Doctoral Dissertations

Date of Award

8-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

E. Ward Plummer

Committee Members

Ted Barnes, James R. Thompson, Robert Compton, David G. Mandrus, Stephen E. Nagler

Abstract

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.

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