Physics

Prof. Tanmoy Das
IISC, Bangalore

Abstract:

One of the fascinating facts about superconductivity is that despite more than 100 years of extensive research and 7 Nobel prizes, we are far behind our main goals. The race continues to both achieving room temperature superconductivity as well as to obtain a theoretical understanding of the mechanism. How can two electrons attract each other when they experience a repulsive Coulomb interaction? By now we know two mechanisms for attractive interaction between the same-charge fermions: Meson mediates an attraction between protons. Phonon mediates attractive potential between electrons with superconductivity. In this talk, I will present a new mechanism of attractive potential between electrons, forming superconductivity. In many intermetallics and heavy-fermion compounds, atoms can possess fractional valency due to valence fluctuation between conduction and localized electrons (such as f-orbitals or flat bands). In such localized f-orbitals, two f-electrons with opposite spins cannot be occupied on the same orbit since they have strong Coulomb repulsion. This means, in the field theory language, a singly occupied f-electron site is attached with an unoccupied f-state (which is a holon gauge field) whose job is to repel another f-electron. However, the unoccupied f-site can be occupied by a conduction electron since the presence of valence fluctuation channel allows mutation between the f- and conduction electrons. We show that the doubly occupied state with f- and conduction electrons condensates like a Cooper pair. I will present this theory along with detailed comparison with recent experimental surprises of conventional superconductivity in heavy-fermion materials where decades old studies predicted unconventional superconductivity.