Dilution effect in correlated electron systems with orbital degeneracy

Theory of dilution effect in orbital ordered system is presented. The e_g orbital model without spin degree of freedom and the spin-orbital coupled model in a three-dimensional simple-cubic lattice are analyzed by the Monte Carlo simulation and the cluster expansion method. In the e_g orbital model without spin degree of freedom, reduction in the orbital ordering temperature due to dilution is steeper than that in the dilute magnet. This is attributed to a modification of the orbital wave function around vacant sites. In the spin-orbital coupled model, it is found that magnetic structure is changed from the A-type antiferromagnetic order into the ferromagnetic one. Orbital-dependent exchange interaction and a sign change in this interaction around vacant sites bring about this phenomenon. Present results explain the recent experiments in transition-metal compounds with orbital dilution.