We present new upper critical field Hc2(T ) data in a broad temperature region 0.3K ≤ T ≤ Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates.The absolute values for all T , in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T ) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors. 74.60.Ec, 74.70.Ad, 74.72Ny The discovery [1,2] of superconductivity in transition metal borocarbides has generated large interest due to their relatively high transition temperatures T c ∼ 15 to 23 K and due to the relation between the mechanisms of superconductivity in these compounds, in cuprates, and in ordinary transition metals. Another highlight is the coexistence of magnetism and superconductivity in some of these compounds containing rare earth elements [3][4][5]. A study of the non-magnetic compounds such as LNi 2 B 2 C, with L=Lu,Y,Th,Sc [6], is a prerequisite for the understanding of their magnetic counterparts. Experimental data for LuNi 2 B 2 C [7] demonstrate beside a maximal positive curvature (PC) of H c2 (T ) near T c , observed also for YNi 2 B 2 C [8,4,9], a weak T -dependent anisotropy within the tetragonal basal plane and a Tindependent out-of-plane anisotropy of the upper critical field H c2 . Both anisotropies have been described [7] in terms of nonlocal corrections to the Ginzburg-Landau (GL) equations. In this picture the PC of H c c2 ( H to the tetragonal c-axis) is caused, almost purely, by the basal plane anisotropy. However, it should be noted that the reported anisotropy of H c2 for YNi 2 B 2 C is significantly smaller than for LuNi 2 B 2 C [7,9,10] whereas its PC is comparable or even larger. Further explanations of the unusual PC of H c2 (T ), such as quasi-2D fluctuations [11], are excluded by the underestimation of H c2 (T ) at low-T [9] and the observed weak anisotropy. The quantum critical point scenario [12] as well as the bipolaronic one [13] can be disregarded because the slope of H c2 (T ) decreases for T →0 (see Fig. 1). Local density approximation (LDA) band structure calculations [14,15] predict a nearly isotropic electronic structure with rather complicated bands near the Fermi level E F . However, in analyzing the superconductivity in terms of an isotropic single-band (ISB) Eliashberg model, the multi-band character and the anisotropic Fermi surface have been widely ignored so far.Here we present and analyze theoretically new data of H c2 (T ) in a broad interval 0.3K≤ T ≤ T c for high purity LuNi 2 B 2 C and YNi 2 B 2 C single crystals. We show that typical features of both compounds, such as H c2 (0) ∼ 8 to 10 T and the unusual PC of H c2 (T ) for T > ∼ 0.5T c , cannot in any way be explained ...
