93 research outputs found
Pressure-induced isostructural phase transition of metal-doped silicon clathrates
We propose an atomistic model for the pressure-induced isostructural phase
transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been
observed at 14 GPa and 23 GPa, respectively. The model explains successfully
the equation of state, transition pressure, change of Raman spectra and
dependence on the doped cations as well as the effects of substituting Si(6c)
atoms with noble metals.Comment: 5 pages, two coumn, 5 figures. See http://www.iitaka.org/down.html
for more informatio
Diamond and -tin structures of Si studied with quantum Monte Carlo calculations
We have used diffusion quantum Monte Carlo (DMC) calculations to study the
pressure-induced phase transition from the diamond to -tin structure in
silicon. The calculations employ the pseudopotential technique and
systematically improvable B-spline basis sets. We show that in order to achieve
a precision of 1 GPa in the transition pressure the non-cancelling errors in
the energies of the two structures must be reduced to 30 meV/atom. Extensive
tests on system size errors, non-local pseudopotential errors, basis-set
incompleteness errors, and other sources of error, performed on periodically
repeated systems of up to 432 atoms, show that all these errors together can be
reduced to well below 30 meV/atom. The calculated DMC transition pressure is
about 3-4 GPa higher than the accepted experimental range of values, and we
argue that the discrepancy may be due to the fixed-node error inherent in DMC
techniques.Comment: 10 pages, 4 figure
Linear response results for phonons and electron-phonon coupling in hcp Sc - spin fluctuations and implications for superconductivity
We present a study of the electronic structure, phonon frequencies and
electron-phonon coupling in hcp Sc under pressure. The electron-phonon coupling
constant is found to increase steadily with pressure in the hcp phase, until
the pressure reaches a value where the hcp phase becomes unstable. Calculations
for the normal pressure ratio predict a phase change somewhere between
calculated pressures of 22 and 30 GPa. The calculated frequencies for the
equilibrium hcp lattice parameters are in good agreement with the inelastic
neutron scattering results. From the measured value of the electronic specific
heat constant and the calculated values of the Fermi level density of states
and electron-phonon coupling constant, we conclude that the electron-paramagnon
coupling constant in hcp Sc should be comparable to the electron-phonon
coupling constant. This indicates that the spin fluctuation effects are strong
enough to suppress superconductivity completely in hcp Sc. Based on estimates
of the electron-paramagnon coupling constants and the calculated or estimated
electron-phonon coupling constants, we argue that the hcp phase may become
superconducting with a very low transition temperature immediately prior to the
transition to the Sc-II phase and that the Sc-II phase should indeed be
superconducting.Comment: To appear in Journal of Physics: Condens. Matter. This is a
substantially revised version of an earlier submission (arXiv:0706.3728v1
[cond-mat.supr-con] 25 Jun 2007) which was withdrawn due to some errors in
the discussion. The revised version addresses those errors and includes
additional result
Consistent Anisotropic Repulsions for Simple Molecules
We extract atom-atom potentials from the effective spherical potentials that
suc cessfully model Hugoniot experiments on molecular fluids, e.g., and
. In the case of the resulting potentials compare very well with the
atom-atom potentials used in studies of solid-state propertie s, while for
they are considerably softer at short distances. Ground state (T=0K) and
room temperatu re calculations performed with the new potential resolve
the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure
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.Comment: 3 pages, 3 figure
Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium
We have investigated the structural sequence of the high-pressure phases of
silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase
transitions. We have used the plane-wave pseudopotential approach to the
density-functional theory implemented within the Vienna ab-initio simulation
package (VASP). We have determined the equilibrium properties of each structure
and the values of the critical parameters including a hysteresis effect at the
phase transitions. The order of the phase transitions has been obtained
alternatively from the pressure dependence of the enthalpy and of the internal
structure parameters. The commonly used tangent construction is shown to be
very unreliable. Our calculations identify a first-order phase transition from
the cd to the beta-tin and from the Imma to the sh phase, and they indicate the
possibility of a second-order phase-transition from the beta-tin to the Imma
phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure
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 has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
Simple Metals at High Pressure
In this lecture we review high-pressure phase transition sequences exhibited
by simple elements, looking at the examples of the main group I, II, IV, V, and
VI elements. General trends are established by analyzing the changes in
coordination number on compression. Experimentally found phase transitions and
crystal structures are discussed with a brief description of the present
theoretical picture.Comment: 22 pages, 4 figures, lecture notes for the lecture given at the Erice
course on High-Pressure Crystallography in June 2009, Sicily, Ital
Mechanical Deformation Induced in Si and GaN Under Berkovich Nanoindentation
Details of Berkovich nanoindentation-induced mechanical deformation mechanisms of single-crystal Si(100) and the metal-organic chemical-vapor deposition (MOCVD) derived GaN thin films have been systematic investigated by means of micro-Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM) techniques. The XTEM samples were prepared by using focused ion beam (FIB) milling to accurately position the cross-section of the nanoindented area. The behaviors of the discontinuities displayed in the loading and unloading segments of the load-displacement curves of Si and GaN thin films performed with a Berkovich diamond indenter tip were explained by the observed microstructure features obtained from XTEM analyses. According to the observations of micro-Raman and XTEM, the nanoindentation-induced mechanical deformation is due primarily to the generation and propagation of dislocations gliding along the pyramidal and basal planes specific to the hexagonal structure of GaN thin films rather than by indentation-induced phase transformations displayed in Si
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