High pressure Raman spectra of Gd,Lu and Y

Abstract

The pressure dependence of Raman phonons was studied for Gd, Lu and Y. The TO phonon mode of the hcp phase goes down in frequency for all three metals on approaching the hcp → Sm-type phase transition. Phonon softening also occurs in the high pressure Sm-type phase.     

Pressure and temperature-dependent raman study of ZnWO4

Abstract

The Raman spectra of monoclinic ZnWO₄ (c⁴ _2h-space group) are reported over wide ranges of pressure (0-24GPa) and temperature (13-970 K). All 18 Raman active pho-nons are observed throughout these ranges. Combining the pressure and temperature Raman data, an identification scheme is suggested which makes possible to distinguish the internal modes (vibrations in the octahedral WO₆ units) from the external ones (pure lattice modes). All phonons harden with pressure, thus showing normal positive dω/dP slopes, and soften with temperature in the region 250 - 970 K (normal negative dω/dT slopes). However, two phonons show a change of slope sign below 300 K, resulting in small positive slopes at low temperatures. Since the pressure dependence of these two phonons is normal and linear, it is concluded that their anomalous, non-linear dependence with temperature is due to anharmonic (temperature induced) phonon interactions rather that volume expansion. It is found that ZnWO₄ remains stable throughout the pressure and temperature ranges.     

The peculiarities of the tunneling conductance and phonon induced features in tetrahedrically bonded semiconductors under pressure

Abstract

Summary The 30 kbar high pressure unit for low temperatures and the digital processing have been applied to the tunneling investigations of Ge and GaAs. The obtained results include the values of the mode Gruneisen constants for zone boundary phonons in <100> direction in Ge, the irregular optical phonons behavior in GaAs, the bistability of tunneling voltage-current curve for GaAs at elevated pressure and the observation of optically induced oscillations on the tunneling conductance of GaAs Schottky barrier. The semi-quantitative model involving the conductance band structure change under pressure and DX-centers in GaAs(Te) is used for the tentative explanation.     

High pressure study of cubic BN and SIC (Raman scattering and EDS)

Abstract

First-order Raman spectra and lattice parameters of BN and SiC were measured simultaneously up to a pressure of 34 and 45 GPa. The bulk moduli, their pressure derivatives and the volume dependence of Gruneisen parameters of LO and TO phonons were obtained. The average Gruneisen parameter for BN does not change much; on the contrary, the same parameter for SiC decreases under pressure. The definite connection between the crystal stability, the pressure behaviour of the optical phonons and the electron structure of the ionic cores is pointed out.     

Brillouin scattering study of methanol at high pressure

Abstract

Brillouin scattering has been used to study methanol as a function of pressure up to 8.4 GPa. Our near-forward-scattering experiments yield the pressure dependence of the velocity of longitudinal acoustic (LA) phonons, while we determine the pressure dependence of the product of the index of refraction and the LA phonon velocity from our backscattering experiments. The pressure dependence of the LA phonon lifetimes is obtained from the linewidth of the measured Brillouin peaks. From our backscattering experiments we find that the normalized phonon attenuation is a decreasing function of pressure.     

Determination of the Mode Grüneisen Parameter of AlN using different Fits on Experimental High Pressure Data

To investigate the pressure dependence of the AlN phonon frequencies Raman spectra of single-crystalline bulk AlN under hydrostatic pressure up to 10 GPa were recorded. The Raman peak positions of the A 1 (TO), E 1 (TO), E 2 (high), A 1 (LO) and quasi-longitudinal optical (QLO) phonons were plotted as a function of pressure. The experimental data was fitted using the traditional parabolic fit (M. Kuball et al . (2001) Appl. Phys. Lett., 78, 724 [1]) and fits to physical models, density, volume, etc. The mode Grüneisen parameters of the different phonons were determined for each fit and significant differences are found between the various fits. Results are compared with recent theoretical calculations (J.-M. Wagner et al . (2000) Phys. Rev. B, 62, 4526 [2]).     

