Exciton luminescence of quasi-two-dimensional solid solutions

Absorption and luminescence of the quantum wells formed by the (Zn-Cd)Se and (Ga-In)As solid solutions are studied in the range of exciton size-quantization ground state. The spectra observed are described by a model assuming the two-dimensional character of fluctuation states in quantum wells and the presence of a percolation threshold within the absorption contour.     

Luminescence of fluctuation tails of disordered solid solutions

The form of the stationary luminescence spectra of excitons, localized by composition fluctuations, in disordered solid solutions under weak excitation is calculated. The tail states for which there are no nonradiative transition channels are distinguished by means of continuum percolation theory. Such states are responsible for the “zero-phonon” luminescence band. The shape of the short-wavelength luminescence band edge is determined mainly by the number of isolated localizing clusters and their smallest complexes, which decreases rapidly near the mobility threshold. The real luminescence spectrum is due to the simultaneous emission of phonons. The phonon emission determines the form of the long-wavelength wing of the emission band. The computed shape of the emission spectrum is compared with the experimental luminescence spectra of the solid solution CdS_(1−c)Se_c. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 274–279 (10 February 1997)     

Exciton luminescence of Cd1−x FexTe solid solutions

Band-edge optical spectra of Cd_1−x Fe_xTe solid solutions differ substantially from those of undoped CdTe. The pattern of the change in photoluminescence spectra with increasing Fe concentration is connected with a change in radiative recombination channels. Fiz. Tverd. Tela (St. Petersburg) 40, 897–899 (May 1998)     

Exciton luminescence of AgBr1−x Cl x solid solutions

Institute of Solid-State Physics, 8, Kengaraga St., Riga, LV 1063, Latvia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 3, pp. 174–177, May–June, 1995.     

Exciton splitting and Fermi resonance in solid solutions

Experimental investigations of the exciton spectra in CH₃I:CD₃I mixed crystals in the Fermi resonance region are made on the well oriented polycrystalline samples. The results obtained are compared with the theory developed.     

Distribution of the density of electron states in TiC-VC and TiC-TiN solid solutions

The published results of a calculation of the energy band structure of TiC, VC, and TiN are utilized in histograms of the density of states in TiC_xN_1–x and Ti_xV_1–xC solid solutions. It is shown that the density of states at the Fermi level varies in different ways with the composition of TiC-VC and TiC-TiN solid solutions and it is shown that the rigid band model is inapplicable to the physical properties of these solid solutions.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 6, pp. 15–19, June, 1980.     

Bounds on the density of states in disordered systems

For a class of tight-binding models governed by short-range one-particle Hamiltonians with site-diagonal and/or off-diagonal disorder and continuous distribution of the matrix elements it is proven that the averaged density of states does neither vanish nor diverge inside the band. This refutes for these models conjectures that the density of states vanishes or diverges at the mobility edge.Work supported in part by the Sonderforschungsbereich 123 Stochastic Mathematical Models of the Deutsche Forschungsgemeinschaft     

Bounds on the density of states in disordered systems

In magnetic systems the radio frequency fieldH ₁ seen by the nuclei is often enhanced by a factor . If the atomic spins are distributed in multi directions, as in Bloch walls or spin glasses, one has to consider a spatial distribution (r) of enhancement factors. The usual way to define a mean enhancement factor is shown to lead to inconsistent results giving rise to errors up to more than 100%. One has to distinguish between two mean factors, and , depending on measuring conditions. Their deviations from the true value are discussed.     

Singularity in the density of states of quasi-one-dimensional disordered systems

We have calculated the correction to the density-of-states in two-dimensional weakly disordered systems. It is shown that the electron-electron interaction gives rise to a singularity in the density-of-states at the Fermi level, the form of which is very sensitive to the width of the sample. The transition from a two-dimensional behaviour to the quasi-one-dimensional regime is discussed. The effects should be observable by electron tunnelling measurements.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

Urbach energy and the tails of the density of states in amorphous semiconductors

Formulas are derived for the Urbach energy as a function of the parameters controlling the slope of the exponential tails in the valence and conduction bands of amorphous semiconductors. It is shown that the Urbach energy remains constant, even if one of these parameters varies over a wide range.     

