The electrical properties of HgTe and HgSe under very high pressure

The electrical properties of HgTe and HgSe have been investigated at pressures up to 200 kbar in an octahedral apparatus. Measurements of the electrical resistivity at room temperature showed that, beyond the well-known transition from the semimetallic to semiconductive state, both become metallic, at 84 kbar and 155 kbar, respectively. The energy gap at various fixed pressures was obtained from the resistance-temperature relationships. The energy gap of semiconducting HgTe decreases monotonically with pressure, the coefficient being ?l.53 × 10^?5$\text{eV}\text{bar}$. The energy gap of HgSe is rather insensitive to pressures up to 75 kbar, above which it decreases continuously ($\text{dE}\text{dP}$ = ?1.59 × 10^?5$\text{eV}\text{bar}$) before vanishing around 150 kbar. At high pressures the temperature coefficient of the resistance in the metallic state is 3.25 ~ 4.70 × $\text{10}{}^{\mathrm{?3}}\text{deg}$ for HgTe, and 5.7 ~ 5.9 × $\text{10}{}^{\mathrm{?3}}\text{deg}$ for HgSe.     

Phase transitions investigation in ZnTe by thermoelectric power measurements at high pressure

The pressure-induced phase transitions were studied in ZnTe by the thermoelectric power (S) technique. For the high-pressure trigonal phase P3₁21 cinnabar the large thermopower values S≈+400 correspond to semiconductor hole conductivity. During a transition into the orthorhombic structure Cmcm the value of S dropped by 40-50 times indicating metallic hole conductivity, like in the high pressure phases of other chalcogenides of II Group (HgSe, HgTe, CdTe) with Cmcm structure. In a transient region between the trigonal and orthorhombic phase (especially under decreasing pressure) a novel phase has been observed with a negative value of S. By analogy with other Zn and Cd chalcogenides whose NaCl phases have an electron type of conductivity the phase observed may have a NaCl structure.     

Thermoelectric properties of the trigonal and orthorhombic modifications of zinc telluride

Thermoelectric measurements are performed to study the phase transformations occurring in ZnTe under high pressure. It is shown that the thermoelectric power S of the cinnabar trigonal phase corresponds to a semiconductor with a hole-type conduction. In the Cmcm orthorhombic phase, the value of S≈+10 μV/K and the sign of the thermoelectric power testify to the metallic hole-type conduction, as in the high-pressure phases of other Group II chalcogenides (HgSe, HgTe, CdTe) with similar crystal lattices. In the transition region between the trigonal and orthorhombic phases, the pressure dependence of the thermoelectric power is found to exhibit an anomaly (a sharp dip), which leads to a change in the sign of S under decreasing pressure. This feature may presumably be related to the formation of the intermediate phase with the NaCl structure, which has an electron-type conduction in other zinc and cadmium chalcogenides.     

Metal-insulator transition in a HgTe quantum well under hydrostatic pressure

The 2D semimetal in a 20 nm (100) HgTe quantum well is characterized by a comparatively low overlap between the conduction and the valence bands induced by lattice mismatch. In the present paper we report the results of transport measurements in this quantum well under hydrostatic pressure of 14.4 kbar. By applying pressure we have further reduced the band overlap, thereby creating favorable conditions for the formation of the excitonic insulator state. As a result, we observed that the metallic-like temperature dependence of the conductivity at lowering temperature sharply changes to the activated behavior, signaling the onset of an excitonic insulator regime.     

On the behaviour of TSeF-TCNQ under pressure

We report b-axis electrical conductivity data for TSeF-TCNQ single crystals from 12 to 300 K under hydrostatic pressures up to 9 kbar. The single anomaly visible in the conductivity at 29 K and the low temperature conductivity gap rise under pressure at the same rate of ～ 6% kbar^-1. It has been found that the pressure dependence of the metal-insulator phase transition is qualitatively consistent with a mean field formulation of the Peierls transition. A Gruneisen constant of 0.64 for TSeF-TCNQ has been derived from this pressure study together with recent optical and compressibility investigations. The pressure dependence of the conductivity anisotropies at room temperature in TSeF-TCNQ and TTF-TCNQ are reported. The magnitude of the anisotropies in the two compounds are found to be essentially the same. The striking result, however, is that the anisotropies in both compounds are found to be independent of pressure up to 9 kbar.     

