61 research outputs found
Renormalization-group approach to superconductivity: from weak to strong electron-phonon coupling
We present the numerical solution of the renormalization group (RG) equations
derived in Ref. [1], for the problem of superconductivity in the presence of
both electron-electron and electron-phonon coupling at zero temperature. We
study the instability of a Fermi liquid to a superconductor and the RG flow of
the couplings in presence of retardation effects and the crossover from weak to
strong coupling. We show that our numerical results provide an ansatz for the
analytic solution of the problem in the asymptotic limits of weak and strong
coupling.Comment: 8 pages, 3 figures, conference proceedings for the Electron
Correlations and Materials Properties, in Kos, Greece, July 5-9, 200
Stimulated superconductivity at strong coupling
Stimulating a system with time dependent sources can enhance instabilities,
thus increasing the critical temperature at which the system transitions to
interesting low-temperature phases such as superconductivity or superfluidity.
After reviewing this phenomenon in non-equilibrium BCS theory (and its marginal
fermi liquid generalization) we analyze the effect in holographic
superconductors. We exhibit a simple regime in which the transition temperature
increases parametrically as we increase the frequency of the time-dependent
source.Comment: 19 pages, 2 figure. v3: Comments, references and one figure added.
Version to appear in JHE
Vortex Core Structure and Dynamics in Layered Superconductors
We investigate the equilibrium and nonequilibrium properties of the core
region of vortices in layered superconductors. We discuss the electronic
structure of singly and doubly quantized vortices for both s-wave and d-wave
pairing symmetry. We consider the intermediate clean regime, where the
vortex-core bound states are broadened into resonances with a width comparable
to or larger than the quantized energy level spacing, and calculate the
response of a vortex core to an {\em a.c.} electromagnetic field for vortices
that are pinned to a metallic defect. We concentrate on the case where the
vortex motion is nonstationary and can be treated by linear response theory.
The response of the order parameter, impurity self energy, induced fields and
currents are obtained by a self-consistent calculation of the distribution
functions and the excitation spectrum. We then obtain the dynamical
conductivity, spatially resolved in the region of the core, for external
frequencies in the range, 0.1\Delta < \hbar\omega \lsim 3\Delta. We also
calculate the dynamically induced charge distribution in the vicinity of the
core. This charge density is related to the nonequilibrium response of the
bound states and collective mode, and dominates the electromagnetic response of
the vortex core.Comment: Presented at the 2000 Workshop on ``Microscopic Structure and
Dynamics of Vortices in Unconventional Superconductors and Superfluids'',
held at the Max Planck Institute for the Physics of Complex Systems in
Dresden, Germany (28 pages with 15 figures). Alternate version with higher
resolution figures:
http://snowmass.phys.nwu.edu/~sauls/Eprints/Dresden2000.htm
Cooling quasiparticles in A(3)C(60) fullerides by excitonic mid-infrared absorption
Long after its discovery, superconductivity in alkali fullerides A(3)C(60) still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium T-c similar to 20 K have been discovered in K3C60 after ultra-short pulsed infrared irradiation-an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to transverse optical phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a 'super-exciton' involving the promotion of one electron from the t(1u) half-filled state to a higher-energy empty t(1g) state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with the Jahn-Teller effect within the enormously degenerate manifold of (t(1u))(2)(t(1g))(1) states. Both long-lived and entropy-rich because they are triplets, the infrared-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much higher temperatures
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