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Distinguished Lecture Series

The Role of Vortices in Limiting Tc in Cuprate Superconductors

Prof. N. P. Ong

by: Prof. N. P. Ong

Date: Friday April 18, 2008

Time: 4:00 pm – 6:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Superconductivity in the copper oxides occurs at temperatures much higher than in all other metals. There is growing evidence that the Cooper pairs actually survive to even higher temperatures. I will discuss Nernst and torque magnetometry experiments which suggest the scenario that, above Tc in the cuprates, long-range phase stiffness is destroyed, rather than the gap order parameter. In the Nernst experiment, the vortex current produced by a temperature gradient generates a Josephson E-field perpendicular to the applied field H. A large Nernst signal eN persisting to a high onset temperature ~130 K is observed in nearly all cuprate families. Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram (x, T, H) in fields up to 45 T. This picture has been confirmed by recent torque magnetometry experiments. In a tilted H, local planar supercurrents associated with vortices above Tc produce a torque that deflects a cantilever. The inferred diamagnetism provides thermodynamic evidence for the vortex liquid picture suggested by the Nernst effect. Recent high-temperature STM experiments providing direct test of these ideas will also be described.

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