We consider the filling factor 5/2 fractional quantum Hall state of spin-polarized fermions in a dirty (mobility μ_b < 10 m^2 V^{-1} s^{-1}) bi-layer quantum well system.We show that the system undergoes a quantum phase transition from the effective two-component state to an effective single-component state, at fixed charge imbalance regulatory parameter Δ_{c} and constant layer separation, as the inter-layer tunneling strength Δ_{SAS} is increased. At finite and constant Δ_{SAS}, a transition from the latter state to the former state is also possible upon increasing the parameter Δ_{c}. We identify the order parameter to describe quantum phase transition as a pseudo-spin component and calculate this with the aid of the Matsubara propagators in the finite-temperature formalism. Our treatment is able to show that, at low temperature(< 0.1 K) and low value of charge imbalance regulatory parameter, there is a competition between the disorder and the inter-layer tunneling strength in the sample. The competition leads to the quasi-particle localization at low tunneling strength.
We study the development of ring luminescence of indirect excitons at macroscopical distances from the central excitation spot in quantum well structures. The Landau model for exciton condensation generalized for particles with finite lifetimes in conditions of inhomogeneous excitation is proposed. The transition between the fragmented and continuous rings and the temperature dependence of the effects are considered. The irradiation of the system by two spatially separated laser spots is simulated as well.
We investigate the spin resonance of electrons in one-sided modulation doped Si_{1-x}Ge_x (x=0-10%)) quantum wells defined by Si_{0.75}Ge_{0.25} barriers. In such structures, the Bychkov-Rashba effect induces an effective magnetic field in the quantum well layer which causes anisotropy of both the g-factor and the spin coherence time. Evaluation of the Rashba coefficient as a function of x yields a monotonic increase. For x=5% the shift in the resonance field exceeds the ESR linewidth already, demonstrating the possibility to use this effect for g-factor tuning to select individual spins in an ensemble.
We theoretically study the polarization-induced band inversion phenomenon in c-plane In-rich InGaN/GaN quantum wells. Our calculations performed using the k·p method with the 8×8 Rashba-Sheka-Pikus Hamiltonian for the structures with the indium content between 90% and 100% show that the reordering of the conduction and valence bands occurs for the quantum well widths below the theoretical values of critical thickness for InGaN layers pseudomorphically grown on GaN substrates.
Using a multiband k·p theory the band structure properties of type-II W-design AlSb/InAs/GaInSb/InAs/AlSb quantum wells on GaSb substrates of various crystallographic orientations have been investigated. Such structures are predicted for the emission in a broad range of mid infrared from below 3 μm to beyond 10 μm. The energy of the fundamental optical transition and the corresponding oscillator strength have been determined in function of the layer structure details and versus the substrate orientation. In addition, the resulting optical anisotropy in such type-II quantum wells has been derived.
We present a detailed analysis of GaAs/AlGaAs terahertz quantum cascade laser in the presence of an intense external magnetic field. One of the objectives in further development of THz quantum cascade laser is the realization of structures operating at higher temperatures. This is difficult to obtain as the operating photon emission energy is smaller than the longitudinal-optical phonon energy in the semiconductor material. With increased temperature, electrons in the upper radiative state gain sufficient in-plane energy to emit an longitudinal-optical phonon, which represents a non-radiative scattering and reduces the optical gain. By applying strong magnetic field, two-dimensional continuous energy subbands become split into series of discrete Landau levels, and at particular values of B it is possible to quench these non-radiative channels. Numerical simulations are performed on two-well design quantum cascade laser operating at 4.6 THz, implemented in GaAs/Al_{0.15}Ga_{0.85}As, and the magnetic field is perpendicular to the epitaxial layers. Strong oscillations of carrier lifetimes for the upper state of the laser transition, as a function of magnetic field are observed, which can be attributed to interface roughness scattering and longitudinal-optical phonon scattering between Landau levels.
