Use of smoke samples in diamond-anvil cells to measure pressure dependence of optical spectra: Application to the ZnO exciton

We present measurements of ZnO exciton peak energies, E₀, at pressures up to 107.3 kbar. Smoke samples consisting of randomly oriented single crystal particles were prepared by oxidizing metallic zinc in air and were collected on one diamond face of a Merrill-Bassett pressure cell. Pressures were measured by the ruby fluorescence technique. In the pressure range between 5 and 90 kbar, our results indicate a consistent linear dependence with dE₀/dP = 2.33 × 10^?3 eV kbar^?1 for both increasing and decreasing pressures. A mixed phase structure is suggested by the observed irregular peak shapes and measured pressure dependence for the sample that had been taken beyond ? 90 kbar where the transformation to the NaCl structure has been reported.     

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.     

Raman study of rutile (TiO2) at high pressures

The Raman spectra and polymorphism of rutile have been investigated under hydrostatic pressures up to 90 kbar at room temperature. A transition previously observed in rutile at 30 kbar in a Drickamer-type cell under nonhydrostatic conditions was observed to begin at approximately 70 kbar in a 4:1 mixture of methanol and ethanol. The small amount (10–20%) of the high-pressure phase synthesized from rutile, however, did not increase even though the sample was left under pressure for a period of 1 month at ambient temperature. On the basis of factor group analysis, in situ powder x-ray diffraction data, and comparison of the Raman spectrum of the high-pressure modification with that of TiO₂-II (α-PbO₂-type structure synthesized from anatase powder at 40 kbar and 400°C), it is evident that a high pressures rutile transforms irreversibly to TiO₂-II.     

Hydrostatic pressure effect on the transformations of amorphous La3Si

Measurements of the relative resistance versus temperature at different hydrostatic pressures up to 30 kbar for amorphous La₃Si have been carried out. The results show that there is a metastable phase only at low pressures and that pressure decreases the crystallization temperature of the stable state (or final phase) from both the amorphous and meta-stable phases. If the pressure is sufficiently high, the transformation from the amorphous to a stable state might occur at room temperature.     

高x值混晶Hg1-xCdxTe光吸收边的压力效应

采用金刚石对顶砧高压装置,在0—36kbar流体静压力范围和室温条件下测量了高x值的P型碲镉汞混晶Hg_0.3Cd_0.7Te光吸收边及其随压力的变化。300K下Hg_0.3Cd_0.7Te光学能隙的实验值与用经验公式的计算值一致。用最小二乘法拟合不同压力下的实验结果,得到Hg_0.3Cd_0.7Te能隙的一阶压力系数α=8.7×10^-11eV/Pa,与用化学键介电 关键词：     

Neutron diffraction investigation of MnAs under high pressure

Powdered MnAs has been investigated by neutron diffraction in a pressure cryostat, at hydrostatic pressures up to 13 kbar and temperatures down to 4.2 K. It has been found that in the orthorhombic MnP type structure, which under pressure is retained at low temperature, a spiral magnetic structure with propagation vector τ_a = 0.125X2πX a¹ at 12.6 kbar is formed.     

Short-range resonant electron scattering in Se- and S-doped GaxIn1-x Sb under hydrostatic pressure

Abstract

Electron scattering mechanisms have been investigated in Se- and S-doped Ga_xIn_1-xSb alloys by measuring the Hall mobilities as a function of hydrostatic pressures up to 25 kbar at temperatures down to 4.2 K. The analysis of the data shows that the Brooks-Herring theory of ionized impurities scattering could fit the experimental variations only overa narrow range of low pressures before carrier freeze-out into a resonant impurity level sets in. At higher pressures, good agreement between theory and experiment could be obtained by introducing an additional scattering mechanism due to the short-range central cell impurity potential.     

Pressure-induced structural transitions in PbI2: A high-pressure Raman and optical absorption study

The effect of pressure on the 2H and 4H polytype of PbI₂ has been investigated by Raman and optical absorption spectroscopy, using the diamond anvil cell. The 2H-polytype undergoes pressure-induced phase transitions at 5 kbar and near 30 kbar. The 4H-polytype exhibits phase transitions near 8 kbar and above 30 kbar. The Raman modes abruptly change at these pressures. The optical absorption edge shifts red at the rate of 15±1 MeV/kbar in the 2H-PbI₂ and at the rate of 7 MeV/kbar in phase II. The latter phase is most likely to possess a 3d-structure and not a layer type. The possible structures for the high pressure phases are discussed.     

Hydrogen bonding in liquid methanol at ambient conditions and at high pressure

Neutron and hard X-ray diffraction measurements on liquid methanol at room temperature are presented at pressures ranging from 1 bar to 9 kbar. Under ambient conditions a complete partial structure factor analysis has been performed. The partial structures are not all in agreement with simulation results. Notably, the data show no sign of a sharp feature in the H_O(O/C) partial, implying that the hydrogen between the hydrogen bonded oxygen is less localized than predicted by the standard simulation potentials. The data have been analysed using a continuous random network approach common in the analysis of the structure of tetrahedral network glasses. The pressure-induced change in the scattering intensity is restricted to the momentum transfer range (0 < Q < 2.5 Å^?1), i.e., there is no discernible change in sharp real-space structures. Up to 9 kbar the change in scattering intensity scales to a good approximation with the density.     

High pressure experiments on AlxGa1−xAs-GaAs quantum-well heterostructure lasers

Hydrostatic pressure experiments on Al_xGa_1?xAsGaAs quantum-well heterostructure (QWH) laser diodes are described. Data are presented giving ～11.5meV/kbar for the bandgap vs. pressure coefficient at lower pressures, with a change to 8.5–9meV/kbar at higher pressures. We suggest that this behavior is caused by biaxial and shear stresses in the active region induced by doping or composition mismatch relative to the confining layers, or between the n and p confining layers themselves. A model consistent with the experimental data is presented.     

