Threshold and probability of impact ionization by electrons in narrow-gap p-type semiconductors with highly degenerate holes

The process of electron-initiated impact ionization of states in the heavy-hole band is investigated theoretically for a p-type narrow-gap semiconductor with energy bands governed by the Kane dispersion law, subject to the condition that the Fermi level of the holes lies in the valence band. The dependence of the minimum electron energy for ionization of a state at the Fermi level in the valence band on the heavy-hole Fermi momentum is determined. The probability of impact ionization for electrons with near-threshold energies is calculated for the case in which the heavy-hole Fermi momentum exceeds the hole threshold momentum for the given ionization process. Relations between the temperatures of holes and electrons with energies of the order of the threshold value are found, thereby establishing the validity domain of the final results. Fiz. Tverd. Tela (St. Petersburg) 39, 275–279 (February 1997)     

Quasiholes in a MgZnO/ZnO Heterojunction as Vacancions

Recombination of two-dimensional electrons of a low density in a MgZnO/ZnO heterojunction with localized valence-band holes is considered. It is suggested that quasiholes appearing in the process of photoluminescence of strongly interacting two-dimensional electrons should be considered as vacancion quasiparticles in a quantum Wigner crystal. Vacancions formed upon the removal of an electron from the crystal are delocalized owing to the tunneling effect. The vacancion energies E(k) form a band of width D that depends on the probability of vacancy tunneling. The width D corresponds to the width of the photoluminescence band of the two-dimensional electron system. The shape of the photoluminescence band of the Wigner crystal is obtained using the tight-binding approximation for the vacancion dispersion relation E(k) is compared with experimental results.

     

反常色散材料光子晶体中光输运的光学控制

研究了光抽运对由铯原子蒸气构成的光子晶体的影响.研究发现,利用较弱光强的线抽运光即可显著地改变这种反常色散光子晶体的透射率.而利用圆偏振光抽运可以进一步降低抽运光的阈值,并在反常色散光子晶体中获得极大的法拉第旋转.反常色散光子晶体的这些特性为光控光开关的研究和制作提供了一条新途径.     

Quantum dots formed in InSb/AlAs and AlSb/AlAs heterostructures

The crystal structure of new self-assembled InSb/AlAs and AlSb/AlAs quantum dots grown by molecularbeam epitaxy has been investigated by transmission electron microscopy. The theoretical calculations of the energy spectrum of the quantum dots have been supplemented by the experimental data on the steady-state and time-resolved photoluminescence spectroscopy. Deposition of 1.5 ML of InSb or AlSb on the AlAs surface carried out in the regime of atomic-layer epitaxy leads to the formation of pseudomorphically strained quantum dots composed of InAlSbAs and AlSbAs alloys, respectively. The quantum dots can have the type-I and type-II energy spectra depending on the composition of the alloy. The ground hole state in the quantum dot belongs to the heavy-hole band and the localization energy of holes is much higher than that of electrons. The ground electron state in the type-I quantum dots belongs to the indirect X_XY valley of the conduction band of the alloy. The ground electron state in the type-II quantum dots belongs to the indirect X valley of the conduction band of the AlAs matrix.     

Conduction electron-exciton complexes in semiconductor single crystals

We consider the possibility of a bound state being formed from the pairing of an excited electron in the conduction band with an exciton in a semiconductor at low temperatures. The model consists of two levels (the valence and conduction bands) for a simple cubic lattice with periodic boundary conditions and the exciton is intermediate between the Wannier and Frenkel type excitons. The exciton which is discussed consistst of a tightly bound electron from the conduction band and a hole from the valence band on the same lattice site. Electrons and holes are, however, allowed to hop independently between nearest-neighbour lattice sites. The dispersion relations which determine the exciton and the electron-exciton modes are solved numerically. It is found that there are two branches for the coupled mode frequencies. This physical picture is analogous to that for polaritons and magnon-phonon modes in crystals.     

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.     

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently.Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.     

