Abstract

This is a talk in two parts. First, I will briefly review where we in New Zealand have got to in manufacturing a broad range of high-Tc products. Then I will address the topic of the title.

A quarter of a century after their discovery the mechanism that pairs electrons in cuprate superconductors remains uncertain, though it is widely believed to be magnetic. If so, the energy scale for the pairing boson and hence for Tc is governed to leading order by J, the antiferromagnetic exchange interaction. We vary J (as measured by two-magnon Raman scattering) by more than 160% by changing ion sizes in LnA2Cu3O7-δ where A=(Ba,Sr) and Ln=(La, ... Lu). Such changes are often referred to as “internal” pressure. Surprisingly, we find Tc_max anticorrelates with J. This is opposite to the effect of external pressure, suggesting that J is not the dominant energy scale governing Tc_max. In contrast, we show there is a clear correlation between Tc_max and the sum of polarisabilities of the constituent ions, weighted by their densities. An immediate consequence is the possibility of a pairing mechanism having its origin with quantized waves of electronic polarization.