Superconducting transitions studied by ultrahigh-resolution photoemission

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low-temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni_1-xPt_x)₂B₂C (x=0.0 and 0.2) shows a difference in the slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi₂B₂C, consistent to magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales.