2,379 research outputs found
Phase transition between d-wave and anisotropic s-wave gaps in high temperature oxides superconductors
We study models for superconductivity with two interactions: due to
antiferromagnetic(AF) fluctuations and due to phonons, in a weak coupling
approach to the high temperature superconductivity. The nature of the two
interactions are considerably different; is positive and sharply peaked
at (,) while is negative and peaked at () due to
weak phonon screening. We numerically find (a) weak BCS attraction is enough to
have high critical temperature if a van Hove anomaly is at work, (b) (AF)
is important to give d-wave superconductivity, (c) the gap order parameter
is constant(s-wave) at extremely overdope region and it
changes to anisotropic s-wave as doping is reduced, (d) there exists a first
order phase transition between d-wave and anisotropic s-wave gaps. These
results are qualitatively in agreement with preceding works; they should be
modified in the strongly underdope region by the presence of antiferromagnetic
fluctuations and ensuing AF pseudogap.Comment: 4 pages in RevTex (double column), 4 figure
Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes
We present a systematic study of the electronic and magnetic properties of
transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon
nanotubes (SWNTs). We considered the adsorption on the external and internal
wall of SWNT and examined the effect of the TM coverage and geometry on the
binding energy and the spin polarization at the Fermi level. All those adsorbed
chains studied have ferromagnetic ground state, but only their specific types
and geometries demonstrated high spin polarization near the Fermi level. Their
magnetic moment and binding energy in the ground state display interesting
variation with the number of electrons of the TM atom. We also show that
specific chains of transition metal atoms adsorbed on a SWNT can lead to
semiconducting properties for the minority spin-bands, but semimetallic for the
majority spin-bands. Spin-polarization is maintained even when the underlying
SWNT is subjected to high radial strain. Spin-dependent electronic structure
becomes discretized when TM atoms are adsorbed on finite segments of SWNTs.
Once coupled with non-magnetic metal electrodes, these magnetic needles or
nanomagnets can perform as spin-dependent resonant tunnelling devices. The
electronic and magnetic properties of these nanomagnets can be engineered
depending on the type and decoration of adsorbed TM atom as well as the size
and symmetry of the tube. Our study is performed by using first-principles
pseudopotential plane wave method within spin-polarized Density Functional
Method.Comment: 8 pages, 6 figures, without proof readin
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Melting properties of a simple tight-binding model of transition metals: I.The region of half-filled d-band
We present calculations of the free energy, and hence the melting properties,
of a simple tight-binding model for transition metals in the region of d-band
filling near the middle of a d-series, the parameters of the model being
designed to mimic molybdenum. The melting properties are calculated for
pressures ranging from ambient to several Mbar. The model is intended to be the
simplest possible tight-binding representation of the two basic parts of the
energy: first, the pairwise repulsion due to Fermi exclusion; and second, the
d-band bonding energy described in terms of an electronic density of states
that depends on structure. In addition to the number of d-electrons, the model
contains four parameters, which are adjusted to fit the pressure dependent
d-band width and the zero-temperature pressure-volume relation of Mo. We show
that the resulting model reproduces well the phonon dispersion relations of Mo
in the body-centred-cubic structure, as well as the radial distribution
function of the high-temperature solid and liquid given by earlier
first-principles simulations. Our free-energy calculations start from the free
energy of the liquid and solid phases of the purely repulsive pair-potential
model, without d-band bonding. The free energy of the full tight-binding model
is obtained from this by thermodynamic integration. The resulting melting
properties of the model are quite close to those given by earlier
first-principles work on Mo. An interpretation of these melting properties is
provided by showing how they are related to those of the purely repulsive
model.Comment: 34 pages, 12 figures. Accepted for publication in Journal of Chemical
Physic
Enhancement of superconductive critical temperatures in almost empty or full bands in two dimensions: possible relevance to beta-HfNCl, C60 and MgB2
We examine possibility of enhancement of superconductive critical temperature
in two-dimensions. The weak coupling BCS theory is applied, especially when the
Fermi level is near the edges of the electronic bands. The attractive
interaction depends on due to screening. The density of states(DOS)
does not have a peak near the bottom of the band, but -dependent
contribution to DOS (electron density on the Fermi surface) has a diverging
peak at the bottom or top. These features lead to significant enhancement of
the critical temperatures. The results are qualitatively consistent with the
superconductive behaviors of HfNCl (\Tc \le 25K) and ZrNCl(\Tc \le 15K),
C with a field-effect transistor configuration (\Tc = 52K), and
MgB (\Tc \approx 40K) which have the unexpectedly high superconductive
critical transition temperatures.Comment: 5 pages,4 figure
Electronic transport in AlMn(Si) and AlCuFe quasicrystals: Break-down of the semiclassical model
The semi-classical Bloch-Boltzmann theory is at the heart of our
understanding of conduction in solids, ranging from metals to semi-conductors.
Physical systems that are beyond the range of applicability of this theory are
thus of fundamental interest. It appears that in quasicrystals and related
complex metallic alloys, a new type of break-down of this theory operates. This
phenomenon is related to the specific propagation of electrons. We develop a
theory of quantum transport that applies to a normal ballistic law but also to
these specific diffusion laws. As we show phenomenological models based on this
theory describe correctly the anomalous conductivity in quasicrystals.
Ab-initio calculations performed on approximants confirm also the validity of
this anomalous quantum diffusion scheme. This provides us with an ab-initio
model of transport in approximants such as alpha-AlMnSi and AlCuFe 1/1 cubic
approximant.Comment: 11 pages, 5 figure
The Coulomb impurity problem in graphene
We address the problem of an unscreened Coulomb charge in graphene, and
calculate the local density of states and displaced charge as a function of
energy and distance from the impurity. This is done non-perturbatively in two
different ways: (1) solving the problem exactly by studying numerically the
tight-binding model on the lattice; (2) using the continuum description in
terms of the 2D Dirac equation. We show that the Dirac equation, when properly
regularized, provides a qualitative and quantitative low energy description of
the problem. The lattice solution shows extra features that cannot be described
by the Dirac equation, namely bound state formation and strong renormalization
of the van Hove singularities.Comment: 3 Figures; minor typo corrections and minor update in Fig. 3
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