Electron spin polarization and conduction band structure of doped ferromagnetic semiconductors

The spin polarization of the conduction electrons of a doped semiconductor (e.g. EuO + x%Gd) is calculated using a moment method together with an alloy analogy. It is shown to be caused by a complicated temperature- and carrier concentration-dependence of the quasiparticle spectrum of the s-f model. Obtained polarization agrees very well with recent photoemission experimental data.     

Temperature dependence of the interband absorption edge of copper

The optical absorption spectra of bulk copper measured by Pells and Shiga have been fitted by means of a model calculation in the interband edge region. The optical gap and the dipole matrix element of the L₃-L'₂(E_F) transition have been determined as a function of the temperature and a comparison with similar results obtained for silver and gold is presented.     

Temperature dependence of absorption edge anisotropy in 2H- MoSe2 layered semiconductors

The absorption-edge anisotropy of 2H- MoSe₂ was studied by photoconductivity (PC) measurements as a function of temperature in the range of 12-300 K. A significant shift towards lower energies has been observed in the PC spectra on the edge plane with respect to those corresponding to the van der Waals (VdW) plane. The parameters that describe the temperature dependence of the absorption edges are evaluated by the Bose-Einstein empirical expression. Effective phonon energy was estimated from the temperature dependence of the Urbach energy. The estimated effective phonon energy for the VdW and edge planes, respectively, can be correlated to the observed Raman active and modes.     

Temperature dependence of the magnetic susceptibility in IV–VI semiconductors

The magnetic susceptibility of IV–VI semiconductors with a quasirelativistic electron spectrum is investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 57–62 (January 1998)     

The density of states and band edge shift of undoped ferromagnetic semiconductors. Real spin 7/2

The perturbation approach for the real spinS=7/2 has been used to study the conduction band density of states for europium chalcogenides starting from the exact atomic limit Green functions. The density of states for an empty conduction band shows the multi-band structure and is temperature dependent. The shift of the optical absorption edge (rs) as a function of temperature has been obtained for EuO and EuS. The results agree relatively well with experiments on EuO. It is shown that thes-f coupling constant is universal for EuO, EuS and EuSe (ferromagnetic phase).     

The spin polarization of conduction electrons in ferromagnetic semiconductors

Within the framework of the s?f exchange model the exact asymptotics of the spin polarization of conduction electrons in the non-degenerate ferromagnetic semiconductor at low temperatures is calculated. It is demonstrated that the temperature dependent corrections to the total density of states are proportional to $\text{T}{}^{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$.     

The dependence of the conduction band edge of the alkali earth metal bismuthates on their composition

We present a comparative analysis of the conduction band edge of the alkali earth metal bismuthates containing Mg, Ca, Sr, and Ba. The conduction band edges were computed using the method suggested by Butler and Ginley. The calculations reveal that they depend on the bismuthate’s composition and vary over a wide range. We demonstrate that the energy of the conduction band increases in the series Ca?→?Sr?→?Ba. It also increases with an increase of the alkali earth metal content. The performed calculations help to determine the potential alkali earth metal bismuthate photocatalysts. The most promising compositions found in this study include strontium and barium bismuthates in which the number of the alkali earth metal atoms in the cationic sublattice exceeds the number of the bismuth atoms.     

Effect of a magnetic field on the phonon-assisted two-photon absorption in semiconductors

The theory of the phonon-assisted two-photon transitions in a magnetic field is developed by means of perturbation theory. Three different band models have been introduced. The electronic transitions are defined by (i) interband-interband indirect transitions in the four-band model, (ii) interband-intraband indirect transitions in the three-band model, and (iii) intraband-intraband indirect transitions in the two-band model. These models differ from each other by the number of bands, the type of the photon matrix elements and the Landau selection rules. The result shows that by increasing the value of the magnetic field, the absorption co-efficient adopting the two-band model is strongly increased, but for a fixed value of the field the four-band model gives the largest contribution.     

Frequency dependence of the ferromagnetic resonance width in magneto-impedance measurements

Giant magneto-impedance has shown large sensitivities that are of great interest in practical applications. Above certain frequencies the ferromagnetic resonance controls the magneto-impedance behavior, and the resonance width limits the maximum achievable magneto-impedance ratio. In this work we present the evolution of the resonance width as a function of the frequency, determined through magneto-impedance and microwave absorption measurements on a NiFe–Au multilayer thin film. The width of the resonance can be fitted to a curve in the form of Δf∼f⁻¹. This is explained by means of a simple model taking into account all inhomogeneities in the sample through a Gaussian distribution of anisotropy fields, as suggested by the shape of the hysteresis loop.     

Temperature dependence of the fundamental absorption edge of ferroelectric crystals of dimethylamine chlorocuprate

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 56, No. 1, pp. 86–89, January, 1992.     

Temperature dependence of the fundamental absorption edge in CuGaSe2

The temperature dependence of the fundamental absorption edge in CuGaSe₂ single crystals was determined in the temperature range from 15 to 300 K. Above about 120 K the gap energy changes linearly with temperature with dE_g/dT = ? (2.1 ± 0.1) eV K^?1. The downshift in dE_g/dT of the I–III–VI₂ compounds compared to their II–VI analogs is discussed accounting for the p-d hybridization of the uppermost valence band.     

The influence of thes-f interaction on the conduction band behaviour in ferromagnetic semiconductors. A theory of the red shift effect

The mechanism of the thermomechanical instability of an elastically slowly strained solid is explored, by a normal mode analysis of temperature and strain fluctuations. The strain dependence of the Grüneisen parameter is found to play a major role, since it controls the rates of damping of the lowest wavevector acoustic excitations pumped by the imposed strain. For -titanium the instability is found to occur at a critical strain comparable with that of the elastic limit. The instability can be envisaged in terms of a fast growing deformation, presumably associated with an acoustic emission.Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, UR7     

Temperature dependence of the rho-meson mass and width

It is shown, to leading order in the virial expansion, that unitarity of the ππ scattering amplitude implies that if the ρ-meson widthΓ _ρ(T) increases with temperature, then the ρ-meson massM _ρ(T) must also increase. The temperature dependence ofM _ρ(T) is explicitly obtained from a simple, but physically motivated, parametrization of theI=J=1 ππ scattering amplitude.     

Theory of hopping conduction in two-dimensional impurity band under strong magnetic field

It is shown that the low-temperature conductivity in two-dimensional impurity band of n-type inversion layer under strong magnetic field is $\text{σ}{}_{\mathrm{xx}}\text{(}\text{min}\text{)=σ}{}_0\text{exp}\text{[?(}\text{T}{}_0\text{T}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{]}$. Therefore the effective conductivity of inversion layer $\text{σ}{}_{\mathrm{<mtext>eff</mtext>}}\text{=}\text{σ}{}^2{}_{\mathrm{xy}}\text{σ}{}_{\mathrm{xx}}\text{(}\text{min}\text{)}$ at Shubnikov-de Haas conductivity minima may be very high at low temperature.     

Temperature dependence of indirect-exciton luminescence in in-plane magnetic field

We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one.