Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

TL;DR

This study investigates how disordered charged defects influence the electronic excitations in doped Mott insulators with orbital degeneracy, revealing persistent spin and orbital order and the formation of soft gap states within the Mott gap.

Contribution

The paper demonstrates that the disordered electronic structure of doped Mott insulators can be accurately modeled with unrestricted Hartree-Fock, capturing defect states, Mott gap robustness, and spin-orbital polarons.

Findings

Atomic multiplet excitations are well reproduced.

Mott gap persists up to high doping levels.

Defect states develop a soft gap at the Fermi energy.

Abstract

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the $R_{1-x}$Ca$_x$VO$_3$ perovskites, where $R=$La,$\dots$,Lu. A fundamental characteristic of these vanadium $d^2$ compounds with partly filled $t_{2g}$ valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-$T_c$ cuprates at low defect concentration. We demonstrate that the disordered electronic structure of doped Mott-Hubbard insulators can be obtained with high precision within the unrestricted Hartree-Fock approximation. In particular: (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect states gap can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. We demonstrate that charge defects trigger small spin-orbital polarons, with their internal kinetic energy responsible for the opening of the soft defect states gap. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio which explains the origin of the robustness of spin and orbital order. Using realistic parameters for La$_{1-x}$Ca$_x$VO$_3$, we show that its soft gap is well reproduced as well as the marginal doping dependence of the position of the chemical potential relative to the center of the lower Hubbard band.