Ferromagnetismo itinerante permanece um problema elusivo em Física. O fenômeno resulta da competição entre interação eletrônica e efeitos de muitos corpos e não pode ser tratado perturbativamente. Particularmente em uma dimensão, teoremas proíbem fases ferromagnéticas em T = 0 para modelos de rede com hopping de primeiros vizinhos. Nos últimos vinte anos, entretanto, apareceram modelos na literatura que estendem o hopping para além de primeiros vizinhos e para os quais ordem ferromagnética foi rigorosamente estabelecida. Praticamente todas as demonstrações da existência de ferromagnetos unidimensionais são feitas em fase isolante (com exceção de casos patológicos, como repulsão infinita). Isto nos levou a investigar o acoplamento entre os setores de spin e carga no regime fortemente interagente quando se dopa o sistema, o que introduz pontos de Fermi pF e -pF. Encontramos, com teoria de perturbação, singularidades logarítmicas na autoenergia do mágnon quando seu momentum é pF ou -pF. Derivamos uma teoria de campo efetiva para o espalhamento em torno desses pontos entre os mágnons e férmions sem spin (que representam o setor de carga). O modelo efetivo é similar ao modelo Kondo, que consiste de uma impureza magnética localizada acoplada localmente com um mar fermiônico por uma interação de troca entre spins. Em nosso modelo, há, na realidade, um pseudospin que indica se o momentum de uma partícula é próximo de pF ou de -pF e o mágnon se comporta como uma impureza móvel. A mobilidade da impureza leva a uma relação de dispersão para os férmions dependente do pseudospin da impureza. / Itinerant ferromagnetism remains an elusive problem in Physics. The phenomenon arises from a competition between electronic interaction and many-body effects and cannot be treated perturbatively. Particularly in 1D, there are rigorous proofs that forbid ferromagnetic phase for lattice models with nearest-neighbours hopping only. In the last twenty years, however, models with hopping beyond nearest-neighbours were proposed in the literature and for which ferromagnetic phase was rigorously established. Virtually every proof of the existence of one-dimensional ferromagnets is done in an insulator phase (disregarding some pathological cases, such as infinite electronic repulsion). That motivated us to investigate the coupling between spin and charge sectors in the strongly interacting regime when we dope the system, introducing two Fermi points, pF and -pF. We found out, through perturbation theory, logarithmic singularities in the magnon selfenergy when its momentum is pF or -pF. To understand them, we derived an effective field theory for the scattering between magnons and spinless fermions (which represent the charge sector) close to these points. The effective model resembles the Kondo model, which describes a magnetic impurity locally coupled to a fermionic sea through spin exchange interaction. In our model, there is actually a pseudospin that indicates if a particle momentum is closest to pF or -pF and the magnon behaves as a mobile impurity. The impurity mobility leads to a fermionic dispersion relation that depends on the impurity pseudospin.

Obtemos a expressão da densidade espectral de uma impureza no modelo de Anderson assimétrico. No regime de Kondo de baixas energias o modelo de Anderson descreve sistemas quânticos com impurezas magnéticas em meios não magnéticos. Para energias características do orbital de impureza simples ou duplamente ocupado iguais a εf e 2εf+U, respectivamente, o modelo é chamado simétrico para 2εf + U = 0 e assimétrico quando 2εf + U ≠ 0. Para o caso simétrico existe uma expressão devido a Frota que descreve ressonância de Kondo na densidade espectral. Todavia, no caso assimétrico a expressão de Frota deixa de ser válida e é necessária uma nova abordagem. Nesta dissertação, a partir de dados numéricos obtidos pelo consagrado método do Grupo de Renormalização Numérico criado por Wilson em 1974 e de tópicos da física de muitos corpos estudamos a assimetria do modelo de Anderson no regime de Kondo e propomos uma expressão mais geral que a de Frota para a densidade espectral da impureza. / We propose an analytical expression for the impurity spectral density in the Kondo regime of the spin-degenerate asymmetric Anderson model. The Anderson model describes quantum systems of magnetic impurities in nonmagnetic metals. For characteristic energies of single and double impurity occupations εf and 2εf+U, respectively, the model is particlehole symmetric for 2εf + U = 0 and asymmetrical 2εf + U ≠ 0. In the symmetric case, there is an expression due to Frota that describes the Kondo resonance in the spectral density. However, in the asymmetric case Frota´s expression is not valid, and a new approach is necessary. In this work, starting with numerical data provided by the Numerical Renormalization Group method developed by Wilson in 1974, we study the asymmetric Anderson model in the Kondo regime and propose an expression for the impurity spectral density that is more general than Frota´s.

