720 research outputs found
Magnetic interference patterns in superconducting junctions: Effects of anharmonic current-phase relations
A microscopic theory of the magnetic-field modulation of critical currents is
developed for plane Josephson junctions with anharmonic current-phase
relations. The results obtained allow examining temperature-dependent
deviations of the modulation from the conventional interference pattern. For
tunneling through localized states in symmetric short junctions with a
pronounced anharmonic behavior, the deviations are obtained and shown to depend
on distribution of channel transparencies. For constant transparency the
deviations vanish not only near Tc, but also at T=0. If Dorokhov bimodal
distribution for transparency eigenvalues holds, the averaged deviation
increases with decreasing temperature and takes its maximum at T=0.Comment: 6 pages, 6 figure
Combined Paramagnetic and Diamagnetic Response of YBCO
It has been predicted that the zero frequency density of states of YBCO in
the superconducting phase can display interesting anisotropy effects when a
magnetic field is applied parallel to the copper-oxide planes, due to the
diamagnetic response of the quasi-particles. In this paper we incorporate
paramagnetism into the theory and show that it lessens the anisotropy and can
even eliminate it altogether. At the same time paramagnetism also changes the
scaling with the square root of the magnetic field first deduced by Volovik
leading to an experimentally testable prediction. We also map out the analytic
structure of the zero frequency density of states as a function of the
diamagnetic and paramagnetic energies. At certain critical magnetic field
values we predict kinks as we vary the magnetic field. However these probably
lie beyond currently accessible field strengths
Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions
The behavior of dc subgap current in magnetic quantum point contact is
discussed for the case of low-transparency junction with different tunnel
probabilities for spin-up () and spin-down ()
electrons. Due to the presence of Andreev bound states in the
system the positions of subgap electric current steps are split at temperature with respect to the
nonmagnetic result . It is found that under the condition
the spin current also manifests subgap
structure, but only for odd values of . The split steps corresponding to
in subgap electric and spin currents are analytically calculated and
the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde
Andreev bound states and tunneling characteristics of a non-centrosymmetric superconductor
The tunneling characteristics of planar junctions between a normal metal and
a non-centrosymmetric superconductor like CePt3Si are examined. It is shown
that the superconducting phase with mixed parity can give rise to
characteristic zero-bias anomalies in certain junction directions. The origin
of these zero-bias anomalies are Andreev bound states at the interface. The
tunneling characteristics for different directions allow to test the structure
of the parity-mixed pairing state.Comment: 4 pages, 3 figure
Low-energy quasiparticle states at superconductor-CDW interfaces
Quasiparticle bound states are found theoretically on transparent interfaces
of d-wave superconductors (dSC) with charge density wave solids (CDW), as well
as s-wave superconductors (sSC) with d-density waves (DDW). These bound states
represent a combined effect of Andreev reflection from the superconducting side
and an unconventional quasiparticle Q-reflection from the density wave solid.
If the order parameter for a density wave state is much less than the Fermi
energy, bound states with almost zero energy take place for an arbitrary
orientation of symmetric interfaces. For larger values of the order parameter,
dispersionless zero-energy states are found only on (110) interfaces. Two
dispersive energy branches of subgap quasiparticle states are obtained for
(100) symmetric interfaces. Andreev low-energy bound states, taking place in
junctions with CDW or DDW interlayers, result in anomalous junction properties,
in particular, the low-temperature behavior of the Josephson critical current.Comment: 6 pages, 2 figure
Bound states at the interface between antiferromagnets and superconductors
We present a detailed theoretical investigation of interfaces and junctions
involving itinerant antiferromagnets. By solving the Bogoliubov-de Gennes
equations with a tight-binding model on a square lattice, we study both the
self-consistent order parameter fields proximate to interfaces between
antiferromagnets (AF) and s-wave (sSC) or d-wave (dSC) superconductors, the
dispersion of quasiparticle subgap states at interfaces and interlayers, and
the local density of states (LDOS) as a function of distance from the
interface. In addition, we present the quasiclassical approach to interfaces
and junctions involving itinerant antiferromagnets developed in an earlier
paper. Analytical results are in excellent agreement with what we obtain
numerically. Strong effects of pair breaking in the presence of low-energy
interface Andreev states are found in particular for AF/sSC interfaces when
interface potentials are not too high. Potential barriers induce additional
extrema in the dispersive quasiparticle spectra with corresponding peaks in the
LDOS. Discrete quasiparticle dispersive levels in AF - normal metal (N) - AF
systems are found to strongly depend on the misorientation angle of the
magnetizations in the two antiferromagnets.Comment: 21 pp, 21 postscript figures, submitted to Phys. Rev.
Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors
Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
for surface impurity concentration , which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration , become centered at zero energy. With
increasing above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.Comment: 9 pages, 7 figure
Andreev reflection in layered structures: implications for high T_c grain boundary Josephson junctions
Andreev reflection is investigated in layered anisotropic normal metal /
superconductor (N/S) systems in the case of an energy gap \Delta in S not
negligible with respect to the Fermi energy E_F, as it probably occurs with
high critical temperature superconductors (HTS). We find that in these limits
retro-reflectivity, which is a fundamental feature of Andreev reflection, is
broken modifying sensitively transport across S/N interfaces. We discuss the
consequences for supercurrents in HTS Josephson junctions and for the midgap
states in S-N contactsComment: 4 pages, 4 figures, to be published in Phys. Rev.
Thermal quantum field theory and the Casimir interaction between dielectrics
The Casimir and van der Waals interaction between two dissimilar thick
dielectric plates is reconsidered on the basis of thermal quantum field theory
in Matsubara formulation. We briefly review two main derivations of the
Lifshitz formula in the framework of thermal quantum field theory without use
of the fluctuation-dissipation theorem. A set of special conditions is
formulated under which these derivations remain valid in the presence of
dissipation. The low-temperature behavior of the Casimir and van der Waals
interactions between dissimilar dielectrics is found analytically from the
Lifshitz theory for both an idealized model of dilute dielectrics and for real
dielectrics with finite static dielectric permittivities. The free energy,
pressure and entropy of the Casimir and van der Waals interactions at low
temperatures demonstrate the same universal dependence on the temperature as
was previously discovered for ideal metals. The entropy vanishes when
temperature goes to zero proving the validity of the Nernst heat theorem. This
solves the long-standing problem on the consistency of the Lifshitz theory with
thermodynamics in the case of dielectric plates. The obtained asymptotic
expressions are compared with numerical computations for both dissimilar and
similar real dielectrics and found to be in excellent agreement. The role of
the zero-frequency term in Matsubara sum is investigated in the case of
dielectric plates. It is shown that the inclusion of conductivity in the model
of dielectric response leads to the violation of the Nernst heat theorem. The
applications of this result to the topical problems of noncontact atomic
friction and the Casimir interaction between real metals are discussed.Comment: 39 pages, 4 figures, to appear in Phys. Rev.
Retarded Casimir-Polder force on an atom near reflecting microstructures
We derive the fully retarded energy shift of a neutral atom in two different
geometries useful for modelling etched microstructures. First we calculate the
energy shift due to a reflecting cylindrical wire, and then we work out the
energy shift due to a semi-infinite reflecting half-plane. We analyze the
results for the wire in various limits of the wire radius and the distance of
the atom from the wire, and obtain simple asymptotic expressions useful for
estimates. For the half-plane we find an exact representation of the
Casimir-Polder interaction in terms of a single, fast converging integral,
which is easy to evaluate numerically.Comment: 12 pages, 8 figure
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