UC Santa Barbara

Strong electron correlations in transition metal oxides give rise to the emergence of a diverse array of transport phenomena and emergent phases such as quantum criticality, nematic order, and magnetism. The synthesis of thin films of these materials using molecular beam epitaxy (MBE) has enabled the discovery of additional phases that are not observed in bulk crystals. Furthermore, the precision offered by MBE over layer thickness in the growth of heterostructures and subtle modifications of the lattice imposed by epitaxial strain open the possibility of tuning these phases. The central aim of the work in this thesis is to explore the relationships between the strongly correlated transport phenomena and emergent phases and the structure of the MBE-grown crystal. This includes tilting and connectivity of the oxygen octahedra in the perovskite structure, quantum well thickness, epitaxial strain, and proximity effects.

The thesis begins with an introduction to the physics of metallic transport in correlated materials with an emphasis on highlighting the phenomena that have been observed in the MBE-grown materials discussed in this work. This includes pseudogap behavior in the density of states, quantum criticality, non-Fermi liquid transport, and Van Hove singularities in the electronic structure. This is followed by an overview of the specific perovskite materials and the hybrid MBE growth technique used to synthesize them.

The body of the experimental work in this thesis is comprised of two main parts. First, investigations of the electron correlations in RTiO3/SrTiO3/RTiO3 (R = Gd, Sm) quantum wells will be discussed. This includes tunneling spectroscopy experiments in which tunnel junction devices were fabricated to the pseudogap phase that emerges in the two-dimensional electron gas residing in the well. A robust quadratic dependence of the Hall angles in these quantum wells is discussed. The second part focuses on the transport properties of the layered strontium ruthenate Ruddlesden-Popper series. A novel method of growing these materials incorporating the use of the volatile precursor RuO4 is introduced. Finally, a study of the electronic and magnetic properties of an epitaxially strained Sr3Ru2O7 film is discussed, where it was found that slight perturbations of the lattice greatly enhance the emergent ordered phases. The thesis concludes with a brief summary and proposals for future research in correlated oxides.