ATOMIC SITE TUNNELING SPECTROSCOPY ON HIGH-T(C) SUPERCONDUCTORS

Superconducting gap structures of Bi2Sr2CaCu2Oy (BSCCO) and YBa2Cu3Oy (YBCO) have been probed by scanning tunneling microscopy (STM) at cryo genic temperatures. The tunneling conductance curves observed on bulk single crystals of BSCCO and epitaxial thin films of YBCO revealed cle ar overshooting peaks and flat bottom regions around V = 0 with quite low zero-bias conductances of approximately 1%. Since the electron tun neling process in STM is essentially incoherent, the present observati on is favored by the s-wave pairing mechanism. However, the conductanc e curves were found to be substantially smeared in comparison with the conventional excitation spectra predicted in the BCS (isotropic s-wav e) superconductors, suggesting gap anisotropy.