Pseudogap and superconducting phase fluctuations in high-Tc superconductors

The relationship between pseudogap and superconducting states of high-TC superconductors has been a focus of interest for a long time. Measurements of the AC conductivity and the Nernst effect have detected vortex-like excitations in a certain temperature range above the superconducting transition temperature TC. Meanwhile, the measurement of susceptibility by the torque technique has also detected weak diamagnetization induced by superconducting fluctuations above TC. These findings strongly support the idea that the pseudogap phase is characterized by a finite pairing amplitude with strong phase fluctuations, and imply that the phase transition at TC is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseuodgap state of high-TC superconductors, the principle of the Nernst effect, and the measurement technique. Then the results of the Nernst effect for different high-TC superconductors are reviewed and discussed. Related theoretical models are also discussed.