Electron-like quasiparticles drive the superconductor-insulator transition in homogeneously disordered thin films

Transport data on Bi, MoGe and PbBi/Ge homogeneously disordered thin films demonstrate that the critical resistivity R_c at the nominal insulator-superconductor transition is linearly proportional to the normal sheet resistance R_N. In addition, the critical magnetic field scales linearly with the superconducting energy gap and is well approximated by H_c2. Because R_N is determined at high temperatures and H_c2 is the pair-breaking field, the two immediate consequences are that electron-like quasiparticles populate the insulating side of the transition and that standard phase-only models are incapable of describing the destruction of the superconducting state. As gapless electronic excitations populate the insulating state, the universality class is no longer the three-dimensional XY model, thereby relaxing the constraints that this model imposes on the critical exponents as well as on the critical resistance, namely R_c = R_Q = h/4e². The lack of a unique critical resistance in homogeneously disordered films can be understood in this context. In the light of recent experiments which observe an intervening metallic state separating the insulator from the superconductor in homogeneously disordered MoGe thin films, we argue that the two transitions that accompany the destruction of superconductivity are superconductor to Bose metal in which phase coherence is lost and Bose metal to localized electron insulator via pair breaking.