED and QED differ only in the replacement of stochastic averages of c-number fields and currents by time-normal averages of the corresponding Heisenberg operators. All relations of QED connecting quantum field to quantum current lack Planck’s constant, and thus coincide with their counterparts in CSED. In Feynman’s terms, one encounters complete disentanglement of the potential and current operators in response picture. Based on this parallelism between QED and CSED, it is natural to expect validity of the Lorentz condition and Maxwell’s equations for the time-normal averages of the potential and current. Things however turn out to be more complicated. Maxwell’s equations under the time-normal ordering can only be demonstrated subject to cancellation of the so-called Schwinger terms by gauge-invariant regularisations. We presume this pattern to be general, formulating this as “commutativity conjecture”. Consistency of the latter with the Heisenberg uncertainty principle is discussed. -- Highlights: •The general structure of interaction in quantum electrodynamics (QED) is analysed. •A detailed parallelism between QED and classical stochastic electrodynamics is shown. •Validity of Maxwell’s equations for fluctuations of the field is discussed. •This validity turns out to be in essence a renormalisation postulate.

  1. Effect of the deposition of cobalt on the optoelectronic properties of quantum-confined In(Ga)As/GaAs heteronanostructures

    SciTech Connect

    Volkova, N. S.; Gorshkov, A. P.; Zdoroveyshchev, A. V.; Istomin, L. A.; Levichev, S. B.

    2015-12-15

    The systematic features of the inf luence of defect formation during the deposition of a cobalt contact on the optoelectronic characteristics of structures containing InAs/GaAs quantum dots and In{sub x}Ga{sub 1–x}As/GaAs quantum wells are studied. From analysis of the temperature dependences of the photosensitivity of the InAs/GaAs quantum-dot structures, the values of the resultant recombination lifetime of photoexcited charge carriers in quantum dots at different conditions of Co deposition and at different structural parameters are determined.

  2. Challenges in the simulation of dye-sensitized ZnO solar cells: quantum confinement, alignment of energy levels and excited state nature at the dye/semiconductor interface.

    PubMed

    Amat, Anna; De Angelis, Filippo

    2012-08-14

    We report a first principles density functional theory/time-dependent density functional theory (DFT/TDDFT) computational investigation on a prototypical perylene dye anchored to realistic ZnO nanostructures, approaching the size of the ZnO nanowires used in dye-sensitized solar cells devices. DFT calculations were performed on (ZnO)(n) clusters of increasing size, with n up to 222, of 1.3 × 1.5 × 3.4 nm dimensions, and for the related dye-sensitized models. We show that quantum confinement in the ZnO nanostructures substantially affects the dye/semiconductor alignment of energy levels, with smaller ZnO models providing unfavourable electron injection. An increasing broadening of the dye LUMO is found moving to larger substrates, substantially contributing to the interfacial electronic coupling. TDDFT excited state calculations for the investigated dye@(ZnO)(222) system are fully consistent with experimental data, quantitatively reproducing the red-shift and broadening of the visible absorption spectrum observed for the ZnO-anchored dye compared to the dye in solution. TDDFT calculations on the fully interacting system also introduce a contribution to the dye/semiconductor admixture, due to configurational excited state mixing. Our results highlight the importance of quantum confinement in dye-sensitized ZnO interfaces, and provide the fundamental insight lying at the heart of the associated DSC devices. PMID:22743544

  3. Electron states in semiconductor quantum dots

    SciTech Connect

    Dhayal, Suman S.; Ramaniah, Lavanya M.; Ruda, Harry E.; Nair, Selvakumar V.

    2014-11-28

    In this work, the electronic structures of quantum dots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications.

  4. Multimodal Mn-doped I-III-VI quantum dots for near infrared fluorescence and magnetic resonance imaging: from synthesis to in vivo application

    NASA Astrophysics Data System (ADS)

    Sitbon, Gary; Bouccara, Sophie; Tasso, Mariana; Francois, Aurélie; Bezdetnaya, Lina; Marchal, Frédéric; Beaumont, Marine; Pons, Thomas

    2014-07-01

    The development of sensitive multimodal contrast agents is a key issue to provide better global, multi-scale images for diagnostic or therapeutic purposes. Here we present the synthesis of Zn-Cu-In-(S, Se)/Zn1-xMnxS core-shell quantum dots (QDs) that can be used as markers for both near-infrared fluorescence imaging and magnetic resonance imaging (MRI). We first present the synthesis of Zn-Cu-In-(S, Se) cores coated with a thick ZnS shell doped with various proportions of Mn. Their emission wavelengths can be tuned over the NIR optical window suitable for deep tissue imaging. The incorporation of manganese ions (up to a few thousand ions per QD) confers them a paramagnetic character, as demonstrated by structural analysis and electron paramagnetic resonance spectroscopy. These QDs maintain their optical properties after transfer to water using ligand exchange. They exhibit T1-relaxivities up to 1400 mM-1 [QD] s-1 at 7 T and 300 K. We finally show that these QDs are suitable multimodal in vivo probes and demonstrate MRI and NIR fluorescence detection of regional lymph nodes in mice.The development of sensitive multimodal contrast agents is a key issue to provide better global, multi-scale images for diagnostic or therapeutic purposes. Here we present the synthesis of Zn-Cu-In-(S, Se)/Zn1-xMnxS core-shell quantum dots (QDs) that can be used as markers for both near-infrared fluorescence imaging and magnetic resonance imaging (MRI). We first present the synthesis of Zn-Cu-In-(S, Se) cores coated with a thick ZnS shell doped with various proportions of Mn. Their emission wavelengths can be tuned over the NIR optical window suitable for deep tissue imaging. The incorporation of manganese ions (up to a few thousand ions per QD) confers them a paramagnetic character, as demonstrated by structural analysis and electron paramagnetic resonance spectroscopy. These QDs maintain their optical properties after transfer to water using ligand exchange. They exhibit T1-relaxivities

  5. Multimodal Mn-doped I-III-VI quantum dots for near infrared fluorescence and magnetic resonance imaging: from synthesis to in vivo application.

    PubMed

    Sitbon, Gary; Bouccara, Sophie; Tasso, Mariana; Francois, Aurélie; Bezdetnaya, Lina; Marchal, Frédéric; Beaumont, Marine; Pons, Thomas

    2014-08-01

    The development of sensitive multimodal contrast agents is a key issue to provide better global, multi-scale images for diagnostic or therapeutic purposes. Here we present the synthesis of Zn-Cu-In-(S, Se)/Zn(1-x)Mn(x)S core-shell quantum dots (QDs) that can be used as markers for both near-infrared fluorescence imaging and magnetic resonance imaging (MRI). We first present the synthesis of Zn-Cu-In-(S, Se) cores coated with a thick ZnS shell doped with various proportions of Mn. Their emission wavelengths can be tuned over the NIR optical window suitable for deep tissue imaging. The incorporation of manganese ions (up to a few thousand ions per QD) confers them a paramagnetic character, as demonstrated by structural analysis and electron paramagnetic resonance spectroscopy. These QDs maintain their optical properties after transfer to water using ligand exchange. They exhibit T1-relaxivities up to 1400 mM(-1) [QD] s(-1) at 7 T and 300 K. We finally show that these QDs are suitable multimodal in vivo probes and demonstrate MRI and NIR fluorescence detection of regional lymph nodes in mice. PMID:24980473

  6. PREFACE: International Conference on Optics of Excitons in Confined Systems

    NASA Astrophysics Data System (ADS)

    Viña, Luis; Tejedor, Carlos; Calleja, José M.

    2010-01-01

    The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43

  7. Benchmark Assessment of Density Functional Methods on Group II-VI MX (M = Zn, Cd; X = S, Se, Te) Quantum Dots.

    PubMed

    Azpiroz, Jon M; Ugalde, Jesus M; Infante, Ivan

    2014-01-14

    In this work, we build a benchmark data set of geometrical parameters, vibrational normal modes, and low-lying excitation energies for MX quantum dots, with M = Cd, Zn, and X = S, Se, Te. The reference database has been constructed by ab initio resolution-of-identity second-order approximate coupled cluster RI-CC2/def2-TZVPP calculations on (MX)6 model molecules in the wurtzite structure. We have tested 26 exchange-correlation density functionals, ranging from local generalized gradient approximation (GGA) and hybrid GGA to meta-GGA, meta-hybrid, and long-range corrected. The best overall functional is the hybrid PBE0 that outperforms all other functionals, especially for excited state energies, which are of particular relevance for the systems studied here. Among the DFT methodologies with no Hartree-Fock exchange, the M06-L is the best one. Local GGA functionals usually provide satisfactory results for geometrical structures and vibrational frequencies but perform rather poorly for excitation energies. Regarding the CdSe cluster, we also present a test of several basis sets that include relativistic effects via effective core potentials (ECPs) or via the ZORA approximation. The best basis sets in terms of computational efficiency and accuracy are the SBKJC and def2-SV(P). The LANL2DZ basis set, commonly employed nowadays on these types of nanoclusters, performs very disappointingly. Finally, we also provide some suggestions on how to perform calculations on larger systems keeping a balance between computational load and accuracy. PMID:26579893

  8. Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots.

    PubMed

    Adegoke, Oluwasesan; Park, Enoch Y

    2016-01-01

    The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27-61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72-93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying. PMID:27250067

  9. Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots

    NASA Astrophysics Data System (ADS)

    Adegoke, Oluwasesan; Park, Enoch Y.

    2016-06-01

    The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27–61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72–93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying.

  10. Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots

    PubMed Central

    Adegoke, Oluwasesan; Park, Enoch Y.

    2016-01-01

    The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27–61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72–93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying. PMID:27250067

  11. Tuning the g-factor of neutral and charged excitons confined to self-assembled (Al,Ga)As shell quantum dots

    SciTech Connect

    Corfdir, P. Van Hattem, B.; Phillips, R. T.; Fontana, Y.; Russo-Averchi, E.; Heiss, M.; Fontcuberta i Morral, A.

    2014-12-01

    We study the neutral exciton (X) and charged exciton (CX) transitions from (Al,Ga)As shell quantum dots located in core-shell nanowires, in the presence of a magnetic field. The g-factors and the diamagnetic coefficients of both the X and the CX depend on the orientation of the field with respect to the nanowire axis. The aspect ratio of the X wavefunction is quantified based on the anisotropy of the diamagnetic coefficient. For specific orientations of the magnetic field, it is possible to cancel the g-factor of the bright states of the X and the CX by means of an inversion of the sign of the hole's g-factor, which is promising for quantum information processing applications.

  12. Influence of the Nuclear Electric Quadrupolar Interaction on the Coherence Time of Hole and Electron Spins Confined in Semiconductor Quantum Dots

    NASA Astrophysics Data System (ADS)

    Hackmann, J.; Glasenapp, Ph.; Greilich, A.; Bayer, M.; Anders, F. B.

    2015-11-01

    The real-time spin dynamics and the spin noise spectra are calculated for p and n -charged quantum dots within an anisotropic central spin model extended by additional nuclear electric quadrupolar interactions and augmented by experimental data. Using realistic estimates for the distribution of coupling constants including an anisotropy parameter, we show that the characteristic long time scale is of the same order for electron and hole spins strongly determined by the quadrupolar interactions even though the analytical form of the spin decay differs significantly consistent with our measurements. The low frequency part of the electron spin noise spectrum is approximately 1 /3 smaller than those for hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by our experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser.

  13. Collagen type VI myopathies.

    PubMed

    Bushby, Kate M D; Collins, James; Hicks, Debbie

    2014-01-01

    Mutations in each of the three collagen VI genes COL6A1, COL6A2 and COL6A3 cause two main types of muscle disorders: Ullrich congenital muscular dystrophy, a severe phenotype, and a mild to moderate phenotype Bethlem myopathy. Recently, two additional phenotypes, including a limb-girdle muscular dystrophy phenotype and an autosomal recessive myosclerosis reported in one family with mutations in COL6A2 have been reported. Collagen VI is an important component of the extracellular matrix which forms a microfibrillar network that is found in close association with the cell and surrounding basement membrane. Collagen VI is also found in the interstitial space of many tissues including muscle, tendon, skin, cartilage, and intervertebral discs. Thus, collagen VI mutations result in disorders with combined muscle and connective tissue involvement, including weakness, joint laxity and contractures, and abnormal skin findings.In this review we highlight the four recognized clinical phenotypes of collagen VI related - myopathies; Ullrich congenital muscular dystrophy (UCMD), Bethlem myopathy (BM), autosomal dominant limb-girdle muscular dystrophy phenotype and autosomal recessive myosclerosis. We discuss the diagnostic criteria of these disorders, the molecular pathogenesis, genetics, treatment, and related disorders. PMID:24443028

  14. Large phonon-drag enhancement induced by narrow quantum confinement at the LaAlO3/SrTiO3 interface

    NASA Astrophysics Data System (ADS)

    Pallecchi, I.; Telesio, F.; Marré, D.; Li, D.; Gariglio, S.; Triscone, J.-M.; Filippetti, A.

    2016-05-01

    The thermoelectric power of the two-dimensional electron system (2DES) at the LaAlO3/SrTiO3 interface is explored below room temperature, in comparison with that of Nb-doped SrTiO3 single crystals. For the interface, we find a region below T =50 K where thermopower is dominated by phonon drag, whose amplitude is hugely amplified with respect to the corresponding bulk value, reaching values ˜mV /K and above. The phonon-drag enhancement at the interface is traced back to the tight carrier confinement of the 2DES, and represents a sharp signature of strong electron-acoustic phonon coupling at the interface.

  15. PHYSICAL FOUNDATIONS OF QUANTUM ELECTRONICS: The distribution function and fluctuations of the number of particles in an ideal Bose gas confined by a trap

    NASA Astrophysics Data System (ADS)

    Alekseev, Vladimir A.

    2001-01-01

    The distribution function ω0(n0) of the number n0 of particles in the condensate of an ideal Bose gas confined by a trap is found. It is shown that at the temperature above the critical one (T > Tc) this function has the usual form ω0(n0) =(1 — eμ)eμno, where μ is the chemical potential in the temperature units. For T < Tc, this distribution changes almost in a jump to a Gaussian distribution, which depends on the trap potential only parametrically. The centre of this function shifts to larger values of n0 with decreasing temperature and its width tends to zero, which corresponds to the suppression of fluctuations.

  16. Deriving k·p parameters from full-Brillouin-zone descriptions: A finite-element envelope function model for quantum-confined wurtzite nanostructures

    SciTech Connect

    Zhou, Xiangyu; Ghione, Giovanni; Bertazzi, Francesco Goano, Michele; Bellotti, Enrico

    2014-07-21

    We present a multiband envelope-function model for wurtzite nanostructures based on a rigorous numerical procedure to determine operator ordering and band parameters from nonlocal empirical pseudopotential calculations. The proposed approach, implemented within a finite-element scheme, leads to well-posed, numerically stable envelope equations that accurately reproduce full-Brillouin-zone subband dispersions of quantum systems. Although demonstrated here for III-nitride nonlocal empirical pseudopotentials, the model provides a general theoretical framework applicable to ab initio electronic structures of wurtzite semiconductors.

  17. Engineering Test Satellite VI (ETS-VI)

    NASA Technical Reports Server (NTRS)

    Horii, M.; Funakawa, K.

    1991-01-01

    The Engineering Test Satellite-VI (ETS-VI) is being developed as the third Japanese three-axis stabilized engineering test satellite to establish the 2-ton geostationary operational satellite bus system and to demonstrate the high performance satellite communication technology for future operational satellites. The satellite is expected to be stationed at 154 deg east latitude. It will be launched from the Tanegashima Space Center in Japan by a type H-II launch vehicle. The Deep Space Network (DSN) will support the prelaunch compatibility test, data interface verification testing, and launch rehersals. The DSN primary support period is from launch through the final AEF plus 1 hour. Contingency support is from final AEF plus 1 hour until launch plus 1 month. The coverage will consist of all the 26-m antennas as prime and the 34-m antennas at Madrid and Canberra as backup. Maximum support will consist of two 8-hour tracks per station for a 7-day period, plus the contingency support, if required. Information is given in tabular form for DSN support, telemetry, command, and tracking support responsibility.

  18. The linear optical properties of a multi-shell spherical quantum dot of a parabolic confinement for cases with and without a hydrogenic impurity

    NASA Astrophysics Data System (ADS)

    Şahin, Mehmet; Köksal, Koray

    2012-12-01

    Throughout this work, we aim to explore the linear optical properties of a semiconductor multi-shell spherical quantum dot with and without a hydrogenic donor impurity. The core and well layers are defined by the parabolic electronic potentials in the radial direction. The energy levels and corresponding wavefunctions of the structure are calculated by using the shooting technique in the framework of the effective-mass approximation. We investigate the intersublevel absorption coefficients of a single electron and the hydrogenic donor impurity comparatively as a function of the photon energy. In addition, we carry out the effect of a donor impurity and the layer thickness on the oscillator strengths and magnitude and position of absorption coefficient peaks. We illustrate the electron probability distribution and variation of the energy levels in cases with and without the impurity for different thicknesses of layers. This kind of structure gives an opportunity to tune and control the absorption coefficient of the system by changing three different thickness parameters. Also it provides a possibility to separate 0s and 1p electrons in different regions of the quantum dot.

  19. Electric field induced optical gain of a hydrogenic impurity in a Cd{sub 0.8}Zn{sub 0.2}Se/ZnSe parabolic quantum dot

    SciTech Connect

    Jasmine, P. Christina Lily; Peter, A. John

    2015-06-24

    The dependence of electric field on the electronic and optical properties is investigated in a Cd{sub 0.8}Zn{sub 0.2}Se/ZnSe quantum dot. The hydrogenic binding energy, in the presence of electric field, is calculated with the spatial confinement effect. The electric field dependent optical gain with the photon energy is found using compact density matrix method. The results show that the electric field has a great influence on the optical properties of II-VI semiconductor quantum dot.

  20. Two blinking mechanisms in highly confined AgInS2 and AgInS2/ZnS quantum dots evaluated by single particle spectroscopy

    NASA Astrophysics Data System (ADS)

    Cichy, B.; Rich, R.; Olejniczak, A.; Gryczynski, Z.; Strek, W.

    2016-02-01

    Ternary AgInS2 quantum dots (QDs) have been found as promising cadmium-free, red-shifted, and tunable luminescent bio-probes with efficient Stokes and anti-Stokes excitations and luminescence lifetimes (ca. 100 ns) convenient for time resolved techniques like fluorescence life-time imaging. Although the spectral properties of the AgInS2 QDs are encouraging, the complex recombination kinetics in the QDs being still far from understood, limits their full utility. In this paper we report on a model describing the recombination pathways responsible for large deviations from the first-order decay law observed commonly in the ternary chalcogenides. The presented results were evaluated by means of individual AgInS2 QD spectroscopy aided by first principles calculations including the electronic structure and structural reconstruction of the QDs. Special attention was devoted to study the impact of the surface charge state on the excited state relaxation and effect of its passivation by Zn2+ ion alloying. Two different blinking mechanisms related to defect-assisted charge imbalance in the QD responsible for fast non-radiative relaxation of the excited states as well as surface recharging of the QD were found as the major causes of deviations from the first-order decay law. Careful optimization of the AgInS2 QDs would help to fabricate new red-shifted and tunable fluorescent bio-probes characterized by low-toxicity, high quantum yield, long luminescence lifetime, and time stability, leading to many novel in vitro and in vivo applications based on fluorescence lifetime imaging (FLIM) and time-gated detection.Ternary AgInS2 quantum dots (QDs) have been found as promising cadmium-free, red-shifted, and tunable luminescent bio-probes with efficient Stokes and anti-Stokes excitations and luminescence lifetimes (ca. 100 ns) convenient for time resolved techniques like fluorescence life-time imaging. Although the spectral properties of the AgInS2 QDs are encouraging, the complex

  1. Intelsat VI Capture Attempt

    NASA Technical Reports Server (NTRS)

    1992-01-01

    The first single crewmember EVA capture attempt of the Intelsat VI as seen from Endeavour's aft flight deck windows. EVA Mission Specialist Pierre Thuot standing on the Remote Manipulator System (RMS) end effector platform, with the satellite capture bar attempting to attach it to the free floating communications satellite.

  2. ESL VI Curriculum Guide.

    ERIC Educational Resources Information Center

    Flander, Leonard

    This curriculum guide for English as a Second Language (ESL) Level VI is the sixth of six in a Guam Community College ESL project series. The other five guides, a companion teacher's guide and pre- and post-tests are available separately (see note). The entire project centers around the Peabody Kits P, Level P, Level 1, Level 2, Level 3, and the…

  3. Chromium(VI)

    Integrated Risk Information System (IRIS)

    Chromium ( VI ) ; CASRN 18540 - 29 - 9 Human health assessment information on a chemical substance is included in the IRIS database only after a comprehensive review of toxicity data , as outlined in the IRIS assessment development process . Sections I ( Health Hazard Assessments for Noncarcinogenic

  4. Quantum warming

    NASA Astrophysics Data System (ADS)

    2009-01-01

    The demonstration in this issue that strong magnetic confinement of electrons can dramatically increase the operating temperature of terahertz quantum cascade lasers is good news for the dream of reaching room temperature. Nature Photonics spoke with Qing Hu about the result and the future prospects.

  5. Condensation of topological defects and confinement

    NASA Astrophysics Data System (ADS)

    Gaete, Patricio; Wotzasek, Clovis

    2004-11-01

    We study the static quantum potential for a theory of antisymmetric tensor fields that results from the condensation of topological defects, within the framework of the gauge-invariant but path-dependent variables formalism. Our calculations show that the interaction energy is the sum of a Yukawa and a linear potentials, leading to the confinement of static probe charges.

  6. Exciton confinement in homo- and heteroepitaxial ZnO/Zn{sub 1-x}Mg{sub x}O quantum wells with x < 0.1

    SciTech Connect

    Laumer, Bernhard; Wassner, Thomas A.; Schuster, Fabian; Stutzmann, Martin; Schoermann, Joerg; Eickhoff, Martin; Rohnke, Marcus; Chernikov, Alexej; Bornwasser, Verena; Koch, Martin; Chatterjee, Sangam

    2011-11-01

    ZnO/Zn{sub 1-x}Mg{sub x}O single quantum well (SQW) structures with well widths d{sub W} between 1.1 nm and 10.4 nm were grown by plasma-assisted molecular beam epitaxy both heteroepitaxially on c-plane sapphire and homoepitaxially on (0001)-oriented bulk ZnO. A significantly reduced Mg incorporation in the top barrier related to the generation of stacking faults is observed for heteroepitaxial samples. Exciton localization is observed for both types of samples, while an enhancement of the exciton binding energy compared to bulk ZnO is only found for homoepitaxial SQWs for 2 nm {<=} d{sub W} {<=} 4 nm. Consistently, for homoepitaxial samples, the carrier dynamics are mainly governed by radiative recombination and carrier cooling processes at temperatures below 170 K, whereas thermally activated non-radiative recombination dominates in heteroepitaxial samples. The effects of polarization-induced electric fields are concealed for Mg concentrations x < 0.1 due to the reduction of the exciton binding energy, the screening by residual carriers as well as the asymmetric barrier structure in heteroepitaxial wells.

