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Title: Quantum magnets with strong spin-orbit interaction probed via neutron and X-ray scattering
Author: Biffin, Alun M.
ISNI:       0000 0004 5361 6124
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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This thesis presents details of x-ray and neutron scattering experiments used to probe quantum magnets with strong spin-orbit interaction. The first of these systems are the three-dimensional iridate compounds, in which the three-fold co-ordination of IrO6 octahedra has been theoretically hypothesized to stabilize anisotropic exchange between Ir4+ ions. This novel interaction between these spin-orbital entangled, Jeff=1/2 moments is described by a Hamiltonian first proposed by Kitaev, and would be the first physical realization of this Hamiltonian in a condensed matter system. This thesis details the determination of the structure of a new polytype within these compounds, the 'stripyhoneycomb' γ-Li2IrO3. Furthermore, through resonant magnetic x-ray diffraction experiments on single crystals of β-Li2IrO3 and γ-Li2IrO3, an incommensurate, non-coplanar structure with counter-rotating moments is found. The counter-rotating moment structure is a rather counter-intuitive result, as it is not stabilizied by Heisenberg exchange between magnetic sites, however, the Kitaev exchange naturally accounts for this feature. As such, these experiments reveal, for the first time, systems which exhibit dominant Kitaev interactions. The ordering wavevector of both β- and γ-Li2IrO3 polytypes are found to be identical, suggesting that the same magnetic interactions are responsible for stabilizing magnetic order in both materials, despite their different lattice topologies. Following this, the spinel FeSc2S4 is considered. Here, despite the presence of strong exchange between Fe2+,/sup>, and the fact that these ions sit in a Jahn-Teller active environment, the system does not order in the spin or orbital degrees of freedom. A 'spin-orbital singlet' has been theoretically proposed to describe the groundstate of this system, and here inelastic neutron scattering (INS) is used to probe the resulting triplon excitations. This allows determination of microscopic parameters in the single ion and exchange Hamiltonians, and moreover experiments in external magnetic field reveal the true spin-and-orbital nature of these triplon excitations. Finally, Ba3CoSb2O9, a physical realization of the canonical triangular antiferromagnet model is explored with INS and the high energy excitations from the 120 degree magnetic structure are found to display significant differences from those calculated by linear spin wave theory, suggesting the presence of quantum dynamics not captured in the 1/S linear spin wave expansion.
Supervisor: Coldea, Radu Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Condensed Matter Physics ; Neutron Scattering ; Resonant Magnetic X-ray Diffraction ; Kitaev Hamiltonian ; Spin Orbit Coupling ; Frustration ; Spin-Orbital Singlet