Odd-Frequency Superconductivity : Peculiar Superconducting Pairs Vanishing at the Same Time(Research)

Abstract

The odd-frequency superconductivity, in which the particles form peculiar Cooper pairs with odd relative time dependence, has attracted growing interest as new class of unconventional superconductors. This exotic pairing state, however, has been suffering from the puzzles on its thermodynamic instability and unphysical electromagnetic responses, if we would apply the standard prescription for the superconducting state. Here, we show that these puzzles can be resolved by reconsidering the underlying conditions for conventional pairing states: (a) the Hermitian relation between gap function and its conjugate counterpart and (b) the zero center-of-mass momentum of a Cooper pair. When one of these conditions are modified appropriately, the bulk odd-frequency superconductivity can be realized as a stable quantum state of matter. We discuss the fundamental aspects of the odd-frequency superconductivity on the basis of recent theoretical development, and remaining issues to be addressed in the future.