Phase transitions in the quantum Hall liquid in a quantum wire

Two scenarios for the collapse of the ν=1 quantum Hall liquid (QHL) state, with the effective quantum wire (QW) width defined by the Fermi vector k_F, are studied. Here, ν for the QW is defined as the filling factor of Landau levels (LL) at the center of the QW. In the first one there is no electron redistribution at critical magnetic field , where the Fermi energy, E_F, coincides with the bottom of the empty upper spin-split LL. For the ν=1 state is unstable due to exchange-correlation effects and lateral confinement. In the second scenario, a transition to the ν=2 state occurs, with much smaller width, at . The latter scenario is analyzed in the Hartree–Fock approximation (HFA). Here the Hartree contribution to the total energy affects drastically due to strong electron redistribution in the QW. In both scenarios, the exchange-enhanced g-factor is suppressed at B_cr. The critical fields, activation energy, and optical g-factor obtained in the first scenario are very close to the measured ones.     

Observation of the quantum Hall effect in cleaved InAs surfaces

We have performed the in-plane magnetotransport measurements on the two-dimensional electron gas at the cleaved p-InAs (1 1 0) surface by deposition of Ag. The surface electron density N_s is determined from the Hall coefficient at . The coverage dependence of N_s is well explained by the assumption that each adsorbed Ag atom denotes one electron into InAs until the surface Fermi level reaches the adsorbate-induced donor level. The electron mobility μ is about and does not show a clear dependence on the coverage over . In the high-magnetic field regime of B>1/μ, Shubnikov–de Hass oscillations were observed. A beating pattern due to the strong spin–orbit interaction appears for high N_s. For lower N_s of , an apparent quantum Hall plateau for ν=4 and vanishing of the longitudinal resistivity were observed around .     

Two types of quantum-confinement characters for the bound states in the InGaN/GaN quantum wells

The (Ga_1−xIn_xN)_Nw(GaN)_Nb single and multiple quantum wells (SQWs and MQWs) are investigated theoretically using the sp³s^? tight-binding (TB) method with inclusion of spin–orbit interaction. This study explores the effects of barrier thickness L_b, well width L_w, indium content x and valence-band offset (VBO) on the quantum confinement (QC) characteristics of the bound states in the well and on the electronic transitions. The calculations are based on the validity of two assumptions: the virtual crystal approximation (VCA) for the structure of the alloyed Ga_1−xIn_xN wells, and the macroscopic theory of elasticity (MTE) for the structure of the computational supercell as a whole. The results demonstrate the following main trends: (1) the existence of two types of QC characteristics for the bound states in the GaInN alloyed wells. The nitrogen p-level (, which is associated with InN TB parametrization), displays a threshold/edge that divides the bound states into two types: (i) block-like localized states (in the energy range , where is the energy gap of zinc-blende GaN) and (ii) singlet-like localized states (in the energy range , where is the energy gap of zinc-blende InN). The confinement energy versus well width L_w is found to follow an exponential rule in the former energy region and a power-law rule in the latter one. A stronger localization should be expected as the level becomes deeper in the quantum well. (2) The TB results of E_g were compared with the available photoluminescence (PL) data of 1-ML and 2-ML thick SQWs. Taking into account the error bars due to the lattice relaxation and interface specific effects, the TB results provide evidence that the high-energy emissions ( and , for 1-ML- and 2-ML-thick wells, respectively) must have originated from wells with fractional filling (i.e., low indium contents). (3) The TB results predict that the indium content would rise as the well width increases. Unfortunately, overcoming this limitation of fractional monolayers is likely to remain beyond the capability of the currently existing growth techniques. The indium content being kept low is a natural authenticity which is the compromise to make in growing ultrathin GaInN/GaN quantum wells free of misfit dislocations.     

