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The quantum anomaly can be written alternatively into a form violating conservation laws or as non-gauge invariant currents seen explicitly on the example of chiral anomaly. By reinterpreting the many-body averaging, the connection to Pauli-Villars regularization is established which gives the anomalous term a new interpretation as arising from quantum fluctuations by many-body correlations at short distances. This is exemplified by using an effective many-body quantum potential which realizes quantum Slater sums by classical calculations. It is shown that these quantum potentials avoid the quantum anomaly but approaches the same anomalous result by many-body correlations. A measure for the quality of quantum potentials is suggested to describe these quantum fluctuations in the mean energy. Consequently quantum anomalies might be a short-cut way of single-particle field theory to account for many-body effects. This conjecture is also supported since the chiral anomaly can be derived by a completely conserving quantum kinetic theory.

authoren Time-periodic perturbations due to classical electromagnetic fields are useful to engineer the topological properties of matter using the Floquet theory. Here, we investigate the effect of quantized electromagnetic fields by focusing on the quantized light–matter interaction on the edge state of a quantum spin Hall insulator. We show that this interaction can be studied using an inhomogeneous Dicke model and that the corresponding Hamiltonian is integrable. A Dicke-type superradiant phase transition occurs at arbitrary weak coupling and the electronic spectrum acquires a finite gap. In addition, when the total number of excitations are fixed, a photocurrent is generated along the edge, pseudoquantized as in the low-frequency limit and decaying as for high frequencies with being the photon frequency.

authoren We demonstrate that the selection of a limited number of single excitonic lines from a dense array of epitaxial quantum dots (QDs) can be realized spectrally via resonant energy transfer from a huge number of small QDs toward a limited set of large nano-islands through their excited levels. Optical and transmission electron microscopy studies support this mechanism. Theoretical modeling describes the architecture of quantum levels in the array of CdSe/ZnSe QDs, which enables such a spectral selection. Schematic of resonant energy transfer between QDs shown together with a plan-view TEM image of a QD array.

The optoelectronic properties of β-SnWO4 are investigated in details using experiments on thin film generated by rapid quenching and the first-principles quantum calculations based on the density functional theory (DFT, including the perturbation approach DFPT) and employing the PBE and the range-separated hybrid exchange–correlation HSE06 functionals. The obtained bandgap, optical absorption coefficient, dielectric constant, and charge-carrier effective masses for β-SnWO4 exhibit data irreconcilable with the reported values: e.g., a large and direct bandgap of 4.30 eV (UV-responsive), inconsistent with the values in the literature (visible-responsive). These properties obtained for β-SnWO4 are distinctive from those for α-SnWO4: an indirect bandgap of 1.52 eV with higher charge mobilities. These data of intrinsic stoichiometric materials suggest that the literature reported nonstoichiometric materials where defects significantly influence the optoelectronic properties.

The phonon-induced polarization of pseudospins in graphene is investigated in this work. On the basis of detailed quantum statistical calculations, it is found that the pseudospin average value always undergoes a sign change as the temperature is lowered through some critical value. This shows that the phonons not only induce a pseudospin polarization, but also lead to a phase transition of pseudospins. This phonon-induced pseudospin polarization is somewhat like the photon-induced polarization. If a circularly polarized light participates in pseudospin polarization besides the phonons, a refrigerating effect would occur.

This work presents a quantum mechanical simulation of an indium gallium nitride (InGaN) based light-emitting diode (LED) using the non-equilibrium Green's function (NEGF) method. Due to the wurtzite crystal structure, such LEDs exhibit strong polarization-induced electric fields at heterointerfaces. Standard simulation methods based on the drift-diffusion (DD) model do not take into account potentially important transport mechanisms such as quantum mechanical tunneling or non-equilibrium carrier distributions in the device. These effects are included in the NEGF model, while the semi-classical models employ parametrized approximations. In principle, the NEGF simulation therefore allows a more realistic view of the physical process that take place in the LEDs.

We extend the method of Silvestrelli [P. L. Silvestrelli, J. Chem. Phys. 139, 054106 (2013)] to approximate long-range van der Waals interactions at the density functional level of theory to periodic systems. The eventual approach is based on a combination of maximally localized Wannier functions with the quantum harmonic oscillator-model. Applying this scheme to study London dispersion forces between graphene and water layers, we find that collective many-body effects beyond simple pair-wise additive interactions are essential to accurately describe van der Waals forces.