  7. Confined SnO2 quantum-dot clusters in graphene sheets as high-performance anodes for lithium-ion batteries.

    PubMed

    Zhu, Chengling; Zhu, Shenmin; Zhang, Kai; Hui, Zeyu; Pan, Hui; Chen, Zhixin; Li, Yao; Zhang, Di; Wang, Da-Wei

    2016-01-01

    Construction of metal oxide nanoparticles as anodes is of special interest for next-generation lithium-ion batteries. The main challenge lies in their rapid capacity fading caused by the structural degradation and instability of solid-electrolyte interphase (SEI) layer during charge/discharge process. Herein, we address these problems by constructing a novel-structured SnO2-based anode. The novel structure consists of mesoporous clusters of SnO2 quantum dots (SnO2 QDs), which are wrapped with reduced graphene oxide (RGO) sheets. The mesopores inside the clusters provide enough room for the expansion and contraction of SnO2 QDs during charge/discharge process while the integral structure of the clusters can be maintained. The wrapping RGO sheets act as electrolyte barrier and conductive reinforcement. When used as an anode, the resultant composite (MQDC-SnO2/RGO) shows an extremely high reversible capacity of 924 mAh g(-1) after 200 cycles at 100 mA g(-1), superior capacity retention (96%), and outstanding rate performance (505 mAh g(-1) after 1000 cycles at 1000 mA g(-1)). Importantly, the materials can be easily scaled up under mild conditions. Our findings pave a new way for the development of metal oxide towards enhanced lithium storage performance. PMID:27181691

  8. Confined SnO2 quantum-dot clusters in graphene sheets as high-performance anodes for lithium-ion batteries

    NASA Astrophysics Data System (ADS)

    Zhu, Chengling; Zhu, Shenmin; Zhang, Kai; Hui, Zeyu; Pan, Hui; Chen, Zhixin; Li, Yao; Zhang, Di; Wang, Da-Wei

    2016-05-01

    Construction of metal oxide nanoparticles as anodes is of special interest for next-generation lithium-ion batteries. The main challenge lies in their rapid capacity fading caused by the structural degradation and instability of solid-electrolyte interphase (SEI) layer during charge/discharge process. Herein, we address these problems by constructing a novel-structured SnO2-based anode. The novel structure consists of mesoporous clusters of SnO2 quantum dots (SnO2 QDs), which are wrapped with reduced graphene oxide (RGO) sheets. The mesopores inside the clusters provide enough room for the expansion and contraction of SnO2 QDs during charge/discharge process while the integral structure of the clusters can be maintained. The wrapping RGO sheets act as electrolyte barrier and conductive reinforcement. When used as an anode, the resultant composite (MQDC-SnO2/RGO) shows an extremely high reversible capacity of 924 mAh g‑1 after 200 cycles at 100 mA g‑1, superior capacity retention (96%), and outstanding rate performance (505 mAh g‑1 after 1000 cycles at 1000 mA g‑1). Importantly, the materials can be easily scaled up under mild conditions. Our findings pave a new way for the development of metal oxide towards enhanced lithium storage performance.

  9. Confined SnO2 quantum-dot clusters in graphene sheets as high-performance anodes for lithium-ion batteries

    PubMed Central

    Zhu, Chengling; Zhu, Shenmin; Zhang, Kai; Hui, Zeyu; Pan, Hui; Chen, Zhixin; Li, Yao; Zhang, Di; Wang, Da-Wei

    2016-01-01

    Construction of metal oxide nanoparticles as anodes is of special interest for next-generation lithium-ion batteries. The main challenge lies in their rapid capacity fading caused by the structural degradation and instability of solid-electrolyte interphase (SEI) layer during charge/discharge process. Herein, we address these problems by constructing a novel-structured SnO2-based anode. The novel structure consists of mesoporous clusters of SnO2 quantum dots (SnO2 QDs), which are wrapped with reduced graphene oxide (RGO) sheets. The mesopores inside the clusters provide enough room for the expansion and contraction of SnO2 QDs during charge/discharge process while the integral structure of the clusters can be maintained. The wrapping RGO sheets act as electrolyte barrier and conductive reinforcement. When used as an anode, the resultant composite (MQDC-SnO2/RGO) shows an extremely high reversible capacity of 924 mAh g−1 after 200 cycles at 100 mA g−1, superior capacity retention (96%), and outstanding rate performance (505 mAh g−1 after 1000 cycles at 1000 mA g−1). Importantly, the materials can be easily scaled up under mild conditions. Our findings pave a new way for the development of metal oxide towards enhanced lithium storage performance. PMID:27181691

  10. O-band quantum-confined Stark effect optical modulator from Ge/Si{sub 0.15}Ge{sub 0.85} quantum wells by well thickness tuning

    SciTech Connect

    Chaisakul, Papichaya E-mail: papichaya.chaisakul@u-psud.fr; Marris-Morini, Delphine Vakarin, Vladyslav; Vivien, Laurent; Frigerio, Jacopo; Chrastina, Daniel; Isella, Giovanni

    2014-11-21

    We report an O-band optical modulator from a Ge/Si{sub 0.15}Ge{sub 0.85} multiple quantum well (MQW). Strong O-band optical modulation in devices commonly operating within E-band wavelength range can be achieved by simply decreasing the quantum well thickness. Both spectral photocurrent and optical transmission studies are performed to evaluate material characteristics and device performance from a surface-illuminated diode and a waveguide modulator, respectively. These results demonstrate the potential of using Ge/Si{sub 0.15}Ge{sub 0.85} MQWs for the realization of future on-chip wavelength-division multiplexing systems with optical modulators operating at different wavelengths over a wide spectral range.

  11. Structure of Ice VI.

    PubMed

    Kamb, B

    1965-10-01

    Ice VI, a high-pressure form of density 1.31 g cm-(3), has a tetragonal cell of dimensions a = 6.27 A, c = 5.79 A, space group P4(2)/nmc, each cell containing ten water molecules. The structure is built up of hydrogen-bonded chdins of water molecules that are analogs of the tectosilicate chains out of which the fibrous zeolites are constructed. The chains in ice VI are linked laterally to one another to form an open, zeolite-like framework. The cavities in this framework are filled with a second framework identical with the first. The two frameworks interpenetrate but do not interconnect, and the complete structure can thus be considered a "self-clathrate." This structural feature is a natural way to achieve high density in tetrahedrally linked framework structures. PMID:17787274

  12. Intelsat VI antenna system

    NASA Astrophysics Data System (ADS)

    Caulfield, M. F.; Lane, S. O.; Taormina, F. A.

    The antenna system design of a series of five new communications satellites known as Intelsat VI is described in detail. Each satellite will utilize 50 transponders operating in the C and K band portions of the frequency spectrum. The transponders are interconnectible using either static switch matrices or a network which provides satellite switched time division multiple access capability. The antenna coverages, characteristics, and special design features are shown and discussed.

  13. Relativistic Quantum Scars

    SciTech Connect

    Huang, Liang; Lai Yingcheng; Ferry, David K.; Goodnick, Stephen M.; Akis, Richard

    2009-07-31

    The concentrations of wave functions about classical periodic orbits, or quantum scars, are a fundamental phenomenon in physics. An open question is whether scarring can occur in relativistic quantum systems. To address this question, we investigate confinements made of graphene whose classical dynamics are chaotic and find unequivocal evidence of relativistic quantum scars. The scarred states can lead to strong conductance fluctuations in the corresponding open quantum dots via the mechanism of resonant transmission.

  14. Entanglement and Quantum Optics with Quantum Dots

    NASA Astrophysics Data System (ADS)

    Burgers, A. P.; Schaibley, J. R.; Steel, D. G.

    2015-06-01

    Quantum dots (QDs) exhibit many characteristics of simpler two-level (or few level) systems, under optical excitation. This makes atomic coherent optical spectroscopy theory and techniques well suited for understanding the behavior of quantum dots. Furthermore, the combination of the solid state nature of quantum dots and their close approximation to atomic systems makes them an attractive platform for quantum information based technologies. In this chapter, we will discuss recent studies using direct detection of light emitted from a quantum dot to investigate coherence properties and confirm entanglement between the emitted photon and an electron spin qubit confined to the QD.

  15. Science-based design of stable quantum dots for energy-efficient lighting

    SciTech Connect

    Martin, James E.; Rohwer, Lauren E. S.; van Swol, Frank B.; Zhou, Xiaowang; Lu, Ping

    2015-09-01

    II-VI quantum dots, such as CdSe and CdTe, are attractive as downconversion materials for solid-state lighting, because of their narrow linewidth, tunable emission. However, for these materials to have acceptable quantum yields (QYs) requires that they be coated with a II-VI shell material whose valence band offset serves to confine the hole to the core. Confinement prevents the hole from accessing surface traps that lead to nonradiative decay of the exciton. Examples of such hole-confined core/shell QDs include CdTe/CdSe and CdSe/CdS. Unfortunately, the shell can also cause problems due to lattice mismatch, which ranges from 4-6% for systems of interest. This lattice mismatch can create significant interface energies at the heterojunction and places the core under radial compression and the shell under tangential tension. At elevated temperatures (~240°C) interfacial diffusion can relax these stresses, as can surface reconstruction, which can expose the core, creating hole traps. But such high temperatures favor the hexagonal Wurtzite structure, which has lower QY than the cubic zinc blende structure, which can be synthesized at lower temperatures, ~140°C. In the absence of alloying the core/shell structure can become metastable, or even unstable, if the shell is too thick. This can cause result in an irregular shell or even island growth. But if the shell is too thin thermallyactivated transport of the hole to surface traps can occur. In our LDRD we have developed a fundamental atomistic modeling capability, based on Stillinger-Weber and Bond-Order potentials we developed for the entire II-VI class. These pseudo-potentials have enabled us to conduct large-scale atomistic simulations that have led to the computation of phase diagrams of II-VI QDs. These phase diagrams demonstrate that at elevated temperatures the zinc blende phase of CdTe with CdSe grown on it epitaxially becomes thermodynamically unstable due to alloying. This is accompanied by a loss of hole

  16. Chemistry of the Colloidal Group II-VI Nanocrystal Synthesis

    SciTech Connect

    Liu, Haitao

    2007-05-17

    In the last two decades, the field of nanoscience andnanotechnology has witnessed tremendous advancement in the synthesis andapplication of group II-VI colloidal nanocrystals. The synthesis based onhigh temperature decomposition of organometallic precursors has becomeone of the most successful methods of making group II-VI colloidalnanocrystals. This method is first demonstrated by Bawendi and coworkersin 1993 to prepare cadmium chalcogenide colloidal quantum dots and laterextended by others to prepare other group II-VI quantum dots as well asanisotropic shaped colloidal nanocrystals, such as nanorod and tetrapod.This dissertation focuses on the chemistry of this type of nanocrystalsynthesis. The synthesis of group II-VI nanocrystals was studied bycharacterizing the molecular structures of the precursors and productsand following their time evolution in the synthesis. Based on theseresults, a mechanism was proposed to account for the 2 reaction betweenthe precursors that presumably produces monomer for the growth ofnanocrystals. Theoretical study based on density functional theorycalculations revealed the detailed free energy landscape of the precursordecomposition and monomerformation pathway. Based on the proposedreaction mechanism, a new synthetic method was designed that uses wateras a novel reagent to control the diameter and the aspect ratio of CdSeand CdS nanorods.

  17. Chemistry of the Colloidal Group II-VI Nanocrystal Synthesis

    SciTech Connect

    Liu, Haitao

    2007-05-17

    In the last two decades, the field of nanoscience andnanotechnology has witnessed tremendous advancement in the synthesis andapplication of group II-VI colloidal nanocrystals. The synthesis based onhigh temperature decomposition of organometallic precursors has becomeone of the most successful methods of making group II-VI colloidalnanocrystals. This methodis first demonstrated by Bawendi and coworkersin 1993 to prepare cadmium chalcogenide colloidal quantum dots and laterextended by others to prepare other group II-VI quantum dots as well asanisotropic shaped colloidal nanocrystals, such as nanorod and tetrapod.This dissertation focuses on the chemistry of this type of nanocrystalsynthesis. The synthesis of group II-VI nanocrystals was studied bycharacterizing the molecular structures of the precursors and productsand following their time evolution in the synthesis. Based on theseresults, a mechanism was proposed to account for the 2 reaction betweenthe precursors that presumably produces monomer for the growth ofnanocrystals. Theoretical study based on density functional theorycalculations revealed the detailed free energy landscape of the precursordecomposition and monomerformation pathway. Based on the proposedreaction mechanism, a new synthetic method was designed that uses wateras a novel reagent to control the diameter and the aspect ratio of CdSeand CdS nanorods.

  18. Chemistry of the colloidal group II-VI nanocrystal synthesis

    NASA Astrophysics Data System (ADS)

    Liu, Haitao

    In the last two decades, the field of nanoscience and nanotechnology has witnessed tremendous advancement in the synthesis and application of group II-VI colloidal nanocrystals. The synthesis based on high temperature decomposition of organometallic precursors has become one of the most successful methods of making group II-VI colloidal nanocrystals. This method is first demonstrated by Bawendi and coworkers in 1993 to prepare cadmium chalcogenide colloidal quantum dots and later extended by others to prepare other group II-VI quantum dots as well as anisotropic shaped colloidal nanocrystals, such as nanorod and tetrapod. This dissertation focuses on the chemistry of this type of nanocrystal synthesis. The synthesis of group II-VI nanocrystals was studied by characterizing the molecular structures of the precursors and products and following their time evolution in the synthesis. Based on these results, a mechanism was proposed to account for the reaction between the precursors that presumably produces monomer for the growth of nanocrystals. Theoretical study based on density functional theory calculations revealed the detailed free energy landscape of the precursor decomposition and monomer formation pathway. Based on the proposed reaction mechanism, a new synthetic method was designed that uses water as a novel reagent to control the diameter and the aspect ratio of CdSe and US nanorods.

  19. Confinement of block copolymers

    SciTech Connect

    1995-12-31

    The following were studied: confinement of block copolymers, free surface confinement, effects of substrate interactions, random copolymers at homopolymer interfaces, phase separation in thin film polymer mixtures, buffing of polymer surfaces, and near edge x-ray absorption fine structure spectroscopy.

  20. Confinement Aquaculture. Final Report.

    ERIC Educational Resources Information Center

    Delaplaine School District, AR.

    The Delaplaine Agriculture Department Confinement Project, begun in June 1988, conducted a confinement aquaculture program by comparing the growth of channel catfish raised in cages in a pond to channel catfish raised in cages in the Black River, Arkansas. The study developed technology that would decrease costs in the domestication of fish, using…

  1. Indoor Confined Feedlots.

    PubMed

    Grooms, Daniel L; Kroll, Lee Anne K

    2015-07-01

    Indoor confined feedlots offer advantages that make them desirable in northern climates where high rainfall and snowfall occur. These facilities increase the risk of certain health risks, including lameness and tail injuries. Closed confinement can also facilitate the rapid spread of infectious disease. Veterinarians can help to manage these health risks by implementing management practices to reduce their occurrence. PMID:26139194

  2. The Azimuthal Dependence of Outflows and Accretion Detected Using O VI Absorption

    NASA Astrophysics Data System (ADS)

    Kacprzak, Glenn G.; Muzahid, Sowgat; Churchill, Christopher W.; Nielsen, Nikole M.; Charlton, Jane C.

    2015-12-01

    We report a bimodality in the azimuthal angle (Φ) distribution of gas around galaxies traced by O vi absorption. We present the mean Φ probability distribution function of 29 Hubble Space Telescope-imaged O vi absorbing (EW > 0.1 Å) and 24 non-absorbing (EW < 0.1 Å) isolated galaxies (0.08 \\lt z \\lt 0.67) within ˜200 kpc of background quasars. We show that equivalent width (EW) is anti-correlated with impact parameter and O vi covering fraction decreases from 80% within 50 kpc to 33% at 200 kpc. The presence of O vi absorption is azimuthally dependent and occurs between ±10°-20° of the galaxy projected major axis and within ±30° of the projected minor axis. We find higher EWs along the projected minor axis with weaker EWs along the project major axis. Highly inclined galaxies have the lowest covering fractions due to minimized outflow/inflow cross-section geometry. Absorbing galaxies also have bluer colors while non-absorbers have redder colors, suggesting that star formation is a key driver in the O vi detection rate. O vi surrounding blue galaxies exists primarily along the projected minor axis with wide opening angles while O vi surrounding red galaxies exists primarily along the projected major axis with smaller opening angles, which may explain why absorption around red galaxies is less frequently detected. Our results are consistent with a circumgalactic medium (CGM) originating from major axis-fed inflows/recycled gas and from minor axis-driven outflows. Non-detected O vi occurs between Φ = 20°-60°, suggesting that O vi is not mixed throughout the CGM and remains confined within the outflows and the disk-plane. We find low O vi covering fractions within +/- 10^\\circ of the projected major axis, suggesting that cool dense gas resides in a narrow planer geometry surrounded by diffuse O vi gas.

  3. Elastic membranes in confinement.

    PubMed

    Bostwick, J B; Miksis, M J; Davis, S H

    2016-07-01

    An elastic membrane stretched between two walls takes a shape defined by its length and the volume of fluid it encloses. Many biological structures, such as cells, mitochondria and coiled DNA, have fine internal structure in which a membrane (or elastic member) is geometrically 'confined' by another object. Here, the two-dimensional shape of an elastic membrane in a 'confining' box is studied by introducing a repulsive confinement pressure that prevents the membrane from intersecting the wall. The stage is set by contrasting confined and unconfined solutions. Continuation methods are then used to compute response diagrams, from which we identify the particular membrane mechanics that generate mitochondria-like shapes. Large confinement pressures yield complex response diagrams with secondary bifurcations and multiple turning points where modal identities may change. Regions in parameter space where such behaviour occurs are then mapped. PMID:27440257

  4. Quantum Dots

    NASA Astrophysics Data System (ADS)

    Tartakovskii, Alexander

    2012-07-01

    Lithographic Techniques: III-V Semiconductors and Carbon: 15. Electrically controlling single spin coherence in semiconductor nanostructures Y. Dovzhenko, K. Wang, M. D. Schroer and J. R. Petta; 16. Theory of electron and nuclear spins in III-V semiconductor and carbon-based dots H. Ribeiro and G. Burkard; 17. Graphene quantum dots: transport experiments and local imaging S. Schnez, J. Guettinger, F. Molitor, C. Stampfer, M. Huefner, T. Ihn and K. Ensslin; Part VI. Single Dots for Future Telecommunications Applications: 18. Electrically operated entangled light sources based on quantum dots R. M. Stevenson, A. J. Bennett and A. J. Shields; 19. Deterministic single quantum dot cavities at telecommunication wavelengths D. Dalacu, K. Mnaymneh, J. Lapointe, G. C. Aers, P. J. Poole, R. L. Williams and S. Hughes; Index.

  5. Fluorescence from Individual PbS Quantum Dots

    NASA Astrophysics Data System (ADS)

    Peterson, Jeffrey

    2005-03-01

    Due to their extremely large electron, hole, and exciton Bohr radii, PbS quantum dots (QDs) can achieve levels of quantum confinement not accessible to III-V and II-VI QDs. Thus, the strong confinement regime is attained for relatively large particles, which may mitigate deleterious surface effects and impart novel properties. PbS QDs are also optically active in the near-infrared region, making these materials potentially useful for telecommunications and biotechnological applications. We will present investigations of single PbS QD fluorescence using far-field microscopy. PbS QDs were synthesized with a size-tunable exciton absorbance ranging between 765 nm and 1800 nm. Of particular note is the ability to synthesize highly luminescent, small radii QDs, allowing for fluorescence detection with high sensitivity silicon CCDs. Upon spincoating QDs onto glass substrates at densities near the single dot level, we observe fluorescence intermittency, or “blinking” and a narrowing of the fluorescence spectra relative to the ensemble, both hallmarks of single fluorophores. The fluorescence energy irreversibly blue shifts with longer integration times and higher excitation intensities, indicative of a photo-induced degradation. Photobleaching of the majority of PbS QDs occurred in 30 sec. An analysis of the blinking statistics will be discussed.

  6. Plasmons under extreme dimensional confinement

    NASA Astrophysics Data System (ADS)

    Weitering, Hanno

    2012-02-01

    In our studies, we explore how surface and bulk plasmons emerge under extreme dimensional confinement, i.e., dimensions that are orders of magnitude smaller than those employed in `nanoplasmonics'. Atomically-smooth ultrathin Mg films were epitaxially grown on Si(111), allowing for atomically-precise tuning of the plasmon response.ootnotetextM.M. "Ozer, E.J. Moon, A.G. Eguiluz, and H.H. Weitering, Phys. Rev. Lett. 106, 197601 (2011). While the single-particle states in these 3-12 monolayer (ML) thick films consist of a series of two-dimensional subbands, the bulk-plasmon response is like that of a thin slice carved from bulk Mg subject to quantum-mechanical boundary conditions. Remarkably, this bulk-like behavior persists all the way down to 3 ML. In the 3-12 ML thickness range, bulk loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. The collective response of the thinnest films is furthermore characterized by a thickness-dependent spectral weight transfer from the high-energy collective modes to the low-energy single-particle excitations, until the bulk plasmon ceases to exist below 3 ML. Surface- and multipole plasmon modes even persist down to 2 ML. These results are striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures. They furthermore suggest the intriguing possibility of tuning resonant plasmon frequencies via precise dimensional control.

  7. Elastic membranes in confinement

    NASA Astrophysics Data System (ADS)

    Bostwick, Joshua; Miksis, Michael; Davis, Stephen

    2014-11-01

    An elastic membrane stretched between two walls takes a shape defined by its length and the volume of fluid it encloses. Many biological structures, such as cells, mitochondria and DNA, have finer internal structure in which a membrane (or elastic member) is geometrically ``confined'' by another object. We study the shape stability of elastic membranes in a ``confining'' box and introduce repulsive van der Waals forces to prevent the membrane from intersecting the wall. We aim to define the parameter space associated with mitochondria-like deformations. We compare the confined to `unconfined' solutions and show how the structure and stability of the membrane shapes changes with the system parameters.

