Bose-Glass and Fermi-Glass

Abstract It is known that two-dimensional superconducting materials undergo a quantum phase transition from a localized state to superconductivity. When the disordered samples are cooled, bosons (Cooper pairs) are generated from Fermi-glass and reach superconductivity through Bose-glass. However, there has been no universal expression representing the transition from Fermi-glass to Bose-glass.Here, we discovered an experimental renormalization group flow from Fermi-glass to Bose- glass in terms of simple β-function analysis. To discuss the universality of this flow, we analyzed manifestly different systems, namely a Nd-based two-dimensional layered perovskite and an ultrathin Pb film. We find that all our experimental data for Fermi-glass fall beautifully into the conventional self-consistent β-function. Surprisingly, however, flows vertical to the conventional β-function were observed in the weakly localized regime of both systems, where localization becomes even weaker. We show that the vertical flow is evidence for the existence of Bose-glass. Consequently, we confirm a universal quantum transition from Bose-glass to Fermi-glass with the new two-dimensional critical sheet resistance close to R□ = h/e2.