Magnetoreflectance and luminescense study of beryllium acceptors in selectively doped GaAs/AlxGa1−xAs multiple quantum wells

We present a magneto-optical study of p-type GaAs/Al_xGa_1−xAs quantum wells doped with Be acceptors over the central one-third of the GaAs layers. Using magneto-reflectance and magneto-luminescence spectroscopies, we have investigated (a) interband Landau transitions and (b) transitions from the conduction band Landau levels to the Be acceptors. Binding energies of the acceptors were determined and the dependence of the impurity ground state energy on magnetic field has been studied.     

Measurement of the miniband width in a superlattice with interband absorption in a magnetic field parallel to the layers

Interband magnetoabsorption measurements of a superlattice with GaAs/ GaAlAs layers, thin compared to the cyclotron radius, in a magnetic field parallel to the layers show interband Landau level transitions exclusively for radiation energies falling within the hole and the electron subband width. The data are quantitatively explained with energy levels of a Kronig-Penney model in a parallel magnetic field, and permit a direct observation of the superlattice bandwidth.     

Recombination radiation from landau states in impactionized GaAs

Recombination radiation from Landau states in impact ionized high-purity GaAs has been observed. The narrow band (～ 3 cm^?1) emission has been magnetically tuned from 80–120 cm^?1, and used to observe rotational transitions in water vapor.     

Carrier dynamics and stimulated radiative terahertz transitions between Landau levels in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau level populations for various values of pumping intensity (tunneling time), magnetic field and structure doping were carried out. The population inversion between zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength. The effect of various scattering mechanisms, both two-particle (electron-electron scattering) and single-particle (acoustic phonon and interface roughness scattering) ones, on level population was studied. The way of lifting the selection rule forbidding the inter-Landau level terahertz transitions of interest and achieving considerable values of the dipole matrix element is proposed.     

Magneto-photoluminescence studies of manganese acceptors in GaAs/AlGaAs multiple quantum wells

We have investigated the photoluminescence associated with residual manganese acceptors in n-type, modulation doped, GaAs/AlGaAs multiple quantum wells. In a magnetic field the luminescence breaks into discrete lines attributed to transitions between conduction band Landau levels and manganese acceptor states. The polarization of the luminescence was studied as function of magnetic field. A simple model based on the spin exchange interaction between the holes and the manganese ions successfully describes the polarization data.     

Confined-acoustic-phonon-assisted cyclotron resonance via multi-photon absorption process in GaAs quantum well structure

Phonon-assisted cyclotron resonance (PACR) in GaAs quantum well (QW) structure is investigated via multi-photon absorption process when electrons interact with the confined acoustic phonon through deformation potential. The additional peaks in the absorption spectrum due to transitions between Landau levels accompanied with the emission and absorption of phonons are indicated. The dependence of absorption power on the temperature, magnetic field and well width is presented. Using profile method, we obtain PACR-linewidth as profiles of the curves. The temperature, magnetic field and well width dependences of the PACR-linewidth are investigated. The results are compared with those in the case of mono-photon absorption process, as well as in the electron-bulk acoustic phonon interaction. The results show that the multi-photon absorption process is strong enough to be detected in PACR.     

Effect of a Coulomb well in (In,Ga)As/GaAs quantum wells

Magnetooptical investigation of exciton transitions in high-quality quantum wells of an (In, Ga)As/GaAs heterosystem has been carried out. Investigation of transmission of free-hanging samples detached from the substrate in the magnetic fields of up to 12 T revealed a rich fine structure associated with various heavy-hole and light-hole exciton transitions. In particular, transitions from the excited states of light holes localized in a Coulomb potential produced by an electron along the heterojunction axis (a Coulomb well) have been detected. Taking into account consistently stresses, formation of Landau levels, the binding energies of excitons (diamagnetic excitons), and the effect of a Coulomb well, we have succeeded to describe the experimental results with the use of a self-consistent variational procedure. As a result, new features in the structure of optical transitions have been explained and the effective masses of electrons and holes of excitons formed by both heavy and light holes have been determined with a high accuracy.     

