Theory of the nuclear magnetic 1/T1 relaxation rate in conventional and unconventional magnets

The search for new phases of quantum matter is at the heart of modern condensed matter physics, and frustrated magnets are one of the most promising classes of material in which to carry out this search. Each distinct phase can be thought of as its own 'universe', with its own set of unique, 'fundamental' excitations. A way of recognising unusual magnetic phases is by the experimental signature of their excitations, and measurement of the NMR T1 relaxation rate is a powerful method of probing these excitations. However, the dynamical information locked in NMR T1 measurements remains notoriously difficult to interpret. The difficulty arises from the fact that information about all possible low-energy spin excitations of the electrons, and their coupling to the nuclear moments, is folded into a single number, 1/T1 In this thesis we first develop a quantitative theory of T1, focusing on the specific example of the collinear antiferromagnet BaFe2As2. One of the most striking features of magnetism in BaFe2As2 is a strong dependence of 1/T1 on the orientation of the applied magnetic field, and we make convincing, quantitative fits to experimental data for different field orientations. This leads to the idea of 'angle-resolved' NMR. The quantum spin-nematic state - the magnetic analogue of the liquid crystal - is an unusual state of matter in which measurement of the T1 relaxation rate promises to be particularly revealing. Such a state has been proposed in the context of a number of magnetic insulators, including the quasi-two dimensional magnet iGa2S4, thin films of 3He, and the spin-chain system LiCuV04 in high magnetic field - but never yet observed in experiment. In all of these cases, the models studied predict an 'antiferroquadrupolar' order, in which spin fluctuations select perpendicular axes on neighbouring sites (or bonds) of the lattice. Progress in understanding these systems has been limited by the difficulty in performing calculations for any realistic microscopic model. With this in mind, we develop a phenomenological, field-theoretical description of anti- ferroquadrupolar spin-nematic order. The resulting action depends only on the symmetry of the order parameter, and so is applicable to a wide range of systems. Observation of the spin-nematic state is complicated by the fact that the order parameter does not break time-reversal symmetry, and is therefore 'invisible' to the tests commonly used to discern magnetic order. However, excitations of the spin-nematic state induce a fluctuating spin- dipole moment, and this can, in principle, be detected by dynamic probes of magnetism, including the NMR T1 relaxation rate. We make predictions for the 'fingerprint' of spin-nematic order in T1 measurements, and a particularly striking finding is the absence of a critical divergence at the onset of ordering. We also make predictions for the signature in inelastic neutron scattering experiments. These predictions could potentially lead to experimental verification of the long- elusive, spin-nematic state.EThOS - Electronic Theses Online ServiceGBUnited Kingdo