Walters, A.C.; (2009) Using X-ray and neutron scattering to study the dynamics of low-dimensional systems. Doctoral thesis , UCL (University College London).

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Abstract

The theoretical and experimental study of low-dimensional systems has dominated much of modern condensed matter physics. Such systems present a range of different phenomena which are not observed in more isotropic, three-dimensional materials. This thesis constitutes the study of the excitations of two types of low-dimensional system: the magnetic excitations in the one-dimensional S = 1/2 antiferromagnetic chain compound Sr2CuO3 as studied using inelastic neutron scattering, and the phonons in quasi-two-dimensional graphite intercalation compounds CaC6 and BaC6 studied using inelastic X-ray scattering. Initially an introduction to the vast field of low-dimensional systems is provided, followed by a description of the neutron and X-ray scattering technique. Spinon excitations in Sr2CuO3 have been measured up to ≈ 600 meV, and are found to be well-described by the Heisenberg model. A new version of the magnetic form factor for Cu2+ in Sr2CuO3 provides an explanation for the considerable reduction in the measured inelastic neutron scattering intensity, which may have significant consequences for past and future work on other related materials. The phonons in superconducting CaC6 (Tc = 11.5 K) are found to be in reasonable agreement with Density Functional Perturbation Theory calculations, but the measured dispersions in non-superconducting BaC6 are significantly different to those predicted. No direct evidence for electron-phonon coupling is found in either compound. It is concluded that the X-ray data supports the validity of the theoretical description of CaC6, but the use of significantly inaccurate lattice parameters for BaC6 in the calculation may provide an explanation of the poor agreement in this case.

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