Bulk electronic structure of single-crystal perovskite oxides studied by soft X-ray angle-resolved photoemission.

The transition-metal oxides (TMOs) are a material class host to a number of intriguing and potentially technologically useful phenomena as a result of many-body correlation effects, from superconductivity, magnetic and orbital ordering, to ferroelectricity and metal-insulator transitions. Here, materials with similar structures and seemingly equivalent electronic configuration often exhibit wildly different properties as a result of strong competition between different ground states from the many degrees of freedom, whose balance can be further tuned through the use of pressure, doping, magnetic fields or temperature. To investigate these materials, we make use of photoelectron spectroscopy (PES), probing elementary excitations possible in the material and thus providing linked information both about the ground state and possible excited states, closely related to the physical properties of a material such as its response to external fields. Angle-resolved PES (ARPES) provides additional momentum information and as a result, it is uniquely suited to investigate the character of the electronic structure of solids as it resolves the dispersion, meaningful in the independent-electron view where crystal momentum is a well-defined quantum number, but which can retain validity even in strongly correlated systems through the concept of quasiparticles. While ARPES is a well-established technique, it is rarely used in the soft X-ray regime (SX-ARPES) due to significant experimental challenges posed. However, the higher energies in SX-ARPES allow it to be significantly more bulk-sensitive, an extremely important fact since the properties of the bulk material and its surface are often extremely, or worse, subtly different. Critically, this permits measurements on single crystals of TMOs, whose surfaces may show roughness or reconstruction, for example as a result of a polar surface compensation, but whose bulk properties are well-defined in contrast to thin films which are additionally subject to substrate effects. We demonstrate on three rather different perovskite oxides, a three-dimensional class of TMOs, that is worthwhile to overcome these issues since it provides access to the true momentum-resolved bulk electronic properties of materials and allows filling noticeable gaps in literature of k-resolved electronic structure measurements for this class of compounds stemming from the impossibility of such measurements at lower energy. A commonality between the materials studied in this thesis is the absence of a strong electronic symmetry-breaking order, such as local-moment antiferromagnetism or charge ordering, that could suppress the existence of sufficiently long-lived quasiparticles to observe dispersion (or equivalently prevent a mobile photo-hole). We first establish that SX-ARPES is indeed capable and suited to measure the bulk-representative electronic structure by measurements on the perfect cubic d1 perovskite ReO3. We present the first k-resolved electronic structure for this material which is rather well explained by band structure, especially close to the Fermi level. In particular, we show and quantify the impact of the significant spin-orbit coupling on the Fermi surface and bands. However, the oxygen bands are less well reproduced by calculations and are correctable by use of hybrid functionals, taken as a sign of spurious self-interaction effects - likely due to the large extent and density differences between delocalised Re 5d and more localised O 2p. We also show that there are signs of some hitherto unknown distortion in ReO3. We then turn to LaNiO3, a metallic oxide in a family of formally d7 rare-earth nickelates which otherwise all undergo metal-insulator and antiferromagnetic (AFM) transitions as well as oxygen bond disproportionation, with a strong competition between these ground states and possible exotic resulting states in the phase diagram. We are able to resolve the dispersion of the eg quasiparticle spectrum along high symmetry cuts of this material as well as its Fermi surface, the latter of which is accurately reproduced by band theory calculations. We investigate the influence of the rhombohedral distortion present in the material through unfolding methods to better compare their influence to measurement, and show how significantly it affects the dispersion, confirming again the importance of single crystals. Its effects are shown to be similar to correlation-induced mass enhancement and their effects are untangled with the help of first DFT+U and later rhombohedral multi-band dynamical mean-field theory (DMFT) calculations. Local correlation effects prove to be the dominant influence on the spectrum, although certain k-dependent mismatches remain, pointing to a possible simultaneous importance of non-local mechanisms. Finally, on the d6 system LaCoO3 that is close to a spin-state transition, we show that this method can also be applied to insulating oxides. Absent a Fermi surface, we naturally concentrate more on the full valence band, where we show that the observed dispersion is well-described by mean-field band methods in the low-spin (LS) regime of LaCoO3 provided that static energy corrections of DFT+U are accounted for (which show a good match to local LS many-body configuration interaction calculations), thus providing k-resolved evidence that one may effectively consider LS LaCoO3 a band insulator, despite possibly strong correlations. We further unveil clear evidence of crystal periodicity doubling by observation of a backfolded oxygen band, and show evidence of a significant asymmetry in the k-resolved lineshape in the valence band and lastly we take a look at the spin state of Co at the surface, which, contrary to prior results, appears to be the same as in the bulk, but which we show to be complicated by significant orbital-shape matrix element effects

