Magnetic and electronic properties of NiAs-type chromium chalcogenides

We have computed spin-dependent energy bands, spin moments and density of states of NiAs-type CrX (X=S, Se and Te) chalcogenides using linear combination of atomic orbitals method within density functional theory as well as full potential augmented plane wave method. In addition, magnetic properties have also been computed using spin polarized relativistic Korringa-Kohn-Rostoker method. We have also obtained the first ever theoretical electron momentum densities of CrX compounds considering linear combination of atomic orbitals and compared the results with the isotropic Compton profiles measured using 20 Ci ¹³⁷Cs Compton spectrometer. The Fermi surface topology and magnetic properties are discussed in terms of majority and minority energy bands and density of states. In addition, to highlight the role of Cr (3d) electrons in such type of chalcogenides, we have also reported the magnetic Compton profile of CrTe using the Korringa-Kohn-Rostoker method.     

Optical properties and electronic structure of europium chalcogenides

The normal incidence reflectivity of europium chalcogenide single crystals and of Gd doped EuO has been measured at room temperature in the spectral region from 250 μm to 12 eV and has been analyzed in terms of the optical constants. In addition, in a reduced spectral region from 0.5 to 6 eV, the optical constants have been evaluated by means of a polarimetric method, as well above as below the magnetic ordering temperature. To enhance the resolution of the magneto-optical transitions, a modulation technique has been applied with a magnetic field as modulating parameter. The Kramers-Kronig relation has been used to analyze the normal incidence reflectivity and the magnetoreflectance spectra in terms of, respectively, the optical constants and the changes in the real and imaginary part of the dielectric response function. For Gd doped EuO the Kramers-Kronig analysis has revealed plasmon and coupled plasmon-phonon modes. The interband transitions of the europium chalcogenides are discussed within the framework of recent APW and OPW energy band calculations. On the other hand we have derived an energy level scheme of the europium chalcogenides from our optical data.     

Calculation of the electronic structure of crystalline and amorphous arsenic

Qualitative differences between crystalline and amorphous arsenic in spectroscopic data are reproduced in chemical pseudopotential calculations of, and a local environment approach to, the density of states of arsenic in the A7 structure and in a three-fold coordinated random network. The changes can be interpreted as a removal of the long interlayer bonds in the crystal, giving a local two-dimensional character to the amorphous density of states.     

The electronic structure of NbO: Theory and experiment

Experimental photoelectric- and X-ray emission spectra obtained from NbO are compared with the molecular orbital calculations of NbO₆^10? cluster using X_α discrete variational method. The agreement between theory and experiment establishes the validity of the cluster approach to determine the electronic structure of the 4d transition metal chalcogenides.     

Structural and electronic properties of transition-metal chalcogenides Mo_5S_4 nanowires

Transition-metal chalcogenide nanowires(TMCN) as a viable candidate for nanoscale applications have been attracting much attention for the last few decades. Starting from the rigid building block of M_6 octahedra(M = transition metal),depending on the way of connection between M_6 and decoration by chalcogenide atoms, multiple types of extended TMCN nanowires can be constructed based on some basic rules of backbone construction proposed here. Note that the well-known Chevrel-phase based M_6X_6 and M_6X_9(X = chalcogenide atom) nanowires, which are among our proposed structures, have been successfully synthesized by experiment and well studied. More interestingly, based on the construction principles, we predict three new structural phases(the cap, edge, and CE phases) of Mo_5S_4, one of which(the edge phase) has been obtained by top-down electron beam lithography on two-dimensional MoS_2, and the CE phase is yet to be synthesized but appears more stable than the edge phase. The stability of the new phases of Mo_5S_4 is further substantiated by crystal orbital overlapping population(COOP), phonon dispersion relation, and thermodynamic calculation. The barrier of the structural transition between different phases of Mo_5S_4 shows that it is very likely to realize an conversion from the experimentally achieved structure to the most stable CE phase. The calculated electronic structure shows an interesting band nesting between valence and conduction bands of the CE Mo_5S_4 phase, suggesting that such a nanowire structure can be well suitable for optoelectronic sensor applications.     