Collective excitations of a periodic Bose condensate in the Wannier representation

We study the dispersion relation of the excitations of a dilute Bose-Einstein condensate confined in a periodic optical potential and its Bloch oscillations in an accelerated frame. The problem is reduced to one-dimensionality through a renormalization of the s-wave scattering length and the solution of the Bogolubov-de Gennes equations is formulated in terms of the appropriate Wannier functions. Some exact properties of a periodic one-dimensional condensate are easily demonstrated: (i) the lowest band at positive energy refers to phase modulations of the condensate and has a linear dispersion relation near the Brillouin zone centre; (ii) the higher bands arise from the superposition of localized excitations with definite phase relationships; and (iii) the wavenumber-dependent current under a constant force in the semiclassical transport regime vanishes at the zone boundaries. Early results by Slater [Phys. Rev. 87, 807 (1952)] on a soluble problem in electron energy bands are used to specify the conditions under which the Wannier functions may be approximated by on-site tight-binding orbitals of harmonic-oscillator form. In this approximation the connections between the low-lying excitations in a lattice and those in a harmonic well are easily visualized. Analytic results are obtained in the tight-binding scheme and are illustrated with simple numerical calculations for the dispersion relation and semiclassical transport in the lowest energy band, at values of the system parameters which are relevant to experiment. Received 3 December 1999 and Received in final form 22 March 2000     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响．计及纵光学(LO)声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子-声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子-声子耦合常数的关系．数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子-声子耦合常数的增大而增大．     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响。计及纵光学（ LO）声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子－声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子－声子耦合常数的关系。数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子－声子耦合常数的增大而增大。     

Electron spectrum and infrared transitions in semiconductor superlattices with a unit cell allowing for quasi-localised carrier states

We studied theoretically the electron spectrum and infrared transitions in a superlattice with a unit cell allowing for quasi-localised carrier states. The dispersion relation and the band structure of such a system have been found. We calculated the dipole matrix element for inter-subband carrier infrared transitions. The wave functions and the electron spectrum in this superlattice show a peculiarity when the energy of a band state approaches the energy of the quasi-localised state in the single cell. The absorption strength peaks up at the respective frequencies.     

Properties of nonadiabatic quantum fluctuations of the strongly coupled electron-phonon systems

A new variational-ansatz of states of electrons and phonons was proposed on the basis of the Holstein model in strongly coupled electron-phonon systems for studying the influence of nonadiabatic phonon fluctuation, arising from the motion and density fluctuation of electrons, on the properties of ground state, uncertainty relation, stability of polarons, charge density wave (CDW) and phonon staggered ordering. The new ansatz represents the correlation among the displacement and squeezing states of phonons and polaron’s state of electrons as well as the squeezing-antisqueezing effect. The correlation and squeezing-antisqueezing effect result in the decrease of ground state energy, enhancement of stability of the systems, increase of binding energy of the polarons, weakening of the growing speed of polaron narrowing of the electron band, increase of the charge density wave order and suppression of the increased tendency of anomalous quantum fluctuation of the phonons in such a case, when compared with the uncorrelated case in the systems. The results obtained show that the ground state determined by the new state ansatz is most stable, thus the new ansatz describing the properties of the coupled electron phonon systems is very relevant and available, especially in strongly coupled and largely squeezed cases. Supported by the National “973” Project of China (Grant No. 2007CB6103)     

Fundamental properties,localization threshold,and the Tomonaga-Luttinger behavior of electrons in nanochains

We provide a fairly complete discussion of the electronic properties of nanochains by modelling the simplest quantum nanowires within a recently proposed approach which combines the Exact Diagonalization in the Fock space with ab initio calculations (EDABI method). In particular, the microscopic parameters of the second-quantized Hamiltonian are determined, and the evolution of the system properties is traced in a systematic manner as a function of the interatomic distance (the lattice parameter, R). Both the many-particle ground state and the dynamical correlation functions are discussed within a single scheme. The principal physical results show: (i) the evolution of the electron momentum distribution and its analysis in terms of the Tomonaga-Luttinger scaling, (ii) the appearance of mixed metallic and insulating features (partial localization) for the half-filled band case, (iii) the appearence of a universal renormalized dispersion relation for the electron energy, which incorporates both the band-structure and the Hubbard-splitting features in the presence of electron interactions, and (iv) the transformation from a highly-conducting nanometallic state to the charge-ordered nanoinsulator in the quarter-filled case. The analysis is performed using the Wannier functions composed of an adjustable Gaussian 1s-like basis set, as well as includes the long-range part of the Coulomb interaction.Received: 19 February 2004, Published online: 12 August 2004PACS: 73.63.-b Electronic transport in nanoscale materials and structures - 31.15.Ar Ab initio calculations - 71.10.Hf Lattice fermion models - 71.27. + a Strongly correlated electron systems     