  8. Confinement of Coulomb balls

    SciTech Connect

    Arp, O.; Block, D.; Klindworth, M.; Piel, A.

    2005-12-15

    A model for the confinement of the recently discovered Coulomb balls is proposed. These spherical three-dimensional plasma crystals are trapped inside a rf discharge under gravity conditions and show an unusual structural order in complex plasmas. Measurements of the thermophoretic force acting on the trapped dust particles and simulations of the plasma properties of the discharge are presented. The proposed model of confinement considers thermophoretic, ion-drag, and electric field forces, and shows excellent agreement with the observations. The findings suggest that self-confinement does not significantly contribute to the structural properties of Coulomb balls.

  9. Polymer Crystallization under Confinement

    NASA Astrophysics Data System (ADS)

    Floudas, George

    Recent efforts indicated that polymer crystallization under confinement can be substantially different from the bulk. This can have important technological applications for the design of polymeric nanofibers with tunable mechanical strength, processability and optical clarity. However, the question of how, why and when polymers crystallize under confinement is not fully answered. Important studies of polymer crystallization confined to droplets and within the spherical nanodomains of block copolymers emphasized the interplay between heterogeneous and homogeneous nucleation. Herein we report on recent studies1-5 of polymer crystallization under hard confinement provided by model self-ordered AAO nanopores. Important open questions here are on the type of nucleation (homogeneous vs. heterogeneous), the size of critical nucleus, the crystal orientation and the possibility to control the overall crystallinity. Providing answers to these questions is of technological relevance for the understanding of nanocomposites containing semicrystalline polymers. In collaboration with Y. Suzuki, H. Duran, M. Steinhart, H.-J. Butt.

  10. Bacteria in Confined Spaces

    NASA Astrophysics Data System (ADS)

    Wilking, Connie; Weitz, David

    2010-03-01

    Bacterial cells can display differentiation between several developmental pathways, from planktonic to matrix-producing, depending upon the colony conditions. We study the confinement of bacteria in hydrogels as well as in liquid-liquid double emulsion droplets and observe the growth and morphology of these colonies as a function of time and environment. Our results can give insight into the behavior of bacterial colonies in confined spaces that can have applications in the areas of food science, cosmetics, and medicine.

  11. Fusion, magnetic confinement

    SciTech Connect

    Berk, H.L.

    1992-08-06

    An overview is presented of the principles of magnetic confinement of plasmas for the purpose of achieving controlled fusion conditions. Sec. 1 discusses the different nuclear fusion reactions which can be exploited in prospective fusion reactors and explains why special technologies need to be developed for the supply of tritium or {sup 3}He, the probable fuels. In Sec. 2 the Lawson condition, a criterion that is a measure of the quality of confinement relative to achieving fusion conditions, is explained. In Sec. 3 fluid equations are used to describe plasma confinement. Specific confinement configurations are considered. In Sec. 4 the orbits of particle sin magneti and electric fields are discussed. In Sec. 5 stability considerations are discussed. It is noted that confinement systems usually need to satisfy stability constraints imposed by ideal magnetohydrodynamic (MHD) theory. The paper culminates with a summary of experimental progress in magnetic confinement. Present experiments in tokamaks have reached the point that the conditions necessary to achieve fusion are being satisfied.

  12. Confined electronic states and their modulations in graphene nanorings

    NASA Astrophysics Data System (ADS)

    Zhu, Jia-Lin; Wang, Xingyuan; Yang, Ning

    2012-09-01

    Confined electronic states in quantum rings formed by spatially modulated finite Dirac gap (FDGQR) in graphene are systematically studied by series-expansion method, and are compared with those in infinite-mass-boundary and one-dimensional quantum rings. The shape-size effect of FDGQR is illustrated to be distinct from that in graphene quantum dots. The Aharonov-Bohm effect in FDGQR is clearly shown by the energy spectrum and the optical-transition probabilities. The FDGQR coupled with the electrostatic-potential induced nanoring is found useful for modulating the Dirac electronic states and the optical-transition probabilities. These results may help us to understand and to control the quantum behaviors of confined electronic states in graphene.

  13. Confining collective motion

    NASA Astrophysics Data System (ADS)

    Bartolo, Denis; Bricard, Antoine; Caussin, Jean-Baptiste; Savoie, Charles; Das, Debasish; Chepizhko, Oleskar; Peruani, Fernando; Saintillan, David

    2014-11-01

    It is well established that geometrical confinement have a significant impact on the structure and the flow properties of complex fluids. Prominent examples include the formation of topological defects in liquid crystals, and the flow instabilities of viscoelastic fluids in curved geometries. In striking contrast very little is known about the macroscopic behavior of confined active fluids. In this talk we show how to motorize plastic colloidal beads and turn them into self-propelled particles. Using microfluidic geometries we demonstrate how confinement impacts their collective motion. Combining quantitative experiments, analytical theory and numerical simulations we show how a population of motile bodies interacting via alignement and repulsive interactions self-organizes into a single heterogeneous macroscopic vortex that lives on the verge of a phase separation.

  14. Order, Disorder and Confinement

    SciTech Connect

    D'Elia, M.; Di Giacomo, A.; Pica, C.

    2006-01-12

    Studying the order of the chiral transition for Nf = 2 is of fundamental importance to understand the mechanism of color confinement. We present results of a numerical investigation on the order of the transition by use of a novel strategy in finite size scaling analysis. The specific heat and a number of susceptibilities are compared with the possible critical behaviours. A second order transition in the O(4) and O(2) universality classes are excluded. Substantial evidence emerges for a first order transition. Results are in agreement with those found by studying the scaling properties of a disorder parameter related to the dual superconductivity mechanism of color confinement.

  15. Self-organized MBE growth of II VI epilayers on patterned GaSb substrates

    NASA Astrophysics Data System (ADS)

    Wissmann, H.; Tran Anh, T.; Rogaschewski, S.; von Ortenberg, M.

    1999-05-01

    We report on the self-organized MBE growth of II-VI epilayers on patterned and unpatterned GaSb substrates resulting in quantum wires and quantum wells, respectively. The HgSe : Fe quantum wires were grown on (0 0 1)GaSb substrates with a buffer of lattice-matched ZnTe 1- xSe x. Due to the anisotropic growth of HgSe on the A-oriented stripes roof-like overgrowth with a definite ridge was obtained. Additional Fe doping in the direct vicinity of the ridge results in a highly conductive quantum wire.

  16. Salivary carbonic anhydrase isoenzyme VI

    PubMed Central

    Kivelä, Jyrki; Parkkila, Seppo; Parkkila, Anna-Kaisa; Leinonen, Jukka; Rajaniemi, Hannu

    1999-01-01

    The carbonic anhydrases (CAs) participate in the maintenance of pH homeostasis in various tissues and biological fluids of the human body by catalysing the reversible reaction CO2+ H2O ⇌ HCO3−+ H+ (Davenport & Fisher, 1938; Davenport, 1939; Maren, 1967). Carbonic anhydrase isoenzyme VI (CA VI) is the only secretory isoenzyme of the mammalian CA gene family. It is exclusively expressed in the serous acinar cells of the parotid and submandibular glands, from where it is secreted into the saliva. In this review, we will discuss recent advances in research focused on the physiological role of salivary CA VI in the oral cavity and upper alimentary canal. PMID:10523402

  17. Collagen VI related muscle disorders

    PubMed Central

    Lampe, A; Bushby, K

    2005-01-01

    Mutations in the genes encoding collagen VI (COL6A1, COL6A2, and COL6A3) cause Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD), two conditions which were previously believed to be completely separate entities. BM is a relatively mild dominantly inherited disorder characterised by proximal weakness and distal joint contractures. UCMD was originally described as an autosomal recessive condition causing severe muscle weakness with proximal joint contractures and distal hyperlaxity. Here we review the clinical phenotypes of BM and UCMD and their diagnosis and management, and provide an overview of the current knowledge of the pathogenesis of collagen VI related disorders. PMID:16141002

  18. Fractional statistics and confinement

    NASA Astrophysics Data System (ADS)

    Gaete, P.; Wotzasek, C.

    2005-02-01

    It is shown that a pointlike composite having charge and magnetic moment displays a confining potential for the static interaction while simultaneously obeying fractional statistics in a pure gauge theory in three dimensions, without a Chern-Simons term. This result is distinct from the Maxwell-Chern-Simons theory that shows a screening nature for the potential.

  19. Plasma confinement at JET

    NASA Astrophysics Data System (ADS)

    Nunes, I.; JET Contributors

    2016-01-01

    Operation with a Be/W wall at JET (JET-ILW) has an impact on scenario development and energy confinement with respect to the carbon wall (JET-C). The main differences observed were (1) strong accumulation of W in the plasma core and (2) the need to mitigate the divertor target temperature to avoid W sputtering by Be and other low Z impurities and (3) a decrease of plasma energy confinement. A major difference is observed on the pedestal pressure, namely a reduction of the pedestal temperature which, due to profile stiffness the plasma core temperature is also reduced leading to a degradation of the global confinement. This effect is more pronounced in low β N scenarios. At high β N, the impact of the wall on the plasma energy confinement is mitigated by the weaker plasma energy degradation with power relative to the IPB98(y, 2) scaling calculated empirically for a CFC first wall. The smaller tolerable impurity concentration for tungsten (<10-5) compared to that of carbon requires the use of electron heating methods to prevent W accumulation in the plasma core region as well as gas puffing to avoid W entering the plasma core by ELM flushing and reduction of the W source by decreasing the target temperature. W source and the target temperature can also be controlled by impurity seeding. Nitrogen and Neon have been used and with both gases the reduction of the W source and the target temperature is observed. Whilst more experiments with Neon are necessary to assess its impact on energy confinement, a partial increase of plasma energy confinement is observed with Nitrogen, through the increase of edge temperature. The challenge for scenario development at JET is to extend the pulse length curtailed by its transient behavior (W accumulation or MHD), but more importantly by the divertor target temperature limits. Re-optimisation of the scenarios to mitigate the effect of the change of wall materials maintaining high global energy confinement similar to JET-C is

  20. The Radioelectric effect in doped superlattices under the influence of confined phonon

    NASA Astrophysics Data System (ADS)

    Quang Bau, Nguyen; Thu Hang, Dao; Long, Do Tuan

    2016-06-01

    The Radioelectric effect in doped superlattices under the influence of confined phonon has been theoretically studied. The analytical expression for the Radioelectric field is obtained by quantum kinetic equation method. The theoretical expression shows that the Radioelectric field in doped superlattices depends on the frequencies and amplitudes of the laser and the linearly polarized electromagnetic wave, the period of the superlattices and especially the quantum number m characterizing the phonon confinement. Numerical calculation is also applied for GaAs:Si/GaAs:Be doped superlattices. It is found that the Radioelectric field is different from that in the normal bulk semiconductor as well as two-dimensional systems in case of unconfined phonon and in case of confined phonon when the contribution of confined potential of doped superlattices and confined phonon is remarkable. The Radioelectric field has multiple resonance peaks and increases as the increasing of quantum number m.

  1. Electrofreezing of confined water.

    PubMed

    Zangi, Ronen; Mark, Alan E

    2004-04-15

    We report results from molecular dynamics simulations of the freezing transition of TIP5P water molecules confined between two parallel plates under the influence of a homogeneous external electric field, with magnitude of 5 V/nm, along the lateral direction. For water confined to a thickness of a trilayer we find two different phases of ice at a temperature of T=280 K. The transformation between the two, proton-ordered, ice phases is found to be a strong first-order transition. The low-density ice phase is built from hexagonal rings parallel to the confining walls and corresponds to the structure of cubic ice. The high-density ice phase has an in-plane rhombic symmetry of the oxygen atoms and larger distortion of hydrogen bond angles. The short-range order of the two ice phases is the same as the local structure of the two bilayer phases of liquid water found recently in the absence of an electric field [J. Chem. Phys. 119, 1694 (2003)]. These high- and low-density phases of water differ in local ordering at the level of the second shell of nearest neighbors. The results reported in this paper, show a close similarity between the local structure of the liquid phase and the short-range order of the corresponding solid phase. This similarity might be enhanced in water due to the deep attractive well characterizing hydrogen bond interactions. We also investigate the low-density ice phase confined to a thickness of 4, 5, and 8 molecular layers under the influence of an electric field at T=300 K. In general, we find that the degree of ordering decreases as the distance between the two confining walls increases. PMID:15267616

  2. Quadrupole interactions in tetraoxoferrates (VI)

    NASA Astrophysics Data System (ADS)

    Dedushenko, Sergey K.; Perfiliev, Yurii D.; Rusakov, Vyacheslav S.; Gapochka, Alexei M.

    2013-05-01

    An applicability of the point charge approach for calculations of quadrupole splittings in Mössbauer spectra of ferrates(VI) was studied. The reasonable correlation between calculated and experimental splittings was observed for the majority of ferrates excepting K3Na(FeO4)2. The comparison of ferrates and chromates was made using calculated nucleus independent coefficient.

  3. From screening to confinement in a gauge-invariant formalism

    NASA Astrophysics Data System (ADS)

    Gaete, Patricio; Schmidt, Iván

    2000-06-01

    Features of screening and confinement are reviewed in two-dimensional quantum electrodynamics. Our discussion is carried out using the gauge-invariant but path-dependent variables formalism. This alternative and useful approach exploits the rich structure of the electromagnetic cloud or dressing around static fermions in a straightforward and simple way.

  4. Molecular-beam epitaxy of heterostructures of wide-gap II–VI compounds for low-threshold lasers with optical and electron pumping

    SciTech Connect

    Sorokin, S. V. Gronin, S. V.; Sedova, I. V.; Rakhlin, M. V.; Baidakova, M. V.; Kop’ev, P. S.; Vainilovich, A. G.; Lutsenko, E. V.; Yablonskii, G. P.; Gamov, N. A.; Zhdanova, E. V.; Zverev, M. M.; Ruvimov, S. S.; Ivanov, S. V.

    2015-03-15

    The paper presents basic approaches in designing and growing by molecular beam epitaxy of (Zn,Mg)(S,Se)-based laser heterostructures with multiple CdSe quantum dot (QD) sheets or ZnCdSe quantum wells (QW). The method of calculation of compensating short-period ZnSSe/ZnSe superlattices (SLs) in both active and waveguide regions of laser heterostructures possessing the different waveguide thickness and different number of active regions is presented. The method allowing reduction of the density of nonequilibrium point defects in the active region of the II–VI laser structures has been proposed. It utilizes the migration enhanced epitaxy mode in growing the ZnSe QW confining the CdSe QD sheet. The threshold power density as low as P{sub thr} ∼ 0.8 kW/cm{sup 2} at T = 300 K has been demonstrated for laser heterostructure with single CdSe QD sheet and asymmetric graded-index waveguide with strain-compensating SLs.

  5. ZnTe/MnTe: A new metastable wide gap II VI heterostructure

    NASA Astrophysics Data System (ADS)

    Durbin, S.; Kobayashi, M.; Fu, Qiang; Pelekanos, N.; Gunshor, R. L.; Nurmikko, A. V.

    1990-04-01

    Optical characteristics of a new metastable wide-gap II-VI semiconductor heterostructure ZnTe/MnTe are reported. Single ZnTe/MnTe quantum wells show strong n = 1 exciton resonance manifesting, for example, in pronounced enhancement of the Raman cross-section. Pseudomorphic nature of the structures is inferred from strain induced shifts in the optical phonon spectrum.

  6. Protostars and Planets VI

    NASA Astrophysics Data System (ADS)

    Beuther, Henrik; Klessen, Ralf S.; Dullemond, Cornelis P.; Henning, Thomas

    star and planet formation. They are used by students to dive into new topics, and they are much valued by experienced researchers as a comprehensive overview of the field with all its interactions. We hope that you will enjoy reading (and learning from) this book as much as we do. The organization of the Protostars and Planets conference was carried out in close collaboration between the Max Planck Institute for Astronomy and the Center for Astronomy of the University Heidelberg, with generous support from the German Science Foundation. This volume is a product of effort and care by many people. First and foremost, we want to acknowledge the 250 contributing authors, as it is only due to their expertise and knowledge that such a comprehensive review compendium in all its depth and breadth is possible. The Protostars and Planets VI conference and this volume was a major undertaking, with support and contributions by many people and institutions. We like to thank the members of the Scientific Advisory Committee who selected the 38 teams and chapters out of more than 120 submitted proposals. Similarly, we are grateful to the reviewers, who provided valuable input and help to the chapter authors. The book would also not have been possible without the great support of Renée Dotson and other staff from USRA’s Lunar and Planetary Institute, who handled the detailed processing of all manuscripts and the production of the book, and of Allyson Carter and other staff from the University of Arizona Press. We are also grateful to Richard Binzel, the General Editor of the Space Science Series, for his constant support during the long process, from the original concept to this final product. Finally, we would like to express a very special thank you to the entire conference local organizing committee, and in particular, Carmen Cuevas and Natali Jurina, for their great commitment to the project and for a very fruitful and enjoyable collaboration.

  7. Heterogeneity of Collagen VI Microfibrils

    PubMed Central

    Maaß, Tobias; Bayley, Christopher P.; Mörgelin, Matthias; Lettmann, Sandra; Bonaldo, Paolo; Paulsson, Mats; Baldock, Clair; Wagener, Raimund

    2016-01-01

    Collagen VI, a collagen with uncharacteristically large N- and C-terminal non-collagenous regions, forms a distinct microfibrillar network in most connective tissues. It was long considered to consist of three genetically distinct α chains (α1, α2, and α3). Intracellularly, heterotrimeric molecules associate to form dimers and tetramers, which are then secreted and assembled to microfibrils. The identification of three novel long collagen VI α chains, α4, α5, and α6, led to the question if and how these may substitute for the long α3 chain in collagen VI assembly. Here, we studied structural features of the novel long chains and analyzed the assembly of these into tetramers and microfibrils. N- and C-terminal globular regions of collagen VI were recombinantly expressed and studied by small angle x-ray scattering (SAXS). Ab initio models of the N-terminal globular regions of the α4, α5, and α6 chains showed a C-shaped structure similar to that found for the α3 chain. Single particle EM nanostructure of the N-terminal globular region of the α4 chain confirmed the C-shaped structure revealed by SAXS. Immuno-EM of collagen VI extracted from tissue revealed that like the α3 chain the novel long chains assemble to homotetramers that are incorporated into mixed microfibrils. Moreover, SAXS models of the C-terminal globular regions of the α1, α2, α4, and α6 chains were generated. Interestingly, the α1, α2, and α4 C-terminal globular regions dimerize. These self-interactions may play a role in tetramer formation. PMID:26742845

  8. Totally confined explosive welding

    NASA Technical Reports Server (NTRS)

    Bement, L. J. (Inventor)

    1978-01-01

    The undesirable by-products of explosive welding are confined and the association noise is reduced by the use of a simple enclosure into which the explosive is placed and in which the explosion occurs. An infrangible enclosure is removably attached to one of the members to be bonded at the point directly opposite the bond area. An explosive is completely confined within the enclosure at a point in close proximity to the member to be bonded and a detonating means is attached to the explosive. The balance of the enclosure, not occupied by explosive, is filled with a shaped material which directs the explosive pressure toward the bond area. A detonator adaptor controls the expansion of the enclosure by the explosive force so that the enclosure at no point experiences a discontinuity in expansion which causes rupture. The use of the technique is practical in the restricted area of a space station.

  9. Quantum Dots: An Experiment for Physical or Materials Chemistry

    ERIC Educational Resources Information Center

    Winkler, L. D.; Arceo, J. F.; Hughes, W. C.; DeGraff, B. A.; Augustine, B. H.

    2005-01-01

    An experiment is conducted for obtaining quantum dots for physical or materials chemistry. This experiment serves to both reinforce the basic concept of quantum confinement and providing a useful bridge between the molecular and solid-state world.

  10. Topological confinement and superconductivity

    SciTech Connect

    Al-hassanieh, Dhaled A; Batista, Cristian D

    2008-01-01

    We derive a Kondo Lattice model with a correlated conduction band from a two-band Hubbard Hamiltonian. This mapping allows us to describe the emergence of a robust pairing mechanism in a model that only contains repulsive interactions. The mechanism is due to topological confinement and results from the interplay between antiferromagnetism and delocalization. By using Density-Matrix-Renormalization-Group (DMRG) we demonstrate that this mechanism leads to dominant superconducting correlations in aID-system.

  11. Classical confined particles

    NASA Technical Reports Server (NTRS)

    Horzela, Andrzej; Kapuscik, Edward

    1993-01-01

    An alternative picture of classical many body mechanics is proposed. In this picture particles possess individual kinematics but are deprived from individual dynamics. Dynamics exists only for the many particle system as a whole. The theory is complete and allows to determine the trajectories of each particle. It is proposed to use our picture as a classical prototype for a realistic theory of confined particles.

  12. Inertial Confinement fusion targets

    NASA Technical Reports Server (NTRS)

    Hendricks, C. D.

    1982-01-01

    Inertial confinement fusion (ICF) targets are made as simple flat discs, as hollow shells or as complicated multilayer structures. Many techniques were devised for producing the targets. Glass and metal shells are made by using drop and bubble techniques. Solid hydrogen shells are also produced by adapting old methods to the solution of modern problems. Some of these techniques, problems, and solutions are discussed. In addition, the applications of many of the techniques to fabrication of ICF targets is presented.

  13. Confined Vortex Scrubber

    SciTech Connect

    Not Available

    1990-02-01

    The program objective is to demonstrate efficient removal of fine particulates to sufficiently low levels to meet proposed small scale coal combustor emission standards. This is to be accomplished using a novel particulate removal device, the Confined Vortex Scrubber. This is the first quarterly technical progress report under this contract. Accordingly, a summary of the cleanup concept and the structure of the program is given here.

  14. Energy confinement in tokamaks

    SciTech Connect

    Sugihara, M.; Singer, C.

    1986-08-01

    A straightforward generalization is made of the ohmic heating energy confinement scalings of Pfeiffer and Waltz and Blackwell et. al. The resulting model is systematically calibrated to published data from limiter tokamaks with ohmic, electron cyclotron, and neutral beam heating. With considerably fewer explicitly adjustable free parameters, this model appears to give a better fit to the available data for limiter discharges than the combined ohmic/auxiliary heating model of Goldston.

  15. Freezing in confined geometries

    NASA Technical Reports Server (NTRS)

    Sokol, P. E.; Ma, W. J.; Herwig, K. W.; Snow, W. M.; Wang, Y.; Koplik, Joel; Banavar, Jayanth R.

