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Simplicity will out: Novel experiment-based expression explains behavior of unconventional superconductors

Posted December 30, 2014

Superconductivity – perhaps the leading example of emergent quantum behavior in matter – was discovered in 1911 but lacked theoretical explanation for almost five decades. In 1957, John Bardeen, Leon Cooper, and John Robert Schrieffer (BCS) developed a microscopic theory of superconductivity1 that came to be known as the BCS theory, which describes superconductivity as a microscopic effect caused by a condensation of Cooper pairs into a boson-like state. BCS theory explains the behavior of what are now known as conventional superconductors – metals for which phonons provide the recently controversial “pairing glue” that leads to the effective attractive quasiparticle interaction responsible for their superconductivity. (Phonons are quantized lattice vibrations, and quasiparticles are mobile electrons or holes in materials; both are quantized elementary excitations.)

A phase diagram for heavy-electron superconductors

A phase diagram for heavy-electron superconductors. In region I, only itinerant heavy electrons exist below TL owing to complete hybridization of the f-moments with background conduction electrons; in region II, collective hybridization is not complete so that heavy electrons coexist with partially hybridized local moments; in region III, these residual moments order antiferromagnetically (AF) at TN and the surviving heavy electrons become superconducting (SC) at a lower temperature, Tc. The coupling of heavy electrons to the magnetic spin fluctuations emanating from the QCP is responsible for the superconductivity in all regions. Credit: Yang Y-F, Pines D (2014) Emergence of superconductivity in heavy-electron materials. Proc Natl Acad Sci USA 111(51):E18178-18182.

As it is wont to do, history is now repeating itself: Unconventional superconductors, in which pairing glue and pairing condensate symmetry differ from conventional superconductors, were discovered in the 1980s – but while it appears from both theory and experiment that electronic spin fluctuations provide the pairing glue for the unconventional superconductivity, a general model remains elusive.

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