Unusual Symmetries in the Kugel-Khomskii Hamiltonian

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Physics
Quantum Physics
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Aharony, Amnon
Entin-Wohlman, Ora
Korenblit, I. Ya
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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.

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2003-08-22
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Physical Review Letters
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At the time of publication, author Taner Yildirim was affiliated with the National Institute of Standards and Technology, Gaithersberg, Maryland. Currently, he is a faculty member in the Materials Science and Engineering Department at the University of Pennsylvania.
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