On the Superconducting Energy Gap in Bi 2 Sr 2 CaCu 2 O 8 Investigated by High-Resolution Angle-Resolved Photoemission

Abstract

The electronic structure of high-T_c Bi₂Sr₂CaCu₂O₈ single crystals in the vicinity of the Fermi level E_F is determined down to about 0.7T_c by high-resolution angle-resolved photoemission spectroscopy applying HeI and synchrotron radiation. Spectra taken with 18 eV photon energy at an emission angle of 9° reveal a clear Fermi edge T > T_c. For T < T_c the emission intensity changes distinctly—at E_F it decreases whereas it increases at about 100 meV below E_F—accompanied by a clear shift of the emission onset to higher binding energy. These observations can be consistently explained by the opening of an energy gap of about 30 meV in the superconducting quasi-particle density of states, yielding a ratio Δ (0)/k_B T_c about twice as large as the BCS value indicating that Bi₂Sr₂CaCu₂O₈ is a strong-coupling superconductor.