Vorticity and Phase Fluctuations in the cuprate superconductors

A recent issue of contention in cuprate physics is the nature of the pairing in the underdoped pseudogap regime. From Nernst and torque experiments, we derive the picture that the pairing is extremely strong across the underdoped regime. In zero magnetic field, the Meissner transition at Tc is caused by the loss of phase rigidity, but the pair condensate, bereft of rigidity, survives up to temperatures as high as  ~130 K. At very low temperatures, a moderate magnetic field (20 T) induces a loss of phase rigidity (via a vortex solid-to-liquid transition), but the pair condensate survives to an upper critical field Hc2 of 150-200 Tesla. The opposite point of view has been proposed from the observation of quantum oscillations in YBCO and other chain-cuprates. In this view, pairing in underdoped YBCO is in the weak regime and easily destroyed by a modest magnetic field of ~20 T. I will explain why a broad array of experiments – not just transport-- favor the strong pairing-strong fluctuation picture.