No description available.

Dans cette thèse, j'ai étudié l'évolution de la surface de Fermi sous l'influence d'un champ magnétique dans des systèmes massifs facilement polarisables à basse température. La première partie est dévouée aux cas du supraconducteur ferromagnétique UCoGe et du supraconducteur paramagnétique CeIrIn5, où la surface de Fermi peut être modifiée sans transition magnétique. Dans UCoGe, plusieurs anomalies successives ont été détectées dans l'effet Seebeck, la résistivité et l'effet Hall, sans transition nette dans l'aimantation. L'observation d'oscillations quantiques montre que ces anomalies sont reliées à des changements de topologie de la surface de Fermi, aussi appelés transitions de Lifshitz. Dans CeIrIn5, une anomalie est détectée dans l'effet Seebeck à HM = 28 T et les oscillations quantiques observées en magnétométrie torque montrent qu'une transition de Lifshitz à lieu à ce champ.Dans la deuxième partie, j'ai étudié comment varie la surface de Fermi à travers une transition magnétique du premier ordre induite par le champ magnétique dans le supraconducteur ferromagnétique URhGe avec le champ selon l'axe de difficile aimantation b et le supraconducteur antiferromagnétique UPd2Al3 avec le champ dans le plan basal. Dans URhGe, l'effet Seebeck permet d'observer un changement de la surface de Fermi à la transition de réorientation des spins à HR = 11.75 T et avec la résistivité confirme le caractère premier ordre de la transition en plus de fournir la localisation dans le diagramme de phase du point tricritique. Dans UPd2Al3, une nouvelle branche de la surface de Fermi est observée dans les oscillations quantiques de de Haas-van Alphen dans l'état antiferromagnétique et l'effet Seebeck montre que la surface de Fermi change à la transition métamagnétique à HM = 18 T. En outre, quatre nouvelles branches sont observées dans la phase polarisée au delà de HM et qui ne peuvent être associées à celles calculées dans les états paramagnétique et antiferromagnétique. / In this thesis, we have studied the evolution of the Fermi surface under the influence of a magnetic field in bulk materials that can be easily polarized at low temperature. The first part was devoted to the cases of the ferromagnetic superconductor UCoGe with a magnetic field applied along the easy magnetization c-axis and the paramagnetic superconductor CeIrIn5 with the field along the c-axis. In UCoGe, several successive anomalies were detected in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition being detected in magnetization. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In CeIrIn5, the thermoelectric power detected an anomaly at HM = 28 T and the quantum oscillations observed in torque magnetometry showed that a Lifshitz transition occurs at this field.In the second part of this thesis, we studied the evolution of the Fermi surface through first order magnetic transitions induced by magnetic field. In the ferromagnetic superconductor URhGe with the field applied along the hard magnetization b-axis and the antiferromagnetic superconductor UPd2Al3 with the field in the basal plane. In URhGe, the thermoelectric power allowed to observe a change in the Fermi surface at the spin reorientation transition at HR = 11.75 T defining the ferromagnetic state and along with resistivity confirmed the first order character of the transition as well as give a location of the tricritical point. In UPd2Al3, a new branch was observed in de Haas-van Alphen experiment in the antiferromagnetic phase and the thermoelectric power showed that the Fermi surface is reconstructed at the metamagnetic transition at HM = 18 T where the antiferromagnetic state is suppressed and could suggest that the Fermi surface changes before this transition. Additionally, four new branches were observed in the polarized paramagnetic phase, above HM, that cannot be associated with calculated branches in the paramagnetic of antiferromagnetic states.

