Evidence for nematic fluctuations in FeSe superconductor: a⁵⁷Fe Mössbauer spectroscopy study

There has been controversy about the driving force of the nematic order in the FeSe superconductor. Here, we present a detailed study of the⁵⁷Fe Mössbauer spectra of FeSe single-crystal powders, focusing on the temperature dependences of the hyperfine parameters in the vicinity of the nematic transition temperature,T_s∼ 90 K. The nematicity-induced splitting ofd_xzandd_yzbands, obtained from the anomalous increase in quadrupole splitting nearT_s, 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 aboveT_sand the establishing way of the associated nematic correlations. Analysis of the linewidths shows that spin fluctuations occur not only below 70 K but also acrossT_s(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 ofT_sand that the lattice degrees of freedom may play an essential role in driving the nematic order for FeSe.