Large magnetoresistance and spin-polarized heavy-mass electron state of the doped valence-bond solid (Ti_1-xV_x)₂O ₃

A heavy-mass electron state is realized in a doped valence bond solid (Ti_1-xV_x)₂O₃. In this system, itinerant holes mediate the mainly ferromagnetic RKKY interaction between the localized magnetic moments and become readily spin-polarized under a magnetic field, while showing large negative magnetoresistance. In spite of the ferromagnetic interaction among the carriers, their effective mass is found to be 1 or 2 orders of magnitude larger than that of usual doped semiconductors. Such strong mass renormalization is ascribable to the polaron formation on the Ti-dimer, where the spin-singlet state is originally formed. Doping dependence of the electronic specific-heat coefficient implies that the dimeric lattice fluctuation or softening is responsible for the enhanced electron-phonon interaction.