Dans cette thèse j’étudie l’impact de la géométrie de micro et nano-domaines biologiques sur les propriétés d'électrodiffusion, ceci à l'aide des équations aux dérivées partielles de Poisson-Nernst-Planck. Je considère des domaines non-triviaux ayant une forme cuspide ou elliptique. Mon objectif est de développer des modèles ainsi que des méthodes mathématiques afin d'étudier les caractéristiques électriques de ces nano/micro-domaines, et ainsi mieux comprendre comment les signaux électriques sont modulés à ces échelles. Dans la première partie j’étudie le voltage à l'équilibre pour un électrolyte dans un domaine borné, et ayant un fort excès de charges positives. Je montre que le premier temps de sortie dans une boule chargée dépend de la surface et non du volume. J’étudie ensuite la géométrie composées d'une boule à laquelle est attachée un domaine cuspide. Je construis ensuite une solution asymptotique pour le voltage dans les cas 2D et 3D et je montre qu’ils sont donnés au premier ordre par la même expression. Enfin, j’obtiens la même conclusion en considérant une géométrie formée d'une ellipse, dont je construis une solution asymptotique du voltage en 2D et 3D. La seconde partie porte sur la modélisation de la compartimentalisation électrique des épines dendritiques. A partir de simulations numériques, je mets en évidence le lien entre la polarisation de concentration dans l'épine et sa géométrie. Je compare ensuite mon modèle à des données de microscopie. Je développe une méthode de déconvolution pour extraire la dynamique rapide du voltage à partir des données de microscopie. Enfin j’estime la résistance du cou et montre que celle-ci ne suit pas la loi d'Ohm. / In this PhD I study how electro-diffusion within biological micro and nano-domains is affected by their shapes using the Poisson-Nernst-Planck (PNP) partial differential equations. I consider non-trivial shapes such as domains with cusp and ellipses. Our goal is to develop models, as well as mathematical tools, to study the electrical properties of micro and nano-domains, to understand better how electrical neuronal signaling is regulated at those scales. In the first part I estimate the steady-state voltage inside an electrolyte confined in a bounded domain, within which we assume an excess of positive charge. I show the mean first passage time in a charged ball depends on the surface and not on the volume. I further study a geometry composed of a ball with an attached cusp-shaped domain. I construct an asymptotic solution for the voltage in 2D and 3D and I show that to leading order expressions for the voltage in 2D and 3D are identical. Finally, I obtain similar conclusion considering an elliptical-shaped domain for which I construct an asymptotic solution for the voltage in 2D and 3D. In the second part, I model the electrical compartmentalization in dendritic spines. Based on numerical simulations, I show how spines non-cylindrical geometry leads to concentration polarization effects. I then compare my model to experimental data of microscopy imaging. I develop a deconvolution method to recover the fast voltage dynamic from the data. I estimate the neck resistance, and we found that, contrary to Ohm's law, the spine neck resistance can be inversely proportional to its radius.

Cette thèse porte sous l'étude sous conditions extrêmes (basse température, fort champ magnétique et haute pression) des composés fortement corrélés du type fermions lourds. Trois composés ont été analysés UCoAl, UGe$_2$ et CeRh$_2$Si$_2$, en utilisant principalement de mesures thermoélectriques; une technique récente et très sensible dans le domaine des fermions lourds. À cette fin, de nouvelles dispositives de mesures de pouvoir thermoélectricité sous pression ont été développés au cours de cette thèse.Concernant le composé d'UCoAl, notre étude a permis d'analyser précisément la transition metamagnétique, induite par le champ magnétique, entre la phase paramagnétique (PM) et la phase ferromagnétique (FM) ainsi que, son évolution sous pression. Ainsi, nos mesures ont permis de compléter le diagramme de phase $(T,P,H)$ et notamment, de mettre en évidence la structure magnétique originale qui apparaît sous pression en forme de ```wings" pas des mesures thermoélectriques.Une fine analyse de la surface de Fermi de la phase FM$2$ d'UGe$_2$ a été réalisée grace à l'observation des oscillations quantiques du pouvoir thermoélectrique. Les résultats obtenues ont été comparés aux études conventionnelles des oscillations quantiques comme de ``de Haas-van Alphen" (dHvA) et de ``Suhbnikov-de Hass" (SdH) effets. Une très bonne accord entre les trois techniques a été constatée. % et montre les avantages d'utilisation des mesures du pouvoir thermoélectrique pour analyser les paramètres microscopiques des fermions lourds.