Optical conductivity evidence of clean-limit superconductivity in LiFeAs

We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.

Figure 1

In-plane infrared reflectivity of LiFeAs. The inset shows the reflectivity at 300 K up to 5 eV.

Figure 2

Optical conductivity of LiFeAs above and below $T_c$. The inset shows the spectral weight below 100 K calculated by integrating ${\sigma }_1$ up to 0.5 eV. Below $T_c$, it shows a drop related to the formation of the superconducting condensate.

Figure 3

Penetration depth of LiFeAs at 5 K (symbols). The circles are the data and the lines show BCS calculations for the penetration depth. The “Dirty limit” simulation has $2\Delta =25{\text{cm}}^{-1}$ and ${\tau }^{-1}=50{\text{cm}}^{-1}$. “Intermediate” and “Clean limit” calculations utilize the same value for the gap and ${\tau }^{-1}$ of 25 ${\text{cm}}^{-1}$ and 10 ${\text{cm}}^{-1}$, respectively. An optical gap of 50 ${\text{cm}}^{-1}$ together with a 10 ${\text{cm}}^{-1}$ scattering rate produce the “Clean limit large gap” simulation.

Figure 4

Drude-Lorentz modeling of the optical conductivity of LiFeAs at 100 K. The thick gray line is the data and the thin black line is a fit with a single Drude term $({\Omega }_p=9980{\text{cm}}^{-1}$ and ${\tau }^{-1}=137{\text{cm}}^{-1})$ and four Lorentz terms [far-infrared (FIR), midinfrared (MIR), and two for higher frequencies (HF)]. Individual contributions are shown as hatched areas. The inset shows the temperature dependence of the dc resistivity (solid line), the Drude term scattering rate (squares), and a linear fit to the ${\tau }^{-1}\left(T\right)$ data (dashed line).