Evidence for multiple superconducting gaps in optimally doped ${\text{BaFe}}_{1.87}{\text{Co}}_{0.13}{\text{As}}_2$ from infrared spectroscopy

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical response of single crystals of the pnictide high-temperature superconductor ${\text{BaFe}}_{1.87}{\text{Co}}_{0.13}{\text{As}}_2$ with $T_c=24.5\text{K}$. Our experimental data provide evidence for multiple superconducting energy gaps and can be well described in terms of three isotropic gaps with $2\Delta /k_B{T}_c$ of 3.1, 4.7, and 9.2. The obtained low-temperature value of the in-plane magnetic penetration depth is 270 nm.

Figure 1

(Color online) (a) Temperature dependence of the FIR reflectivity, $R\left(\omega \right)$ and (b) optical conductivity ${\sigma }_1\left(\omega \right)$. Solid circles show ${\sigma }^{\text{dc}}$ as reported for optimally doped ${\text{BaFe}}_{1-x}{\text{Co}}_x{\text{As}}_2$ (Ref. 26).

Figure 2

(Color online) Self-consist determination of the SC plasma frequency, ${\omega }_{p,\text{SC}}$. The measured ${\epsilon }_1^{5\text{K}}$ (symbols) is shown to compare well with the expected spectrum (solid line) that has been derived from the measured ${\epsilon }_1^{30\text{K}}$ by subtracting the missing inductive response due to $\Delta {\sigma }_1={\sigma }_1^{30\text{K}}-{\sigma }_1^{5\text{K}}$ (see the inset) to obtain ${\epsilon }^{reg}$ (dotted line) and adding the contribution ${\epsilon }^{\delta }$ (dashed line) of the SC delta function.

Figure 3

(Color) Simulation with a BCS-type model of multiple isotropic gaps (Ref. 31). Comparison of the experimental spectra of the (a) conductivity and [(b) and (c)] reflectivity ratio $R\left(T\right)/R\left(30\text{K}\right)$ (symbols) with the simulated spectra (lines) as obtained by first fitting the normal state response with a Drude-Lorentz model and then imposing one, two, or three independent isotropic BCS-type energy gaps (parameters are given in Table ). Arrows mark clear features due to the gap edges at $2\Delta$. The inset shows the corresponding experimental (symbol) and simulated (red line) spectra of the dielectric function.