Observation of granularity inYNi2B2C superconductors containing normal impurity phases

Cryogenic scanning tunneling microscopy is used to study the local electronic properties of intermetallic ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C superconductors. Samples which were prepared with Ni deficiency showed a sharp superconducting transition, onset around 15 K, while x-ray-diffraction measurements clearly indicated a presence of a nonsuperconducting ${\mathrm{YB}}_2$ ${\mathrm{C}}_2$ phase. Our scanning tunneling spectroscopy measurements on these samples show strong spatial variations of the tunneling conductance, consistent with granular materials. In particular, we observe reproducible transitions between normal regions to superconducting regions, having a gap parameter of 2.55±0.05 meV at 4.2 K, on length scales of 20 nm. In addition, the I-V characteristics often manifest single-electron tunneling effects, also typical of granular systems.