The final public oral examination of Yayu Wang will take place on Friday, June 18, 2004 at 10:30 a.m. in Room 202. The examining committee will consist of Professors N. P. Ong, P. Anderson, D. Huse, M. Z. Hasan and C. Nappi. Any other members of the University wishing to attend the examination may do so.

The thesis of Yayu Wang, entitled "Nernst effect in high temperature superconductors", has been placed on deposit.

Any member of the University wishing to read the thesis may do so. Any objections should be submitted to me in writing. The principal advisor for this work was N.P. Ong.

ABSTRACT

This thesis presents a study of the Nernst effect in high temperature superconductors. The vortex Nernst measurements have been carried out on a variety of high T_c cuprates to very high magnetic fields. These results provide vital information about the properties and relations of the pseudogap phase and superconducting phase in high T_c superconductors. Our first result is the existence of vortex-like excitations at temperatures much higher than T_c0, the zero filed transition temperature, in the cuprates. This anomalous behavior is most dramatic in the underdoped regime (0.10 < x < 0.12) where vortex-like signal persists to more than 100 K above T_c0. This finding suggests that in the putative normal state of cuprates, although bulk Meissner effect is absent and resistivity looks normal, the amplitude of the Cooper pairing is still sizable. The transition at T_c0 is driven by the loss of long range phase coherence rather than the disappearance of superconducting condensate. The high field Nernst effect offers a reliable way to determine the upper critical field H_c2 of high T_c cuprates and many unusual properties are uncovered. For cuprates with relatively large hole density (x> 0.15), we found that H_c2 is almost temperature independent for T < T_c0. This is in strong contrast to the H_c2-T relation of conventional superconductors. Moreover, using a scaling analysis, we have demonstrated that H_c2 increases with decreasing hole density x in this doping range, implying a stronger pairing potential at lower doping. Both of these findings are in excellent agreement with the gap amplitude \Delta measured by ARPES and STM.

In the severely underdoped regime (x < 0.12), some new features become apparent and they imply that the vortex Nernst signal is comprised of two distinct contributions. The first is from coherent regions with long range phase coherence and relatively low upper critical field, more like the superconducting phase; the second is from phase incoherent regions with much larger field scales, indicative of the pseudogap phase. Analysis of our data shows that with rising temperature, the superconducting phase gives weight to the pseudogap phase. The conversion between these two phases strongly suggests that they are closely related to each other rather than being competing orders. Moreover, the upper critical field H_c2 of the superconducting phase scales with the onset temperature T^{\nu}_{onset} of the vortx-like excitations.

Daniel Marlow
Chair, Dept. of Physics