Quantum magnetism and quantum phase transitions

Abstract:

We explore the magnetic excitations of the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal high-resolution inelastic neutron scattering. Sharp magnons with no decay are observed throughout reciprocal space, with a strongly renormalized dispersion and multiple soft modes compared to linear spin wave theory. We propose an empirical parametrization that can quantitatively capture the complete dispersions in the three-dimensional Brillouin zone and explicitly show that the dispersion renormalizations have the direct consequence that one to two magnon decays are avoided throughout reciprocal space, whereas such decays would be allowed for the unrenormalized dispersions. At higher energies, we observe a very strong continuum of excitations with highly-structured intensity modulations extending up at least 4x the maximum one-magnon energy. The one-magnon intensities decrease much faster upon increasing energy than predicted by linear spin wave theory and the higher-energy continuum contains much more intensity than can be accounted for by a two-magnon cross-section, suggesting a significant transfer of spectral weight from the high-energy magnons into the higher-energy continuum states. We attribute the strong dispersion renormalizations and substantial transfer of spectral weight to continuum states to the effect of quantum fluctuations and interactions beyond the spin wave approximation, and make connections to theoretical approaches that might capture such effects. Finally, through measurements in a strong applied magnetic field, we find evidence for magnetic domains with opposite senses for the spin rotation in the 120 degree ordered ground state, as expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense of spin rotation is selected via spontaneous symmetry breaking.

Abstract:

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T-S = 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of similar to 1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

Abstract:

This thesis presents inelastic neutron scattering (INS) studies of spin dynamics in two different Ising-like quantum magnetic materials as a function of applied magnetic field.

The first material explored is the ferromagnetic Ising chain CoNb₂O₆. The excitations in an Ising chain are independent domain wall quasiparticles, however, additional interactions beyond pure one-dimensional Ising exchange can lead to the formation of a variety of bound states in different regimes of applied magnetic field. The excitations in CoNb₂O₆ are first studied as a function of applied near-longitudinal field. This causes domain walls to become confined into bound states, with the confinement potential tuned by the applied field, from the limit of weak field where there is a Zeeman ladder of bound states to strong field where only two remain. Next, a quantum spin Hamiltonian is refined based on a global fit to the spectrum observed in zero field and in high field applied along two orthogonal directions. A novel method is developed using a combination of linear spinwave theory and exact diagonalization on finite chains to capture the observed bound states as well as the weak interactions between the different chains. Calculations using the refined Hamiltonian quantitatively reproduce not only the spectra to which the data were fit, but also the rich behaviour observed in low transverse field. An intuitive physical picture and fully analytically solved model is proposed for this behaviour based on novel bound states stabilized by the competition between an off-diagonal exchange and transverse field. Finally, the magnetization of CoNb₂O₆ is measured as a function of field up to 16 T, observing evidence for the proximity to the expected transverse-field induced quantum critical point.

The other material studied in this thesis is the frustrated triangular lattice quantum Ising-like antiferromagnet Na₂BaCo(PO₄)₂, whose spectrum is measured using INS as a function of transverse field. In high field, sharp magnons are observed and a spin Hamiltonian is refined based on the observed magnon dispersions. At fields below the transition to the field polarized phase, strong continuum scattering is observed, which dominates at low fields where no magnons are observed. The observed spectrum cannot be explained even qualitatively at low field using a linear spinwave approach including one and two magnon excitations.