Superconducting energy gap and c-axis plasma frequency of (Nd,Sm)O0.82F0.18FeAs superconductors from infrared ellipsometry

TL;DR

This study uses infrared ellipsometry to investigate the anisotropic superconducting properties of Nd and Sm-based pnictide superconductors, revealing a large energy gap and high anisotropy similar to cuprate high-temperature superconductors.

Contribution

First infrared ellipsometry measurements of the c-axis response in (Nd,Sm)O0.82F0.18FeAs superconductors, demonstrating strong anisotropy and a large superconducting energy gap.

Findings

c-axis plasma frequency upper limit ≤ 260 cm⁻¹

c-axis magnetic penetration depth ≥ 6 μm

anisotropy ratio λ_c/λ_ab ≥ 30

Abstract

We present ellipsometric measurements of the far-infrared dielectric response of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar as in the cuprate high-temperature superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis superconducting plasma frequency of $\omega_{{\rm pl},c}^{\rm SC}\leq 260\cm$ which corresponds to a lower limit of the c-axis magnetic penetration depth of $\lambda_c\geq6\mum$ and an anisotropy of $\lambda_c/\lambda_{ab}\geq 30$ as compared to $\lambda_{ab}=185$ nm from muon spin rotation [A. Drew {\it et al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra exhibit the signatures of a gap-like suppression of the conductivity in the superconducting state with a large gap magnitude of $2\Delta\approx300\cm$ (37 meV) and a ratio of $2\Delta/k_{\rm B}\tc\approx8$ that is suggestive of strong coupling.