108 research outputs found
Role of the impurity-potential range in disordered d-wave superconductors
We analyze how the range of disorder affects the localization properties of
quasiparticles in a two-dimensional d-wave superconductor within the standard
non-linear sigma-model approach to disordered systems. We show that for purely
long-range disorder, which only induces intra-node scattering processes, the
approach is free from the ambiguities which often beset the disordered
Dirac-fermion theories, and gives rise to a Wess-Zumino-Novikov-Witten action
leading to vanishing density of states and finite conductivities. We also study
the crossover induced by internode scattering due to a short range component of
the disorder, thus providing a coherent non-linear sigma-model description in
agreement with all the various findings of different approaches.Comment: 38 pages, 1 figur
Multiple Andreev reflections in a quantum dot coupled to superconducting leads: Effect of spin-orbit coupling
We study the out of equilibrium current through a multilevel quantum dot contacted to two superconducting leads and in the presence of Rashba and Dresselhaus spin-orbit couplings, in the regime of strong dot-lead coupling. The multiple Andreev reflection (MAR) subgap peaks in the current-voltage characteristics are found to be modified (but not suppressed) by the spin-orbit interaction in a way that it strongly depends on the shape of the dot confining potential. In a perfectly isotropic dot the MAR peaks are enhanced when the strength αR and αD of Rashba and Dresselhaus terms are equal. When the anisotropy of the dot confining potential increases, the dependence of the subgap structure on the spin-orbit angle decreases. Furthermore, when an in-plane magnetic field is applied to a strongly anisotropic dot, the peaks of the nonlinear conductance oscillate as a function of the magnetic-field angle and the location of the maxima and minima allows for a straightforward read-out of the spin-orbit angle $theta
Observation of Umklapp processes in non-crystalline materials
Umklapp processes are known to exist in cristalline materials, where they
control important properties such as thermal conductivity, heat capacity and
electrical conductivity. In this work we report the provocative observation of
Umklapp processes in a non-periodical system, namely liquid Lithium. The lack
of a well defined periodicity seems then not to prevent the existence of these
scattering processes mechanisms provided that the local order of the systems
i.e. the maxima of the static structure factor supply the equivalent of a
reciprocal lattice vector in the case of cristalline materials.Comment: 13 pages P
Critical temperature of non-interacting Bose gases on disordered lattices
For a non-interacting Bose gas on a lattice we compute the shift of the
critical temperature for condensation when random-bond and onsite disorder are
present. We evidence that the shift depends on the space dimensionality D and
the filling fraction f. For D -> infinity (infinite-range model), using results
from the theory of random matrices, we show that the shift of the critical
temperature is negative, depends on f, and vanishes only for large f. The
connections with analogous results obtained for the spherical model are
discussed. For D=3 we find that, for large f, the critical temperature Tc is
enhanced by disorder and that the relative shift does not sensibly depend on f;
at variance, for small f, Tc decreases in agreement with the results obtained
for a Bose gas in the continuum. We also provide numerical estimates for the
shift of the critical temperature due to disorder induced on a non-interacting
Bose gas by a bichromatic incommensurate potential.Comment: 18 pages, 8 figures; Fig. 8 improved adding results for another value
of q (q=830/1076
The evolution of vibrational excitations in glassy systems
The equations of the mode-coupling theory (MCT) for ideal liquid-glass
transitions are used for a discussion of the evolution of the
density-fluctuation spectra of glass-forming systems for frequencies within the
dynamical window between the band of high-frequency motion and the band of
low-frequency-structural-relaxation processes. It is shown that the strong
interaction between density fluctuations with microscopic wave length and the
arrested glass structure causes an anomalous-oscillation peak, which exhibits
the properties of the so-called boson peak. It produces an elastic modulus
which governs the hybridization of density fluctuations of mesoscopic wave
length with the boson-peak oscillations. This leads to the existence of
high-frequency sound with properties as found by X-ray-scattering spectroscopy
of glasses and glassy liquids. The results of the theory are demonstrated for a
model of the hard-sphere system. It is also derived that certain schematic MCT
models, whose spectra for the stiff-glass states can be expressed by elementary
formulas, provide reasonable approximations for the solutions of the general
MCT equations.Comment: 50 pages, 17 postscript files including 18 figures, to be published
in Phys. Rev.
Quantum-critical pairing with varying exponents
We analyse the onset temperature T_p for the pairing in cuprate
superconductors at small doping, when tendency towards antiferromagnetism is
strong. We consider the model of Moon and Sachdev (MS), which assumes that
electron and hole pockets survive in a paramagnetic phase. Within this model,
the pairing between fermions is mediated by a gauge boson, whose propagator
remains massless in a paramagnet. We relate the MS model to a generic
\gamma-model of quantum-critical pairing with the pairing kernel \lambda
(\Omega) \propto 1/\Omega^{\gamma}. We show that, over some range of
parameters, the MS model is equivalent to the \gamma-model with \gamma =1/3
(\lambda (\Omega) \propto \Omega^{-1/3}). We find, however, that the parameter
range where this analogy works is bounded on both ends. At larger deviations
from a magnetic phase, the MS model becomes equivalent to the \gamma-model with
varying \gamma >1/3, whose value depends on the distance to a magnetic
transition and approaches \gamma =1 deep in a paramagnetic phase. Very near the
transition, the MS model becomes equivalent to the \gamma-model with varying
\gamma <1/3. Right at the magnetic QCP, the MS model is equivalent to the
\gamma-model with \gamma =0+ (\lambda (\Omega) \propto \log \Omega), which is
the model for color superconductivity. Using this analogy, we verified the
formula for T_c derived for color superconductivity.Comment: 10 pages, 8 figures, submitted to JLTP for a focused issue on Quantum
Phase Transition
Fermi surface instabilities at finite Temperature
We present a new method to detect Fermi surface instabilities for interacting
systems at finite temperature. We first apply it to a list of cases studied
previously, recovering already known results in a very economic way, and
obtaining most of the information on the phase diagram analytically. As an
example, in the continuum limit we obtain the critical temperature as an
implicit function of the magnetic field and the chemical potential
. By applying the method to a model proposed to describe reentrant
behavior in , we reproduce the phase diagram obtained
experimentally and show the presence of a non-Fermi Liquid region at
temperatures above the nematic phase.Comment: 10 pages, 10 figure
A Green's function approach to transmission of massless Dirac fermions in graphene through an array of random scatterers
We consider the transmission of massless Dirac fermions through an array of
short range scatterers which are modeled as randomly positioned -
function like potentials along the x-axis. We particularly discuss the
interplay between disorder-induced localization that is the hallmark of a
non-relativistic system and two important properties of such massless Dirac
fermions, namely, complete transmission at normal incidence and periodic
dependence of transmission coefficient on the strength of the barrier that
leads to a periodic resonant transmission. This leads to two different types of
conductance behavior as a function of the system size at the resonant and the
off-resonance strengths of the delta function potential. We explain this
behavior of the conductance in terms of the transmission through a pair of such
barriers using a Green's function based approach. The method helps to
understand such disordered transport in terms of well known optical phenomena
such as Fabry Perot resonances.Comment: 22 double spaced single column pages. 15 .eps figure
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
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