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Superfluid density reveals a quantum critical point between d-wave superconductivity and a Mott insulator

In the phase diagram of the high /T/c cuprates important clues for connecting the Mott insulator and the /d/-wave superconducting state can be found in the evolution of the superfluid density with doping. Superfluid density is one of the most fundamental properties of a superconductor, measuring its ability to resist perturbations to the phase of the superfluid wavefunction.  Recent breakthroughs in the preparation of high purity YBCO crystals let us continuously tune doping over a 20 K range of Tc in a single sample, without altering oxygen content and without changing cation disorder.  These materials have high chemical homogeneity, allowing us to maintain tight control over doping and produce samples with sharp superconducting transitions in the limit that /T/c goes to zero.  Using microwave cavity perturbation, we have made measurements of the superfluid density at a series of closely spaced doping intervals as we continuously tune to the boundary of the superconducting phase. The results show that the approach to the insulating state is controlled by (3+1)-dimensional fluctuations of a strongly-paired superconductor in the vicinity of a quantum critical point.