Donor-acceptor pair capture cross section for highly excited electrons in polar semiconductors

The capture cross section σ (R) for Donor-Acceptor Pairs (DAPs) with pair separation distance R is improved for the case of hot carriers by taking into account the LO-phonon interaction so that an energy dependend cross section σ(E_k, R) results. This latter process, which is of importance for a non-equilibrium carrier distribution, yields a preferential occupation of close DAPs thus provoking a blue shift and a deformation of the main emission band if compared with line profiles where only thermalized carriers are involved.     

Current relaxation processes in highly excited semiconductors

We compare the conductivity relaxation time due to electron-hole scattering to the one due to electron-phonon scattering. We first calculate the variations of both scatterings with the electron-hole plasma density from the dilute to the metal-like regime, using the appropriate screenings of the interaction potentials; we find that, unexpectedly, the electron-hole collisions dominate the electron-phonon scattering, not at very large density, but instead, at intermediate density, when the plasma becomes degenerate.     

Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

Time evolution of electron temperature and hot LO-phonon distribution function in laser excited polar semiconductors

The numerical analysis is presented of the time evolution of the conduction electron temperature and of the hot LO-phonon population in polar semiconductors GaAs, CdTe and ZnS excited by a short laser pulse. The paper is based on the equations describing the evolution of the nonequilibrium electron-phonon system including the electron energy collision broadening and the electron screening of the electron-phonon interaction. The connection between the hot phonon population in the range of small values of the wave vector and the wave vector dependence of the screening function is pointed out.     

An approach to the nonequilibrium theory of highly excited semiconductors

We present an investigation of the optical properties of semiconductors, which are excited by a strong laser pulse. On the basis of a nonequilibrium Green's function technique (Keldysh formalism) we study the system of electrons and holes, interacting among themselves and with the exciting pulse. A set of coupled equations for the one-particle Green's function and the transverse and longitudinal polarizability is derived, which is valid for arbitrary excitation conditions. For an initially prepared state, which is not too far from a quasiequilibrium state of the excited system, the theory is elaborated in detail, using a screened Hartree-Fock approximation scheme. Within this approximation earlier results of various theoretical approaches are found to be special cases of the equilibrium limit of the present theory.     

On the dielectric function in zero-gap semiconductors

The q and ω dependent dielectric function is examined in the random phase approximation for zinc-blende type, zero-gap semiconductors such as HgTe or HgSe. A detailed analysis of the properties of this function based on the analytical formulae obtained for the case of a highly degenerate free-electron gas is performed. In the limiting cases of the static and dynamic dielectric functions a comparison to previous theories is given.     

Magnetic field dependent dielectric function for zero-gap semiconductors

Using the random phase approximation and the Yafet wave functions the inter and intraband static dielectric functions of degenerate zero-gap semiconductors placed in a magnetic field are calculated in the long wavelength limit. It is shown that the values of the dielectric function for gapless semiconductors differ considerably from the result obtained previously for the one-band model with parabolic energy dispersion ands-type wave functions. Finally the dielectric function is used to find the static screening of a point impurity charge.     

Non-equilibrium distribution function in polar direct gap semiconductors

Electron distribution function (EDF) for non-equilibrium configurations in polar direct gap semiconductors is obtained in the temperature range k_BT? ? ω_Lo. The sample is assumed to be free of impurities and crystal defects. Electron-phonon coupling is the main relaxation mechanism. Master equation is solved approximately and analitycal expressions for EDF are obtained. Average electron energy as a function of temperature is discussed.     

Picosecond dynamics of IR reflection and transmission by highly excited semiconductors

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 3, pp. 438–444, 1990.     

A simple theory for the effects of plasma screening on the optical spectra of highly excited semiconductors

A partly phenomenological theory is developed to describe the nonlinear optical properties of laser-excited semiconductors in the spectral vicinity of the fundamental absorption edge under conditions where each optically generated electron-hole pair interacts with the electron-hole plasma. A closed expression for the density- and temperature-dependent absorption spectrum is derived. The resulting equations are evaluated for the example of highly excited GaAs.     

Calculation of highly excited eigenstates of chaotic quantum systems

Computational methods for the calculation of a large number of eigenstates of coupled oscillator system are developed and discussed. These calculations have enabled us to identify and investigate properties of an infinite set of states sharply localized in configuration space in this system. Some of the results and their significance are discussed. Extensions to three-dimensional systems are also briefly considered.     

From coherently excited highly correlated states to incoherent relaxation processes in semiconductors

Recent theories of highly excited semiconductors are based on two formalisms, referring to complementary experimental conditions, the real-time nonequilibrium Green's function techniques and the coherently controlled truncation of the many-particle problem. We present a novel many-particle theory containing both of these methods as limiting cases. As a first example of its application, we investigate four-particle correlations in a strong magnetic field including dephasing resulting from the growth of incoherent one-particle distribution functions. Our results are the first rigorous solution concerning formation and decay of four-particle correlations in semiconductors. They are in excellent agreement with experimental data.     

Calculation of the dielectric function of Li metal

The inverse microscopic dielectric function of Li metal is evaluated within the time dependent local density approximation (TLDA) using one-electron energies and wave-functions obtained by SCF band structure calculations. Even at larger values of transferred momentum the influence of exchange and correlation is found to be quite small. Local fields are approximated by considering all elements of the dielectric matrix corresponding to the first and second shell of the reciprocal lattice vectors. Their influence is found to be orientation-dependent but in most cases quite small. However, in some cases they even cause a change of the shape of the inverse dielectric function. Our theoretical results agree quite well with measurements of the dynamic structure function by use of inelastic X-ray scattering with transferred energy up to 18 eV. Unrealistic oscillations at still higher energies as well as the narrow width of the peaks indicate the limits of a one-particle scheme.     

Anisotropic dielectric function in polar nanoregions of relaxor ferroelectrics

The Letter suggests treating the infrared reflectivity spectra of single crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally frozen polarization makes the dielectric function strongly anisotropic in the phonon frequency range and the random orientation of the polarization at nanoscopic scale requires taking into account the inhomogeneous depolarization field. Employing a simple effective medium approximation (the Bruggeman symmetrical formula) turns out to be sufficient for reproducing all principal features of room temperature reflectivity of Pb(Mg(1/3)Nb(2/3))O3. One of the reflectivity bands is identified as a geometrical resonance entirely related to the nanoscale polarization inhomogeneity. The approach provides a general guide for systematic determination of the polar mode frequencies split by the inhomogeneous polarization at the nanometer scale.     

The dielectric function of holes in semiconductors of zinc-blende structure

A closed analytic expression for the q and ω dependent dielectric function of the degenerate gas of holes in a zinc-blende or diamond structure semi-conductor is obtained in the random phase approximation (RPA). The limits of q → 0 and ω → 0 are examined and a comparison with previous developments is given. Significant deviations from the earlier results are found.