Electronic Excitations In Solids Research Articles

Coherent transient spectroscopy of excitons in GaAs-AlGaAs quantum wells

Coherent transient optical spectroscopy techniques provide a means of studying the relaxation of electronic excitations in solids. Using these techniques the authors examine the relaxation of excitons in GaAs-AlGaAs multiple quantum wells at low temperature. Localization of excitons due to interface disorder results in inhomogeneous broadening of the absorption spectrum and relaxation due to migration between localization sites. The temperature dependence and energy dependence of the relaxation indicate the presence of complex scattering mechanisms. The dependence of the optical response on the polarization of the incident excitation fields is shown to result in a dramatic change of the dephasing and broadening characteristics of the optical response. This response is interpreted in terms of the presence of both localized and delocalized excitons. Mechanisms for the polarization dependence are discussed. >

Mechanisms of atomic processes induced by electronic excitation in solids

We describe the mechanisms of atomic processes induced by valence-electron excitation in solids. It is pointed out that localization of elementary excitation and instability of the localized excitation are the crucial steps for inducing atomic processes. The mechanisms of localization and instability, both of which are induced by electron-phonon interaction are discussed. The high-density effects for three classes of solids, classified according to the mode of localization, are discussed.

On the computation of electronic excitations in solids

An approximation scheme is proposed for calculating electronic excitations in solids. It is described within a projection operator formalism of the Mori-Zwanzig type. The approximation consists in successively limiting the space of dynamical variables which are treated. The selection of the variables is done by making use of the local character of the correlation hole which is surrounding an electron. The method is distinct from a perturbation expansion and can be related to a variational ansatz via the Sauermann functional. The computation of the spectral density of the Green's function is reduced to the diagonalization of matrices. Their dimension depends on the required degree of accuracy of the calculations. The present method can be considered as an extension to excited states of a previously developed. Local Approach for ground state calculations.

Coherent Energy Transfer in Solids

Periodicity is the property of crystalline solids that makes possible the propagation of electronic excitation in solids as coherent wavepackets in the limit of weak exciton-lattice interactions. In this review, we briefly discuss the theoretical founda­ tion of coherent energy migration in the various limits of the exciton-lattice inter­ action. In addition, we revicw and evaluate experimental evidence for coherence from a variety of sources. This latter area is the focus of this review because the experimental detection and characterization of coherent energy transfer has been a field of such great interest and moderate controversy in recent years. In the last dozen years several general reviews on the properties of molecular solids have been published. These include those written by Hochstrasser (1), Robinson (2), Kopelman (3), and El-Sayed (4). Soos (5) has reviewed excitation in organic charge-transfer crystals. More recently, Silbey (6) has written an excellent review on electronic energy transfer processes in molecular crystals. Although his review has much in common with ours, he focused on formal developments in the theory of exciton-phonon interactions while we are concerned primarily with the theory and its relation to experimental observations of coherence and exciton­ phonon coupling. While experimental evidence is of recent vintage, the prediction of coherence in molecular solids dates back to the work of Frenkel in 1931. In his original paper on excitons (7), Frenkel develops three important insights. First, he pointed out that the electronic excitation can be delocalized and, as a result, be described by a series of Bloch wave functions in analogy to the lattice modes of the crystal. Second, as a consequence of delocalization, the superposition of several of the modes will form a localized wavepacket that propagates through the lattice with a group velocity

Energy losses by slow ions and atoms to electronic excitation in solids

We consider the theory of energy losses by slow ions and atoms to electronic excitations in an electron gas. Predictions of the theory are compared with experimental data on ion penetration in several different solids. We also compare the stopping power obtained from linear-response theory with that found from numerically computed phase shifts for electron scattering on the screened potential of an ion. The stopping power of an electron gas for slow, singly ionized He atoms is calculated from linear-response theory, by using a wave function for the bound electron determined self-consistently in the electron gas.

Radiationless relaxation in solids as measured by a heat-pulse technique

The heat resulting from the radiationless relaxation of electronic excitations in solids can be followed thermometrically on a submicrosecond time scale using a superconducting bolometer as detector. Details of the technique are described, and illustrated with the relaxation profiles of Cr(III) ion excited in Al2O3, LiTaO3, and MgO host lattices.

Theory of light scattering from electronic excitations in solids

The effect of the periodic potential and the residual Coulomb interaction on the inelastic scattering of light from pure electronic excitations in solids has been investigated by using the diagrammatic perturbation theory. It is shown that for a degenerate electron gas single-particle excitations with no spin reversals are heavily screened and almost all the scattering is from the collective plasma mode for momentum transfers small compared to Fermi-Thomas momentum. However, there is no screening of single-particle spin-density fluctuations. This is an important intra-band process for III–V semiconductors with an appreciable spin-orbit splitting of the valence band. For high concentration of carriers, the nonparabolicity of the band structure also contributes to the intra-band single-particle scattering. For low concentrations or at high temperatures, when plasma mode is no longer a well-defined normal mode of the system, these two processes are not important. In this case there is no screening of the single-particle excitations from electrons in a spherical parabolic band. For inter-band excitations, the effect of Coulomb interaction is shown to be negligible in the long-wavelength limit.