Anisotropic s-wave superconductors studied by angle-resolved photoemission spectroscopy

We review our angle-resolved photoemission spectroscopy (ARPES) results across superconducting transitions in 2H–NbSe2 and MgB2, and also show recent results on YNi2B2C. For 2H–NbSe2 and MgB2, we observe that the superconducting gap exhibits clear Fermi surface (FS) sheet dependence for two types of FS sheets with different characters, thus providing evidence that the FS sheet dependence is a general feature for a superconductor having multiple FS sheets of different character. In YNi2B2C, we show experimental FSs, and momentum-dependent superconducting gap showing large anisotropy observed on a single FS. These ARPES results provide direct evidence for and importance of momentum-dependent superconducting gaps of phonon-mediated superconductors.