Optical properties of the optimally doped Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{2}$As$_{2}$)$_5$ single crystal

TL;DR

This study investigates the optical properties of an optimally doped iron-based superconductor, revealing multiple superconducting gaps and a pseudogap evolution consistent with preformed Cooper pairs.

Contribution

It provides detailed optical conductivity measurements and theoretical modeling showing the presence of multiple superconducting gaps and pseudogap behavior in this compound.

Findings

Presence of multiple bands at the Fermi level.

Observation of pseudogap evolving into superconducting gap below Tc.

Identification of two distinct superconducting gaps with different magnitudes.

Abstract

We have measured the reflectivity of the optimally doped Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{10}$As$_{10}$) single crystal ($T_c$ = 32.8K) over the broad frequency range from 40 to 12000 $cm^{-1}$ and for temperatures from 8 to 300 K. The optical conductivity spectra of the low frequency region ($< 1,000$ $cm^{-1}$) in the normal state (80 K $< T \leq$ 300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low frequency Drude spectra develops pseudogap (PG) hump structure at around $\approx 100$ $cm^{-1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below $T_c$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above $T_c$ into the SC gap structure below $T_c$. The extracted two SC gap sizes are $\Delta_S$ = 4.9 $meV$ and $\Delta_L$ = 14.2 $meV$, suggesting Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{10}$As$_{10}$) as a multiple gap superconductor with a mixed character of the weak coupling and strong coupling superconductivity.