报告摘要：

The goal of our research is to unveil the mystery of high temperature superconductivity. Currently, the key issues are the nature of the pseudogap phase and its relation to the superconducting state.  We address these questions using two different experimental techniques: the vortex Nernst effect and fluctuation induced diamagnetism.  Very surprisingly, both results reveal that vortex-like excitations exist at anomalously high temperature and field scales, especially in the underdoped regime of the phase diagram. These findings emphasize the importance of the phase degree of freedom in understanding high Tc superconductivity.  The physical picture we proposed is that in underdoped cuprates, the phase transition at Tc is driven by the loss of long-range phase coherence.  A significant portion of the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations.