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

We review our recent measurements of the complex AC conductivity of thin InOx 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 omega = 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) Omega/square) 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. (C) 2007 Elsevier B.V. All rights reserved.