Multilayered cuprate superconductor Ba2Ca5Cu6O12(O1−x,Fx)2 studied by temperature-dependent scanning tunneling microscopy and spectroscopy

Abstract

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multilayered cuprate superconductor Ba2Ca5Cu6O12(O1−x,Fx)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 ΔS and large gaps (pseudogaps) ΔL. 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 ΔS follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2ΔS(T=0)/kBTc 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 ΔL survives in the normal state and even increases with T above Tc. The T dependencies of the spatial distributions for both relevant gaps (Δ map), as well as for each gap separately (ΔS and ΔL), were obtained. From the histogram of Δ map, the averaged gap values were found to be Δ¯S=∼24 meV and Δ¯L=∼79 meV. The smaller gap ΔS shows a spatially homogeneous distribution while the larger gap ΔL is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length ξΔL of ΔL 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 ΔL is strongly affected by the disorder of the apical O/F.

Source-reported materials — not catalogue approval

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