Neutron scattering in the novel quantum magnets LiVO? and DMACuCl? /

by 1970- Tian, Wei

The behavior of magnetic systems in the extreme quantum limit is one of the most interesting forefront areas in condensed matter physics. This dissertation investigates two particularly interesting quantum magnets: LiVO2 and DMACuCl3. Systematic studies were performed on single crystal samples using different experimental techniques, especially inelastic neutron scattering. Detailed experimental results and corresponding model calculations are presented and discussed in this dissertation. LiVO2 is a good candidate to study the interplay between “magnetic frustration” and orbital ordering. V3+ ions in LiVO2 form a triangular lattice involving threefold degenerate t2g orbitals. LiVO2 undergoes a first order phase transition at Tt = 500 K accompanied by a large reduction of the magnetic susceptibility in the low temperature phase. The origin of this phase transition has been associated with a peculiar type of t2g orbital ordering resulting in the formation of vanadium trimer clusters with a spin-singlet ground state. Our inelastic neutron scattering experiments reveal multiple magnetic excitations with energy transfer as large as several hundreds of meV associated with the low temperature orbitally ordered phase. This can not be explained by a simple “spin-only” isolated trimer model. Model calculations including a Kugel-Khomskii orbital-ordering term yield qualitatively similar spectra suggesting that a new type of excitation, which we refer to as “cluster orbiton”, has been observed in LiVO2. iv DMACuCl3 was proposed as a S = 1/2 FM-AFM alternating-sign chain along the a-axis. However, our inelastic neutron scattering experiments indicate that the strong magnetic coupling in DMACuCl3 is along the b-axis, and the magnetic excitation spectrum is dominated by the FM-AFM alternating chain with JFM = -1.2(1) meV and JAFM = 1.3(2) meV. Furthermore, recent studies show that DMACuCl3 undergoes a structural phase transition at Tt = 285 K. Below Tt, the low temperature crystal structure suggests that DMACuCl3 contains two types of dimers: FM and AFM dimers. Inelastic neutron scattering results suggest a two chain model along the b-axis for DMACuCl3. This research is conducted under the supervision of the author’s advisors: Dr. David G. Mandrus at material science and technology division and Dr. Stephen E. Nagler at center for neutron scattering, Oak Ridge National Laboratory. v