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Center for Nonlinear Studies |
In transition metal compounds with relatively high (e.g. cubic) symmetry, a rich possibility has also been contemplated, in which not only spin but also orbital states of localized electrons fluctuate. Such a ``Spin Orbital Liquid'' (SOL) was proposed in LaTiO$_3$. Probably the best candidate for a SOL is FeSc$_2$S$_4$, a spinel compound (with the general structure AB$_2$X$_4$), in which only the A sites form a magnetically/orbitally active diamond sublattice. In recent years, a variety of such A-site spinels, e.g. CoAl$_2$O$_4$ and MnSc$_2$S$_4$, were also found to be frustrated, forming a "spiral spin liquid'' at certain temperature range. FeSc$_2$S$_4$ stands out markedly amongst this class of compounds in exhibiting a much broader liquid regime, extending down to the lowest measured temperatures. We present a theory of spin and orbital physics in the A-site spinel compound FeSc$_2$S$_4$ , 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" (SOS), and exchange, which favors magnetic and orbital ordering. Separating the SOS from the ordered state is a quantum critical point (QCP). We argue that FeSc$_2$S$_4$ is close to this QCP on the SOS side. The full phase diagram of the model includes a commensurate-incommensurate transition within the ordered phase. A variety of comparison to and suggestion for experiments, including the low temperature specific heat, neutron scattering, NMR T1 relaxation time and magnetic phase diagram, are discussed.
Host: Ivar Martin