High-Resolution Photoemission Study of the Low-Energy Excitations Reflecting the Superconducting State of Bi-Sr-Ca-Cu-O Single Crystals

uv-photoemission spectra (hv=21. 2 eV) of single-crystal specimens of the high-Tc, superconductor Bi2Sr2CaCu 2O8 have been measured with a resolution of 20 meV in the normal state at 105 K and in the superconducting state at 15 K. In a narrow energy range of 120 meV below the Fermi energy, the low-temperature spectra reveal unambiguously the low-lying quasiparticle excitations rellecting the super-conducting state. A straightforward analysis of these spectra provides strong evidence for the existence of a superconducting gap with a reduced value 2Δ/k B T c≃8