UC Santa Barbara

Perovskite materials, having the simple ABO3 chemical formula, show a wide variety of electronic properties depending on the choice of the A and/or B cations. Multiple orders (e.g. electronic, magnetic and structural) with similar energy scales frequently occur in a single perovskite material. Cooperation between neighboring orders is believed to enhance the existing orders or, more excitingly, give rise to new phenomena.

SrTiO3 is the first oxide superconductor to be discovered, but the nature of its superconducting state has been a longstanding subject of debate. SrTiO3 is an incipient ferroelectric and becomes superconductive upon doping, which means that superconductivity is near ferroelectricity in this material. In this thesis, the superconducting states of doped SrTiO3 thin films grown by molecular beam epitaxy are investigated. Epitaxially strained SrTiO3 films tuned into a ferroelectric ground state show a record high superconducting transition temperature and field. Untangling the intertwined relationship between superconductivity and ferroelectricity in SrTiO3 could pave the way towards understanding the nature of the superconducting state.

Antiferromagnetic ordering can potentially bridge topological phenomena and spintronics. EuTiO3 is a tunable antiferromagnet. In this thesis, the topological states of doped EuTiO3 thin films grown by molecular beam epitaxy are investigated. Unusual anomalous Hall effects are observed in doped EuTiO3. The non-monotonic Hall signal was attributed to topological Hall effect. The magnitude and sign of these Hall effects strongly depend on the carrier concentration. The strong dependence on the carrier concentration makes EuTiO3 of potential interest for topological field-effect devices. It is also shown that the magnetic field can systematically control the symmetry of electronic states in EuTiO3. Finally, the formation of a two-dimensional electron system at the SmTiO3/EuTiO3 interface is studied. The results open interesting possibilities for epitaxial heterostructures that combine topological states, antiferromagnetic ordering, and other phenomena.

Discovery of a wide range of emergent phenomena at interfaces between complex oxides generated excitement in last decade. In this thesis, the polar discontinuity at the interface of two perovskite titanates, SrTiO3 and SmTiO3, is investigated. The polar discontinuity is compensated with half-electron donation per interfacial unit cell to the SrTiO3 conduction band, potentially leading to electron-electron correlation effects. At the SmTiO3/SrTiO3 interface, a gate voltage induced metal-insulator transition is observed. Increasing the thickness of SmTiO3 or SrTiO3 lowers the resistance below the Mott-Ioffe-Regel limit at low temperatures. In these structures an abrupt metal-insulator transition is observed in a narrow temperature window which does not fit conventional description.