Phase diagram of electron-hole systems: Interplay between exciton Mott transition and quantum pair condensation

Quasi-thermal-equilibrium states of electron-hole (e-h) systems in photoexcited insulators are studied from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type pair-condensed state, and the exciton Mott transition between an insulating exciton/biexciton gas phase and a metallic e-h plasma phase. We determine the quasi-equilibrium phase diagram of the e-h system at zero and finite temperatures with applying the dynamical mean-field theory (DMFT) to the e-h Hubbard model with both repulsive and attractive on-site interactions. Effects of inter-site interactions on the exciton Mott transition are also clarified with applying the extended DMFT to the extended e-h Hubbard model.     

Macroscopic Quantum State in Optically Driven Semiconductors: Crossover from Electron-Hole BCS to Exciton Bose Condensation

Luminescence spectra from high-density electron-hole ( e-h ) systems at zero temperature are studied. The calculated spectra clearly show the crossover between the e-h BCS state and the exciton Bose-Einstein condensate; this behavior analyzed neither with the BCS-like mean-field theory nor with the interacting Boson model. The strong phase fluctuation associated with the center-of-mass motion of e-h pairs is considered with the generalized random phase approximation combined with the Bethe-Salpeter equation for e-h pairs. The calculated spectra show that the broad spectral components from the e-h BCS state splits into P- and P 2 -lines with decreasing e-h density. The calculated density dependence of the band-gap renormalization excellently agrees with experiments for CuCl and ZnO.     

Electron-hole plasma generation and evolution in semiconductors

We discuss various physical problems related to dense electron-hole (e-h) plasmas in silicon-like semiconductors such as: the e-h plasma energy in the quantum and classical limits including the actual band structure; the dielectric constant using the appropriate plasma frequency mass and the variation of the plasma relaxation time due to e-h collisions; the plasma generation including its nonlinear aspect when the e-h density passes the plasma frequency density of the pump beam; the e-h plasma evolution due to Auger recombination and impact ionization as well as the e-h plasma hydrodynamics; finally, we will discuss also the possibility of a very fascinating melting at T=0.     

Formation of a high T(c) electron-hole liquid in diamond

We report on the observation of electron-hole ( e-h) liquid (EHL) in diamond by time-resolved luminescence measurements under an intense femtosecond photoexcitation above the band gap. The EHL luminescence band is observed below the e-h plasma band, showing a finite rise time of several tens of picoseconds. The rise time, which corresponds to the nucleation and the growth of the e-h droplets, plummets on approaching the EHL critical temperature. Time-resolved spectral shape analysis reveals a very high carrier density of 1x10(20) cm(-3) and very high critical temperature of T(c) = 165 K of EHL.     

Coulomb effects on the gain and absorption spectra of the electron-hole plasma in GaAs

Many-body effects due to electron-hole (e-h) attraction and self-energy corrections are investigated on gain and absorption line shapes of degenerate e-h plasma in direct-gap semiconductors. It is demonstrated for GaAs that a large enhancement in experimental gain and absorption coefficients near crossover, which is not reproduced in single-particle treatments, is accounted for by the excitonic e-h interaction. The self-energy corrections, containing the renormalization due to e-e and e-phonon interactions, reduce the direct band gap in GaAs. Their weak $\text{k}$ dependence further improves agreement with experiment.     

Optical properties of a coherent phase in electron-hole systems: Stimulated light scattering and multibeam processes

A theoretical study is reported of stimulated light scattering, including wave-vector reversal and anomalous transmission, by a coherent phase in electron-hole (e-h) systems of low and high charge-carrier density. For these two cases the coherent phase is taken to be a Bose-Einstein condensate of excitons or a BCS-like state of e-h pairs, respectively. The scattering mechanism is laser-induced coherent recombination of two excitons or two coherent e-h pairs, respectively. The e-h system is assumed to exist within a GaAs/AlGaAs double quantum well or bulk GaAs. The emission rate of two photons with antiparallel momenta is estimated. Multiphoton emission due to multiexciton coherent recombination is covered. Methods for detecting the effects predicted are proposed.     

Separate Mott and liquid-gas transitions in a coulomb gas: Photoexcited germanium

The phase diagram of photoexcited electron-hole (e-h) pairs, which are confined to a strain well in stressed Ge, is investigated via measurements of energy spectra and spatial distributions of e-h recombination luminescence. Above a triple-point temperature of 3.7 ± 0.2K, a Mott (metal-insulator) transition is observed at lower densities than the liquid-gas (LG) transition. The measured critical temperatures of the Mott and LG transitions are 6.5 ± 0.5K and 4.5 ± 0.2K respectively.     

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.     

Fast optical preparation, control, and readout of a single quantum dot spin

We propose and demonstrate the sequential initialization, optical control, and readout of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged-exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent readout of the hole spin state.     

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − x Ge x /Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si_{1 ? x} Ge _x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si_{1 ? x} Ge _x layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T ⁺ and biexcitons, in the exciton gas.     

