Low Temperature Upper Critical Field Anomalies in Clean Superconductors
  We interpret the upper-critical-field anomalies observed in some high- temperature superconductors as resulting from the proximity to a zero-temperature quantum critical point. We estimate the shape of the phase boundary between the normal and the superconducting phase by modeling the zero temperature critical point as the second-order endpoint of the first-order melting line of the vortex lattice. 
Gabriel Kotliar and Chandra Varma