Hydrostatic-pressure-induced degradation of MBE CdTe/CdMgTe heterostructures grown on GaAs substrate

Abstract

Modulation doped CdTe/Cd_1?xMg_xTe heterostructures grown on GaAs substrates were studied by means of magnetotransport measurements performed under hydrostatic pressure, as well as X-ray diffraction and cross-sectional transmission electron microscopy completed before and after pressure experiments. We have shown that hydrostatic pressure leads to the creation of dislocations in the CdTe/Cd_1?xMg_xTe structure in the vicinity of the interface between the II-VI structure and the substrate. The dislocation-enhanced internal stress leads to internal microfractures, resulting in a permanent damage of the heterostructure.     

Effect of strain-induced electronic topological transitions on the superconducting properties of La 2 - x Sr x CuO 4 thin films

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-T _c cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of T _c in most cuprates. In particular, a nonmonotonic dependence of T _c on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed. Received 1st October 2001 and Received in final form 5 December 2001     

Raman studies of the high pressure effects in high Tc superconductors

Abstract

The effect of a hydrostatic pressure (up to 6GPa) on the Raman spectra of YBa₂Cu₃O_x(x = 6.5 and overdoped) single crystals is investigated in the temperature range 77-300 K in connection with all available data in the literature for the other oxygen concentrations. All five strong modes (4A_g+1B_1g) are examined for both oxygen doping and compared with previously published results. From the systematic investigation of the phonon spectral characteristics with the application of pressure, the formation or disappearance of the phases is examined. The results provide a spectroscopic support for the changes induced by the hydrostatic pressure on the chain ordering and the corresponding phases.     

Pressure induced anisotropic magnetovolume phenomena in Lu2Fe17 Intermetallics

Magnetisation and magnetic susceptibility of a Lu₂Fe₁₇ single crystal have been studied under hydrostatic pressure up to 1.2 GPa at temperatures down to 5 K using a SQUID magnetometer. The ferromagnetic phase of Lu₂Fe₁₇ is suppressed rapidly above a critical pressure P _C = 0.4 GPa in the whole temperature range below the critical temperature T _C . A magnetic phase diagram of Lu₂Fe₁₇ has been constructed using results of the magnetic susceptibility measurements under pressure. A pressure induced incommensurate antiferromagnetic phase exhibits metamagnetic transitions with the increasing critical magnetic field H _C under pressure. Taking into account recent neutron diffraction data, the pressure induced anisotropic changes of the lattice parameters of the Lu₂Fe₁₇ are discussed.     

Raman scattering study of interlayer bonding in CdI2 AND SnS2 under hydrostatic pressure: analysis by use of Van Der Waals interaction

The pressure dependence of inter- and intra-layer modes in 4H-CdI₂, 4HSnS₂ and 18R-SnS₂ has been studied using Raman scattering techniques. X-ray diffraction measurements on 4H-CdI₂ give the variation of lattice parameters under hydrostatic pressure. The relationship between interlayer bond-length and bond-strength is obtained from a combination of the Raman scattering and X-ray diffraction results. The results are analyzed by use of a short range 6-exp type potential. Van der Waals constants for the anion-anion interaction and polarizabilities of the anion in CdI₂ and SnS₂ are determined from fitting the calculation to the experimental results.     

Neutron investigations of the magnetic properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S under pressure up to 4.2 GPa

Fe _x Mn_1?x S belongs to the group of strong electron correlations compounds MnO. We present here experimental results for the antiferromagnetic iron–manganese sulfide system, based on X-ray and neutron diffraction studies. The neutron diffraction investigations were carried out at ambient conditions and at hydrostatic pressures up to 4.2 GPa in the temperature range from 65 to 300 K. Our results indicate that the Néel temperature of α-MnS increases up to room temperature by applying chemical (x _Fe) or weak hydrostatic pressure P. In Fe_0.27Mn_0.73S, the Néel temperature increases from 205(2) K (P = 0 GPa) to 280(2) K (P = 4.2 GPa) and the magnetization at 100 K decreases by a factor of 2.5 when the hydrostatic pressure increases from 0 to 4.2 GPa.     

Compression behavior of NiO in a diamond cell

Abstract

The compressibility of synthetic polycrystalline NiO was studied in a diamond anvil cell at room temperature utilizing two different X-ray sources. A standard film with a conventional X-ray source and the energy dispersive X-ray diffraction (EDXRD) method with synchrotron radiation were used for data acquisition. In the film method, the sample was compressed in a 4:1 methanol to ethanol solution up to 7 GPa with ruby fluorescence as a pressure calibrant. In the energy dispersive method, NiO powder was mixed with gold and compressed in two different conditions: gasketed and ungasketed up to 30 GPa. In the gasketed run, water was used as the pressure transmitting medium while gold was used as pressure calibrant in both runs.

