UC Berkeley

Transition metal oxides are fascinating and complex materials that have puzzled condensed matter physicists for many years due to their many different electronically ordered ground states. Albeit separated by external or internal parameters, these nearly degenerate phases are sometimes intertwined in competition or coexistence, which can itself lead to further phase emergence.

A unique feature of the 5d iridium oxide materials is that spin-orbit and Coulomb interactions are of comparable strength and therefore compete vigorously. In a honeycomb structure, this competition gives rise to strong magnetic frustration due to their edge-shared bonding environment, in which quantum interference of the two exchange paths favors a strongly anisotropic, Ising-like exchange between neighboring spin-1/2 moments. Such an interaction couples different orthogonal spin components for the three nearest neighbors and, as a consequence, no single exchange direction can be simultaneously satisfied, leading to strong frustration described by the Kitaev Hamiltonian. This model results in the infinitely degenerate and extremely exotic quantum spin liquid state, whose excitations range from being Majorana-like to fractionalized fermionic excitations. In this thesis, I examine the magnetic properties of the three-dimensional Kitaev candidate materials in the Harmonic Honeycomb family, using a combination of thermodynamics and resonant x-ray scattering techniques.