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_(16)O_(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, T_c is the superconducting critical temperature and k_B 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 (A map), as well as for each gap separately (∆_s and ∆_L), were obtained. From the histogram of A 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 ζ_(∆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.