3,739 research outputs found
Selective transmission of Dirac electrons and ballistic magnetoresistance of \textit{n-p} junctions in graphene
We show that an electrostatically created n-p junction separating the
electron and hole gas regions in a graphene monolayer transmits only those
quasiparticles that approach it almost perpendicularly to the n-p interface.
Such a selective transmission of carriers by a single n-p junction would
manifest itself in non-local magnetoresistance effect in arrays of such
junctions and determines the unusual Fano factor in the current noise universal
for the n-p junctions in graphene.Comment: 4 pages, 2 fig
Two mesoscopic models of two interacting electrons
We study two simple mesoscopic models of interacting two electrons; first one
consists of two quantum coherent parallel conductors with long-range Coulomb
interaction in some localized region and the other is of an interacting quantum
dot (QD) side-coupled to a noninteracting quantum wire. We evaluate exact
two-particle scattering matrix as well as two-particle current which are
relevant in a two-particle scattering experiment in these models. Finally we
show that the on-site repulsive interaction in the QD filters out the
spin-singlet two-electron state from the mixed two-electron input states in the
side-coupled QD model.Comment: 8 pages, 5 figures, revised version, to appear in Phys. Rev.
Temperature- and quantum phonon effects on Holstein-Hubbard bipolarons
The one-dimensional Holstein-Hubbard model with two electrons of opposite
spin is studied using an extension of a recently developed quantum Monte Carlo
method, and a very simple yet rewarding variational approach, both based on a
canonically transformed Hamiltonian. The quantum Monte Carlo method yields very
accurate results in the regime of small but finite phonon frequencies,
characteristic of many strongly correlated materials such as, e.g., the
cuprates and the manganites. The influence of electron-electron repulsion,
phonon frequency and temperature on the bipolaron state is investigated.
Thermal dissociation of the intersite bipolaron is observed at high
temperatures, and its relation to an existing theory of the manganites is
discussed.Comment: 12 pages, 7 figures; final version, accepted for publication in Phys.
Rev.
Statistics of transmission in one-dimensional disordered systems: universal characteristics of states in the fluctuation tails
We numerically study the distribution function of the conductance
(transmission) in the one-dimensional tight-binding Anderson and
periodic-on-average superlattice models in the region of fluctuation states
where single parameter scaling is not valid. We show that the scaling
properties of the distribution function depend upon the relation between the
system's length and the length determined by the integral density of
states. For long enough systems, , the distribution can still be
described within a new scaling approach based upon the ratio of the
localization length and . In an intermediate interval of the
system's length , , the variance of the Lyapunov
exponent does not follow the predictions of the central limit theorem and this
scaling becomes invalid.Comment: 22 pages, 12 eps figure
Generalized Lyapunov Exponent and Transmission Statistics in One-dimensional Gaussian Correlated Potentials
Distribution of the transmission coefficient T of a long system with a
correlated Gaussian disorder is studied analytically and numerically in terms
of the generalized Lyapunov exponent (LE) and the cumulants of lnT. The effect
of the disorder correlations on these quantities is considered in weak,
moderate and strong disorder for different models of correlation. Scaling
relations between the cumulants of lnT are obtained. The cumulants are treated
analytically within the semiclassical approximation in strong disorder, and
numerically for an arbitrary strength of the disorder. A small correlation
scale approximation is developed for calculation of the generalized LE in a
general correlated disorder. An essential effect of the disorder correlations
on the transmission statistics is found. In particular, obtained relations
between the cumulants and between them and the generalized LE show that, beyond
weak disorder, transmission fluctuations and deviation of their distribution
from the log-normal form (in a long but finite system) are greatly enhanced due
to the disorder correlations. Parametric dependence of these effects upon the
correlation scale is presented.Comment: 18 pages, 11 figure
Nonrelativistic Quantum Analysis of the Charged Particle-Dyon System on a Conical Spacetime
In this paper we develop the nonrelativistic quantum analysis of the charged
particle-dyon system in the spacetime produced by an idealized cosmic string.
In order to do that, we assume that the dyon is superposed to the cosmic
string. Considering this peculiar configuration {\it conical} monopole
harmonics are constructed, which are a generalizations of previous monopole
harmonics obtained by Wu and Yang(1976 {\it Nucl. Phys. B} {\bf 107} 365)
defined on a conical three-geometry. Bound and scattering wave functions are
explicitly derived. As to bound states, we present the energy spectrum of the
system, and analyze how the presence of the topological defect modifies
obtained result. We also analyze this system admitting the presence of an extra
isotropic harmonic potential acting on the particle. We show that the presence
of this potential produces significant changes in the energy spectrum of the
system.Comment: Paper accepted for publication in Classical and Quantum Gravit
Phase transitions in simplified models with long-range interactions
We study the origin of phase transitions in some simplified models with long
range interactions. For the ring model, we show that a possible new phase
transition predicted in a recent paper by Nardini and Casetti from an energy
landscape analysis does not occur. Instead of such phase transitions we observe
a sharp, although without any non-analiticity, change from a core-halo to an
only core configuration in the spatial distribution functions for low energies.
By introducing a new class of solvable simplified models without any critical
points in the potential energy, we show that a similar behaviour to the ring
model is obtained, with a first order phase transition from an almost
homogeneous high energy phase to a clustered phase, and the same core-halo to
core configuration transition at lower energies. We discuss the origin of these
features of the simplified models, and show that the first order phase
transition comes from the maximization of the entropy of the system as a
function of energy an an order parameter, as previously discussed by Kastner,
which seems to be the main mechanism causing phase transitions in long-range
interacting systems
Theory of the Diamagnetism Above the Critical Temperature for Cuprates
Recently experiments on high critical temperature superconductors has shown
that the doping levels and the superconducting gap are usually not uniform
properties but strongly dependent on their positions inside a given sample.
Local superconducting regions develop at the pseudogap temperature () and
upon cooling, grow continuously. As one of the consequences a large diamagnetic
signal above the critical temperature () has been measured by different
groups. Here we apply a critical-state model for the magnetic response to the
local superconducting domains between and and show that the
resulting diamagnetic signal is in agreement with the experimental results.Comment: published versio
Policy and Institutional Support for CA Development (Examples from Europe, Africa, Asia)
Policy and Institutional Support for CA Development (Examples from Europe, Africa, Asia
Random-Matrix Theory of Electron Transport in Disordered Wires with Symplectic Symmetry
The conductance of disordered wires with symplectic symmetry is studied by a
random-matrix approach. It has been believed that Anderson localization
inevitably arises in ordinary disordered wires. A counterexample is recently
found in the systems with symplectic symmetry, where one perfectly conducting
channel is present even in the long-wire limit when the number of conducting
channels is odd. This indicates that the odd-channel case is essentially
different from the ordinary even-channel case. To study such differences, we
derive the DMPK equation for transmission eigenvalues for both the even- and
odd- channel cases. The behavior of dimensionless conductance is investigated
on the basis of the resulting equation. In the short-wire regime, we find that
the weak-antilocalization correction to the conductance in the odd-channel case
is equivalent to that in the even-channel case. We also find that the variance
does not depend on whether the number of channels is even or odd. In the
long-wire regime, it is shown that the dimensionless conductance in the
even-channel case decays exponentially as --> 0 with increasing system
length, while --> 1 in the odd-channel case. We evaluate the decay
length for the even- and odd-channel cases and find a clear even-odd
difference. These results indicate that the perfectly conducting channel
induces clear even-odd differences in the long-wire regime.Comment: 28pages, 5figures, Accepted for publication in J. Phys. Soc. Jp
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