Peculiarities of the Nematic Transition in FeSe\(_{0.675}\)Te\(_{0.3}\)S\(_{0.025}\) and Its Proximity to the Quantum Critical Point

Abstract

\(^{77}\)Se NMR measurements confirm that in Fe(Se,Te) a change in the type of nematic transition is observed near its complete suppression. We present the studies of structural phase transitions in FeSe\(_{0.675}\)Te\(_{0.3}\)S\(_{0.025}\) 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. Similar to Fe(Se,Te) with a close tellurium content, the resistivity at low temperatures for the studied FeSe\(_{0.675}\)Te\(_{0.3}\)S\(_{0.025}\) is proportional to the square of the temperature, while for pure FeSe below the structural transition it depends almost linearly on temperature. \(^{77}\)Se NMR proves that these changes are associated with changes in the type of ordering. 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 existence of the nematicity change point (NCP) in Fe(Se,Te) at a low Te content and make the phase diagram of quasi-binary Fe(Se,Te) compounds generally consistent with the phase diagram of FeSe under pressure. Thus, in terms of chemical pressure, the effect of isovalent substitution in the series of Fe(Te,Se,S) compounds on the type of ordering can be caused by the anisotropic part of the deformation.