Physics Colloquium

The superconducting state is characterized by a macroscopic wave function that resists distortions to its phase. This "London rigidity" is analogous to the rigidity of familiar solids. The destruction of this rigidity determines the observed critical temperature Tc (rather than the vanishing of the pair condensate as in BCS superconductors). In zero magnetic field, the destruction occurs because of spontaneous vortex formation. The existence of the vortex liquid above Tc informs the whole phase diagram of this fascinating material. I will review 2 experiments, the Nernst effect and torque magnetometry, which have been most effective for illuminating this issue. If time permits, I will also discuss the vortex liquid in the the quantum limit at very low temperature and low doping.