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.

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.

In dieser Arbeit werden Messungen elektrischer, thermischer und insbesondere thermoelektrischer Transportkoeffizienten in topologischen Weyl-Halbmetall-Kandidaten sowie in eisenbasierten Hochtemperatur-Supraleitern vorgestellt, analysiert und diskutiert. In TaAs und TaP, zwei Weyl-Halbmetall-Kandidaten mit gebrochener Inversionssymmetrie, liefert das anomale Verhalten des Nernst-Signals in Abhängigkeit des Magnetfeldes Hinweise auf die Existenz von Weyl-Punkten in der Nähe der Fermi-Fläche, wobei sich die Verschiebung des chemischen Potenzials sowie ein Lifshitz-Übergang detektieren lassen. Die Temperaturabhängigkeit des Nernst-Signales erlaubt außerdem Rückschlüsse auf den Abstand der Weyl-Punkte zur Fermi-Fläche. In Mn3Ge, einem Weyl-Halbmetall-Kandidaten mit gebrochener Zeitumkehrsymmetrie, zeigt sich für alle gemessenen Temperaturen ein komplett anomales Verhalten des Nernst-Signals in Abhängigkeit des Magnetfeldes mit einer rechteckigen Hysterese bei kleinen Feldern, welches im Vergleich mit Daten der Magnetisierung einen eindeutigen Hinweis auf die Präsenz von Weyl-Punkten in diesem Material liefert. Mithilfe eines minimalen theoretischen Modells zweier Weyl-Punkte in der Nähe der Fermi-Fläche lässt sich eine Anpassungsformel für die Temperaturabhängigkeit des Nernst-Signals gewinnen, aus welcher sowohl geometrische Parameter der Bandstruktur als auch die Stärke der Berry-Krümmung an der Fermi-Energie extrahiert werden können. Für eisenbasierte Supraleiter besteht seit Langem der Verdacht, dass nematische Fluktuationen einen Einfluss auf die Supraleitung haben. Hier vorgestellte theoretische Betrachtungen zeigen im Rahmen eines Zweibandmodells eindeutig, dass eine endliche nematische Kopplung zu einer starken Erhöhung und einer nicht-monotonen Abhängigkeit des Nernst-Koeffizienten von der Dotierung führt, welcher ein Maximum über dem supraleitenden Dom aufweist. Dies wird anhand von Nernst-Messungen in Co-dotiertem LaFeAsO bestätigt. Ein Vergleich der Ergebnisse des Nernst-Effekts mit Elasto-Widerstandsmessungen enthüllt eine erstaunliche Ähnlichkeit der Dotierabhängigkeiten. Die Daten werden außerdem mit Messungen des Nernst-Effekts an Rh-dotiertem BaFe2As2 verglichen, wobei ebenfalls eine Erhöhung im Bereich optimaler Dotierung nachgewiesen werden kann. In Rh-dotietem BaFe2As2 zeigt sich jedoch ein Unterschied zwischen Elasto-Widerstands- und Nernst-Messungen, woraus abgeleitet wird, dass Elasto-Widerstandsmessungen kein vollständiges Bild der nematischen Fluktuation liefern. Der Nernst-Effekt ist hingegen aufgrund der Sensitivität auf nematische Fluktuationen universell in zwei Vertretern verschiedener Familien eisenbasierter Supraleiter maximal im Bereich des supraleitenden Doms. Dies liefert, zusammen mit den theoretischen Betrachtungen, einen starken Hinweis auf den Einfluss nematischer Fluktuationen auf die Supraleitung. / In this work the electric, thermal, and thermoelectric transport properties of several topological Weyl semimetal candidates and iron-based superconductors are investigated. In TaAs and TaP, two Weyl semimetal candidates with broken inversion symmetry, the Nernst signal exhibits anomalous behaviour as a function of magnetic field, consistent with Weyl points close to the Fermi surface. Furthermore, a shift of the chemical potential and a Lifshitz transition are detected. The temperature dependence of the Nernst signal allows for an estimation of the energy of the Weyl points with respect to the Fermi level. In Mn3Ge, a Weyl semimetal candidate with broken time reversal symmetry, the Nernst signal shows completely anomalous behaviour as a function of magnetic field that can be obtained at all measured temperatures. At low fields the signal exhibits a rectangular hysteresis cycle. A comparison with magnetization measurements evidently shows that these effects are caused by Weyl points lying close to the Fermi surface. With the help of a minimal model of two Weyl points in the vicinity of the Fermi level, a fitting formula of the temperature dependence of the Nernst signal can be obtained. The fit provides geometrical properties of the band structure, such as the $\boldsymbol{k}$-space separation of the Weyl points, their energy with respect to the Fermi level as well as the strength of the Berry curvature close to the Fermi energy. For a long time nematic fluctuations have been suspected to influence superconductivity in iron-based superconductors. A theoretical analysis, with the help of a two-band model, shows clearly that a finite nematic coupling causes a strong enhancement and non-monotonic behaviour of the Nernst coefficient, which develops a maximum above the superconducting dome. These findings are confirmed by Nernst measurements in Co-doped LaFeAsO. A comparison with elasto-resistivity measurements shows a stunning similarity of the doping dependencies of both quantities. Furthermore the data are compared with measurements on Rh-doped BaFe2As2, which also exhibits an enhancement of the Nernst coefficient in the region of optimal doping. However, in Rh-doped BaFe2As2 a difference between elastoresistivity and Nernst measurements is obtained, indicating that the elasto-resistivity measurements are not universally sensitive to nematic fluctuations. The Nernst effect, on the other hand, is enhanced in the vicinity of the superconducting dome in two members of different families of iron-based superconductors. Together with theoretical insights, these results provide strong evidence for the influence of nematic fluctuations on superconductivity in the iron-based superconductors.