We report the influence of 2.4 MeV proton irradiation on the photoluminescence (PL) of self-assembled InAs/InP quantum wires (QWRs) and quantum wells (QWs), which show PL emission at similar wavelengths. The proton irradiation leads to an extinction of the PL intensity and to a deterioration of the thermal stability of the PL both in QWR and QW samples. However, the QWRs tend to exhibit higher radiation hardness, especially at low temperatures and upon just above-bandgap excitation.

Electronic transport through the Corbino disk in a doped graphene–substrate system is studied taking into account intrinsic spin–orbit interaction (SOI) and perpendicular magnetic field. By combining the intrinsic SOI and substrate–graphene effects, we are able to break the Kramers degeneracy associated to degrees of freedom and to control the spin-polarized resonant transmission channels. The latter manifests itself in both conductance and Fano factor characteristics. As an astonishing result, we have pointed out a new universal limit for the quantum shot noise, related to this particular topology.

Spin noise spectroscopy is a promising technique for revealing the microscopic nature of spin dephasing processes in quantum dots (QDs). We compare the spin-noise in an ensemble of singly charged QDs calculated by two complementary approaches. The Chebyshev polynomial expansion technique (CET) accounts for the full quantum mechanical fluctuation of the nuclear spin bath and a semi-classical approach (SCA) is based on the averaging the electron spin dynamics over all different static Overhauser field configurations. We observe a remarkable agreement between both methods in the high-frequency part of the spectra determined by static nuclear fields. The low-frequency part is determined by the long time fluctuations of the Overhauser field. We find small differences in the spectra depending on the distribution of hyperfine couplings. The spin-noise spectra in strong enough magnetic fields where the nuclear dynamics is quenched calculated by two complimentary approaches are in perfect agreement.

In this paper, we review the theory of spin switching for systems of different sizes, from a quantum mechanical master equation approach for small atomic clusters to the quasiclassical description of spin dynamics by a stochastic Landau–Lifshits–Gilbert equation in metallic nanoscale magnetic systems. We present characteristic results emphasizing the role of the quantum character of spin at different scales. Comparing the quantum mechanical and the quasiclassical approach to spin switching, we draw analogies between the factors affecting quantum and classical spin dynamics. We analyze assumptions used in each approach and emphasize the limits of applicability of the quantum mechanical and of the quasiclassical descriptions.

An analytical study on controlling quantum breathers in one-dimensional ferromagnetic XXZ spin chains through an oblique magnetic field is presented in this paper. Based on the time-dependent Hartree approximation and the semidiscrete multiple-scale method, we prove the existence of quantum breathers, analyze conditions for their appearance, and discuss their properties. Our results show that the appearance and features of quantum breathers can be controlled by varying the value of the oblique angle θ.

Magnetic skyrmions, topologically protected vortex-like configurations in spin textures, are of wide conceptual and practical appeal for quantum information technologies, notably in relation to the making of so-called race-track memory devices. Skyrmions can be created, steered and destroyed with magnetic fields and/or (spin) currents. Here we focus on the latter mechanism, analyzed via a microscopic treatment of the skyrmion-current interaction. The system we consider is an isolated skyrmion in a square-lattice cluster, interacting with electrons spins in a current-carrying quantum wire. For the theoretical description, we employ a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff-Baym ansatz (GKBA). This is combined with a treatment of skyrmions based on classical localized spins, with the skyrmion motion described via Ehrenfest dynamics. With our mixed quantum-classical scheme, we assess how time-dependent currents can affect the skyrmion dynamics, and how this in turn depends on electron-electron and spin-orbit interactions in the wire. Our study shows the usefulness of a quantum-classical treatment of skyrmion steering via currents, as a way for example to validate/extract an effective, classical-only, description of skyrmion dynamics from a microscopic quantum modeling of the skyrmion-current interaction.

We develop an efficient numerical method to study the quantum critical behavior of disordered systems with order-parameter symmetry in the large-N limit. It is based on the iterative solution of the large-N saddle-point equations combined with a fast algorithm for inverting the arising large sparse random matrices. As an example, we consider the superconductor-metal quantum phase transition in disordered nanowires. We study the behavior of various observables near the quantum phase transition. Our results agree with recent renormalization group predictions, i.e., the transition is governed by an infinite-randomness critical point, accompanied by quantum Griffiths singularities. In contrast to the existing numerical approach to this problem, our method gives direct access to the temperature dependencies of observables. Moreover, our algorithm is highly efficient because the numerical effort for each iteration scales linearly with the system size. This allows us to study larger systems, with up to 1024 sites, than previous methods. We also discuss generalizations to higher dimensions and other systems including the itinerant antiferromagnetic transitions in disordered metals.