Pressure Dependence of Acoustic Modes in AgGaSe 2

The pressure dependence of the transverse acoustic TA[0 0 1] and TA[1 0 0] phonons and the low-frequency optical v 3 phonons of AgGaSe 2 has been measured at pressures up to 4.3 GPa by inelastic neutron scattering. The strong frequency decreases in the whole Brillouin zone of the TA modes show generally a weak nonlinearity. Depending on the wavevector, the softening prior to the phase transition at 2.6 GPa also affects the optical mode to a less extent.     

Raman scattering in metals up to 50 GPa

Abstract

Measurements of k ≈ 0 optical phonons by Raman scattering are reported for Zn metal and for metallic high pressure phases of Si and Ge up to 50 GPa. Mode Griineisen parameters are determined and the experimental results are compared with theoretical results.     

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.     

Technical Reports: Scaling Laws in High-temperature Superconductors as Revealed Through Infrared Spectroscopy

Superconductivity refers to a fascinating state of matter where the electrical resistivity is precisely zero. Originally discovered in elemental metals such as mercury and tin in the early part of the last century, the mechanism of superconductivity was elusive and nearly 50 years passed before a comprehensive theory for superconductivity in metals was proposed by Bardeen, Cooper and Schrieffer [1] (the “BCS” theory). In a normal metal, the resistivity is determined by the elastic scattering of carriers. However, when a metal becomes a superconductor, the charge carriers are no longer single electrons, but rather pairs of electrons (“Cooper pairs”), which are bound together by a phonon interaction (phonons are the vibrations of the atomic lattice), and flow without resistance.     

Mechanism for thermal conductivity in energetic displacement cascades

Abstract

Temperature relaxation inside and outside the energetic displacement cascade in fast neutron-irradiated metals is consistently described. The characteristic time of the energy transfer between phonons and electrons in the damaged area is calculated. The space–time temperature distribution in the cascade damaged area in Fe and Ni is presented. The electron–phonon coupling is shown to play an important role in the evolution of the damaged area due to non-equilibrium between the local phonon and electron systems at the beginning of the cooling phase of the displacement cascade.     

Damping of optical phonons in metals and strongly doped semiconductors

The electron-phonon-induced damping of optical phonons arising in metals and strongly doped semiconductors under laser irradiation is investigated. The damping of both short-wave KV_F > ω₀ and long-wave KV_F < ω₀ optical phonons is calculated; K is the wavevector, ω₀ is the frequency of the optical phonon; V_F is the Fermi velocity. The electron- phonon-induced damping is important if the frequency of the optical phonon is larger than two frequencies of acoustic phonons of all branches in the range of the whole Brillouin zone. The damping of a soft transverse optical phonon in narrow-gap ferroelectric-semiconductors is also defined by the electron-phonon interaction. In other cases the main relaxation process for optical phonons in metals is the decay into two acoustic phonons due to lattice anharmonicity.     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Pressure Effects in Phonon Modes and Structure of A II B 2 III C 4 IV Compounds and Combinations

A II B 2 III C 4 VI compounds have been prepared with A II =Zn, Cd, Mn, B III =In, and C VI =S, Se. These atomic substitutions induce modifications in the structure and frequency for some of the phonons. These compounds have been studied by Raman spectroscopy at ambient conditions and under hydrostatic pressures. All phonons appear in the spectral region below 400 cm m 1 . High-pressure measurements have shown the existence of soft modes. New modes seem to appear at high pressures due to the phonon splitting. All low frequency phonons of the Zn 0.6 Mn 0.4 In 2 S 4 compound show a sudden modification in the frequency dependence on pressure above ～2 GPa. Based on the Raman results we propose an assignment for the modes and try to extract information about the structure and the possible phase transitions.     

