The combined scanning tunneling microscopy/spectroscopy (STM/STS) and the break-junction tunneling spectroscopy (BJTS) measurements of the three-layer Pb doped Bi2223 cuprate superconductor were carried out.

Surface-sensitive STS measurements [1] revealed the spatial inhomogeneous energy gap Δ distributions having several-nm size of the patch structures with the averaged magnitude of Δ_ave ~ 46 meV. The spatial distributions of the local density of states (LDOS maps) show the periodic modulations with both the energy dispersive and non-dispersive types. The features of the former one can be identified as the quasi-particle interference scattering vectors associated with the d_x²_-y² gap symmetry, showing features essentially similar to those widely reported for the two-layer Bi based cuprates.

In order to investigate the relation between these STS gap features and bulk superconductivity, the BJTS measurements of superconductor-insulator-superconductor (SIS) junctions were carried out. In the measurements, we referred the zero bias (ZB) peak as being due to the Josephson tunneling by fabricating the junctions where manifestations of both the gap peaks and the ZB peak were found for the single conductance-voltage curve. With increasing the temperature, the ZB peaks were reduced and vanished just below T_c (≈ 105 K), reflecting the bulk superconductivity. Simultaneously, from the temperature dependence of the gap features from 4.2 K to 200 K, the gap value of Δ ≈ 35 meV at 4.2 K persists across the T_c, suggesting its non-superconductivity origin. This value is the smaller one in the low temperature (5 K) distribution observed by STS.

[1] A. Sugimoto et al, Supercond. Sci. Technol. 33, 095011 (2020).

Keywords: Bi2223, STM/STS, Tunnel spectroscopy, Josephson junction