The Superconducting Gap Behavior in the Antiferromagnetic Nickel-Borocarbide Compounds RNi2B2C (R=Dy, Ho, Er, Tm) Studied by Point-Contacts Spectroscopy

An general survey of the superconducting (SC) gap study in the title compounds by point-contact (PC) spectroscopy is presented. The SC gap was determined from dV/dI of PCs employing the well-known theory of conductivity for normal metal-superconductor PCs accounting Andreev reflection. The theory was modified by including pair-breaking effects considering the presence of magnetic rare-earth ions. A possible multiband structure of these compounds was also taken into account. The PC study of the gap in the Er-compound (T-N similar or equal to 6 K < T-c similar or equal to 11 K) gives evidence for the presence of two SC gaps. Additionally, a distinct decrease of both gaps is revealed for R = Er in the antiferromagnetic (AF) state. For R = Tm (T-N similar or equal to 1.5K < T-c similar or equal to 10.5 K) a decrease of the SC gap is observed below 4-5 K, while for R = Dy (T-N similar or equal to 10.5K > T-c similar or equal to 6.5 K) the SC gap has a BCS-like dependence in the AF state. The SC gap for R = Ho (T-N similar or equal to 5.2K < T-c similar or equal to 8.5 K) exhibits below T* similar or equal to 5.6K a single- band BCS-like dependence vanishing above T*, where a specific magnetic order occurs. The difference in the SC gap behavior in the title compounds is attributed to different AF ordering.