16,504 research outputs found
Magnetic susceptibility of ultra-small superconductor grains
For assemblies of superconductor nanograins, the magnetic response is
analyzed as a function of both temperature and magnetic field. In order to
describe the interaction energy of electron pairs for a huge number of
many-particle states, involved in calculations, we develop a simple
approximation, based on the Richardson solution for the reduced BCS Hamiltonian
and applicable over a wide range of the grain sizes and interaction strengths
at arbitrary distributions of single-electron energy levels in a grain. Our
study is focused upon ultra-small grains, where both the mean value of the
nearest-neighbor spacing of single-electron energy levels in a grain and
variations of this spacing from grain to grain significantly exceed the
superconducting gap in bulk samples of the same material. For these ultra-small
superconductor grains, the overall profiles of the magnetic susceptibility as a
function of magnetic field and temperature are demonstrated to be qualitatively
different from those for normal grains. We show that the analyzed signatures of
pairing correlations are sufficiently stable with respect to variations of the
average value of the grain size and its dispersion over an assembly of
nanograins. The presence of these signatures does not depend on a particular
choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses:
[email protected], [email protected], [email protected]
Quantum-to-classical crossover for Andreev billiards in a magnetic field
We extend the existing quasiclassical theory for the superconducting
proximity effect in a chaotic quantum dot, to include a time-reversal-symmetry
breaking magnetic field. Random-matrix theory (RMT) breaks down once the
Ehrenfest time becomes longer than the mean time between
Andreev reflections. As a consequence, the critical field at which the
excitation gap closes drops below the RMT prediction as is
increased. Our quasiclassical results are supported by comparison with a fully
quantum mechanical simulation of a stroboscopic model (the Andreev kicked
rotator).Comment: 11 pages, 10 figure
Coexistence of superconductivity and ferromagnetism in two dimensions
Ferromagnetism is usually considered to be incompatible with conventional
superconductivity, as it destroys the singlet correlations responsible for the
pairing interaction. Superconductivity and ferromagnetism are known to coexist
in only a few bulk rare-earth materials. Here we report evidence for their
coexistence in a two-dimensional system: the interface between two bulk
insulators, LaAlO (LAO) and SrTiO (STO), a system that has been studied
intensively recently. Magnetoresistance, Hall and electric-field dependence
measurements suggest that there are two distinct bands of charge carriers that
contribute to the interface conductivity. The sensitivity of properties of the
interface to an electric field make this a fascinating system for the study of
the interplay between superconductivity and magnetism.Comment: 4 pages, 4 figure
Monomial integrals on the classical groups
This paper presents a powerfull method to integrate general monomials on the
classical groups with respect to their invariant (Haar) measure. The method has
first been applied to the orthogonal group in [J. Math. Phys. 43, 3342 (2002)],
and is here used to obtain similar integration formulas for the unitary and the
unitary symplectic group. The integration formulas turn out to be of similar
form. They are all recursive, where the recursion parameter is the number of
column (row) vectors from which the elements in the monomial are taken. This is
an important difference to other integration methods. The integration formulas
are easily implemented in a computer algebra environment, which allows to
obtain analytical expressions very efficiently. Those expressions contain the
matrix dimension as a free parameter.Comment: 16 page
Neuropilins 1 and 2 mediate neointimal hyperplasia and re-endothelialization following arterial injury
AIMS: Neuropilins 1 and 2 (NRP1 and NRP2) play crucial roles in endothelial cell migration contributing to angiogenesis and vascular development. Both NRPs are also expressed by cultured vascular smooth muscle cells (VSMCs) and are implicated in VSMC migration stimulated by PDGF-BB, but it is unknown whether NRPs are relevant for VSMC function in vivo. We investigated the role of NRPs in the rat carotid balloon injury model, in which endothelial denudation and arterial stretch induce neointimal hyperplasia involving VSMC migration and proliferation. METHODS AND RESULTS: NRP1 and NRP2 mRNAs and proteins increased significantly following arterial injury, and immunofluorescent staining revealed neointimal NRP expression. Down-regulation of NRP1 and NRP2 using shRNA significantly reduced neointimal hyperplasia following injury. Furthermore, inhibition of NRP1 by adenovirally overexpressing a loss-of-function NRP1 mutant lacking the cytoplasmic domain (ΔC) reduced neointimal hyperplasia, whereas wild-type (WT) NRP1 had no effect. NRP-targeted shRNAs impaired, while overexpression of NRP1 WT and NRP1 ΔC enhanced, arterial re-endothelialization 14 days after injury. Knockdown of either NRP1 or NRP2 inhibited PDGF-BB-induced rat VSMC migration, whereas knockdown of NRP2, but not NRP1, reduced proliferation of cultured rat VSMC and neointimal VSMC in vivo. NRP knockdown also reduced the phosphorylation of PDGFα and PDGFβ receptors in rat VSMC, which mediate VSMC migration and proliferation. CONCLUSION: NRP1 and NRP2 play important roles in the regulation of neointimal hyperplasia in vivo by modulating VSMC migration (via NRP1 and NRP2) and proliferation (via NRP2), independently of the role of NRPs in re-endothelialization
Ballistic Electron Quantum Transport in Presence of a Disordered Background
Effect of a complicated many-body environment is analyzed on the electron
random scattering by a 2D mesoscopic open ballistic structure. A new mechanism
of decoherence is proposed. The temperature of the environment is supposed to
be zero whereas the energy of the incoming particle can be close to or
somewhat above the Fermi surface in the environment. The single-particle
doorway resonance states excited in the structure via external channels are
damped not only because of escape through such channels but also due to the
ulterior population of the long-lived environmental states. Transmission of an
electron with a given incoming through the structure turns out to be
an incoherent sum of the flow formed by the interfering damped doorway
resonances and the retarded flow of the particles re-emitted into the structure
by the environment. Though the number of the particles is conserved in each
individual event of transmission, there exists a probability that some part of
the electron's energy can be absorbed due to environmental many-body effects.
