Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

Polar discontinuities occurring at interfaces between two different materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates or charge transfer. In the latter case, one may expect a local electronic doping of one material: one sparkling example is the two-dimensional electron liquid (2DEL) appearing in SrTiO$_3$ once covered by a polar LaAlO$_3$ layer. Here we show that tuning the formal polarisation of a (La,Al)$_{1-x}$(Sr,Ti)$_x$O$_3$ (LASTO:$x$) overlayer through chemical composition modifies the quantum confinement of the 2DEL in SrTiO$_3$ and its electronic band structure. The analysis of the behaviour in magnetic field of superconducting field-effect devices reveals, in agreement with $ab\ initio$ calculations and self-consistent Poisson-Schr\"odinger modelling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on interface charge densities. These results not only strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO$_3$/SrTiO$_3$ interface, but also demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.Comment: 18 pages, 4 figures, 1 ancillary file (Supporting Information

First principles study of heavily doped full Heusler Fe2YZ for high thermoelectric power factor

Using a combination of first-principles calculations based on density functional theory and Boltzmann semi-classical transport theory, we investigate the properties of heavily $n$-type doped full Heusler Fe2YZ1-xAx compounds. Using a supercell approach and including explicitly the dopant impurities, we recover in some cases giant thermoelectric power factors as previously predicted under doping within the rigid band approximations [Phys. Rev. Lett. 114, 136601 (2015)]. In other cases, however, we highlight that the system evolves toward a ferromagnetic half-metallic ground state so that the power factor is strongly modified. We rationalize the appearance of this magnetic instability, showing that it consistent with the Stoner model. The uncovered properties of the heavily doped phases of the studied Fe2YZ1-xAx compounds appear promising for Seebeck and spin-Seebeck applications

Low-dimensional d-states electron systems: insights from first-principles

The present thesis focuses on the physics of various solid-state systems sharing the common feature of involving 3d electrons with a low-dimensional aspect for transport, and studied using Density Functional Theory. Exploiting an original hybrid functional approach for the exchange-correlation energy, with improved accuracy compared to local/semi-local functionals, we present the seminal two-dimensional electron system (2DES) at the (001) interface of band insulators SrTiO3 and LaAlO3 , and review two of the most popular hypotheses about its origin, namely the electric-field driven Zener breakdown model and polarity-induced surface oxygen vacancies model. This analysis is extended to the interface between SrTiO3 and the (Sr1−xLax)(Ti1−xAlx)O3 alloy. We also study, based on experiments and theoretical modelling, how the composition of the alloy overlayer affects the charge density of the 2DES. We then address the effect of structural confinement on the 2DES when the host layer thickness is reduced toward the very-thin limit, and how such effects are witnessed in angle-resolved photo-emission spectroscopy experiments. We study the effects of capping the SrTiO 3 /LaAlO 3 het- erostructures with SrTiO 3 , highlighting how experiments may be interpreted from the aforementioned electric-field driven models. This work also focuses on the thermoelec- tric properties of layered oxides, specifically Ca3Co4O9 and SrTiO3 -based superlattices, discussing the relevance of their layered structure for improving the thermoelectric prop- erties. Importing the concepts of low-dimensional transport found in SrTiO3 -based sys- tems to the iron-based Heusler Fe2YZ family, we explore the effect of electron doping, highlighting magnetic instabilities related to their Fe 3d orbitals, which impact signif- icantly the thermoelectric properties. Finally, shifting our attention on Fe2TiSn, we rationalize experimental results provided by collaborators from first-principles, address- ing the role of native defects and their relevance for tailoring transport

Business plan : création d’une ferme aquaponique.

Notre projet consiste en la mise en place d’une ferme aquaponique en milieu urbain ou péri-urbain avec distribution des produits en circuit court auprès des restaurants, points de ventes locaux mais également directement aux consommateurs. Dans ce mémoire, nous exposerons l’ensemble des recherches et travaux effectués qui nous ont conduits à poser nos choix stratégiques relatifs aux 4P du marketing mix : Price, Product, Place and Promotion.Master [120] en sciences de gestion (horaire décalé à Mons), Université catholique de Louvain, 2017La diffusion de ce mémoire n'est pas autorisée par l'institutio

Polarity-field driven conductivity in SrTiO3/LaAlO3: A hybrid functional study

The origin of the 2-dimensional electron system (2DES) appearing at the (001) interface of band insulators $\rm SrTiO_3$ and $\rm LaAlO_3$ has been rationalized in the framework of a polar catastrophe scenario. This implies the existence of a critical thickness of polar $\rm LaAlO_3$ overlayer ($4~\rm u.c.$) for the appearance of the 2DES: polar catastrophe for thick $\rm LaAlO_3$ overlayer is avoided either through a Zener breakdown or a stabilization of donor defects at the $\rm LaAlO_3$ surface, both providing electrons to dope the substrate. The observation of a critical thickness is observed in experiments, supporting these hypotheses. Yet, there remains an open debate about which of these possible mechanisms actually occurs first. Using hybrid functional Density Functional Theory, we re-examine these mechanisms at the same level of approximation. Particularly, we clarify the role of donor defects in these heterostructures, and argue that, under usual growth conditions, electric-field driven stabilization of oxygen vacancies and hydrogen adsorbates at the LAO surface occur at a smaller LAO thickness than required for Zener breakdown

Thermoelectric properties of heavily-doped Fe2YZ full-Heusler compounds

Fe2YZ full-Heusler compounds were recently predicted to exhibit very large thermoelectric power factors [Phys. Rev. Lett. 114, 136601 (2015)]. Combining first-principles density functional theory calculations within a hybrid functional approach and Boltzmann semi-classical transport theory, we investigate the limits of the rigid band doping approximation, and the properties of heavily n-type doped full-Heusler Fe2YZ1-xAx compounds. Using a supercell approach and including explicit dopant impurities, we recover in some cases giant thermoelectric power factors, as previously predicted within the rigid band approximation. In others the effect on the electronic structure is very different from rigid shifts. Interestingly, we find that some systems present a magnetic instability consistent with the Stoner model, and evolve towards a ferromagnetic half-metallic ground state, with a strongly modified power factor. Our results show the promise of the heavily-doped phases of the studied Fe2YZ1-xAx compounds for Seebeck and spin-dependent Seebeck applications