Evidence for nematic fluctuations in FeSe superconductor: a 57Fe Mössbauer spectroscopy study.

Abstract

There has been controversy about the driving force of the nematic order in the FeSe superconductor. Here, we present a detailed study of the 57Fe Mössbauer spectra of FeSe single-crystal powders, focusing on the temperature dependences of the hyperfine parameters in the vicinity of the nematic transition temperature, Ts ~ 90 K. The nematicity-induced splitting of dxz and dyz bands, obtained from the anomalous increase in quadrupole splitting near Ts, starts at 143 K. The temperature evolution of the lattice dynamics, deduced from the recoilless fractions and second-order Doppler shifts, is found to undergo successively two segments of phonon-softening (160 K - 105 K) and phonon-hardening (105 K - 90 K), related to the appearance of local orthorhombic distortions above Ts and the establishing way of the associated nematic correlations. Analysis of the linewidths shows that spin fluctuations occur not only below 70 K but also across Ts (105 K-70 K), accompanied by the non-Fermi liquid behavior of the electrons. The results demonstrate the strong interactions between lattice, spin, and electron degrees of freedom in the vicinity of Ts and that the lattice degrees of freedom may play an essential role in driving the nematic order for FeSe.