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SUMMARY:Magnetic versus dielectric pairing in cuprate superconductors - Je
 ff Tallon\,MacDiarmid Institute for Advanced Materials and Nanotechnology
DTSTART:20120815T101500Z
DTEND:20120815T111500Z
UID:TALK38468@talks.cam.ac.uk
CONTACT:Michael Sutherland
DESCRIPTION:This is a talk in two parts. First\, I will briefly review whe
 re we in New Zealand have got to in manufacturing a broad range of high-Tc
  products. Then I\nwill address the topic of the title.\n\nA quarter of a 
 century after their discovery the mechanism that pairs electrons in cuprat
 e superconductors remains uncertain\, though it is widely believed to be m
 agnetic. If so\, the energy scale for the pairing boson and hence for Tc i
 s governed to leading order by J\, the antiferromagnetic exchange interact
 ion. We vary J (as measured by two-magnon Raman scattering) by more than 1
 60% by changing ion sizes in LnA2Cu3O7-δ where A=(Ba\,Sr) and Ln=(La\, ..
 . Lu). Such\nchanges are often referred to as "internal" pressure. Surpris
 ingly\, we find Tc_max anticorrelates with J. This is opposite to the effe
 ct of external pressure\, suggesting that J is not the dominant energy sca
 le governing Tc_max. In contrast\, we show there is a clear correlation be
 tween Tc_max and the sum of polarisabilities of the constituent ions\, wei
 ghted by their densities. An immediate consequence is the possibility of a
  pairing mechanism having its origin with quantized waves of electronic po
 larization.\n\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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