Estudamos o calor específico de um sistema formado por duas impurezas adsorvidas, sem spin, em meio fermiônico (banda de condução do metal) e que contém um buraco (elétron) tunelando entre elas. Modelamos esse sistema por dois níveis acoplados e que sofrem interação Coulombiana com a banda de condução. Através da análise das curvas de calor específico, investigamos a alteração (renormalização) da taxa de tunelamento em função da interação eletrostática entre os elétrons da banda de condução e o buraco tunelante e da separação entre as impurezas. Utilizamos o Hamiltoniano de Kondo de tunelamento para representar esse modelo e usamos o Grupo de Renormalização Numérico para diagonalizá-lo. Analisamos a influência de cada termo do Hamiltoniano na renormalização da taxa de tunelamento e verificamos que a troca de paridade das funções de onda do buraco tunelante e dos elétrons da banda desempenha papel essencial. Encontramos uma expressão que combina a distância entre as impurezas e a interação Coulombiana em um único parâmetro (a), de tal forma que sistemas diferentes mas que apresentam o mesmo a e a mesma taxa de tunelamento livre têm a mesma curva de calor específico. / We calculate the specific heat of the two-spinless impurity coupled to a fermionic bath. The model takes into account the tunneling of a hole between the impurities. The two-level system representing the impurities is coupled electrostatically with the conduction electrons. Through the specific heat curves, we analyse the renormalization of the tunneling rate as a function of the Coulomb interaction and distance between impurities. The Numerical Renormalization Group is used to diagonalize the tunneling Hamiltonian proposed by Kondo. We analyse the role of each term of the Hamiltonian in the renormalization of the bare tunneling rate and we stress the importance of the exchange parity between impurity states and conduction states. Finally, a parameter a, is found which combines the distance between impurities and Coulomb interaction in such a way that every curve is specified only by a and the bare tunneling rate.

Orientador: Carlos Rettori / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T05:10:39Z (GMT). No. of bitstreams: 1 Garcia_FernandoAssis_D.pdf: 2669059 bytes, checksum: 122a24668135bef8b9e2ee683190a28a (MD5) Previous issue date: 2010 / Resumo: Esta tese aborda o problema geral dos efeitos de campos cristalino e rattling modes em compostos do tipo skutterudite. Emprega, predominantemente, a técnica de ressonância de spin eletrônico (ESR, ou EPR na literatura clássica) a fim de sondar a simetria local dos estados eletrônicos associados a íons de terras raras nestes compostos e investigar as inomogeneidades decorrentes da dinâmica (rattling modes) destes íons. Desenvolvemos o nosso estudo predominantemente sob o sistema Ce1-xRxFe4P12 (R = Gd, Dy, Er, Yb), usualmente descrito como um semicondutor com gap G ¿ 0.15 eV. Nossos resultados apontam que a correta determinação dos efeitos de campo cristalino requer uma nova abordagem no estudo de compostos com simetria Th. O grupo pontual Th é um dos cinco grupos pontuais cúbicos (O, Oh, T, Th, Td) e é obtido pelo produto T x sh, onde T descreve a simetria de um tetraedro regular. Desta maneira, Th não possui as operações 6C4 e 3C¿2 e precisamos utilizar um novo parâmetro de sexta ordem para descrever todos os invariantes da simetria do grupo pontual Th Este novo parâmetro causa uma mistura na simetria do estado fundamental de sistemas cúbicos que dá origem a fenômenos não triviais discutidos na tese. Resultado igualmente importante é que, a partir do estudo deste novo parâmetro, fomos capazes de mapear a inomogeneidade de campo cristalino decorrente da dinâmica dos íons de terra rara. Desta maneira, a técnica de ESR foi utilizada pela primeira vez no estudo da dinâmica de íons em sistemas do tipo gaiola. Os estudos em sistemas S (L = 0) mostraram ainda que os efeitos desta dinâmica nos espectros de ESR não depende da presença deste parâmetro, sendo algo mais geral. Esta tese examina estes dois importantes aspectos relacionados a estrutura cristalina da família das skutterudites e discute estes resultados no contexto das possíveis aplicações destes compostos na construção de dispositivos termoelétricos