Visualising emergent phenomena at oxide interfaces

Knowledge of atomic-level details of structure, chemistry, and electronic states is paramount for a comprehensive understanding of emergent properties at oxide interfaces. We utilise a novel methodology based on atomic-scale electron energy loss spectroscopy (EELS) to spatially map the electronic states tied to the formation of a two-dimensional electron gas (2DEG) at the prototypical non-polar/polar $TiO_2$/$LaAlO_3$ interface. Combined with differential phase contrast analysis we directly visualise the microscopic locations of ions and charge and find that 2DEG states and $Ti^{3+}$ defect states exhibit different spatial distributions. Supported by density functional theory (DFT) and inelastic scattering simulations we examine the role of oxygen vacancies in 2DEG formation. Our work presents a general pathway to directly image emergent phenomena at interfaces using this unique combination of arising microscopy techniques with machine learning assisted data analysis procedures.Comment: 17 pages, 10 figure

Direct-ARPES and STM investigation of FeSe thin film growth by Nd:YAG laser

Funding: D.M. acknowledges the receipt of a fellowship from the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. R.A. and A.B. acknowledges the support by the Austrian Science Fund (FWF) through Projects No. P26830, No. P31423 and H2020 NFFA-Europe 654360.Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.Publisher PDFPeer reviewe

Evidence of a 2D Electron Gas in a Single-Unit-Cell of Anatase TiO2 (001)

The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO2), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO2 thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO2 is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO2 and give insight in formation mechanism of electronic states at the surface of TMO

Synthesis and properties of highly metallic orbital-ordered A-site manganites

In perovskite oxide materials, because of the insertion of multiple valence states ions (e.g., Mn in manganites) at atomic A-site (i.e., at the center of perovskite cubic cell), an enhancement of the ferromagnetic metallic state together with a strong orbital order of Mn-ions is established. Such a feature goes beyond the conventional theoretical framework for which the kinetic energy of the free charge carriers prevents the occurrence of a long-range orbital order. We do provide a complete physical characterization of these so-called A-site manganites by comparing transport and structural properties of La0.7Sr0.3MnO3 and LaMnO3 thin films with different excess Mn content and different oxygen content. A viable route to successfully grow such class of materials as a function of temperature and oxygen environment is provided. The observed multi-order phase coexistence opens unexplored perspectives toward the synthesis of new intrinsic multi-functional materials

Physical properties of La0.7Ba0.3MnO3−delta complex oxide thin films grown by pulsed laser deposition technique

We report on transport properties of oxide manganite La0.7Ba0.3MnO3−delta (LBMO) thin films deposited by pulsed laser deposition (PLD) technique. Detailed analysis of heavy-ion stoichiometric composition has been carried out as a function of laser-pulse