Observation of the interaction between Landau levels of different two-dimensional subbands in GaAs in a normal magnetic field

Tunnel current measurements between strongly disordered two-dimensional electron systems in a perpendicular magnetic field are presented. Two-dimensional electron accumulation layers are formed by an extremely narrow layer of Si donors (Si delta doping) in GaAs on either side of an AlGaAs tunnel barrier. Strong interaction between Landau levels of the two-dimensional subbands in each accumulation layer is observed as an anticrossing of the related peak positions in the tunnel current vs. voltage curves as a function of magnetic field. The splitting of the interacting Landau levels is about 10 meV, which cannot be explained by nonparabolicity of the conduction band in GaAs. A possible reason for the observed interaction connected with the collective excitations in the 2DES is discussed.     

第二类相变的对称理论

本文评述了第二类相变的Landau理论。详细地阐明了Landau理论中包括子群链判据、Landau判据和Lifshitz判据在内的群论公式,扼要地给出有关结构相变的重整化群分析的方法。     

Resonant polaron coupling of the n = 1 Landau level and the 2p+ donor state in GaAs

The n = 0 → > n = 1 Landau level and 1s ?2p⁺ impurity transitions in GaAs were investigated up to energies above the optical phonon energy ?Ω_LO and d.c. magnetic fields up to 25 T. Pinning of both transitions to an energy slightly above and below ?Ω_LO was observed. At an energy very close to ?Ω_LO two additional impurity transitions are found. These features are attributed to the resonant polaron effect which leads to hybridization and dipole selection rule breakdown. Also the spin doublet splitting of both transitions were resolved showing a strong magnetic field dependence which can not be explained by nonparabolicity of the conduction band alone.     

Interband resonant tunneling in superconductor heterostructures in a quantizing magnetic field

The resonant tunneling of electrons through quasistationary levels in the valence band of a quantum well in double-barrier structures based on III–V materials with type-II heterojunctions is considered in a quantizing magnetic field directed perpendicularly to the interfaces. The transmission coefficients of the tunnel structure for transitions from states corresponding to different Landau levels are calculated using the Kane model. It is shown that transitions with a unit change in the Landau level index n as a result of mixing of the wave functions of states with opposite spin orientations are possible on the interfaces due to spin-orbit coupling. The probability of such transitions can be comparable to the probability of transitions without a change in the Landau level index for InAs/AlGaSb/GaSb resonant-tunneling structures. Fiz. Tverd. Tela (St. Petersburg) 40, 2121–2126 (November 1998)     

Hysteresis and first-order phase transition in the two-dimensional electron gas

We report experimental evidence of the first-order phase transitions in the two-dimensional electron gas formed in a gated wide GaAs/AlGaAs quantum well at even-integer quantum-Hall states. At the filling factor values of ν=2,4 and low temperatures, crossing of Landau levels through the application of the gate bias yields a suppression of the quantum-Hall-state excitation gap and hysteretical behaviour of the diagonal resistivity in up and down sweeps of the magnetic field. Detailed many-body calculations indicate the occurrence of a first-order phase transition and allow the determination of the exact properties of the electron ground states involved in the transition.     

Study of shallow impurity states in compound semiconductors using high resolution photoluminescence spectroscopy

In this paper we review briefly the use of high resolution photoluminescence to study the behavior of shallow impurity states in compound semiconductors. As an illustration we focus our review on GaAs. The binding energies of the ground state and of several low-lying excited states of the impurity centers are determined by studying the radiative transitions associated with excitons bound to neutral donors or acceptors. The difference between the binding energies of different donors in GaAs is rather small. Thus to resolve transitions associated with different chemical donors a magnetic field is used. This has the effect of sharpening the transitions as well as increasing the separation between them. One can identify donors in samples with total impurity concentrations as high as 5X10¹⁵/cm³. The binding energies of different chemical acceptors in GaAs are much higher. Thus the radiative transitions associated with excitons bound to neutral acceptors can be resolved in zero magnetic field. Energy levels of shallow donors and acceptors in GaAs are reviewed.     

