Magnetic and orbital correlations in a two-site molecule

Abstract

We analyze the role of orbital degeneracy in possible magnetic and orbital instabilities by solving exactly a two-site molecule with two orbitals of either e g or t 2g symmetry at quarter-filling. As a generic feature of both models one finds that the spin and orbital correlations have opposite signs in the low temperature regime when the orbitals are degenerate, in agreement with the Goodenough-Kanamori rules. While Hund's exchange coupling J H induces ferromagnetic spin correlations in both models, it is more efficient for t 2g orbitals where the orbital quantum number is conserved along the hopping processes. We show that the ground state and finite temperature properties may change even qualitatively with increasing Coulomb interaction when the crystal field splitting of the two orbitals is finite, and the Goodenough-Kanamori rules may not be followed.Published in J. Phys.: Condens. Matter 18, 7449-7469 (2006).

Introduction to Effective Low‐Energy Hamiltonians in Condensed Matter Physics and Chemistry

“…However, if one studies models with more than one site and moves away from the atomic limit (t = 0) one finds that there is a subtle competition between the kinetic (hopping) term and the Hund's rule coupling which means that the high spin state is not always the lowest energy state. Many such interesting effects can be understood on the basis of a two site generalisation of this two orbital model (56).…”

Magnetic ordering in the striped nickelate La5/3Sr1/3NiO4: A band structure point of view

“…It therefore is unlikely to design a realistic model without accounting for this self-consistency. Nevertheless, a large number of calculations for the twoband Hubbard model clearly point at competing phases, see [31,32] and the references therein.…”