Physics @ Berkeley
Events in Physics
Title: CM Special Seminar    (Special)
Start Date: 02/01/2006
Time: 3:00 pm
Location: 375 LeConte
Speaker: Michael Hermele
Affiliation: MIT
Contact Person: Ashvin Vishwanath   (510) 643-3952

The dc Josephson effect refers to the dissipationless electrical current -- the supercurrent -- that can be sustained across a weak link connecting two bulk superconductors.  This effect probes the nature of the superconducting state, which depends crucially on spatial dimensionality. For bulk (i.e. three-dimensional) superconductors, the superconductivity
is most robust, and the Josephson effect is sustained even at nonzero temperature.  However, in wires and thin films, thermal and quantum fluctuations play a crucial role.  In superconducting wires, these effects qualitatively modify the electrical transport across a weak link. Despite several recent experiments involving weak links between thin-film
superconductors, little theoretical attention has been paid to the electrical conduction in such systems.

I will discuss recent work [1,2] in which a system of two superconducting thin films connected by a point contact was analyzed.  Remarkably, the Josephson effect is absent at nonzero temperature.  The point contact resistance is nonzero and varies with temperature in a nearly activated fashion, with a universal energy barrier set by the superfluid stiffness characterizing the films.  I will discuss the derivation of this result
and try to give some intuition for the underlying physics, which can best be pictured in terms of quantum phase slip events.  In addition, I will discuss considerations relevant to future experimental tests of the predicted behavior.

[1]  Michael Hermele, Gil Refael, Matthew P. A. Fisher and Paul M. Goldbart, Nature Physics 1, 117 (2005).
[2]  Michael Hermele, Gil Refael, Matthew P. A. Fisher and Paul M. Goldbart, cond-mat/0511210.