Superfluid (phase) stiffness: The ignored energy scale in Superconductivity

The superconducting state is composed of two key ingredients: Pairs of electrons that form Bosonic bound state called Cooper pairs and the condensation of Cooper pairs into phase coherent state. Consequently, two kinds of excitations can ultimately destroy a superconducting state: Quasiparticle excitations produced by breaking Cooper pairs and phase fluctuation that destroys the phase coherence. While conventional theories of superconductivity consider the first excitation in detail, the second excitation has traditionally received much less attention. However, a series experiments in the last decade in disordered and low dimensional superconductors has brought phase excitation back into focus. In this talk I will describe various facets of this problem, including the paradigmatic Berezinski-Kosterlitz-Thouless transition as well as the less explored long wavelength phase fluctuations.

References:

Phys. Rev. B 102, 060501(R) (2020); Phys. Rev. Lett. 106, 047001 (2011).