1,121 research outputs found
Characteristic energies, transition temperatures, and switching effects in clean SNS graphene nanostructures
We study proximity effects in clean nanoscale superconductor-normal
metal-superconductor (SNS) graphene heterostructures using a
self-consistent numerical solution to the continuum Dirac Bogoliubov-de Gennes
(DBdG) equations. We obtain results for the pair amplitude and the local
density of states (DOS), as a function of doping and of the geometrical
parameters determining the width of the structures. The superconducting
correlations are found to penetrate the normal graphene layers even when there
is extreme mismatch in the normal and superconducting doping levels, where
specular Andreev reflection dominates. The local DOS exhibits peculiar
features, which we discuss, arising from the Dirac cone dispersion relation and
from the interplay between the superconducting and Thouless energy scales. The
corresponding characteristic energies emerge in the form of resonant peaks in
the local DOS, that depend strongly on the doping level, as does the energy
gap, which declines sharply as the relative difference in doping between the S
and N regions is reduced. We also linearize the DBdG equations and develop an
essentially analytical method that determines the critical temperature of
an \sns nanostructure self-consistently. We find that for S regions that occupy
a fraction of the coherence length, can undergo substantial variations as
a function of the relative doping. At finite temperatures and by manipulating
the doping levels, the self consistent pair amplitudes reveal dramatic
transitions between a superconducting and resistive normal state of the
structure. Such behavior suggests the possibility of using the proposed system
as a carbon-based superconducting switch, turning superconductivity on or off
by tuning the relative doping levels.Comment: 13 pages, figures include
Angular Dependence of the Superconducting Transition Temperature in Ferromagnet-Superconductor-Ferromagnet Trilayers
The superconducting transition temperature, , of a ferromagnet (F) -
superconductor (S) - ferromagnet trilayer depends on the mutual orientation of
the magnetic moments of the F layers. This effect has been previously observed
in F/S/F systems as a difference between parallel and antiparallel
configurations of the F layers. Here we report measurements of in
CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments
of the CuNi ferromagnets. The observed angular dependence of is in
qualitative agreement with a F/S proximity theory that accounts for the odd
triplet component of the condensate predicted to arise for non-collinear
orientation of the magnetic moments of the F layers.Comment: 4 + \epsilon pages including 4 figures. To appear in Phys. Rev. Let
Micro Black Holes and the Democratic Transition
Unitarity implies that the evaporation of microscopic quasi-classical black
holes cannot be universal in different particle species. This creates a puzzle,
since it conflicts with the thermal nature of quasi-classical black holes,
according to which all the species should see the same horizon and be produced
with the same Hawking temperatures. We resolve this puzzle by showing that for
the microscopic black holes, on top the usual quantum evaporation time, there
is a new time-scale which characterizes a purely classical process during which
the black hole looses the ability to differentiate among the species, and
becomes democratic. We demonstrate this phenomenon in a well-understood
framework of large extra dimensions, with a number of parallel branes. An
initially non-democratic black hole is the one localized on one of the branes,
with its high-dimensional Schwarzschild radius being much shorter than the
interbrane distance. Such a black hole seemingly cannot evaporate into the
species localized on the other branes, that are beyond its reach. We
demonstrate that in reality the system evolves classically in time, in such a
way that the black hole accretes the neighboring branes. The end result is a
completely democratic static configuration, in which all the branes share the
same black hole, and all the species are produced with the same Hawking
temperature. Thus, just like their macroscopic counterparts, the microscopic
black holes are universal bridges to the hidden sector physics.Comment: 35 pages, 5 figure
Superconducting proximity effects in metals with a repulsive pairing interaction
Studies of the superconducting proximity effect in normal
conductor/superconductor junctions almost universally assume no
effective electron-electron coupling in the region. While such an
approximation leads to a simple description of the proximity effect, it is
unclear how it could be rigorously justified. We reveal a much more complex
picture of the proximity effect in bilayers, where is a clean s-wave
BCS superconductor and is a simple metal with a repulsive effective
electron coupling. We elucidate the proximity effect behavior using a highly
accurate method to self-consistently solve the Bogoliubov-deGennes equations.
We present our results for a wide range of values of the interface scattering,
the Fermi wave vector mismatch, the temperature, and the ratio of the
effective interaction strengths in the and region. We find that the
repulsive interaction, represented by a negative , strongly alters the
signatures of the proximity effect as can be seen in the spatial dependence of
the Cooper pair amplitude and the pair potential, as well as in the local
density of states near the interface.Comment: 12 pages, including 10 figures. To appear in Phys. Rev.
