Critical superconductors

Abstract

Bogomolnyi critical point, originally introduced in string theories, is fundamental to superconductivity. At the critical temperature T_c it marks the sharp border between ideally diamagnetic bulk type-I superconductors and type-II ones that can host vortices, while itself it harbors infinitely degenerate distributions of magnetic flux in the superconducting state. Here we show that below T_c the physics of the Bogomolnyi critical point projects onto a wider range of microscopic parameters, even extremely wide for multiband superconductors. In this critical range, the degeneracy of the superconducting state at Tc is lifted into a sequence of novel topological equilibria and the customary understanding of superconducting phenomena does not suffice. As a radical departure from traditional views, we introduce the paradigm of critical superconductors, discuss their distinct magnetic properties, advocate their subdivision in terms of possible intermediate states, and demonstrate direct relevance of this paradigm to superconducting materials of prime interest today.