Investigations of the band line-up in semimetallic InAs---GaSb superlattices under hydrostatic pressure

Using magneto-optical methods, we have measured the energy separation between the subbands in a semimettalic InAs---GaSb superlattice associated with the InAs conduction band (CB) and the GaSb valence band (VB) as a function of hydrostatic pressure up to 11kBar. The CB-VB energy overlap (Δ) at the InAs---GaSb interface is found to depend less on pressure than predicted on grounds of usual assumptions concerning band energy shifts induced by pressure.Also inter-subband coupling has been observed between the bands in different adjacent materials. This coupling is found to lead to anti-crossing in the subband dispersion relation.     

Electronic band structure of calcium selenide under pressure

Energy band structures under pressure of calcium selenide (CaSe) were calculated using the plane-wave pseudopotential code CASTEP. The results show a progressive transition from a direct to an indirect gap semiconductor at a pressure of about 2 GPa, in the B1 phase. An insulator-conductor change was also observed at 70 GPa, in the B2 phase. Concerning CaSe, these two results could not be evidenced in previous literature. Hence, our work is a first attempt in this direction.     

Theoretical band structure calculation and superconductivity of NbC under pressure

Abstract

We report a theoretical calculation of the band structure and superconductivity of niobium carbide in the NaCl structure under pressure. The effect of pressure on the band structure is obtained by means of the self-consistent linear muffin-tin orbital method. The parameters necessary to calculate the superconducting transition temperature (T_c) are taken from our band structure results. The dependence of total energy on volume is calculated and is in good agreement with other earlier works. The calculated value of the cell parameter is in agreement with the experimental value (8.45 a.u). McMillan formula is used to calculate the value of T_c The calculated values of T_c are compared with the available experimental data.     

Elemental sulfur under high hydrostatic pressure. An up-to-date Raman study

We report a high pressure Raman study of orthorhombic elemental sulfur from ambient pressure to ～ 25 GPa. Using a near infrared laser and low laser intensity on the scattering volume, we achieve off-resonant conditions up to larger pressures in comparison with previous studies. Raman spectra were recorded over the full spectral range including external (librational, translational) and internal (bond bending and bond stretching) modes. Drastic changes are observed as regards the peak frequencies, relative intensities and band splitting of degenerate modes. The main outcome of the present study is the observation of a “structural” transition at ～ 16 GPa manifested as slope changes of certain frequencies and sudden relative intensities changes. The present findings are discussed in the context of previous pressure Raman studies and comparison with existing X-ray diffraction as well as ab initio molecular dynamics results is attempted.     

Photoluminescence on CdSe and CdTe under hydrostatic pressure

Photoluminescence measurements on CdSe and CdTe samples have been made under hydrostatic pressure up to the phase transition. Transition pressure values are reported. Nonlinear pressure dependence of the direct gap is observed for both materials. Pressure coefficients are in good agreement with values calculated from the dielectric theory of the chemical Bond.     

Electrophysical properties of composite films based on multiwalled carbon nanotubes under hydrostatic pressure

The effect hydrostatic pressure of up to 9 GPa has on the electrical behavior of polymer composites based on multiwalled carbon nanotubes (MWCNT) is investigated at room temperature. The ratio R _9GPa/R ₀ ≈ 7 for samples with resistance in the kilo-ohm range is higher than for samples in the ohm range. Pressure-induced MWCNT features at P ≈ 1.5 GPa are associated with the deformation of the circular form of nanotube walls, in agreement with theoretical predictions. Measurements of direct and reverse resistance and current voltage characteristics under pressure demonstrate the reversibility of electrophysical properties, which could find practical application in creating pressure sensors.     

Effect of overlap of localized orbitals on the band structure of insulators under pressure

A new method for calculating the energy spectra of compressed insulators based on the cluster expansion is proposed. The dependence on the compression of the spectra of both the conduction band and the valence bands is uniquely determined by the values of the overlap integrals of the orbitals of pairs of isolated atoms. The overlap integrals and the matrices defined by them are calculated numerically for inert-gas crystals and their properties and values are analyzed for different lattice constants. Numerical calculations of the bands of the compressed crystal in the various proposed models are carried out and discussed for neon. Fiz. Tverd. Tela (St. Petersburg) 40, 1464–1472 (August 1998)     

Photonic band structure in the nearly plane wave approximation

For photons propagating in a periodic dielectric lattice, the dispersion curve forms photonic bands separated by forbidden gaps. When the dielectric lattice deviates only slightly from being homogenous, the photonic band structure resembles the linear dispersion relation for photons folded into the first Brillouin zone, i.e., the so-called empty lattice bands. Using group theoretical technique, we calculate the splitting of the accidental degeneracies in the empty lattice bands at symmetry points for a simple cubic dielectric lattice. Received 23 June 1998     

LaAg under hydrostatic pressure: Superconductivity and phase transformation

The electrical resistivity of LaAg has been measured from 1–300 K at hydrostatic pressures to 12.4 kbar and the superconducting transition temperature T_c determined inductively to 23 kbar. For P ? 6.2 kbar a lattice transformation is observed at a temperature T_M which increases rapidly with pressure. T_c shows an oscillatory pressure dependence, increasing initially from T_c (0) = 1.062 K. There is no obvious correlation between the pressure dependence of T_M and T_c.     

