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Creation and characterization of mixed valence states in transition-metal oxides are a fundamental approach to search for the unprecedented electronic and magnetic properties. In contrast to complex oxides, mixed-valence simple oxides tend to form binary or ternary phases, and turning a valence from one to next must be accompanied by structural transformations owing to a lower tolerance for oxygen non-stoichiometry. In this paper, epitaxial growth and transport properties of low-valence titanate thin films are reported to shed light on recently discovered superconducting γ-phase Ti3O5 (γ-Ti3O5). Single-phase TiO and Ti2O3 films and eutectic films including TiO, Ti2O3, and γ-Ti3O5 phases were independently grown on α-Al2O3 (0001) substrates by using pulsed-laser deposition. The X-ray diffraction measurements revealed clear epitaxial relationships with substrates and among three eutectic phases. Temperature dependence of the resistivity revealed that the γ-Ti3O5-rich films exhibited superconductivity with a maximum of transition temperature (TC) of 6.3 K. Distinct effects of the phase fraction on TC are found between TiO- and Ti2O3-enriched samples, suggesting the complex mechanisms of the superconducting proximity effect. [ABSTRACT FROM AUTHOR]

We investigated heteroepitaxial growth of Si-doped Ga2O3 films on MgO (100) substrates by pulsed-laser deposition as a function of growth temperature (Tg) to find a strong correlation between the structural and electronic properties. The films were found to contain cubic γ-phase and monoclinic β-phase, the latter of which indicated rotational twin domains when grown at higher Tg. The formation of the metastable γ-phase and twin-domain structure in the stable β-phase are discussed in terms of the in-plane epitaxial relationships with a square MgO lattice, while crystallinity of the β-phase degraded monotonically with decreasing Tg. The room-temperature conductivity indicated a maximum at the middle of Tg, where the β-Ga2O3 layer was relatively highly crystalline and free from the twin-domain structure. Moreover, both crystallinity and conductivity of β-Ga2O3 films on the MgO substrates were found superior to those on α-Al2O3 (0001) substrates. A ratio of the conductivity, attained to the highest quantity on each substrate, was almost three orders of magnitude. [ABSTRACT FROM AUTHOR]

Polar-nonpolar interfaces between insulating LaFeO3 (LFO) and semiconducting SrTiO3 (STO) were investigated to elucidate effects of built-in potential on photocarrier dynamics during water oxidation reactions. The LFO films were grown on the TiO2- and SrO-terminated (001) Nb-doped STO substrates by using pulsed-laser deposition. The photocurrent-voltage curves varied depending on the interface termination. Potential profiles across the interface were established from these curves as well as capacitance-voltage curves. The presence of a depletion (accumulation) region near the SrO- (TiO2-) terminated interface facilitates (suppresses) the extraction of photocarriers generated in STO. On the other hand, the difference in the built-in potential in LFO barely reflects the magnitude of the photocurrent. [ABSTRACT FROM AUTHOR]

The authors have investigated the thickness-dependent physical properties of PrNiO3 ultrathin films epitaxially grown on LaAlO3 substrates. The strained PrNiO3 films exhibit metallic behavior and do not show any indication of temperature-driven metal-insulator transition (MIT) in bulk form, whereas an insulating ground state is realized in a thin limit. In situ photoemission measurements reveal that the observed thickness-dependent MIT is caused by the reduction in bandwidth due to the dimensional control of the films. These results strongly suggest that the MIT in PrNiO3 films can be controlled by changing the dimensionality under epitaxial constraint. [ABSTRACT FROM AUTHOR]