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Phys Rev Lett. 2009 Mar 6;102(9):096406. Epub 2009 Mar 5.

Spin-orbital singlet and quantum critical point on the diamond lattice: FeSc2S4.

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Physics Department, University of California, Santa Barbara, California 93106, USA.


We present a theory of spin and orbital physics in the A-site spinel compound FeSc2S4, which experimentally exhibits a broad "spin-orbital liquid" regime. A spin-orbital Hamiltonian is derived from a combination of microscopic consideration and symmetry analysis. We demonstrate a keen competition between spin-orbit interactions, which favor formation of a local "spin-orbital singlet," and exchange, which favors magnetic and orbital ordering. Separating the spin-orbital singlet from the ordered state is a quantum critical point. We argue that FeSc2S4 is close to this quantum critical point on the spin-orbital singlet side. The full phase diagram includes a commensurate-incommensurate transition within the ordered phase. A variety of comparisons to and suggestions for experiments are discussed.

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