13,407 research outputs found
Performance of the modified Becke-Johnson potential
Very recently, in the 2011 version of the Wien2K code, the long standing
shortcome of the codes based on Density Functional Theory, namely, its
impossibility to account for the experimental band gap value of semiconductors,
was overcome. The novelty is the introduction of a new exchange and correlation
potential, the modified Becke-Johnson potential (mBJLDA). In this paper, we
report our detailed analysis of this recent work. We calculated using this
code, the band structure of forty one semiconductors and found an important
improvement in the overall agreement with experiment as Tran and Blaha [{\em
Phys. Rev. Lett.} 102, 226401 (2009)] did before for a more reduced set of
semiconductors. We find, nevertheless, within this enhanced set, that the
deviation from the experimental gap value can reach even much more than 20%, in
some cases. Furthermore, since there is no exchange and correlation energy term
from which the mBJLDA potential can be deduced, a direct optimization procedure
to get the lattice parameter in a consistent way is not possible as in the
usual theory. These authors suggest that a LDA or a GGA optimization procedure
is used previous to a band structure calculation and the resulting lattice
parameter introduced into the 2011 code. This choice is important since small
percentage differences in the lattice parameter can give rise to quite higher
percentage deviations from experiment in the predicted band gap value.Comment: 10 pages, 2 figures, 5 Table
Hydromagnetic Instability in plane Couette Flow
We study the stability of a compressible magnetic plane Couette flow and show
that compressibility profoundly alters the stability properties if the magnetic
field has a component perpendicular to the direction of flow. The necessary
condition of a newly found instability can be satisfied in a wide variety of
flows in laboratory and astrophysical conditions. The instability can operate
even in a very strong magnetic field which entirely suppresses other MHD
instabilities. The growth time of this instability can be rather short and
reach shear timescales.Comment: 6 pages, 5 figures. To appear on PR
High-growth-rate magnetohydrodynamic instability in differentially rotating compressible flow
The transport of angular momentum in the outward direction is the fundamental
requirement for accretion to proceed in an accretion disc. This objective can
be achieved if the accretion flow is turbulent. Instabilities are one of the
sources for the turbulence. We study a differentially rotating compressive flow
in the presence of non vanishing radial and azimuthal magnetic field and
demonstrate the occurrence of a high growth rate instability. This instability
operates in a region where magnetic energy density exceeds the rotational
energy density
Observation of ground-state quantum beats in atomic spontaneous emission
We report ground-state quantum beats in spontaneous emission from a
continuously driven atomic ensemble. Beats are visible only in an intensity
autocorrelation and evidence spontaneously generated coherence in radiative
decay. Our measurement realizes a quantum eraser where a first photon detection
prepares a superposition and a second erases the "which-path" information in
the intermediate state.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Letter
Equation of state of cubic boron nitride at high pressures and temperatures
We report accurate measurements of the equation of state (EOS) of cubic boron
nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range
500-900 K. Experiments were performed on single-crystals embedded in a
quasi-hydrostatic pressure medium (helium or neon). Comparison between the
present EOS data at 295 K and literature allows us to critically review the
recent calibrations of the ruby standard. The full P-V-T data set can be
represented by a Mie-Gr\"{u}neisen model, which enables us to extract all
relevant thermodynamic parameters: bulk modulus and its first
pressure-derivative, thermal expansion coefficient, thermal Gr\"{u}neisen
parameter and its volume dependence. This equation of state is used to
determine the isothermal Gr\"{u}neisen mode parameter of the Raman TO band. A
new formulation of the pressure scale based on this Raman mode, using
physically-constrained parameters, is deduced.Comment: 8 pages, 7 figure
Superconductivity in the Sn-Ba-Sr-Y-Cu-O system
