arXiv subjects

Russell J. Hemley

Publications and source records attributed to Russell J. Hemley.

50 records · Page 3Linked to original sources

arXiv2012

Equation of state and Raman-active $E_{2g}$ lattice phonon in phases I, II, and III of solid hydrogen and deuterium

We present results of lattice dynamics calculations of the $P-V$ equation of state and the pressure dependence of the Raman-active $E_{2g}$ lattice phonon for $p-$H$_2$ and $o-$D$_2$ in a wide pressure range up to $\sim$2 Mbar using our recently developed semi-empirical many-body potential, and density-functional theory. Comparison with existing body of experimental and theoretical results showed that the employed many-body potential is a reliable basis for high-precision calculations for phases I, II, and III of solid hydrogens.

cond-mat.mtrl-sci↗

arXiv2007

Brillouin scattering and molecular dynamics study of the elastic properties of Pb(Mg1/3Nb2/3)O3

We report the complete set of elastic constants and the bulk modulus for single crystal Pb(Mg1/3Nb2/3)O3 (PMN) at room temperature obtained from Brillouin spectroscopy and molecular dynamics (MD) simulations. The bulk modulus from Brillouin is found to be 103 GPa, in a good agreement with earlier x-ray studies. We also derived the refractive index along all principal axes and found PMN to be optically isotropic, with a refractive index of 2.52 +/- 0.02. PMN shows mechanical anisotropy with A=1.7. The MD simulations of PMN using the random site model overestimate the elastic constants by 20-50 GPa and the bulk modulus is 148 GPa, but the mechanical anisotropy matches the Brillouin results of A = 1.7. We also determined the elastic constants for various models of PMN and we find variation in the elastic constants based on chemical ordering.

cond-mat.mtrl-sci↗

arXiv2005

Crystal structures of superconducting phases of S and Se

Compressed S and Se are studied by x-ray diffraction with synchrotron radiation up to 160 GPa. The S-IV phase is shown to have a body-centered monoclinic structure and to be stable between 83 and 150 GPa on pressure increase, where it transform to S-V with a primitive rhombohedral $β$-Po structure. Observation of the modulation reflections in S-IV up to 135 GPa shows that its crystal structure is incommensurately modulated, as recently reported for Se and Te. The modulated body-centered monoclinic phase of Se, Se-IV, is shown to transform to the $β$-Po phase Se-V at around 80 GPa. Se-V transforms to a body-centered cubic phase at 140 GPa in accordance with previous studies. Pressure dependence of the structural parameters of these high-pressure phases in S and Se are discussed in relation to their superconducting behaviour.

cond-mat.other↗

arXiv2004

High-pressure phase relations of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ single crystals

We have investigated the pressure dependence of the superconducting transition temperature $T_{c}$ up to 18 GPa of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ single crystals ranging from the highly underdoped through the nearly optimally doped to the highly overdoped level. For all three samples studied, $T_{c}$ is found to increase initially and then saturate at a critical pressure $P_{c}$ but decrease modestly with further compression. Oxygen doping tends to reduce the increase in $T_{c}$ and $P_{c}$. A new high-pressure phase diagram between the saturated $T_{c}$ and $P_{c}$ is then obtained. Theoretical interpretation is given by using the competition between the hole carrier density and pairing interaction strength based on the high-pressure transport data of the resistivity and Hall coefficient in this system.

cond-mat.supr-con↗

arXiv2004

Pressure-induced phonon frequency shifts in transition-metal nitrides

We report the first experiments on the high pressure phonon spectra of transition-metal nitrides HfN, ZrN, and NbN, obtained by Raman scattering measurements. Two pronounced bands, which are related to the acoustic part at low frequency around 200 cm$^{-1}$ and the optical part at high frequency around 550 cm$^{-1}$ of the phonon spectrum, respectively, shift to high frequency values with increasing pressure. An analysis of the results allows us to reproduce the experimental pressure dependence of the superconducting transition temperature $T_{c}$ of ZrN and NbN.

cond-mat.supr-con↗

arXiv2002

New Methods for Investigating Superconductivity at Very High Pressures

We review recent advances in diamond anvil cell development, focusing on the details of magnetic susceptibility techniques. We have developed and implemented highly sensitive technique that allowed observations of superconductivity using magnetic susceptibility at megabar pressures, including the observation of a Tc in sulfur to above 230 GPa. The advancement of the technique should enable the most demanding diamond anvil cell applications. The Tc(P) data for simple s-p metals (Se,S, MgB_2) are presented and analyzed. The possibility for a phonon-assisted electronic topological transition (ETT) in MgB_2 at high pressure is discussed.

