OBSERVATION OF GRANULARITY IN YNI2B2C SUPERCONDUCTORS CONTAINING NORMAL IMPURITY PHASES

Cryogenic scanning tunneling microscopy is used to study the local ele ctronic properties of intermetallic YNi2B2C superconductors. Samples w hich were prepared with Ni deficiency showed a sharp superconducting t ransition, onset around 15 K, while x-ray-diffraction measurements cle arly indicated a presence of a nonsuperconducting YB2C2 phase. Our sca nning tunneling spectroscopy measurements on these samples show strong spatial variations of the tunneling conductance, consistent with gran ular materials. In particular, we observe reproducible transitions bet ween normal regions to superconducting regions, having a gap parameter of 2.55+/-0.05 meV at 4.2 K, on length scales of 20 Mn. In addition, the I-V characteristics often manifest single-electron tunneling effec ts, also typical of,granular systems.