Theory of Raman scattering of light on spin-flip and electronic excitation in Europium chalcogenides

The inelastic scattering of light in magnetic semiconductors from the family of the Europium chalcogenides is discussed in relation with spin-orbit coupling and d-f exchange interaction. It is shown that the Raman processes connected with spin-flip and other electronic excitations can occur in the energy range of 0.1-0.5 eV. In this case and for the typical values of d-f exchange interaction and spin-orbit coupling parameter lying between 0.05 and 0.1 eV, the values of the differentional cross-sections are between 10^-9 - 10^-11 cm²sr^-1 sec. The selection rules for the polarization of the incident and scattered photons as well as the Raman tensors for the four allowed transitions are derived. The possibility of applying spin-flip Raman processes in magnetic semiconductors in the tuning of electromagnetic radiation in Raman lasers is analyzed briefly.     

First principles study of structural,elastic and electronic structural properties of strontium chalcogenides

Structural, elastic and electronic properties of strontium chalcogenides SrX (X = O, S and Se) in the B1 (NaCl) and B2 (CsCl) phases were investigated in the present work. The calculations were performed using density functional theory (DFT) within generalized gradient approximation (GGA) using scalar relativistic Vanderbilt-type ultrasoft pseudopotentials. Results for structural properties of both phases, the pressure at which transition from B1 to B2 phase occurs and the volume compression ratio for each compound were reported. Elastic properties of the B1 phase of these compounds, such as elastic constants C₁₁, C₁₂, and C₄₄, shear modulus (G), Young's modulus (E), Poisson's ratio (σ), Kleinman parameter (ξ), and anisotropy factor (A) were also calculated at ambient conditions. The band gaps and density of states were studied too for the B1 structure of these compounds. The present results were compared with the available experimental and other theoretical results, and found to be in satisfactory agreement with them.     

On the electronic structure of Ag chalcogenides

The electronic structure of Ag chalcogenides in the α phase, which exhibit an interesting, electronic semiconducting behaviour as well as the fast ion transport, is discussed on the basis of an energy band structure calculation. As a simplest way of simulating the effect of the Ag ions on the electronic states, some hypothetical crystalline compounds are constructed such as the perovskite, the sodium chloride and the flourite structures. The absolute magnitude of the calculated conduction electron effective mass is quite small irrespectively of the structures, about 10% of the free-electron mass, in semiquantitative agreement with experiments. A deviation from an effective mass approximation near the conduction band bottom is found to be appreciable, and to explain reasonably well experimental results. An origin of these features of the conduction band is a rather strong hybridization of the Ag 5s band and the chalcogen s band. The calculation also shows that the hybridization of the Ag 4d band and the chalcogen p band can affect the absolute magnitude of the hole effective mass appreciably, and that the energy band gap depends sensitively on these s-s and d-p hybridization effects.     

Optical properties of grooved silicon microstructures: Theory and experiment

The reflection spectra of grooved silicon structures consisting of alternating silicon walls and grooves (air channels) with a period of a = 4–6 μm are studied experimentally and theoretically in the mid-IR spectral range (2–25 μm) upon irradiation of samples by normally incident light polarized along and perpendicular to silicon layers. The calculation is performed by the scattering matrix method taking into account Rayleigh scattering losses in a grooved layer by adding imaginary parts to the refractive indices of silicon and air in grooved regions. The experimental and calculated reflection spectra are in good agreement in the entire spectral range studied. The analysis of experimental and calculated spectra gave close values of the effective refractive indices and birefringence of the studied structures in the long-wavelength spectral region. The values calculated in the effective medium model in the long-wavelength approximation (λ ≫ a) gave considerably understated values. The obtained results confirm the efficiency of the scattering matrix method for describing the optical properties of silicon microstructures.     

