Putting the squeeze on superconductivity

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Science  13 Jan 2017:
Vol. 355, Issue 6321, pp. 133
DOI: 10.1126/science.aak9803

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Superconductivity is a fascinating quantum state of matter that has captured the imagination of physicists for over a century. The pioneering theoretical work of Bardeen, Cooper, and Schrieffer described the superconducting state as a phase-coherent condensate of electrons bound into Cooper pairs, whose distinguishing hallmark, the order parameter, reflects the symmetry of the pair wave function. According to the laws of quantum mechanics, the wave function describing a pair of electrons must possess either even or odd parity. The vast majority of conventional low-temperature superconductors possess order parameters that are approximately isotropic (s-wave), in which the electron spins form a singlet state and hence are of even parity. The small handful of high-temperature superconductors, such as the cuprate and iron-based families, have more complex pairing symmetries (such as d-wave symmetry) but nonetheless are also described by an even-parity state. Odd-parity superconductors are much more elusive and may harbor exotic properties such as Majorana fermions and non-Abelian statistics. On page 148 of this issue, Steppke et al. (1) apply extremely large uniaxial pressures to single crystals of Sr2RuO4, one of the strongest candidates for odd-parity superconductivity, and find a dramatic enhancement of the superconducting transition temperature (Tc) as well as an unexpected change in the superconducting properties under large strains.