Interplay between lattice and spin states degree of freedom in the FeSe superconductor: Dynamic spin state instabilities

Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large (similar to 6.5%) hardening of the B-1g (Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with a gradual decrease of the iron paramagnetic moment. Ab initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature T-s and its change below T-s is discussed in relation with the opening of an energy gap between low (S = 0) and higher spin states which prevents magnetic order in FeSe. The very narrow phonon linewidths compared to observations in FeTe suggest the absence of intermediate spin states in the fluctuating spin state manifold in FeSe. In the tetragonal phase under approaching T-s we observe the development of a quasielastic Raman response that is typical for the presence of nematic fluctuations. DOI: 10.1103/PhysRevB.87.144508