1,649 research outputs found
Hydrodynamic Waves in Regions with Smooth Loss of Convexity of Isentropes. General Phenomenological Theory
General phenomenological theory of hydrodynamic waves in regions with smooth
loss of convexity of isentropes is developed based on the fact that for most
media these regions in p-V plane are anomalously small. Accordingly the waves
are usually weak and can be described in the manner analogous to that for weak
shock waves of compression. The corresponding generalized Burgers equation is
derived and analyzed. The exact solution of the equation for steady shock waves
of rarefaction is obtained and discusses.Comment: RevTeX, 4 two-column pages, no figure
Structural, elastic and electronic properties of new layered superconductor HfCuGe2 in comparison with isostructural HfCuSi2, ZrCuGe2, and ZrCuSi2 from first-principles calculations
Very recently, low-temperature superconductivity was discovered for the intermetallic compound HfCuGe2 (2013; Cava RJ, et al, EPL 101:67001.), which was declared as "a non-magnetic analog of the 1111 iron pnictides". Herein, by means of the first-principles calculations, we have examined in detail the structural, elastic, and electronic properties of HfCuGe2, as well as of the isostructural and isoelectronic phases ZrCuGe2, HfCuSi2, and ZrCuSi2, which are analyzed in comparison with a set of 1111-like phases. The obtained close similarity of the electronic factors, namely, the topologies of the near-Fermi bands, the Fermi surfaces, as well as the DOS values at the Fermi level for superconducting HfCuGe2 and other examined 112 phases allowed us to assume that low-temperature superconductivity may be expected also for ZrCuGe2, HfCuSi2, and ZrCuSi2. © 2013 Elsevier Ltd. All rights reserved
Correlated band structure of superconducting NdFeAsO0.9F0.1: Dynamical mean-field study
We report the LDA + DMFT (method combining local density approximation with dynamical mean-field theory) results for spectral properties of the superconductor NdFeAsO0.9F0.1 in the paramagnetic phase. The calculated momentum-resolved spectral functions are in good agreement with angle-resolved photoemission spectra (ARPES). The obtained effective quasiparticle mass enhancement (m*/m = 1.4) is smaller than the one in isostructural parent compound LaFeAsO which critical temperature under the same fluorine doping (LaFeAsO0.9F0.1) is two times lower. Our results demonstrate that in quaternary FeAs-based superconductors of the same class, changes of the crystal structure caused by substitution of one rare-earth atom, implicitly result in reduction of the electronic correlation strength. © 2013 Pleiades Publishing, Inc
Extended DFT+U+V method with on-site and inter-site electronic interactions
In this article we introduce a generalization of the popular DFT+U method
based on the extended Hubbard model that includes on-site and inter-site
electronic interactions. The novel corrective Hamiltonian is designed to study
systems for which electrons are not completely localized on atomic states
(according to the general scheme of Mott localization) and hybridization
between orbitals from different sites plays an important role. The application
of the extended functional to archetypal Mott - charge-transfer (NiO) and
covalently bonded insulators (Si and GaAs) demonstrates its accuracy and
versatility and the possibility to obtain a unifying and equally accurate
description for a broad range of very diverse systems
First-order transition between a small-gap semiconductor and a ferromagnetic metal in the isoelectronic alloys FeSiGe
The contrasting groundstates of isoelectronic and isostructural FeSi and FeGe
can be explained within an extended local density approximation scheme (LDA+U)
by an appropriate choice of the onsite Coulomb repulsion, on the Fe-sites.
A minimal two-band model with interband interactions allows us to obtain a
phase diagram for the alloys FeSiGe. Treating the model in a mean
field approximation, gives a first order transition between a small-gap
semiconductor and a ferromagnetic metal as a function of magnetic field,
temperature, and concentration, . Unusually the transition from metal to
insulator is driven by broadening, not narrowing, the bands and it is the
metallic state that shows magnetic order.Comment: 4 pages, 5 figure
Classical spin model of the relaxation dynamics of rare-earth doped permalloy
In this paper, the ultrafast dynamic behavior of rare-earth doped permalloy is investigated using an atomistic spin model with Langevin dynamics. In line with experimental work, the effective Gilbert damping is calculated from transverse relaxation simulations, which shows that rare-earth doping causes an increase in the damping. Analytic theory suggests that this increase in damping would lead to a decrease in the demagnetization time. However, longitudinal relaxation calculations show an increase with doping concentration instead. The simulations are in a good agreement with previous experimental work of Radu et al. [Radu et al., Phys. Rev. Lett. 102, 117201 (2009)]. The longitudinal relaxation time of the magnetization is shown to be driven by the interaction between the transition metal and the laser-excited conduction electrons, whereas the effective damping is predominantly determined by the slower interaction between the rare-earth elements and the phonon heat bath. We conclude that for complex materials, it is evidently important not to expect a single damping parameter but to consider the energy transfer channel relevant to the technique and time scale of the measurement
Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic-Smectic-A Phase Transition
We have conducted two quantitative tests of predictions based on the
Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase
transitions. First, we explore the effect of an external magnetic field on the
nematic-smectic-A (NA) transition in a liquid crystal. Second, we examine the
dependence of the first-order discontinuity as a function of mixture
concentration in pure 8CB and three 8CB-10CB mixtures. We find the first
quantitative evidence for deviations from the HLM theory.Comment: 4 pages, 2 figure
Electron-lattice kinetics of metals heated by ultrashort laser pulses
We propose a kinetic model of transient nonequilibrium phenomena in metals
exposed to ultrashort laser pulses when heated electrons affect the lattice
through direct electron-phonon interaction. This model describes the
destruction of a metal under intense laser pumping. We derive the system of
equations for the metal, which consists of hot electrons and a cold lattice.
Hot electrons are described with the help of the Boltzmann equation and
equation of thermoconductivity. We use the equations of motion for lattice
displacements with the electron force included. The lattice deformation is
estimated immediately after the laser pulse up to the time of electron
temperature relaxation. An estimate shows that the ablation regime can be
achieved.Comment: 7 pages; Revtex. to appear in JETP 88, #1 (1999
TiOCl, an orbital-ordered system?
We present first principles density functional calculations and downfolding
studies of the electronic and magnetic properties of the layered quantum spin
system
TiOCl. We discuss explicitely the nature of the exchange pathes and attempt
to clarify the concept of orbital ordering in this material. An analysis of the
electronic structure of slightly distorted structures according to the phononic
modes allowed in this material suggests that this system is subject to large
orbital fluctuations driven by the electron-phonon coupling. Based on these
results, we propose a microscopic explanation of the behavior of TiOCl near the
phase transition to a spin-gapped system.Comment: Some figures are compressed, for higher quality please contact the
author
Parametrization of dark energy equation of state Revisited
A comparative study of various parametrizations of the dark energy equation
of state is made. Astrophysical constraints from LSS, CMB and BBN are laid down
to test the physical viability and cosmological compatibility of these
parametrizations. A critical evaluation of the 4-index parametrizations reveals
that Hannestad-M\"{o}rtsell as well as Lee parametrizations are simple and
transparent in probing the evolution of the dark energy during the expansion
history of the universe and they satisfy the LSS, CMB and BBN constraints on
the dark energy density parameter for the best fit values.Comment: 11 page
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