Electron spin resonance of the two-dimensional metallic state and the quantum Hall state in a Si/SiGe quantum well

We report electrically detected electron spin resonance (ESR) measurements of a high mobility two-dimensional (2D) electron system formed in a Si/SiGe quantum well, with millimeter wave in a high magnetic field . The negative ESR signal observed under an in-plane magnetic field gives direct evidence that the spin polarization leads to a resistance increase in the 2D metallic state. Suppression of spin decoherence was observed in the quantum Hall state at the Landau level filling factor ν=2. Strength of the nuclear magnetic field in the resonance is evaluated to be less than , much smaller than that reported for GaAs/AlGaAs heterostructures.     

Anomalous quantum Hall effect induced by nearby quantum dots

Transport measurements in high magnetic fields have been performed on two-dimensional electron system (2DES) separated by a thin barrier layer from a layer of InAs self-assembled quantum dots (QDs). Clear feature of quantum Hall effect was observed in spite of presence of QDs nearby 2DES. However, both magnetoresistance, ρ_xx, and Hall resistance, ρ_xy, are suppressed significantly only in the magnetic field range of filling factor in 2DES ν<1 and voltage applied on a front gate . The results indicate that the electron state in QDs induces spin-flip process in 2DES.     

Elliptic quantum group , Hopf algebroid structure and elliptic hypergeometric series

We propose a new realization of the elliptic quantum group equipped with the H-Hopf algebroid structure on the basis of the elliptic algebra . The algebra has a constructive definition in terms of the Drinfeld generators of the quantum affine algebra and a Heisenberg algebra. This yields a systematic construction of both finite- and infinite-dimensional dynamical representations and their parallel structures to . In particular we give a classification theorem of the finite-dimensional irreducible pseudo-highest weight representations stated in terms of an elliptic analogue of the Drinfeld polynomials. We also investigate a structure of the tensor product of two evaluation representations and derive an elliptic analogue of the Clebsch–Gordan coefficients. We show that it is expressed by using the very-well-poised balanced elliptic hypergeometric series .     

Anisotropic magnetotransport near the quantum Hall state

We report experimental results on an anisotropic magnetotransport of a bilayer electron system at the Landau level filling factor under tilted magnetic fields. We find that the magnetoresistance changes when the direction of applied in-plane field B is changed with respect to the direction of the current while keeping the strength of B fixed. This anisotropy only appears at the ground state where interlayer electron tunneling is permitted. Therefore, this phenomenon indicates that bilayer systems at with interlayer tunneling inherently have a dissipation mechanism related to the direction of the in-plane field.     

Characterization of [ZnO]m[ZnMgO]n multiple quantum wells grown by molecular beam epitaxy

Thermal stability of single-crystalline [ZnO]_m[Zn_0.7Mg_0.3O]_n multiple quantum wells (MQWs) grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy is reported. X-ray diffraction analysis revealed that these MQWs were grown as designed with a fixed Zn_0.7Mg_0.3O barrier width of and a series of ZnO well widths of . Cathodoluminescence spectra from these MQWs consisted of two major peaks; one was the emission from the bound excitons in Zn_0.7Mg_0.3O barrier layers, and the other was that from the confined excitons in ZnO well layers. These structural and optical properties were found to be dramatically changed by the ex situ annealing treatments over 700 °C. These changes were presumably due to the onset of phase separation of the Zn_0.7Mg_0.3O barrier layers with pronounced Mg diffusion toward the ZnO wells.     

Magnetooptical investigations of single self assembled In0.3Ga0.7As quantum dots

We present results from magnetooptical investigations of large elongated single self assembled In_0.3Ga_0.7As quantum dots with a low surface density of . Compared to conventional In_0.6Ga_0.4As quantum dots the dimension of the investigated dots is enlarged by nearly one order of magnitude using a low strain In_0.3Ga_0.7As nucleation layer. In addition, the exciton exhibits a smaller g-factor of 0–0.4 and a larger diamagnetic coefficient of 20– in Faraday geometry, reflecting the increased extension of the exciton wavefunction, with respect to In_0.6Ga_0.4As quantum dots. From power dependent investigations we observe biexciton binding energies ranging from 1.7 to 1.9 meV. Excited state emission appears typically 2–5 meV above the ground state which is consistent with the increased dimensions of the structure. Furthermore we find linear polarization degrees of up to 0.6 from exciton emission of the elongated quantum dot structure.     

