Doping-Induced Ferromagnetism and Possible Triplet Pairing in d4 Mott Insulators

We study the effects of electron doping in Mott insulators containing d(4) ions such as Ru4+, Os4+, Rh5+, and Ir5+ with J = 0 singlet ground state. Depending on the strength of the spin-orbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J = 1 triplet states of t(2g)(4). We find that the interaction between J excitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic metal. Close to the ferromagnetic phase, the low-energy spin response is dominated by intense paramagnon excitations that may act as mediators of a triplet pairing.