We have developed a comprehensive rate equations based model for calculating the optical gain in the active region of a quantum cascade laser in magnetic field perpendicular to the structure layers, which takes into account all the relevant scattering channels. The model is applied to gain-optimized quantum cascade laser active region, obtained by a systematic optimization procedure based on the use of genetic algorithm, which we have previously set up for designing novel structures and improving performance of existing ones. It has proven to be very efficient in generating optimal structures which emit radiation at specified wavelengths corresponding to absorption fingerprints of particular harmful pollutants found in the atmosphere. We also illustrate another interesting prospective application of quantum cascade laser-type structures: the design of metamaterials with tunable complex permittivity, based on amplification via intersubband transitions. In this case, the role of the magnetic field is to assist the attainment of sufficient optical gain (population inversion), necessary to effectively manipulate the permittivity and fulfill the conditions for negative refraction (left-handedness).
Gorbatsevich et al. and Kibis suggested that a number of interesting galvano-magnetic effects could be observed in quantum structures where the symmetry with respect to the space coordinates inversion and time-reversal are broken simultaneously. In the paper of Kibis for example, the infinite triangular quantum well in an external magnetic field was considered and the anisotropy of electron momentum transfer due to interaction with phonons was predicted. The role of magnetic field was to provide the time-invariance breaking. In this work we considered the effect of anisotropy of electron momentum transfer due to interaction with polarized light using more realistic model of finite triangular quantum well. This anisotropy leads to the anisotropy of the real part of photoconductivity and as it follows from our calculations, the effect though not very great, could be measurable for the attainable values of magnetic field B≈5 T and the widths of quantum well.
We present a method for systematic optimization of quantum cascade laser active region, based on the use of the genetic algorithm. The method aims at obtaining a gain-maximized structure, designed to emit radiation at specified wavelengths suitable for direct absorption by pollutant gasses present in the ambient air. After the initial optimization stage, we introduce a strong external magnetic field to tune the laser output properties and to slightly modify the emission wavelength to match the absorption lines of additional compounds. The magnetic field is applied perpendicularly to the epitaxial layers, thus causing two-dimensional continuous energy subbands to split into series of discrete Landau levels. This affects all the relevant relaxation processes in the structure and consequently the lifetime of carriers in the upper laser level. Furthermore, strong effects of band nonparabolicity result in subtle changes of the lasing wavelength at magnetic fields which maximize the gain, thus providing a path for fine-tuning of the output radiation properties. Numerical results are presented for GaAs/Al_{x}Ga_{1-x}As based quantum cascade laser structures designed to emit at particular wavelengths in the mid-infrared part of the spectrum.
In this work we studied the charge carriers behaviour in quantum structures where the symmetry with respect to space coordinates and time-reversal symmetry are broken simultaneously. As the model of such structures we considered finite semiparabolic quantum well (we considered earlier the case of triangular QW) placed in external magnetic field. We have shown by numerical analysis that the energy spectra of charge carriers in such structures are anisotropic with respect to in-plane (transverse) motion ϵ_{n}( + k_{x}) ≠ ϵ_{n}(-k_{x}). This leads to the anisotropy of charge carriers in-plane momentum transfer which, in its turn leads to the anisotropy of photoconductivity σ( + k_{x}) ≠ σ(-k_{x}) and as it follows from our calculations, the effect though not very great, could be measurable for the magnetic field of about few T.
In the photoluminescence excitation spectra of two-dimensional valence holes with large spin gap and strong disorder we find evidence for quantum Hall ferromagnetism and small skyrmions around the Landau level filling factorν=1. This interpretation is supported by numerical calculations.
We discuss the properties of resonant tunneling diode with resonant levels in the quantum well. The energy levels are formed inside the well as a consequence of quantization of the states between two potential barriers. We solved the Schrödinger equation for the multilayer structure and found the energy of resonant level as a function of the width of quantum well for different parameters of energy profile in the equilibrium. The results present the dependence of spin splitting in the quantum well of nonmagnetic semiconductor on the spin polarization of electrodes.
In this paper we show that intersubband scattering can lead to apparent inconsistency of the experimental results obtained by means of classical and quantum transport measurements and this discrepancy is entirely connected with the usage of classical formulae to describe magnetic field dependence of a conductivity tensor. We prove that there is no contradiction in our observations and that the models describing quantum oscillations and magnetic-field dependence of the conductivity tensor, which are present in the literature, complement each other.
