Low-temperature upper-critical-field anomalies in clean superconductors

We interpret the upper-critical-held 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 end point of the first-order melting line of the vortex lattice.