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

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Abstract
The electronic structure of high-Tc Bi2Sr2CaCu2O8 single crystals in the vicinity of the Fermi level EF is determined down to about 0.7Tc 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 > Tc. For T < Tc the emission intensity changes distinctly—at EF it decreases whereas it increases at about 100 meV below EF—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)/kB Tc about twice as large as the BCS value indicating that Bi2Sr2CaCu2O8 is a strong-coupling superconductor.