Gosuly, SP; (2017) Neutron Scattering Studies of Low-Dimensional Quantum Spin Systems. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes how neutron scattering is used to reveal aspects of the physics of three experimental realizations of model systems with localized quantum spins and low-dimensional magnetic character of exchange interactions. The rst studied system is (5CAP)2CuCl4 (CAPCC), a realization of the famous S=1/2 square-lattice Heisenberg antiferromagnet. High-resolution time-of- ight inelastic neutron scattering is used to measure the excitation spectrum in applied magnetic eld. A t of the magnon dispersion predicted by linear spin-wave theory, to the dispersion measured in the eld-polarised state, gives the parametrization of the low-energy e ective Hamiltonian in terms of exchange couplings. These results establish that CAPCC is a quasi-2D square-lattice quantum Heisenberg antiferromagnet with a leading nearest neighbour Heisenberg exchange J = 0:100(2) meV, but with signi cant inter-planar coupling J0 = 0:032(2) meV. The excitation spectrum measured in a magnetic eld B = 0:85 Bs, just below the saturation eld, provides experimental evidence that supports the theoretical prediction of eldinduced magnon decays in CAPCC. The second investigated system is Y2BaNiO5, an excellent realization of the S=1 Heisenberg antiferromagnetic chain. Very high-resolution inelastic neutron spinecho is used to extend the magnon lifetime study by measuring the temperature dependence of the lifetime of triplet excitations and blue shift of the gap energy. The results are compared with theoretical predictions based on the non-linear sigma model, which fails to describe the measured lifetimes and blue shift over the whole range of measured temperatures. The residual measured lifetime as T ! 0 is attributed to chain-breaking defects and a mean chain length of 148 Ni-Ni sites is extracted. The possible limitation of the neutron spin-echo technique is highlighted when the excitation lineshape is asymmetric and the curvature of the dispersion surface probed by the resolution ellipsoid is large. The third studied system is Cs2CoCl4, an e ective S=1/2 XXZ antiferromagnetic spin chain. In a rst part, time-of- ight magnetic neutron di raction is used to extract the quantum correlations along the chain and an entanglement witness quantity that is theoretically predicted to signal an entanglement transition. In a second part, a triple-axis neutron technique is used to investigate the magnetic eld evolution of the magnetic ordering in Cs2CoCl4, when a magnetic eld is applied along the chain direction. Spin-wave excitations measured in the eld-polarised state provide the parameters of the low-energy spin Hamiltonian which are responsible for the observed complex ordering.

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