Systematic temperature-dependent optical studies of optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (OP Bi2212) single crystals have been carried out. An electron-boson spectral function peaked at 43 meV is derived in the normal state at 100 K, indicating that the most significant spectral feature in OP Bi2212 is a resonance at 43 meV. The optical data below $T_c$ show that this boson is directly involved in the pair formation in OP Bi2212. Origins of the 43 meV mode are discussed.

• Received 21 August 2002

DOI:https://doi.org/10.1103/PhysRevB.66.144514