Nematic quantum critical point without magnetism in FeSe_1-xS_x superconductors

In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe_1-xS_x superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near x ≈0.17, the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe_1-xS_x as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.