PRECISE STM SPECTROSCOPY ON THE SUPERCONDUCTING PHASE OF BI2SR2CACU2O8

We carried out electron tunnelling spectroscopy for single crystals of Bi2Sr2CaCu2O8 in the superconducting phase with use of STM. The tunnelling differential conductance curve with low noise was obtained for the cleaved surface and compared with that for the lateral surface. The two spectra were found to be essentially identical. It is concluded that tunnelling spectra obtained by our STM experiment contain contributions from every wave number and represent the total electronic density of states. From the functional form of the tunnelling conductance, the simple d wave was excluded for the symmetry of the superconducting pair wave function. The conductance curve obtained was well explained by the model, in which the gap is finite on the whole Fermi surface while it is highly anisotropic in k-space. It was suggested that the symmetry of the superconducting pair wave function is of the mixed symmetry of the s and d waves.