We have developed a numerically exact approach to compute real-time path integral expressions for quantum transport problems out of equilibrium. The scheme is based on a deterministic iterative summation of the path integral (ISPI) for the generating function of nonequilibrium observables of interest. We apply the scheme to compute the charge current or dynamical quantities of small strongly correlated quantum systems as a quantum dot or molecules that are tunnel coupled to leads. Self-energies due to the leads, being nonlocal in time, are fully taken into account within a finite memory time, thereby including non-Markovian effects. Numerical results are extrapolated first to vanishing (Trotter) time discretization and, second, to infinite memory time. The method is applied to nonequilibrium transport through a single-impurity Anderson dot in the first place. We benchmark our results in various regimes of the rich parameter space. In the respective regime of validity, iterative summation of real-time path integrals (ISPI) results are shown to match those of other state-of-the art methods. Especially, we have chosen the mixed valence regime of the Anderson model to compare ISPI to time dependent density matrix renormalization group (tDMRG) and functional RG calculations. Secondly, we determine the nonequilibrium current through a molecular junction in presence of a vibrational mode. We have found an exact mapping of the single impurity Anderson–Holstein model to an effective spin-1 problem. In analytically tractable regimes, as the adiabatic phonon or weak molecule-lead coupling regime, we reproduce known perturbative results. Studying the crossover regime between those limits shows that the Franck–Condon blockade persists in the quantum limit. At low temperature, the Franck–Condon steps in the characteristics are smeared due to nonequilibrium conditions. The third system under investigation here is the magnetic Anderson model which consists of a spinful single-orbital quantum dot with an incorporated quantum mechanical spin–1/2 magnetic impurity. Coulomb interaction together with the exchange coupling of the magnetic impurity with the electron spins strongly influence the dynamics. We investigate the nonequilibrium tunneling current through the system as a function of exchange and Coulomb interaction as well as the real-time impurity polarization. From the real-time evolution of physical observables, we are able to determine characteristics of the time-dependent nonequilibrium current and the relaxation dynamics of the impurity. These examples illustrate that the ISPI technique is particularly well suited for the quantum regime, when all time and energy scales are of the same order of magnitude.

The singularity of quantum correlations, such as quantum entanglement(QE) and quantum discord(QD), has been widely regarded as a valuable indicator for quantum phase transition(QPT) in low-dimensional quantum systems. In this paper, for a spin ladder system with ring exchange, we find that the singularity of QD (or QE) could not indicate the critical points of the system. Instead, the QD shows a novel odd-even effect in some phases, which can be used to detect the phase boundary points. The size effect is related to the symmetry breaking of the ladder.

In order to identify the performance limitations of InGaAs/AlAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependencies of the threshold current (I) were undertaken. Using the theoretical optical phonon current (I) and carrier leakage (I) to fit the measured threshold current at various pressures, we show that the electron scattering from the top lasing level to the upper L-minima gives rise to the increase in I with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3% of I at RT and is negligible at 100K. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate the dynamical properties of the two-channel Anderson model using the noncrossing approximation (NCA) supplemented by numerical renormalization-group calculations. We provide evidence supporting the conventional wisdom that the NCA gives reliable results for the standard two-channel Anderson model of a magnetic impurity in a metal. We extend the analysis to the pseudogap two-channel model describing a semi-metallic host with a density of states that vanishes in power-law fashion at the Fermi energy. This model exhibits continuous quantum phase transitions between weak- and strong-coupling phases. The NCA is shown to reproduce the correct qualitative features of the pseudogap model, including the phase diagram, and to yield critical exponents in excellent agreement with the NRG and exact results. The forms of the dynamical magnetic susceptibility and impurity Green's function at the fixed points are suggestive of frequency-over-temperature scaling.

