Upper Critical Field Peculiarities of Superconducting ${\mathrm{YNi}}_2{B}_{2}C$ and ${\mathrm{LuNi}}_2{B}_{2}C$

We present new upper critical field $H_{c2\mathrm{}}\left(T\right)$ data in a broad temperature region $0.3\mathrm{K}\le T\le T_c$ for ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ single crystals with well characterized low impurity scattering rates. The absolute values for all $T$, in particular, $H_{c2\mathrm{}}\left(0\right)$, and the sizable positive curvature (PC) of $H_{c2\mathrm{}}\left(T\right)$ 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 $T_c$ gives strong evidence for clean limit type-II superconductors.