The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as LaAlO 3 and SrTiO 3 has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, the fundamental origins of the 2DES are still not understood, e.g., the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation on the depth profile of 2DES-relevant Ti 3d interface carriers using depth-and element-specific techniques like standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti 3d interface carriers, which give rise to the 2DES are located within three unit cells from the n-type interface in the SrTiO 3 layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first three unit cells of the opposing LaAlO 3 layer (∼10 Å). Our findings provide a microscopic picture of how the localized and mobile Ti 3d interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the LaAlO 3 /SrTiO 3 interface have disparate explanations as originating from different types of interface carriers.

PhySH: Complex oxides; transition metal oxides; X-ray absorption; X-ray magnetic circular dichroism; thin films; scanning transmission electron microscopy. Abstract We report charge-transfer up to a single electron per interfacial unit cell across non-polar heterointerfaces from the Mott insulator LaTiO 3 to the charge transfer insulator LaCoO 3 . In high-quality bi-and tri-layer systems grown using pulsed laser deposition, soft X-ray absorption, dichroism and STEM-EELS are used to probe the cobalt 3d-electron count and provide an element-specific investigation of the magnetic properties. The experiments prove a deterministically-tunable charge transfer process acting in the LaCoO 3 within three unit cells of the heterointerface, able to generate full conversion to 3d 7 divalent Co, which displays a paramagnetic ground state. The number of LaTiO 3 |LaCoO 3 interfaces, the thickness of an additional 'break' layer between the LaTiO 3 and LaCoO 3 , and the LaCoO 3 film thickness itself in tri-layers provide a trio of sensitive control knobs for the charge transfer process, illustrating the efficacy of O2p-band alignment as a guiding principle for property design in complex oxide heterointerfaces.

We present 90 meV resolved Co 2p3d resonant inelastic x-ray scattering linear dichroism spectra of strained LaCoO 3 films and a LaCoO 3 single crystal. A polarization-dependent low-energy excitation is observed at ∼0.2 eV on the tensile-strained LaCoO 3 /SrTiO 3 film, while it is not observed in either bulk LaCoO 3 or the compressive-strained LaCoO 3 /LaAlO 3 film. Guided by cluster calculations, we are able to distinguish the spin-state manifolds close to their transition point of Co 3+ ions in LaCoO 3 systems. Through a polarization analysis, we show that the spin state can easily flip from a low-spin 1 A 1g state in an octahedral symmetry to the high-spin 5 B 2g or 5 E g states with a small tetragonal distortion. A mixture of spin states suggests that the high-spin Co 3+ plays an important role in long-range ferromagnetic order on both tensile-and compressive-strained LaCoO 3 films.

Angle-dependent 2p3d resonant inelastic X-ray scattering spectra of a LaCoO3 single crystal and a 55 nm LaCoO3 film on a SrTiO3 substrate are presented. Theoretical calculation shows that, with ∼20 meV resolved Co 2p3d resonant inelastic X-ray scattering (RIXS), the excited states of the isotropic 1A1g (O h ) ground state are split by 3d spin–orbit coupling, which can be distinguished via their angular dependence. However, strong self-absorption and saturation effects distort the spectra of the LaCoO3 single crystal and limit the observation of small angular dependence. In contrast, the RIXS on 55 nm LaCoO3 shows less self-absorption effects and preserves the angular dependence of the excited states.

This chapter describes optimization of the Pulsed Laser Deposition process to fabricate combinations of ultra-thin LaCoO3 and LaTiO3. Normally, a low O2 background pressure is key in avoiding La2Ti2O7 phase formation in LaTiO3. Since these pressures likely result in oxygen vacancy formation in LaCoO3, a strategy was adopted to grow both materials in an intermediate pressure of 2e-3 mbar. Assisted by the use of a LaAlO3 buffer layer and by keeping the thickness of LaTiO3 fixed at 4 unit cells. With fixed pressure, the other deposition parameters were optimized to yield a low surface roughness and the correct perovskite structure. The samples were monitored during growth by Reflection High-Energy Electron Diffraction and characterized ex-situ using Atomic Force Microscopy, Scanning Transmission Electron Microscopy and X-Ray Diffraction.