Mena, Mattia; (2018) Dynamic correlations in model quantum magnets in various dimensions investigated with neutron scattering. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Several properties of a system of correlated spins contribute to the macroscopic behaviour, and amongst them are those of quantum versus classical magnetism, frustration or dimensionality of the system, Ising or Heisenberg interactions, which on their own can lead to a rich and diverse phenomenology. This thesis investigates three materials in order to explain the diverse manifestation of their magnetic excitations and relate it to their physical properties. The excited spectrum of an ideal antiferromagnetic Ising chain is either bow-tie-shaped continua or a series of discrete cosine-like modes, depending on the presence or absence of a staggered magnetic eld. In one physical realization of this system, the hexagonal halide RbCoCl3, which possesses three distinct magnetic phases, both a continuum and discrete Ising spins are observed in the lowest ordered magnetic phase, while only a continuum is found in the other phases. We propose that this is due to the nearest and next-nearest interactions perpendicular to the plane, which lead to either long-range or short-range order. In this compound an unstaggered ordered Ising chain can be observed down to 0 K, allowing for studies of the thermal Ising chain and providing a material where the e ect of frustration can be tuned with temperature and a ects the dispersion. In the ferromagnetic insulating pyrochlore Lu2V2O7 magnons display a novel thermal equivalent of the Hall e ect induced by the Dzyaloshinskii- Moriya interaction (DMI). In an in-depth study of its spectrum, we have measured the full spectrum of Lu2V2O7 and con rmed that a minimal Hamiltonian is su cient to describe it, obtaining the ratio between the ferromagnetic interaction and the DMI, which is the parameter controlling the magnon Hall e ect. SrCu2(BO3)2 is a two-dimensional frustrated square compound where the spins form antiferromagnetic dimerized singlets whose elementary excitation is an intra-dimeric singlet-to-triplet transition. Temperature causes the excited spectrum to decrease in intensity at a temperature much lower than the gap. The exchange between the polarized shells created around the dimers has been suggested to provide the main decay channel for the excited triplets. In this thesis we investigate the ~ Q-dependence of this decay channel. Only part of the results summarized in this thesis have been published, in a PRL paper on the magnon dispersion of Lu2V2O7 of which I am rst author. I am currently drafting a paper on RbCoCl3, with an additional paper authored by N. Hanni on the subject having been already submitted to PRL. Furthermore, I am collaborating on a joint publication on SrCu2(BO3)2 with D. Lancon.

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