108 research outputs found
: a complete model for the chain sub-system
A second neighbor model for the chain subsystem of the
has been extracted from ab-initio calculations. This
model does not use periodic approximation but describes the entire chain
through the use of the four-dimensional crystallographic description. Second
neighbors interactions are found to be of same order than the first neighbors
ones. The computed values of the second neighbors magnetic interaction are
coherent with experimental estimations of the intra-dimer magnetic
interactions, even if slightly smaller. The reasons of this underestimation are
detailed. The computed model allowed us to understand the origin of the chain
dimerisation and predicts correctly the relative occurrence of dimers and free
spins. The orbitals respectively supporting the magnetic electrons and the
holes have been found to be essentially supported by the copper 3d orbitals
(spins) and the surrounding oxygen orbitals (holes), thus giving a strong
footing to the existence of Zhang-Rice singlets
An ab initio study of magneto-electric coupling of
The present paper proposes the direct calculation of the microscopic
contributions to the magneto-electric coupling, using ab initio methods. The
electrostrictive and the Dzyaloshinskii-Moriya contributions were evaluated
individually. For this purpose a specific method was designed, combining DFT
calculations and embedded fragments, explicitely correlated, quantum chemical
calculations. This method allowed us to calculate the evolution of the magnetic
couplings as a function of an applied electric field. We found that in the Dzyaloshinskii-Moriya contribution to the magneto-electric effect
is three orders of magnitude weaker than the electrostrictive contribution.
Strictive effects are thus dominant in the magnetic exchange evolution under an
applied electric field, and by extension on the magneto-electric effect. These
effects remain however quite small and the modifications of the magnetic
excitations under an applied electric field will be difficult to observe
experimentally. Another important conclusion is that the amplitude of the
magneto-electric effect is very small. Indeed, it can be shown that the linear
magneto-electric tensor is null due to the inter-layer symmetry operations.Comment: J. Phys. Cond. Matter 201
A 2D quantum antiferromagnet with a four-fold degenerate valence-bond-solid ground state
We study the competition between antiferromagnetic order and valence bond
solid formation in a two-dimensional frustrated spin-1/2 model. The J1-J2 model
on the square lattice is further frustrated by introducing products of
three-spin projectors which stabilize four dimer-product states as degenerate
ground states. These four states are reminiscent of the dimerized singlet
ground state of the Shastry-Sutherland model. Using exact diagonalizations, we
study the evolution of the ground state by varying theratio of interactions.
For a large range of parameters (J2 > 0.25J1), our model shows a direct
transition between the valence-bond-solid phase and the collinear
antiferromagnetic phase. For small values of J2, several intermediate phases
appear which are also analyzed
Charge order in the incommensurate compounds
The present paper studies, using ab-initio calculations, the influence of the
incommensurate structural modulations on the low energy physics of the
oxides. On-site, nearest-neighbor and
next-nearest-neighbor effective parameters were computed within a model
based on the copper magnetic orbitals. The structural modulations appear to be
the key degree of freedom, responsible for the low energy properties such as
the electron localization, the formation of dimers in the compound or the
anti-ferromagnetic order in the compound
Electronic structure of the incommensurate compound
We extracted, from strongly-correlated ab-initio calculations, a complete
model for the chain subsystem of the
incommensurate compound. A second neighbor model has been determined as
a function of the fourth crystallographic parameter , for both low and
room temperature crystallographic structures. The analysis of the obtained
model shows the crucial importance of the structural modulations on the
electronic structure through the on-site energies and the magnetic
interactions. The structural distortions are characterized by their long range
effect on the cited parameters that hinder the reliability of analyses such as
BVS. One of the most striking results is the existence of antiferromagnetic
nearest-neighbor interactions for metal-ligand-metal angles of . A
detailed analysis of the electron localization and spin arrangement is
presented as a function of the chain to ladder hole transfer and of the
temperature. The obtained spin arrangement is in agreement with
antiferromagnetic correlations in the chain direction at low temperature
Entropy Driven Atomic Motion in Laser-Excited Bismuth
