Sixfold enhancement of superconductivity in a tunable electronic nematic system*

The electronic nematic phase - in which electronic degrees of freedom lower the crystal rotational symmetry - is commonly observed in high-temperature superconductors. However, understanding the role of nematicity and nematic fluctuations in Cooper pairing is often made more 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 that is not magnetic, and show that the enhancement is directly born out of strong nematic fluctuations associated with a quantum phase transition. We present measurements of the resistance as a function of strain in Ba$_{1-x}$Sr$_{x}$Ni$_2$As$_2$ to show that strontium substitution promotes an electronically-driven nematic order in this system. In addition, the complete suppression of that order to absolute zero temperature leads to an 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 investigate the role of nematicity in boosting superconductivity.