Photoemission Spectroscopy of the Strong-Coupling Superconducting Transitions in Lead and Niobium

We study the changes in the electronic structure associated with low- $T_c$ strong-coupling superconducting transitions in Pb and Nb using ultrahigh-resolution (2.3 meV) temperature-dependent (5.3–12.0 K) photoemission spectroscopy. We observe peaks in the density of states on entering the superconducting-phase accompanying gap formation and spectacular redistribution of spectral weight at low energy scales as a function of temperature. The well-known peak-dip feature of the high- $T_c$ cuprates is seen in Pb, making it a characteristic of strong-coupling superconductivity.