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Superconductivity refers to a phenomenon where pairs of electron
(called Cooper pairs) “condense” in a macroscopic quantum state
below a certain temperature giving rise to a material with zero
electrical resistance. On the other hand, disorder tends to localize
electrons in a metal, giving rise to an insulating ground state in the
limit of strong disorder. The competition between these two opposing
effects in strongly disordered superconductors has been a recurring
theme in quantum many-body physics. Several emergent phenomena such as
magnetic field driven transition from a superconductor to an insulator,
the persistence of Cooper pairs even after the superconducting state is
destroyed, and unusual frequency dependence of the electrical
conductivity have been observed in strongly disordered superconductors.
The TIFR story of disordered superconductors begins about 15 years ago
through a seminal theoretical paper by Amit Ghosal, Mohit Randeria and
Nandini Trivedi [Phys. Rev. Lett. 81, 3940 [1] (1998)] which opened a
new paradigm. They suggested that in the presence of strong disorder the
superconducting state spontaneously segregates into superconducting and
insulating island both made of Cooper pairs. Consequently, the
transition from a superconducting to a non-superconducting state is not
governed by disappearance of the Cooper pairs as in conventional
superconductors, but rather by the coupling between these
superconducting islands. Over the past 5 years a series of experiments,
using low temperature Scanning tunneling spectroscopy, Broadband
microwave spectroscopy and penetration depth measurements performed in
our laboratory provide broad validation of this scenario. In this talk,
I will take you through the physics of disordered superconductors,
highlighting our experiments and the toys that we had to build as past
of our quest. |