Recherche de nouvelles résonances a haute masse dans l'état final dilepton et calibration du démonstrateur pour l'upgrade Phase-1 du calorimètre électromagnétique dans l'expérience ATLAS

The Standard Model (SM) of particle physics was tested to a high precision at various experiments in the last decades and no significant deviation has been found so far. The LHC is a perfect machine to search for new phenomena, as its energy reach is considerably higher than that of previous accelerators. The so-called Run 2 of the LHC covers the data taking period between 2015 and 2018, during which proton-proton collisions at a centre-of-mass energy of 13 TeV were recorded. The full Run 2 integrated luminosity to be used by the ATLAS analyses is 139 fb⁻¹.Many theories beyond the Standard Model predict additional resonances that can decay leptonically. A search for dielectron and dimuon resonances in the 250 GeV to 6 TeV mass range, based on the full Run 2 dataset, is presented in this thesis. The background estimation in the analysis is performed by fitting a functional form to the data. A generic signal shape is chosen in order to facilitate reinterpretations of the fiducial cross-section limits. Lower resonance mass limits are set for benchmark models, and reach 4.5 TeV for the E6-motivated Z′ᵩ boson in the dilepton channel. This search is one of the flagship analyses of the ATLAS experiment.In addition to the dileptonic decay modes, heavy new resonances can decay into other final states such as pairs of SM W and Z bosons, VH pairs with V∈{W,Z} and H as the SM Higgs boson, or pairs of a lepton and a neutrino. Combining searches in various decay channels extends the discovery reach by exploiting their complementarity. The combination of the VV, VH, lv and dilepton final states with 36.1 fb⁻¹ of 2015-2016 data is detailed in this document.The increased instantaneous luminosity of the LHC in the next data-taking period (Run 3) requires an upgrade of the ATLAS liquid argon (LAr) calorimeter trigger electronics. The new electronics increase tenfold the granularity of the LAr information provided to the first level of the trigger system and will enable a more advanced selection at the hardware level of the trigger. To test pre-prototypes of the upgraded electronics, an in-situ demonstrator system was installed at the start of Run 2 to collect data with the new read-out in parallel to the current one. The energy and timing calibration of the demonstrator system is presented in this thesis. Further detailed studies, performed with data collected in 2017, include the comparison of the energy and timing reconstruction between the output of the upgraded system and the standard LAr read-out, event-level shower shape information and pulse shape predictions, among other measurements. A good agreement with the expectation is found, demonstrating the readiness for the Run 3 data-taking period.Le Modèle standard (MS) de la physique des particules a été testé avec une grande précision par diverses expériences au cours des dernières décennies et aucun écart significatif n'a été constaté jusqu'à présent. Le LHC est une machine parfaite pour la recherche de nouveaux phénomènes, car il permet d'atteindre des énergies considérablement plus élevées que celles des accélérateurs précédents. Le Run 2 du LHC couvre la période de collecte des données allant de 2015 à 2018, au cours de laquelle des collisions proton-proton d'une énergie au centre de la masse de 13 TeV ont été enregistrées. La luminosité intégrée totale utilisable par les analyses ATLAS pour le Run 2 est de 139 fb⁻¹.De nombreuses théories au-delà du Modèle standard prédisent des résonances supplémentaires qui peuvent se désintégrer leptoniquement. Cette thèse présente une recherche de résonances dans les états finaux diélectron et dimuon dans la fourchette de masse de 250 GeV à 6 TeV, basée sur l'ensemble de données Run 2. L'estimation du bruit de fond dans l'analyse est effectuée en ajustant une forme fonctionnelle aux données. Une forme de signal générique est choisie afin de faciliter la réinterprétation des limites sur la section efficace dans le volume fiduciel. Des limites inférieures pour la masse de nouvelles résonances sont mesurées pour des modèles de référence. Pour le boson Z′ᵩ motivé par la symétrie E6, la limite mesurée dans le canal dilepton est de 4.5 TeV. Cette recherche est une des analyses les plus importantes de l'expérience ATLAS.En plus des modes de désintégration dileptoniques, de nouvelles résonances à haute masse peuvent se désintégrer dans d'autres états finaux tels que des paires de bosons W et Z du MS, des paires VH avec V∈{W,Z} et le boson de Higgs H du MS, ou des paires lepton-neutrino. La combinaison de recherches dans différents canaux de désintégration permet d'étendre le potentiel de la découverte en exploitant leur complémentarité. La combinaison des états finaux VV, VH, lv et dilepton avec 36.1 fb⁻¹ de données de 2015-2016 est détaillée dans ce document.L'augmentation de la luminosité instantanée du LHC au cours de la prochaine période de prise de données (Run 3) nécessite une mise à niveau de l'électronique de déclenchement du calorimètre à argon liquide (LAr) d'ATLAS. La nouvelle électronique décuple la granularité de l'information LAr fournie au premier niveau du système de déclenchement et permettra une sélection plus fine au niveau hardware du déclenchement. Pour tester les pré-prototypes de l'électronique mise à niveau, un système de démonstration in-situ a été installé au début du Run 2, afin de collecter les données avec la nouvelle lecture en parallèle à l'actuelle. L'étalonnage de l'énergie et de l'information en temps du système démonstrateur sont présentés dans cette thèse. D'autres études détaillées, réalisées à partir de données recueillies en 2017, comprennent la comparaison de la reconstruction de l'énergie et de l'information en temps entre le système démonstrateur et le système LAr standard, l'information sur la forme des cascades au niveau de l'événement et la prédiction de la forme des impulsions d'ionisation, parmi d'autres mesures. Un bon accord avec les performances attendues est obtenu, démontrant l'état de préparation du système pour la prise de données du Run 3

