Date of this Version
Physical Review Letters
The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d1 ions having threefold degenerate t2g orbitals. Since orbitals do not couple along “inactive” axes, perpendicular to the orbital planes, the total number of electrons in |α⟩ orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.
Harris, A., Yildirim, T., Aharony, A., Entin-Wohlman, O., & Korenblit, I. (2003). Unusual Symmetries in the Kugel-Khomskii Hamiltonian. Physical Review Letters, 91 (8), 087206-1-087206-4. http://dx.doi.org/10.1103/PhysRevLett.91.087206
Date Posted: 20 August 2015
This document has been peer reviewed.