Creation of electronic excitations and defects by vuv radiation (6-40 eV) in wide-gap solids

The processes of multiplication of electronic excitations (MEE), connected with the creation of secondary excitons or electron-hole (e-h) pairs by hot conduction electrons, are realized in wide-gap metal halides and oxides. In oxides, secondary e-h pairs can be also formed by 27-40 v eV photons due to L 1 VV Auger transitions (with the participation of 2s oxygen holes). The excitation spectra of luminescence and the creation spectra of electron F centres or hole V centres have been measured for Na 6 Al 6 Si 6 O 24 (NaI) 2x sodalites and MgO:Be, respectively, at 8-80 v K. A high local density of excitations has been revealed under MEE conditions in KBr and Br sodalites with self-trapping excitons and holes.     

应变纤锌矿GaN/A10.15Ga0.85N柱形量子点的光学性质：流体静压力效应

在有效质量近似下,考虑强的内建电场和应变对材料参量的影响,变分研究了流体静压力对有限高势垒应变纤锌矿GaN/Al0.15Ga0.85N柱形量子点中重空穴激子的结合能、发光波长和电子空穴复合率的影响.数值结果表明,激子结合能和电子空穴复合率随流体静压力的增大而近线性增大,发光波长随流体静压力的增大而单调减小.在量子点尺寸较小的情况下,流体静压力对激子结合能和电子空穴复合率的影响更明显.由于应变效应,为了获得有效的电子-空穴复合过程,GaN量子点的高度必须小于5.5 nm.     

Screening of the electron-hole interaction in quantum well structures

Recent optical non-linearities in GaAs/Ga_1?xAl_xAs quantum well structures have been attributed to the screening of the electron-hole interaction in such structures by the free carriers created. We here present the results of a variational calculation of the ground state energy of an electron-hole pair confined to move in two dimensions when screening of the Coulomb interaction between the electron-hole pair by the free carriers present is taken into account by using the screened potential obtained by Stern and Howard for hydrogenic impurities in semiconducting inversion layers. It is found that the binding energy of the 2D exciton decreases less rapidly with the screening parameter than is the case for a 3D exciton and that the 2D exciton remains bound even for large values of the screening parameter. This is in contrast to the case in bulk semiconductors where the electron-hole pair no longer bind into an exciton when the screening length of the free carriers becomes less than the Bohr radius of the exciton.     

On the instability of the one-dimensional Peierls insulator in uniform electric field

Behaviour of the quasi-one-dimensional Peierls insulator (PI) with a doubled lattice period (of the trans-polyacetylene type) is studied in the uniform electric field The electric field polarizes the insulator thus decreasing the gap Δ in the quasiparticle spectrum and creates by tunnelling charge carriers pairs. Depending on the field strength direct creation of soliton-antisoliton pairs or electron-hole pairs (e-h) takes place. The (e-h) pairs deform the lattice forming polarons. For the cis-type polyacetylene the superinsulator phase with soliton confinement exists. The latter is characterized by a great number of soliton-antisoliton quantum levels. Because of the charges generation the process of insulator-to-metal transition becomes continuous.     

Fine structure of highly charged excitons in semiconductor quantum dots

An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.     

应变纤锌矿GaN/A10.15Ga0.85N柱形量子点的光学性质:流体静压力效应

在有效质量近似下,考虑强的内建电场和应变对材料参量的影响,变分研究了流体静压力对有限高势垒应变纤锌矿GaN/Al_0.15Ga_0.85N柱形量子点中重空穴激子的结合能、发光波长和电子空穴复合率的影响.数值结果表明,激子结合能和电子空穴复合率随流体静压力的增大而近线性增大,发光波长随流体静压力的增大而单调减小.在量子点尺寸较小的情况下,流体静压力对激子结合能和电子空穴复合率的影响更明显.由于应变效应,为了获得有效的电子-空穴复合过程,GaN量子点的高度必须小于5.5 nm.     

硅BC8量子点太阳能电池中的多重激子效应

多重激子效应是指在纳米半导体晶体中,量子点吸收一个高能光子而产生多个电子-空穴对的过程,该效应可以提高单结太阳电池能量转换效率。利用碰撞电离机制和费米统计模型计算了工作温度300 K的单结硅BC8量子点太阳能电池在AM1.5G太阳光谱下的能量转换效率。对于波长在280~580 nm的入射光,多重激子效应可以大幅增强硅BC8量子点直径d>5.0 nm的量子点太阳电池的能量转换效率。硅纳米量子点的直径d=6.3~6.4 nm时,最大能量转换效率为51.6%。     

Exciton and biexciton fine structure in single elongated islands grown on a vicinal surface

We report a microphotoluminescence study of the exciton and the biexciton localized in very elongated islands formed by well-width fluctuations in a thin CdTe/CdMgTe quantum well grown on a vicinal surface. The electron-hole exchange interaction in a local reduced symmetry splits the exciton states. The resulting transitions are linearly polarized along the two orthogonal principal axes of the island. The valence band mixing induced by the elongated shape of the potential leads to a strong polarization anisotropy and to the observation of dark exciton states under magnetic field. The biexciton-exciton transition reproduces all the fine structure of the exciton state including the transition of the biexciton to the dark exciton state.