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Distinguished Lecture Series
Are all HTS Josephson Junctions the Same?
by: Dr. Cathy Foley
Date: Wednesday July 16, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Josephson junctions are the basis of all active superconducting electronics. Since the discovery of high temperature superconducting (HTS) materials and YBCO, in particular, a number of different methods of junction fabrication have been devised on a range of different substrates; grain boundary step-edges, bi-epitaxial, bi-crystal and ramp junctions are common examples. The properties of these junctions vary with differences in the range of critical currents and normal resistance that are achievable, their response to magnetic fields and the amount of s-or d-wave phase shifting across the junction. Superconducting electronic applications are broad ranging including SQUIDs for magnetometry, gradiometry, macroscopic quantum state formation for quantum computer qubits, and microwave and terahertz resonators and detectors. However the requirements of the Josephson junction for each of these applications are quite different. This paper will review four different Josephson junction types and what their properties are by considering the impact of the junction morphology and the substrate material on their demonstrated characteristics. We will report on various devices fabricated at CSIRO and use some data from the literature. We will show that these different junctions have different s- and d-wave contributions as well as other properties that make different junctions more appropriate for each specific application in superconducting electronics.
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Distinguished Lecture Series
The Role of Vortices in Limiting Tc in Cuprate Superconductors
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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