Melting and structure of the vortex solid in strongly anisotropic layered superconductors with random columnar pins
We study the melting transition of the low-temperature vortex solid in
strongly anisotropic layered superconductors with a concentration of random
columnar pinning centers small enough so that the areal density of the pins is
much less than that of the vortex lines. Both the external magnetic field and
the columnar pins are assumed to be oriented perpendicular to the layers Our
method, involving numerical minimization of a model free energy functional,
yields not only the free energy values at the local minima of the functional
but also the detailed density distribution of the system at each minimum: this
allows us to study in detail the structure of the different phases. We find
that at these pin concentrations and low temperatures, the thermodynamically
stable state is a topologically ordered Bragg glass. This nearly crystalline
state melts into an interstitial liquid (a liquid in which a small fraction of
vortex lines remain localized at the pinning centers) in two steps, so that the
Bragg glass and the liquid are separated by a narrow phase that we identify
from analysis of its density structure as a polycrystalline Bose glass. Both
the Bragg glass to Bose glass and the Bose glass to interstitial liquid
transitions are first-order. We also find that a local melting temperature
defined using a criterion based on the degree of localization of the vortex
lines exhibits spatial variations similar to those observed in recent
experiments.Comment: 17 page
Proximity effects and triplet correlations in Ferromagnet/Ferromagnet/Superconductor nanostructures
We report the results of a study of superconducting proximity effects in
clean Ferromagnet/Ferromagnet/Superconductor ()
heterostructures, where the pairing state in S is a conventional singlet
s-wave. We numerically find the self-consistent solutions of the Bogoliubov-de
Gennes (BdG) equations and use these solutions to calculate the relevant
physical quantities. By linearizing the BdG equations, we obtain the
superconducting transition temperatures as a function of the angle
between the exchange fields in and . We find that
the results for in systems are clearly different
from those in systems, where monotonically increases
with and is highest for antiparallel magnetizations. Here,
is in general a non-monotonic function, and often has a minimum
near . For certain values of the exchange field and
layer thicknesses, the system exhibits reentrant superconductivity with
: it transitions from superconducting to normal, and then returns to a
superconducting state again with increasing . This phenomenon is
substantiated by a calculation of the condensation energy. We compute, in
addition to the ordinary singlet pair amplitude, the induced odd triplet
pairing amplitudes. The results indicate a connection between equal-spin
triplet pairing and the singlet pairing state that characterizes . We find
also that the induced triplet amplitudes can be very long-ranged in both the S
and F sides and characterize their range. We discuss the average density of
states for both the magnetic and the S regions, and its relation to the pairing
amplitudes and . The local magnetization vector, which exhibits reverse
proximity effects, is also investigated.Comment: 14 pages including 11 figure
Adapting integrity checking techniques for concurrent operation executions
One challenge for achieving executable models is preserving the integrity of the data. That is, given a structural model describing the constraints that the data should satisfy, and a behavioral model describing the operations that might change the data, the integrity checking problem consists in ensuring that, after executing the modeled operations, none of the specified constraints is violated. A multitude of techniques have been presented so far to solve the integrity checking problem. However, to the best of our knowledge, all of them assume that operations are not executed concurrently. As we are going to see, concurrent operation executions might lead to violations not detected by these techniques. In this paper, we present a technique for detecting and serializing those operations that can cause a constraint violation when executed concurrently , so that, previous incremental techniques, exploiting our approach, can be safely applied in systems with concurrent operation executions guaranteeing the integrity of the data.Peer ReviewedPostprint (author's final draft
Distributions of Conductance and Shot Noise and Associated Phase Transitions
For a chaotic cavity with two indentical leads each supporting N channels, we
compute analytically, for large N, the full distribution of the conductance and
the shot noise power and show that in both cases there is a central Gaussian
region flanked on both sides by non-Gaussian tails. The distribution is weakly
singular at the junction of Gaussian and non-Gaussian regimes, a direct
consequence of two phase transitions in an associated Coulomb gas problem.Comment: 5 pages, 3 figures include
Tunneling conductance in Superconductor/Ferromagnet junctions: a self consistent approach
We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor
junctions via a fully self-consistent numerical solution of the microscopic
Bogoliubov-DeGennes equations. We present results for a relevant range of
values of the Fermi wavevector mismatch (FWM), the spin polarization, and the
interfacial scattering strength. For nonzero spin polarization, the conductance
curves vary nonmonotonically with FWM. The FWM dependence of the
self-consistent results is stronger than that previously found in
non-self-consistent calculations, since, in the self-consistent case, the
effective scattering potential near the interface depends on the FWM. The
dependence on interfacial scattering is monotonic. These results confirm that
it is impossible to characterize both the the FWM and the interfacial
scattering by a single effective parameter and that analysis of experimental
data via the use of such one-parameter models is unreliable.Comment: 12 pages, including 8 figure
Structure and kinematics of the Ayora-Cofrentes Diapir (eastern Betics). Role of basement faulting in the salt and suprasalt deformation of the Mesozoic cover
The Ayora-Cofrentes Diapir is a 34 km long N-trending salt wall cutting the Valencian Domain that is made by Middle to Upper Triassic salt. On both sides, it is flanked by parallel half grabens offsetting the subtabular strata of the Jurassic to Cretaceous suprasalt carbonate successions. Based on detailed geological mapping and cross-sections, one of them supported by a new magnetotelluric profile, this study analyses the structure and kinematics of this salt wall with the purpose of establishing the role played by the subsalt structure in its development; and, thus, help in the interpretation of diapirs currently incorporated in fold and thrust belts. In this regard, the study evidences the presence of a subsalt basement fault (the Ayora Fault), active as extensional during the Early Jurassic, and reactivated with a reverse throw sometime between the latest Cretaceous and middle Miocene. Both motions are older than the salt wall growth (middle-late Miocene). This strongly suggest that the Ayora-Cofrentes Diapir was not triggered by the vertical motion of the underlying Ayora basement fault but by thin-skinned processes for which this pre-existing basement fault appears to have played a crucial role as salt/suprasal strain localizer
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