PT-symmetry breaking in resonant tunneling heterostructures

We present fermionic model based on symmetric resonant tunneling heterostructure, which demonstrates spontaneous symmetry breaking in respect to combined operations of space inversion (P) and time reversal (T). PT-symmetry breaking manifests itself in resonance coalescence (collapse of resonances). We show that resonant energies are determined by eigenvalues of auxiliary pseudo-Hermitian PT-invariant Hamiltonian.     

Brownian motion in a quantizing magnetic field

A model previously discussed by the author to study Brownian motion of charged carriers in a quantizing magnetic field is extended to include a Landau level-dependent friction parameter. A phase-space Fokker-Planck equation is used to derive a generalized diffusion equation describing spatial diffusion of the carriers, coupled with random jumps between adjacent Landau levels. This partial differential-difference equation is solved analytically. The longitudinal “global” diffusion coefficient is calculated and shown to be enhanced over the value in the extreme quantum limit.     

Righi-Leduc effect in a quantizing magnetic field

The Righi-Leduc effect in semiconductors with a Kane dispersion law in the presence of strong, quantizing, magnetic fields is studied theoretically. The explicit form of the dependence on the magnetic field, temperature, and concentration in arbitrary quantizing magnetic fields is established for semiconductors with a nondegenerate electron gas in the approximation of small nonparabolicity. A simple formula that is applicable for all strong magnetic fields, including quantizing fields, is derived for the Righi-Leduc coefficient in the case of strongly degenerate semiconductors with an arbitrary nonparabolic band. It is shown that in order to determine the photon part of the thermal conductivity ,ph directly from experiment it is best to employ samples with a nondegenerate electron gas in strong, but nonquantizing, magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 102–107, July, 1988.     

Resonant tunneling of electrons between two-dimensional systems of different densities in a quantizing magnetic field

The results of experimental investigation of the vertical electron transport in a GaAs/Al_0.3Ga_0.7As/GaAs single-barrier tunneling heterostructure with a doped barrier are presented. Two-dimensional accumulation layers appear on different sides of the barrier as a result of the ionization of Si donors in the barrier layer. The nonmonotonic shift of the current peak is found in the I–V curve of the tunneling diode in a magnetic field perpendicular to the planes of two-dimensional layers. Such a behavior is shown to be successfully explained in the model of appearing the Coulomb pseudogap and the pinning of the spin-split Landau levels at the Fermi levels of the contacts. In this explanation, it is necessary to assume that the Landé factor is independent of the filling factors of the Landau levels and is g* = 7.5 for both layers.     

Local field dynamics in a resonant quantum tunneling system of magnetic molecules

We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual correlation effects in these systems. Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998     

Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures

Using the transfer matrix method and the effective-mass approximation, the effect of resonant states on spin transport is studied in ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric and magnetic fields. The numerical results show that the ZnMnSe layers, which act as spin filters, polarize the electric currents. Variation of thickness of the central ZnSe layer shifts the resonant levels and exhibits an oscillatory behavior in spin current densities. It is also shown that the spin polarization of the tunneling current in geometrical asymmetry of the heterostructure where two ZnMnSe layers have different Mn concentrations, depends strongly on the thickness and the applied bias.     

Cold neutral plasma in a quantizing magnetic field

The chemical potential of a relativistic and an ultrarelativistic electron gas and the energy of an ultrarelativistic electron gas at T = 0 are expressed analytically as functions of the magnetic field. The possible application of these expressions to the investigation of super-compressed matter in the presence of superstrong magnetic fields is discussed.     

Time dependence of resonant tunneling in heterostructures with a three-trough energy spectrum

The time dependence of resonant electron tunneling inGaAs andAlAs quantum heterostructures is studied for a three-trough model. From an analysis of transmitted and reflected wave phases, the spectra of tunneling and reflection times are obtained. The tunneling of a wave packet through a two-barrier heterostructure is modeled by numerical solution of the nonstationary Schrödinger equation.     

Thermo-emf of InSb in a transverse quantizing magnetic field

We have measured the thermo-emf of n-type indium antimonide in a transverse quantizing magnetic field. The results are explained using a theory that includes splitting of the Landau levels.Physics Institute of the Dagestanskii Center, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 26–29, March, 1993.     

Nonohmic behavior of semiconductors in a quantizing magnetic field

We have investigated the nonohmic resistivity of a nondegenerate semiconductor in quantizing magnetic fields for the case where acoustic phonons are the dominant scattering mechanism. The type of I-V characteristics found depends upon which of three mechanism are dominant. The three mechanisms are due to collisional broadening, inelasticities due to the finite phonon energy and phonon drag. When collistion broadening is important, the nonlinearities in the current voltage characteristic arise only from electron heating, while when the inelasticities are dominant, there is also an intrinsic nonlinearity in the characteristic. Finally, when phonon drag is dominant, high frequency acoustoelectric amplification will occur when the Hall velocity exceeds the sound velocity, i.e. V_H > S.For the case where inelasticities dominate, a region of negative differential resistance is obtained that should persist even when there is considerable optical phonon scattering.     

Zeeman splitting of single semiconductor impurities in resonant tunneling heterostructures

Zeeman splitting of the ground state of single impurities in the quantum wells of resonant tunneling heterostructures is reported. We determine the absolute magnitude of the effective magnetic spin splitting factorg* for a single impurity in a 44 Å Al_0.27Ga_0.73As/GaAs/Al_0.27Ga_0.73As quantum well to be 0.28±0.02. This system also allows for independent measurement of the electron tunneling rates through the two potential barriers and estimation of the occupation probability of the impurity state in the quantum well.     

Voltage-controlled spin selection in a magnetic resonant tunneling diode

We have fabricated all II-VI semiconductor resonant tunneling diodes based on the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the quantum well, and studied their current-voltage characteristics. When subjected to an external magnetic field the resulting spin splitting of the levels in the quantum well leads to a splitting of the transmission resonance into two separate peaks. This is interpreted as evidence of tunneling transport through spin polarized levels, and could be the first step towards a voltage controlled spin filter.     

Percolating granular superconductor in a magnetic field

An array of Josephson tunnel junctions is studied as a model for granular superconductors. We determine the critical line in the (H, T)-plane, separating the phase-coherent and incoherent region as a function of the magnetic field. Both positional disorder and junction-dilution are treated. Approaching the percolation threshold from above, the critical exponent of the upper critical field is found to be 0.60±0.03.