Multilayered cuprate superconductor Ba_2Ca_5Cu_6O_(12)(O_(1−x),F_x)_2 studied by temperature-dependent scanning tunneling microscopy and spectroscopy

摘要

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multilayered cuprate superconductor Ba_2Ca_5Cu_6O_(12)(O_(1−x) ,F_x )_2. STM topography revealed random spot structures with the characteristic length ≤0.5 nm. The conductance spectra dI/dV (V ) show the coexistence of smaller gaps △_S and large gaps (pseudogaps) △_L. The pseudogap-related features in the superconducting state were traced with the spatial resolution of ～0.07 nm. Here, I and V are the tunnel current and bias voltage, respectively. The temperature, T , dependence of △_S follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2△_S (T = 0)/k_BT_c equals to 4.9, which is smaller than those of other cuprate superconductors. Here, Tc is the superconducting critical temperature and kB is the Boltzmann constant. The larger gap △_L survives in the normal state and even increases with T above T_c. The T dependencies of the spatial distributions for both relevant gaps (△map), as well as for each gap separately (△_S and △_L), were obtained. From the histogram of △ map, the averaged gap values were found to be △_S = ～24 meV and △_L = ～79 meV. The smaller gap △_S shows a spatially homogeneous distribution while the larger gap △_L is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length ξ_(△L) of △_L correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that △_L is strongly affected by the disorder of the apical O/F.