While structure refinement is routinely achieved for simple bulk materials, the accurate structural determination still poses challenges for thin films due on the one hand to the small amount of material deposited on the thicker substrate and, on the
other hand, to the intricate epitaxial relationships that substantially complicate standard X-ray diffraction analysis. Using a combined approach, we analyze the crystal structure of epitaxial LaVO$_3$ thin films grown on (100)-oriented SrTiO$_3$. Transmission electron microscopy study reveals that the thin films are epitaxially grown on SrTiO$_3$ and points to the presence of 90$^{circ}$ oriented domains. The mapping of the reciprocal space obtained by high resolution X-ray diffraction permits refinement of the lattice parameters. We finally deduce that strain accommodation imposes a monoclinic structure onto the LaVO$_3$ film. The reciprocal space maps are numerically processed and the extracted data computed to refine the atomic positions, which are compared to those obtained using precession electron diffraction tomography. We discuss the obtained results and our methodological approach as a promising thin film structure determination for complex systems.
Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO3) is the bedrock of the emerging field of oxide electronics. SrTiO3 is the preferred template to create exotic two-dimensional (2D) phases of electron matter
at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. Here we show, using angle-resolved photoemission spectroscopy (ARPES), that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3, independent of bulk carrier densities over more than seven decades, including the undoped insulating material. This 2DEG is confined within a region of ~5 unit cells with a sheet carrier density of ~0.35 electrons per a^2 (a is the cubic lattice parameter). We unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. The similarity of this 2DEG with those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices, and a novel route to generate 2DEGs at surfaces of transition-metal oxides.
Using a low-temperature conductive-tip atomic force microscope in cross-section geometry we have characterized the local transport properties of the metallic electron gas that forms at the interface between LaAlO3 and SrTiO3. At low temperature, we f
ind that the carriers do not spread away from the interface but are confined within ~10 nm, just like at room temperature. Simulations taking into account both the large temperature and electric-field dependence of the permittivity of SrTiO3 predict a confinement over a few nm for sheet carrier densities larger than ~6 10^13 cm-2. We discuss the experimental and simulations results in terms of a multi-band carrier system. Remarkably, the Fermi wavelength estimated from Hall measurements is ~16 nm, indicating that the electron gas in on the verge of two-dimensionality.
We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature.
The value of the superconducting gap is 15 +- 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation in the cuprates, pointing to possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.