% Les inconvénients de cette technique sont aussi présentés.Finalement, dans le système CeRh$_2$Si$_2$, la suppression du domaine antiferromagnétique (AF) sous champ magnétique $H_c sim 26$T et sous pression $Psim 1$GPa a été étudiée. Un très fort changement de la surface de Fermi à $H_c$ correspondant à la transition de l'ordre AF vers une phase paramagnétique polarisée (PPM), a été observé. Sous pression, des fluctuations magnétiques et une reconstruction de la surface de Fermi apparaissent autour de $P_c$. Ces fluctuations cachent la nature de la suppression de l'ordre AF vers un ordre paramagnétique (PM). L'étude du diagramme de phase $(T,H,P)$ révèle que les phases PM et PPM sont différentes, cependant des points en commun demeurent. / Thermopower is a technique whose importance is related to the possibility of directly measuring electronic properties of the systems, as it is sensitive to the derivative of the density of states. In this work, the low temperature regime of strongly correlated electron systems has been studied using this technique. For that, a new pressure-field thermopower device was developed, and used, to determine $(T,P,H)$ phase diagrams of the itinerant ferromagnets UCoAl and UGe$_2$, and of the weak antiferromagnet CeRh$_2$Si$_2$.For example, in the case of UCoAl, this same technique was used to analyze the metamagnetic transition from paramagnetic (PM) to ferromagnetic (FM) phases and to study its evolution towards the quantum critical end point. The existence of exotic magnetic excitations in the ground state and around the critical end point were also evidenced.On the compound CeRh$_2$Si$_2$, the suppression of the antiferromagnetic (AF) order by magnetic fields and pressures was explored. A strong change of the Fermi surface at $H_c$, the field at which the suppression of the AF into the paramagnetic polarized (PPM) phase, was observed. We show that under pressure, the magnetic fluctuations around the critical pressure $P_c$ masked the Fermi surface reconstruction of the AF phase into the PM phase. The analysis of the $(T,P,H)$ phase diagram revealed that the non-ordered phases of this compound (PM and PPM) are different, therefore pressure and field behave as different suppressor mechanisms.In the UGe$_2$ compound, the analysis of its Fermi surface by thermopower quantum oscillations was performed as a last example of the utility and of the importance of this technique. To the best of the author knowledge, this is the first time that this technique was used in heavy fermion systems. A comparison to traditional probes such as de Haas-van Alphen and Shubnikov–de Haas effects was done. We observed a good agreement between them and we explain the advantages and the disadvantages of thermopower quantum oscillations technique over the traditional probes.

Les mécanismes produisant les signaux observés par la méthode de Polarisation Provoquée (PP) sur une large gamme de fréquences (entre 1 mHz et 20 kHz) ne sont pas encore complètement identifiés. Deux sujets particuliers ont été abordés dans cette thèse. L'origine du signal observé à haute fréquence (>1 kHz) a été analysé en effectuant des mesures de la résistivité complexe sur de l'eau à différentes salinités. Les résultats montrent des écarts sur la phase par rapport à la réponse attendue à haute fréquence. Ils dépendent du type d'électrode de mesures et de la conductivité du milieu. Un modèle basé sur un circuit électrique équivalent a été proposé pour modéliser ces effets. Nous avons aussi exploré le mécanisme responsable de la polarisation en présence de grains semi-conducteurs en analysant la dépendance de temps de relaxation. La réponse spectrale d'un milieu sableux saturé a été étudiée en variant la concentration et le type de l'électrolyte, la taille, le type et la quantité de grains semi-conducteurs insérés. En utilisant la méthode des éléments finis, nous avons entrepris une simulation numérique 2D basée sur la solution des équations de Poisson-Nernst-Planck dans le domaine temporel et spectral. Les résultats expérimentaux sont conformes à ceux issus de la simulation numérique et montrent une décroissance comparable du temps de relaxation avec l'augmentation de la concentration de l'électrolyte. Finalement, une campagne géophysique de terrain sur un site paléo-miniers contenant des grains semi-conducteurs complète l'approche de laboratoire. Des mesures de PP temporelles permettent de délimiter les zones de scories sur le site et d'en estimer le volume. / The physical mechanisms responsible for the induced polarization response over the frequency range (from 1 mHz to 20 kHz) are not completely understood. In particular, within the framework of this thesis, two subjects have been addressed. The origin of the signal observed at high frequency (HF) (>1 kHz) was analyzed by carrying out Spectral IP measurements on tap water samples. A phase deviation from the expected response has been observed at HF. The resulted deviation in phase appears to be dependent on the measuring electrode type (potential electrodes) and conductivity of the medium. A model based on an equivalent electrical circuit and designed to represent HF response, has been proposed to correct these effects. The mechanism responsible for the polarization in a medium containing semi-conductor grains has been investigated by analyzing the dependence of the relaxation time. We carried out experimental measurements on a sand medium containing different types of semi-conductors. The spectral response was studied by varying the concentration and type of the electrolyte, the size and content of semi-conductor grains. By using the finite element method, a 2D numerical simulation based on Poisson-Nernst-Planck equations was performed in time and frequency domains. The experimental results are qualitatively in accordance with numerical simulation. It showed a comparable decrease in the relaxation time when increasing the electrolyte concentration. Finally, field measurements on a paleo-mining site containing semi-conductor grains have been acquired. Time-domain IP measurements allowed us to define the zones of slag in the site and led to estimate the slag volume.

