158 research outputs found
Structural and superconducting transition in selenium under high pressures
First-principles calculations are performed for electronic structures of two
high pressure phases of solid selenium, -Po and bcc.
Our calculation reproduces well the pressure-induced phase transition from
-Po to bcc observed in selenium.
The calculated transition pressure is 30 GPa lower than the observed one, but
the calculated pressure dependence of the lattice parameters agrees fairly well
with the observations in a wide range of pressure.
We estimate the superconducting transition temperature of both
the -Po and the bcc phases by calculating the phonon dispersion and the
electron-phonon interaction on the basis of density-functional perturbation
theory.
The calculated shows a characteristic pressure dependence, i.e.
it is rather pressure independent in the -Po phase, shows a
discontinuous jump at the transition from -Po to bcc, and then decreases
rapidly with increasing pressure in the bcc phase.Comment: 8 pages, 11 figure
Electronic Structure and Magnetic Properties of Solids
We review basic computational techniques for simulations of various magnetic
properties of solids. Several applications to compute magnetic anisotropy
energy, spin wave spectra, magnetic susceptibilities and temperature dependent
magnetisations for a number of real systems are presented for illustrative
purposes.Comment: Review article; To appear in Journal of Computational Crystallograph
Linear-response theory and lattice dynamics: a muffin-tin orbital approach
A detailed description of a method for calculating static linear-response
functions in the problem of lattice dynamics is presented. The method is based
on density functional theory and it uses linear muffin-tin orbitals as a basis
for representing first-order corrections to the one-electron wave functions. As
an application we calculate phonon dispersions in Si and NbC and find good
agreement with experiments.Comment: 18 pages, Revtex, 2 ps figures, uuencoded, gzip'ed, tar'ed fil
Linear Response Calculations of Spin Fluctuations
A variational formulation of the time--dependent linear response based on the
Sternheimer method is developed in order to make practical ab initio
calculations of dynamical spin susceptibilities of solids. Using gradient
density functional and a muffin-tin-orbital representation, the efficiency of
the approach is demonstrated by applications to selected magnetic and strongly
paramagnetic metals. The results are found to be consistent with experiment and
are compared with previous theoretical calculations.Comment: 11 pages, RevTex; 3 Figures, postscript, high-resolution printing
(~1200dpi) is desire
Cellular Dynamical Mean Field Approach to Strongly Correlated Systems
We propose a cellular version of dynamical-mean field theory which gives a
natural generalization of its original single-site construction and is
formulated in different sets of variables. We show how non-orthogonality of the
tight-binding basis sets enters the problem and prove that the resulting
equations lead to manifestly causal self energies.Comment: RevTex, 4 pages, 1 embedded figur
Precise measurement of energy of the first excited state of 115Sn (Eexc = 497.3 keV)
Single beta decay of 115In to the first excited level of 115Sn (Eexc = 497.3
keV) is known as beta decay with the lowest Q value. To determine the Q
precisely, one has to measure very accurately the Eexc value. A sample of tin
enriched in 115Sn to 50.7% was irradiated by proton beam at the U-120
accelerator of INR, Kyiv. The 115Sb radioactive isotope, created in
115Sn(p,n)115Sb reaction, decays with T1/2 = 32 min to 115Sn populating the 497
keV level with ~ 96% probability. The total statistics of ~ 10^5 counts
collected in the 497 keV peak in series of measurements, exact description of
the peak shape and precisely known calibration points around the 497 keV peak
allowed to obtain the value Eexc = 497.342(3) keV, which is the most precise
to-date. This leads to the following Q* value for the decay 115In to 115Sn*: Q*
= 147 +- 10 eV.Comment: 6 pages, 3 figure
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.Comment: 6 pages, LaTeX and 5 figures in the postscript forma
S=1/2 chains and spin-Peierls transition in TiOCl
We study TiOCl as an example of an S=1/2 layered Mott insulator. From our
analysis of new susceptibility data, combined with LDA and LDA+U band structure
calculations, we conclude that orbital ordering produces quasi-one-dimensional
spin chains and that TiOCl is a new example of Heisenberg-chains which undergo
a spin-Peierls transition. The energy scale is an order of magnitude larger
than that of previously known examples. The effects of non-magnetic Sc
impurities are explained using a model of broken finite chains.Comment: 5 pages, 5 figures (color); details on crystal growth added; to be
published in Phys. Rev.
Acoustical-Mode-Driven Electron-Phonon Coupling in Transition-Metal Diborides
We show that the electron-phonon coupling in the transition-metal diborides
NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in
contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio
results, described in terms of phonon dispersion, linewidth, and partial
electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2
have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is
an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g}
frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA
frequency at A for TaB2 is almost half of that of MgB2 or NbB2.Comment: 4 pages, 4 figures, and 1 tabl
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