Since Bednorz and Muller discovered high-T(sub c) superconductivity in the La-Ba-Cu-O compound, several families of superconducting oxides have been synthesized. Here, researchers report the results of search for superconductivity in the compounds based on tin, which has a lone electron pair like Bi, Tl, Pb. The following compounds were synthesized: Sn1Ba1Sr1Cu3Ox, Sn1Ba1Ca1Cu3Ox, Sn1Ba1Mg1Cu3Ox, Sn1Sr1Ca1Cu3Ox, Sn1Sr1Mg1Cu3Ox, Sn1Ca1Mg1Cu3Ox. The initial components were oxides and carbonates of the appropriate elements. Standard firing-grinding procedure was used. Final heating was carried out at 960 C during 12 hours. Then the samples were cooled inside the furnace. All the synthesis cycles were carried out in air atmosphere. Among the synthesized compounds only Sn1Ba1Sr1Cu3Ox showed remarkable conductivity. Other compounds were practically dielectrics. Presence of a possible superconductivity in Sn1Ba1Sr1Cu3Ox was defined by using the Meissner effect. At low temperature a deviation from paramagnetic behavior is observed. The hysteresis loops obtained at lower temperatures undoubtly testify to the presence of a superconductive phase in the sample. However, the part of the superconductive phase in the Sn1Ba1Sr1Cu3Ox ceramic turned out to be small, less than 2 percent, which agrees with the estimation from magnetic data. In order to increase the content of the superconductive phase two-valent cations Ba, Sr were partially substituted by univalent (K) and three-valent ones (Y)
Non-equilibrium statistical mechanics of classical nuclei interacting with the quantum electron gas
Kinetic equations governing time evolution of positions and momenta of atoms
in extended systems are derived using quantum-classical ensembles within the
Non-Equilibrium Statistical Operator Method (NESOM). Ions are treated
classically, while their electrons quantum mechanically; however, the
statistical operator is not factorised in any way and no simplifying
assumptions are made concerning the electronic subsystem. Using this method, we
derive kinetic equations of motion for the classical degrees of freedom (atoms)
which account fully for the interaction and energy exchange with the quantum
variables (electrons). Our equations, alongside the usual Newtonian-like terms
normally associated with the Ehrenfest dynamics, contain additional terms,
proportional to the atoms velocities, which can be associated with the
electronic friction. Possible ways of calculating the friction forces which are
shown to be given via complicated non-equilibrium correlation functions, are
discussed. In particular, we demonstrate that the correlation functions are
directly related to the thermodynamic Matsubara Green's functions, and this
relationship allows for the diagrammatic methods to be used in treating
electron-electron interaction perturbatively when calculating the correlation
functions. This work also generalises previous attempts, mostly based on model
systems, of introducing the electronic friction into Molecular Dynamics
equations of atoms.Comment: 18 page
Quantum, Multi-Body Effects and Nuclear Reaction Rates in Plasmas
Detailed calculations of the contribution from off-shell effects to the
quasiclassical tunneling of fusing particles are provided. It is shown that
these effects change the Gamow rates of certain nuclear reactions in dense
plasma by several orders of magnitude.Comment: 11 pages; change of content: added clarification of one of the
important steps in the derivatio
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
Nonlinear magneto-optical resonances at D1 excitation of 85Rb and 87Rb in an extremely thin cell
Nonlinear magneto-optical resonances have been measured in an extremely thin
cell (ETC) for the D1 transition of rubidium in an atomic vapor of natural
isotopic composition. All hyperfine transitions of both isotopes have been
studied for a wide range of laser power densities, laser detunings, and ETC
wall separations. Dark resonances in the laser induced fluorescence (LIF) were
observed as expected when the ground state total angular momentum F_g was
greater than or equal to the excited state total angular momentum F_e. Unlike
the case of ordinary cells, the width and contrast of dark resonances formed in
the ETC dramatically depended on the detuning of the laser from the exact
atomic transition. A theoretical model based on the optical Bloch equations was
applied to calculate the shapes of the resonance curves. The model averaged
over the contributions from different atomic velocity groups, considered all
neighboring hyperfine transitions, took into account the splitting and mixing
of magnetic sublevels in an external magnetic field, and included a detailed
treatment of the coherence properties of the laser radiation. Such a
theoretical approach had successfully described nonlinear magneto-optical
resonances in ordinary vapor cells. Although the values of certain model
parameters in the ETC differed significantly from the case of ordinary cells,
the same physical processes were used to model both cases. However, to describe
the resonances in the ETC, key parameters such as the transit relaxation rate
and Doppler width had to be modified in accordance with the ETC's unique
features. Agreement between the measured and calculated resonance curves was
satisfactory for the ETC, though not as good as in the case of ordinary cells.Comment: v2: substantial changes and expanded theoretical model; 13 pages, 10
figures; accepted for publication in Physical Review
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