Georgopoulou, Madeleine; (2023) Novel Ground States and Excitations in Claringbullite and Averieivite Quantum Magnets: Routes to Kagome Spin Liquids. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Quantum spin liquids (QSLs) are novel states of matter that exhibit exotic electronic ground states and fractionalized excitations as a consequence of many-body entanglement. They do not break symmetry or any Landau-type order parameters and are instead better characterised by their projective symmetry groups. An ideal candidate is the two-dimensional S = 1/2 kagome Heisenberg antiferromagnet (KHAFM), where quantum spins form a geometrically frustrated lattice of corner- sharing triangles. Research into experimental realisations of QSLs was fuelled in 1973 when Phillip W. Anderson proposed that these states could underpin the transition to high-T_C superconductivity. The syntheses, crystal and magnetic structures, bulk magnetometry and inelastic neutron scattering (INS) studies of two series of frustrated S = 1/2 magnets are presented. The claringbullite series, Zn_{x}Cu_{4–x}(OD)_6FCl, includes the x = 0 sample that is orthorhombically distorted and the x = 1 material that represents the ‘ideal’ KHAFM. Seven exchange interactions, including Dzyaloshinskii-Moryia, were used to model the gapped spin wave excitations of the x = 0 sample, using semi-classical linear spin wave theory (LSWT). The x = 1 sample has gapless (E ≥ 0.27 meV) diffuse magnetic scattering and is found to be a good quantum spin liquid candidate, where the strongest correlations are between the kagome layers. The averievite series, Zn_{x}Cu_{5–x}(VO_4)_2O_2CsCl, includes the x = 0 sample that has a monoclinic distortion, and the x = 1 and x = 2 materials (the latter is the KHAFM) where the kagome lattices are found to be distorted by an in-plane rotation of equilateral triangles. Preliminary LSWT calculations based on two Heisenberg exchanges and a collinear magnetic structure, capture the main features of the averievite spin waves. Despite a transition in the magnetic susceptibility of x = 1 averievite and the spin wave-like excitations, no magnetic Bragg peaks are observed, suggesting a glassy ground state. For x = 2 averievite, the gapless (E ≥ 0.3 meV) diffuse scattering evidences a QSL and obeys a scaling behaviour, indicating proximity to a quantum critical point.

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