It was the purpose of this thesis to evaluate two-dimensional (2D) magneto-thermoelectric (MTE) phenomena in thinner regime. Mostly this work was motivated by the recent discovery of MTE properties in transition metal dichalcogenides (TMD). In general, TMD thin films have attracted much attention due to their very good electrical, optical, and electrochemical properties. However the total amount of studies of the MTE in TMD is rather small compared to the other properties, such as electric, opto-electric, and catalyst. Hence, in this thesis, we aimed to evaluate the MTE properties in TMD materials. Before we started to measure TMDs, we established a measurement platform and studied MTE properties in ferromagnetic CoFeB, and Weyl semimetal Co2MnGa.:1. Introduction a. Physical background i. Seebeck Effect ii. Anomalous Hall Effect and Anomalous Nernst Effect iii. Mott relation 2. Sample Preparation and evaluation a. Physical vapor deposition b. Mechanical Exfoliation c. Patterning Process 3. Data Evaluation 4. State of the art in Transition Metal Dichalcogenids a. Introduction b. TMD in use c. Magneto-thermoelectric properties in TMD 5. Magneto-thermoelectrical properties in CoFeB thin film a. Introduction b. Results and Discussion c. Conclusion 6. Anomalous Nernst and Anomalous Hall effect in Co2MnGa thin film a. Introduction b. Results and Discussion c. Summary 7. Anomalous Hall effect in exfoliated VS2 flake a. Introduction b. Experiment c. Results and Discussion d. Summary 8. Summary Acknowledgement and References

Magnetoresistive effects are powerful tools for studying the intricate structure of solid state electronic systems, and have many applications in our current information technology. In particular, the electronic system reflects the crystal symmetry and the orbital structure of the atoms of a given solid, and thus is crucial to understanding magnetism, superconductivity and many other effects which are of key interest to current solid state research. Consequently, studies of the electrical transport properties of solid state matter allow to evaluate this imprint and in turn draw conclusions about the interactions within a material. In this thesis, we will exploit the capabilities of magnetotransport measurements to infer the properties of a multitude of magnetic systems. In turn, this allows us to push the understanding of transport phenomena in magnetic materials. The first part of this work is focused on the magnetoresistance observed in spin Hall active metals in contact with a magnetic insulator. In such bilayers, the interfacial spin accumulation caused by the spin Hall effect in the metal can interact with the magnetic insulator, giving rise to interesting magnetoresistive effects. In the framework of this thesis, bilayers with several magnetic insulators are studied, including antiferomagnets, ferrimagnets and paramagnets (disordered magnets). For the disordered magnetic insulators, we find that the established spin Hall magnetoresistance framework does not allow to consistently describe the observed transport response. Consequently, we propose an alternative explanation of the magnetoresistance in such heterostructures, using the Hanle magnetoresistance and assuming an interface which has a finite electrical conductivity. This alternative model can serve to generalize the theory of the spin Hall magnetoresistance, providing addition information on the microscopic picture for the loss of the transverse spin component. Additionally, by partly removing the magnetic insulator and studying the ensuing changes, we verify that magnons are crucial for the observation of a non-local magnetoresistance in bilayers of a magnetic insulator and a metal. Finally, the local and non-local spin Seebeck effect (i.e. the electric field generated by a thermally driven pure spin current) is investigated in bilayers of Cr2O3 and Pt where the occurrence of a spin superfluid ground state was reported. In our sample, however, the transport response is consistent with the antiferromagnetic spin Seebeck effect mediated by the small magnetic field induced magnetization also reported for other antiferromagnet/metal heterostructures. As such, we cannot verify the presence of a spin superfluid ground state in the system. In the second part of this thesis, the topological properties of the electronic system and the related changes of the magnetoelectric and magnetothermal transport response are investigated. To that end, we first demonstrate a novel measurement technique, the alternating thermal gradient technique, allowing to separate the relevant thermovoltages from spurious other voltages generated within the measurement setup. We employ this novel technique for measuring the topological Nernst effect in Mn 1.8 PtSn and show the possibility to combine the magnetoelectric and magnetothermal transport response to evaluate the presence of topological transport signatures without requiring magnetization measurements. Additionally, we show that the anomalous Nernst effect in the non-collinear antiferromagnet Mn3Sn is connected to the antiferromagnetic domain structure: Using spatially resolved measurements of the anomalous Nernst effect, direct access to the antiferromagnetic domain structure is demonstrated. Additionally, a thermally assisted domain writing scheme is implemented, allowing the preparation of Mn3Sn into a defined antiferromagnetic domain state.

