UPPER CRITICAL-FIELD PECULIARITIES OF SUPERCONDUCTING YNI2B2C AND LUNI2B2C

We present new upper critical field H-c2(T) data in a broad temperatur e region 0.3 K less than or equal to T less than or equal to T-c for L uNi2B2C and YNi2B2C single crystals with well characterized low impuri ty scattering rates. absolute values for all T, in particular, H-c2(0) , and the sizable positive curvature (PC) of H-c2(T) at high and inter mediate T are explained quantitatively within an effective two-band mo del. The failure of the isotropic single-band approach is discussed in detail. Supported by de Haas-van Alphen data, the superconductivity r eveals direct insight into details of the electronic structure. The ob served maximal PC near T-c gives strong evidence for clean limit type- II superconductors. [S0031-9007(98)05338-1].