VACUUM TUNNELING SPECTROSCOPY OF HIGH-TEMPERATURE SUPERCONDUCTORS - ACRITICAL-STUDY

Using scanning tunneling spectroscopy (STS) the local density of state s (LDOS) of Bi2Sr2Ca1Cu2O8+delta [Bi(2212)] and Y1Ba2Cu3O7-delta (YBaC uO) monocrystals were studied at low temperature (down to 4.2 K). Due to the high quality of the vacuum tunneling junctions the pair-breakin g effects are consequently reduced, and rather fine features of the su perconducting LDOS could be analyzed. For Bi(2212) it is shown that, i n order to fit properly the experimental data, d(x2-y2) 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 natu ral surface contamination of the samples. Both the fundamental problem of pairing symmetry and the influence of experimental conditions on v acuum STS measurements are discussed.