Low Temperature STM/STS Studies of High Jc Superconductors

We have carried out low‐temperature scanning tunneling microscopy/spectroscopy (LT‐STM/STS) studies on the cleaved surface of very high critical‐current density (Jc) superconductors, (Nd, Eu, Gd)Ba2Cu3Oy (NEG123) melt textured crystals (Jc ∼7 × 108 A/m2 at 77 K) and Bi2Sr2CaCu2O8+δ (Bi2212) single crystals (Jc ∼1 × 1010 A/m2 at 20 K). For NEG123, STS results at 77 K clearly showed that the spatial distribution of superconducting gap value A?? was homogeneous, though a nano‐scale lamellar structure was observed in STM images at room temperature. On the other hand, the STS gap map for Bi2212 showed an inhomogeneous distribution for Δ, namely, a lamellar structure along the b‐axis with periodicity about 15 nm. The inhomogenity in gap value Δ along the b‐axis is supposed to have relation to Ca element concentration and the strong pinning effect in this crystal.