High-resolution ARPES study of quasi-particles in high-T _c superconductors

We have performed systematic and comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on Bi-system high-T _c superconductors to study the quasi-particles related to and/or responsible for the occurrence of the superconductivity. We have experimentally determined the full energy dispersion and the coherence factors of the Bogoliubov quasi-particles, which show a good quantitative agreement with the prediction from the BCS theory. This proves the basic validity of the BCS theory in the broad sense to describe the high-T _c superconductivity. We have experimentally identified two different bosonic modes in Bi-system high-T _c superconductors. One produces a 'small' kink in the limited momentum region around the nodal cut and another is related to a 'large' kink which exists in a relatively wide momentum region with the stronger magnitude closer to the (π, 0) point. The observed momentum and temperature dependence of the kinks as well as the impurity effect show that the large kink is of magnetic origin and closely related to the high-T _c superconductivity.