Superconducting Gap of Filled Skutterudite Superconductor LaRu4P12 Studied by Sub-meV Resolution Photoemission Spectroscopy

We study the superconducting gap of the filled skutterudite superconductor LaRu 4 P 12 (transition temperature T c =7.2 K) using a newly developed sub-meV resolution photoemission spectrometer with a vacuum-UV laser as a light source. The result at 4.8 K shows the opening of a superconducting gap, whose spectral shape is well explained with an isotropic s-wave Dynes function using a gap size of 1.3 meV. This corresponds to the reduced gap 2Δ(0)/ k B T c of 4.2. The temperature dependence of the superconducting gap shows fairly good agreement with the conventional BCS theory. The present results are consistent with those of a moderately strong-coupling and isotropic s-wave superconductor.