In such a case the electron can disappear from the resonance energy interval
and elude observation at the fixed transmission energy thus resulting
in seeming loss of particles, violation of the time reversal symmetry and, as a
consequence, suppression of the weak localization. The both decoherence and
absorption phenomena are treated within the framework of a unit microscopic
model based on the general theory of the resonance scattering. All the effects
discussed are controlled by the only parameter: the spreading width of the
doorway resonances, that uniquely determines the decoherence rateComment: 7 pages, 1 figure. The published version. A figure has been added;
the list of references has been improved. Some explanatory remarks have been
include
2*2 random matrix ensembles with reduced symmetry: From Hermitian to PT-symmetric matrices
A possibly fruitful extension of conventional random matrix ensembles is
proposed by imposing symmetry constraints on conventional Hermitian matrices or
parity-time- (PT-) symmetric matrices. To illustrate the main idea, we first
study 2*2 complex Hermitian matrix ensembles with O(2) invariant constraints,
yielding novel level-spacing statistics such as singular distributions,
half-Gaussian distribution, distributions interpolating between GOE (Gaussian
Orthogonal Ensemble) distribution and half Gaussian distributions, as well as
gapped-GOE distribution. Such a symmetry-reduction strategy is then used to
explore 2*2 PT-symmetric matrix ensembles with real eigenvalues. In particular,
PT-symmetric random matrix ensembles with U(2) invariance can be constructed,
with the conventional complex Hermitian random matrix ensemble being a special
case. In two examples of PT-symmetric random matrix ensembles, the
level-spacing distributions are found to be the standard GUE (Gaussian Unitary
Ensemble) statistics or "truncated-GUE" statistics
Ehrenfest-time dependence of counting statistics for chaotic ballistic systems
Transport properties of open chaotic ballistic systems and their statistics
can be expressed in terms of the scattering matrix connecting incoming and
outgoing wavefunctions. Here we calculate the dependence of correlation
functions of arbitrarily many pairs of scattering matrices at different
energies on the Ehrenfest time using trajectory based semiclassical methods.
This enables us to verify the prediction from effective random matrix theory
that one part of the correlation function obtains an exponential damping
depending on the Ehrenfest time, while also allowing us to obtain the
additional contribution which arises from bands of always correlated
trajectories. The resulting Ehrenfest-time dependence, responsible e.g. for
secondary gaps in the density of states of Andreev billiards, can also be seen
to have strong effects on other transport quantities like the distribution of
delay times.Comment: Refereed version. 15 pages, 14 figure
Superconductor-proximity effect in chaotic and integrable billiards
We explore the effects of the proximity to a superconductor on the level
density of a billiard for the two extreme cases that the classical motion in
the billiard is chaotic or integrable. In zero magnetic field and for a uniform
phase in the superconductor, a chaotic billiard has an excitation gap equal to
the Thouless energy. In contrast, an integrable (rectangular or circular)
billiard has a reduced density of states near the Fermi level, but no gap. We
present numerical calculations for both cases in support of our analytical
results. For the chaotic case, we calculate how the gap closes as a function of
magnetic field or phase difference.Comment: 4 pages, RevTeX, 2 Encapsulated Postscript figures. To be published
by Physica Scripta in the proceedings of the "17th Nordic Semiconductor
Meeting", held in Trondheim, June 199
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