Two-dimensional semimetal-insulator transition in HgTe-based quantum wells induced by a longitudinal magnetic field

A metal-insulator transition in a two-dimensional semimetal based on HgTe quantum wells is discovered. The transition is induced by a magnetic field applied parallel to the plane of the quantum well. The threshold behavior of the activation energy as a function of the magnetic-field strength and an abrupt reduction of the Hall resistance at the onset of the transition suggest that the observed effect originates from the formation of an excitonic insulator.     

Two-dimensional electron-hole system in a HgTe-based quantum well

A two-dimensional electron-hole system consisting of light high-mobility electrons with a density of N _s = (4–7) × 10¹⁰ cm^?2 and a mobility of μ _n = (4–6) × 10⁵ cm²/V s and heavier low-mobility holes with a density of P _s = (0.7–1.6) × 10¹¹ cm^?2 and a mobility of μ _p = (3–7) × 10⁴ cm²/V s has been discovered in a quantum well based on mercury telluride with the (013) surface orientation. The system exhibits a number of specific magnetotransport properties in both the classical magnetotransport (positive magnetoresistance and alternating Hall effect) and the quantum Hall effect regime. These properties are associated with the coexistence of two-dimensional electrons and holes.     

Electron spin resonance in GaSb-InAs-GaSb semimetal quantum wells

Resonance of the photo-emf induced by far-IR light in the presence of a magnetic field oriented parallel to the plane of the well is observed in a GaSb-InAs-GaSb semimetal quantum well. It is inferred that the effect is due to optical transitions between sublevels of the first size-quantization level. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 768–773 (25 November 1998)     

Manifestation of a semimetallic state in cyclotron resonance in low-symmetry HgTe-based quantum wells

Cyclotron-resonance measurements in 21-nm-thick HgTe/CdHgTe quantum wells of different crystallographic orientations have been performed. It has been found that, in contrast to the structures with the (001) orientation of the quantum-well plane, (013)-oriented quantum wells are semimetallic and their absorption spectra exhibit both electron and hole cyclotron-resonance lines. The simultaneous presence of the two types of charge carriers originates from an overlap between the upper heavy-hole quantum-confinement subbands hh1 and hh2. This overlap is caused by the strong interaction of these subbands with the Dyakonov-Khaetskii interface state. Calculations carried out using the eight-band kp-Hamiltonian indicate that, for known values of the band-structure parameters, the overlap between hh2 and hh1 subbands does not occur; this result is in agreement with the cyclotron-resonance data for (001)-oriented structures. The enhanced interaction between heavy-hole and interface states owing to the existence of steps at low-symmetry heterointerfaces may be the mechanism responsible for the appearance of an overlap between subbands in HgTe quantum wells with orientation different from (001).     

Two-dimensional electrons in spirally rolled-up quantum wells

An effective Hamiltonian is obtained to describe the motion of a one-dimensional quantum particle along an arbitrary plane curve. Calculations are made of the energy levels and the polarization dependence of the electromagnetic wave absorption in a spirally rolled-up quantum well.     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

Two-dimensional surface magnetism in the bulk paramagnetic intermetallic alloy CoAl(100)

Utilizing a combination of the in situ magneto-optical Kerr effect and scanning tunneling microscopy and spectroscopy measurements, we show that the (100) surface of the B2 bulk paramagnetic CoAl is an excellent representation of a two-dimensional ferromagnet. The order-parameter critical exponent beta=0.22+/-0.02 is determined, which is the universal signature of a finite-size two-dimensional XY behavior. The Curie temperature is found to be T(c)=90 K. The magnetism can be explained by the appearance of Co antisite atoms at the surface.     

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.     

Spin-sensitive bleaching and monopolar spin orientation in quantum wells

Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-type GaAs/AlGaAs quantum well structures. The absorption of circularly polarized radiation saturates at lower intensities than that of linearly polarized light due to monopolar spin orientation in the first heavy-hole subband. Spin relaxation times of holes in p-type material in the range of tens of ps were derived from the intensity dependence of the absorption.     

Two-dimensional correlation spectroscopy of two-exciton resonances in semiconductor quantum wells

We propose a three-pulse coherent ultrafast optical technique that is particularly sensitive to two-exciton correlations. Two Liouville-space pathways for the density matrix contribute to this signal which reveals double quantum coherences when displayed as a two-dimensional correlation plot. Two-exciton couplings spread the cross peaks along both axes, creating a characteristic highly resolved pattern. This level of detail is not available from conventional one-dimensional four-wave mixing or other two-dimensional correlation spectroscopy signals such as the photo echo, in which two-exciton couplings show up along a single axis and are highly congested.     