We introduce a thermodynamical model based on the two-temperature approach in order to fully understand the dynamics of the coherent A(1g) phonon in laser-excited bismuth. Using this model, we simulate the time evolution of (111) Bragg peak intensities measured by Fritz et al. [Science 315, 633 (2007)] in femtosecond x-ray diffraction experiments performed on a bismuth film for different laser fluences. The agreement between theoretical and experimental results is striking not only because we use fluences very close to the experimental ones but also because most of the model parameters are obtained from ab initio calculations performed for different electron temperatures
The role of dynamical polarization of the ligand to metal charge transfer excitations in {\em ab initio} determination of effective exchange parameters
The role of the bridging ligand on the effective Heisenberg coupling
parameters is analyzed in detail. This analysis strongly suggests that the
ligand-to-metal charge transfer excitations are responsible for a large part of
the final value of the magnetic coupling constant. This permits to suggest a
new variant of the Difference Dedicated Configuration Interaction (DDCI)
method, presently one of the most accurate and reliable for the evaluation of
magnetic effective interactions. This new method treats the bridging ligand
orbitals mediating the interaction at the same level than the magnetic orbitals
and preserves the high quality of the DDCI results while being much less
computationally demanding. The numerical accuracy of the new approach is
illustrated on various systems with one or two magnetic electrons per magnetic
center. The fact that accurate results can be obtained using a rather reduced
configuration interaction space opens the possibility to study more complex
systems with many magnetic centers and/or many electrons per center.Comment: 7 pages, 4 figure
Spin orbitronics using alloy materials with tailored spin and charge dependent properties
L’objectif de la thèse est de développer un système de type jonction tunnel tout oxyde à base de La2/3Sr1/3MnO3 (LSMO) où il serait possible de contrôler l’aimantation des électrodes magnétiques par des processus à faible consommation d’énergie. Des jonctions tunnel épitaxiées de LSMO/SrTiO3/LSMO ont été obtenues montrant un double renversement de l’aimantation à température ambiante et un taux de magnétorésistance de 71 % à 10 K. En exerçant une contrainte sur le LSMO par le substrat il a été possible de moduler l’anisotropie des couches magnétiques. Des anisotropies perpendiculaire ou dans le plan ont pu être obtenues. Afin de contrôler le renversement des moments magnétiques dans une des électrodes ferromagnétiques trois options ont été envisagées : l’utilisation de l’injection de spin à partir d’un métal à fort couplage spin-orbite (Pt, Ir) ou d’un oxyde contenant de tels ions (ici Ru dans SrRuO3), et l’utilisation du Bi2FeCrO6, un oxyde multiféroïque pouvant présenter un couplage magnétoélectrique. Malgré des résultats prometteurs, aucune solution n’a permis des tests sur des jonctions afin d’estimer leur efficacité. L’objectif final n’est pas encore atteint mais des avancées intéressantes ont été faites afin d’envisager des dispositifs permettant le stockage et la manipulation de l’information.The objective of this work is to develop La2/3Sr1/3MnO3 (LSMO) based all-oxide magnetic tunnel junction systems where it would be possible to control the magnetization of magnetic electrodes by low energy consumption processes. Epitaxial tunnel junctions of LSMO/SrTiO3/LSMO were obtained showing a double magnetization switching at room temperature and a magnetoresistance ratio of 71 % at 10 K. Using strain engineering, it was possible to modulate the anisotropy of the LSMO magnetic layers. Perpendicular or in plane anisotropies could be thus obtained. In order to control the reversal of the magnetic moments in one of the ferromagnetic electrodes three options were considered : the use of spin injection from a metal with a strong spin-orbit coupling (Pt, Ir) or an oxide containing this type of ions (here Ru in SrRuO3), and the use of Bi2FeCrO6 multiferroic oxide that may exhibit a magnetoelectric coupling. Despite promising results, no solution has allowed tests on junctions to be carried out to estimate their effectiveness. Although the final objective is not yet achieved, interesting progress has been made on the way to information storage and manipulation devices
Caractérisation du Trouble de l'Usage de l'Alcool : Une approche transculturelle bio-psycho-sociale en France, au Bénin et au Togo
Alcohol has detrimental effects on health, leading to significant morbidity as well as economic and social consequences. Disorders related to its consumption rank among the leading causes of disability worldwide. However, epidemiological data on Alcohol Use Disorder (AUD) in sub-Saharan Africa remains scarce. This thesis aims to characterize AUD in France, Benin, and Togo to optimize prevention and treatment strategies. Initially, a systematic review found that only 16 studies have been conducted in the general African population, none of which used the DSM-5 criteria. The prevalence in this region ranges from 0.1% to 33.2%, highlighting a major public health issue. Subsequently, a survey using DSM-5 criteria was conducted in the general populations of Togo and Benin. In Togo, the AUD