Hydrostatic compression of NiO in both diffraction methods yields a bulk modulus (K _o) of 187 ± 7 GPa assuming K′ = 4. The compression of gasketed NiO of the synchrotron experiment, however, showed an obvious break at pressure exceeding 4 GPa due to the loss of hydrostaticity. NiO in a nonhydrostatic condition behaves with less compressibility than the hydrostatic results with a nominal K _o of 238 ± 10 GPa. The lower compressibility of NiO in synchrotron runs is attributed to the uniaxial loading effect which was more easily detected by the EDXRD geometry. The discrepancy in the bulk modulus can be attributed to the contrast in the shear strength between the sample and pressure medium and the Poisson effect of the sample under uniaxial loading.     

High pressure raman study of Y-124

In this work, a micro-Raman study under high hydrostatic pressure (up to ～5.5 GPa) has been carried on YBa₂Cu₄O₈ and Y(Ba, Sr)₂Cu₄O₈ single crystals at room temperature. In both samples, seven strong modes, of A_g symmetry, and one weak, of B_3g symmetry, have been observed and examined in connection with previously published results concerning YBa₂Cu₄O₈. With the Sr substitution for Ba, the ambient pressure measurements show an upward shift in energy for all modes, except those that involve vibrations of the plane and apex oxygen atoms. With increasing hydrostatic pressure all phonons shift to higher energies. Anomalous nonlinear pressure behaviour has been observed for three phonons, which is correlated with the pressure dependence of T _c of these compounds.     

Design of a device for the simultaneous application of uniaxial stress and hydrostatic pressure

Abstract

The design of a device for the simultaneous application of uniaxial stress and hydrostatic pressure is presented. We discuss its usefulness for the studies of semiconductor devices and high T_c superconductors.     

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.     

Weak 2D hole localization on the (0001)Te surface under hydrostatic pressure

Effect of hydrostatic pressure on the anomalous magnetoresistance (AMR) of the two-dimensional (2D) hole layer on (0001) surface of Te has been studied. Application of hydrostatic pressure was found to reverse the sign of AMR of the 2D hole system from positive to negative. The results are interpreted in terms of weak localization theory for noninteracting particles, modified by taking into account the specific features of tellurium valence-band energy spectrum, namely, lifted spin degeneracy, many-valley nature, and trigonal spectrum distortion on the (0001) plane. The pressure dependences of the characteristic relaxation times representing the parameters of the theory have been found. It is shown that hydrostatic pressure symmetrizes the 2D hole spectrum at the (0001)Te surface, which manifests itself, in particular, in a reduced trigonal distortion of the spectrum. The pressure dependence of the spin-orbit splitting between the midpoint of the gap separating the two uppermost valence subbands and the lowest, spin-degenerate subband, Δ₁(P), has been determined. Fiz. Tverd. Tela (St. Petersburg) 39, 568–572 (March 1997)     

Investigations of texture formation in geomaterials by neutron diffraction with high pressure chambers

Abstract

This paper suggests a classification of texture in quartzitic rocks. Possible mechanisms of texture formation and their relation to tectonic processes are discussed.

A scheme for the experimental varification of some models describing the evolution of deformation structures in geological materials in dependence on various erxternal conditions (high pressures, temperatures, various loadings, etc.) acting on a sample is proposed.

To investigate “in situ” the mechanisms of texture formation in rocks, high pressure devices are under construction. They will be built of a special T_i ? Z_r alloy, which has zero coherent scattering lenght and therefore is well suited for neutron diffraction investigations. Two different devices are proposed. The first one allows neutron diffraction measurements of sample volumes up to 4 cm³, a hydrostatic pressure of 1, 5 GPa, a temperature of 300° C, and uniaxial compression up to 50 kN. Pressure temperature and axial load are measured inside the chamber. Besides during the experiment, diagrams of load, elastic wave velocities and acoustic emission will be recorded. The second chamber is designed to investigate the mechanisms of texture formation in polycrystal samples (rocks or their imitations) at temperatures up to 800° C and axial compression with a force of up to 150 kN.     

