In a 1987 Science paper Phil Anderson made a radical proposal, stimulated by the discovery of high-Tc superconductivity in layered copper oxides. [A meausure of the influence of Anderson's paper is that it has been cited more than 4000 times]. My version of Anderson's proposal is:
The fluctuating spin singlet pairs [produced by the exchange interaction] in the [Mott] insulating state become charged superconducting pairs when the insulating state is destroyed by doping, frustration or reduced correlations.
These fluctuations are enhanced by spin frustration and low dimensionality.
To me the idea is radical and rather counter-intuitive because in one sense insulating and superconducting states are so different. One has infinite conductivity and the other zero.
Ben Powell and I have been invited to write a review article for Reports in Progress in Physics concerning organic charge transfer salts which can be described by a Hubbard model on the anisotropic triangular lattice. [An earlier review of related materials is here].
A key issue is whether these materials can be used as tuneable systems to test ideas about the interplay of superconductivity, Mott insulation, quantum fluctuations and spin frustration.
I'm doubtful about Prof. Anderson's theory. True, it has been cited many times, but 23 years later his theory has not been confirmed, nor has it led us closer to understanding high temp superconductivity. Prof. Anderson himself is still working on additional research on this very subject. See, for example, his strong comments posted at the Aspen Center for Physics blog--August 2007, "Novel Aspects of Superconductivity."
ReplyDeleteI have suggested a competing theory. My suggestions appear on this Condensed Concepts blogsite, following the May 15, 2009 entry entitled "A New State of Matter: The Superinsulator."
My theory borrows from Art Winfree, who wrote seminal articles on coupled oscillators in the late 1960s, in which he explained how there is a tendency for biological oscillators to synchronize precisely, in certain exact patterns. I argue that his theory may be applied to states of matter in physics, particularly in cases involving precise synchrony. Superconductors and superinsulators exhibit precise synchrony. The oscillations that synchronize might be simple and obvious (e.g., spin as in magnetism) or more esoteric (as in quantum fluctuations, the synchronization of which could explain the many new exotic states of matter.
Prof. McKenzie's comment that insulating states and superconducting states are so different, while correct, is also indirectly supportive of my theory. How can opposite states arise in the same material? Our traditional understanding of materials says this is impossible. But Winfree's theory says that varying forms of synchrony may emerge, and they are in many cases likely to be exactly synchronous or exactly antisynchronous. So Winfree would say that this result--exact opposite states, in the same matter--is highly intuitive.
Winfree's math was developed further by Steve Strogatz, of Cornell. See his book "Sync" and see his December 1991 Scientific American article for more information on Winfree's theory.