Band-selective clean-limit and dirty-limit superconductivity with nodeless gaps in the bilayer iron-based superconductor CsCa2Fe4As4F2

Abstract

The optical properties of the new iron-based superconductor CsCa2Fe4As4F2 with Tc≃29 K have been determined. In the normal state a good description of the low-frequency response is obtained with a superposition of two Drude components of which one has a very low scattering rate (narrow Drude peak) and the other a rather large one (broad Drude peak). Well below Tc≃29 K, a pronounced gap feature is observed which involves a complete suppression of the optical conductivity below ∼110cm−1 and thus is characteristic of a nodeless superconducting state. The optical response of the broad Drude component can be described with a dirty-limit Mattis-Bardeen-type response with a single isotropic gap of 2Δ≃14 meV. To the contrary, the response of the narrow Drude component is in the ultraclean limit and its entire spectral weight is transferred to the zero-frequency δ(ω) function that accounts for the loss-free response of the condensate. These observations provide clear evidence for a band-selective coexistence of clean- and dirty-limit superconductivity with nodeless gaps in CsCa2Fe4As4F2.

Source-reported materials — not catalogue approval

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