In-plane tunneling spectroscopy of the high temperature superconductor, Bi-2212.

摘要

A novel technique for tunneling into the a-b plane of the layered cuprate (copper-oxide) superconductors is described and applied to the study of the in-plane tunneling spectra of the high temperature superconductor {dollar}rm Bisb2Srsb2CaCusb2Osb8{dollar} (Bi-2212). This technique involves using a home-built low-temperature scanning tunneling microscope to form mechanical tunneling junctions with well-prepared edges of single crystals. Our research group has pioneered the use of these cross-junctions, and the results presented in this dissertation constitute the first systematic investigation employing this technique.; Using this cross-junction tunneling technique, the superconducting energy gap has been measured along various directions within the a-b plane of Bi-2212. The results show unambiguously that the in-plane energy gap is highly anisotropic. The measured energy gap {dollar}Deltasb{lcub}rm p-p{rcub}{dollar} is largest along the a-axis direction, with {dollar}Deltasb{lcub}rm p-p{rcub}approx 36{dollar} meV and decreases gradually with {dollar}Deltasb{lcub}rm p-p{rcub}approx 20{dollar} meV along the direction 45{dollar}spcirc{dollar} from the a-axis. The anisotropy pattern is found to have a four-fold symmetry. Using these results, an angular mapping of the in-plane gap anisotropy is presented and compared to theoretical predictions as well as results from other experiments. Effects due to the gap anisotropy on the tunneling spectra are also discussed.; In another set or experiments, the effects of Pb doping on the tunneling spectra of Bi-2212 were studied. Doping Bi-2212 with Pb causes a partial substitution of the Bi atoms between the conducting Cu-O{dollar}sb2{dollar} planes, and modifies some of the electronic and structural properties of the material. In this study it was found that while Pb doping decreases T{dollar}sb{lcub}rm c{rcub}{dollar} profoundly, it has a minimal effect on the in-plane energy gap, leading to a significant increase in the reduced gap, {dollar}rm2Delta /ksb{lcub}B{rcub}Tsb{lcub}c{rcub}.{dollar} This result is consistent with other studies of the effects of in-plane dopants, but is the first evidence of such an effect attributable to defects introduced out of the plane. Inspection of the physical evidence leads to the conclusion that the interlayer coupling has been modified. This finding indicates a connection between the mechanism responsible for high temperature superconductivity in the cuprates and the interlayer coupling between the Cu-O{dollar}sb2{dollar} planes.; For both doped and undoped Bi-2212, substantial differences between the observed spectra and predictions from BCS theory are observed. While the BCS theory has been extremely successful in explaining conventional superconductivity, the agreement with data from high temperature cuprate superconductors has been poor. The present results indicate that either a substantial modification to BCS or a new theoretical framework is needed to explain superconductivity in the high temperature superconductors.