On spectral density of electronic states in disordered systems

In the present paper, the averaged spectral density of electronic states, 〈?(k, E)〉, has been computed for three-dimensional disordered systems with spherical well type atomic potentials. The formula for 〈?(k, E)〉 is derived by using a realistic perturbational approach that takes propagator modification into account, together with a handy approximation due toBallentine. 〈?(k, E)〉 has been computed for various sets of potential strength and concentration, and the dependence of 〈?(k, E)〉 upon these parameters has been discussed. The motivation behind the computations reported here, is to examine the circumstances under which the electron motion in disordered systems is expected to be almost free electronic. Our principal finding is that a simultaneous existence of weak electron-scatterer interaction-potential and high density of scatterers would tend to make the motion of an electron in a disordered system almost like that of a free one.     

Quasiequilibrium states of solid solutions

A phenomenological theory of quasiequilibrium states of solid solutions with at least three components is constructed under the assumption that the atomic mobilities of the components differ in order of magnitude. The short-range parts of the interatomic interactions are taken into account by introducing atomic self-volumes. The long-range parts of the potentials are taken into account in the effective-field approximation. The equations for the quasiequilibrium components in the solutions are derived neglecting the mobility of the least mobile component. It is shown that, for a nonuniform distribution of this component, the nonequilibrium critical temperature ceases being unique within the sample and acquires a local character. In consequence, opposing processes occur in the system in the temperature interval between T _c ^min and T _c ^max — homogenization in some sections and formation of interphase boundaries in others. Fiz. Tverd. Tela (St. Petersburg) 41, 60–63 (January 1999)     

Low energy density of states in several disordered systems

Disordered systems of arbitrary dimension consisting of randomly distributed scatterers are studied in the low energy limit (Roger Waxler, Ph.D. thesis, Columbia University). We evaluate the density of electronic energy levels for long range, attractive, and electric dipole scatterers.     

Pair breaking and density of states in disordered superconductors

It is shown that pair breaking occurs in a disordered superconductor due to spatial variations of the order parameter, although the system is time reversal invariant. The pair breaking effect is reflected by the occurence of some interesting fine structure in the one particle density of states. Discrete bound states and split off impurity bands show up in the single impurity case and for very dilute alloys, respectively. For finite alloy concentrations the calculations are done within the CPA. Although principally important, the fine structure is concentrated in an energy range too narrow to be detected experimentally.     

On the calculation of electron density of states in disordered metals

It is demonstrated that in a full nonlocal pseudopotential calculation of the density of states, the results from the standard formula, $\text{N(E) = N(E}{}_{\mathrm{<mtext>FE</mtext>}}\text{)k (}\text{?E}{}_{\mathrm{<mtext>k</mtext>}}\text{?k}\text{)}{}^{\mathrm{?1}}$, do not differ significantly from those using the Green-function method and are reliable for simple liquid metals, despite the fact that there is an uncertainty in treating E_k as a dispersion function.     

Gap inhomogeneities and the density of states in disordered d-wave superconductors

We report on a numerical study of disorder effects in 2D d-wave BCS superconductors. We compare exact numerical solutions of the Bogoliubov-de Gennes (BdG) equations for the density of states rho(E) with the standard T-matrix approximation. Local suppression of the order parameter near impurity sites, which occurs in self-consistent solutions of the BdG equations, leads to apparent power-law behavior rho(E) approximately |E|(alpha) with nonuniversal alpha over an energy scale comparable to the single-impurity resonance energy Omega(0). We show that the novel effects arise from static spatial correlations between the order parameter and the impurity distribution.     

Optical spectra and a tail in the density of states of the disordered solid solution ZnSe1−c Tec

A calculational scheme is presented to determine the density of states in the fluctuation tail for the disordered solid solution ZnSe_1−c Te_c at concentrations below the threshold for percolation over sublattice sites. Zero-phonon absorption and luminescence bands in the region of the exciton ground state are found using an approach developed earlier. Phonon coupling is taken into account, and vibronic absorption and luminescence bands are obtained. Experimental data are shown to be in a good agreement with the calculations. Fiz. Tverd. Tela (St. Petersburg) 40, 1420–1425 (August 1998)