194 research outputs found
Quasiparticle trapping in Meissner and vortex states of mesoscopic superconductors
Nowadays superconductors serve in numerous applications, from high-field
magnets to ultra-sensitive detectors of radiation. Mesoscopic superconducting
devices, i.e. those with nanoscale dimensions, are in a special position as
they are easily driven out of equilibrium under typical operating conditions.
The out-of-equilibrium superconductors are characterized by non-equilibrium
quasiparticles. These extra excitations can compromise the performance of
mesoscopic devices by introducing, e.g., leakage currents or decreased
coherence times in quantum devices. By applying an external magnetic field, one
can conveniently suppress or redistribute the population of excess
quasiparticles. In this article we present an experimental demonstration and a
theoretical analysis of such effective control of quasiparticles, resulting in
electron cooling both in the Meissner and vortex states of a mesoscopic
superconductor. We introduce a theoretical model of quasiparticle dynamics
which is in quantitative agreement with the experimental data
The congruence kernel of an arithmetic lattice in a rank one algebraic group over a local field
Let k be a global field and let k_v be the completion of k with respect to v,
a non-archimedean place of k. Let \mathbf{G} be a connected, simply-connected
algebraic group over k, which is absolutely almost simple of k_v-rank 1. Let
G=\mathbf{G}(k_v). Let \Gamma be an arithmetic lattice in G and let C=C(\Gamma)
be its congruence kernel. Lubotzky has shown that C is infinite, confirming an
earlier conjecture of Serre. Here we provide complete solution of the
congruence subgroup problem for \Gamm$ by determining the structure of C. It is
shown that C is a free profinite product, one of whose factors is
\hat{F}_{\omega}, the free profinite group on countably many generators. The
most surprising conclusion from our results is that the structure of C depends
only on the characteristic of k. The structure of C is already known for a
number of special cases. Perhaps the most important of these is the
(non-uniform) example \Gamma=SL_2(\mathcal{O}(S)), where \mathcal{O}(S) is the
ring of S-integers in k, with S=\{v\}, which plays a central role in the theory
of Drinfeld modules. The proof makes use of a decomposition theorem of
Lubotzky, arising from the action of \Gamma on the Bruhat-Tits tree associated
with G.Comment: 27 pages, 5 figures, to appear in J. Reine Angew. Mat
Anisotropy and effective dimensionality crossover of the fluctuation conductivity of hybrid superconductor/ferromagnet structures
We study the fluctuation conductivity of a superconducting film, which is
placed to perpendicular non-uniform magnetic field with the amplitude
induced by the ferromagnet with domain structure. The conductivity tensor is
shown to be essentially anisotropic. The magnitude of this anisotropy is
governed by the temperature and the typical width of magnetic domains . For
the difference between diagonal fluctuation
conductivity components along the domain walls and
across them has the order of . In the
opposite case for the fluctuation conductivity tensor reveals
effective dimensionality crossover from standard two-dimensional
behavior well above the critical temperature to the one-dimensional
one close to for or to the
dependence for . In the intermediate case
for a fixed temperature shift from the dependence
is shown to have a minimum at
while is a monotonically increasing function.Comment: 11 pages, 8 figure
Conductance of 1D quantum wires with anomalous electron-wavefunction localization
We study the statistics of the conductance through one-dimensional
disordered systems where electron wavefunctions decay spatially as for , being a constant. In
contrast to the conventional Anderson localization where and the conductance statistics is determined by a single
parameter: the mean free path, here we show that when the wave function is
anomalously localized () the full statistics of the conductance is
determined by the average and the power . Our theoretical
predictions are verified numerically by using a random hopping tight-binding
model at zero energy, where due to the presence of chiral symmetry in the
lattice there exists anomalous localization; this case corresponds to the
particular value . To test our theory for other values of
, we introduce a statistical model for the random hopping in the tight
binding Hamiltonian.Comment: 6 pages, 8 figures. Few changes in the presentation and references
updated. Published in PRB, Phys. Rev. B 85, 235450 (2012
Anderson localization of one-dimensional hybrid particles
We solve the Anderson localization problem on a two-leg ladder by the
Fokker-Planck equation approach. The solution is exact in the weak disorder
limit at a fixed inter-chain coupling. The study is motivated by progress in
investigating the hybrid particles such as cavity polaritons. This application
corresponds to parametrically different intra-chain hopping integrals (a "fast"
chain coupled to a "slow" chain). We show that the canonical
Dorokhov-Mello-Pereyra-Kumar (DMPK) equation is insufficient for this problem.
Indeed, the angular variables describing the eigenvectors of the transmission
matrix enter into an extended DMPK equation in a non-trivial way, being
entangled with the two transmission eigenvalues. This extended DMPK equation is
solved analytically and the two Lyapunov exponents are obtained as functions of
the parameters of the disordered ladder. The main result of the paper is that
near the resonance energy, where the dispersion curves of the two decoupled and
disorder-free chains intersect, the localization properties of the ladder are
dominated by those of the slow chain. Away from the resonance they are
dominated by the fast chain: a local excitation on the slow chain may travel a
distance of the order of the localization length of the fast chain.Comment: 31 pages, 13 figure
Theory of the Eigler-swith
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.Comment: 13 pages (including 2 figures in separate LaTeX-files with
ps-\specials), REVTEX 3.
Statistics of the One-Electron Current in a One-Dimensional Mesoscopic Ring at Arbitrary Magnetic Fields
The set of moments and the distribution function of the one-electron current
in a one-dimensional disordered ring with arbitrary magnetic flux are
calculated.Comment: 10 pages; Plain TeX; IFUM 448/FT; to appear in J. Stat. Phy
Localization length in Dorokhov's microscopic model of multichannel wires
We derive exact quantum expressions for the localization length for
weak disorder in two- and three chain tight-binding systems coupled by random
nearest-neighbour interchain hopping terms and including random energies of the
atomic sites. These quasi-1D systems are the two- and three channel versions of
Dorokhov's model of localization in a wire of periodically arranged atomic
chains. We find that for the considered systems with
, where is Thouless' quantum expression for the inverse
localization length in a single 1D Anderson chain, for weak disorder. The
inverse localization length is defined from the exponential decay of the
two-probe Landauer conductance, which is determined from an earlier transfer
matrix solution of the Schr\"{o}dinger equation in a Bloch basis. Our exact
expressions above differ qualitatively from Dorokhov's localization length
identified as the length scaling parameter in his scaling description of the
distribution of the participation ratio. For N=3 we also discuss the case where
the coupled chains are arranged on a strip rather than periodically on a tube.
From the transfer matrix treatment we also obtain reflection coefficients
matrices which allow us to find mean free paths and to discuss their relation
to localization lengths in the two- and three channel systems
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