Strong interactions, narrow bands, and dominant spin-orbit coupling in Mott insulating quadruple perovskite CaCo3V4O12

摘要

We investigate the electronic and magnetic structures and the character anddirection of spin and orbital moments of the recently synthesized quadrupleperovskite cobaltovanadate CaCo3V4O12 using a selection of methods from densityfunctional theory. Implementing the generalized gradient approximation and theHubbard U correction (GGA+U), ferromagnetic spin alignment leads tohalf-metallicity rather than the observed narrow gap insulating behavior.Including spin-orbit coupling (SOC) leaves a half-semimetallic spectrum whichis essentially Mott insulating. SOC is crucial for the Mott insulatingcharacter of the V d^1 ion, breaking the d_{m=\pm 1} degeneracy and also givinga substantial orbital moment. Evidence is obtained of the large orbital momentson Co that have been inferred from the measured susceptibility. Switching tothe orbital polarization (OP) functional, GGA+OP+SOC also displays cleartendencies toward very large orbital moments but in its own distinctive manner.In both approaches, application of SOC, which requires specification of thedirection of the spin, introduces large differences in the orbital moments ofthe three Co ions in the primitive cell. We study a fictitious but simplercousin compound Ca3CoV4O12 (Ca replacing two of the Co atoms) to probe in moretransparent fashion the interplay of spin and orbital degrees of freedom withthe local environment of the planar CoO4 units. The observation that theunderlying mechanisms seems to be local to a CoO4 plaquette, and that there isvery strong coupling of the size of the orbital moment to the spin direction,strongly suggest non-collinear spins, not only on Co but on the V sublattice aswell.