STM TUNNELING SPECTROSCOPY ON HIGH-TC SUPERCONDUCTORS

STM tunneling spectroscopy has been performed on the bulk single cryst als of BiSrCaCuO (BSCCO) and the epitaxial thin films of YBaCuO (YBCO) at cryogenic temperatures. The STM images and tunneling spectra obser ved on the (001) surfaces can be classified into three cases; 1) Atomi c image is visible. However, the tunneling spectrum shows semiconducti ng or smeared superconducting gap structures, depending on the tip-sam ple distance. 2) Clear atomic image can not be obtained. But, the tunn eling spectrum shows flat bottom region with quite low zero bias condu ctance. 3) Tunneling spectra demonstrate gapless behavior, independent of the tip-sample separation. These observations support the quasi-2D electronic picture in which a-wave like 2D superconducting layers are coupled with each other through the Josephson effect.