Studies on atomic images and tunneling spectra of Bi_2Sr_2CaCu_2O_(8+ delta ) by STM/STS

摘要

In STM experiments on the cleaved surface of Bi_2Sr_2CaCu_2O_(8+ delta ), the semiconducting Bi-O and metallic Cu-O planes have been seletively observed at bias voltages larger and smaller than the Bi-O plane semiconducting gap E_g/e, respectively. The Cu-O plane STM images of crystals with the bole concentration p approx 0.14, characterized by cross stripes, are consistent with the Cu-O bonds. We have performed STS experiments while observing STM images of the superconducting Cu-O plane, and observed tunneling spectra consistent with an energy gap of d_x2_(-y)2 symmetry. The spectral shape at low voltages was very reproducible for changing tip heights as far as the tip was not brought so close to the specimen as to give rise to some pair-breaking effects. We have also examined the p dependence of 2 DELTA _0. In the underdoped region below p approx p_o giving the maximum T_c, 2 DELTA _o tends to increase with decreasing p although T_c is largely reduced, which is in qualtiative agreement with the results of photoemission experiments by Shen et al..