    1992-01-01

    Results of detailed structural studies, using elastic neutron scattering, of the freezing of liquid O2 and D2 in porous vycor glass, are presented. The experimental studies have been complemented by computer simulations of the dynamics of freezing of a Lennard-Jones liquid in narrow channels bounded by molecular walls. Results point to a new simple physical interpretation of freezing in confined geometries.

  16. In situ grown quantum-wire lasers

    NASA Astrophysics Data System (ADS)

    Coldren, L. A.; Gossard, A. C.; English, J. H.; Mui, D.; Corzine, S. W.

    1994-04-01

    This program concentrated on developing techniques to better understand and fabricate quantum-confined structures. The goal was to create the enabling technology for more efficient semiconductor lasers and integrated optoelectronic circuits. Over the contract period, significant advances occurred in the development of quantum-confined lasers, UHV in-situ processing technology, and the underlying theory for quantum-confined laser structures. The quantum-confined laser work included both quantum-wire laser and vertical-cavity laser development. This latter effort also required substantial improvements in the MBE growth technology. Much of this technology is now ready for transfer to industry. In fact, a number of joint projects with industry are underway, as a result of this program.

  17. Electroluminescence of quantum-dash-based quantum cascade laser structures

    SciTech Connect

    Liverini, V.; Bismuto, A.; Nevou, L.; Beck, M.; Faist, J.

    2011-12-23

    We developed two mid-infrared quantum cascade structures based on InAs quantum dashes. The dashes were embedded either in AlInGaAs lattice-matched to InP or in tensile-strained AlInAs. The devices emit between 7 and 11 {mu}m and are a step forward in the development of quantum cascade lasers based on 3-D confined active regions.

  18. Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

    NASA Astrophysics Data System (ADS)

    Barman, B.; Oszwałdowski, R.; Schweidenback, L.; Russ, A. H.; Pientka, J. M.; Tsai, Y.; Chou, W.-C.; Fan, W. C.; Murphy, J. R.; Cartwright, A. N.; Sellers, I. R.; Petukhov, A. G.; Žutić, I.; McCombe, B. D.; Petrou, A.

    2015-07-01

    We used continuous wave photoluminescence (cw-PL) and time-resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe /(Zn ,Mn )Se system the holes are confined in the nonmagnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn ,Mn )Te /ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding nonmagnetic ZnSe matrix. The magnetic polaron formation energies EMP in both systems were measured from the temporal redshift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe /(Zn ,Mn )Se system the magnetic polaron shows conventional behavior with EMP decreasing with increasing temperature T and increasing magnetic field B . In contrast, EMP in the (Zn ,Mn )Te /ZnSe system has unconventional dependence on temperature T and magnetic field B ; EMP is weakly dependent on T as well as on B . We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.

  19. Nanowire terahertz quantum cascade lasers

    SciTech Connect

    Grange, Thomas

    2014-10-06

    Quantum cascade lasers made of nanowire axial heterostructures are proposed. The dissipative quantum dynamics of their carriers is theoretically investigated using non-equilibrium Green functions. Their transport and gain properties are calculated for varying nanowire thickness, from the classical-wire regime to the quantum-wire regime. Our calculation shows that the lateral quantum confinement provided by the nanowires allows an increase of the maximum operation temperature and a strong reduction of the current density threshold compared to conventional terahertz quantum cascade lasers.

  20. Summary report of session VI

    SciTech Connect

    Weiren Chou et al.

    2002-08-19

    This report gives a brief review of the presentations in Session VI of the Ecloud'02 Workshop and summarizes the major points during the discussions. Some points (e.g., the critical mass phenomenon) are not conclusive and even controversial. But it has been agreed that further investigations are warranted. The topic of Session VI in the Ecloud'02 workshop is ''Discussions of future studies, collaborations and possible solutions.'' Half of the session is devoted to presentations, another half to discussions. This report will focus on the latter. There are six presentations: (1) R. Macek, Possible cures to the e-cloud problem; (2) G. Rumolo, Driving the electron-cloud instability by an electron cooler; (3) U. Iriso Ariz, RF test benches for electron-cloud studies; (4) F. Caspers, Stealth clearing electrodes; (5) F. Ruggiero, Future electron-cloud studies at CERN; and (6) E. Perevedentsev, Beam-beam and transverse impedance model.

  1. Directional Mechanosensing in Myosin VI

    NASA Astrophysics Data System (ADS)

    Yang, Yubo; Tehver, Riina

    2013-03-01

    Myosin is a family of versatile motor proteins that perform various tasks, such as organelle transport, anchoring and cell deformation. Although the general mechanism of the motors has been fairly well established, details on dynamic aspects like force response of the motor, and force propagation are yet to be fully understood. In this poster, we present the response of the ATP binding region to force exerted on the tail domain in order to test the proposed tension-dependent gating mechanism of myosin VI processive motion. We employed the Self-Organized Polymer model in a computer simulation to explore the effect. Current results show that the ATP binding domain of myosin VI indeed exhibits tension dependence - both structurally and dynamically.

  2. Growth and characterization of CdTe single quantum wells confined by Cd 1- xZn xTe alloy and short period CdTe/ZnTe superlattice

    NASA Astrophysics Data System (ADS)

    Magnea, N.; Lentz, G.; Mariette, H.; Feuillet, G.; Dal'bo, F.; Tuffigo, H.

    1989-02-01

    A comparative study of structural and optical properties of CdTe single quantum wells has been performed. Very efficient CdTe-Cd 1-xZn xTe (x Zn ≤ 0.2) wells have been grown and short period superlattices appear as a promising substitute of the alloy.

  3. Electron Confinement in Cylindrical Potential Well

    NASA Astrophysics Data System (ADS)

    Baltenkov, A. S.; Msezane, A. Z.

    2016-05-01

    We show that studying the solutions of the wave equation for an electron confined in a cylindrical potential well offers the possibility to analyze the confinement behavior of an electron executing one- or two-dimensional motion in the remaining three-dimensional space within the framework of the same mathematical model of the potential well. Some low-lying electronic states with different symmetries are considered and the corresponding wave functions are calculated. The behavior of their nodes and their peak positions with respect to the parameters of the cylindrical well is analyzed. Additionally, the momentum distributions of electrons in these states are calculated. The limiting cases of the ratio of the cylinder length H to its radius R0 are considered; when H significantly exceeds R0 and when R0 is much greater than H. The possible application of the results obtained here for the description of the general features in the behavior of electrons in nanowires with metallic type of conductivity (or nanotubes) and ultrathin epitaxial films (or graphene sheets) are discussed. Possible experiments are suggested as well where the quantum confinement can be manifested. Work supported by the Uzbek Foundation (ASB) and by the U.S. DOE, Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, Office of Energy Research (AZM).

  4. Confined vortex scrubber

    SciTech Connect

    Not Available

    1990-07-01

    The program objective is to demonstrate efficient removal of fine particulates to sufficiently low levels to meet proposed small scale coal combustor emission standards using a cleanup technology appropriate to small scale coal combustors. This to be accomplished using a novel particulate removal device, the Confined Vortex Scrubber (CVS), which consists of a cylindrical vortex chamber with tangential flue gas inlets. The clean gas exit is via vortex finder outlets, one at either end of the tube. Liquid is introduced into the chamber and is confined within the vortex chamber by the centrifugal force generated by the gas flow itself. This confined liquid forms a layer through which the flue gas is then forced to bubble, producing a strong gas/liquid interaction, high inertial separation forces and efficient particulate cleanup. During this quarter a comprehensive series of cleanup experiments have been made for three CVS configurations. The first CVS configuration tested gave very efficient fine particulate removal at the design air mass flow rate (1 MM BUT/hr combustor exhaust flow), but had over 20{double prime}WC pressure drop. The first CVS configuration was then re-designed to produce the same very efficient particulate collection performance at a lower pressure drop. The current CVS configuration produces 99.4 percent cleanup of ultra-fine fly ash at the design air mass flow at a pressure drop of 12 {double prime}WC with a liquid/air flow ratio of 0.31/m{sup 3}. Unlike venturi scrubbers, the collection performance of the CVS is insensitive to dust loading and to liquid/air flow ratio.

  5. Duality and confinement in D = 3 models driven by condensation of topological defects

    NASA Astrophysics Data System (ADS)

    Gaete, topological defects [rapid communication] P.; Wotzasek, C.

    2005-10-01

    We study the interplay of duality and confinement in Maxwell-like three-dimensional models induced by the condensation of topological defects driven by quantum fluctuations. To this end we check for the confinement phenomenon, in both sides of the duality, using the static quantum potential as a testing ground. Our calculations are done within the framework of the gauge-invariant but path-dependent variables formalism which are alternative to the Wilson loop approach. Our results show that the interaction energy contains a linear term leading to the confinement of static probe charges.

  6. Confinement Contains Condensates

    SciTech Connect

    Brodsky, Stanley J.; Roberts, Craig D.; Shrock, Robert; Tandy, Peter C.

    2012-03-12

    Dynamical chiral symmetry breaking and its connection to the generation of hadron masses has historically been viewed as a vacuum phenomenon. We argue that confinement makes such a position untenable. If quark-hadron duality is a reality in QCD, then condensates, those quantities that have commonly been viewed as constant empirical mass-scales that fill all spacetime, are instead wholly contained within hadrons; i.e., they are a property of hadrons themselves and expressed, e.g., in their Bethe-Salpeter or light-front wave functions. We explain that this paradigm is consistent with empirical evidence, and incidentally expose misconceptions in a recent Comment.

  7. Confinement Vessel Dynamic Analysis

    SciTech Connect

    R. Robert Stevens; Stephen P. Rojas

    1999-08-01

    A series of hydrodynamic and structural analyses of a spherical confinement vessel has been performed. The analyses used a hydrodynamic code to estimate the dynamic blast pressures at the vessel's internal surfaces caused by the detonation of a mass of high explosive, then used those blast pressures as applied loads in an explicit finite element model to simulate the vessel's structural response. Numerous load cases were considered. Particular attention was paid to the bolted port connections and the O-ring pressure seals. The analysis methods and results are discussed, and comparisons to experimental results are made.

  8. Determining Individual Mineral Contributions To U(VI) Adsorption In A Contaminated Aquifer Sediment: A Fluorescence Spectroscopy Study

    SciTech Connect

    Wang, Zheming; Zachara, John M.; Boily, Jean F.; Xia, Yuanxian; Resch, Charles T.; Moore, Dean A.; Liu, Chongxuan

    2011-05-15

    The adsorption and speciation of U(VI) was investigated on contaminated, fine grained sediment materials from the Hanford 300 area (SPP1 GWF) in simulated groundwater using cryogenic laser-induced U(VI) fluorescence spectroscopy combined with chemometric analysis. A series of reference minerals (montmorillonite, illite, Michigan chlorite, North Carolina chlorite, California clinochlore, quartz and synthetic 6-line ferrihydrite) was used for comparison that represents the mineralogical constituents of SPP1 GWF. Surface area-normalized Kd values were measured at U(VI) concentrations of 5x10-7 mol L-1 and 5x10-6 mol L-1, respectively, that displayed the following affinity series: 6-line-ferrihydrite > North Carolina chlorite ≈ California clinochlore > Michigan chlorite ≈ quartz > montmorillonite ≈ illite ≈ SPP1 GWF. Both time-resolved spectra and asynchronous two-dimensional (2D) correlation analysis of SPP1 GWF at different delay times indicated that two major adsorbed U(VI) species were present in the sediment that resembled U(VI) adsorbed on quartz and phyllosilicates. Simulations of the normalized fluorescence spectra confirmed that the speciation of SPP1 GWF was best represented by a linear combination of U(VI) adsorbed on quartz (90%) and phyllosilicates (10%). However, the fluorescence quantum yield for U(VI) adsorbed on phyllosilicates was lower than quartz and, consequently, its fractional contribution to speciation may be underestimated. Spectral comparison with literature data suggested that U(VI) exists primarily as inner-sphere U(VI) complexes with surface silanol groups on quartz while U(VI) on phyllosilicates was consistent with the formation of surface U(VI) tricarbonate complexes.

  9. Mobility in geometrically confined membranes.

    PubMed

    Domanov, Yegor A; Aimon, Sophie; Toombes, Gilman E S; Renner, Marianne; Quemeneur, François; Triller, Antoine; Turner, Matthew S; Bassereau, Patricia

    2011-08-01

    Lipid and protein lateral mobility is essential for biological function. Our theoretical understanding of this mobility can be traced to the seminal work of Saffman and Delbrück, who predicted a logarithmic dependence of the protein diffusion coefficient (i) on the inverse of the size of the protein and (ii) on the "membrane size" for membranes of finite size [Saffman P, Delbrück M (1975) Proc Natl Acad Sci USA 72:3111-3113]. Although the experimental proof of the first prediction is a matter of debate, the second has not previously been thought to be experimentally accessible. Here, we construct just such a geometrically confined membrane by forming lipid bilayer nanotubes of controlled radii connected to giant liposomes. We followed the diffusion of individual molecules in the tubular membrane using single particle tracking of quantum dots coupled to lipids or voltage-gated potassium channels KvAP, while changing the membrane tube radius from approximately 250 to 10 nm. We found that both lipid and protein diffusion was slower in tubular membranes with smaller radii. The protein diffusion coefficient decreased as much as 5-fold compared to diffusion on the effectively flat membrane of the giant liposomes. Both lipid and protein diffusion data are consistent with the predictions of a hydrodynamic theory that extends the work of Saffman and Delbrück to cylindrical geometries. This study therefore provides strong experimental support for the ubiquitous Saffman-Delbrück theory and elucidates the role of membrane geometry and size in regulating lateral diffusion. PMID:21768336

  10. The Properties of Confined Water and Fluid Flow at the Nanoscale

    SciTech Connect

    Schwegler, E; Reed, J; Lau, E; Prendergast, D; Galli, G; Grossman, J C; Cicero, G

    2009-03-09

    This project has been focused on the development of accurate computational tools to study fluids in confined, nanoscale geometries, and the application of these techniques to probe the structural and electronic properties of water confined between hydrophilic and hydrophobic substrates, including the presence of simple ions at the interfaces. In particular, we have used a series of ab-initio molecular dynamics simulations and quantum Monte Carlo calculations to build an understanding of how hydrogen bonding and solvation are modified at the nanoscale. The properties of confined water affect a wide range of scientific and technological problems - including protein folding, cell-membrane flow, materials properties in confined media and nanofluidic devices.

  11. The cooling of confined ions driven by laser beams

    SciTech Connect

    Reyna, L.G.; Sobehart, J.R.

    1993-07-01

    We finalize the dynamics of confined ions driven by a quantized radiation field. The ions can absorb photons from an incident laser beam and relax back to the ground state by either induced emissions or spontaneous emissions. Here we assume that the absorption of photons is immediately followed by spontaneous emissions, resulting in single-level ions perturbed by the exchange of momentum with the radiation field. The probability distribution of the ions is calculated using singular expansions in the low noise asymptotic limit. The present calculations reproduce the quantum results in the limit of heavy particles in static traps, and the classical results of ions in radio-frequency confining wells.

  12. Amoeboid motion in confined geometry

    NASA Astrophysics Data System (ADS)

    Wu, Hao; Thiébaud, M.; Hu, W.-F.; Farutin, A.; Rafaï, S.; Lai, M.-C.; Peyla, P.; Misbah, C.

    2015-11-01

    Many eukaryotic cells undergo frequent shape changes (described as amoeboid motion) that enable them to move forward. We investigate the effect of confinement on a minimal model of amoeboid swimmer. A complex picture emerges: (i) The swimmer's nature (i.e., either pusher or puller) can be modified by confinement, thus suggesting that this is not an intrinsic property of the swimmer. This swimming nature transition stems from intricate internal degrees of freedom of membrane deformation. (ii) The swimming speed might increase with increasing confinement before decreasing again for stronger confinements. (iii) A straight amoeoboid swimmer's trajectory in the channel can become unstable, and ample lateral excursions of the swimmer prevail. This happens for both pusher- and puller-type swimmers. For weak confinement, these excursions are symmetric, while they become asymmetric at stronger confinement, whereby the swimmer is located closer to one of the two walls. In this study, we combine numerical and theoretical analyses.

  13. Lead salt quantum effect structures

    NASA Astrophysics Data System (ADS)

    Partin, Dale L.

    1988-08-01

    Lead salt (V-VI) compounds have been grown epitaxially by a variety of growth techniques, such as molecular-beam epitaxy and hot-wall epitaxy. Recently, compositional superlattices and quantum-well heterostructures have been grown that exhibit strong quantum optical effects. These structures have been used to fabricate midinfrared diode lasers with greatly improved operating temperatures. Thus, it appears that these devices will continue to maintain a significant advantage over II-VI and III-V compound diode lasers. Doping superlattices have been made which possess enhanced minority carier properties. Ferromagnetic ordering in PbSnTe-MnTe alloys suggests potential areas for future work in magnetic field sensitivity devices. Lead salt quantum-effect structures are included.

  14. Deforming baryons into confining strings

    NASA Astrophysics Data System (ADS)

    Hartnoll, Sean A.; Portugues, Rubén

    2004-09-01

    We find explicit probe D3-brane solutions in the infrared of the Maldacena-Nuñez background. The solutions describe deformed baryon vertices: q external quarks are separated in spacetime from the remaining N-q. As the separation is taken to infinity we recover known solutions describing infinite confining strings in N=1 gauge theory. We present results for the mass of finite confining strings as a function of length. We also find probe D2-brane solutions in a confining type IIA geometry, the reduction of a G2 holonomy M theory background. The relation between these deformed baryons and confining strings is not as straightforward.

  15. Myosin VI: cellular functions and motor properties.

    PubMed Central

    Roberts, Rhys; Lister, Ida; Schmitz, Stephan; Walker, Matthew; Veigel, Claudia; Trinick, John; Buss, Folma; Kendrick-Jones, John

    2004-01-01

    Myosin VI has been localized in membrane ruffles at the leading edge of cells, at the trans-Golgi network compartment of the Golgi complex and in clathrin-coated pits or vesicles, indicating that it functions in a wide variety of intracellular processes. Myosin VI moves along actin filaments towards their minus end, which is the opposite direction to all of the other myosins so far studied (to our knowledge), and is therefore thought to have unique properties and functions. To investigate the cellular roles of myosin VI, we identified various myosin VI binding partners and are currently characterizing their interactions within the cell. As an alternative approach, we have expressed and purified full-length myosin VI and studied its in vitro properties. Previous studies assumed that myosin VI was a dimer, but our biochemical, biophysical and electron microscopic studies reveal that myosin VI can exist as a stable monomer. We observed, using an optical tweezers force transducer, that monomeric myosin VI is a non-processive motor which, despite a relatively short lever arm, generates a large working stroke of 18 nm. Whether monomer and/or dimer forms of myosin VI exist in cells and their possible functions will be discussed. PMID:15647169

  16. Phase VI Glove Durability Testing

    NASA Technical Reports Server (NTRS)

    Mitchell, Kathryn C.

    2010-01-01

    The current state-of-the-art space suit gloves, the Phase VI gloves, have an operational life of 25 - 8 hour Extravehicular Activities (EVAs) in a clean, controlled ISS environment. Future planetary outpost missions create the need for space suit gloves which can endure up to 90 - 8 hour traditional EVAs or 576 - 45 minute suit port-based EVAs in a dirty, uncontrolled planetary environment. Prior to developing improved space suit gloves for use in planetary environments, it is necessary to understand how the current state-of-the-art performs in these environments. The Phase VI glove operational life has traditionally been certified through cycle testing consisting of ISS-based tasks in a clean environment, and glove durability while performing planetary EVA tasks in a dirty environment has not previously been characterized. Testing was performed in the spring of 2010 by the NASA Johnson Space Center Crew and Thermal Systems Division to characterize the durability of the Phase VI Glove and identify areas of the glove design which need improvement to meet the requirements of future NASA missions. Lunar simulant was used in this test to help replicate the dirty lunar environment, and generic planetary surface EVA tasks were performed during testing. A total of 50 manned, pressurized test sessions were completed in the Extravehicular Mobility Unit (EMU) using one pair of Phase VI gloves as the test article. The 50 test sessions were designed to mimic the total amount of pressurized cycling the gloves would experience over a 6 month planetary outpost mission. The gloves were inspected at periodic intervals throughout testing, to assess their condition at various stages in the test and to monitor the gloves for failures. Additionally, motion capture and force data were collected during 18 of the 50 test sessions to assess the accuracy of the cycle model predictions used in testing and to feed into the development of improved cycle model tables. This paper provides a

  17. Phase VI Glove Durability Testing

    NASA Technical Reports Server (NTRS)

    Mitchell, Kathryn

    2011-01-01

    The current state-of-the-art space suit gloves, the Phase VI gloves, have an operational life of 25 -- 8 hour Extravehicular Activities (EVAs) in a dust free, manufactured microgravity EVA environment. Future planetary outpost missions create the need for space suit gloves which can endure up to 90 -- 8 hour traditional EVAs or 576 -- 45 minute suit port-based EVAs in a dirty, uncontrolled planetary environment. Prior to developing improved space suit gloves for use in planetary environments, it is necessary to understand how the current state-of-the-art performs in these environments. The Phase VI glove operational life has traditionally been certified through cycle testing consisting of International Space Station (ISS)-based EVA tasks in a clean environment, and glove durability while performing planetary EVA tasks in a dirty environment has not previously been characterized. Testing was performed in the spring of 2010 by the NASA Johnson Space Center (JSC) Crew and Thermal Systems Division (CTSD) to characterize the durability of the Phase VI Glove and identify areas of the glove design which need improvement to meet the requirements of future NASA missions. Lunar simulant was used in this test to help replicate the dirty lunar environment, and generic planetary surface EVA tasks were performed during testing. A total of 50 manned, pressurized test sessions were completed in the Extravehicular Mobility Unit (EMU) using one pair of Phase VI gloves as the test article. The 50 test sessions were designed to mimic the total amount of pressurized cycling the gloves would experience over a 6 month planetary outpost mission. The gloves were inspected periodically throughout testing, to assess their condition at various stages in the test and to monitor the gloves for failures. Additionally, motion capture and force data were collected during 18 of the 50 test sessions to assess the accuracy of the cycle model predictions used in testing and to feed into the

  18. II-VI Materials-Based High Performance Intersubband Devices

    NASA Astrophysics Data System (ADS)

    Ravikumar, Arvind Pawan

    Mid-infrared (mid-IR) light is of vital technological importance because of its application in trace-gas absorption spectroscopy, imaging, free-space communication or infrared countermeasures. Thus the ability to generate and detect mid-IR light at low cost and preferably, at room temperature is of utmost importance. High performance quantum cascade (QC) lasers - mid-IR light sources based on optical transitions in thin quantum wells, and intersubband infrared detectors - namely the quantum well infrared photodetectors (QWIPs) and quantum cascade detectors (QCDs), have rapidly advanced, due to excellent material quality of III-V materials. In spite of this tremendous success, there lie challenges such as lack of efficient short-wavelength emitters or broadband detectors - challenges that arise from intrinsic materials properties. As a central theme in this thesis, we look at a new class of materials, the II-VI based ZnCdSe/ZnCdMgSe system, to close technological gaps and develop high performance infrared light sources and detectors in the entire mid-IR regime. To that end, we first demonstrate the flexibility that the combination of II-VI materials and band structure engineering allows by developing various QWIPs, QCDs and QC emitters at different wavelengths, not easily achieved by other materials. The performance of these first-of-their-kind detectors is already comparable to existing commercial solutions. To fully realize the potential of this new material system, we also developed a room-temperature broadband infrared detector detecting between 3 and 6 mum with record responsivity. With this technology, it is now possible to monolithically integrate high performance mid-IR lasers and detectors for on-chip applications. One of the challenges with all intersubband detectors is that they do not absorb normally incident light, like most conventional detectors. In order to make intersubband detectors attractive to commercial exploration, we develop a novel method to

  19. Quantum Computation and Quantum Information

    NASA Astrophysics Data System (ADS)

    Nielsen, Michael A.; Chuang, Isaac L.