Strongly correlated electron systems at the border of magnetism are of active current interest, particularly because the accompanying quantum criticality provides a route towards both strange-metal non-Fermi liquid behavior and unconventional superconductivity. Among the many important questions is whether the magnetism acts simply as a source of fluctuations in the textbook Landau framework, or instead serves as a proxy for some unexpected new physics. We put into this general context the recent developments on quantum phase transitions in antiferromagnetic (AF) heavy fermion metals. Among these are the extensive recent theoretical and experimental studies on the physics of Kondo destruction in a class of beyond-Landau quantum critical points (QCPs). Also discussed are the theoretical basis for a global phase diagram of AF heavy fermion metals, and the recent surge of materials suitable for studying this phase diagram. Furthermore, we address the generalization of this global phase diagram to the case of Kondo insulators, and consider the future prospect to study the interplay among Kondo coherence, magnetism, and topological states. Finally, we touch upon related issues beyond the AF settings, arising in mixed valent, ferromagnetic, quadrupolar, or spin glass f-electron systems, as well as some general issues on emergent phases near QCPs.

Materials close to a quantum-critical point – a zero-temperature phase transition – exhibit anomalous thermodynamic properties also at finite temperatures. Close to a magnetic field-induced quantum-critical point, for example, the finite-temperature entropy ST shows strong variations upon varying the magnetic field. Here we discuss the possibility to use this accumulation of entropy around a field-induced quantum-critical point for realizing an efficient magnetic cooling. Our proof-of-principle demonstration is based on measurements and theoretical calculations of the magnetocaloric properties of low-dimensional spin-1/2 antiferromagnets close to their field-induced quantum-critical points. We present results of the magnetocaloric effect ΓB = T−1(∂T/∂B)S ≈ const as a function of both field and temperature in the vicinity of the quantum-critical point and discuss various performance characteristics, such as range of operation, efficiency and hold time. These figures are compared with those of a state-of-the-art paramagnetic coolant.

We present band structure calculations of the non-magnetic compound LuRh2Si2 which serves as a reference for YbRh2Si2, a prototypical material for the investigation of quantum critical points. The relative z position of the Si atoms is found to have a strong impact on the band structure and the Fermi surface topology. We find the total energy to be minimized for z = 0.381c whereas a comparison of predicted extremal orbits with Shubnikov–de Haas frequencies shows best agreement at the experimental value z = 0.379c. We therefore recommend usage of z = 0.379c for future electronic structure calculations of LuRh2Si2 and YbRh2Si2.

We study the evolution of the magnetic ground state close to the putative quantum critical point in CeCoGe2.2Si0.8 by using electrical resistivity and specific heat measurements under the same high hydrostatic pressure conditions. The electrical resistivity shows that the antiferromagnetic ordering temperature, TN, is suppressed above PC = 5.9 kbar, with the emergence of non-Fermi liquid behavior. On the other hand, the specific heat shows two different magnetic transitions at 0 kbar, one at TN and another one at Tl ≈ 0.3 K. Both transition temperatures are reduced by pressure but remain finite up to at least 7.2 kbar. The height of the specific heat anomaly at TN, ΔCN, is strongly reduced above PC. These features suggest that the quantum criticality in CeCoGe2.2Si0.8 is governed by disorder.

The intermetallic compound CeAgSb2 is a rare example of a ferromagnetically ordered Kondo lattice system. The transition temperature of 9.6 K in zero field is sensitive to the application of magnetic fields perpendicular to the easy axis. Therefore transverse field tuning could be used as an effective way to investigate quantum critical behaviour. In this work we present combined measurements of the specific heat and the magnetocaloric effect which allows a comprehensive thermodynamical study of the system. The entropy landscape is constructed as a function of temperature and field tuning.

A double-cascade-type four-level system of semiconductor multiple quantum wells (MQWs) was constructed with biexcitons and excitons. The nonlinear optical properties for amplification, absorption, and dispersion of 2-weak fields in this scheme are investigated. It shows that the amplification, absorption, and dispersion responses of 2-weak fields can be achieved by appropriately adjusting the relative phase, the probe detuning, and the two control Rabi frequencies. The investigation is much more practical than its atomic counterpart because of its flexible design and the widely adjustable parameters. It may provide a new possibility in technological applications for the light amplifier and optical switch working on quantum coherence effects in MQW solid-state systems.