Effect of conduction band nonparabolicity on the optical properties in a single quantum well under hydrostatic pressure and electric field

The effect of conduction band nonparabolicity on the linear and nonlinear optical properties such as absorption coefficients, and changes in the refractive index are calculated in the Al_0.3Ga_0.7As/GaAs heterostructure-based symmetric rectangular quantum well under applied hydrostatic pressure and electric field. The electron envelope functions and energies are calculated in the effective mass equation including the conduction band nonparabolicity. The linear and nonlinear optical properties have been calculated in the density matrix formalism with two-level approximation. The conduction band nonparabolicity shifts the positions of the optical properties and decreases their strength compared to those without this correction. Both the optical properties are enhanced with the applied hydrostatic pressure. While the absorption coefficients are bleached under the combined effect of high pressure and electric field, the bleaching effect is reduced when nonparabolicity is included.     

Effect of interdiffusion on the band structure and absorption coefficient of a one-dimensional semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential, band structure and absorption coefficient of electromagnetic radiation of a one-dimensional superlattice, composed of GaAs/Ga_1?x Al _x As quantum wells with the initially rectangular potential profile, is studied within the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the widening of the energy minibands and to the blueshift of the absorption spectrum observed in experiments.     

Semi-empirical lcao band structure calculation on the transition of diamond into the metallic state under pressure

The band structure for tetragonally distorted diamond is calculated by the Extended Hückel Theory. The indirect band gap of diamond shrinks with increasing strain and vanishes at e_zz = -0.17. This result is in very good agreement with high pressure experiments on diamond of Vereshchagin et al. as well as with theoretical results for the pressure-temperature phase diagram for diamond by Van Vechten.     

蓝宝石在高压下光学性质及能带结构的从头算模拟

采用量子力学从头算和赝势平面波基组以及GGA-PBE交换相关函数对蓝宝石(α- Al2O3)窗口材料在0－1000GPa静水压力作用下光学性质及能带结构进行了研究。结果表明, 在静水压力作用下可以观察到蓝宝石一直保持良好的透明性,前沿带隙随着压力的增大先变大后变小,而且部分键长有明显变化,即蓝宝石单晶结构在加压过程中发生了原子位置移动而进行了位置重构。     

蓝宝石在高压下光学性质及能带结构的从头算模拟

采用量子力学从头算和赝势平面波基组以及GGA-PBE交换相关函数对蓝宝石(α- Al2O3)窗口材料在0－1000GPa静水压力作用下光学性质及能带结构进行了研究。结果表明, 在静水压力作用下可以观察到蓝宝石一直保持良好的透明性,前沿带隙随着压力的增大先变大后变小,而且部分键长有明显变化,即蓝宝石单晶结构在加压过程中发生了原子位置移动而进行了位置重构。     

Resistivity of positive-J spin glasses: Study ofLaGd under hydrostatic pressure

Results of a new calculation of the resistivity in the noise model of spin glasses, based on the –J S interaction forpositive J, are compared with measurements of the resistivity in an fcc La-8at% Gd alloy under hydrostatic pressures up to 12 kbar, the first positive-J resistivity investigations under pressure. A positive value ofJ that decreases with increasing pressure, in accord with other experiments, leads to quantitative agreement between theory and experiment if one uses a large electronic density of states at the Fermi energy, in accord with specific heat measurements and electronic band structure calculations in fcc La. Impurity potential scattering is found to contribute significantly to the resistivity. Its size and pressure variation is determined.This work is supported in part by the Deutsche Forschungs-gemeinschaft.     

Two comments on the structure function approximation for deep-inelastic scattering

The mechanism of radiative return to resonance can be effectively used to describe radiative corrections in terms of the Drell-Yan process. The iteration procedure is proposed. It is shown that the y→1 kinematic region can be described in terms of modified structure functions and the Sudakov formfactor, which significantly changes the result obtained both in the lowest order and with allowance for all leading orders of perturbation theory.     

Luminescence of BaBr2:Eu under hydrostatic pressure

Luminescence spectra of BaBr₂:Eu²⁺ have been measured under pressures up to 27 GPa at room temperature. In the low-pressure range a red-shift of the 5d-4f transition of −225 cm⁻¹/GPa is observed. From 3 to 10 GPa a phase mixture of the original orthorhombic phase and the high-pressure monoclinic phase gives rise to two luminescence bands. Above 10 GPa the crystal is completely transformed to its high-pressure phase where two different Eu²⁺ sites exist of which, however, only one 5d-4f transition is detected. This transition exhibits a red-shift of −200 cm⁻¹/GPa. The shifts are compared with other literature data.