Search for new heavy resonances in the dilepton final state and calibration of the LAr Phase-1 Upgrade demonstrator at the ATLAS experiment

The Standard Model (SM) of particle physics was tested to a high precision at various experiments in the last decades and no significant deviation has been found so far. The LHC is a perfect machine to search for new phenomena, as its energy reach is considerably higher than that of previous accelerators. The so-called Run 2 of the LHC covers the data taking period between 2015 and 2018, during which proton-proton collisions at a centre-of-mass energy of $13\,\mathrm{TeV}$ were recorded. The full Run 2 integrated luminosity to be used by the ATLAS analyses is $139\,\mathrm{fb}^{-1}$. Many theories beyond the Standard Model predict additional resonances that can decay leptonically. A search for dielectron and dimuon resonances in the $250\,\mathrm{GeV}$ to $6\,\mathrm{TeV}$ mass range, based on the full Run 2 dataset, is presented in this thesis. The background estimation in the analysis is performed by fitting a functional form to the data. A generic signal shape is chosen in order to facilitate reinterpretations of the fiducial cross-section limits. Lower resonance mass limits are set for benchmark models, and reach $4.5\,\mathrm{TeV}$ for the $\mathrm{E}_6$-motivated $Z'_\psi$ boson in the dilepton channel. This search is one of the flagship analyses of the ATLAS experiment. In addition to the dileptonic decay modes, heavy new resonances can decay into other final states such as pairs of SM $W$ and $Z$ bosons, $VH$ pairs with $V \in \lbrace W, Z \rbrace$ and $H$ as the SM Higgs boson, or pairs of a lepton and a neutrino. Combining searches in various decay channels extends the discovery reach by exploiting their complementarity. The combination of the $VV$, $VH$, $\ell\nu$ and dilepton final states with $36.1\,\mathrm{fb}^{-1}$ of $2015-2016$ data is detailed in this document. The increased instantaneous luminosity of the LHC in the next data-taking period (Run 3) requires an upgrade of the ATLAS liquid argon (LAr) calorimeter trigger electronics. The new electronics increase tenfold the granularity of the LAr information provided to the first level of the trigger system and will enable a more advanced selection at the hardware level of the trigger. To test pre-prototypes of the upgraded electronics, an in-situ demonstrator system was installed at the start of Run 2 to collect data with the new read-out in parallel to the current one. The energy and timing calibration of the demonstrator system is presented in this thesis. Further detailed studies, performed with data collected in 2017, include the comparison of the energy and timing reconstruction between the output of the upgraded system and the standard LAr read-out, event-level shower shape information and pulse shape predictions, among other measurements. A good agreement with the expectation is found, demonstrating the readiness for the Run 3 data-taking period

Recherche de nouvelles résonances a haute masse dans l'état final dilepton et calibration du démonstrateur pour l'upgrade Phase-1 du calorimètre électromagnétique dans l'expérience ATLAS

