The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba1−xSrxNi2As2 substitution series to show that strontium substitution promotes an electronically driven B1g nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity.
I.High-temperature superconductivity in both cuprate [1, 2] and iron-based materials [3-5] emerges from a notably complex normal state. Though magnetic spin fluctuations are commonly believed to drive Cooper pairing in both of these families, the common occurrence of a rotational symmetry-breaking nematic phase has captured increasing attention in recent years [6,7]. In contrast to a conventional structural transition, overwhelming evidence suggests that the nematic phase in these compounds is promoted by an electronic instability rather than lattice softening [8,9].Theoretical analyses have shown that fluctuations associated with such an electronic nematic phase, particularly near a putative quantum critical point, can significantly enhance superconductivity [10][11][12][13][14]. Being peaked at zero wave-vector, nematic fluctuations favor pairing instabilities in several symmetry channels, in contrast to the case of magnetic fluctuations. Experiments have indeed shown a striking enhancement of nematic fluctuations centered at optimal tuning of superconductivity in a number of iron-based superconductors [8,9], and a strong tendency towards nematicity in high T c cuprate materials [15][16][17]. However, the overarching presence of magnetic fluctuations emanating from proximate antiferromagnetic instabilities complicates drawing any isolated relation between enhanced pairing and nematicity in most nematic materials. The FeSe 1−x S x substitution series is one exception, where the system exhibits both superconductivity and nematicity in the absence of magnetic order [18]. However, in this series, the superconducting transition temperature T c is virtually unaffected by tuning through the nematic quantum critical point [18,19], leaving ...