Theory of the isostructural extension-contraction phase transition in proper ferroelastics

Using the phenomenological Landau theory of phase transitions, a diagram of states and anomalies of physical characteristics are investigated in the area of conditions corresponding to isostructural transitions in a high-symmetric phase. A cubic phase with a two-component order parameter describing the ferroelastic extension-compression transition is considered as an example.     

Totally symmetric order parameter in the framework of the phenomenological theory of phase transitions: Ferroelastics

A new approach is proposed for constructing the phenomenological theory of phase transitions. The approach is based on the classical Landau theory with allowance made for the order parameter that corresponds to changes in the charge distribution probability density of a crystal and does not affect the symmetry of the high-symmetry phase. It is demonstrated that this approach makes it possible to describe phase transitions in terms of a nonequilibrium polynomial Landau potential of degree four in the components of the order parameters. The capabilities of the proposed approach are illustrated with three systems that undergo ferroelastic phase transitions.     

Mesoscopic effects in the quantum Hall regime

We report results of a study of (integer) quantum Hall transitions in a single or multiple Landau levels for non-interacting electrons in disordered two-dimensional systems, obtained by projecting a tight-binding Hamiltonian to the corresponding magnetic subbands. In finite-size systems, we find that mesoscopic effects often dominate, leading to apparent non-universal scaling behavior in higher Landau levels. This is because localization length, which grows exponentially with Landau level index, exceeds the system sizes amenable to the numerical study at present. When band mixing between multiple Landau levels is present, mesoscopic effects cause a crossover from a sequence of quantum Hall transitions for weak disorder to classical behavior for strong disorder. This behavior may be of relevance to experimentally observed transitions between quantum Hall states and the insulating phase at low magnetic fields.     

Phase diagrams of Heusler alloys with inversion of the exchange interaction

The phase diagrams of magnetically ordered Heusler alloys with the inversion of the exchange interaction have been studied using the phenomenological Landau theory of phase transitions. It has been shown that the shape of the phase diagrams in these alloys depends on the magnitudes and signs of the phenomenological parameters in the Landau functional. At certain magnitudes and signs of the parameters of the Landau functional, the phase diagrams have thermodynamic paths that allow one to explain the experimentally observed sequences of the phase transitions in the Heusler alloys.     

Magneto-transport study of Landau level broadening in a gated AlGaAs/GaAs parabolic quantum well structure

We study the Landau level broadening by analyzing the Shubnikov-de Haas oscillations in a gated AlGaAs/GaAs parabolic quantum well structure when only one electronic subband is occupied. Small-angle scattering is determined to be important in this system. The Shubnikov-de Haas oscillations are described equally well by employing Gaussian or Lorentzian broadening of the Landau levels at low magnetic field where the quantum localization effect is not important. A possible explanation is that the electron-electron interactions lead to the overlapping of adjacent Landau levels and one can not distinguish between the two broadening types.     

Comments on the use of the Landau theory in some order-order transitions

The application of the Landau theory of phase transitions to a two-sublattice magnet results in a broad range of magnetic transitions. The use of a two-component order parameter enables one to describe both order-disorder and order-order transitions. In particular, the metamagnetic transitions exhibit the thermal hysteresis. It is shown that the Kittel model is a special case of the Landau expansion. Finally, an alternative one-component order parameter expansion is discussed.     

Carrier kinetics and population inversion in Landau level system in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau levels population for various values of pumping intensity (tunneling time), magnetic field and the structure doping were carried out. The effect of various scattering mechanisms, as two-electron (electron–electron scattering) as single-electron (acoustic phonon and interface roughness scattering) ones on level population was studied. The population inversion between the zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength thus providing the possibility of wide range tunable stimulated terahertz emission.