    2010-12-01

    Part I. Fundamental Concepts: 1. Introduction and overview; 2. Introduction to quantum mechanics; 3. Introduction to computer science; Part II. Quantum Computation: 4. Quantum circuits; 5. The quantum Fourier transform and its application; 6. Quantum search algorithms; 7. Quantum computers: physical realization; Part III. Quantum Information: 8. Quantum noise and quantum operations; 9. Distance measures for quantum information; 10. Quantum error-correction; 11. Entropy and information; 12. Quantum information theory; Appendices; References; Index.

  20. Bound states in continuum: Quantum dots in a quantum well

    NASA Astrophysics Data System (ADS)

    Prodanović, Nikola; Milanović, Vitomir; Ikonić, Zoran; Indjin, Dragan; Harrison, Paul

    2013-11-01

    We report on the existence of a bound state in the continuum (BIC) of quantum rods (QR). QRs are novel elongated InGaAs quantum dot nanostructures embedded in the shallower InGaAs quantum well. BIC appears as an excited confined dot state and energetically above the bottom of a well subband continuum. We prove that high height-to-diameter QR aspect ratio and the presence of a quantum well are indispensable conditions for accommodating the BIC. QRs are unique semiconductor nanostructures, exhibiting this mathematical curiosity predicted 83 years ago by Wigner and von Neumann.

  1. Aortic and Mitral Valve Involvement in Maroteaux-Lamy Syndrome VI: Surgical Implications in the Enzyme Replacement Therapy Era.

    PubMed

    Torre, Salvatore; Scarpelli, Mauro; Salviati, Alessandro; Buffone, Ebba; Faggian, Giuseppe; Luciani, Giovanni Battista

    2016-07-01

    Open-heart operations in patients with mucopolysaccharidoses are exceedingly rare and pose distinct clinical challenges. Few reports exist of valve replacement in type VI mucopolysaccharidosis, mostly entailing combined mitral and aortic valve replacement. Here reported is the case of a young woman with mitral and aortic valve disease, in whom the surgical procedure was confined to the aortic valve. The rationale behind this strategy, particularly in light of the benefits offered by specific enzyme replacement therapy of type VI mucopolysaccharidosis, is discussed. PMID:27343522

  2. Lateral Quantum Dots for Quantum Information Processing

    NASA Astrophysics Data System (ADS)

    House, Matthew Gregory

    The possibility of building a computer that takes advantage of the most subtle nature of quantum physics has been driving a lot of research in atomic and solid state physics for some time. It is still not clear what physical system or systems can be used for this purpose. One possibility that has been attracting significant attention from researchers is to use the spin state of an electron confined in a semiconductor quantum dot. The electron spin is magnetic in nature, so it naturally is well isolated from electrical fluctuations that can a loss of quantum coherence. It can also be manipulated electrically, by taking advantage of the exchange interaction. In this work we describe several experiments we have done to study the electron spin properties of lateral quantum dots. We have developed lateral quantum dot devices based on the silicon metal-oxide-semiconductor transistor, and studied the physics of electrons confined in these quantum dots. We measured the electron spin excited state lifetime, which was found to be as long as 30 ms at the lowest magnetic fields that we could measure. We fabricated and characterized a silicon double quantum dot. Using this double quantum dot design, we fabricated devices which combined a silicon double quantum dot with a superconducting microwave resonator. The microwave resonator was found to be sensitive to two-dimensional electrons in the transistor channel, which we measured and characterized. We developed a new method for extracting information from random telegraph signals, which are produced when we observe thermal fluctuations of electrons in quantum dots. The new statistical method, based on the hidden Markov model, allows us to detect spin-dependent effects in such fluctuations even though we are not able to directly observe the electron spin. We use this analysis technique on data from two experiments involving gallium arsenide quantum dots and use it to measure spin-dependent tunneling rates. Our results advance the

  3. Conventional versus unconventional magnetic polarons: ZnMnTe/ZnSe and ZnTe/ZnMnSe quantum dots

    NASA Astrophysics Data System (ADS)

    Barman, B.; Tsai, Y.; Scrace, T.; Murphy, J. R.; Cartwright, A. N.; Pientka, J. M.; Zutic, I.; McCombe, B. D.; Petrou, A.; Sellers, I. R.; Oszwaldowski, R.; Petukhov, A.; Fan, W. C.; Chou, W. C.; Yang, C. S.

    2014-08-01

    We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, EMP , in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with EMP=35 meV. Furthermore, EMP has unconventionally weak dependence of on both temperature T and magnetic field Bappl . In contrast, magnetic polarons in sample B show conventional characteristics with EMP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.

  4. Electronic properties of Hg1-xCdxSe lens-shaped quantum dots under external fields

    NASA Astrophysics Data System (ADS)

    Herrera, J. R.; Gutierrez, W.; Miranda, D. A.

    2016-02-01

    Hg1-xCdxSe are II-VI semiconductors alloys with optoelectronic properties that depend upon the molar fraction x, which can be further controlled by nanostructuring. In this work one electron confined in a zero-dimensional lens-shaped nanostructure of Hg1-xCdxSe surrounded by a matrix of different molar fraction is analyzed and its electronic properties are studied under external magnetic and electric fields. Our system was modeled by means of the 3D Schrodinger equation in the framework of the effective mass approximation, which was solved using a finite element method. The model is described by a discontinuous space with Ben Daniel-Duke boundary conditions. We calculated the energy spectrum and the corresponding probability density of the electron for some low-lying energy levels as a function of: electric field strength on plane and magnetic field strength applied along the growth direction. Also, the effect of finite confinement potential was studied in presence of a uniform magnetic field. Our results shown that the electronic properties of Hg1-xCdxSe quantum dots are highly sensitive to a threading magnetic field because the degenerate energy levels are split. On the other hand, the effect of electric and magnetic fields applied simultaneously on a quantum dot can increase the system stability against external perturbation, e.g. thermal interactions.

  5. Genetics Home Reference: collagen VI-related myopathy

    MedlinePlus

    ... Genetics Home Health Conditions collagen VI-related myopathy collagen VI-related myopathy Enable Javascript to view the ... boxes. Download PDF Open All Close All Description Collagen VI-related myopathy is a group of disorders ...

  6. Confined helium on Lagrange meshes.

    PubMed

    Baye, D; Dohet-Eraly, J

    2015-12-21

    The Lagrange-mesh method has the simplicity of a calculation on a mesh and can have the accuracy of a variational method. It is applied to the study of a confined helium atom. Two types of confinement are considered. Soft confinements by potentials are studied in perimetric coordinates. Hard confinement in impenetrable spherical cavities is studied in a system of rescaled perimetric coordinates varying in [0,1] intervals. Energies and mean values of the distances between electrons and between an electron and the helium nucleus are calculated. A high accuracy of 11 to 15 significant figures is obtained with small computing times. Pressures acting on the confined atom are also computed. For sphere radii smaller than 1, their relative accuracies are better than 10(-10). For larger radii up to 10, they progressively decrease to 10(-3), still improving the best literature results. PMID:25732054

  7. The role of nanopores on U(VI) sorption and redox behavior in U(VI)-contaminated subsurface sediments

    SciTech Connect

    Xu, Huifang; Roden, Eric E.; Kemner, Kenneth M.; Jung, Hun-Bok; Konishi, Hiromi; Boyanov, Maxim; Sun, Yubing; Mishra, Bhoopesh

    2013-10-16

    Most reactive surfaces in clay-dominated sediments are present within nanopores (pores of nm dimension). The behavior of geological fluids and minerals in nanopores is significantly different from those in normal non-nanoporous environments. The effect of nanopore surfaces on U(VI) sorption/desorption and reduction is likely to be significant in clay-rich subsurface environments. Our research results from both model nanopore system and natural sediments from both model system (synthetic nanopore alumina) and sediments from the ORNL Field Research Center prove that U(VI) sorption on nanopore surfaces can be greatly enhanced by nanopore confinement environments. The results from the project provide advanced mechanistic, quantitative information on the physiochemical controls on uranium sorption and redox behavior in subsurface sediments. The influence of nanopore surfaces on coupled uranium sorption/desorption and reduction processes is significant in virtually all subsurface environments, because most reactive surfaces are in fact nanopore surfaces. The results will enhance transfer of our laboratory-based research to a major field research initiative where reductive uranium immobilization is being investigated. Our results will also provide the basic science for developing in-situ colloidal barrier of nanoporous alumina in support of environmental remediation and long term stewardship of DOE sites.

  8. Probing semiconductor confined excitons decay into surface plasmon polaritons

    NASA Astrophysics Data System (ADS)

    Sobreira, F. W. A.; de Oliveira, E. R. Cardozo; Teodoro, M. D.; Marques, G. E.; Marega, E.

    2016-04-01

    The study of the interaction of surface plasmon polaritons (SPPs) with quantum emitters has become very important in the last few years. The ability to design optical devices as well as investigate the physics of strongly interacting systems is some of its useful applications. In this paper, we will show some results on the decay of excitons confined in a InAs/GaAs quantum dot into SPP modes confined in a metallic thin film made of Au, an important step toward the investigation of the basic features of the SPP-exciton interaction. The lifetime of the ground state level was investigated and shown to decrease with the presence of the metallic film.

  9. 29 CFR 1915.1026 - Chromium (VI).

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 29 Labor 7 2014-07-01 2014-07-01 false Chromium (VI). 1915.1026 Section 1915.1026 Labor Regulations Relating to Labor (Continued) OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) OCCUPATIONAL SAFETY AND HEALTH STANDARDS FOR SHIPYARD EMPLOYMENT Toxic and Hazardous Substances § 1915.1026 Chromium (VI). (a) Scope....

  10. 29 CFR 1910.1026 - Chromium (VI).

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 29 Labor 6 2012-07-01 2012-07-01 false Chromium (VI). 1910.1026 Section 1910.1026 Labor Regulations Relating to Labor (Continued) OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) OCCUPATIONAL SAFETY AND HEALTH STANDARDS (CONTINUED) Toxic and Hazardous Substances § 1910.1026 Chromium (VI). (a) Scope. (1) This...

  11. 29 CFR 1926.1126 - Chromium (VI).

    Code of Federal Regulations, 2013 CFR

    2013-07-01

    ... 29 Labor 8 2013-07-01 2013-07-01 false Chromium (VI). 1926.1126 Section 1926.1126 Labor Regulations Relating to Labor (Continued) OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) SAFETY AND HEALTH REGULATIONS FOR CONSTRUCTION Toxic and Hazardous Substances § 1926.1126 Chromium (VI). (a) Scope. (1) This...

  12. [Occupational exposure to chromium(VI) compounds].

    PubMed

    Skowroń, Jolanta; Konieczko, Katarzyna

    2015-01-01

    This article discusses the effect of chromium(VI) (Cr(VI)) on human health under conditions of acute and chronic exposure in the workplace. Chromium(VI) compounds as carcinogens and/or mutagens pose a direct danger to people exposed to them. If carcinogens cannot be eliminated from the work and living environments, their exposure should be reduced to a minimum. In the European Union the proposed binding occupational exposure limit value (BOELV) for chromium(VI) of 0.025 mg/m³ is still associated with high cancer risk. Based on the Scientific Commitee of Occupational Exposure Limits (SCOEL) document chromium(VI) concentrations at 0.025 mg/m³ increases the risk of lung cancer in 2-14 cases per 1000 exposed workers. Exposure to chromium(VI) compounds expressed in Cr(VI) of 0.01 mg Cr(VI)/m3; is responsible for the increased number of lung cancer cases in 1-6 per 1000 people employed in this condition for the whole period of professional activity. PMID:26325053

  13. Atomic-scale confinement of resonant optical fields.

    PubMed

    Kern, Johannes; Grossmann, Swen; Tarakina, Nadezda V; Häckel, Tim; Emmerling, Monika; Kamp, Martin; Huang, Jer-Shing; Biagioni, Paolo; Prangsma, Jord C; Hecht, Bert

    2012-11-14

    In the presence of matter, there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding resonant intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically defined gaps reaching below 0.5 nm. The existence of atomically confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and antisymmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically resolved spectroscopic imaging, deeply nonlinear optics, ultrasensing, cavity optomechanics, as well as for the realization of novel quantum-optical devices. PMID:22984927

  14. Electric field suppression of ultracold confined chemical reactions

    SciTech Connect

    Quemener, Goulven; Bohn, John L.

    2010-06-15

    We consider ultracold collisions of polar molecules confined in a one-dimensional optical lattice. Using a quantum scattering formalism and a frame transformation method, we calculate elastic and chemical quenching rate constants for fermionic molecules. Taking {sup 40}K{sup 87}Rb molecules as a prototype, we find that the rate of quenching collisions is enhanced at zero electric field as the confinement is increased but that this rate is suppressed when the electric field is turned on. For molecules with 500 nK of collision energy, for realistic molecular densities, and for achievable experimental electric fields and trap confinements, we predict lifetimes for KRb molecules to be 1 s. We find a ratio of elastic to quenching collision rates of about 100, which may be sufficient to achieve efficient evaporative cooling of polar KRb molecules.

  15. Confinement of gigahertz sound and light in Tamm plasmon resonators

    NASA Astrophysics Data System (ADS)

    Villafañe, V.; Bruchhausen, A. E.; Jusserand, B.; Senellart, P.; Lemaître, A.; Fainstein, A.

    2015-10-01

    We demonstrate theoretically and by pump-probe picosecond acoustics experiments the simultaneous confinement of light and gigahertz sound in Tamm plasmon resonators, formed by depositing a thin layer of Au onto a GaAs/AlGaAs Bragg reflector. The cavity has InGaAs quantum dots (QDs) embedded at the maximum of the confined optical field in the first GaAs layer. The different sound generation and detection mechanisms are theoretically analyzed. It is shown that the Au layer absorption and the resonant excitation of the QDs are the more efficient light-sound transducers for the coupling of near-infrared light with the confined acoustic modes, while the displacement of the interfaces is the main back-action mechanism at these energies. The prospects for the compact realization of optomechanical resonators based on Tamm plasmon cavities are discussed.

  16. Quantum Phase Extraction in Isospectral Electronic Nanostructures

    SciTech Connect

    Moon, Christopher

    2010-04-28

    Quantum phase is not a direct observable and is usually determined by interferometric methods. We present a method to map complete electron wave functions, including internal quantum phase information, from measured single-state probability densities. We harness the mathematical discovery of drum-like manifolds bearing different shapes but identical resonances, and construct quantum isospectral nanostructures possessing matching electronic structure but divergent physical structure. Quantum measurement (scanning tunneling microscopy) of these 'quantum drums' [degenerate two-dimensional electron states on the Cu(111) surface confined by individually positioned CO molecules] reveals that isospectrality provides an extra topological degree of freedom enabling robust quantum state transplantation and phase extraction.

  17. Inertial confinement fusion

    SciTech Connect

    Powers, L.; Condouris, R.; Kotowski, M.; Murphy, P.W.

    1992-01-01

    This issue of the ICF Quarterly contains seven articles that describe recent progress in Lawrence Livermore National Laboratory's ICF program. The Department of Energy recently initiated an effort to design a 1--2 MJ glass laser, the proposed National Ignition Facility (NIF). These articles span various aspects of a program which is aimed at moving forward toward such a facility by continuing to use the Nova laser to gain understanding of NIF-relevant target physics, by developing concepts for an NIF laser driver, and by envisioning a variety of applications for larger ICF facilities. This report discusses research on the following topics: Stimulated Rotational Raman Scattering in Nitrogen; A Maxwell Equation Solver in LASNEX for the Simulation of Moderately Intense Ultrashort Pulse Experiments; Measurements of Radial Heat-Wave Propagation in Laser-Produced Plasmas; Laser-Seeded Modulation Growth on Directly Driven Foils; Stimulated Raman Scattering in Large-Aperture, High-Fluence Frequency-Conversion Crystals; Fission Product Hazard Reduction Using Inertial Fusion Energy; Use of Inertial Confinement Fusion for Nuclear Weapons Effects Simulations.

  18. Thermostating highly confined fluids.

    PubMed

    Bernardi, Stefano; Todd, B D; Searles, Debra J

    2010-06-28

    In this work we show how different use of thermostating devices and modeling of walls influence the mechanical and dynamical properties of confined nanofluids. We consider a two dimensional fluid undergoing Couette flow using nonequilibrium molecular dynamics simulations. Because the system is highly inhomogeneous, the density shows strong fluctuations across the channel. We compare the dynamics produced by applying a thermostating device directly to the fluid with that obtained when the wall is thermostated, considering also the effects of using rigid walls. This comparison involves an analysis of the chaoticity of the fluid and evaluation of mechanical properties across the channel. We look at two thermostating devices with either rigid or vibrating atomic walls and compare them with a system only thermostated by conduction through vibrating atomic walls. Sensitive changes are observed in the xy component of the pressure tensor, streaming velocity, and density across the pore and the Lyapunov localization of the fluid. We also find that the fluid slip can be significantly reduced by rigid walls. Our results suggest caution in interpreting the results of systems in which fluid atoms are thermostated and/or wall atoms are constrained to be rigid, such as, for example, water inside carbon nanotubes. PMID:20590213

  19. Casimir effects for classical and quantum liquids in slab geometry: A brief review

    SciTech Connect

    Biswas, Shyamal

    2015-05-15

    We analytically explore Casimir effects for confinement of classical and quantum fluctuations in slab (film) geometry (i) for classical (critical) fluctuations over {sup 4}He liquid around the λ point, and (ii) for quantum (phonon) fluctuations of Bogoliubov excitations over an interacting Bose-Einstein condensate. We also briefly review Casimir effects for confinement of quantum vacuum fluctuations confined to two plates of different geometries.

  20. The Radiolysis of AmVI Solutions

    SciTech Connect

    Bruce J. Mincher

    2013-06-01

    The reduction of bismuthate-produced AmVI by 60Co gamma-rays was measured using post-irradiation UV/Vis spectroscopy. The reduction of AmVI by radiolysis was rapid, producing AmV as the sole product. Relatively low absorbed doses in the ~0.3 kGy range quantitatively reduced a solution of 2.5 x 10-4 M AmVI. The addition of bismuthate to samples during irradiation did not appear to protect AmVI from radiolytic reduction during these experiments. It was also shown here that AmV is very stable toward radiation. The quantitative reduction of the AmVI concentration here corresponds to 1.4 hours of exposure to a process solution, however the actual americium concentrations will be higher and the expected contact times short when using centrifugal contactors. Thus, the reduction rate found in these initial experiments may not be excessive.

  1. Transport through graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Güttinger, J.; Molitor, F.; Stampfer, C.; Schnez, S.; Jacobsen, A.; Dröscher, S.; Ihn, T.; Ensslin, K.

    2012-12-01

    We review transport experiments on graphene quantum dots and narrow graphene constrictions. In a quantum dot, electrons are confined in all lateral dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. The recently isolated two-dimensional carbon allotrope graphene is an interesting host to study quantum phenomena, due to its novel electronic properties and the expected weak interaction of the electron spin with the material. Graphene quantum dots are fabricated by etching mono-layer flakes into small islands (diameter 60-350 nm) with narrow connections to contacts (width 20-75 nm), serving as tunneling barriers for transport spectroscopy. Electron confinement in graphene quantum dots is observed by measuring Coulomb blockade and transport through excited states, a manifestation of quantum confinement. Measurements in a magnetic field perpendicular to the sample plane allowed to identify the regime with only a few charge carriers in the dot (electron-hole transition), and the crossover to the formation of the graphene specific zero-energy Landau level at high fields. After rotation of the sample into parallel magnetic field orientation, Zeeman spin splitting with a g-factor of g ≈ 2 is measured. The filling sequence of subsequent spin states is similar to what was found in GaAs and related to the non-negligible influence of exchange interactions among the electrons.

  2. Confinement & Stability in MAST

    NASA Astrophysics Data System (ADS)

    Akers, Rob

    2001-10-01

    Transition to H-mode has been achieved in the MAST spherical tokamak (ST) for both ohmically and neutral beam heated plasmas (P_NBI ~ 0.5-1.5MW), resulting in double-null diverted discharges containing both regular and irregular edge localised modes (ELMs). The observed L-H power threshold is ~10 times higher than predicted by established empirical scalings. L-H transition in MAST is accompanied by a sharp increase in edge density gradient, the efficient conversion of internal electron Bernstein waves into free space waves, the onset and saturation of edge poloidal rotation and a marked decrease in turbulence. During ELM free periods, a reduction in outboard power deposition width is observed using a Langmuir probe array. A novel divertor structure has been installed to counter the resulting increase in target heat-flux by applying a toroidally varying potential to the divertor plasma, theory suggesting that convective broadening of the scrape off layer will take place. Global confinement in H-mode is found to routinely exceed the international IPB(y,2) scaling, even for discharges approaching the Greenwald density. In an attempt to further extend the density range (densities in excess of Greenwald having been achieved for plasma currents up to 0.8MA) a multi-pellet injector has been installed at the low-field-side. In addition, high field side fuelling can be supplied via a gas-feed located at the centre-column mid-plane, this technique having been found to significantly enhance H-mode accessibility and quality. A range of stability issues will be discussed, including vertical displacement events, the rich variety of high frequency MHD seen in MAST and the physics of the Neoclassical Tearing Mode. This work was funded by the UK Department of Trade and Industry and by EURATOM. The NBI equipment is on loan from ORNL and the pellet injector was provided by FOM.