We study the antiferromagnetic phase of the three-dimensional (3D) Hubbard model with nearest-neighbor hopping on a bipartite cubic lattice. We use the quantum SU(2) × U(1) rotor approach that yields a fully self-consistent treatment of the antiferromagnetic state that respects the symmetry properties of the model and satisfies the Mermin–Wagner theorem. As our theory describes the evolution from a Slater (U ≪ t) to a Mott–Heisenberg (U ≫ t) antiferromagnet, we present the phase diagram of the antiferromagnetic Hubbard model as a function of the crossover parameter U/t.

Our previous numerical diagonalization study on the magnetization processes of the kagome- and triangular-lattice antiferromagnets revealed that the former system exhibits a new field-induced phenomenon, called a magnetization ramp, at 1/3 of the saturation magnetization, while the latter one a conventional magnetization plateau. To clarify the difference between the magnetization ramp and plateau, we consider a generalized lattice model with two exchange interactions J1 and J2, which corresponds to the kagome lattice for J2 = 0 and triangular one for J2 = J1. The present numerical diagonalization study revealed that a quantum phase transition between the magnetization ramp and plateau occurs about J2/J1 ∼ 0.1.

The quantum phase crossover in a spin ladder model with four-spin exchange is investigated. Previous studies show that the crossover cannot be detected by the singularity or finite-size analysis of ground-state observables for finite ladders. In this work, we find that the first-excited-state fidelity shows a sudden drop exactly at the crossover point, regardless of the length of the ladder. It suggests that the excited-state fidelity for finite systems is very effective in detecting the crossover in infinite systems. The underlying mechanism is understood through the analysis of re-construction of the low-lying energy levels of the systems.

The Bose–Hubbard model (BHM) is an archetypal quantum lattice system exhibiting a quantum phase transition between its superfluid (SF) and Mott-insulator (MI) phase. Unlike in higher dimensions the phase diagram of the BHM in one dimension possesses regions in which increasing the hopping amplitude can result in a transition from MI to SF and then back to a MI. This type of re-entrance is well known in classical systems like liquid crystals yet its origin in quantum systems is still not well understood. Moreover, this unusual re-entrant character of the BHM is not easily captured in approximate analytical or numerical calculations. Here we study in detail the predictions of three different and widely used approximations; a multi-site mean-field decoupling, a finite-sized cluster calculation, and a real-space renormalization group (RG) approach. It is found that mean-field calculations do not reproduce re-entrance while finite-sized clusters display a precursor to re-entrance. Here we show for the first time that RG does capture the re-entrant feature and constitutes one of the simplest approximation able to do so. The differing abilities of these approaches reveals the importance of describing short-ranged correlations relevant to the kinetic energy of a MI in a particle-number symmetric way.

We formulate a general method to analyze hybridization modes in metamaterials by employing electrical circuit models. By rewriting circuit equations for coupled inductor–capacitor circuit networks in the bra-ket notation, we show that there exists a good analogy between circuit models and quantum tight-binding models in solid-state physics. Our picture is also applicable to transmission systems that do not have tightly bound eigenmodes. This analogy enables the synthesis of various electromagnetic metamaterials having dispersion relations similar to those for electrons in solids and consequently allows the systematic construction of metamaterials with desired characteristics. In this paper, we focus on the formation of flat bands in metamaterials with specific symmetries. The flat band, which corresponds to a slow group velocity of light or a heavy photon, is useful in designing functional metamaterials.

We present and discuss a general density-matrix description of energy-dissipation and decoherence phenomena in open quantum systems, able to overcome the intrinsic limitations of the conventional Markov approximation. In particular, the proposed alternative adiabatic scheme does not threaten positivity at any time. The key idea of our approach rests in the temporal symmetrization and coarse graining of the scattering term in the Liouville-von Neumann equation, before applying the reduction procedure over the environment degrees of freedom. The resulting dynamics is genuinely Lindblad-like and recovers the Fermi's golden rule features in the semiclassical limit. Applications to the prototypical case of a semiconductor quantum dot exposed to incoherent phonon excitation peaked around a central mode are discussed, highlighting the success of our formalism with respect to the critical issues of the conventional Markov limit.

We present a study of the spin transport at low-temperature in the quantum S = 1 one- and two-dimensional Heisenberg antiferromagnet with the Dzyaloshinskii–Moriya interaction (DM). The spin conductivity is calculated using the Schwinger boson representation and the Kubo formalism of transport in the one-dimensional case and using the self-consistent harmonic approximation and the Kubo formalism in the two-dimensional case. The objective is to determine the regular part of the spin conductivity and the Drude weight DS. This determines if the spin transport has a ballistic or diffusive character. We calculate the regular part of the spin conductivity, σreg(ω), as function of the frequency at T = 0 only and the Drude weight at zero and finite temperatures. We obtained that the spin transport is governed by the multimagnon processes in all cases analyzed since the symmetry of the isotropic Heisenberg antiferromagnetic model is broken with the addition of the DM term.

