Observation of Band Renormalization Effects in Hole-Doped High-T_c Superconductors

We report a systematic high-resolution angle-resolved photoemission spectroscopy on high-T_c superconductors Bi₂Sr₂Ca_n-1Cu_nO_2n+4 (n=1–3) to study the origin of many-body interactions responsible for superconductivity. For n=2 and 3, a sudden change in the energy dispersion, so called “kink”, becomes pronounced on approaching (π,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. For n=1, the kink shows no significant temperature dependence even across T_c. This could suggest that the coupling of electrons with Q=(π,π) magnetic mode is dominant in the superconducting state for multilayered cuprates, while the interactions at the normal state and that of single-layered cuprates have a different origin.