Green's function variational approach to orbital polarons in KCuF3

Abstract

We develop an $e_g$ orbital, $t$-$J$-like model of a single charge doped into a two-dimensional plane with ferromagnetic spin order and alternating orbital order, and present its solution by Green's functions in the variational approximation framework. The model is designed to represent the orbital physics within ferromagnetic $(a,b)$ planes of KCuF${}_3$ and K${}_2$CuF${}_4$. The variational approximation (VA) relies on the systematic generation of equations of motion for the Green's function, taking into account the real-space constraints coming from the exclusion of doubly occupied sites. This method is compared to the firmly established self-consistent Born approximation, and to the variational cluster approximation (VCA) which relies on the itinerant regime of the model. We find that the present variational approximation captures the essential aspects of the spectral weight distribution of the coherent quasiparticle state and gives a result similar to the VCA, while also reproducing well the momentum dependence of the spectral moments. In contrast, the spectral function obtained within the self-consistent Born approximation is more incoherent and its quasiparticle is heavier, at strong effective couplings, than observed with VCA and VA.

arXiv comment14 pages, 9 figures

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Cited by in corpus (7)

• Overcoming the Sign Problem at Finite Temperature: Quantum Tensor Network for the Orbital $e_g$ Model on an Infinite Square Lattice
• Defect-induced orbital polarization and collapse of orbital order in doped vanadium perovskites
• Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy
• From "Weak" to "Strong" Hole Confinement in a Mott Insulator
• Orbiton-magnon interplay in the spin-orbital polarons of KCuF3 and LaMnO3
• Spectral properties of spin-orbital polarons as a fingerprint of orbital order
• The Green function variational approximation: Significance of physical constraints