Plasmonic nanoantennas can enhance the radiative decay rate of quantum emitters via the Purcell-effect. Similar to their radiofrequency equivalents, they can also direct the emitted light into preferential directions. In this paper we first investigate plasmonic Yagi-Uda antennas that are able to confine light to and direct light from subwavelength size volumes. Hence, enhanced transition rates and directed emission are expected when near-field coupling between quantum emitters and the antennas is achieved. Second, we present suitable techniques to couple different quantum systems to plasmonic antennas. We use top-down fabrication techniques to achieve positioning of individual quantum emitters relative to plasmonic nanostructures with an accuracy better than 10 nm. We assure a sufficiently small distance for an efficient near-field coupling of the transition dipole to the plasmonic nanoantenna, which is, however, large enough not to quench the transition. The hybrid system using quantum dots, molecules, or nitrogen-vacancy (NV)-centers in diamond can serve as an efficient single photon source. It is suitable for high-speed information transfer at optical frequencies on the nanoscale for future applications.

This paper presents an inductive method for the microscopic description of quantum dot (QD) QED. Our description reproduces known effects up to an arbitrary accuracy, and is extendable to typical semiconductor effects, like many electron- and phonon-interactions. As an application, this method is used to theoretically examine quantum coherence phenomena and their impact on photon statistics for a A-type semiconductnr QD strongly coupled to a single mode cavity and simultaneously excited with an external laser.

The initial steps of photosynthesis require the absorption and subsequent transfer of energy through an intricate network of pigment–protein complexes. Held within the protein scaffold of these complexes, chromophore molecules are densely packed and fixed in specific geometries relative to one another resulting in Coulombic coupling. Excitation energy transfer through these systems can be accomplished with near unity quantum efficiency [Wraight and Clayton, Biochim. Biophys. Acta 333, 246 (1974)]. While replication of this feat is desirable for artificial photosynthesis, the mechanism by which nature achieves this efficiency is unknown. Recent experiments have revealed the presence of long-lived quantum coherences in photosynthetic pigment–protein complexes spanning bacterial and plant species with a variety of functions and compositions. Its ubiquitous presence and wavelike energy transfer implicate quantum coherence as key to the high efficiency achieved by photosynthesis.

We study self-induced coherence (SIC) in a system composed of two coupled quantum dots (QDs). SIC consists in a coherent transfer of excitation between two systems (atoms or QDs) resulting from their collective interaction with the quantum electromagnetic vacuum. This leads to population trapping in a delocalized, optically inactive state. We focus on the effect of a difference in transition energies and coupling between the two emitters on the evolution of exciton occupation in the two QD system.

We have investigated the exciton spin dynamics in an ultrathin CdTe/ZnTe single quantum well (QW) and self-assembled quantum dots (QDs) by observing time-resolved Kerr rotation (TRKR). At temperatures low enough to suppress the thermal escape of holes from the quantum-confinement structures, excitons dominate the TRKR signal. It shows characteristic dependence on the magnetic field, which is distinctly different from that of isolated electrons. From the data, we have estimated the electron–hole exchange energy to be 137 µeV in the QW, and larger than 190 µeV in the QDs.

We present quantum-based simulations of single grain boundary reflectivity of electrons in metals, Cu and Ag. We examine twin and non-twin grain boundaries using non-equilibrium Green's function and first principles methods. We also investigate the mechanism of reflectivity in grain boundaries by modeling atomic vacancies, disorder, and orientation and find that the change in grain orientation and disorder in the boundary itself both contribute significantly to reflectivity. We find that grain boundary reflectivity may vary widely depending on the grain boundary structure consistent with experimental results. Finally, we examine the reflectivity from multiple grain boundaries and find that grain boundary reflectivity may depend on neighboring grain boundaries. This study based on detailed numerical techniques reveals some potential limitations in the independent grain boundary assumptions of the Mayadas-Shatzkes model.

