Observation of pseudogaplike feature above $T_c$ in LiFeAs by ultrafast optical spectroscopy

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature $T_c$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1–2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

Figure 1

The temperature-dependent dc magnetic susceptibility of LiFeAs measured in field cooling (FC) and zero field cooling (ZFC) modes. The insets show the magnetic susceptibility around superconducting transition temperature.

Figure 2

(a) The time-resolved optical reflectivity from 7 to 20 K. (b)The processed traces at 7, 10, and 13 K after subtraction of the trace at 16 K (just above $T_c$ = 15 K of LiFeAs) to reveal the SC quasiparticle relaxation.

Figure 3

(a) The density of thermally excited quasiparticles, $n_T\left(t\right)$; (b) the magnitudes of $M_{\mathrm{SC}}$ described in the text are shown in black dots. The fitting lines with $\Delta \left(0\right)$ = 3.5, 5.2, and 6.0 meV are also shown.

Figure 4

(a) The time-resolved optical reflectivity from 4 to 150 K. The normalized traces are shown in (b) for above 99 K and in (c) for below 99 K. The traces above 99 K in (c) are similar while there is a nonmonotonic temperature dependence of a broad hump below 99 K as highlighted by the orange dashed ellipse. The trace at 99 K is shown in both (a) and (b) as a reference for comparison.

Figure 5

The processed traces for fitting the pseudogaplike feature $B\left(t\right)$ at (a) 5 K, (b) 20 K, (c) 40 K, (d) 51 K, (e) 64 K, and (f) 99 K. The fitting curves are shown in dashed lines.

Figure 6

The temperature dependence of normalized $M_{\mathrm{PG}}$ are shown in black dots. The red line represents the fitted curve with ${\Delta }_{\mathrm{PG}}=18\mathrm{meV}$ in Eq. (2). The corresponding relaxation time ${\tau }_{\mathrm{PG}}$ is shown in the inset.