Effects of Co doping on antiferromagnetic structure in

We performed the elastic neutron scattering experiments on the mixed compounds CeRh_1-xCo_xIn₅, and found that doping Co into CeRhIn₅ dramatically changes the antiferromagnetic (AF) structure. The incommensurate AF state with the propagation vector of observed in pure CeRhIn₅ is suppressed with increasing x, and new AF states with an incommensurate and a commensurate modulations simultaneously develop near the AF quantum critical point: x_c0.8. These results suggest that the AF correlations with the q_c and q₁ modulations enhanced in the intermediate Co concentrations may play a crucial role in the evolution of the superconductivity observed above x0.4.     

Electronic structure of (0 0 1) AlN/GaN quantum wells by means of a spsd empirical tight-binding Hamiltonian

We have studied the electronic band structure of (0 0 1) AlN/GaN quantum wells by means of a sp³s^∗d⁵ empirical tight-binding Hamiltonian with nearest-neighbor interactions, including spin-orbit coupling and the effects of strain together with the surface Green function matching method. We have analyzed quantum wells with a thickness in the range 2 ? n ? 50, n being the number of principal layers of GaN in the well region. Results are presented for the point and the direction of the 2D Brillouin zone. The orbital character and the spatial localization of the different states have been also studied.     

Resonant Auger decay study of core-excited OCS

The present work aims at characterizing short-lived C1s⁽⁻¹⁾π^*(1) core-excited states of the OCS molecule based on the analysis of the vibrational fine structure and lineshape profiles of the high-resolution resonant Auger decay spectra recorded at the excitation energies along the C1s→π^* resonance in the binding energy region 15–19 eV. Very different behavior in terms of lineshape and resonant enhancement is observed for the , and final states. This is explained by (1) the variation in the C–O bond lengths for the states involved in the electronic relaxation and (2) different contributions in terms of Mulliken population to the molecular orbitals determining the electronic character of the corresponding states. Since the final-state geometries are known from a number of previous experiments and ab initio calculations, the geometry of the C1s⁽⁻¹⁾π^*(1) intermediate states can be predicted in analogy with e.g. the N₂, CO₂ and N₂O molecules.     

Minimal length in quantum mechanics and non-Hermitian Hamiltonian systems

Deformations of the canonical commutation relations lead to non-Hermitian momentum and position operators and therefore almost inevitably to non-Hermitian Hamiltonians. We demonstrate that such type of deformed quantum mechanical systems may be treated in a similar framework as quasi/pseudo and/or -symmetric systems, which have recently attracted much attention. For a newly proposed deformation of exponential type we compute the minimal uncertainty and minimal length, which are essential in almost all approaches to quantum gravity.     

Quantum mechanics of Klein-Gordon-type fields and quantum cosmology

With a view to address some of the basic problems of quantum cosmology, we formulate the quantum mechanics of the solutions of a Klein-Gordon-type field equation: (∂_t²+D)ψ(t)=0, where and D is a positive-definite operator acting in a Hilbert space . In particular, we determine all the positive-definite inner products on the space of the solutions of such an equation and establish their physical equivalence. This specifies the Hilbert space structure of uniquely. We use a simple realization of the latter to construct the observables of the theory explicitly. The field equation does not fix the choice of a Hamiltonian operator unless it is supplemented by an underlying classical system and a quantization scheme supported by a correspondence principle. In general, there are infinitely many choices for the Hamiltonian each leading to a different notion of time-evolution in . Among these is a particular choice that generates t-translations in and identifies t with time whenever D is t-independent. For a t-dependent D, we show that regardless of the choice of the inner product the t-translations do not correspond to unitary evolutions in , and t cannot be identified with time. We apply these ideas to develop a formulation of quantum cosmology based on the Wheeler-DeWitt equation for a Friedman-Robertson-Walker model coupled to a real scalar field with an arbitrary positive confining potential. In particular, we offer a complete solution of the Hilbert space problem, construct the observables, use a position-like observable to introduce the wave functions of the universe (which differ from the Wheeler-DeWitt fields), reformulate the corresponding quantum theory in terms of the latter, reduce the problem of the identification of time to the determination of a Hamiltonian operator acting in , show that the factor-ordering problem is irrelevant for the kinematics of the quantum theory, and propose a formulation of the dynamics. Our method is based on the central postulates of nonrelativistic quantum mechanics, especially the quest for a genuine probabilistic interpretation and a unitary Schrödinger time-evolution. It generalizes to arbitrary minisuperspace (spatially homogeneous) models and provides a way of unifying the two main approaches to the canonical quantum cosmology based on these models, namely quantization before and after imposing the Hamiltonian constraint.     