The structure of FeCl2 at high pressures

A full three-dimensional structure determination of FeCl₂ has been performed at hydrostatic pressures up to 6.4 kbar at room temperature. These studies demonstrate the feasibility of four-circle neutron diffraction studies under pressure, and show that above 5.8 kbar FeCl₂ has a structure isomorphous to FeBr₂. The pressure dependence of chlorine position parameter and lattice parameters has been studied over the entire pressure range.     

Vibrational spectroscopy at high pressures. Part 50. A Raman scattering study of MSCN (M = K,Rb, Cs,NH4) at high pressures

The phase behaviour of the title compounds was studied at high pressures using Raman spectroscopy. The isostructural materials KSCN and RbSCN show two phase changes (12.5 and 45.5 kbar; 10.5 and 37.5 kbar, respectively) which appear to be second order in type. Structures and transition mechanisms are proposed for each phase. CsSCN has a different orthorhombic structure and shows only one phase change in the range studied, at 23.5 ± 1.5 kbar, which also seems to be second order. The known phase transition in NH₄SCN at 2 kbar has been confirmed, and a new one found at 10 kbar. A monoclinic cell is proposed for the new phase.     

Behavior of the p-T phase diagram of the organic conductor (ET)4Hg3I8

The conductivity of single crystals of the organic conductor (ET)₄Hg₃I₈ [ET-bis(ethylendithio)tetrathiafulvalene] has been investigated at temperatures from 4.2 to 360 K and pressures of up to 75 kbar. Two first-order phase transitions have been detected at room temperature at pressures of 2.75 and 6.7 kbar. On the basis of the experimental data, the p-Tphase diagram for the first-order phase transitions has been plotted. The unusual shape of the phase diagram (a slow monotonic growth of the transition temperature with a slope dT/dp=4 deg/kbar followed by a sharp drop around the point p ₀=6.5 kbar, T ₀=324 K) has been analyzed using the Landau theory of second-order phase transitions. Our analysis supports the hypothesis of a second-order phase transition around this point and also exhibits satisfactory agreement between calculations and the experimental curves of the first-order phase transitions. Zh. éksp. Teor. Fiz. 115, 2190–2196 (June 1999)     

Pressure induced insulator-metal transition in quasi one-dimensional niobium tetraiodide

The resistivity of a quasi one-dimensional semiconductor NbI₄ was measured at high pressures. The resistivity decreased monotonously with increasing pressure. The activation energy of 0.12 eV at atmospheric pressure decreased to 0.06 eV at 100 kbar and to 0.02 eV at 130 kbar. The temperature coefficient of resistivity becomes positive at about 150 kbar.     

Heavy fermion state and pressure-induced resistive behaviour in CeRu2Si2

Pressure-induced variation in the resistive behaviour of CeRu₂Si₂, a Kondo-lattice system, has been studied in the temperature interval 1.5K – 300K. At low temperatures, resistivity exhibits (i) a linear temperature dependence at low applied pressures (P ?2.26 kbar) and (ii) a quadratic temperature dependence

(T) ～ AT² at higher pressures (6.5 kbar ? P ? 16.8 kbar). The coefficient A decrease, which in turn implies that the spin fluctuation temperature T_sf increases, with increasing applied pressure. The temperature coefficient γ of specific heat C = γ T at low temperature (0.3K ? T ? 1.6K) is quite large (385 mJ/mole-K²) confirming the heavy fermi liquid nature of the material.     

Photodarkening in glassy As2S3 under pressure

Photodarkening phenomena have been studied in glassy As₂S₃ films under pressure up to 3 kbar. A remarkable shift of the absorption edge to lower energy by the band gap illumination is observed under pressure. The shift is quenched even after releasing pressure. X-ray diffraction studies have also been performed at various pressures. The first sharp diffraction peak disappears at 50 kbar. The origin of the observed enhancement of the photodarkening may be associated with the increase in the interlayer correlation by application of pressure.     

Dependence of temperature-dependent electrical resistivity of SrIrO3 on hydrostatic pressure to 9.1 kbar

Non-Fermi-liquid behavior and close proximity to a quantum critical point in the 5d transition metal iridate SrIrO₃ at ambient pressure motivate our search for possible anomalous behavior in its transport properties under pressure. The electrical resistivity in the ab-plane of a single crystal of SrIrO₃ has been measured over the temperature range 1.35–285 K at both ambient and 9.1 kbar hydrostatic pressure. The resistivity decreases slightly over the entire temperature range, but no superconducting transition or changes in the non-Fermi-liquid behavior are observed under pressure. It is estimated that significantly higher pressures are likely required before sizable changes in the properties of SrIrO₃ will occur.     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

Effect of hydrostatic pressure on the Raman spectrum of anatase (TiO2)

Raman spectra of anatase have been investigated under pressures up to 60 kbar and at room temperature. A pressure-induced phase transition is observed at pressures above 25.6 kbar. The 197 cm^-1 mode (at 1 atm.) of six Raman active modes exhibits anomalous pressure dependence in which the frequency decreases with increasing pressure. This mode may be significant in the phase transition. The remaining modes show usual behavior.     

Effect of pressure on the Kondo temperature of Cu(Fe)

The electrical resistivity of Cu with Fe impurities has been measured from 1.4 K–40 K under pressures to 82 kbar. When plotted versus the reduced temperature T/T_K, the data at all pressures overlap, confirming the existence of a universal resistivity curve.