Pseudogap and superconducting phase fluctuations in high-T_c superconductors

The relationship between pseudogap and superconducting states of high-T_C 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 T_C. Meanwhile, the measurement of susceptibility by the torque technique has also detected weak diamagnetization induced by superconducting fluctuations above T_C. 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 T_C is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseuodgap state of high-T_C superconductors, the principle of the Nernst effect, and the measurement technique. Then the results of the Nernst effect for different high-T_C superconductors are reviewed and discussed. Related theoretical models are also discussed.