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Has lightning struck twice?


John Bardeen, who would have celebrated his 100th birthday on 23 May, was the only person to have won two Nobel prizes in physics — one for the transistor and the other for the microscopic explanation of superconductivity, known as the Bardeen–Cooper–Schrieffer theory (BCS). For about 30 years, all superconductors more or less behaved according to BCS, none of them violating the predicted 30 K maximum transition temperature Tc. However, in 1986, along came the copper-oxide-based superconductors, with Tc rapidly reaching 164 K (under pressure). According to David Pines, Bardeen was delighted, as he had proposed several (non-BCS) models for superconductivity of purely electronic origin — with spin fluctuations replacing phonons as the pairing glue — and he thought a novel mechanism had to be at work in the cuprates.

Recently, a new and unexpected family of superconductors, based on FeP or FeAs layers, has seemed to further challenge BCS. In the case of non-superconducting LaOFeAs, with alternating stacks of LaO and FeAs layers, fluorine doping into the LaO layer induces superconductivity at 26 K (ref. 1). Under pressure2, the superconductivity survives up to 43 K. Chemically exchanging a smaller ion for La mimics applied pressure, and indeed, with Sm substitution, the transition temperature increases to 43 K (ref. 3). With further tweaking4, SmO1–xFxFeAs reaches a Tc of 55 K. Oxygen vacancies also seem to have an important role in raising Tc.

As the rush continues to push Tc ever higher, what of the physics of these superconductors? At first sight, Fe is an unusual ingredient, as ferromagnetism and superconductivity are generally viewed as being antagonistic. Placing a superconductor in a magnetic field will kill the superconductivity eventually.

And to complicate matters, antiferromagnetic correlations are present as well. Experimental probes5,6,7,8 detect a structural phase transition near 150 K and an antiferromagnetic transition — with long-range spin-density wave order — closer to 130 K in undoped LaOFeAs. With increasing F-doping, both of these features move to lower temperature, weakening and eventually disappearing. As superconductivity appears while the other two wane, the magnetism and superconductivity do seem to be competing states.

The similarities between these superconductors and the cuprates are suggestive, but it's too early to jump to conclusions, or say whether the FeAs family will dethrone the cuprates in terms of Tc. From Bardeen's approach to superconductivity, Pines ventures that Bardeen “would have suspected that a magnetic mechanism could be at work, but he would have wanted some direct evidence for this. After considering various experimental probes, he would likely have encouraged his colleagues to produce sufficiently pure samples that NMR experiments could look for a build-up of antiferromagnetic spin fluctuations in the normal state, analogous to that seen in the cuprates, and perhaps, as a bonus, provide a tentative identification of an unconventional pairing state. Then, he would have waited for the experimental results before embarking on detailed model calculations.”


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Chiao, M. Has lightning struck twice?. Nature Phys 4, 446 (2008).

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