1,246 research outputs found
Localization of Gauge Fields and Monopole Tunnelling
We study the dynamical localization of a massless gauge field on a
lower-dimensional surface (2-brane). In flat space, the necessary and
sufficient condition for this phenomenon is the existence of confinement in the
bulk. The resulting configuration is equivalent to a dual Josephson junction.
This duality leads to an interesting puzzle, as it implies that a localized
massless theory, even in the Abelian case, must become confining at
exponentially large distances. Through the use of topological arguments we
clarify the physics behind this large-distance confinement and identify the
instantons of the brane world-volume theory that are responsible for its
appearance. We show that they correspond to the (condensed) bulk magnetic
charges (monopoles), that occasionally tunnel through the brane and induce weak
confinement of the brane theory. We consider the possible generalization of
this effect to higher dimensions and discuss phenomenological bounds on the
confinement of electric charges at exponentially large distances within our
Universe.Comment: 11 pages, 3 figures, improvements in the presentation, version to
appear in Physical Review
Universal point contact resistance between thin-film superconductors
A system comprising two superconducting thin films connected by a point
contact is considered. The contact resistance is calculated as a function of
temperature and film geometry, and is found to vanish rapidly with temperature,
according to a universal, nearly activated form, becoming strictly zero only at
zero temperature. At the lowest temperatures, the activation barrier is set
primarily by the superfluid stiffness in the films, and displays only a weak
(i.e., logarithmic) temperature dependence. The Josephson effect is thus
destroyed, albeit only weakly, as a consequence of the power-law-correlated
superconducting fluctuations present in the films below the
Berezinskii-Kosterlitz-Thouless transition temperature. The behavior of the
resistance is discussed, both in various limiting regimes and as it crosses
over between these regimes. Details are presented of a minimal model of the
films and the contact, and of the calculation of the resistance. A formulation
in terms of quantum phase-slip events is employed, which is natural and
effective in the limit of a good contact. However, it is also shown to be
effective even when the contact is poor and is, indeed, indispensable, as the
system always behaves as if it were in the good-contact limit at low enough
temperature. A simple mechanical analogy is introduced to provide some
heuristic understanding of the nearly-activated temperature dependence of the
resistance. Prospects for experimental tests of the predicted behavior are
discussed, and numerical estimates relevant to anticipated experimental
settings are provided.Comment: 29 pages (single column format), 7 figure
The Josephson effect throughout the BCS-BEC crossover
We study the stationary Josephson effect for neutral fermions across the
BCS-BEC crossover, by solving numerically the Bogoliubov-de Gennes equations at
zero temperature. The Josephson current is found to be considerably enhanced
for all barriers at about unitarity. For vanishing barrier, the Josephson
critical current approaches the Landau limiting value which, depending on the
coupling, is determined by either pair-breaking or sound-mode excitations. In
the coupling range from the BCS limit to unitarity, a procedure is proposed to
extract the pairing gap from the Landau limiting current.Comment: 4 pages, 3 figures; improved version to appear in Phys. Rev. Let
Macroscopic Symmetry Group Describes Josephson Tunneling in Twinned Crystals
A macroscopic symmetry group describing the superconducting state of an
orthorhombically twinned crystal of YBCO is introduced. This macroscopic
symmetry group is different for different symmetries of twin boundaries.
Josephson tunneling experiments performed on twinned crystals of YBCO determine
this macroscopic symmetry group and hence determine the twin boundary symmetry
(but do not experimentally determine whether the microscopic order parameter is
primarily d- or s-wave). A consequence of the odd-symmetry twin boundaries in
YBCO is the stability of vortices containing one half an elementary flux
quantum at the intersection of a twin boundary and certain grain boundaries.Comment: 6 pages, to be published in the Proceedings of the MOS96 Conference
in the Journal of Low Temperature Physic
Collective Excitations of Bose-Einstein Condensates in a Double-Well Potential
We investigate collective excitations of Bose-Einstein condensates at
absolute zero in a double-well trap. We solve the Bogoliubov equations with a
double-well trap, and show that the crossover from the dipole mode to the
Josephson plasma mode occurs in the lowest energy excitation. It is found that
the anomalous tunneling property of low energy excitations is crucial to the
crossover.Comment: 14 pages, 6 figure
Cooper Pair Shape in Normal-metal/Superconductor Junctions
In s-wave superconductors the Cooper pair wave function is isotropic in
momentum space. This property may also be expected for Cooper pairs entering a
normal metal from a superconductor due to the proximity effect. We show,
however, that such a deduction is incorrect and the pairing function in a
normal metal is surprisingly anisotropic because of quasiparticle interference.
We calculate angle resolved quasiparticle density of states in NS bilayers
which reflects such anisotropic shape of the pairing function. We also propose
a magneto-tunneling spectroscopy experiment which could confirm our
predictions
Electrodynamics of Abrikosov vortices: the Field Theoretical Formulation
Electrodynamic phenomena related to vortices in superconductors have been
studied since their prediction by Abrikosov, and seem to hold no fundamental
mysteries. However, most of the effects are treated separately, with no guiding
principle. We demonstrate that the relativistic vortex worldsheet in spacetime
is the object that naturally conveys all electric and magnetic information, for
which we obtain simple and concise equations. Breaking Lorentz invariance leads
to down-to-earth Abrikosov vortices, and special limits of these equations
include for instance dynamic Meissner screening and the AC Josephson relation.
On a deeper level, we explore the electrodynamics of two-form sources in the
absence of electric monopoles, in which the electromagnetic field strength
itself acquires the characteristics of a gauge field. This novel framework
leaves room for unexpected surprises.Comment: LaTeX, 23 pages, 5 figure
Giant Josephson current through a single bound state in a superconducting tunnel junction
We study the microscopic structure of the Josephson current in a single-mode
tunnel junction with a wide quasiclassical tunnel barrier. In such a junction
each Andreev bound state carries a current of magnitude proportional to the
{\em amplitude} of the normal electron transmission through the junction.
Tremendous enhancement of the bound state current is caused by the resonance
coupling of superconducting bound states at both superconductor-insulator
interfaces of the junction. The possibility of experimental observation of the
single bound state current is discussed.Comment: 11 pages, [aps,preprint]{revtex
The phase-dependent linear conductance of a superconducting quantum point contact
The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data.Comment: 12 pages (revtex) + 1 postscript figure. Submitted to Phys. Rev. Let
Pion Propagation near the QCD Chiral Phase Transition
We point out that, in analogy with spin waves in antiferromagnets, all
parameters describing the real-time propagation of soft pions at temperatures
below the QCD chiral phase transition can be expressed in terms of static
correlators. This allows, in principle, the determination of the soft pion
dispersion relation on the lattice. Using scaling and universality arguments,
we determine the critical behavior of the parameters of pion propagation. We
predict that when the critical temperature is approached from below, the pole
mass of the pion drops despite the growth of the pion screening mass. This fact
is attributed to the decrease of the pion velocity near the phase transition.Comment: 8 pages (single column), RevTeX; added references, version to be
published in PR
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