Features of the density of states of high-Tcsuperconductors probed by vacuum tunneling

A scanning tunneling microscope has been used to study ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{x}}$ single crystals with transition temperatures of 85 K. Images of atoms and topographical features were obtained at low temperatures and spectra were acquired in the vacuum tunneling mode. Interpretations of the conductance data suggest that 2Δ/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$≊8.2. Most significantly, an additional conductance peak at 90 meV, suggesting a localized state, is observed. The latter would appear to support the marginal-Fermi-liquid picture.