High-resolution photoemission study of low-T_c superconductors: Phonon-induced electronic structures in low-T_c superconductors and comparison with the results of high-T_c cuprates

Recent advances in the energy resolution and cryogenic techniques used in photoemission spectroscopy have enabled us to directly observe the superconducting electronic structure of superconductors with a relatively lower transition temperature (low-T_c). In this paper, we report photoemission results on low-T_c superconductors measured with an ultrahigh-resolution low-temperature photoemission spectrometer. The result of Nb₃Al exhibits a superconducting gap and a phonon-induced peak-dip-hump structure characteristic of the strong-coupling superconducting spectral function. From measurements of Ba_0.67K_0.33BiO₃, we observe isotropic s-wave superconducting gap opening below T_c as well as depletion of intensity near the Fermi level above T_c, which can be attributed to a phonon-induced pseudogap. We will compare these photoemission results with those of high-T_c superconductors.