High resolution angle-resolved photoemission is used to study the electronic structure of optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. These studies, with an energy resolution of 8 meV, show that the sharp peak observed in the superconducting state has an intrinsic width of the order of 14 meV. Detailed temperature dependent studies reveal that this peak is fixed in binding energy at all temperatures at which it is observable and further that it exists at temperatures even above the superconducting transition temperature $T_c$. However, our analysis indicates that with the onset of long range phase coherence a gap emerges in the spectral response between the main incoherent peak and the Fermi level.

• Received 9 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2179