Abstract
Cleaved surfaces of Bi 2Sr 2CaCu 2O 8+ δ ( T c=86 K) have been investigated by low temperature scanning tunneling spectroscopy. The clean surface has been prepared by cleaving the sample at liquid helium temperature in a vacuum. The intrinsic superconducting density of states has been obtained in the tunneling spectra exhibiting a superconducting gap with a metallic background; tunneling spectra with semiconducting backgrounds are not intrinsic. The tunneling spectra have a broad bump just outside a superconducting gap. The background conductance is slightly asymmetric with respect to the Fermi energy E F; the density of states above E F is smaller than that below E F. Because of the broad bump, the spectra cannot be fitted by the theoretical bulk density of states of s-wave or d-wave superconductors. The broad bump in the tunneling spectra can be seen to consist of two peaks at bias voltages of ∼30 meV and ∼40 meV. The shape of measured spectra is similar to the spectra calculated in the 2D Hubbard model.
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