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 ($\sim 6.5\%$) 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.

Figure 1

Polarized Raman spectra of FeSe at room temperature. The inset shows the atomic displacement patterns of the Raman-active phonon modes.

Figure 2

Polarized Raman spectra of FeSe at $T=7$ K. The left inset shows a comparison between FeSe and FeTe at $T=50$ K. The right inset shows the temperature evolution of the ($aa$) polarized spectra.

Figure 3

Temperature dependence of the $A_{1g}$(Se) and $B_{1g}$(Fe) phonon line parameters in FeSe. (a), (b) Phonon frequencies. Black dashed lines correspond to Grüneisen dependencies with ${\gamma }_{A1g\left(\mathrm{Se}\right)}=1.04$, ${\gamma }_{B1g\left(\mathrm{Fe}\right)}=5.09$, and ${\gamma }_{B1g\left(\mathrm{Fe}\right)}=0.9$. (c), (d) Linewidths (FWHM). Black dashed lines correspond to fits with Eq. (1). (e), (f) Integrated intensities.

Figure 4

Temperature-dependent Raman spectra of FeSe. The low-energy quasielastic spectral weight does not show a pronounced temperature evolution in the ($aa$), ($xx$), and ($xy$) channels of scattering. In the ($ab$) channel ($B_{2g}$ channel in the tetragonal setting) quasielastic scattering is not present at high temperature but develops below 150 K.

Figure 5

(a) Differential Raman spectra from 150 K in ($ab$) polarization. The zero levels are shown by the horizontal lines with the same color as the spectra. (b) Integrated intensity of the quasielastic tail. (c) Energy of the hump as function of temperature.