Phonon spectrum of a naphthalene crystal at a high pressure: Influence of shortened distances on the lattice and intramolecular vibrations

The Raman spectra of a naphthalene crystal have been measured at room temperature in the pressure range up to 20 GPa. The pressure shift and Grüneisen parameters for intermolecular and intramolecular phonons have been determined. The maximum rate of the pressure shift for intermolecular phonons is 44 cm^?1/GPa, and the rate of the pressure shift for intramolecular phonons lies in the range from 1 to 11 cm^?1/GPa for different modes. The pressure dependence of the phonon frequencies for direct and inverse pressure variations has a hysteresis in the pressure range from 2.5 to 16.5 GPa. It has been shown that the linear dependence of the intermolecular phonon frequency on the crystal density has a peculiarity, which indicates a possible phase transition at a pressure of 3.5 GPa. The pressure dependence of intramolecular phonons related to the stretching vibrations of hydrogen atoms exhibits features that are characteristic of intermolecular phonons, which is associated with the influence of shortened distances between the hydrogen atoms of the neighboring molecules on the intermolecular interaction potential.     

Pressure Effects in the Raman Spectra of La 2−x Sr x CuO 4

Micro-Raman measurements under hydrostatic pressures up to 6 GPa have been carried out on high-quality La 2 m x Sr x CuO 4 polycrystalline compounds with Sr concentration up to x =0.45. The zz scattering polarization has been investigated, where two strong modes due to La/Sr and the apex oxygen, and (in the low Sr concentrations) the soft mode at ～100 cm m 1 are observed. The frequency of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate d y /d p and the phonon width have been detected depending on the amount of doping. For x =0.45 a considerable increase in the width of both strong phonons with pressure was found, which must reflect a separation into phases, since this concentration is close to the solubility limit. The relative intensity of the strong phonons was investigated in connection with its correlation to the transition temperature.     

Absence of low-temperature anomaly on the melting curve of <Superscript>4</Superscript>He

The melting pressure and pressure in the liquid at a constant density of ultrapure ⁴He (0.3 ppb of ³He impurities) have been measured with an accuracy of about 0.5 μbar in the temperature range from 10 to 320 mK. The measurements show that the anomaly on the melting curve below 80 mK, which was recently observed [I. A. Todoshchenko et al., Phys. Rev. Lett. 97, 165302 (2006)], is entirely due to an anomaly in the elastic modulus of Be-Cu from which our pressure gauge is made. Thus, the melting pressure of ⁴He follows the T ⁴ law due to phonons in the whole temperature range from 10 to 320 mK without any attribute of a supersolid transition. The text was submitted by the authors in English.     

Free and bound excitons in silver bromide under hydrostatic pressure

Abstract

Employing resonant Raman and luminescence spectroscopy, various exciton states and phonon modes are studied at low temperature in AgBr under hydrostatic pressure up to 0.7 GPa. The deformation potential for the indirect free exciton gap and mode Grüneisen parameters for various phonons are determined. Excitons bound to neutral donors and isoelectronic iodine are found to essentially derive from L-point valence band states.     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

The superconducting and the electrical properties of single crystals Bi2Sr2CuCa2oXat high pressure

Abstract

We have investigated the effect of pressure on lattice parameters and Raman-active phonons of the molecular crystal xenon difluride by powder x-ray diffraction and first-order Raman scattering, respectively. The ambient pressure tetragonal crystal structure remains stable up to the maximum pressure of 50 GPa. The compressibility is found to be strongly anisotropic. The frequencies of the intramolecular A1_g stretching mode and the E_g libration increase sublinearly under pressure. ding the a-and c-axis compression data, we have determined the dependence of phonon frequencies on lattice parameters. The high-frequency A1_g mode shows a pronounced nonlinear scaling with c-axis lattice parameter, with particularly small changes during initial compression. This effect is attributed to the interplay between intra-and intermolecular interactions during compression