Gap Opening and Orbital Modification of Superconducting FeSe above the Structural Distortion

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a $\sim 36\mathrm{meV}$ energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

Figure 1

Real part of the optical conductivity [$\sigma (\omega )={\sigma }_1(\omega )+i{\sigma }_2(\omega )$]. The arrow depicts the photon energy adopted in the steady-state and the transient reflectivity experiments. The inset shows $T$-dependent optical reflectivity (solid dots) and Hall coefficient $R_H$ (open dots), where the solid line guides the eye.

Figure 2

(a) Picosecond response of transient optical reflectivity $\Delta R/R$ measured at different $T$ (solid lines from top to bottom: 20, 40, 60, 80, 100, 120, 140, 180, 220, 260, 297 K). The data are normalized and vertically shifted for clarity. The dotted lines depict relaxation processes. (b) $T$-dependent ${\tau }_{\mathrm{slow}}^{-1}$ and $A_{\mathrm{fast}}$ (dots). The dotted lines are the descriptions of TTM for poor metals in the high-$T$ (HT) and low-$T$ (LT) limits with the same $c\mathrm{\text{−}}p$ coupling constant $\lambda$. The solid line is the Rothwarf-Taylor (RT) fit. The line in the inset shows the fitting curve of Kabanov bottleneck model. (c) $T$-dependent fitting variables $A_{\mathrm{slow}}$, $A_{\mathrm{step}}$, and ${\tau }_{\mathrm{fast}}$.

Figure 3

(a) Transient optical reflectivity changes measured at different temperatures (solid lines), which are vertically shifted for clarity. The dashed lines depict the decrease of the oscillation frequency at lower $T$. (b) $T$-dependent oscillation amplitude $A_p$. (c) $T$-dependent effective stiffness. The fitting curve is based on the formula $C_{\mathrm{eff}}=C_{\mathrm{eff}}^0(T-\Theta )/(T-T_s)$ [28], where $C_{\mathrm{eff}}^0$ is the high-$T$ effective stiffness and $\Theta$ is a fitting variable.