Estimation of effective diffusion coefficients is essential to be able to describe the diffusive transport of solutes in porous media. It has been shown in theory that in the case of uncharged porous materials the effective diffusion coefficient of solutes is a function of the pore morphology of the material and can be described by their tortuosity (tensor). To estimate the apparent diffusion coefficients, the values of tortuosity and porosity should be known first. In contrast with calculation of porosity, which can be easily obtained, estimation of tortuosity is intricate, particularly with increasing micro-geometry complexity in porous media. Moreover, many engineering materials (e.g, clays and shales) are characterized by electrical surface charges on particles of the porous material which can strongly affect the diffusive transport properties of ions. For these materials, estimation of effective diffusion coefficients have been mostly based on phenomenological equations with no link to underlying microscale properties of these charged materials although a few recent studies have used alternative methods to obtain the diffusion parameters. / In the first part of this thesis a numerical method based on a recently proposed up-scaled Poisson-Nernst-Planck type of equation (PNP) and its microscale counterpart is employed to estimate the tortuosity and thus the effective and apparent diffusion coefficients in thin charged membranes. Beside this, a new mathematical approach for estimation of tortuosity is applied and validated. This mathematical approach is also derived while upscaling of micro-scale Poisson-Nernst-Planck system of equations using the volume averaging method. A variety of different pore 2D and 3D micro-geometries together with different electrochemical conditions are studied here. To validate the new approaches, the relation between porosity and tortuosity has been obtained using a multi-scale approach and compared with published results. These include comparison with the results from a recently developed numerical method that is based on macro and micro-scale PNP equations. / Results confirm that the tortuosity value is the same for porous media with electrically uncharged and charged particles but only when using a consistent set of PNP equations. The effects of charged particles are captured by the ratio of average concentration to effective intrinsic concentration in the macroscopic PNP equations. Using this ratio allows to consistently take into account electro-chemical interactions of ions and charges on particles and so excludes any ambiguity generally encountered in phenomenological equations. / Steady-state diffusion studies dominate this thesis; however, understanding of transient ion transport in porous media is also important. The last section of this thesis briefly introduces transient diffusion through bentonite. To do so, the micro Nernst-Planck equation with electro-neutrality condition (NPE) is solved for a porous medium which consists of compacted bentonite. This system has been studied before in another research using an experimental approach and the results are available for both transient and steady-state phases. Three different conditions are assumed for NPE governing equations and then the numerical results from these three conditions are compared to the experimental values and analytical phenomenological solution. The tortuosity is treated as a fitting parameter and the effective diffusion coefficient can be calculated based on these tortuosity values. The results show that including a sorption term in the NPE equations can render similar results as the experimental values in transient and steady state phases. Also, as a fitting parameter, the tortuosity values were found varying with background concentration. This highlights the need to monitor multiple diffusing ion fluxes and membrane potential to fully characterize electro-diffusive transport from fundamental principles (which have been investigated in first part of this thesis) rather than phenomenological equations for predictive studies. / This research has lead to two different journal articles submissions, one already accepted in Computers and Geotechnics (October 22, 2009, 5-yrs Impact Factor 0.884) and the other one still under review.

