683 research outputs found
Reply to comment on "Simple one-dimensional model of heat conduction which obeys Fourier's law"
In this reply we answer the comment by A. Dhar (cond-mat/0203077) on our
Letter "Simple one dimensional model of heat conduction which obeys Fourier's
law" (Phys. Rev. Lett. 86, 5486 (2001), cond-mat/0104453)Comment: 1 pag., 1 fi
A simple one-dimensional model of heat conduction which obeys Fourier's law
We present the computer simulation results of a chain of hard point particles
with alternating masses interacting on its extremes with two thermal baths at
different temperatures. We found that the system obeys Fourier's law at the
thermodynamic limit. This result is against the actual belief that one
dimensional systems with momentum conservative dynamics and nonzero pressure
have infinite thermal conductivity. It seems that thermal resistivity occurs in
our system due to a cooperative behavior in which light particles tend to
absorb much more energy than the heavier ones.Comment: 5 pages, 4 figures, to be published in PR
Geometric, electronic, and magnetic structure of CoFeSi: Curie temperature and magnetic moment measurements and calculations
In this work a simple concept was used for a systematic search for new
materials with high spin polarization. It is based on two semi-empirical
models. Firstly, the Slater-Pauling rule was used for estimation of the
magnetic moment. This model is well supported by electronic structure
calculations. The second model was found particularly for Co based Heusler
compounds when comparing their magnetic properties. It turned out that these
compounds exhibit seemingly a linear dependence of the Curie temperature as
function of the magnetic moment. Stimulated by these models, CoFeSi was
revisited. The compound was investigated in detail concerning its geometrical
and magnetic structure by means of X-ray diffraction, X-ray absorption and
M\"o\ss bauer spectroscopies as well as high and low temperature magnetometry.
The measurements revealed that it is, currently, the material with the highest
magnetic moment () and Curie-temperature (1100K) in the classes of
Heusler compounds as well as half-metallic ferromagnets. The experimental
findings are supported by detailed electronic structure calculations
Self-Trapped Exciton Defects in a Charge Density Wave: Electronic Excitations of BaBiO3
In the previous paper, it was shown that holes doped into BaBiO3 self-trap as
small polarons and bipolarons. These point defects are energetically favorable
partly because they undo locally the strain in the charge-density-wave (Peierls
insulator) ground state. In this paper the neutral excitations of the same
model are discussed. The lowest electronic excitation is predicted to be a
self-trapped exciton, consisting of an electron and a hole located on adjacent
Bi atoms. This excitation has been seen experimentally (but not identified as
such) via the Urbach tail in optical absorption, and the multi-phonon spectrum
of the ``breathing mode'' seen in Raman scattering. These two phenomena occur
because of the Franck-Condon effect associated with oxygen displacement in the
excited state.Comment: 5 pages with 7 embedded figures. See also cond-mat/0108089 on
polarons and bipolarons in BaBiO3 contains background informatio
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds
The XYZ half-Heusler crystal structure can conveniently be described as a
tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X
species. This description is well suited to understand the electronic structure
of semiconducting 8-electron compounds such as LiAlSi (formulated
Li[AlSi]) or semiconducting 18-electron compounds such as TiCoSb
(formulated Ti[CoSb]). The basis for this is that [AlSi]
(with the same electron count as Si) and [CoSb] (the same electron
count as GaSb), are both structurally and electronically, zinc-blende
semiconductors. The electronic structure of half-metallic ferromagnets in this
structure type can then be described as semiconductors with stuffing magnetic
ions which have a local moment: For example, 22 electron MnNiSb can be written
Mn[NiSb]. The tendency in the 18 electron compound for a
semiconducting gap -- believed to arise from strong covalency -- is carried
over in MnNiSb to a tendency for a gap in one spin direction. Here we similarly
propose the systematic examination of 18-electron hexagonal compounds for
semiconducting gaps; these would be the "stuffed wurtzite" analogues of the
"stuffed zinc blende" half-Heusler compounds. These semiconductors could then
serve as the basis for possibly new families of half-metallic compounds,
attained through appropriate replacement of non-magnetic ions by magnetic ones.
These semiconductors and semimetals with tunable charge carrier concentrations
could also be interesting in the context of magnetoresistive and thermoelectric
materials.Comment: 11 pages, 6 figures, of which 4 are colou
"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7
We point out that all current Josephson-junction experiments probing directly
the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in
terms of the bilayer antiferromagnetic spin fluctuation model, which renders
the superconducting state with the order parameters of extended symmetry,
but with the opposite signs in the bonding and antibonding Cu-O plane bands.
The essential part of our interpretation includes the Cu-O chain band which
would have the order parameter of the same sign as antibonding plane band. We
show that in this case net Josephson currents along and perpendicular to the
chains have the phase shift equal to pi.Comment: 4 pages, revtex, 1 figure uuencoded (POSTSCRIPT figure replaced - the
previous file did not print Greek letters correctly
Electronic Raman scattering in YBCO and other superconducting cuprates
Superconductivity induced structures in the electronic Raman spectra of
high-Tc superconductors are computed using the results of ab initio LDA-LMTO
three-dimensional band structure calculations via numerical integrations of the
mass fluctuations, either in the whole 3D Brillouin zone or limiting the
integrations to the Fermi surface. The results of both calculations are rather
similar, the Brillouin zone integration yielding additional weak structures
related to the extended van Hove singularities. Similar calculations have been
performed for the normal state of these high-Tc cuprates. Polarization
configurations have been investigated and the results have been compared to
experimental spectra. The assumption of a simple d_(x^2-y^2)-like gap function
allows us to explain a number of experimental features but is hard to reconcile
with the relative positions of the A1g and B1g peaks.Comment: 14 pages, LaTeX (RevTeX), 5 PostScript figures, uses multicol.sty,
submitted to PR
Ground State Theory of delta-Pu
Correlation effects are important for making predictions in the delta phase
of Pu. Using a realistic treatment of the intra-atomic Coulomb correlations we
address the long-standing problem of computing ground state properties. The
equilibrium volume is obtained in good agreement with experiment when taking
into account Hubbard U of the order 4 eV. For this U, the calculation predicts
a 5f5 atomic-like configuration with L=5, S=5/2, and J=5/2 and shows a nearly
complete compensation between spin and orbital magnetic moments.Comment: 4 pages, 1 postscript figure, 1 jpg figure (viewable via Netscape,
IE
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