Disordered Superconductors: A TIFR Story

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 (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 7 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. Very recently, we also observed the first indication of the "Higgs mode" in strongly disordered superconductors from TeraHerz conductivity and tunnelling measurements. 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.