Inelastic Neutron Scattering Studies of Novel Quantum Magnets

Inelastic neutron scattering was used to study the magnetic excitation spectrum of three quantum magnets: (i) the double perovskite Ba2FeReO6; (ii) the two-dimensional square lattice Heisenberg antiferromagnet Sr2CuO2Cl2; and (iii) the quasi-two-dimensional frustrated two-leg ladder BiCu2PO6.We have conducted inelastic neutron scattering measurements on powder samples of the double perovskite compound Ba2FeReO6. The measurements revealed two well defined dispersing spin wave modes. No excitation gap was observable and the spectrum can be explained with a local moment model incorporating the interactions of Fe spins with spin-orbital locked degrees of freedom on the Re site. The results reveal that both significant electronic correlations and spin-orbit coupling on the Re site play a significant role in the spin dynamics of Ba2FeReO6.High resolution neutron scattering measurements of magnetic excitations in the parent cuprate Sr2CuO2Cl2 reveal a significant dispersion and momentum dependent damping of the zone boundary magnons. We directly compare our measurements with previous resonant inelastic x-ray scattering measurements and find a ~25 meV discrepancy between the two techniques for the measured zone boundary energy at (1/2, 0). The deviations are greatest precisely in the region of phase space where the magnon damping is strongest. This comparison shows that the inelastic x-ray spectrum must contain significant contributions from higher energy excitations not previously considered. Our measurements demonstrate that the high energy continuum of magnetic fluctuations is a ubiquitous feature of the magnetic spectrum among insulating monolayer cuprates, and that these excitations couple to both inelastic neutron and light scattering.A comprehensive series of inelastic neutron scattering measurements was used to investigate spin excitations in the frustrated two-leg ladder compound BiCu2PO6. The measurements revealed six branches of steeply dispersing triplon excitations. Dynamic spin correlations in BiCu2PO6 are the quantum analog of the incommensurate spiral order that appears in classical frustrated magnets and the dispersion minimum occurs at an incommensurate wavevector with a gap of 1.67(2) meV. Neutron scattering measurements in applied magnetic field show that the triplon degeneracy in zero field is completely broken by anisotropic magnetic interactions and each branch was observed to exhibit an anomalous Zeeman behavior. The lowest energy excitation spectrum and anomalous Zeeman splitting is captured by a bond-operator theory, incorporating very significant anisotropic magnetic exchange interactions. At high energies, we have also observed a continuum of two-triplon scattering and a unique two-triplon bound state excitation. Anharmonic terms in the magnetic Hamiltonian of BiCu2PO6 result in a spectacular spontaneous decay of the triplon quasiparticles.Ph.D