Direct observation of quantum superconducting fluctuations across the 2D superconductor-insulator transition

Abstract

We review our recent measurements of the complex AC conductivity of thin InO x films studied as a function of magnetic field through the nominal 2D superconductor-insulator transition. These measurements—the first to probe anything other than the ω = 0 response of these archetypical systems—reveal a significant finite frequency superfluid stiffness well into the insulating regime. Unlike conventional fluctuation superconductivity in which thermal fluctuations can give a superconducting response in regions of parameter space that do not exhibit long range order, these fluctuations are temperature independent as T → 0 and are exhibited in samples where the resistance is large (greater than 10 6 Ω / □ ) and strongly diverging. We interpret this as the first direct observation of quantum superconducting fluctuations around an insulating ground state. This system serves as a prototype for other insulating states of matter that derive from superconductors.