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

Abstract

uv-photoemission spectra ($h\ensuremath{\nu}=21.2$ 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 $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}\ensuremath{\simeq}8$.