Phase-fluctuations in disordered superconductors

Understanding the role of interaction and disorder is at the heart of understanding of collective behavior of many-body quantum systems, such as High temperature superconductors, quantum Hall effect and superfluid He. In superconductors, where the order parameter is described by a complex order parameter with an amplitude and phase ( ), the presence of strong disorder poses an intriguing question: Is the destruction of the superconducting state always associated with the vanishing or the amplitude or could strong phase fluctuations destroy the superconducting state even when the amplitude of the superconducting pairing remains finite? The latter, if true, would give rise to novel electronic states with finite superconducting pairing amplitude but no global superconductivity.
In this colloquium I will describe our attempts to answer this question, by extracting all the key parameters of the superconducting state, on a set of NbN thin films with controlled amount of disorder. I will focus on a number of experimental tools, such the low-temperature scanning tunneling microscope, a two coil mutual inductance technique for the measurement of penetration depth and low temperature cryostats with extremely high stability that we had to develop as part of this quest. Finally, I will show the signatures that unambiguously identify phase fluctuations as a dominant mechanism for the destruction of superconductivity in disordered superconductors.
References:
Phys. Rev. B 77, 214503 (2008)
Phys. Rev. B 79, 094509 (2009)
Appl. Phys. Lett. 94, 262501 (2009)
Phys. Rev. B 80, 134514 (2009)
Appl. Phys. Lett. 96, 072509 (2010)
arXiv:1006.4143.