Abstract
In many unconventional superconductors, nematic quantum fluctuations are strongest where the critical temperature is highest, inviting the conjecture that nematicity plays an important role in the pairing mechanism. Recently, Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ has been identified as a tunable nematic system that provides an ideal testing ground for this proposition. We therefore propose several sharp empirical tests, supported by quantitative calculations in a simple model of Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$. The most stringent predictions concern experiments under uniaxial strain, which has recently emerged as a powerful tuning parameter in the study of correlated materials. Since uniaxial strain so precisely targets nematic fluctuations, such experiments may provide compelling evidence for nematic-mediated pairing, analogous to the isotope effect in conventional superconductors.
Highlights
Considerable debate remains regarding the pairing mechanisms of most unconventional superconductors
We have shown that even weak coupling of the electrons near the Fermi surface to nematic fluctuations can explain the observed increase in the Tc of Ba1−xSrxNi2As2 as x is reduced from 1.0 to 0.7
(1) nematic fluctuations promote gap anisotropy, whose signatures are observable in tunneling conductance, specific heat, and the gap to Tc ratio, among others; (2) uniaxial strain substantially affects Tc and other superconducting properties, by altering both the band structure and the strength of the nematic-mediated interaction
Summary
Considerable debate remains regarding the pairing mechanisms of most unconventional superconductors. Many hypothesize that fluctuations of some order parameter(s), such as magnetism, mediate the bulk of the pairing interaction These hypotheses are plausible, in part, because the superconducting region of the phase diagram is often close to various other forms of long-range order. The other set concerns the effects of uniaxial strain, which explicitly breaks lattice rotation symmetry and reduces the strength of nematic fluctuations. Both sets of predictions are essentially general, but those regarding strain are sharper, may apply to a broader variety of materials, and are timely in light of the increasing use of strain as an experimental tuning parameter [21]
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