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Excitations and Criticality in Quantum Magnets

Merchant, PLH; (2013) Excitations and Criticality in Quantum Magnets. Doctoral thesis , UCL (University College London). Green open access

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This thesis describes the neutron scattering studies of three model magnetic systems; the coupled spin dimer compound TlCuCl3, the frustrated spin ladder material BiCu2PO6 and the impurity-doped spin ladder material BiCu_2(1-x)Zn2xPO6. TlCuCl3 is a realisation of a continuously tunable model magnet, where applied hydrostatic pressure can drive the system from a state of disorder into long-range magnetic order with the emergence of an excitation at the quantum critical point that corresponds to longitudinal fluctuations of the ordered moment. The study of the excitations in TlCuCl3 is now extended to finite temperatures. The results are summarised in Chapter 4, where similarities are reported between the quantum phase and thermal phase transitions. Spin ladder systems provide an exciting opportunity to study aspects of low-dimensional physics. With model magnets previously constrained to the limits of `strong' exchange (~ 100 meV) in the cuprates and `weak' exchange (~1 meV) in the metal-organics, the new spin ladder BiCu2PO6 offers the opportunity for study of spin ladder physics in the `intermediate' exchange regime (~ 10 meV). Inelastic neutron scattering studies of this system are presented in Chapter 5, where the magnon dispersion, exchange geometry and anisotropy are deduced from analysis of the excitation energies. Substitution of the Cu2+ sites in BiCu2PO6 with non-magnetic impurities Zn2+ results in the creation of BiCu2(1-x)Zn2xPO6, where a single S = 1/2 moment is liberated for each impurity. These moments are shown to demonstrate long-range correlations and magnetic ordering below a characteristic temperature, TN. Single crystal samples with x = 0.01, 0.03 and 0.05 have been investigated and structural studies of each are reported in Chapter 6. The field and temperature dependence of the observed long-range order is reported as well as a magnetic structure determination and studies of the impurity dependence of the coherence of the magnetic order.

Type: Thesis (Doctoral)
Title: Excitations and Criticality in Quantum Magnets
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright restricted material has been removed from the e-thesis
Keywords: quantum magnetism, neutron scattering, quantum criticality
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1388128
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