Isomorphic In_{0.52}Al_{0.48}As/In_{0.53}Ga_{0.47}As/In_{0.52}Al_{0.48}As quantum well structure on InP substrate were grown by molecular beam epitaxy. We investigated the electron transport properties and mobility enhancement in the structures by changing of doping level, the width d of quantum well In_{0.53}Ga_{0.47}As or by illumination using light with λ = 668 nm. Persistent photoconductivity was observed in all samples due to spatial separation of carriers. We used the Shubnikov-de Haas effect to analyze subband electron concentration and mobility. The maximal mobility was observed for quantum well width d = 16 nm.
The new approach to the understanding of intrashallow donor transition in the reduced dimensionality systems is presented. The magnetospectroscopy experiments done on the CdTe/CdMgTe quantum well based samples, uniformly n-doped, show indications that the surprising lack of spectral sensitivity on applied photon energy can be understood as a result of sample response coming from its different regions. This "non spectroscopic" behaviour (in a sense of the Zeeman splitting) is a consequence of the properties of systems with reduced dimensionality where variety of centre locations in the structure results in continuous density of states available for absorption.
We report on the results of our simulations of Γ-X scattering in GaAs/AlGaAs heterostructures, discussing the importance of the mole fraction, doping density, and lattice and electron temperature in determining the scattering rates. We consider three systems, a single quantum well (for the investigation of Γ-X scattering), a double quantum well (to compare the Γ-X-G and Γ-Γ scattering rates), and an example of a GaAs/AlGaAs mid-infrared quantum cascade laser. Our simulations suggest that Γ-X scattering can be significant at room temperature but falls off rapidly at lower temperatures. One important factor determining the scattering rate is found to be the energy difference between the Γ- and X-states.
We report transmission measurements of GaN quantum well grown on sapphire substrate in the 220-325 GHz frequency band at low temperatures. A significant enhancement of the transmitted beam intensity with the applied voltage on the devices under test is found.
Recombination of excitons and positive trions is studied by two-beam photoluminescence of a two-dimensional hole gas in a high magnetic field. The singlet, dark-triplet and bright-triplet states of a free trion are resolved, and their binding energies are determined. Recombination of acceptor-bound trions is also detected, including a low-energy cyclotron replica, corresponding to a hole shake-up process. Identification of all these different transitions was possible by analysis of optical selection rules and the comparison of experimental spectra with realistic numerical calculations.
We study the scattering of torsional waves through a quasi-one-dimensional cavity both from the experimental and theoretical points of view. The experiment consists of an elastic rod with square cross-section. In order to form a cavity, a notch at a certain distance of one end of the rod was grooved. To absorb the waves, at the other side of the rod, a wedge, covered by an absorbing foam, was machined. In the theoretical description, the scattering matrix S of the torsional waves was obtained. The distribution of S is given by Poisson's kernel. The theoretical predictions show an excellent agreement with the experimental results. This experiment corresponds, in quantum mechanics, to the scattering by a delta potential, in one dimension, located at a certain distance from an impenetrable wall.
Carrier recombination dynamics in polar and nonpolar GaN epilayers and GaN/AlGaN multiple quantum wells grown over sapphire substrates with various crystallographic orientation were studied under high photoexcitation by 20 ps laser pulses. The transient of luminescence featured a significant enhancement in nonradiative recombination of free carriers for nonpolar a-plane GaN epilayers compared to conventional c-plane samples. The epitaxial lateral overgrowth technique was demonstrated to significantly improve the quality of nonpolar a-plane films. This was proved by more than 40-fold increase in luminescence decay time (430 ps compared to ≤10 ps in the ordinary a-plane epilayer). Under high-excitation regime, a complete screening of built-in electric field by free carriers in multiple quantum wells grown on c-plane and r-plane sapphire substrates was achieved. Under such high excitation, luminescence efficiency and carrier lifetime of multiple quantum wells was shown to be determined by the substrate quality.
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