  3. Psychopathological effects of solitary confinement.

    PubMed

    Grassian, S

    1983-11-01

    Psychopathological reactions to solitary confinement were extensively described by nineteenth-century German clinicians. In the United States there have been several legal challenges to the use of solitary confinement, based on allegations that it may have serious psychiatric consequences. The recent medical literature on this subject has been scarce. The author describes psychiatric symptoms that appeared in 14 inmates exposed to periods of increased social isolation and sensory restriction in solitary confinement and asserts that these symptoms form a major, clinically distinguishable psychiatric syndrome. PMID:6624990

  4. ITER EDA design confinement capability

    NASA Astrophysics Data System (ADS)

    Uckan, N. A.

    Major device parameters for ITER-EDA and CDA are given in this paper. Ignition capability of the EDA (and CDA) operational scenarios is evaluated using both the 1 1/2-D time-dependent transport simulations and 0-D global models under different confinement ((chi((gradient)(T)(sub e)(sub crit)), empirical global energy confinement scalings, chi(empirical), etc.) assumptions. Results from some of these transport simulations and confinement assessments are summarized in and compared with the ITER CDA results.

  5. PREFACE: VI Mexican School on Gravitation and Mathematical Physics

    NASA Astrophysics Data System (ADS)

    Alcubierre, Miguel; Cervantes-Cota, Jorge L.; Montesinos, Merced

    2005-01-01

    The Mexican School on Gravitation and Mathematical Physics, sponsored by the Mexican Physical Society, started in the early 1990s. The aim of the school is to consider different topics at the frontiers of current research on gravitation, field theory and mathematical physics. It is held every two years and a different theme is chosen on each occasion. The school, which is oriented towards advanced graduate students and non-expert researchers, has been gaining reputation because of the quality of the lectures given by leaders in each field. On previous occasions, the subjects covered have been Supergravity and Mathematical Physics, Branes, Black Holes, Early Universe and Observational Cosmology, and the speakers have included A. Ashtekar, A. Balachandran, J. Barrow, B. Carter, P. Chrusciel, G. Gibbons, M. Heusler, W. Israel, F. Müller-Hoisen, R. Kallosh, A. Linde, Y. Neeman, R. Myers, A. Peet, L. Randall, C. Rovelli, L. Smolin, R. Sorkin, P. Van Niewenhuizen, R. Wald, among other top ranked physicists. Let us now turn our attention to the current edition of the school. The two great pillars of twentieth century physics are quantum mechanics and the general theory of relativity (GR) and, in spite of their great independent successes, it has been enormously difficult to combine them into a single theoretical framework. It seems that the nonlinearities of GR and its attractive nature pose severe problems to understand its possible quantum nature, e.g. renormalization issues. In order to solve this and other quantization problems, as well as to try to unify gravity with the other quantum fields (electromagnetic, weak, and strong fields), in the past decades several different approaches to quantum gravity have been developed. In view of the fundamental importance of such topics today, the theme Approaches to Quantum Gravity was chosen for the VI Mexican School on Gravitation and Mathematical Physics. We considered that subjects like loop quantum gravity (quantum

  6. Synthetic Developments of Nontoxic Quantum Dots.

    PubMed

    Das, Adita; Snee, Preston T

    2016-03-01

    Semiconductor nanocrystals, or quantum dots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantum dots. In this Review, we select three major classes of nontoxic quantum dots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantum dots composed of these material systems. PMID:26548450

  7. Inhibition of a U(VI)- and sulfate-reducing consortia by U(VI).

    PubMed

    Nyman, Jennifer L; Wu, Hsin-I; Gentile, Margaret E; Kitanidis, Peter K; Criddle, Craig S

    2007-09-15

    The stimulation of microbial U(VI) reduction is currently being investigated as a means to reduce uranium's mobility in groundwater, but little is known about the concentration at which U(VI) might inhibit microbial activity, or the effect of U(VI) on bacterial community structure. We investigated these questions with an ethanol-fed U(VI)- and sulfate-reducing enrichment developed from sediment from the site of an ongoing field biostimulation experiment at Area 3 of the Oak Ridge Field Research Center (FRC). Sets of triplicate enrichments were spiked with increasing concentrations of U(VI) (from 49 microm to 9.2 mM). As the U(VI) concentration increased to 224 microM, the culture's production of acetate from ethanol slowed, and at or above 1.6 mM U(VI) little acetate was produced over the time frame of the experiment. An uncoupling inhibition model was applied to the data, and the inhibition coefficient for U(VI), Ku, was found to be approximately 100 microM U(VI), or 24 mg/L, indicating the inhibitory effect is relevant at highly contaminated sites. Microbial community structure at the conclusion of the experiment was analyzed with terminal restriction fragment length polymorphism (T-RFLP) analysis. T-RFs associated with Desulfovibrio-like organisms decreased in relative abundance with increasing U(VI) concentration, whereas Clostridia-like T-RFs increased. PMID:17948804

  8. Emissions of chromium (VI) from arc welding.

    PubMed

    Heung, William; Yun, Myoung-Jin; Chang, Daniel P Y; Green, Peter G; Halm, Chris

    2007-02-01

    The presence of Cr in the +6 oxidation state (Cr[VI]) is still observed in ambient air samples in California despite steps taken to reduce emissions from plating operations. One known source of emission of Cr(VI) is welding, especially with high Cr-content materials, such as stainless steels. An experimental effort was undertaken to expand and update Cr(VI) emission factors by conducting tests on four types of arc-welding operations: gas-metal arc welding (GMAW), shielded metal arc welding (SMAW), fluxcore arc welding, and pulsed GMAW. Standard American Welding Society hood results were compared with a total enclosure method that permitted isokinetic sampling for particle size-cut measurement, as well as total collection of the aerosol. The fraction of Cr(VI) emitted per unit mass of Cr electrode consumed was determined. Consistent with AP-42 data, initial results indicate that a significant fraction of the total Cr in the aerosol is in the +6 oxidation state. The fraction of Cr(VI) and total aerosol mass produced by the different arc welding methods varies with the type of welding process used. Self-shielded electrodes that do not use a shield gas, for example, SMAW, produce greater amounts of Cr(VI) per unit mass of electrode consumed. The formation of Cr(VI) from standard electrode wires used for welding mild steel was below the method detection limit after eliminating an artifact in the analytical method used. PMID:17355086

  9. Role of InxGa1-xAs Layer Composition in Modifying Strain Fields and Carrier Confinement Potentials in a Close-stacked InAs/GaAs Quantum Dot System

    NASA Astrophysics Data System (ADS)

    Lee, Woong; Yoo, Yo-Han; Shin, Hyunho; Myoung, Jae-Min

    2005-05-01

    Role of the composition of InxGa1-xAs strain-relief layer (SRL) in controlling the strain fields and consequent modification of band structures in a close-stacked multi-layer InAs/GaAs quantum dot (QD) system was investigated within the framework of continuum elasticity and model solid theory. It was predicted that strains in the QDs are significantly relieved in proportion to the In concentration in the InxGa1-xAs SRLs between InAs QD and GaAs cap layer. The relaxation of strains caused substantial shift of the conduction band edge in the QDs mainly by the relief of hydrostatic strain component resulting in narrower bandgap within the QDs with increasing In concentration. It is interpreted that such strain relaxation and subsequent band structure modifications are responsible for the experimentally observed redshift of photo-luminescence (PL) spectra elsewhere. Therefore, together with existing experimental work, it is confirmed that conduction band edges of QD systems can be tailored by the control of the SRL composition allowing more flexibility in bandgap engineering.

  10. Alternative approaches to plasma confinement

    NASA Technical Reports Server (NTRS)

    Roth, J. R.

    1978-01-01

    The paper discusses 20 plasma confinement schemes each representing an alternative to the tokamak fusion reactor. Attention is given to: (1) tokamak-like devices (TORMAC, Topolotron, and the Extrap concept), (2) stellarator-like devices (Torsatron and twisted-coil stellarators), (3) mirror machines (Astron and reversed-field devices, the 2XII B experiment, laser-heated solenoids, the LITE experiment, the Kaktus-Surmac concept), (4) bumpy tori (hot electron bumpy torus, toroidal minimum-B configurations), (5) electrostatically assisted confinement (electrostatically stuffed cusps and mirrors, electrostatically assisted toroidal confinement), (6) the Migma concept, and (7) wall-confined plasmas. The plasma parameters of the devices are presented and the advantages and disadvantages of each are listed.

  11. Tandem mirror plasma confinement apparatus

    DOEpatents

    Fowler, T. Kenneth

    1978-11-14

    Apparatus and method for confining a plasma in a center mirror cell by use of two end mirror cells as positively charged end stoppers to minimize leakage of positive particles from the ends of the center mirror cell.

  12. Determining individual mineral contributions to U(VI) adsorption in a contaminated aquifer sediment: A fluorescence spectroscopy study

    NASA Astrophysics Data System (ADS)

    Wang, Zheming; Zachara, John M.; Boily, Jean-François; Xia, Yuanxian; Resch, Tom C.; Moore, Dean A.; Liu, C.

    2011-05-01

    The adsorption and speciation of U(VI) was investigated on contaminated, fine grained sediment materials from the Hanford 300 area (SPP1 GWF) in simulated groundwater using cryogenic laser-induced U(VI) fluorescence spectroscopy combined with chemometric analysis. A series of reference minerals (montmorillonite, illite, Michigan chlorite, North Carolina chlorite, California clinochlore, quartz and synthetic 6-line ferrihydrite) was used for comparison that represents the mineralogical constituents of SPP1 GWF. Surface area-normalized Kd values were measured at U(VI) concentrations of 5 × 10 -7 and 5 × 10 -6 mol L -1 that displayed the following affinity series: 6-line-ferrihydrite > North Carolina chlorite ≈ California clinochlore > quartz ≈ Michigan chlorite > illite > montmorillonite. Both time-resolved spectra and asynchronous two-dimensional (2D) correlation analysis of SPP1 GWF at different delay times indicated that two major adsorbed U(VI) species were present in the sediment that resembled U(VI) adsorbed on quartz and phyllosilicates. Simulations of the normalized fluorescence spectra confirmed that the speciation of SPP1 GWF was best represented by a linear combination of U(VI) adsorbed on quartz (90%) and phyllosilicates (10%). However, the fluorescence quantum yield for U(VI) adsorbed on phyllosilicates was lower than quartz and, consequently, its fractional contribution to speciation may be underestimated. Spectral comparison with literature data suggested that U(VI) exist primarily as inner-sphere complexes with surface silanol groups on quartz and as surface U(VI) tricarbonate complexes on phyllosilicates.

  13. A mechanochemical model for myosin VI

    NASA Astrophysics Data System (ADS)

    Tehver, Riina; Jack, Amanda; Lowe, Ian

    Myosin VI is a motor protein that transports cellular cargo along actin filaments. This transport takes place as a result of a coordinated mechano-chemical cycle that is controlled by external variables including imposed force and nucleotide concentrations. We present a model that captures the different dynamic pathways that myosin VI can take in response to these variables. The results of our model for experimentally observable quantities, such as the motor velocity or run length, agree with available experimental data, and we can also make predictions beyond the tested regimes. Using the model, we study how myosin VI reacts to its environment and test its operational efficiency.

  14. Alternative approaches to plasma confinement

    NASA Technical Reports Server (NTRS)

    Roth, J. R.

    1977-01-01

    The potential applications of fusion reactors, the desirable properties of reactors intended for various applications, and the limitations of the Tokamak concept are discussed. The principles and characteristics of 20 distinct alternative confinement concepts are described, each of which may be an alternative to the Tokamak. The devices are classed as Tokamak-like, stellarator-like, mirror machines, bumpy tori, electrostatically assisted, migma concept, and wall-confined plasma.

  15. Solvent cavitation under solvophobic confinement.

    PubMed

    Ashbaugh, Henry S

    2013-08-14

    The stability of liquids under solvophobic confinement can tip in favor of the vapor phase, nucleating a liquid-to-vapor phase transition that induces attractive forces between confining surfaces. In the case of water adjacent to hydrophobic surfaces, experimental and theoretical evidence support confinement-mediated evaporation stabilization of biomolecular and colloidal assemblies. The macroscopic thermodynamic theory of cavitation under confinement establishes the connection between the size of the confining surfaces, interfacial free energies, and bulk solvent pressure with the critical evaporation separation and interfacial forces. While molecular simulations have confirmed the broad theoretical trends, a quantitative comparison based on independent measurements of the interfacial free energies and liquid-vapor coexistence properties has, to the best of our knowledge, not yet been performed. To overcome the challenges of simulating a large number of systems to validate scaling predictions for a three-dimensional fluid, we simulate both the forces and liquid-vapor coexistence properties of a two-dimensional Lennard-Jones fluid confined between solvophobic plates over a range of plate sizes and reservoir pressures. Our simulations quantitatively agree with theoretical predictions for solvent-mediated forces and critical evaporation separations once the length dependence of the solvation free energy of an individual confining plate is taken into account. The effective solid-liquid line tension length dependence results from molecular scale correlations for solvating microscopic plates and asymptotically decays to the macroscopic value for plates longer than 150 solvent diameters. The success of the macroscopic thermodynamic theory at describing two-dimensional liquids suggests application to surfactant monolayers to experimentally confirm confinement-mediated cavitation. PMID:23947875

  16. Electrochromic nanocrystal quantum dots.

    PubMed

    Wang, C; Shim, M; Guyot-Sionnest, P

    2001-03-23

    Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence. PMID:11264530

  17. Quantum Theory and Beyond

    NASA Astrophysics Data System (ADS)

    Bastin, Ted

    2009-07-01

    List of participants; Preface; Part I. Introduction: 1. The function of the colloquium - editorial; 2. The conceptual problem of quantum theory from the experimentalist's point of view O. R. Frisch; Part II. Niels Bohr and Complementarity: The Place of the Classical Language: 3. The Copenhagen interpretation C. F. von Weizsäcker; 4. On Bohr's views concerning the quantum theory D. Bohm; Part III. The Measurement Problem: 5. Quantal observation in statistical interpretation H. J. Groenewold; 6. Macroscopic physics, quantum mechanics and quantum theory of measurement G. M. Prosperi; 7. Comment on the Daneri-Loinger-Prosperi quantum theory of measurement Jeffrey Bub; 8. The phenomenology of observation and explanation in quantum theory J. H. M. Whiteman; 9. Measurement theory and complex systems M. A. Garstens; Part IV. New Directions within Quantum Theory: What does the Quantum Theoretical Formalism Really Tell Us?: 10. On the role of hidden variables in the fundamental structure of physics D. Bohm; 11. Beyond what? Discussion: space-time order within existing quantum theory C. W. Kilmister; 12. Definability and measurability in quantum theory Yakir Aharonov and Aage Petersen; 13. The bootstrap idea and the foundations of quantum theory Geoffrey F. Chew; Part V. A Fresh Start?: 14. Angular momentum: an approach to combinatorial space-time Roger Penrose; 15. A note on discreteness, phase space and cohomology theory B. J. Hiley; 16. Cohomology of observations R. H. Atkin; 17. The origin of half-integral spin in a discrete physical space Ted Bastin; Part VI. Philosophical Papers: 18. The unity of physics C. F. von Weizsäcker; 19. A philosophical obstacle to the rise of new theories in microphysics Mario Bunge; 20. The incompleteness of quantum mechanics or the emperor's missing clothes H. R. Post; 21. How does a particle get from A to B?; Ted Bastin; 22. Informational generalization of entropy in physics Jerome Rothstein; 23. Can life explain quantum mechanics? H. H

  18. Numerical Studies of Properties of Confined Helium

    NASA Technical Reports Server (NTRS)

    Manousakis, Efstratios

    2003-01-01

    We carry out state of the art simulations of properties of confined liquid helium near the superfluid transition to a degree of accuracy which allows to make predictions for the outcome of fundamental physics experiments in microgravity. First we report our results for the finite-size scaling behavior of heat capacity of superfluids for cubic and parallel-plate geometry. This allows us to study the crossover from zero and two dimensions to three dimensions. Our calculated scaling functions are in good agreement with recently measured specific heat scaling functions for the above mentioned geometries. We also present our results of a quantum simulation of submonolayer of molecular hydrogen deposited on an ideal graphite substrate using path-integral quantum Monte Carlo simulation. We find that the monolayer phase diagram is rich and very similar to that of helium monolayer. We are able to uncover the main features of the complex monolayer phase diagram, such as the commensurate solid phases and the commensurate to incommensurate transition, in agreement with the experiments and to find some features which are missing from the experimental analysis.

  19. First principle thousand atom quantum dot calculations

    SciTech Connect

    Wang, Lin-Wang; Li, Jingbo

    2004-03-30

    A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantum dots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantum dot band gaps are parametrized for future references.

  20. 40 CFR Appendix Vi to Part 266 - Stack Plume Rise

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 40 Protection of Environment 28 2012-07-01 2012-07-01 false Stack Plume Rise VI Appendix VI to Part 266 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED... FACILITIES Pt. 266, App. VI Appendix VI to Part 266—Stack Plume Rise Flow rate (m3/s) Exhaust Temperature...

  1. 40 CFR Appendix Vi to Part 266 - Stack Plume Rise

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 40 Protection of Environment 27 2014-07-01 2014-07-01 false Stack Plume Rise VI Appendix VI to Part 266 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED... FACILITIES Pt. 266, App. VI Appendix VI to Part 266—Stack Plume Rise Flow rate (m3/s) Exhaust Temperature...

  2. 40 CFR Appendix Vi to Part 266 - Stack Plume Rise

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ... 40 Protection of Environment 26 2010-07-01 2010-07-01 false Stack Plume Rise VI Appendix VI to Part 266 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED... FACILITIES Pt. 266, App. VI Appendix VI to Part 266—Stack Plume Rise Flow rate (m3/s) Exhaust Temperature...

  3. 40 CFR Appendix Vi to Part 266 - Stack Plume Rise

    Code of Federal Regulations, 2011 CFR

    2011-07-01

    ... 40 Protection of Environment 27 2011-07-01 2011-07-01 false Stack Plume Rise VI Appendix VI to Part 266 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED... FACILITIES Pt. 266, App. VI Appendix VI to Part 266—Stack Plume Rise Flow rate (m3/s) Exhaust Temperature...

  4. 40 CFR Appendix Vi to Part 266 - Stack Plume Rise

    Code of Federal Regulations, 2013 CFR

    2013-07-01

    ... 40 Protection of Environment 28 2013-07-01 2013-07-01 false Stack Plume Rise VI Appendix VI to Part 266 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED... FACILITIES Pt. 266, App. VI Appendix VI to Part 266—Stack Plume Rise Flow rate (m3/s) Exhaust Temperature...

  5. 40 CFR 144.18 - Requirements for Class VI wells.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 40 Protection of Environment 23 2014-07-01 2014-07-01 false Requirements for Class VI wells. 144.18 Section 144.18 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) WATER... Requirements for Class VI wells. Owners or operators of Class VI wells must obtain a permit. Class VI...

  6. 19 CFR Annex Vi to Part 351 - Countervailing Investigations Timeline

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ... 19 Customs Duties 3 2013-04-01 2013-04-01 false Countervailing Investigations Timeline VI Annex VI to Part 351 Customs Duties INTERNATIONAL TRADE ADMINISTRATION, DEPARTMENT OF COMMERCE ANTIDUMPING AND COUNTERVAILING DUTIES Pt. 351, Annex VI Annex VI to Part 351—Countervailing Investigations Timeline ER19MY97.000...

  7. 40 CFR 144.18 - Requirements for Class VI wells.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 40 Protection of Environment 24 2012-07-01 2012-07-01 false Requirements for Class VI wells. 144.18 Section 144.18 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) WATER... Requirements for Class VI wells. Owners or operators of Class VI wells must obtain a permit. Class VI...

  8. 19 CFR Annex Vi to Part 351 - Countervailing Investigations Timeline

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ... 19 Customs Duties 3 2012-04-01 2012-04-01 false Countervailing Investigations Timeline VI Annex VI to Part 351 Customs Duties INTERNATIONAL TRADE ADMINISTRATION, DEPARTMENT OF COMMERCE ANTIDUMPING AND COUNTERVAILING DUTIES Pt. 351, Annex VI Annex VI to Part 351—Countervailing Investigations Timeline ER19MY97.000...

  9. 24 CFR 971.11 - HOPE VI developments.

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 24 Housing and Urban Development 4 2011-04-01 2011-04-01 false HOPE VI developments. 971.11... § 971.11 HOPE VI developments. Developments with HOPE VI implementation grants that have approved HOPE VI revitalization plans will be treated as having shown the ability to achieve long-term...

  10. 40 CFR 144.18 - Requirements for Class VI wells.

    Code of Federal Regulations, 2013 CFR

    2013-07-01

    ... 40 Protection of Environment 24 2013-07-01 2013-07-01 false Requirements for Class VI wells. 144.18 Section 144.18 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) WATER... Requirements for Class VI wells. Owners or operators of Class VI wells must obtain a permit. Class VI...

  11. 40 CFR 144.18 - Requirements for Class VI wells.

    Code of Federal Regulations, 2011 CFR

    2011-07-01

    ... 40 Protection of Environment 23 2011-07-01 2011-07-01 false Requirements for Class VI wells. 144.18 Section 144.18 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) WATER... Requirements for Class VI wells. Owners or operators of Class VI wells must obtain a permit. Class VI...

  12. 24 CFR 971.11 - HOPE VI developments.

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ... 24 Housing and Urban Development 4 2012-04-01 2012-04-01 false HOPE VI developments. 971.11... § 971.11 HOPE VI developments. Developments with HOPE VI implementation grants that have approved HOPE VI revitalization plans will be treated as having shown the ability to achieve long-term...

  13. 19 CFR Annex Vi to Part 351 - Countervailing Investigations Timeline

    Code of Federal Regulations, 2014 CFR

    2014-04-01

    ... 19 Customs Duties 3 2014-04-01 2014-04-01 false Countervailing Investigations Timeline VI Annex VI to Part 351 Customs Duties INTERNATIONAL TRADE ADMINISTRATION, DEPARTMENT OF COMMERCE ANTIDUMPING AND COUNTERVAILING DUTIES Pt. 351, Annex VI Annex VI to Part 351—Countervailing Investigations Timeline ER19MY97.000...

  14. 24 CFR 971.11 - HOPE VI developments.

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ... 24 Housing and Urban Development 4 2013-04-01 2013-04-01 false HOPE VI developments. 971.11... § 971.11 HOPE VI developments. Developments with HOPE VI implementation grants that have approved HOPE VI revitalization plans will be treated as having shown the ability to achieve long-term...

