ENERGY-GAP OF QUATERNARY COMPOUND SUPERCONDUCTOR YNI2B2C AND LUNI2B2C BY POINT-CONTACT SPECTROSCOPY

Tunneling spectra of a single crystalline YNi2B2C (T-c = 15.4 K) and LuNi2B2C (T-c = 15.7 K) have been measured by a point-contact spectroscopy using a low-temperature scanning tunneling microscope. The conductance curve of these materials shows a smeared gap structure with an almost linear density of states inside of the gap region as well as a sizable zero-bias conductance. The linear conductance inside the gap region has been explained by a proximity-induced energy gap in the degraded surface normal layer. The resulting energy gap at 4.2 K is 3.2 meV and 3.3 meV for YNi2B2C and LuNi2B2C, respectively. A simple analysis using a smeared BCS density of states, on the other hand, yields energy gap of 2.55 meV and 2.65 meV. Temperature dependence of the energy gap in LuNi2B2C shows a reasonable agreement with the BCS type temperature dependence except a slight deviation near T-c.