Multilayered cuprate superconductor Ba${}_2$Ca${}_5$Cu${}_6$O${}_{12}$(O${}_{1-x}$,F${}_x$)${}_2$ studied by temperature-dependent scanning tunneling microscopy and spectroscopy

Abstract

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multi-layered cuprate superconductor Ba${}_2$Ca${}_5$Cu${}_6$O${}_{12}$(O${}_{1-x}$,F${}_x$)${}_2$. STM topography revealed random spot structures with the characteristic length $\le 0.5$ nm. The conductance spectra dI/dV(V) show the coexistence of smaller gaps ${\Delta }_S$ and large gaps (pseudogaps) ${\Delta }_L$. The pseudogap-related features in the superconducting state were traced with the spatial resolution of $\sim$ 0.07 nm. Here, $I$ and $V$ are the tunnel current and bias voltage, respectively. The temperature, $T$, dependence of ${\Delta }_S$ follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2${\Delta }_S(T=0)/k_B{T}_c$ 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 ${\Delta }_L$ survives in the normal state and even increases with $T$ above $T_c$. The $T$ dependences of the spatial distributions for both relevant gaps ($\Delta$ map), as well as for each gap separately (${\Delta }_S$ and ${\Delta }_L$) were obtained. From the histogram of $\Delta$ map, the averaged gap values were found to be ${\bar{\Delta }}_S=\sim 24$ meV and ${\bar{\Delta }}_L=\sim 79$ meV. The smaller gap ${\Delta }_S$ shows a spatially homogeneous distribution while the larger gap ${\Delta }_L$ is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length ${\xi }_{{\Delta }_L}$ of ${\Delta }_L$ 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 ${\Delta }_L$ is strongly affected by the disorder of the apical O/F.

arXiv comment14 pages, 6 figures

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