Semiclassical energy levels of electrons in metals with band degeneracy lines

We show that in calculating the semiclassical energy levels of electrons in metals located in a magnetic field, one must determine whether or not the corresponding electron paths in the space of wave vectors k are attached to a band degeneracy line. Calculations in the two possible cases, i.e., with and without such attachment, differ by |e|ℏ/2m*c, where e is the electron charge and m* is the cyclotron mass of the electron. This shift in the energy levels is of a topological nature, and its existence depends neither on the specific form of the electron dispersion relation ε(k) near the electron path nor on the shape or size of this path. The reason for this shift lies in the fact that the electron orbit is attached to the band degeneracy line, which is the line of singular points of the Bloch wave functions. In many respects this effect is similar to the Aharonov-Bohm effect if the band degeneracy line is considered an infinitely thin “solenoid.” This shift in energy levels should become apparent in studies of oscillation phenomena in metals. We give examples of metals in which the conditions for observing the shift is probably the most favorable. Zh. éksp. Teor. Fiz. 114, 1375–1392 (October 1998)     

Theory for slightly doped antiferromagnetic mott insulators

New trial wave functions, constructed explicitly from the unique Mott insulating state with antiferromagnetic order, are proposed to describe the ground state of a Mott insulator slightly doped with holes or electrons. A rigid band is observed as charged quasiparticles with well-defined momenta being realized in these states. These states have much less superconducting correlations than previously studied ones. Small Fermi patches obtained are consistent with recent experiments on high T(c) cuprates doped lightly with holes or electrons.     

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ～5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ～1.99 THz (8.2 meV) and ～2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ～3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ～17 nW per 1 W of the input power.     

Electronic states in a Pöschl–Teller-like quantum well: Combined effects of electric field, hydrostatic pressure, and temperature

The work is devoted to present a theoretical study of the influences of external probes, such as applied electric field and hydrostatic pressure, on the electron and hole states in a Pöschl–Teller quantum well. The calculations have been done in the framework of the variational method. The dependence of the ground state energy of an electron and/or hole confined in the quantum well has been obtained as a function of the applied electric field and hydrostatic pressure. Different values of the asymmetry parameters of the Pöschl–Teller potential as well as temperature have been considered. It is shown that as a result of the increase in the electric field there is an augment of the ground state energy, and also that by increasing the quantum well width the effects of applied electric field are strengthened. It is obtained from the calculations that the ground state energy is a decreasing (increasing) function of the hydrostatic pressure (temperature). It is found that in the high pressure regime the energy grows with pressure, which is a previously unknown result. In the case of holes, the energy is always an increasing function both of the pressure and the temperature. Besides, the behavior of the photoluminescence peak energies associated to transitions between the ground states of electrons and heavy holes in the system is also reported.     

Polarization of valence band holes in the (Ga,Mn)as diluted magnetic semiconductor

We report on direct measurements of the impurity band hole polarization in the diluted magnetic semiconductor (Ga,Mn)As. The polarization of impurity band holes in a magnetic field is strongly enhanced by antiferromagnetic exchange interaction with Mn ions. The temperature dependence of the hole polarization shows a strong increase of this polarization below the Curie temperature. We show that the ground state of the impurity band is formed by uniaxial stress split F=+/-1 states of antiferromagnetically coupled Mn ions (S=5/2) and valence band holes (J=3/2). The gap between the Mn acceptor related impurity band and the valence band is directly measured in a wide range of Mn content.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Luminescence property studies of α-Al2O3 by means of nanosecond time-resolved VUV spectroscopy

The luminescence spectra of corundum monocrystals grown by different methods are investigated by means of a time-resolved spectroscopy method at temperatures 90 K and 300 K. The existence of fast and slow emission in the VUV luminescence spectra of irradiated and nonirradiated crystals was observed. We observed luminescence bands with a maximum at 326 nm produced by F ⁺ centers. A new type of fast luminescence at the band of 270 nm was found. This is known as cross-luminescence and is connected with the recombination of valence band electrons with the holes in the low ground band. It was shown that the band of 410 nm isn't due to to anionic centers (F-centers), but is determined by the short lifetime center of emission (F ^- -centers). Received 20 October 1998 and Received in final form 20 January 1999