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

Uv-photoemission spectra ($h\nu =21.2\mathrm{}$ eV) of single-crystal specimens of the high-$T_c$ superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ 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 reflecting the superconducting state. A straightforward analysis of these spectra provides strong evidence for the existence of a superconducting gap with a reduced value $\frac{2\Delta }{k_B{T}_c}\simeq 8$.