Photoemission spectroscopic evidence of gap anisotropy in an f-electron superconductor.

We used low-temperature ultrahigh-resolution ($360\text{ }\text{ }\ensuremath{\mu}\mathrm{e}\mathrm{V}$) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an $f$-electron superconductor $\mathrm{C}\mathrm{e}\mathrm{R}{\mathrm{u}}_{2}$. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an $f$-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in $\mathrm{C}\mathrm{e}\mathrm{R}{\mathrm{u}}_{2}$, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated $d$- and $f$-electron superconductors.