Peculiarities of the nematic transition in FeSe0.675Te0.3S0.025 and its proximity to the quantum critical point

Abstract In this work, we present the studies of structural phase transitions in FeSe0.675Te0.3S0.025 crystals. The data obtained indicate a significant change in the behavior of many characteristics during the transition in this composition compared to the case of the unsubstituted FeSe. The resistivity at low temperatures for the studied FeSe0.675Te0.3S0.025 is proportional to the square of the temperature, while for pure FeSe below the structural transition it depends almost linearly on temperature. 77Se NMR studies also confirm a change in the type of phase transition. The NMR data showed a noticeable line broadening below the structural transition and an anomaly in the temperature dependence of the relaxation rate, which was not observed in FeSe. Our results confirm the quantum criticality of Fe(Se,Te) at a low Te content or existence of the nematicity change point (NCP). This makes the phase diagram of quasi-binary Fe(Se,Te) compounds generally consistent with the phase diagram of FeSe under pressure. In both cases, there is a local minimum in the superconducting critical temperature near NCP and a strong increase in the superconducting critical temperature near the point of complete suppression of nematicity.