The electronic structure of liquid silver chalcogenides: a tight-binding approach

We present a computational study of the electronic structure of the stoichiometric liquid zero-gap semiconductors [Formula: see text], [Formula: see text] and [Formula: see text]. The geometry of the fluids is described by the primitive model of charged hard spheres; the electronic structure is modelled using a tight-binding Hamiltonian. The density of states is computed considering the Madelung potential fluctuations and the topological disorder characteristic of an ionic fluid. Only the introduction of nonzero tight-binding hopping matrix elements - equivalent to the formation of chemical bonds - induces a pseudogap between the chalcogenide conduction band and the silver valence band. The Fermi level can be located in a region of a small density of states; eigenstates at [Formula: see text] are likely to exhibit disorder-induced localization.     

Opto-electronic properties of gallium chalcogenides

A study of opto-electronic properties of gallium chalcogenides (GaS, GaSe and GaTe) is presented here. An investigation into the various effective charge parameters ($\text{Z}{}_{\mathrm{<mtext>eff</mtext>}}\text{, e}{}^1{}_{\mathrm{s}}\text{, e}{}^1{}_{\mathrm{T}}\text{and}\text{e}{}^1{}_{\mathrm{i}}$ and their correlation with ionicity have been done. The energy dependence of optical effective charge parameter (n_eff) have been discussed with respect to the band structure of respective chalcogenides. The low frequency optical dielectric constants (?₀₀) have been derived using reflectivity data and the obtained values of ?₀₀ have been found in agreement with the values obtained using Moss relation as also with the reported experimental data. The difference in the arrangement of fundamental structures of GaS and GaSe on one hand and GaTe on the other hand does not seem to effectively change the opto-eleetronic properties and the fundamental structure (which is common in all the three chalcogenides) is mainly responsible for determining these properties. The effect of surface conditions on the values of n_eff and ?∞ have been evaluated for GaTe. Using derived values of n_eff and ?_∞ the value of Penn gap have been determined. The Penn gap thus obtained is shown to be in agreement with the reflectivity data. In the passing we also show that essentially the Varshni formula explains fairly well the temperature dependence of the energy gap for gallium chalcogenides and the constants involved are evaluated.     

The thermoelectric properties of quaternary chalcogenides

Abstract

Single phase compounds of the form 1–2₂-3–6, were fabricated using powder metallurgical techniques. Measurements were made of the thermoelectric power, Hall coefficient, electrical conductivity, microhardness and optical reflectance. The electronic properties are discussed in terms of alloy and ionized impurity scattering.     

Magnetic and electrical properties of crystalline materials based on indium and copper chalcogenides in a wide range of temperatures and pressures

The effect of temperatures (2–300 K) and high pressures (to 50 GPa) on the electrical and magnetic properties of crystalline materials based on copper and indium chalcogenides with the general formula (InB)_1?x (CuAB ₂)_x, where A = As, Sb; and B = S, Se, and also crystalline CuInSe₂ and CuInS₂, has been studied.     

Photoelectric and electric properties of four-component copper chalcogenides

The results of investigation of the electrophysical and photoelectric properties of complex copper chalcogenides are presented, namely, the properties of CuSnAsSe₃, which exhibits ferroelectric properties, and CuInAsS₃, which exhibits ionic conductivity. The spectral and temperature regions of photosensitivity of these crystals are determined. The depth of the level of carrier trapping centers, which manifest themselves under thermal activation, are evaluated from the analysis of thermally stimulated conductivity (TSC) curves in CuInAsS₃.     

The effective properties of macroscopically nonuniform ferromagnetic composites: Theory and numerical experiment

Various theoretical models (self-consistent field, local linearization, and percolation theory methods and an analytic solution of the linear problem for an ordered medium) for calculating the magnetostatic properties of two-phase composites containing one ferromagnetic phase were considered. The concentration and field dependences of the effective magnetic permeability were found. A method for determining the coercive force and remanent magnetization as functions of the ferromagnetic phase concentration was suggested. Numerical experiments were performed for composites with a periodic distribution of circular inclusions. The results were compared with the analytically calculated effective magnetic permeability.