The Standard Model (SM) of particle physics was tested to a high precision at various experiments in the last decades and no significant deviation has been found so far. The LHC is a perfect machine to search for new phenomena, as its energy reach is considerably higher than that of previous accelerators. The so-called Run 2 of the LHC covers the data taking period between 2015 and 2018, during which proton-proton collisions at a centre-of-mass energy of 13 TeV were recorded. The full Run 2 integrated luminosity to be used by the ATLAS analyses is 139 fb⁻¹.Many theories beyond the Standard Model predict additional resonances that can decay leptonically. A search for dielectron and dimuon resonances in the 250 GeV to 6 TeV mass range, based on the full Run 2 dataset, is presented in this thesis. The background estimation in the analysis is performed by fitting a functional form to the data. A generic signal shape is chosen in order to facilitate reinterpretations of the fiducial cross-section limits. Lower resonance mass limits are set for benchmark models, and reach 4.5 TeV for the E6-motivated Z′ᵩ boson in the dilepton channel. This search is one of the flagship analyses of the ATLAS experiment.In addition to the dileptonic decay modes, heavy new resonances can decay into other final states such as pairs of SM W and Z bosons, VH pairs with V∈{W,Z} and H as the SM Higgs boson, or pairs of a lepton and a neutrino. Combining searches in various decay channels extends the discovery reach by exploiting their complementarity. The combination of the VV, VH, lv and dilepton final states with 36.1 fb⁻¹ of 2015-2016 data is detailed in this document.The increased instantaneous luminosity of the LHC in the next data-taking period (Run 3) requires an upgrade of the ATLAS liquid argon (LAr) calorimeter trigger electronics. The new electronics increase tenfold the granularity of the LAr information provided to the first level of the trigger system and will enable a more advanced selection at the hardware level of the trigger. To test pre-prototypes of the upgraded electronics, an in-situ demonstrator system was installed at the start of Run 2 to collect data with the new read-out in parallel to the current one. The energy and timing calibration of the demonstrator system is presented in this thesis. Further detailed studies, performed with data collected in 2017, include the comparison of the energy and timing reconstruction between the output of the upgraded system and the standard LAr read-out, event-level shower shape information and pulse shape predictions, among other measurements. A good agreement with the expectation is found, demonstrating the readiness for the Run 3 data-taking period.Le Modèle standard (MS) de la physique des particules a été testé avec une grande précision par diverses expériences au cours des dernières décennies et aucun écart significatif n'a été constaté jusqu'à présent. Le LHC est une machine parfaite pour la recherche de nouveaux phénomènes, car il permet d'atteindre des énergies considérablement plus élevées que celles des accélérateurs précédents. Le Run 2 du LHC couvre la période de collecte des données allant de 2015 à 2018, au cours de laquelle des collisions proton-proton d'une énergie au centre de la masse de 13 TeV ont été enregistrées. La luminosité intégrée totale utilisable par les analyses ATLAS pour le Run 2 est de 139 fb⁻¹.De nombreuses théories au-delà du Modèle standard prédisent des résonances supplémentaires qui peuvent se désintégrer leptoniquement. Cette thèse présente une recherche de résonances dans les états finaux diélectron et dimuon dans la fourchette de masse de 250 GeV à 6 TeV, basée sur l'ensemble de données Run 2. L'estimation du bruit de fond dans l'analyse est effectuée en ajustant une forme fonctionnelle aux données. Une forme de signal générique est choisie afin de faciliter la réinterprétation des limites sur la section efficace dans le volume fiduciel. Des limites inférieures pour la masse de nouvelles résonances sont mesurées pour des modèles de référence. Pour le boson Z′ᵩ motivé par la symétrie E6, la limite mesurée dans le canal dilepton est de 4.5 TeV. Cette recherche est une des analyses les plus importantes de l'expérience ATLAS.En plus des modes de désintégration dileptoniques, de nouvelles résonances à haute masse peuvent se désintégrer dans d'autres états finaux tels que des paires de bosons W et Z du MS, des paires VH avec V∈{W,Z} et le boson de Higgs H du MS, ou des paires lepton-neutrino. La combinaison de recherches dans différents canaux de désintégration permet d'étendre le potentiel de la découverte en exploitant leur complémentarité. La combinaison des états finaux VV, VH, lv et dilepton avec 36.1 fb⁻¹ de données de 2015-2016 est détaillée dans ce document.L'augmentation de la luminosité instantanée du LHC au cours de la prochaine période de prise de données (Run 3) nécessite une mise à niveau de l'électronique de déclenchement du calorimètre à argon liquide (LAr) d'ATLAS. La nouvelle électronique décuple la granularité de l'information LAr fournie au premier niveau du système de déclenchement et permettra une sélection plus fine au niveau hardware du déclenchement. Pour tester les pré-prototypes de l'électronique mise à niveau, un système de démonstration in-situ a été installé au début du Run 2, afin de collecter les données avec la nouvelle lecture en parallèle à l'actuelle. L'étalonnage de l'énergie et de l'information en temps du système démonstrateur sont présentés dans cette thèse. D'autres études détaillées, réalisées à partir de données recueillies en 2017, comprennent la comparaison de la reconstruction de l'énergie et de l'information en temps entre le système démonstrateur et le système LAr standard, l'information sur la forme des cascades au niveau de l'événement et la prédiction de la forme des impulsions d'ionisation, parmi d'autres mesures. Un bon accord avec les performances attendues est obtenu, démontrant l'état de préparation du système pour la prise de données du Run 3