Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi₂Sr₂CaCu₂O_8+δ Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi₂Sr₂CaCu₂O_8+δ (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 ( $T_c=91$ K) at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form ( $\cos (2\Phi$ )). It can be alternatively fitted by including a high-order term ( $\cos (6\Phi$ )) 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 Bi₂Sr₂CuO_6+δ 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. (paper)