210 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
Generalized Darboux transformations for the KP equation with self-consistent sources
The KP equation with self-consistent sources (KPESCS) is treated in the
framework of the constrained KP equation. This offers a natural way to obtain
the Lax representation for the KPESCS. Based on the conjugate Lax pairs, we
construct the generalized binary Darboux transformation with arbitrary
functions in time for the KPESCS which, in contrast with the binary Darboux
transformation of the KP equation, provides a non-auto-B\"{a}cklund
transformation between two KPESCSs with different degrees. The formula for
N-times repeated generalized binary Darboux transformation is proposed and
enables us to find the N-soliton solution and lump solution as well as some
other solutions of the KPESCS.Comment: 20 pages, no figure
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
The Solutions of the NLS Equations with Self-Consistent Sources
We construct the generalized Darboux transformation with arbitrary functions
in time for the AKNS equation with self-consistent sources (AKNSESCS)
which, in contrast with the Darboux transformation for the AKNS equation,
provides a non-auto-B\"{a}cklund transformation between two AKNSESCSs with
different degrees of sources. The formula for N-times repeated generalized
Darboux transformation is proposed. By reduction the generalized Darboux
transformation with arbitrary functions in time for the Nonlinear
Schr\"{o}dinger equation with self-consistent sources (NLSESCS) is obtained and
enables us to find the dark soliton, bright soliton and positon solutions for
NLSESCS and NLSESCS. The properties of these solution are analyzed.Comment: 24 pages, 3 figures, to appear in Journal of Physics A: Mathematical
and Genera
Inverse proximity effect in semiconductor Majorana nanowires
We study the influence of the inverse proximity effect on the
superconductivity nucleation in hybrid structures consisting of the
semiconducting nanowires placed in contact with a thin superconducting film and
discuss the resulting restrictions on the operation of Majorana-based devices.
A strong paramagnetic effect for electrons entering the semiconductor together
with spin-orbit coupling and van Hove singularities in the electronic density
of states in the wire are responsible for the suppression of superconducting
correlations in the low field domain and for the reentrant superconductivity at
high magnetic fields in the topologically nontrivial regime. The growth of the
critical temperature in the latter case continues up to the upper critical
field destroying the pairing inside the superconducting film due to either
orbital or paramagnetic mechanism. The suppression of the homogeneous
superconducting state near the boundary between the topological and
non-topological regimes provides the conditions favorable for the
Fulde-Ferrel-Larkin-Ovchinnikov instability.Comment: 19 pages, 4 figures, 31 references, accepted to Beilstein Journal of
Nanotechnology. Thematic Series on "Topological Materials
Disorder-induced trapping and anti-trapping of vortices in type-II superconductors
We study the features of the superconductivity nucleation and vortex
configurations in superconductors with modulated disorder. Using the
Ginzburg-Landau-type theory with spatially varying diffusion coefficient, we
uncover and explain the switching between the vortex-defect attraction to the
repulsion upon the increase in the external magnetic field. It is shown that
for rather weak applied magnetic fields, a superconducting nucleus localized
near the region with the suppressed diffusion coefficient possesses a nonzero
vorticity whereas the increase in the magnetic field can lead to a transition
into the state with zero winding number. We demonstrate the manifestations of
this switching phenomenon in superconductors with a large number of defects by
performing numerical simulations of the vortex structures in superconductors
with periodic spatial profiles of the diffusion coefficient. The obtained
results clarify the physics of the vortex arrangement in several classes of the
superconducting materials including one-dimensional superlattices and
nanopatterned superconductors with regular arrays of the defects characterized
by the increased concentration of nonmagnetic impurities.Comment: 13 pages, 9 figure
Giant oscillations of energy levels in mesoscopic superconductors
The interplay of geometrical and Andreev quantization in mesoscopic
superconductors leads to giant mesoscopic oscillations of energy levels as
functions of the Fermi momentum and/or sample size. Quantization rules are
formulated for closed quasiparticle trajectories in the presence of normal
scattering at the sample boundaries. Two generic examples of mesoscopic systems
are studied: (i) one dimensional Andreev states in a quantum box, (ii) a single
vortex in a mesoscopic cylinder.Comment: 4 pages, 3 figure
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