Resonant and surface polaritons in quantum wells

Summary We show how the breaking of the translational invariance in a quantum well modifies the concept of polariton with respect to that defined for bulk material. Polaritons in quantum wells result from the combination of the exciton states with the radiation field. They are here obtained as the solutions of Maxwell equations with retardation, provided an appropriate nonlocal response function is used for the electric susceptibility, and Maxwell boundary conditions are imposed. We find two types of polaritons depending on the values of the in-plane wavevectork _II: those atk _II<ω/v (wherev=c/n is the velocity of light in the sample) are resonant with the radiation field in the barrier and those atk _II>ω/v cannot be coupled to waves in the barrier. In both cases explicit expressions are given for radiative shifts and radiative broadenings as functions ofk _II. Numerical results are obtained for GaAs-Ga_1−x Al _x As and for CuCl quantum wells and new experiments are suggested. The existence of resonant and surface polaritons justifies an interpretation of the temperature dependence of the radiative lifetime suggested by the same authors. It also decreases the radiative efficiency in the direction perpendicular to the planes and increases the radiative efficiency parallel to the planes with increasing temperature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

声表面波对GaAs(110)量子阱发光特性的调制

结合声表面波和光致发光谱在低温(15K)下对非故意掺杂的GaAs(110)量子阱结构的发光特性进行了研究.实验结果表明，由于声表面波的作用GaAs(110)量子阱的发光强度减弱,并且其对应的重空穴能级出现了分裂的现象，当施加的声波强度P_rf达到20dBm时，能级分裂ΔE达到了10meV.进一步讨论了声表面波对GaAs(110)量子阱圆偏振光自旋注入的影响. 关键词： 发光 GaAs量子阱 声表面波 自旋极化     

Measurement of cyclotron masses of spin-orbit-split quasi-two-dimensional holes in GaAs(100) narrow quantum wells

The magnetic field dependence of the cyclotron mass of heavy holes in asymmetric GaAs(100) quantum wells is measured by optical detection of resonant microwave absorption for various concentrations of quasi-two-dimensional holes. The effect of spin-orbit splitting on the cyclotron masses of heavy holes is discovered and investigated. The energy spectrum of holes is calculated on the basis of experimental data. The energy range in which spin-plasmon oscillations are observed in hole systems with various concentrations is predicted.     

Two-dimensional electron-hole liquid in single Si quantum wells with large electronic and dielectric confinement

We report a luminescence study of the electronic properties of the 2D electron-hole liquid in crystalline Si quantum wells with SiO2 dielectric barriers. The Fermi-Dirac condensation of e-h pairs into a metallic liquid is strongly enhanced by spatial localization. We present experimental evidence for the formation of liquid nanodroplets, with size increasing with e-h pair density. The quantum confined regime is observed for well width below 15 nm. The data are analyzed in a confinement model that takes account of the band-gap renormalization by 2D many-body effects and the increase of the Coulomb interactions due to the dielectric mismatch between the Si well and the SiO2 barriers.     

Dielectric enhancement of magnetoexcitons in surface quantum wells

Magnetoexcitons in InGaAs/GaAs surface quantum wells in a quantizing magnetic field are investigated theoretically and experimentally. Dielectric enhancement of an exciton with decreasing thickness of the barrier layer is demonstrated, and the dependence of the effect on the strength of the magnetic field is analyzed for the 1s and 2s states. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 47–51 (10 July 1996)     

Comparison of the quantum confined stark effect for (100)- and (311)A-oriented GaAs/AlGaAs quantum wells using electroreflectance spectroscopy

The excitonic subband transitions of (100)- and (311)A-oriented GaAs/AlGaAs single quantum wells (SQW's) are investigated using electroreflectance spectroscopy. The strong Franz-Keldysh-Oscillations arising from the undoped AlGaAs barrier layers are analyzed to determine the field strength within the SQW's directly from the measurements with high accuracy. Both, the orientation-dependent splitting between the first heavy and light hole subbands and their field-induced shifts are compared with the results of k·p calculations. Best agreement between experiment and theory is obtained for a set of Luttinger parameters corresponding to heavy and light hole masses of 0.55m₀ and 0.084m₀, respectively, along the [311] direction.     

Anisotropic spin transport in (110) GaAs quantum wells

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.