Superconducting rare earth transition metal borocarbides

We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the a-axis is responsible for the pair breaking in HoxR1-xNi2B2C; R = Y, Lu samples. The effect of doping (Cu. Co) at the transition metal site is considered experimentally and theoretically. The possible role of correlation effects due to the presence of the transition metal component in determining the electronic structure is discussed comparing the band structure calculation results with various electronic spectroscopies as well as de Haas-van Alphen data. Important thermodynamic properties of these systems are analyzed within multiband Eliashberg theory with special emphasis on the upper critical field H-c2(T) and the specific heat. In particular, the unusual positive curvature of H-c2(T) near T-c observed for high-quality single crystals, polycrystalline samples of YNi2B2C, LuNi2B2C as well as to a somewhat reduced extent also for the mixed system Y1-xLuxNi2B2C is explained microscopically. It is shown that in these well-defined samples the clean limit of type II superconductors is achieved. The values of H-c2(T) and of its positive curvature near T, (as determined both resistively and from magnetization as well as from specific heat measurements is an intrinsic quantity generic for such samples) decrease with growing impurity content. Both quantities thus provide a direct measure of the sample quality. (C) 1999 Elsevier Science B.V. All rights reserved.