We present a detailed depth-sensitive study of the evolution in correlated electron behavior from the surface of the prototypical correlated oxide, SrxCa1-xVO3, to its bulk. Photoemission measurements of varying surface sensitivity are employed to directly compare both the spectral weight and energetics of the correlated electron features, and resonant soft x-ray emission spectroscopy is used as a bulk-sensitive reference. The surface component, which still contributes significantly to photoemission at 2.2 keV, is characterized by a transfer of spectral weight into the incoherent lower Hubbard band and the corresponding shift of these states towards lower binding energy.

We present a simultaneous photoemission spectroscopic, low-energy electron diffraction and low-energy electron microscopic study of the metal-insulator transition of strained VO2. The fraction of rutile structure is extracted from the microscopic measurements throughout the transition, and compared with the fraction of the metallic electrons from photoemission data. We find that at intermediate temperatures, while the system is predominantly monoclinic-like in structure, the electronic component of the transition is much further advanced. Our results provide direct evidence for a monoclinic-like metallic phase of VO2 that is easily accessible at ambient temperatures and pressures.

Ionic piezoresistance, the effect of lattice strain on ionic conductivity, is an important concept that needs to be harnessed to engineer the next generation of fast ionic conductors. To date there have been many reports of strain affecting changes in the level of ionic conductivity in solid electrolytes. The fundamental understanding is, however, still lacking, with limited experimental quantification of the magnitude of the effect. Here, we propose using the ionic piezoresistive coefficient, the constant of proportionality between the strain state and the change in conductivity, as a quantitative measure of this effect and detail a novel technique we have developed to quantify this in high temperature ionically conducting materials.

We report the epitaxial growth of γ-Al2O3 on SrTiO3 (STO) substrates by atomic layer deposition (ALD). The ALD growth of γ-Al2O3 on STO(001) single crystal substrates was performed at a temperature of 345 °C. Trimethylaluminum and water were used as co-reactants. In-situ reflection high-energy electron diffraction and ex-situ x-ray diffraction were used to determine the crystallinity of the Al2O3 films. In-situ x-ray photoelectron spectroscopy was used to characterize the Al2O3/STO heterointerface. The formation of a Ti3+ feature is observed in the Ti 2p spectrum of STO after the first few ALD cycles of Al2O3 and even after exposure of the STO substrate to trimethylaluminum alone at 345 °C. The presence of a Ti3+ feature is a direct indication of oxygen vacancies at the Al2O3/STO heterointerface, which provide the carriers for the quasi-two dimensional electron gas at the interface.