Doped two orbital chains with strong Hund's rule couplings - Ferromagnetism, spin gap, singlet and triplet pairings

Different models for doping of two-orbital chains with mobile S = 1/2 fermions and strong, ferromagnetic (FM) Hund's rule couplings stabilizing the S = 1 spins are investigated by density matrix renormalization group (DMRG) methods. The competition between antiferromagnetic (AF) and FM order leads to a rich phase diagram with a narrow FM region for weak AF couplings and strongly enhanced triplet pairing correlations. Without a level difference between the orbitals, the spin gap persists upon doping, whereas gapless spin excitations are generated by interactions among itinerant polarons in the presence of a level difference. In the charge sector we find dominant singlet pairing correlations without a level difference, whereas upon the inclusion of a Coulomb repulsion between the orbitals or with a level difference, charge density-wave (CDW) correlations decay slowest. The string correlation functions remain finite upon doping for all models.