Competition between Carrier Injection and Structural Distortions in Electron-Doped Perovskite Nickelate Thin Films

The discovery of superconductivity in doped infinite-layer nickelate thin films has brought increased attention to the behavior of the doped perovskite phase. Despite this interest, the majority of existing studies pertain to hole-doped perovskite rare-earth nickelate thin films, while most electron-doping studies have been performed on bulk materials so far. To tackle this imbalance, a detailed study that addresses doping of NdNiO thin films using A-site substitution is presented, using Pb as a dopant and taking advantage of its valence-skipping nature. Through a combination of complementary techniques including X-ray diffraction, transport measurements, X-ray absorption spectroscopy, electron energy-loss spectroscopy and scanning transmission electron microscopy, the valence of Pb in the NdPbNiO structure is confirmed to be 4+, and the behavior of the doped thin films is found to be controlled by a competition between carrier injection and structural distortions, which respectively reduce and increase the metal-to-insulator transition temperature. This work provides a systematic study of electron doping in NdNiO, demonstrating that A-site substitution with Pb is an appropriate method for such doping in perovskite rare-earth nickelate systems

Competition between Carrier Injection and Structural Distortions in Electron‐Doped Perovskite Nickelate Thin Films

The discovery of superconductivity in doped infinite‐layer nickelate thin films has brought increased attention to the behavior of the doped perovskite phase. Despite this interest, the majority of existing studies pertain to hole‐doped perovskite rare‐earth nickelate thin films, while most electron‐doping studies have been performed on bulk materials so far. To tackle this imbalance, a detailed study that addresses doping of NdNiO$_{3}$ thin films using A‐site substitution is presented, using Pb as a dopant and taking advantage of its valence‐skipping nature. Through a combination of complementary techniques including X‐ray diffraction, transport measurements, X‐ray absorption spectroscopy, electron energy‐loss spectroscopy and scanning transmission electron microscopy, the valence of Pb in the Nd$_{1−x}$Pb$_{x}$NiO$_{3}$ structure is confirmed to be 4+, and the behavior of the doped thin films is found to be controlled by a competition between carrier injection and structural distortions, which respectively reduce and increase the metal‐to‐insulator transition temperature. This work provides a systematic study of electron doping in NdNiO$_{3}$, demonstrating that A‐site substitution with Pb is an appropriate method for such doping in perovskite rare‐earth nickelate systems

Adidbot, un robot qui vous veut du bien. Conception d’un chatbot destiné à soutenir les apprentissages, dans le cadre d’un cours de formation à distance

L’objectif de ce mémoire est le développement d'un chatbot éducatif dans le contexte de l'enseignement hybride et à distance. Ce travail met en évidence les avantages potentiels des chatbots dans le domaine de l'éducation, tels que l'amélioration de l'expérience d'apprentissage et la personnalisation de l'assistance pédagogique. L'importance de l'analyse des besoins des enseignants et des apprenants, ainsi que de l'évaluation continue du chatbot, sont également mis en évidence dans cette recherche. Le mémoire décrit le processus de développement d'un chatbot éducatif, en mettant l'accent sur la définition des objectifs pédagogiques, la conception conversationnelle, la collecte et la structuration des données, l'intégration du contenu éducatif, ainsi que l'évaluation et l'amélioration de l'outil. Enfin, le document présente trois scénarios d'utilisation du chatbot dans le cadre d'un cours universitaire à distance. Ces scénarios impliquent l'interaction entre le chatbot et les étudiant·e·s, en recueillant, lors des entretiens effectués, des informations sur leurs attentes, leurs besoins, leurs émotions et en fournissant des réponses pré-définies en fonction du contexte de l'apprentissage. Les données obtenues et l’évaluation du chatbot ont notamment permis de mettre en évidence certaines insuffisances qui requièrent une attention particulière de la part du concepteur en vue d'améliorer son acceptabilité et d'encourager une adoption plus large par les futurs utilisateurs.</p

Paramagnetic Nd sublattice and thickness-dependent ferromagnetism in Nd2NiMnO6 double perovskite thin films

We investigate ferromagnetic and insulating thin films of the B-site ordered double perovskite Nd2NiMnO6 (NNMO) grown by radio frequency off-axis magnetron sputtering. The films grow epitaxially strained on a selection of substrates and display a strain-independent and bulklike TC of 200K at a thickness of 30 unit cells. We explore the thickness dependence of the NNMO/SrTiO3(001) system and find ferromagnetism down to ultralow thicknesses of only 3 unit cells (∼1.2nm). Below 10 unit cells, the magnetic properties deteriorate due to an interfacial charge transfer caused by the polar discontinuity at the NNMO/SrTiO3 interface. A detailed x-ray magnetic circular dichroism study allows us to separate the magnetic components into a robust ferromagnetic Ni/Mn sublattice and a paramagnetic Nd sublattice.This research was supported by the Swiss National Science Foundation under Projects No. PP00P2_170564 and No. 206021_150784. XAS and XMCD measurements were performed at ALBA BL29 BOREAS under proposal 2020024223-2 and at SLS X07MA XTreme under proposal 20210318.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Top‐Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in bulk materials but can also lead to degraded properties in ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control charge transfer will be beneficial. Here, an appropriate top-interface design allows for remote manipulation of the charge configuration of the buried interface and concurrent restoration of the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2–5 unit cell LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine-tune the electronic features of complex multilayered heterostructures

Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces.

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in the bulk material but can also lead to degraded properties in the ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control the charge transfer would be beneficial. Here, an appropriate top-interface design allows for remotely manipulating the charge configuration of the buried interface and concurrently restoring the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2 NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2-5 unit cells LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine tune the electronic features of complex multi-layered heterostructures. This article is protected by copyright. All rights reserved

Two-carrier Magnetoresistance: Applications to Ca3Ru2O7

Ambipolar transport is a commonly occurring theme in semimetals and semiconductors. Here we present an analytical formulation of the conductivity for a two-band system. Electron and hole carrier densities and their respective conductivities are mapped into a two-dimensional unit-less phase space. Provided that one of the carrier densities is known, the dimensionless phase space can be probed through magnetoresistance measurements. This formulation of the two-band model for conductivity is applied to magnetoresistance experiments on Ca3Ru2O7. While previous such measurements focused on the low-temperature limit, we cover a broad temperature range and find negative magnetoresistance in an intermediate interval below the electronic transition at 48 K. The low-temperature magnetoresistance in Ca3Ru2O7 is consistent with a two-band structure. However, the model fails to describe the full temperature and magnetic field dependence. Negative magnetoresistance found in an intermediate temperature range is, for example, not captured by this model. We thus conclude that the electronic and magnetic structure in this intermediate temperature range render the system beyond the most simple two-band model