  15. 24 CFR 971.11 - HOPE VI developments.

    Code of Federal Regulations, 2014 CFR

    2014-04-01

    ... 24 Housing and Urban Development 4 2014-04-01 2014-04-01 false HOPE VI developments. 971.11... § 971.11 HOPE VI developments. Developments with HOPE VI implementation grants that have approved HOPE VI revitalization plans will be treated as having shown the ability to achieve long-term...

  16. 19 CFR Annex Vi to Part 351 - Countervailing Investigations Timeline

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 19 Customs Duties 3 2010-04-01 2010-04-01 false Countervailing Investigations Timeline VI Annex VI to Part 351 Customs Duties INTERNATIONAL TRADE ADMINISTRATION, DEPARTMENT OF COMMERCE ANTIDUMPING AND COUNTERVAILING DUTIES Pt. 351, Annex VI Annex VI to Part 351—Countervailing Investigations Timeline ER19MY97.000...

  17. 19 CFR Annex Vi to Part 351 - Countervailing Investigations Timeline

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 19 Customs Duties 3 2011-04-01 2011-04-01 false Countervailing Investigations Timeline VI Annex VI to Part 351 Customs Duties INTERNATIONAL TRADE ADMINISTRATION, DEPARTMENT OF COMMERCE ANTIDUMPING AND COUNTERVAILING DUTIES Pt. 351, Annex VI Annex VI to Part 351—Countervailing Investigations Timeline ER19MY97.000...

  18. Quench dynamics in confined 1 + 1-dimensional systems

    NASA Astrophysics Data System (ADS)

    Engelhardt, Dalit

    2016-03-01

    We present a framework for investigating the response of conformally invariant confined 1 + 1-dimensional systems to a quantum quench. While conformal invariance is generally destroyed in a global quantum quench, systems that can be described as or mapped to integrable deformations of a CFT may present special instances where a conformal field theory-based analysis could provide useful insight into the non-equilibrium dynamics. We investigate this possibility by considering a quench analogous to that of the quantum Newton’s Cradle experiment (Kinoshita et al 2006 Nature 440 900) and demonstrating qualitative agreement between observables derived in the CFT framework and those of the experimental system. We propose that this agreement may be a feature of the proximity of the experimental system to an integrable deformation of a c = 1 CFT.

  19. ETS-VI multibeam satellite communications systems

    NASA Astrophysics Data System (ADS)

    Kawai, Makoto; Tanaka, Masayoshi; Ohtomo, Isao

    1989-10-01

    The fixed and mobile satellite communications systems of the Japanese Engineering Test Satellite-VI (ETS-VI) are described. The system requirements are outlined along with the system configuration. The ETS-VI multibeam system employs three frequency bands. When used for Ka-band fixed communications, it covers the Japanese main islands with thirteen 0.3-degree-wide spot beam. Four of the beams are active for ETS-VI. When used for S-band mobile communications, five beams cover the area within 200 nautical miles from the Japanese coast. The C-band beam for fixed communications covers the central area of the Japanese main islands with a single beam. The onboard antenna system is described along with the transponders and their associated onboard systems. A discussion of the system technology follows, covering the TDMA transmisssion system, the relay function, rainfall compensation, and the antenna and propagation performance.

  20. XAS investigations of Fe(VI).

    SciTech Connect

    Kemner, K. M.; Kelly, S. D.; Orlandini, K. A.; Tsapin, A. I.; Goldfeld, M. G.; Perfiliev, Y. D.; Nealson, K. H.; Environmental Research; APS-USR; Jet Propulsion Lab.; Moscow State Univ.

    2001-03-01

    Recent attention has been given to a reexamination of results from the early Viking missions to Mars that suggested the presence of one or more strong oxidants in Martian soil. Since Fe is one of the main constituents of the Martian surface and Fe(VI) is known to be a highly reactive, strong oxidant, we have made XANES and EXAFS measurements of Fe(II), Fe(III), Fe(IV), and Fe(VI) in solid and solution forms. Results from these studies indicate a pre-edge XANES feature from Fe(VI) samples similar to that commonly seen from Cr(VI) samples. Results of first shell analysis indicate a linear relationship between the Fe-O bond length and Fe valence state.

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

    NASA Astrophysics Data System (ADS)

    Vaseghi, B.; Hashemi, H.

    2016-06-01

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

  2. Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs quantum wells and quantum dots

    NASA Technical Reports Server (NTRS)

    Leon, R.; Swift, G. M.; Magness, B.; Taylor, W. A.; Tang, Y. S.; Wang, K. L.; Dowd, P.; Zhang, Y. H.

    2000-01-01

    The photoluminescence emission from InGaAs/GaAs quantum-well and quantum-dot (QD) structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented here show a significant enhancement in radiation tolerance with three-dimensional quantum confinement.

  3. PREFACE: Water in confined geometries

    NASA Astrophysics Data System (ADS)

    Rovere, Mauro

    2004-11-01

    The study of water confined in complex systems in solid or gel phases and/or in contact with macromolecules is relevant to many important processes ranging from industrial applications such as catalysis and soil chemistry, to biological processes such as protein folding or ionic transport in membranes. Thermodynamics, phase behaviour and the molecular mobility of water have been observed to change upon confinement depending on the properties of the substrate. In particular, polar substrates perturb the hydrogen bond network of water, inducing large changes in the properties upon freezing. Understanding how the connected random hydrogen bond network of bulk water is modified when water is confined in small cavities inside a substrate material is very important for studies of stability and the enzymatic activity of proteins, oil recovery or heterogeneous catalysis, where water-substrate interactions play a fundamental role. The modifications of the short-range order in the liquid depend on the nature of the water-substrate interaction, hydrophilic or hydrophobic, as well as on its spatial range and on the geometry of the substrate. Despite extensive study, both experimentally and by computer simulation, there remain a number of open problems. In the many experimental studies of confined water, those performed on water in Vycor are of particular interest for computer simulation and theoretical studies since Vycor is a porous silica glass characterized by a quite sharp distribution of pore sizes and a strong capability to absorb water. It can be considered as a good candidate for studying the general behaviour of water in hydrophilic nanopores. But there there have been a number of studies of water confined in more complex substrates, where the interpretation of experiments and computer simulation is more difficult, such as in zeolites or in aerogels or in contact with membranes. Of the many problems to consider we can mention the study of supercooled water. It is

  4. Biotreatment of chromium (VI) effluents

    SciTech Connect

    Tavares, T.; Neto, P.; Martins, C.

    1995-12-31

    The presence of heavy metals in industrial wastewaters is still a serious problem for some local small and medium size industries. Particularly electroplating and tanneries produce highly concentrated chromium effluents, which are treated by traditional physico-chemical processes. Those are able to reduce the total chromium concentration from some hundreds of mg.l{sup {minus}1} to very low concentrations, but the allowable final value of 0.1 mg.l{sup {minus}1} is hardly obtained as the referred processes become too costly for those small and medium size industries. The aim of these studies is the definition of an efficient system, economically attractive and friendly to the environment, based on the ability of some microorganisms to concentrate heavy metals. This system would be used as a final treatment step to remove low concentrations of hexavalent chromium. Three different bacteria were used in batch systems to evaluate their resistance to Cr(VI) and their ability to reduce it to the trivalent form. The results were compared with those obtained with microorganisms isolated from sludge of treatment plants receiving wastewater loaded with chromium. One of those bacteria was supported on granular activated carbon and the biofilm was optimized in terms of adhesion and removal efficiency. The chromium adsorption capacity of the support was also studied as albeit it is known that adsorption is not used for heavy metals removal, granular activated carbon is an excellent immobilization support for the biofilm and certainly has some responsibility on the chromium fixation process.

  5. In-well pumped mid-infrared PbTe/CdTe quantum well vertical external cavity surface emitting lasers

    NASA Astrophysics Data System (ADS)

    Khiar, A.; Volobuev, V.; Witzan, M.; Hochreiner, A.; Eibelhuber, M.; Springholz, G.

    2014-06-01

    Optical in-well pumped mid-infrared vertical external cavity surface emitting lasers based on PbTe quantum wells embedded in CdTe barriers are realized. In contrast to the usual ternary barrier materials of lead salt lasers such as PbEuTe of PbSrTe, the combination of narrow-gap PbTe with wide-gap CdTe offers an extremely large carrier confinement, preventing charge carrier leakage from the quantum wells. In addition, optical in-well pumping can be achieved with cost effective and readily available near infrared lasers. Free carrier absorption, which is a strong loss mechanism in the mid-infrared, is strongly reduced due to the insulating property of CdTe. Lasing is observed from 85 K to 300 K covering a wavelength range of 3.3-4.2 μm. The best laser performance is achieved for quantum well thicknesses of 20 nm. At low temperature, the threshold power is around 100 mWP and the output power more than 700 mWP. The significance of various charge carrier loss mechanisms are analyzed by modeling the device performance. Although Auger losses are quite low in IV-VI semiconductors, an Auger coefficient of CA = 3.5 × 10-27 cm6 s-1 was estimated for the laser structure, which is attributed to the large conduction band offset.

  6. Structure development models of ETS-VI

    NASA Astrophysics Data System (ADS)

    Katagi, Tsuguhiko; Tsujihata, Akio; Nishio, Masanobu; Kuwao, Fumihiro; Tsukashima, Takashi; Katoh, Tatsuo; Akaeda, Tadayoshi

    Japan's Engineering Test Satellite (ETS) VI has been designed to conduct several communications experiments relevant to future direct-broadcasting satellites and is currently in its design finalization phase. Two structure-development models have accordingly been devised for static and dynamic loading tests, respectively. Results are presented from the ETS VI development models' modal survey, acoustics, sinusoid vibration, pyrotechnic shock, alignment, and mass properties tests.

  7. Dermal ultrastructure in collagen VI myopathy.

    PubMed

    Hermanns-Lê, Trinh; Piérard, Gérald E; Piérard-Franchimont, Claudine; Delvenne, Philippe

    2014-04-01

    The COL VI mutations are responsible for a spectrum of myopathies. The authors report cutaneous ultrastructural alterations in a patient with COL6A2 myopathy. The changes include variations in size of collagen fibrils, flower-like sections of collagen fibrils, as well as thickening of vessel and nerve basement membranes. Electron microscopy of a skin biopsy contributes to the diagnosis of COL VI myopathies. PMID:24134684

  8. O VI IN THE LOCAL INTERSTELLAR MEDIUM

    SciTech Connect

    Barstow, M. A.; Boyce, D. D.; Barstow, J. K.; Forbes, A. E.; Preval, S.; Welsh, B. Y.; Lallement, R.

    2010-11-10

    We report the results of a search for O VI absorption in the spectra of 80 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In approximately 35% of cases (where photospheric material is detected), the velocity differences between the interstellar and photospheric components were beneath the resolution of the FUSE spectrographs. Therefore, in 65% of these stars the interstellar and photospheric contributions could be separated and the nature of the O VI component unambiguously determined. Furthermore, in other examples, where the spectra were of a high signal-to-noise, no photospheric material was found and any O VI detected was assumed to be interstellar. Building on the earlier work of Oegerle et al. and Savage and Lehner, we have increased the number of detections of interstellar O VI and, for the first time, compared their locations with both the soft X-ray background emission and new detailed maps of the distribution of neutral gas within the local interstellar medium. We find no strong evidence to support a spatial correlation between O VI and SXRB emission. In all but a few cases, the interstellar O VI was located at or beyond the boundaries of the local cavity. Hence, any T {approx} 300,000 K gas responsible for the O VI absorption may reside at the interface between the cavity and surrounding medium or in that medium itself. Consequently, it appears that there is much less O VI-bearing gas than previously stated within the inner rarefied regions of the local interstellar cavity.

  9. ORNL fission product release tests VI-6

    SciTech Connect

    Osborne, M.F.; Lorenz, R.A.; Collins, J.L.; Lee, C.S.

    1991-01-01

    The ORNL fission product release tests investigate release and transport of the major fission products from high-burnup fuel under LWR accident conditions. The two most recent tests (VI-4 and VI-5) were conducted in hydrogen. In three previous tests in this series (VI-1, VI-2, and VI-3), which had been conducted in steam, the oxidized Zircaloy cladding remained largely intact and acted as a barrier to steam reaction with the UO{sub 2}. Test VI-6 was designed to insure significant oxidation of the UO{sub 2} fuel, which has been shown to enhance release of certain fission products, especially molybdenum and ruthenium. The BR3 fuel specimen used in test VI-6 will be heated in hydrogen to 2300 K; the Zircaloy cladding is expected to melt and runoff at {approximately}2150 K. Upon reaching the 2300 K test temperature, the test atmosphere will be changed to steam, and that temperature will be maintained for 60 min, with the three collection trains being operated for 2-, 18-, and 40-min periods. The releases of {sup 85}Kr and {sup 137}Cs will be monitored continuously throughout the test. Posttest analyses of the material collected on the three trains will provide results on the release and transport of Mo, Ru, Sb, Te, Ba, Ce, and Eu as a function of time at 2300 K. Continuous monitoring of the hydrogen produced during the steam atmosphere period at high temperature will provide a measure of the oxidation rate of the cladding and fuel. Following delays in approval of the safety documentation and in decontamination of the hot cell and test apparatus, test VI-6 will be conducted in late May.

  10. Optical Study of Exciton Localization Phenomena in Semimagnetic Semiconductors and Their Multiple Quantum Wells.

    NASA Astrophysics Data System (ADS)

    Zhang, Xi-Cheng

    1986-12-01

    The results of picosecond photomodulation and photoluminescence spectroscopies in novel II-VI semimagnetic semiconductors Cd(,1-x)Mn(,x)Te (x < 0.50) bulk and multiple quantum well (MQW) samples are presented. By studying excitonic emission near the bandgap of semiconductors, it is found that excitons can be confined or localized by alloy potential fluctuations, quantum well confinements, local strain of heterointerfaces and energy self-trapping. Steady-state photoluminescence in undoped CdTe/Cd(,1 -x)Mn(,x)Te MQW samples at low temperature shows intense excitonic emission where their radiative quantum efficiencies are two or three orders of magnitude larger than that of the high quality CdTe bulk samples. Time-resolved photoluminescence shows that the excitons have relatively short lifetime (500 picosecond). High quantum efficiency and short exciton lifetime suggest that the radiative recombination is a dominating factor in the excitonic-decay processes in the MQW samples. In general, excitonic emission energies in CdMnTe MQW samples are lower than the free exciton energies (typically 20-40 meV lower as noted from the reflectance spectra). The behavior of these emissions under an external magnetic field (up to 36 tesla) shows that excitons prefer to be localized at the heterointerfaces rather than at the center of the wells in MQW samples. The kinetics of the free and the heterointerface localized excitons in the Cd(,1-x)Mn(,x)Te/Cd(,1-y)Mn(,y)Te MQW samples have been studied by using a transient photoluminescence technique. Exciton lifetimes have been measured in several samples with various quantum well widths. The trapping time of the free exciton localized at the interface has been observed in the wide quantum well samples. The average energy loss rate of localized excitons has been calculated. The resonance excitation spectra of steady-state and transient luminescence show that the exciton spectra are spatially inhomogeneously broadened. An external magnetic

  11. Ferrate(VI) oxidation of aqueous cyanide

    SciTech Connect

    Sharma, V.K.; Rivera, W.; Smith, J.O.; O`Brien, B.

    1998-09-01

    The rates of oxidation of cyanide with Fe(VI) were measured as a function of pH and temperature. The reaction was found to be first order for each reactant. The rates decrease with increasing pH. The energy of activation was found to be 38.9 {+-} 1.0 kJ mol{sup {minus}1} at pH 9.0. The removal of cyanide by oxidation with Fe(VI) was studied at pH 7.5, 9.0, and 12.0. Fe(VI) removal efficiency was greater at pH 9.0 than at pH 7.5 and 12.0. At pH 9.0, Fe(VI) molar consumption was nearly equal to that of oxidized cyanide. Cyanate and nitrite ions were identified as the products of the reaction at pH 7.5. The experiments indicated 1:1 stoichiometric conversion of cyanide to nitrite ion at pH 9.0 and 12.0. Experiments were conducted to test the Fe(VI) removal efficiency of cyanide in electroplating rinsewater. The results indicate that Fe(VI) has the potential to serve as a reliable and safe oxidative treatment for removing cyanide in wastewater effluent.

  12. The confinement induced resonance in spin-orbit coupled cold atoms with Raman coupling

    PubMed Central

    Zhang, Yi-Cai; Song, Shu-Wei; Liu, Wu-Ming

    2014-01-01

    The confinement induced resonance provides an indispensable tool for the realization of the low-dimensional strongly interacting quantum system. Here, we investigate the confinement induced resonance in spin-orbit coupled cold atoms with Raman coupling. We find that the quasi-bound levels induced by the spin-orbit coupling and Raman coupling result in the Feshbach-type resonances. For sufficiently large Raman coupling, the bound states in one dimension exist only for sufficiently strong attractive interaction. Furthermore, the bound states in quasi-one dimension exist only for sufficient large ratio of the length scale of confinement to three dimensional s-wave scattering length. The Raman coupling substantially changes the confinement-induced resonance position. We give a proposal to realize confinement induced resonance through increasing Raman coupling strength in experiments. PMID:24862314

  13. Local-density approximation for confined bosons in an optical lattice

    SciTech Connect

    Bergkvist, Sara; Henelius, Patrik; Rosengren, Anders

    2004-11-01

    We investigate local and global properties of the one-dimensional Bose-Hubbard model with an external confining potential, describing an atomic condensate in an optical lattice. Using quantum Monte Carlo techniques we demonstrate that a local-density approximation, which relates the unconfined and the confined model, yields quantitatively correct results in most of the interesting parameter range. We also examine claims of universal behavior in the confined system, and demonstrate the origin of a previously calculated fine structure in the experimentally accessible momentum distribution.

  14. Phase Sensitivity of Atomic Josephson Junctions with a Bosonic Species Confined by a Double-Well Potential

    NASA Astrophysics Data System (ADS)

    Zhu, Hua-Gui; Huang, Guo-Qiang; Luo, Cui-Lan

    2016-02-01

    We investigate the reciprocal of the mean quantum Fisher information per particle (RMQFIP) and phase sensitivity of atomic Josephson junctions with a bosonic species confined by a double-well potential. Here we are focus on the Rabi oscillation energy's influence on RMQFIP and phase sensitivity. The better quantum entanglement and phase sensitivity may be achieved by decreasing the Rabi oscillation energy.

  15. Phonon confinement in Ge nanocrystals in silicon oxide matrix

    NASA Astrophysics Data System (ADS)

    Jie, Yiaxiong; Wee, A. T. S.; Huan, C. H. A.; Shen, Z. X.; Choi, W. K.

    2011-02-01

    Spherical Ge nanocrystals well-dispersed in amorphous silicon oxide matrix have been synthesized with different sizes, and significant size-dependent Raman shift and broadening have been observed. The lattice constant of Ge nanocrystals well-bonded to silicon oxide matrix has been characterized nearly size-independent. With our proposed stress generation and relaxation mechanisms, stress effects in our samples have been analyzed to be insignificant with respect to phonon confinement effects. The phenomenological model introduced by [Richter, Wang, and Ley, Solid State Commun. 39, 625 (1981] with Gaussian weighting function and TO2 phonon dispersion function has been found to give a quite good description of the measured size-dependence of Raman shift and broadening. A 3-peak fitting method has been proposed to determine Ge nanocrystal size and film crystallinity. After physically quantizing quantum-confined one-dimensional elastic waves, we have deduced that each quantum-confined phonon possesses an instantaneous momentum of a given magnitude ℏk with an equal chance of being either positive or negative and momentum conservation is retained in an electron-phonon scattering process. Therefore, on the basis of the first-principle microscopic model and our experimental results, we deduced that Raman scattering in spherical nanocrystals is a concurrent two-phonon process, one phonon generation and one phonon transition.

  16. Plutonium(VI) accumulation and reduction by lichen biomass: correlation with U(VI).

    PubMed

    Ohnuki, Toshihiko; Aoyagi, Hisao; Kitatsuji, Yoshihiro; Samadfam, Mohammad; Kimura, Yasuhiko; William Purvis, O

    2004-01-01

    The uptake of plutonium(VI) and uranium(VI) by lichen biomass was studied in the foliose lichen Parmotrema tinctorum to elucidate the migration behavior of Pu and U in the terrestrial environment. Pu and U uptake by P. tinctorum averaged 0.040+/-0.010 and 0.055+/-0.015 g gdry (-1), respectively, after 96 h incubation with 4.0 x 10(14) mol 1(-1) Pu solutions of pH 3, 4 and 5. SEM observations showed that the accumulated Pu is evenly distributed on the upper and lower surfaces of P. tinctorum, in contrast to U(VI), which accumulated in both cortical and medullary layers. UV/VIS absorption spectroscopy demonstrates that a fraction of Pu(VI) in the solution is reduced to Pu(V) by the organic substances released from P. tinctorum, and the accumulated Pu on the surface is reduced to Pu(IV), while U(VI) keeps the oxidation state of VI. Since the solubility of Pu(IV) hydroxides is very low, reduced Pu(VI) does not penetrate to the medullary layers, but is probably precipitated as Pu(IV) hydroxides on the cortical lichen surface. It is concluded that the uptake and reduction of Pu(VI) by lichens is important to determine the mobilization and oxidation states of Pu in the terrestrial environment. PMID:15381325

  17. Confined Visible Optical Tamm States

    NASA Astrophysics Data System (ADS)

    Feng, F.; Ouaret, K.; Portalupi, S.; Lafosse, X.; Nasilovski, M.; de Marcillac, W. Daney; Frigerio, J.-M.; Schwob, C.; Dubertret, B.; Maître, A.; Senellart, P.; Coolen, L.

    2016-05-01

    Optical Tamm states are two-dimensional (2D) electromagnetic modes propagating at the interface between a Bragg mirror and a metallic film. When a thin (a few tens of nm) metallic micron-radius disk is deposited on a Bragg mirror, optical Tamm states can be confined below the disk surface, creating a Tamm-states cavity. We describe here the photoluminescence properties of colloidal semiconductor nanocrystals embedded in a Tamm cavity. Tamm states confinement effects are demonstrated and analysed as a function of the disk diameter, and compared with finite-elements simulations.

  18. CONFINEMENT OF HIGH TEMPERATURE PLASMA

    DOEpatents

    Koenig, H.R.

