Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi_2Sr_2CaCu_2O_8+\delta Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi_2Sr_2CaCu_2O_8+\delta (Bi2212) superconductor in the superconducting and pseudogap states by super-high resolution laser-based angle-resolved photoemission spectroscopy. The precise determination of the superconducting gap for the nearly optimally doped Bi2212 (T_\rm c=91 K) at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form (\cos(2\it \Phi)). It can be alternatively fitted by including a high-order term (\cos(6\it \Phi)) in which the next nearest-neighbor interaction is considered. We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi_2Sr_2CuO_6+\delta superconductors. This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors. These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.