Hole doping of a Mott insulator with orbital degrees of freedom

Abstract

We study the effects of hole doping on one-dimensional Mott insulators with orbital degrees of freedom. We describe the system in terms of a generalized $t\text{\ensuremath{-}}J$ model. At a specific point in parameter space the model becomes integrable in analogy to the one-band supersymmetric $t\text{\ensuremath{-}}J$ model. We use the Bethe ansatz to derive a set of nonlinear integral equations which allow us to study the thermodynamics exactly. Moving away from this special point in parameter space we use the density-matrix renormalization group applied to transfer matrices to study the evolution of various phases of the undoped system with doping and temperature. Finally, we study a one-dimensional version of a realistic model for cubic titanates which includes the anisotropy of the orbital sector due to Hund's coupling. We find a transition from a phase with antiferromagnetically correlated spins to a phase where the spins are fully ferromagnetically polarized, a strong tendency toward phase separation at large Hund's coupling, as well as the possibility of instability toward triplet superconductivity.