prevalence was 12.4% (CI 95%: 9.5-15.7) and 8.7% in Benin (CI 95%: 6.4-11.7). The primary comorbidities in Togo are suicidal risk, depression, gambling addiction, and smoking, with male gender and certain scores as risk factors. In Benin, the main disorders are suicidal risk, smoking, and psychotic syndrome. Lastly, a survey was carried out in hospitals in France, Togo, and Benin. In France, the majority of AUD patients are 50-year-old men, with 82% being smokers. In Benin, they are mostly young Christian men living in urban areas. In Togo, they are men in their forties working in cities. Additionally, a study was conducted to explore variations in BDNF and Pro-BDNF in patients suffering from major depressive episodes and AUD.L'alcool a des effets néfastes sur la santé, engendrant une significative morbidité ainsi que des conséquences économiques et sociales. Les troubles de sa consommation comptent parmi les principales causes d'incapacité à travers le monde. Cependant, les données épidémiologiques concernant le Trouble de l’Usage de l’Alcool (TUA) en Afrique subsaharienne sont encore insuffisantes. Cette thèse cherche à caractériser le TUA en France, au Bénin, et au Togo pour optimiser les stratégies de prévention et de traitement. D'abord, une revue systématique a révélé que seulement 16 études ont été menées auprès de la population générale africaine, sans que l'une d'elles n'utilise les critères DSM-5. La prévalence dans cette région fluctue entre 0,1% et 33,2%, soulignant un majeur problème de santé publique. Ensuite, une enquête en population générale au Togo et au Bénin utilisant les critères DSM-5 a été effectuée. Au Togo, la prévalence du TUA était de 12,4% (IC 95% : 9,5-15,7) et de 8,7% au Bénin (IC 95% : 6,4-11,7). Les comorbidités principales au Togo sont le risque suicidaire, la dépression, l'addiction aux jeux et le tabagisme, avec le genre masculin et certains scores comme facteurs de risque. Au Bénin, les troubles principaux sont le risque suicidaire, le tabagisme, et le syndrome psychotique. Enfin, une enquête en milieu hospitalier en France, Togo, et Bénin a été menée. En France, la majorité des patients TUA sont des hommes de 50 ans, 82% étant fumeurs. Au Bénin, il s'agit surtout de jeunes hommes chrétiens vivant en ville. Au Togo, ce sont des hommes d'une quarantaine d'années, travaillant en ville. Par ailleurs, une étude a été menée pour explorer les variations de BDNF et Pro-BDNF chez les patients souffrant d’épisode dépressif caractérisé et de TUA
Hétérostructures épitaxiées avec des propriétés dépendantes de spin et de charges pour des applications en spintronique
The objective of this work is to develop La2/3Sr1/3MnO3 (LSMO) based all-oxide magnetic tunnel junction systems where it would be possible to control the magnetization of magnetic electrodes by low energy consumption processes. Epitaxial tunnel junctions of LSMO/SrTiO3/LSMO were obtained showing a double magnetization switching at room temperature and a magnetoresistance ratio of 71 % at 10 K. Using strain engineering, it was possible to modulate the anisotropy of the LSMO magnetic layers. Perpendicular or in plane anisotropies could be thus obtained. In order to control the reversal of the magnetic moments in one of the ferromagnetic electrodes three options were considered : the use of spin injection from a metal with a strong spin-orbit coupling (Pt, Ir) or an oxide containing this type of ions (here Ru in SrRuO3), and the use of Bi2FeCrO6 multiferroic oxide that may exhibit a magnetoelectric coupling. Despite promising results, no solution has allowed tests on junctions to be carried out to estimate their effectiveness. Although the final objective is not yet achieved, interesting progress has been made on the way to information storage and manipulation devices.L’objectif de la thèse est de développer un système de type jonction tunnel tout oxyde à base de La2/3Sr1/3MnO3 (LSMO) où il serait possible de contrôler l’aimantation des électrodes magnétiques par des processus à faible consommation d’énergie. Des jonctions tunnel épitaxiées de LSMO/SrTiO3/LSMO ont été obtenues montrant un double renversement de l’aimantation à température ambiante et un taux de magnétorésistance de 71 % à 10 K. En exerçant une contrainte sur le LSMO par le substrat il a été possible de moduler l’anisotropie des couches magnétiques. Des anisotropies perpendiculaire ou dans le plan ont pu être obtenues. Afin de contrôler le renversement des moments magnétiques dans une des électrodes ferromagnétiques trois options ont été envisagées : l’utilisation de l’injection de spin à partir d’un métal à fort couplage spin-orbite (Pt, Ir) ou d’un oxyde contenant de tels ions (ici Ru dans SrRuO3), et l’utilisation du Bi2FeCrO6, un oxyde multiféroïque pouvant présenter un couplage magnétoélectrique. Malgré des résultats prometteurs, aucune solution n’a permis des tests sur des jonctions afin d’estimer leur efficacité. L’objectif final n’est pas encore atteint mais des avancées intéressantes ont été faites afin d’envisager des dispositifs permettant le stockage et la manipulation de l’information
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