Orbital Rotations induced by Charges of Polarons and Defects in Doped Vanadates

TL;DR

This paper investigates how charges from doped holes and defects induce orbital rotations and disrupt orbital order in doped vanadate perovskites, revealing that defect-induced orbital rotations primarily cause the decline in orbital order.

Contribution

It demonstrates that defect-induced orbital rotations, rather than charge carriers alone, are the main factor behind the loss of orbital order in doped vanadates.

Findings

Defect Coulomb fields induce anisotropic orbital rotations.

Orbital rotations modify spin-orbital polaron clouds.

Orbital order decline mainly stems from defect-induced orbital rotations.

Abstract

We explore the competiton of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degenerate orbitals. These rotations modify the spin-orbital polaron clouds and compete with orbital rotations induced by defects. Both mechanisms lead to a mixing of orbitals, and cause the suppression of the asymmetry of kinetic energy in the $C$-type magnetic phase. We find that the gradual decline of orbital order with doping, a characteristic feature of the vanadates, however, has its origin not predominantly in the charge carriers, but in the off-diagonal couplings of orbital rotations induced by the charges of the doped ions.