Observation of a BCS Spectral Function in a Conventional Superconductor by Photoelectron Spectroscopy

We present high-resolution photoelectron spectra on the $A15\mathrm{}$-type conventional superconductor $\mathrm{V}{}_3\mathrm{Si}$, where—for the first time—both singularities of the BCS density of states can be resolved by photoemission spectroscopy (PES). With a transition temperature of about $T_c\approx 17\mathrm{K}$ the gap ${\Delta }_{\mathrm{gap}}$ of this compound has a magnitude of approximately 5 meV. A measurement by PES on this small energy scale requires a very high energy resolution $(\Delta E\lesssim 5\mathrm{meV})$ and sample temperatures significantly below $T_c$.