Neutron Diffraction Study of the p - T Phase Diagram for Erbium

Electrical resistivity measurements performed on a single crystal of erbium as a function of temperature and hydrostatic pressure have provided a preliminary p - T phase diagram. The results have been interpreted in terms of a model for the magnetic structures of Er deduced from neutron diffraction studies at ambient pressure. This model predicted the existence of a magnetic structure with a wave vector of Q =2/7 c * at 4.2 K, when the applied pressure is larger than 3 kbar. This paper reports a neutron diffraction study of erbium made in the temperature range of 4 to 100 K, at pressures between 0.5 and 6 kbar. We have observed the predicted suppression of the low temperature conical ferromagnetic phase and the emergence of a new phase with Q =8/33 c *. The neutron diffraction measurements has enabled us to identify the various phases that develop from the cycloidal phases previously observed at atmospheric pressure.     

A high pressure Raman study of terbium molybdate

Abstract

Tb₂(MoO₄)₃ has been studied by Raman spectroscopy under hydrostatic pressure up to 9 GPa at room temperature. The measurements reveal two phase transitions, one at around 2 GPa and another one above 5 GPa. The first phase transition is associated with an increase in the coordination number of Mo while the second is probably a transition to an amorphous phase in which only a wide band originating from Mo-O vibrations remains. This behaviour is irreversible as the Raman spectrum of the initial structure is not recovered at atmospheric pressure.     

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.     

High-pressure Raman study of SnGeS3

Abstract

We report Raman-scattering studies of SnGeS₃ under hydrostatic pressures up to 19.5 GPa. An assignment to internal-external modes is proposed, based on the pressure slopes obtained. Our data show evidence for two critical pressures, one around 7 GPa and a second one around 12 GPa. The material renders itself Raman inactive at 19.5 GPa. The observed changes are reversible upon pressure release.     

Observation of metastable effects in pseudomorphic heterostructures InCaAs/InAlAs

Abstract

The results of electrical transport measurements (Hall effect and resistivity) performed on In_.65Ga_.35As-In Al_.48 As modulation doped heterostructure (with Si-doped InAlAs donor layer) and on In _.52Al _.48As thick layer are presented as a function of temperature (77–300K range) and hydrostatic pressure (up to 1300 MPa). At low temperature, the persistent photoconductivity (PPC) and metastable donor states occupation effects were observed. Due to the pressure induced decrease of the two-dimensional electron gas concentration the pronounced increase in the amplitude of observed effects was obtained. The above results are discussed in terms of DX-like centers and/or interface/surface states in the QW structure.     

Elastic and plastic deformation of NaCl and Ni3Al polycrystals during compression in a multi-anvil apparatus

Abstract

The compression behaviour in a multi-anvil apparatus of pure NaCl and of a foil of Ni₃Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction. At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. A saturation of microstrains observed in NaCl at 2 GPa is thus attributed to brittle fracture setting in at uniaxial stresses of around 0.3 GPa. Ni₃Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa. The compression behaviour of both Ni₃Al and NaCl is identical to that previously determined in a diamond anvil cell. While a multi-anvil device thus has the advantage, compared with a diamond anvil cell, of constant pressure and stress throughout the sample chamber, microstrains in poly-crystalline samples arise in both devices. Samples in a multi-anvil apparatus thus need to be mixed with a pressure medium and to consist of essentially single crystals just as in a diamond anvil cell. Annealing experiments at high pressures confirm that the release of the uniaxial stress component in the pressure medium does not cause a release of microstrains in the embedded sample if the latter has been plastically deformed. Annealing for the purpose of attaining hydrostatic conditions in compression studies thus has to be carried out with care.     

The SrB4O7:Sm2+ optical sensor and the pressure homogeneization through thermal cycles in diamond anvil cells

Abstract

Preliminary results, up to 550 kbar, on a thermal-cycling relaxation process of the 4.Methanol-1.Ethanol pressure-transmitting medium, are presented. The homogeneization is monitored by linewidth measurements of the ⁵D₀-⁷F₀ emission line of SrB₄O₇:Sm²⁺, recently proposed as an “almost ideal” high pressure calibrant. Above 100 kbar at 20°C, the line broadens linearly with increaisng pressure. Upon heating, at a fixed pressure, the linewidth is reduced to a minimum (“hydrostatic”) value provided a sufficient temperature is reached and this narrowing is irreversible upon cooling. For T=300°C this relaxation is observed for P (250 kbar, incomplete at 340 kbar and not observed at 550 kbar.     

Pressure dependence of the soft modes in the model ferroelastics Hg2I2

The pressure dependence of the soft modes in the paraphase (boundary of the Brillouin zone, X point) and the ferrophase (center of the Brillouin zone, Γ point) is studied in the model ferroelastic crystals Hg₂I₂. An anomalous linear pressure dependence of the soft-mode frequency in the paraphase (p<p _c ) is observed. The results obtained and the nature of the ferroelastic phase transition induced in Hg₂I₂ crystals by a high hydrostatic pressure are discussed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 227–231 (25 February 1996)