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Probing the Superconducting Order Parameter of High-Tc Superconductor Bi2Sr2CaCu2O8+δ by Scanning Josephson Tunneling Microscopy

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eScholarship - University of California
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The technique of scanning tunneling microscopy (STM) with a normal metal tip has recently been used to study the high transition temperature (T<sub>C</sub>) superconducting cuprates and has revealed many fascinating and complex features of quasiparticle excited states of these materials. For conventional superconductors, the Bardeen-Cooper-Schrieffer theory connects the pair amplitude and the superconducting gap as measured from the quasiparticle excitation spectra, while for the high-T<sub>C</sub> materials there is still no theory to connect these quantities. We are unable to make any quantitative analysis of the superconducting ground state from the quasiparticle data.Josephson tunneling is the tunneling of the Cooper pairs between two superconductors and the Josephson current directly relates to the superconducting pair wave function amplitude. In this thesis, we have developed the superconducting STM as a local Josephson probe and carried out direct measurements of the superconducting pair amplitude of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> single crystals via the c-axis Josephson tunneling between Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> and a conventional superconducting STM tip. Josephson measurements at different surface locations of overdoped Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> yield local values for the Josephson I<sub>C</sub>R<sub>N</sub> product, indicating an inhomogeneous structure of the I<sub>C</sub>R<sub>N</sub> product in overdoped Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> on a nanometer length scale. The corresponding energy gap was also measured at the same locations and an unexpected inverse correlation is observed between the local I<sub>C</sub>R<sub>N</sub> product and the local energy gap. Our interpretation of the I<sub>C</sub>R<sub>N</sub> vs. the energy gap relation with the phase fluctuation model for the phase diagram of high-T<sub>C</sub> superconducting cuprates will be presented. A preliminary study of the high current density effect on the density of states of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> will also be reported.The effect of cleaving the Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> surface on its electronic structure is also discussed. This is motivated by the question that the gap inhomogeneity observed by STM is intrinsic property of this material or induced by the cleaving. Since the superconducting tunneling probes the depth of a coherence length into the sample surface and Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> has a very short c-axis coherence length, it's important to address this question. I will present some preliminary results of the superconducting STM studies on chemically etched Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> surfaces.


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