Vacuum tunneling spectroscopy of high-temperature superconductors: A critical study

Using scanning tunneling spectroscopy (STS) the local density of states (LDOS) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ [Bi(2212)] and ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBaCuO) monocrystals were studied at low temperature (down to 4.2 K). Due to the high quality of the vacuum tunneling junctions the pair-breaking effects are consequently reduced, and rather fine features of the superconducting LDOS could be analyzed. For Bi(2212) it is shown that, in order to fit properly the experimental data, ${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$ pairing (or a very anisotropic s pairing) as well as a strong in-plane anisotropy of the Fermi surface are required. Although YBaCuO STS spectra look more ``d like,'' no precise analysis was possible because of the high natural surface contamination of the samples. Both the fundamental problem of pairing symmetry and the influence of experimental conditions on vacuum STS measurements are discussed. \textcopyright{} 1996 The American Physical Society.

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