High pressure transport properties of the topological insulator Bi2Se3

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.     

Highconducting oxide ceramics bimevox: Synthesis, structure, and properties

Materials on the basis of BIMEVOX compounds which are stable within wide concentration and temperature regions have been obtained. Refinement of crystal structure of different BIMEVOX modifications obtained at different temperatures and partial oxygen pressures has been carried out. The most promising solid solutions have been identified according to total conductivity value as well as to linearity of conductivity dependence on temperature and independence of that on partial oxygen pressure.     

On the dielectric function in zero-gap semiconductors

The q and ω dependent dielectric function is examined in the random phase approximation for zinc-blende type, zero-gap semiconductors such as HgTe or HgSe. A detailed analysis of the properties of this function based on the analytical formulae obtained for the case of a highly degenerate free-electron gas is performed. In the limiting cases of the static and dynamic dielectric functions a comparison to previous theories is given.     

Temperature study of interband Γ6→Γ8 magnetoabsorption in HgSe

The magnetoabsorption experiments have been performed on HgSe samples at several temperatures between 10 and 80 K in magnetic fields up to 70kG. Because of an anomalous structure of the Γ₆ valence band, which is due to the small value of the spin orbit splitting Δ, several anomalous features have been observed. The interpretation of the results is based on the many level model of Pidgeon and Brown. A set of band parameters and the energy gap dependence on temperature has been found.     

Effect of pressure and temperature on the thermal conductivity of siltstone and dolomite

The effective thermal conductivity of rocks (siltstone and dolomite) at high pressures of up to 250 MPa and temperatures of 275–523 K is investigated. It is established that the degree of crystallization of rock-forming substances affects the temperature dependence of thermal conductivity. The thermal conductivity of amorphous and crystalline components in the structure of rock is calculated.     

Electrons and holes in HgTe and Hg0.82Cd0.18Te with controlled deviations from stoichiometry

The deviations from the stoichiometric composition of HgTe and Hg_0.82Cd_0.18Te crystals have been controlled by heat treatment under Hg vapor pressure. The magnetic field dependence of the Hall coefficient always shows the presence of two different sets of electrons and one set of holes. A low mobility electron is shown to belong to the conduction band. Vapor pressure dependence of hole concentration in HgTe shows that the concentration of nonstoichiometric defects decreases with increasing Hg vapor pressure, but the hole concentration is always higher than the electron concentration. In the case of Hg_0.82Cd_0.18Te, the electron concentration exceeds the hole concentration at high Hg vapor pressures. The dependence of the conduction band electron mobility in HgTe upon carrier density shows that the scattering by holes and impurities is predominant. In Hg_0.82Cd_0.18Te, however, optical phonon scattering is dominant when the deviation from stoichiometry is small and the effect of residual impurities can be neglected, and scattering by holes is dominant when the hole concentration is over $\text{10}{}^{17}\text{cm}{}^3$.     

Density of states in the impurity d band and inelastic electron scattering by a system of mixed-valence iron ions in HgSe: Fe crystals

The resistivity and the Hall coefficient of HgSe: Fe crystals with various iron content are experimentally studied in the temperature range 1.3≤T≤300 K and in magnetic fields up to 60 kOe. The temperature dependences of the density and mobility of conduction electrons in these crystals are determined. The influence of spatial charge ordering in the system of mixed-valence iron ions on impurity states in HgSe: Fe crystals is considered. The density of states in the impurity d band is theoretically analyzed, and inelastic electron scattering in which bi-and trivalent iron ions are recharged is discussed. It is shown that the experimentally detected features in the dependence of the density and mobility of electrons on temperature and iron impurity content can be explained by the influence of Coulomb correlations in the mixed-valence iron ion system on the structure of the impurity d band.

