Abstract
This thesis presents the work and results of four elastic and inelastic X-ray and neutron scattering studies on single crystals. The topics, the studied compounds and the applied scattering techniques are manifold. However all of those have a common theme: The characterisation of ordered phases, their characteristic excitations and the their effects on material properties in layered 3d- and 4d-transition metal oxides by neutron and X-ray scattering techniques. The presented work particularly focuses on interesting states such as unconventional superconductivity, magnetism and charge density wave order in square-lattice quantum materials, to compare with model Hamiltonians, to determine the natures of the states, or to characterise the interactions between orders.A neutron time-of-flight spectroscopy study performed at the MAPS instrument at the ISIS Neutron and Muon Source in the UK, reveals an anomalous high-energy spin-wave dispersion in square-lattice S = 1 antiferromagnet La2NiO4, which resembles the anomalous dispersion observed in multiple quantum antiferromagnets. A continuation of this study, at the SEQUOIA instrument at the Spallation Neutron Source at the Oak Ridge National Laboratory in the USA, showed no other signatures of quantum antiferromagentism such as a high-energy scattering continuum or missing spectral weights. Instead it showed that the spectral weights are well described by a classical one-magnon + semi-classical two-magnon model, including antiferromagnetic second-nearest-neighbour Heisenberg coupling and strong anisotropy.
Sr2RuO4 is an unconventional superconductor, which was long though to be solid state analogue of the superfluid 3He A-phase, exhibiting a chiral spin-triple state with an odd-parity order parameter and an out-of-plane d-vector. Two polarised neutron diffraction experiments, performed on the instrument IN20 at the Institut Laue–Langevin in France, are presented, which not only reject the previously thought state, but also show a large jump in the spin-susceptibility at Hc2, due a first order transition induced by Pauli paramagnetism. But especially, they reveal a large low temperature and low magnetic field spin susceptibility, in the superconducting state, consistent with an odd-parity state with in-plane d-vector or with an even-parity state with residual density of states.
Multiple X-ray diffraction studies were performed at beamline i16 at the Diamond Light Source in the UK, at the sector 6-ID-D at the Advanced Photon Source at the Argonne National Laboratory in the USA, and at the beamline P24 at the PETRA III at the DESY facility in Germany to uncover the atomic displacements associated with the charge density wave (CDW) order in the compound La1.675Eu0.2Sr0.125CuO4, a derivative of the high-Tc superconductor La2−xSrxCuO4. A stripe-like in-plane atomic displacement is observed which is directly related to the stripe-like hole-accumulation of the charge density ordering. In addition, large collective out-of-plane modulations, which appear to screen the charge order and relax internal strain induced by the stripe-like displacements, are observed. At last, clear signatures of coupling, of the microscopic structure, to distortions affiliated with the low-temperature tetragonal (LTT) structure are revealed. These appear as periodic modulations of the CuO6-octahedra tiltings and hence, stabilise the S3 irreducible representation of the LTT structure.
In the last experimental chapter two triple-axis spectroscopy experiments on the high-Tc superconductor YBa2Cu3O6.65 (YBCO) are presented. These were performed on the instrument IN8 at the Institut Laue–Langevin in France. Although this is a very well characterised system, we present a novel excitation at ≈ 7meV, which has not yet been observed in inelastic neutron scattering. It is very weak, appears strongest at small Q and is rather dispersionless throughout the whole Brillouin zone. It was attempted to map out the excitation and we observed an unusual temperature scaling but the nature of the excitation remains elusive.
| Date of Award | 13 May 2022 |
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| Original language | English |
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| Supervisor | Nigel E Hussey (Supervisor) & Stephen M Hayden (Supervisor) |