Many ultrathin films of transition metal oxides exhibit nonmetallic behavior, in contrast to their metallic bulk counterpart, thus displaying a metal-insulator transition (MIT) as the film thickness is reduced. The nature of this MIT has been a long-standing issue in the epitaxial oxide research community. Here, we report the processing dependence of the critical thickness ($t_{\mathrm{c}}$) of MIT and the origin of the insulating phase in $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films. A $t_{\mathrm{c}}$ of 3 unit cells (u.c.) is achieved by minimizing oxygen vacancies under optimal growth conditions, diminishing the epitaxial strain with a tunable buffer layer and suppressing surface strain by film capping. The electrical transport measurements demonstrate that the nonmetallic behavior in LSMO thin films is an unavoidable result of localization initiated by inherent disorder but amplified by the reduction in dimensionality.

The recent observation of replica bands in single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ by angle-resolved photoemission spectroscopy (ARPES) has triggered intense discussions concerning the potential influence of the FeSe electrons coupling with substrate phonons on the superconducting transition temperature. Here we provide strong evidence that the replica bands observed in the single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer surface phonons in ionic materials in general. The photoelectron energy loss in ARPES on single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ is calculated using the demonstrated successful semiclassical dielectric theory in describing low energy electron energy loss spectroscopy of ionic insulators. Our result shows that the observed replica bands are mostly a result of extrinsic photoelectron energy loss and not a result of the electron phonon interaction of the Fe $d$ electrons with the substrate phonons. The strong enhancement of the superconducting transition temperature in these monolayers remains an open question.

Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti²⁺. We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti–Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

We report on a combined scanning tunneling microscopy and density functional theory calculation study of the ${\mathrm{SrTiO}}_3(110)-(4\times 1)$ surface. It is found that antiphase domains are formed along the $[1\overline{1}0]$-oriented stripes on the surface. The domain boundaries are decorated by defect pairs consisting of ${\mathrm{Ti}}_2{\mathrm{O}}_3$ vacancies and Sr adatoms, which relieve the residual stress. The formation energy of and interactions between vacancies result in a defect superstructure. It is suggested that the density and distributions of defects can be tuned by strain engineering, providing a flexible platform for the designed growth of complex oxide materials.

The surface reconstruction of SrTiO3(110) is studied with scanning tunneling microscopy and density functional theory (DFT) calculations. The reversible phase transition between (4\ifmmode\times\else\texttimes\fi{}1) and (5\ifmmode\times\else\texttimes\fi{}1) is controlled by adjusting the surface metal concentration [Sr] or [Ti]. Resolving the atomic structures of the surface, DFT calculations verify that the phase stability changes upon the chemical potential of Sr or Ti. In particular, the density of oxygen vacancies is low on the thermodynamically stabilized SrTiO3(110) surface.

High quality single crystals of ${\mathrm{BaBiO}}_3$ were grown by congruent melting technique and characterized with x-ray diffraction, x-ray photoemission, and transport property studies. The perovskite oxide ${\mathrm{BaBiO}}_3$ is a negative charge transfer gap high $T_c$ oxide parent superconducting compound exhibiting self-doping of holes into the oxygen $2p$ band. We study the low energy scale valence and conduction bands in detail from both a theoretical perspective as well as through x ray, absorption/emission, and photoelectron spectroscopies. X-ray spectroscopy verifies the results of density functional theory (DFT) regarding the overall band structure featuring strong O $2p$ character of the empty antibonding combination of the hybridized Bi $6s$ and O $2p$ states. From the analysis of the core level line shapes we conclude that the dominant O $2p$-Bi $6s$ hybridization energy scale determines the low energy scale electronic structure. This analysis provides further insight into the importance of self-doped oxygen $2p$ states in this high $T_c$ family of oxides.