cond-mat.supr-con↗

arXiv2001

Raman Scattering of Metals to Very High Pressures

We review recent development in Raman techniques and diamond anvil cell technology, that allow measurements of Raman of metals in the megabar pressure range. The Raman technique includes holographic transmission optics and single-stage spectrographs with CCD array detectors. Use of synthetic high-purity diamonds and anglular excitation geometry reduces spurious background scattering substantially, which makes observable very weak Raman peaks in metals. We give several examples of studies including the hexagonal close packed (hcp) $ε$-Fe, Fe$_{(1-x)}$Ni${_x}$, and Re and the recently discovered high-temperature superconductor MgB$_2$. The shift of phonon modes with pressure is used for determination of the mode-Grüneisen parameters, estimation of shear elastic moduli and provide information on structural and electronic transitions.

cond-mat.mtrl-sci↗

arXiv2001

New Phases of Solid Nitrogen

We report the discovery of a new class of molecular phases of solid nitrogen at high pressures and temperatures by Raman and infrared spectroscopy and powder x-ray diffraction. Unlike the molecular phases consisting of disk- and sphere-like molecular disorder ($δ$-, $ε$- and $ζ$-N$_2$) and reportedly stable over a wide P-T range, one of the new phases ($ι$) is diatomic with disk-like molecules. A second new phase (the higher pressure $θ$-phase) is characterized by strong intermolecular interactions and infrared vibron absorption. Both phases exhibit wide P-T ranges of stability and metastability.

cond-mat.mtrl-sci↗

arXiv2001

Spectroscopic Studies of the Vibrational and Electronic Properties of Solid Hydrogen to 285 GPa

We report Raman scattering and visible to near-infrared absorption spectra of solid hydrogen under static pressure up to 285 GPa at 85-140 K. We obtain pressure dependences of vibron and phonon modes in agreement with previously determined to lower pressures. The results indicate the stability of the ordered molecular phase III to the highest pressure reached and provide constraints on the insulator-to-metal transition pressure.

cond-mat.mtrl-sci↗

arXiv2001

Superconductivity in the Chalcogens up to Multimegabar Pressures

Highly sensitive magnetic susceptibility techniques were used to measure the superconducting transition temperatures in S up to 231($\pm$5) GPa. S transforms to a superconductor with T$_c$ of 10 K and has a discontinuity in T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition. Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200 GPa and then slowly decreases with pressure to 15 K at 230 GPa. This trend in the pressure dependence parallels the behavior of the heavier members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K. Similiarities in the T_c dependences for S, Se, and Te, and the implications for oxygen are discussed.

cond-mat.supr-con↗

arXiv2001

Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa

The partial density of vibrational states has been measured for Fe in compressed FeO (wüstite) using nuclear resonant inelastic x-ray scattering. Substantial changes have been observed in the overall shape of the density of states close to the magnetic transiton around 20 GPa from the paramagnetic (low pressure) to the antiferromagnetic (high pressure) state. Our data indicate a substantial softening of the aggregate sound velocities far below the transition, starting between 5 and 10 GPa. This is consistent with recent radial x-ray diffraction measurements of the elastic constants in FeO. The results indicate that strong magnetoelastic coupling in FeO is the driving force behind the changes in the phonon spectrum of FeO.

cond-mat.str-el↗

arXiv2001

High-Pressure Amorphous Nitrogen

The phase diagram and stability limits of diatomic solid nitrogen have been explored in a wide pressure--temperature range by several optical spectroscopic techniques. A newly characterized narrow-gap semiconducting phase $η$ has been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational and optical properties of the $η$ phase produced under these conditions indicate that it is largely amorphous and back transforms to a new molecular phase. The band gap of the $η$ phase is found to decrease with pressure indicating possible metallization by band overlap above 280 GPa.

cond-mat.mtrl-sci↗

arXiv2001

Raman spectrum and lattice parameters of MgB2 as a function of pressure

We report Raman spectra and synchrotron x-ray diffraction measurements of lattice parameters of polycrystalline MgB2 under hydrostatic pressure conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is observed that exhibits a large linear pressure shift of its frequency. The large mode damping and Gruneisen parameter indicate a highly anharmonic nature of the mode, broadly consistent with theoretical predictions for the E2g in-plane boron stretching mode. The results obtained may provide additional constraints on the electron-phonon coupling in the system.

cond-mat.supr-con↗

arXiv2000

Accuracy of equation of state formulations

The accuracy of equation-of-state formulations is compared for theoretical total energies or experimental pressure-volume measurements for H$_{2}$, Ne, Pt, and Ta. This spans the entire range of compression found for minerals and vo latiles in the Earth. The Vinet equation is found to be most accurate. The origin of the behavior of different equation-of-state formulations is discussed. It is shown that subtle phase transitions can be detected by examining the residuals from an equation-of-state fit. A change in the electronic structure of Ta is found at high pressures using this procedure, and a possible new transition in H$_{2}$.

cond-mat.mtrl-sci↗