Chapter 204 Characterization of cuprate superconductors using tunneling spectra and scanning tunneling microscopy

Abstract

Summary Recent achievements in the tunneling study for cuprate superconductors are summarized as follows: (1) In the high-temperature oxide superconductors, in particular Bi2212 and T12223, alternating stacking of superconducting and blocking layers is recognized as intrinsic Josephson junctions. Tunneling currents flowing through small junctions have been extensively investigated, indicating Josephson plasma excitation, characteristic for cuprate superconductors. (2) Vacuum tunneling has been performed utilizing scanning tunneling microscope at low temperature. Removing the topmost surface by cleavage in vacuum or an inert gas atmosphere prior to tunneling measurements has been demonstrated to be indispensable to observe intrinsic superconducting gap structure in Bi2212. The reported superconducting gap spectra have been analyzed in the framework of d-wave superconductivity. It has been found that results can be, in principle, described in terms of the d-wave order parameter. (3) Hole doping and temperature dependence have been further investigated. Gap-like structures have been reported for underdoped Bi2212 samples above Tc. These gap-like structures should be understood in accordance with the “pseudogap” observed in other types of measurements. (4) Using STM, inner vortex structures of the differential conductance were successfully visualized for Bi2212 and Y123 crystals. The inner vortex excitation spectra are found to be quite different from those expected from a simple d-wave mechanism. (5) Phase-sensitive measurements have been carried out for Y123 crystals with different orientations. Josephson tunneling at the interface between conventional and high-temperature superconductors has also been investigated. The observed tunneling spectra are highly dependent on geometry; some results agree with theoretical predictions from d-wave symmetry, while others do not. Better theoretical models, including the admixture of d-wave and s-wave order parameters, have been proposed.