Orbitally induced string formation in the spin-orbital polarons
Abstract
We study the spectral function of a single hole doped into the (a,b) plane of the Mott insulator LaVO3, with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active \dyz,dzx\ 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 eg systems, as e.g. in K2CuF4 or (to some extent) in LaMnO3, and that the hole may be seen as confined in a string-like 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.