Exciton phase transitions in semiconductor quantum wells with disc-shaped electrode

Phase transitions in a system of indirect excitons in semiconductor double quantum wells are studied for a set-up when one of the electrodes is of finite size and, in particular, has the shape of a disc. At voltage a region under the rim of the disc is created where excitons have lower energy, thus providing a macroscopic trap attractive for excitons while being repulsive for charged particles. The theory of the formation of patterns of the excitonic condensed phase under the disc is built based on the assumption of the existence of the inter-exciton range where the interaction between them is attractive. The finite value of the exciton lifetime is taken into account serving as a limiting factor for the size of the islands of the condensed phase. The calculations reveal complex restructuring of the patterns of the spatial distribution of exciton density with increasing pumping intensity: from the structureless gaseous phase to separate islands of the condensed phase within the gaseous phase, then to islands of the gaseous phase in the bulk of the condensed phase and finally to the continuous condensed phase.     

Intersubband spin-orbit coupling and spin splitting in symmetric quantum wells

In semiconductors with inversion asymmetry, spin-orbit coupling gives rise to the well-known Dresselhaus and Rashba effects. If one considers quantum wells with two or more conduction subbands, an additional, intersubband-induced spin-orbit term appears whose strength is comparable to the Rashba coupling, and which remains finite for symmetric structures. We show that the conduction band spin splitting due to this intersubband spin-orbit coupling term is negligible for typical III-V quantum wells.     

Exciton quantization in parabolic quantum wells

The exciton wavefunction in parabolic quantum wells is calculated using variational techniques and effective mass theory. The influences of the potential shape and of confinement on the exciton binding energies are studied. The results are in good agreement with previous calculations. The oscillator-strength of excitons in GaAs/Ga_1-xAl_xAS quantum wells has a maximum value very close to the cross-over from three to two dimensions.     

Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Intraband optical transitions of holes in strained SiGe quantum wells

Optical absorption spectra of polarized infrared radiation in p-type SiGe quantum-well structures have been investigated. Three types of structures with different magnitudes and signs of biaxial elastic strains have been studied. The light absorption factor has been found to double in the region of interband transitions under the in-plane tensile strain of the quantum-well heterojunction. The results are attributed to a change in the order of light- and heavy-hole bands caused by the tensile strain and to the formation of the ground state of charge carriers in the light-hole band.     

Exciton condensate emission in double quantum wells

Studies of the photoluminescence spectra of spatially indirect excitons in coupled quantum wells revealed that the emission has a directional pattern dependent on the external electric field and pumping power. The experimentally detected correlation between the spectral emission parameters of spatially indirect excitons, namely, the concentration (phase state) of such excitons, the line half-width, the degree of linear polarization, and the existence of a directional pattern, permits a suggestion that the spontaneous photoluminescence of spatially indirect excitons in the condensed state is of a coherent nature.     

Exciton states and interband optical transitions in ZnO/MgZnO quantum dots

Within the framework of the effective-mass approximation, the exciton states confined in wurtzite ZnO/MgZnO quantum dot (QD) are calculated using a variational procedure, including three-dimensional confinement of carriers in the QD and the strong built-in electric field effect due to the piezoelectricity and spontaneous polarizations. The exciton binding energy and the electron-hole recombination rate as functions of the height (or radius) of the QD are studied. Numerical results show that the strong built-in electric field leads to a remarkable electron-hole spatial separation, and this effect has a significant influence on the exciton states and optical properties of wurtzite ZnO/MgZnO QD.     

Exciton optical absorption in disordered,strained InGaAs/GaAs single quantum wells

The absorption coefficient spectrum of undoped, disordered InGaAs/GaAs single quantum wells is calculated within the parabolic band approximation in this compressively strained structure. Results are presented for light propagating normal to and along the plane of the quantum well, taking into consideration the 1S-like exciton and all bound states, including the 2D enhancement Sommerfeld factor. The results presented here show that the exciton peaks in TE polarization remain constant with disordering and exhibit a larger wavelength shift than in TM polarization. The absorption edge in disordered InGaAs/GaAs, which is a function of the strain and interdiffusion, can be tailored to the desired wavelength around 1.0 μm. These results can be of interest in the design of photonic devices on a single substrate.     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

Doubley-resonant coherent coupling of infrared radiation via intersubband transitions in quantum wells

The influence of a strong coupling field on the linear response of a modulation-doped quantum well to a weak probe field is studied theoretically for the case when the coupling field frequencies are comparable to those of bound-bound and bound quasi-bound transitions within the conduction subbands. Model calculations suggest that line-center reductions in the ground-to-coupled excited-subband absorption of up to 80% can be coherently induced with CO₂ laser intensities of ∼ 20 MW/cm². The effect requires the infrared coupling field to be polarized in the growth direction, and the nature of the modifications in the linear response depends strongly oft the parity and relaxation rates of the excited subband states.     

Rashba自旋轨道耦合下square-octagon晶格的拓扑相变

本文研究了各向同性square-octagon晶格在内禀自旋轨道耦合、Rashba自旋轨道耦合和交换场作用下的拓扑相变,同时引入陈数和自旋陈数对系统进行拓扑分类.系统在自旋轨道耦合和交换场的影响下会出现许多拓扑非平庸态,包括时间反演对称破缺的量子自旋霍尔态和量子反常霍尔态.特别的是,在时间反演对称破缺的量子自旋霍尔效应中,无能隙螺旋边缘态依然能够完好存在.调节交换场或者填充因子的大小会导致系统发生从时间反演对称破缺的量子自旋霍尔态到自旋过滤的量子反常霍尔态的拓扑相变.边缘态能谱和自旋谱的性质与陈数和自旋陈数的拓扑刻画完全一致.这些研究成果为自旋量子操控提供了一个有趣的途径.     

Exciton condensation in quantum wells: Temperature effects

Our previous theory of the formation of exciton condensed phases in a two-dimensional system is applied to the analysis of the temperature dependence of a number of parameters of the system. The pumping intensity-temperature phase diagram is constructed, some of its features characteristic of indirect excitons having a dipole moment are established, the dependences of the intensity of the radiation band corresponding to the condensed phase on the pumping intensity and temperature are calculated, and the results are compared with experimental data.