Superconducting Transition Metal Borocarbides

Abstract

We present an overview of the present knowledge of the electronic structure of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds. The calculated highly anisotropic Fermi surfaces exhibit clear similarities such as nested regions but also significant distinctions. Electrons from the nested parts of the Fermi surface affect several properties in the superconducting state. We report theoretical calculations which emphasize the relevance of these electrons to the mechanism of superconductivity. Structural parameters for ScNi2B2C derived from total energy calculation allow to discriminate conflicting experimental structural reports. The importance 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 such as X-ray absorption spectroscopy (XAS). The excellent agreement with the XAS data suggests a minor importance of correlation effects compared with the cuprate superconductors. Thermodynamic properties of these systems erg theory with special emphasis on the upper critical fieldH _{c 2} (T)and the specific heat. In particular, the unusual positive curvature ofH _{c 2} (T)near T_Tobserved for high-quality single crystals, polycrystalline samples of YNiB₂C₂, LuNiB₂C₂as well as to a somewhat reduced extent also for the mixed system Yi__x Lu_xNiB₂C₂is explained microscopically. The values ofH _{c 2}(T) and of its positive curvature near T, are intrinsic quantities generic for such samples. Both quantities decrease with growing disorder and thus provide a direct measure of the sample quality.