Structural and electronic behavior of Sr2GdRuO6 complex perovskite

We report experimental and theoretical study of crystallographic lattice and electronic structure of Sr₂GdRuO₆ complex perovskite, which is used as precursor in the fabrication process of superconducting ruthenocuprate RuSr₂GdCu₂O₈. Samples were produced by the standard solid state reaction. Rietveld refinement of experimental X-ray diffraction patterns shows that material crystallizes in a monoclinic structure, which belongs to the P2₁/n (#14) space group, with lattice parameters , , , and tilt angle β=90.258. Calculations of electronic structure were performed by the density functional theory. The exchange and correlation potentials were included through the LDA+U approximation. Density of states (DOS) study was carried out considering the two spin polarizations. Results show Gd are majority responsible for the magnetic character in this material, but Ru contribution is also relevant because d-orbital is closer to Fermi level. Theoretical results evidence that Sr₂GdRuO₆ material behaves as a magnetic semiconductor, with 20μ_B effective magnetic moment.     

Giant optical anisotropy in R-plane GaN/AlGaN quantum wells caused by valence band mixing effect

This study investigates the optical anisotropy spectrum in the R-plane (i.e., the -oriented layer plane) of GaN/Al_0.2Ga_0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in -oriented GaN/Al_0.2Ga_0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x^′-polarization to y^′-polarization as the well width increases.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

Korringa-like nuclear spin–lattice relaxation in a 2DES at

Via a resistively detected NMR technique, the nuclear spin–lattice relaxation time T₁ of ⁷¹Ga has been measured in a GaAs/AlGaAs heterostructure containing two weakly coupled 2D electron systems (2DES) at low temperatures, each at Landau level filling . Incomplete electronic spin polarization, which has been reported previously for low density 2DESs at , should facilitate hyperfine-coupled nuclear spin relaxation owing to the presence of both electron spin states at the Fermi level. Composite fermion theory suggests a Korringa-law temperature dependence: T₁T=constant is expected for temperatures . Our measurements show that for temperatures in the range , T₁ rises less rapidly with falling temperature than this law predicts. This may suggest the existence of alternate nuclear spin relaxation mechanisms in this system. Also, our data allows for an estimate of the composite fermion mass.     

Three equations of state for solids considering thermal effect

 The Einstein model to consider thermal effect in universal equations of state (UEOS) is modified. It is proposed that the zero-point vibration term should be deleted in a thermal UEOS, and the parameters cannot be directly taken as experimental data at a reference temperature, V_R, B_R, and , but their values at absolute zero temperature, V₀, B₀, and . An approach is proposed to solve V₀, B₀, and from V_R, B_R, and . The approaches are applied to three typical universal EOSs, including the Baonza, mGLJ and Morse EOSs. The numerical results show that the solved values of parameters are almost identical for different EOSs. And the thermo-physical properties predicted through different EOSs are almost identical at zero- and low-pressure conditions, once the same approach and input experimental data are used to solve the parameters. It is concluded that the prediction of thermo-physical properties at zero- and low-pressure conditions cannot be taken as the criteria to judge the applicability of a universal EOS.