Orbitally induced string formation in the spin-orbital polarons

We study the spectral function of a single hole doped into the ab plane of the Mott insulator LaVO₃, with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active {d_yz,d_zx} orbitals. Starting from the respective t-J model, with spin-orbital superexchange and effective three-site hopping terms, we derive the polaron Hamiltonian and show that a hole couples simultaneously to the collective excitations of the AF/AO phase, magnons, and orbitons. Next, we solve this polaron problem using the self-consistent Born approximation and find a stable quasiparticle solution—a spin-orbital polaron. We show that the spin-orbital polaron resembles the orbital polaron found in e_g systems, as e.g., in K₂CuF₄ or (to some extent) in LaMnO₃, and that the hole may be seen as confined in a stringlike potential. However, the spins also play a crucial role in the formation of this polaron—we explain how the orbital degrees of freedom: (i) confine the spin dynamics acting on the hole as the classical Ising spins and (ii) generate the string potential which is of the joint spin-orbital character. Finally, we discuss the impact of the results presented here on the understanding of the phase diagrams of the lightly doped cubic vanadates.