Electronic structures and instabilities of ZrNCl and HfNCl: implications for superconductivity in the doped compounds

Amongst the many rules of thumb that prevail in the search for new superconductors is a belief that square nets of metal atoms are crucial to the achievement of high transition temperatures. The recent finding of superconductivity below 26 K in some intercalated beta-HfNCl phases suggests that high transition temperatures might be achievable in compounds with triangular or graphite-like nets as well. Here we present key features of high level ab initio band structure calculations on the insulating ZrNCl and HfNCl parent compounds. Electron doping through intercalation has been modeled within a rigid-band framework. Features in the energy isosurfaces of the 'doped' compounds are examined for nesting instabilities of the kind that have been implicated in the superconducting properties of high-T-c compounds with square nets. Despite very different electron counts, bonding patterns and atom topologies, it would seem that certain important aspects of the Fermi surfaces of the superconducting Zr(Hf)NCl phases are in many respects very similar to the Fermi surfaces of the cuprate high-T-c superconductors.