6,539 research outputs found
Comment on the anisotropic impurity scattering in superconductors
We discuss a case of a strong anisotropic impurity scattering within the
model introduced in our previous paper [Phys. Rev. B54, 15463 (1996)] and
clarify our former statement about a possible enhancement of the critical
temperature in this scattering regime. We show that for the anisotropy of the
impurity potential determined by the functions from the non-identity
irreducible representations of the crystal point group, the critical
temperature decreases with the impurity scattering rate which is consistent
with a generally understood role of disorder in superconductors.Comment: 6 pages, RevTe
Maximum screening fields of superconducting multilayer structures
It is shown that a multilayer comprised of alternating thin superconducting
and insulating layers on a thick substrate can fully screen the applied
magnetic field exceeding the superheating fields of both the
superconducting layers and the substrate, the maximum Meissner field is
achieved at an optimum multilayer thickness. For instance, a dirty layer of
thickness m at the Nb surface could increase mT
of a clean Nb up to mT. Optimized multilayers of NbSn, NbN,
some of the iron pnictides, or alloyed Nb deposited onto the surface of the Nb
resonator cavities could potentially double the rf breakdown field, pushing the
peak accelerating electric fields above 100 MV/m while protecting the cavity
from dendritic thermomagnetic avalanches caused by local penetration of
vortices
Pair-breaking in iron-pnictides
The puzzling features of the slopes of the upper critical field at the
critical temperature , , and of the
specific heat jump of iron-pnictides are interpreted as
caused by a strong pair-breaking
Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os{1-x}Ru{x})4Sb12
In the skutterudite compounds the anharmonic 'rattling' oscillations of
4f-host ions in the surrounding Sb12 cages are found to have significant
influence on the low temperature properties. Recently specific heat analysis of
Pr(Os{1-x}Ru{x})4Sb12 has shown that the energy of crystalline electric field
(CEF) singlet-triplet excitations increases strongly with Ru-concentration x
and crosses the almost constant rattling mode frequency at about x ~
0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic
effects in inelastic neutron scattering (INS) intensity and quadrupolar
susceptibility. Furthermore the Ru- concentration dependence of the
superconducting Tc, notably the minimum at intermediate x is explained as a
crossover effect from pairforming aspherical Coulomb scattering to pairbreaking
exchange scattering.Comment: 12 pages, 5 figures; to appear in Phys. Rev.
Unusual field and temperature dependence of Hall effect in graphene
We calculate the classic Hall conductivity and mobility of the undoped and
doped (or in the gate voltage) graphene as a function of temperature, magnetic
field, and carrier concentration. Carrier collisions with defects and acoustic
phonons are taken into account. The Hall resistivity varies almost linearly
with temperature. The magnetic field dependence of resistivity and mobility is
anomalous in weak magnetic fields. There is the square root contribution from
the field in the resistivity. The Hall mobility diverges logarithmically with
the field for low doping.Comment: 4 pages, 5 figures, typos correcte
Weak anisotropic impurity scattering in unconventional superconductors
The effect of weak anisotropic (momentum-dependent) impurity scattering in
unconventional superconductors has been investigated. It is shown that the
anisotropic scattering can lead either to a small reduction or a small
enhancement of the isotropic pair-breaking effect. The influence of the
anisotropy of the scattering potential becomes significant for the order
parameters with large Fermi surface average values. In that case an unexpected
enhancement (up to 10%) of the critical temperature over the critical
temperature in the absence of impurities is predicted for a small impurity
concentration.Comment: 12 pages, RevTeX, 3 PostScript figure
Influence of high magnetic fields on superconducting transition of one-dimensional Nb and MoGe nanowires
The effects of strong magnetic field on superconducting Nb and MoGe nanowires
with diameter nm have been studied. We have found that the
Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase
slips is applicable in a wide range of magnetic fields and describes well the
temperature dependence of the wire resistance, over eleven orders of magnitude.
The field dependence of the critical temperature, , extracted from the
LAMH fits is in good quantitative agreement with the theory of pair-breaking
perturbations that takes into account both spin and orbital contributions. The
extracted spin-orbit scattering time agrees with an estimate , where is the elastic scattering time and
is the atomic number.Comment: accepted for publication in Physical Review Letter
Differential conductance of point contacts between an iron-based superconductor and a normal metal
We present a theoretical description of the differential conductance of point
contacts between a normal metal and a multi-band superconductor with extended
s\pm pairing symmetry. We demonstrate that the interband impurity scattering
broadens the coherent peak near the superconducting gap and significantly
reduces its height even at relatively low scattering rates. This broadening is
consistent with a number of recent experiments performed for both tunnel
junctions and larger diffusive contacts. Our theory helps to better evaluate
the energy gap of iron-based superconductors from point contact Andreev
spectroscopy measurements.Comment: 5 pages, 4 figure
Ultra-cold fermions in real or fictitious magnetic fields: The BCS-BEC evolution and the type-I--type-II transition
We study ultra-cold neutral fermion superfluids in the presence of fictitious
magnetic fields, as well as charged fermion superfluids in the presence of real
magnetic fields. Charged fermion superfluids undergo a phase transition from
type-I to type-II superfluidity, where the magnetic properties of the
superfluid change from being a perfect diamagnet without vortices to a partial
diamagnet with the emergence of the Abrikosov vortex lattice. The transition
from type-I to type-II superfluidity is tunned by changing the scattering
parameter (interaction) for fixed density. We also find that neutral fermion
superfluids such as Li and K are extreme type-II superfluids, and
that they are more robust to the penetration of a fictitious magnetic field in
the BCS-BEC crossover region near unitarity, where the critical fictitious
magnetic field reaches a maximum as a function of the scattering parameter
(interaction).Comment: 4+ pages with 2 figure
Fermion States on the Sphere
We solve for the spectrum and eigenfunctions of Dirac operator on the sphere.
The eigenvalues are nonzero whole numbers. The eigenfunctions are two-component
spinors which may be classified by representations of the SU(2) group with
half-integer angular momenta. They are not the conventional spherical spinors
but special linear combinations of those.Comment: Talk given at the Fifth Workshop on Quantum Field Theory under the
Influence of External Conditions. 4p
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