     

CO_2水合物导热特性的分子动力学模拟

使用分子动力学模拟方法在NVT系综下对结构完整CO_2水合物以及结构缺陷CO_2水合物进行了导热模拟计算.对于结构完整的CO_2水合物,在200-230 K温度区间内,体系导热系数由0.4684 W·m~(-1)·K~(-1)变化到0.4836 W·m~(-1)·K~(-1),温度相关性较弱;而在230-280 K温度区间内,体系导热系数由0.4836 W·m~(-1)·K~(-1)变化到0.7494 W·m~(-1)·K~(-1),温度相关性变强;另外,通过计算功率图谱发现主体分子对水合物体系的导热贡献更大.对于结构缺陷CO_2水合物,发现晶穴占有率和笼形结构缺陷对体系导热均有一定影响,空笼晶胞导热系数约为完整晶胞导热系数的86.67%,体系的导热能力主要取决于主体结构的性质.     

Pressure-induced electronic and structural transformations in bulk GeSe2 glass

The pressure dependence of the electrical resistivity of bulk GeSe₂ glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure havinga = 4·06 A. The electrical conductivity has also been studied as a function of temperature at various pressures.     

Study under high pressure on the Hall effect in the magnetically ordering Kondo lattice material CeAl2

Measurements of the Hall effect in CeAl₂ have been performed between 1.5 K and 4.2 K under pressures up to 2 GPa. The temperature dependence of the Hall coefficient has been found to show increasingly distinct structure with rising pressure, which is obviously correlated to the magnetic ordering transition in this material. Under pressures exceeding about 1.6 GPa the Hall coefficient changes its sign in the temperature range under consideration.     

Electrical conductivity of xenon at megabar pressures

The electrical transport properties of solid xenon were directly measured at pressures up to 155 GPa and temperatures from 300 K to 27 mK. The temperature dependence of resistance changed from semiconducting to metallic at pressures between 121 and 138 GPa, revealing direct proof of metallization of a rare-gas solid by electrical transport measurements. Anomalies in the conductivity are observed at low temperatures in the vicinity of the transition such that purely metallic behavior is observed only at 155 GPa over the entire temperature range.     

Electrical conductivity and equations of states of β-rhombohedral boron in the megabar dynamic pressure range

The pressure dependence of the conductivity of boron under conditions of a stepwise shock compression of megabaric range is studied. With this purpose, the following problems have been solved. The conductivity of boron has been measured in the range of dynamic pressures, where boron has different high-pressure phases. The equations of state of β-rhombohedral and amorphous boron have been constructed in a megabaric pressure range. The thermodynamic states of boron in the conditions of these experiments are calculated, which, in combination with the measurement data, made it possible to determine the change in the boron conductivity in the conditions of strong stepwise shock compression at dynamic pressures to 110 GPa. The increase in the conductivity of polycrystalline boron at megabar pressures is interpreted as a result of a nonmetal–metal transition.

     

Core-shell structure and quantum effect of CdSe/HgSe/CdSe quantum dot quantum well

Core-shell structure CdSe/HgSe/CdSe quantum dot quantum well (QDQW) nanocrystals were firstly synthesized and well controlled by colloidal chemical methods. High-resolution transmission electron microscope (HRTEM) photographs show that the CdSe core has a wurtzite crystal structure and the shell of HgSe has a hexagonal structure. The enhancement and blue shift of photoluminescence (PL) peak was observed which was attributed to the quantum confinement effect of carriers in the HgSe well.     

Temperature dependence of energy gaps in some II–VI compounds

The Brooks-Yu theory is applied to the calculation of the temperature dependence of the energy band gaps in some II–VI compounds. The theory is applied in the framework of the empirical pseudo-potential method, and utilizes a recent lattice dynamical calculation of the Debye-Waller factors. The temperature dependence of several critical-point band gaps are determined and compared with a vailable experimental data. In general, reasonable agreement is obtained. The interesting case of HgTe, which exhibits an anomalous positive temperature coefficient of the fundamental band gap, is also discussed.