ANDREEV-REFLECTION STUDY OF SUPERCONDUCTING RNI(2)B(2)C COMPOUNDS

A comparative study has been made of the current-voltage characteristi cs of RNi(2)B(2)C-Ag point contacts with R = Y, Er, and Ho, at various temperatures and magnetic fields. At low temperatures the double-mini mum structure in the differential resistance dV/dI, characteristic for Andreev-reflection processes, can be well described by the Blonder-Ti nkham-Klapwijk (BTK) model, yielding an energy gap Delta = 2.42 +/- 0. 07, 1.7 +/- 0.2, and 1.04 +/- 0.06 meV for, respectively, the Y-, Er-, and He-based compounds. For the Y- and Er-based compounds, the temper ature dependence of the spectra is in quantitative agreement with the BTK prediction. However for the He-based compound, an anomalous temper ature dependence with two superconducting transitions at 8.5 and 6.5K has been observed. The ratio 2 Delta/k(B)T(c) amounts about 3.7 for al l compounds (providing one takes T-c similar or equal to 6.5K for the critical temperature of the Ho compound), corresponding to a moderate electron-phonon coupling strength.