A novel, efficient method for calculating the temperature dependencies of the linear dielectric functions of semiconductor systems and its application are presented. The method follows an intuitive and natural path with ab-initio finite temperature molecular dynamics providing the thermally perturbed atomic configurations, which are used as structural inputs for calculating the dielectric function. The effect of lattice dynamics, including quantum zero point vibration, on the electronic bands and dielectric function of crystalline (c-) GaAs and Si as well as hydrogenated amorphous Si (a-Si:H) is discussed. Our theoretical results for bulk c-GaAs and c-Si in the range from 0 to 1000 K are in good overall agreement with highly accurate ellipsometric measurements. The implementation of the method resolves a serious discrepancy in energy and line shape between experiment and the latest optical models, allof which neglect lattice dynamics, and provides information on the indirect gap and indirect optical transitions in c-Si. For a-Si:H, the calculated temperature dependent optical response combined with the vibrational spectroscopy provides detailed insight into electronic, dynamical properties, and stability of this important prototypical amorphous semiconductor material At semiconductor surfaces, dynamical effects are expected to be even more pronounced due to reduced atom coordination and reconstruction. This is demonstrated for C(111) 2 × 1, an intensively studied but controversial surface of the quantum diamond crystal.

In this paper we review the recent achievements in terms of GaN/ AlGaN-based intersubband (ISB) physics and devices. We first discuss the design issues and in particular the band bending effect and its influence on the ISB absorption. We then illustrate recent achievements in terms of III-nitride ISB devices with special emphasis on electro-optical modulator and a quantum cascade detector operating in the near-infrared. We show that these devices offer high frequency response at telecommunication wavelengths.

The quantum criticality and thermodynamics for the trimerized spin-1/2 isotropic XY chain with three-spin interactions in an external magnetic field are investigated by means of the Green's function theory combined with Jordan-Wigner transformation. The ground-state phase diagrams are explored, in which various phases are identified and described by typical M-h curves. Therein, two cusps emerge for strong three-spin interactions in two gapless phases at low and high fields, respectively. Moreover, the spin correlations and two-site entanglement entropy are calculated for a further understanding of quantum phase transition (QPT). It is also found that the magnetic-field-driven quantum criticality is closely related to the energy spectrum, in which an energy gap responsible for the appearance of 1/3 magnetization plateau can be opened up by three-spin interactions. The critical behavior disappears when the temperature becomes nonzero, yielding only a crossover behavior. In addition, the gapped low-lying excitations are responsible for the observed thermodynamic behaviors, wherein a structure with three peaks in the temperature dependence of specific heat is unveiled. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

It is shown that the Bethe-Salpeter approach, the Bardeen, Cooper, and Schrieffer (BCS) based vertex method, and a generalized random-phase approximation (GRPA) to the many-electron problem in the presence of a condensed quantum phase yield the same theoretical excitation spectrum ω(Q) to collective modes. This spectrum reveals a secondary peak in optical absorption in semiconductors that can be understood as signaling the existence of an excitonic Bose-Einstein condensate (BEC). The analysis shows as well that there is an additional, non-trivial linearly-dispersive " moving" Cooper-pair solution for superconductors in both weak and strong coupling.

Ultracold quantum gases in optical lattices can be used as novel model system for the exploration of strongly correlated many-body phases of bosonic or fermionic particles. Here we report on the realization of metallic, Mott-insulating and band insulating phases in ultracold spin mixtures of fermionic 40 K atoms. The experimental results are compared to ab initio dynamical mean field theory (DMFT) calculations.

The applicability of mean field models of ferroelectric and ferromagnetic quantum critical points is examined for a selection of d-electron systems. Crucially, we find that the tendency of the effective interaction between critical fluctuation modes to become attractive and anomalous as the ordering temperatures tend to absolute zero results in particularly complex and striking phenomena. The multiplicity of quantum critical fields at the border of metallic ferromagnetism, in particular, is discussed here.

The Hall effect of LuRh2Si2--the non-magnetic homologue of the heavy-fermion material YbRh2Si2--is studied with two different setups: In the conventional single-field geometry, the field dependence is analyzed in terms of the differential Hall coefficient. Beyond that, the recently developed crossed-field experiment allows to examine the linear-response Hall coefficient as a function of magnetic field. The results reveal the expected analogy between both experiments which corroborates the equivalent findings in YbRh2Si2. This emphasizes the applicability to investigate field-induced quantum critical points with both methods.