    1963-05-01

    The confinement of a high temperature plasma in a stellarator in which the magnetic confinement has tended to shift the plasma from the center of the curved, U-shaped end loops is described. Magnetic means are provided for counteracting this tendency of the plasma to be shifted away from the center of the end loops, and in one embodiment this magnetic means is a longitudinally extending magnetic field such as is provided by two sets of parallel conductors bent to follow the U-shaped curvature of the end loops and energized oppositely on the inside and outside of this curvature. (AEC)

  19. Low Pressure Phase Transitions in Wurtzite CdSe Quantum Dots

    NASA Astrophysics Data System (ADS)

    Meulenberg, Robert W.; Strouse, Geoffrey F.

    2001-03-01

    Over the last several years, significant efforts in understanding the effects of high pressures on quantum dots (QDs) have been reported. It has been shown that the high-pressure phase transition from wurtzite (WZ) to rock-salt for CdSe QDs is doubled (3 - 6 GPa), but the energy dependence of the absorption edge is near that of the bulk value (partialE/partialP 45 meV/GPa). Upon release of pressure, mixtures of both hexagonal and cubic (WZ and zinc blende (ZB), respectively) structures are seen, due to the low energy of interconversion of the lattice. Surprisingly, ZB is rarely observed for II-VI nanomaterials although it is thermodynamically preferred and moderate to low pressures should induce a WZ -> ZB phase transition. Experiments with pressures in lower pressure ranges (< 1 GPa) have been ignored and may give insight into these types of low energy phase transitions. We report findings of QD size dependent pressure coefficients and postulate that changes in the band structure of quantum confined semiconductors (which lead to these changes in the pressure coefficient) are a function of the compressibility and defect nature of the material, which induce surface reconstruction events. We present optical absorption and photoluminescence data, as well as time-resolved luminescence data to infer to the mechanism of the pressure dependence.

  20. Building solids inside nano-space: from confined amorphous through confined solvate to confined 'metastable' polymorph.

    PubMed

    Nartowski, K P; Tedder, J; Braun, D E; Fábián, L; Khimyak, Y Z

    2015-10-14

    The nanocrystallisation of complex molecules inside mesoporous hosts and control over the resulting structure is a significant challenge. To date the largest organic molecule crystallised inside the nano-pores is a known pharmaceutical intermediate - ROY (259.3 g mol(-1)). In this work we demonstrate smart manipulation of the phase of a larger confined pharmaceutical - indomethacin (IMC, 357.8 g mol(-1)), a substance with known conformational flexibility and complex polymorphic behaviour. We show the detailed structural analysis and the control of solid state transformations of encapsulated molecules inside the pores of mesoscopic cellular foam (MCF, pore size ca. 29 nm) and controlled pore glass (CPG, pore size ca. 55 nm). Starting from confined amorphous IMC we drive crystallisation into a confined methanol solvate, which upon vacuum drying leads to the stabilised rare form V of IMC inside the MCF host. In contrast to the pure form, encapsulated form V does not transform into a more stable polymorph upon heating. The size of the constraining pores and the drug concentration within the pores determine whether the amorphous state of the drug is stabilised or it recrystallises into confined nanocrystals. The work presents, in a critical manner, an application of complementary techniques (DSC, PXRD, solid-state NMR, N2 adsorption) to confirm unambiguously the phase transitions under confinement and offers a comprehensive strategy towards the formation and control of nano-crystalline encapsulated organic solids. PMID:26280634

  1. Quantum interference in plasmonic circuits.

    PubMed

    Heeres, Reinier W; Kouwenhoven, Leo P; Zwiller, Valery

    2013-10-01

    Surface plasmon polaritons (plasmons) are a combination of light and a collective oscillation of the free electron plasma at metal/dielectric interfaces. This interaction allows subwavelength confinement of light beyond the diffraction limit inherent to dielectric structures. As a result, the intensity of the electromagnetic field is enhanced, with the possibility to increase the strength of the optical interactions between waveguides, light sources and detectors. Plasmons maintain non-classical photon statistics and preserve entanglement upon transmission through thin, patterned metallic films or weakly confining waveguides. For quantum applications, it is essential that plasmons behave as indistinguishable quantum particles. Here we report on a quantum interference experiment in a nanoscale plasmonic circuit consisting of an on-chip plasmon beamsplitter with integrated superconducting single-photon detectors to allow efficient single plasmon detection. We demonstrate a quantum-mechanical interaction between pairs of indistinguishable surface plasmons by observing Hong-Ou-Mandel (HOM) interference, a hallmark non-classical interference effect that is the basis of linear optics-based quantum computation. Our work shows that it is feasible to shrink quantum optical experiments to the nanoscale and offers a promising route towards subwavelength quantum optical networks. PMID:23934097

  2. Quantum interference in plasmonic circuits

    NASA Astrophysics Data System (ADS)

    Heeres, Reinier W.; Kouwenhoven, Leo P.; Zwiller, Valery

    2013-10-01

    Surface plasmon polaritons (plasmons) are a combination of light and a collective oscillation of the free electron plasma at metal/dielectric interfaces. This interaction allows subwavelength confinement of light beyond the diffraction limit inherent to dielectric structures. As a result, the intensity of the electromagnetic field is enhanced, with the possibility to increase the strength of the optical interactions between waveguides, light sources and detectors. Plasmons maintain non-classical photon statistics and preserve entanglement upon transmission through thin, patterned metallic films or weakly confining waveguides. For quantum applications, it is essential that plasmons behave as indistinguishable quantum particles. Here we report on a quantum interference experiment in a nanoscale plasmonic circuit consisting of an on-chip plasmon beamsplitter with integrated superconducting single-photon detectors to allow efficient single plasmon detection. We demonstrate a quantum-mechanical interaction between pairs of indistinguishable surface plasmons by observing Hong-Ou-Mandel (HOM) interference, a hallmark non-classical interference effect that is the basis of linear optics-based quantum computation. Our work shows that it is feasible to shrink quantum optical experiments to the nanoscale and offers a promising route towards subwavelength quantum optical networks.

  3. Spatial confinement effects on ultrathin semiconducting polymer heterojunction thin films

    SciTech Connect

    Xuejun Zhang; Jenekhe, S.A.

    1996-12-31

    Thin and ultrathin films of electroactive and photoactive polymers are of growing interest for applications in electronic and optoelectronic devices such as thin film transistors, light emitting diodes, solar cells, and xerographic photoreceptors. Although spatial confinement effects on the electronic, optical, optoelectronic, magnetic, and mechanical properties of inorganic semiconductors, metals, oxides, and ceramics are well known and understood, very little is currently known about nanoscale size effects in electroactive and photoactive polymers. Therefore, we recently initiated studies aimed at the understanding of spatial confinement effects on electroactive and photoactive nanostructured polymers and related thin film devices. We have extensively investigated layered nanoscale semiconducting polymer heterojunctions by applying several experimental techniques including photoluminescence, optical absorption, transient absorption, electroluminescence, cyclic voltammetry, and current-voltage measurements. Our findings reveal clear evidence of spatial confinement effects, including: dramatic enhancement of photoconductivity in ultrathin films; enhancement of electroluminescence efficiency and performance characteristics in nanoscale heterojunction devices; observation of novel phenomena in nanoscale devices. These spatial confinement effects in nanostructured semiconducting polymers can be understood in terms of classical charge transport and interfacial processes without invoking quantum size effects.

  4. Structural and electronic properties of sodium clusters under confinement

    NASA Astrophysics Data System (ADS)

    Nagare, Balasaheb J.; Kanhere, Dilip G.; Chacko, Sajeev

    2015-02-01

    Using real-space density functional theory, electronic structure and equilibrium geometries of sodium clusters in the size range of 2-20 atoms have been calculated as a function of confinement. We have examined the evolution of the five lowest isomers as a function of volume for six different compressions. The minimum volume considered is about 1 /15 to 1 /10 of the free-space box volume. We observe a strong tendency for isomeric transitions in many cases, with the higher isomers evolving into the ground state under confinement. In general the clusters tend to become more spherical. The changes in the total energies and the geometries are not significant until the volume gets reduced beyond the 1/3 of the original volume. In this sense, the clusters are not easy to compress. Once the critical volume is reached, the changes in the total energies and geometries are rapid. It turns out that the increase in the total energy is mainly due to the ion-ion and Hartree energies of electrons. We also address how anisotropic confinement affects the geometry of clusters. We further show that geometries obtained with anisotropic confinement are strongly supported by the simulation of clusters inside a carbon nanotube using a hybrid quantum-mechanical and molecular-mechanics approach.

  5. Quantum superchemistry: Role of trapping profile and quantum statistics

    SciTech Connect

    Olsen, M.K.

    2004-01-01

    The process of Raman photoassociation of a trapped atomic condensate to form condensed molecules has been labeled superchemistry because it can occur at 0 K and experiences coherent bosonic stimulation. We show here that the differences from ordinary chemical processes go even deeper, with the conversion rates depending on the quantum state of the reactants, as expressed by the Wigner function. We consider different initial quantum states of the trapped atomic condensate and different forms of the confining potentials, demonstrating the importance of the quantum statistics and the extra degrees of freedom which massive particles and trapping potentials make available over the analogous optical process of second-harmonic generation. We show that both mean-field analyses and quantum calculations using an inappropriate initial condition can make inaccurate predictions for a given system. This is possible whether using a spatially dependent analysis or a zero-dimensional approach as commonly used in quantum optics.

  6. ViA: a perceptual visualization assistant

    NASA Astrophysics Data System (ADS)

    Healey, Chris G.; St. Amant, Robert; Elhaddad, Mahmoud S.

    2000-05-01

    This paper describes an automated visualized assistant called ViA. ViA is designed to help users construct perceptually optical visualizations to represent, explore, and analyze large, complex, multidimensional datasets. We have approached this problem by studying what is known about the control of human visual attention. By harnessing the low-level human visual system, we can support our dual goals of rapid and accurate visualization. Perceptual guidelines that we have built using psychophysical experiments form the basis for ViA. ViA uses modified mixed-initiative planning algorithms from artificial intelligence to search of perceptually optical data attribute to visual feature mappings. Our perceptual guidelines are integrated into evaluation engines that provide evaluation weights for a given data-feature mapping, and hints on how that mapping might be improved. ViA begins by asking users a set of simple questions about their dataset and the analysis tasks they want to perform. Answers to these questions are used in combination with the evaluation engines to identify and intelligently pursue promising data-feature mappings. The result is an automatically-generated set of mappings that are perceptually salient, but that also respect the context of the dataset and users' preferences about how they want to visualize their data.

  7. Dislocation dynamics in confined geometry

    NASA Astrophysics Data System (ADS)

    Gómez-García, D.; Devincre, B.; Kubin, L.

    1999-05-01

    A simulation of dislocation dynamics has been used to calculate the critical stress for a threading dislocation moving in a confined geometry. The optimum conditions for conducting simulations in systems of various sizes, down to the nanometer range, are defined. The results are critically compared with the available theoretical and numerical estimates for the problem of dislocation motion in capped layers.

  8. Dirac equations with confining potentials

    NASA Astrophysics Data System (ADS)

    Noble, J. H.; Jentschura, U. D.

    2015-01-01

    This paper is devoted to a study of relativistic eigenstates of Dirac particles which are simultaneously bound by a static Coulomb potential and added linear confining potentials. Under certain conditions, despite the addition of radially symmetric, linear confining potentials, specific bound-state energies surprisingly preserve their exact Dirac-Coulomb values. The generality of the "preservation mechanism" is investigated. To this end, a Foldy-Wouthuysen transformation is used to calculate the corrections to the spin-orbit coupling induced by the linear confining potentials. We find that the matrix elements of the effective operators obtained from the scalar, and time-like confining potentials mutually cancel for specific ratios of the prefactors of the effective operators, which must be tailored to the preservation mechanism. The result of the Foldy-Wouthuysen transformation is used to verify that the preservation is restricted (for a given Hamiltonian) to only one reference state, rather than traceable to a more general relationship among the obtained effective low-energy operators. The results derived from the nonrelativistic effective operators are compared to the fully relativistic radial Dirac equations. Furthermore, we show that the preservation mechanism does not affect antiparticle (negative-energy) states.

  9. Mirror Confinement Systems: project summaries

    SciTech Connect

    Not Available

    1980-07-01

    This report contains descriptions of the projects supported by the Mirror Confinement Systems (MCS) Division of the Office of Fusion Energy. The individual project summaries were prepared by the principal investigators, in collaboration with MCS staff office, and include objectives and milestones for each project. In addition to project summaries, statements of Division objectives and budget summaries are also provided.

  10. Momentum Confinement at Low Torque

    SciTech Connect

    Solomon, W M; Burrell, K H; deGrassie, J S; Budny, R; Groebner, R J; Heidbrink, W W; Kinsey, J E; Kramer, G J; Makowski, M A; Mikkelsen, D; Nazikian, R; Petty, C C; Politzer, P A; Scott, S D; Van Zeeland, M A; Zarnstorff, M C

    2007-06-26

    Momentum confinement was investigated on DIII-D as a function of applied neutral beam torque at constant normalized {beta}{sub N}, by varying the mix of co (parallel to the plasma current) and counter neutral beams. Under balanced neutral beam injection (i.e. zero total torque to the plasma), the plasma maintains a significant rotation in the co-direction. This 'intrinsic' rotation can be modeled as being due to an offset in the applied torque (i.e. an 'anomalous torque'). This anomalous torque appears to have a magnitude comparable to one co-neutral beam source. The presence of such an anomalous torque source must be taken into account to obtain meaningful quantities describing momentum transport, such as the global momentum confinement time and local diffusivities. Studies of the mechanical angular momentum in ELMing H-mode plasmas with elevated q{sub min} show that the momentum confinement time improves as the torque is reduced. In hybrid plasmas, the opposite effect is observed, namely that momentum confinement improves at high torque/rotation. The relative importance of E x B shearing between the two is modeled using GLF23 and may suggest a possible explanation.

  11. Coulomb gauge confinement in the heavy quark limit

    SciTech Connect

    Popovici, C.; Watson, P.; Reinhardt, H.

    2010-05-15

    The relationship between the nonperturbative Green's functions of Yang-Mills theory and the confinement potential is investigated. By rewriting the generating functional of quantum chromodynamics in terms of a heavy quark mass expansion in Coulomb gauge, restricting to leading order in this expansion and considering only the two-point functions of the Yang-Mills sector, the rainbow-ladder approximation to the gap and Bethe-Salpeter equations is shown to be exact in this case and an analytic, nonperturbative solution is presented. It is found that there is a direct connection between the string tension and the temporal gluon propagator. Further, it is shown that for the 4-point quark correlation functions, only confined bound states of color-singlet quark-antiquark (meson) and quark-quark (baryon) pairs exist.

  12. O VI absorption in interstellar cloud surfaces

    NASA Technical Reports Server (NTRS)

    Cowie, L. L.; Jenkins, E. B.; Songaila, A.; York, D. G.

    1979-01-01

    The velocity profiles of O VI absorption lines of 24 stars, observed in early Copernicus surveys, have been compared with the line profiles of Si III (1206.51 A) and N II (1083.99 A). The velocity structures of the O VI lines appear to be correlated with those of the material in the lower ionization stages. It is argued that the O VI absorption arises in the coronal gas of the conductive interface between hot gas, responsible for extended, soft X-ray emission, and cooler interstellar clouds. The velocity broadening of both sets of lines is attributed to motions of the cloud surfaces induced by pressure fluctuations in the interstellar medium.

  13. Confinement and Structural Changes in Vertically Aligned Dust Structures

    NASA Astrophysics Data System (ADS)

    Hyde, Truell

    2013-10-01

    In physics, confinement is known to influence collective system behavior. Examples include coulomb crystal variants such as those formed from ions or dust particles (classical), electrons in quantum dots (quantum) and the structural changes observed in vertically aligned dust particle systems formed within a glass box placed on the lower electrode of a Gaseous Electronics Conference (GEC) rf reference cell. Recent experimental studies have expanded the above to include the biological domain by showing that the stability and dynamics of proteins confined through encapsulation and enzyme molecules placed in inorganic cavities such as those found in biosensors are also directly influenced by their confinement. In this paper, the self-assembly and subsequent collective behavior of structures formed from n, charged dust particles interacting with one another and located within a glass box placed on the lower, powered electrode of a GEC rf reference cell is discussed. Self-organized formation of vertically aligned one-dimensional chains, two-dimensional zigzag structures, and three-dimensional helical structures of triangular, quadrangular, pentagonal, hexagonal, and heptagonal symmetries are shown to occur. System evolution is shown to progress from one-dimensional chain structures, through a zigzag transition to a two-dimensional, spindle like structures, and then to various three-dimensional, helical structures exhibiting various symmetries. Stable configurations are shown to be strongly dependent upon system confinement. The critical conditions for structural transitions as well as the basic symmetry exhibited by the one-, two-, and three-dimensional structures that subsequently develop will be shown to be in good agreement with molecular dynamics simulations.

  14. Telegraph noise in coupled quantum dot circuits induced by a quantum point contact.

    PubMed

    Taubert, D; Pioro-Ladrière, M; Schröer, D; Harbusch, D; Sachrajda, A S; Ludwig, S

    2008-05-01

    Charge detection utilizing a highly biased quantum point contact has become the most effective probe for studying few electron quantum dot circuits. Measurements on double and triple quantum dot circuits is performed to clarify a back action role of charge sensing on the confined electrons. The quantum point contact triggers inelastic transitions, which occur quite generally. Under specific device and measurement conditions these transitions manifest themselves as bounded regimes of telegraph noise within a stability diagram. A nonequilibrium transition from artificial atomic to molecular behavior is identified. Consequences for quantum information applications are discussed. PMID:18518321

  15. Fisher Information and Shannon Entropy in Confined 1D Harmonic Oscillator

    SciTech Connect

    Stevanovic, Ljiljana

    2010-01-21

    Study of the linear harmonic oscillator confined in the square well with impenetrable walls is of great interest since its application for modeling parabolic quantum well semiconductor heterostructures. Fisher information and Shannon entropy, as a complexity measure for its ground and some excited energy levels are reported here.

  16. Confinement transition to density wave order in metallic doped spin liquids

    NASA Astrophysics Data System (ADS)

    Patel, Aavishkar A.; Chowdhury, Debanjan; Allais, Andrea; Sachdev, Subir

    2016-04-01

    Insulating quantum spin liquids can undergo a confinement transition to a valence bond solid via the condensation of topological excitations of the associated gauge theory. We extend the theory of such transitions to fractionalized Fermi liquids (FL*): These are metallic doped spin liquids in which the Fermi surfaces only have gauge neutral quasiparticles. Using insights from a duality transform on a doped quantum dimer model for the U(1)-FL* state, we show that projective symmetry group of the theory of the topological excitations remains unmodified, but the Fermi surfaces can lead to additional frustrating interactions. We propose a theory for the confinement transition of Z2-FL* states via the condensation of visons. A variety of confining, incommensurate density wave states are possible, including some that are similar to the incommensurate d -form factor density wave order observed in several recent experiments on the cuprate superconductors.

  17. Room-temperature efficient light detection by amorphous Ge quantum wells

    PubMed Central

    2013-01-01

    In this work, ultrathin amorphous Ge films (2 to 30 nm in thickness) embedded in SiO2 layers were grown by magnetron sputtering and employed as proficient light sensitizer in photodetector devices. A noteworthy modification of the visible photon absorption is evidenced due to quantum confinement effects which cause both a blueshift (from 0.8 to 1.8 eV) in the bandgap and an enhancement (up to three times) in the optical oscillator strength of confined carriers. The reported quantum confinement effects have been exploited to enhance light detection by Ge quantum wells, as demonstrated by photodetectors with an internal quantum efficiency of 70%. PMID:23496870

  18. Real-time dynamics of the confining string

    NASA Astrophysics Data System (ADS)

    Loshaj, Frasher

    Quantum chromodynamics (QCD) describes the interaction of quarks and gluons, which are charged under the color group. Due to confinement of color charge, only colorless hadrons are observed in experiment. At very short distances (hard processes), perturbation theory is a valid tool for calculations and predictions can be made which agree well with experiment. Confinement, which is not yet understood from first principles, is important even for hard processes, because after the perturbative evolution is finished, the final colored particles combine to create the final state hadrons. There are many effective theories of confinement developed over the years. We will consider the Abelian projection; the gauge theory becomes Abelian-like and the theory contains magnetic monopoles. Confinement happens due to the dual Meissner effect, where dual in this case means the roles of the electric and magnetic fields are reversed. The field between charges resembles that of an Abrikosov-Nielsen-Olesen vortex or string. Based on the Abelian nature of the confining string, because fermion zero modes are localized along the vortex and by considering very energetic jets, we assume that the dynamics along this string is described by massless quantum electrodynamics in 1+1 dimensions. This theory shares with QCD many important properties: confinement, chiral symmetry breaking, theta-vacuum, and is exactly soluble. We use the model to compute the fragmentation functions of jets in electron-positron annihilation and after fixing two adjustable parameters, we study the modification of fragmentation functions of jets in the QCD medium. We address an important puzzle in hadron scattering: the soft photon yield in processes with hadrons in the final state is much larger than what is expected from the Low theorem. We find that soft photons produced from currents induced during the real-time dynamics of jet fragmentation can contribute in the enhancement of photons. We compare the result with

  19. Switching-on quantum size effects in silicon nanocrystals.

    PubMed

    Sun, Wei; Qian, Chenxi; Wang, Liwei; Wei, Muan; Mastronardi, Melanie L; Casillas, Gilberto; Breu, Josef; Ozin, Geoffrey A

    2015-01-27

    The size-dependence of the absolute luminescence quantum yield of size-separated silicon nanocrystals reveals a "volcano" behavior, which switches on around 5 nm, peaks at near 3.7-3.9 nm, and decreases thereafter. These three regions respectively define: i) the transition from bulk to strongly quantum confined emissive silicon, ii) increasing confinement enhancing radiative recombination, and iii) increasing contributions favoring non-radiative recombination. PMID:25472530

  20. Quantum correlation via quantum coherence

    NASA Astrophysics Data System (ADS)

    Yu, Chang-shui; Zhang, Yang; Zhao, Haiqing

    2014-06-01

    Quantum correlation includes quantum entanglement and quantum discord. Both entanglement and discord have a common necessary condition—quantum coherence or quantum superposition. In this paper, we attempt to give an alternative understanding of how quantum correlation is related to quantum coherence. We divide the coherence of a quantum state into several classes and find the complete coincidence between geometric (symmetric and asymmetric) quantum discords and some particular classes of quantum coherence. We propose a revised measure for total coherence and find that this measure can lead to a symmetric version of geometric quantum correlation, which is analytic for two qubits. In particular, this measure can also arrive at a monogamy equality on the distribution of quantum coherence. Finally, we also quantify a remaining type of quantum coherence and find that for two qubits, it is directly connected with quantum nonlocality.