L'effet Nernst, bien que peu exploité depuis sa découverte en 1886, a acquis récemment une place importante dans le domaine des électrons corrélés. Au cours de cette thèse, nous avons utilisé l'effet Nernst afin d'étudier deux exemples de systèmes corrélés, un supraconducteur NbxSi1-x et un fermion lourd PrFe4P12. Dans l'étude des films amorphes supraconducteurs de Nb0.15Si0.85, le signal Nernst observé est en parfait accord avec la prédiction théorique de Ussiskhin, Sondhi et Huse (USH) dans la limite de faible champ magnétique et près de Tc. La théorie USH qui se fonde sur l'existence de paires de Cooper au temps de vie fini au-dessus de Tc, relie directement le coefficient Nernst à la longueur de corrélation à champ nul, c-à-d la taille des paires de Cooper fluctuantes. L'étude approfondie des données a montré que, de façon plus générale, le signal Nernst est déterminé par une seule longueur, la longueur de corrélation à toute température et tout champ magnétique. La théorie USH n'est que la limite bas champ d'une théorie plus générale qui relierait le coefficient Nernst à la longueur de corrélation. Ces résultats démontrent que le signal Nernst observé au-dessus de Tc jusqu'à très haute température (30 xTc) et jusqu'à très haut champ magnétique (3 X Bc2) dans les films amorphes de Nb0.15Si0.85 est généré par les fluctuations supraconductrices de type paires de Cooper fluctuantes. La seconde étude que nous avons effectuée dans le composé fermion lourd PrFe4P12 nous a permis de caractériser les phases qui apparaissent dans ce matériau à basse température. L'amplitude exceptionnelle de l'effet Nernst observée dans la phase ordonnée à bas champ magnétique est la conséquence de trois facteurs indépendants : une faible densité de porteurs, une augmentation de la masse effective et un grand libre parcours moyen. Ce comportement est caractéristique d'un fermion lourd semi-métallique. L'augmentation importante du pouvoir thermoélectrique dans la phase ordonnée est révélatrice d'une destruction importante de la surface de Fermi. La phase qui apparaît à haut champ magnétique pour la direction [111] semble également liée à une restructuration de la surface de Fermi, bien que moins importante, associée à un comportement non liquide de Fermi. Le changement de signe de l'effet Nernst lors de l'apparition de la phase à haut champ magnétique pourrait s'interpréter comme le signe d'une transition métamagnétique.

Made available in DSpace on 2014-12-17T15:15:04Z (GMT). No. of bitstreams: 1 JoseCCN_DISSERT.pdf: 4506673 bytes, checksum: 52e5774ef6550be4717b8297c50b7163 (MD5) Previous issue date: 2014-05-09 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Quando um material ferromagn?tico ? submetido a um gradiente de temperatura e a um campo magn?tico surge o chamado efeito Nernst-An?malo, medido atrav?s da voltagem que surge na amostra magn?tica. Este efeito atualmente vem sendo investigado em materiais para aplica??o em spintr?nica e caloritr?nica [1]. Atualmente, os materiais chamados de Heusler s?o os mais promissores para essa nova ?rea de pesquisa. Neste trabalho, investigamos as curvas de voltagem associadas ao efeito Nernst-An?malo com anisotropia magneto-cristalina, em termos da voltagem versus campo magn?tico aplicado e da voltagem versus ?ngulo planar. Analisamos tr?s tipos de anisotropias: c?bica (100), que apresenta simetria C4, uniaxial que ? do tipo C2 e a c?bica (110), que tamb?m ? do tipo C2. O objetivo foi comprovar que o uso da anisotropia c?bica (110) ? equivalente a anisotropia (100) adicionada de uma uniaxial. Especialmente, quando a constante de anisotropia uniaxial ? considerada grande, cerca de 40% da constante de anisotropia c?bica (110), e o seu eixo f?cil est? a 90? do eixo f?cil da (100). Os resultados demonstram essa total equival?ncia e produz uma nova interpreta??o baseada no uso da anisotropia c?bica (110)