3,496 research outputs found
Thermodynamics of vortex lines in layered superconductors
We study the dissipative thermodynamics of vortex lines in layered
superconductors within a simple string model in the dilute limit of negligible
vortex interactions and compute the specific heat in presence of
arbitrary dissipation. The interplay of dissipation, inertia and elasticity is
shown to control the qualitative thermodynamical behavior and their relative
amount determines two very distinct regimes for the specific heat. In the
dissipation dominated case we find a behavior for a large
interval of temperature below .Comment: 10 pages, RevTe
Handbook on string decay
We explain simple semi-classical rules to estimate the lifetime of any given
highly-excited quantum state of the string spectrum in flat spacetime. We
discuss both the decays by splitting into two massive states and by massless
emission. As an application, we study a solution describing a rotating and
pulsating ellipse which becomes folded at an instant of time -- the ``squashing
ellipse''. This string interpolates between the folded string with maximum
angular momentum and the pulsating circular string. We explicitly compute the
quantum decay rate for the corresponding quantum state, and verify the basic
rules that we propose. Finally, we give a more general (4-parameter) family of
closed string solutions representing rotating and pulsating elliptical strings.Comment: 18 pages, 9 figures. Final version appeared in JHE
Massless radiation from Strings: quantum spectrum average statistics and cusp-kink configurations
We derive general formulae for computing the average spectrum for Bosonic or
Fermionic massless emission from generic or particular sets of closed
superstring quantum states, among the many occurring at a given large value of
the number operator. In particular we look for states that can produce a
Bosonic spectrum resembling the classical spectrum expected for peculiar
cusp-like or kink-like classical configurations, and we perform a statistical
counting of their average number. The results can be relevant in the framework
of possible observations of the radiation emitted by cosmic strings.Comment: 13 pages, 4 figures, improved explanations, an appendix added on
rotating folded strin
Vortex Quantum Nucleation and Tunneling in Superconducting Thin Films: Role of Dissipation and Periodic Pinning
We investigate the phenomenon of decay of a supercurrent in a superconducting
thin film in the absence of an applied magnetic field. The resulting
zero-temperature resistance derives from two equally possible mechanisms: 1)
quantum tunneling of vortices from the edges of the sample; and 2) homogeneous
quantum nucleation of vortex-antivortex pairs in the bulk of the sample,
arising from the instability of the Magnus field's ``vacuum''. We study both
situations in the case where quantum dissipation dominates over the inertia of
the vortices. We find that the vortex tunneling and nucleation rates have a
very rapid dependence on the current density driven through the sample.
Accordingly, whilst normally the superconductor is essentially resistance-free,
for the high current densities that can be reached in high- films a
measurable resistance might develop. We show that edge-tunneling appears
favoured, but the presence of pinning centres and of thermal fluctuations leads
to an enhancement of the nucleation rates. In the case where a periodic pinning
potential is artificially introduced in the sample, we show that
current-oscillations will develop indicating an effect specific to the
nucleation mechanism where the vortex pair-production rate, thus the
resistance, becomes sensitive to the corrugation of the pinning substrate. In
all situations, we give estimates for the observability of the studied
phenomena.Comment: 8 pages (LaTeX), 2 postscript figures. Invited talk to the SATT8 (8th
Italian Meeting on High-T_c Superconductivity), Como (Italy), Villa Olmo, 1-4
October 1996, to be published in La Rivista del Nuovo Cimento
Sommerfeld enhancement from Goldstone pseudo-scalar exchange
We point out that the exchange of a Goldstone pseudo-scalar can provide an
enhancement in the dark matter annihilation rate capable of explaining the
excess flux seen in high energy cosmic ray data. The mechanism of enhancement
involves the coupling of s and d waves through the tensor force that is very
strong and, in fact, singular at short distances. The results indicate that
large enhancements require some amount of fine tuning. We also discuss the
enhancement due to other singular attractive potentials, such as WIMP models
with a permanent electric dipole.Comment: 14 pages, 4 figures, v2 includes contact informatio
Microscopic Oscillations in the Quantum Nucleation of Vortices Subject to Periodic Pinning Potential in a Thin Superconductor
We present a theory for the decay of a supercurrent through nucleation of
vortex-antivortex pairs in a two-dimensional superconductor in the presence of
dissipation and of a periodic pinning potential. Through a powerful quantum
electrodynamics formulation of the problem we show that the nucleation rate
develops oscillations in its current-density dependence which are connected to
the pinning periodicity. A remnant of the dissipation-driven localization
transition is present, and an estimate of the nucleation rate suggests that
these effects might be observable in real thin superconductors.Comment: REVTeX file, 4 pages in two-column mode, 1 Postscript figure, to
appear in Phys.Rev.B (Rapid Communications
Electromagnetism and multiple-valued loop-dependent wave functionals
We quantize the Maxwell theory in the presence of a electric charge in a
"dual" Loop Representation, i.e. a geometric representation of magnetic
Faraday's lines. It is found that the theory can be seen as a theory without
sources, except by the fact that the wave functional becomes multivalued. This
can be seen as the dual counterpart of what occurs in Maxwell theory with a
magnetic pole, when it is quantized in the ordinary Loop Representation. The
multivaluedness can be seen as a result of the multiply-connectedness of the
configuration space of the quantum theory.Comment: 5 page
Edge Tunneling of Vortices in Superconducting Thin Films
We investigate the phenomenon of the decay of a supercurrent due to the
zero-temperature quantum tunneling of vortices from the edge in a thin
superconducting film in the absence of an external magnetic field. An explicit
formula is derived for the tunneling rate of vortices, which are subject to the
Magnus force induced by the supercurrent, through the Coulomb-like potential
barrier binding them to the film's edge. Our approach ensues from the
non-relativistic version of a Schwinger-type calculation for the decay of the
2D vacuum previously employed for describing vortex-antivortex pair-nucleation
in the bulk of the sample. In the dissipation-dominated limit, our explicit
edge-tunneling formula yields numerical estimates which are compared with those
obtained for bulk-nucleation to show that both mechanisms are possible for the
decay of a supercurrent.Comment: REVTeX file, 15 pages, 1 Postscript figure; to appear in Phys.Rev.
Black holes as D3-branes on Calabi-Yau threefolds
We show how an extremal Reissner-Nordstrom black hole can be obtained by
wrapping a dyonic D3-brane on a Calabi-Yau manifold. In the orbifold limit
T^6/Z_3, we explicitly show the correspondence between the solution of the
supergravity equations of motion and the D-brane boundary state description of
such a black hole.Comment: 14 pages, LaTex, minor corrections, version to appear on Phys. Lett.
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