Probing superconducting energy gap from infrared spectroscopy on a Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal with T$_c$=37 K

TL;DR

This study uses infrared spectroscopy to investigate the superconducting energy gaps in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals, confirming the presence of two gaps and measuring the penetration depth, aligning with previous ARPES results.

Contribution

First infrared spectroscopy measurement directly probing the superconducting gaps in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals, providing quantitative gap values and validating the spectral weight sum rule.

Findings

Observation of two distinct superconducting gaps in infrared spectra.

Determination of the penetration depth as approximately 2000 Å.

Validation of the Ferrell-Glover-Timkham sum rule at low energy scales.

Abstract

We performed optical spectroscopy measurement on a superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal with T$_c$=37 K. Formation of the superconducting energy gaps in the far-infared reflectance spectra below T$_c$ is clearly observed. The gap amplitudes match well with the two distinct superconducting gaps observed in angle-resolved photoemission spectroscopy experiments on different Fermi surfaces. We determined absolute value of the penetration depth at 10 K as $\lambda\simeq2000 \AA$. A spectral weight analysis shows that the Ferrell-Glover-Timkham sum rule is satisfied at low energy scale, less than 6$\Delta$.