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The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν > 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations. [ABSTRACT FROM AUTHOR]

For certain conditions of layer thickness, the interface between GdTiO3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas(2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7, with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening. [ABSTRACT FROM AUTHOR]

The band gaps of rare-earth titanates are commonly reported to be 0.2-0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials. [ABSTRACT FROM AUTHOR]

SrTiO3 (STO) is at the core of recent discoveries of two-dimensional electron gas (2DEG) formation at complex oxide interfaces, with the 2DEG residing on the STO side. Experimental results for ultrathin STO layers inserted in GdTiO3 reveal a transition from metallic to insulating behavior, and suggest a strong interplay between electron-electron interaction and lattice distortions. Using first-principles calculations, we show that the metal-to-insulator transition is a bulk property of STO that emerges at extreme doping levels. We find that doping with 1/4 electron per Ti atom produces a metallic phase as expected, but that adding 1/2 electron per Ti results in a charge-ordered Mott-insulating phase. The effects of electron-electron interactions and lattice distortions are disentangled, and the Mott-insulator phase is shown to occur in STO/LaAlO3 and STO/GdTiO3 heterostructures with ultrathin STO layers. [ABSTRACT FROM AUTHOR]

The polar discontinuity at the SrTiO3/LaAlO3 interface (STO/LAO) can in principle sustain an electron density of 3.3 × 1014 cm-2 (0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schrodinger-Poisson simulations we show that the problem lies in the asymmetric nature of the structure, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, or to fully passivate the LAO surface. Our insights apply to oxide interfaces in general, explaining for instance why the SrTiO3/GdTiO3 interface has been found to exhibit the full density of 3.3 × 1014 cm-2. [ABSTRACT FROM AUTHOR]

The electronic structure of native defects and impurities in GdTiO3, a rare-earth titanate Mott insulator, is studied using density functional theory with a hybrid functional. Among native defects, the cation vacancies have the lowest formation energies in oxygen-rich conditions and oxygen vacancies have the lowest formation energy in oxygen-poor conditions. Among the impurities, SrGd,Hi, and CO have low formation energies. A common feature of the native defects and impurities is that they lead to the formation of small hole polarons, which explains the frequent observation of p-type hopping conductivity in the rare-earth titanates. These small hole polarons also lead to optical absorption and act as electron traps in devices. [ABSTRACT FROM AUTHOR]

For certain conditions of layer thickness, the interface between GdTiO3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas(2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7, with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening. [ABSTRACT FROM AUTHOR]