e também no estudo de fenômenos fortemente correlacionados / Abstract: This thesis addresses the general problem of crystall field effects and rattling modes in skut- terudite compounds. It employs, predominantly, the electron spin resonance technique (ESR or EPR, in the classical literature) aiming to probe the local point symmetry of the electronic states associated to the rare earth ions and the inohomogenities implied by their dynamics (rattling modes). Our study was mainly directed to the Ce 1-xRxFe4P12 (R = Gd, Dy, Er, Yb) compounds, usually described as a small gap semiconductor (G ¿ 0.15 eV). Our results pointed that the correct determination of the crystall field effects requires a new approach in the case of compounds with Th symmetry. The point group Th is one the five cubic point groups (O, 0h, T, Th, Td) being generated by the product T x sh, where T describes the full symmetry of a regular tetrahedron. In this sense, Th lacks the symmetry operations 6C4 e 3C¿2 and we need a new sixth order parameter, to describe all the invariants of point group T^ symmetry. This new parameter gives rise to a mix of the ground state symmetry of cubic systems, implying in non trivial phenomena discussed in this these. Interesting enough, following these results, we were able to map the crystall field ino-homogenities due to the dynamics of the rare earth ion within the skutterudite cage. In this sense, the ESR techinique was introduced as a probe to the guest ion dynamics in cage systems. Investigation of S systems (L = 0) showed that the effects of this dynamics in our experiment does not require the presence of this new sixth order parameter, being associated to a more general role of the rattling modes. We examine these two important aspects related to crystal structure of the skutterudite family and discuss the results in the context of possible applications of these compounds in the construction of thermoelectric devices and also in the study of strongly correlated phenomena / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Dans cette thèse, je me suis principalement intéressé aux effets liés aux impuretés magnétiques dans les métaux et supraconducteurs. Dans le premier chapitre je présenterai l’effet Kondo, celui-ci se produit lorsqu’une impureté magnétique présente un couplage antiferromagnétique avec le métal qu’elle pollue. Les électrons forment alors, à suffisamment basse température, un nuage écrantant le magnétisme de l’impureté. Le deuxième chapitre portera sur les états de Yu-Shiba-Rusinov produits par des impuretés magnétiques dans un supraconducteur. Dans ce cas, l’impureté brise localement l’ordre supraconducteur et un état quantique est crée à l’intérieur du gap.Certains matériaux dits topologiques peuvent comporter des états quantiques protégés à leurs bords contre les perturbations extérieures. Dans les deux derniers chapitres, je présenterai les propriétés des supraconducteurs topologiques et leurs états de bords. Dans le troisième chapitre je présenterai les différentes phases topologiques que peut comporter un supraconducteur avec un paramètre d’ordre complexe mélangeant ondes p et s en présence d’un champ Zeeman. Dans le quatrième et dernier chapitre je présenterai une étude des états de bords que peut comporter ce type de supraconducteurs. / In this thesis, I was mainly interested in the effects related to magnetic impurities in metals and superconductors.In the first chapter I will present the Kondo effect, this effect occurs when a magnetic impurity exhibits an antiferromagnetic coupling with the metal that it pollutes. The electrons then form, at a sufficiently low temperature, a cloud screening the magnetism of the impurity. The second chapter will focus on the states of Yu-Shiba-Rusinov products by magnetic impurities in a superconductor. In this case, the impurity locally breaks the superconducting order and a quantum state is created inside the gap.Some so-called topological materials may include quantum states protected at their edges against external perturbations. In the last two chapters, I will present the properties of topological superconductors and their edge states. In the third chapter I will present the different topological phases that can include a superconductor with a parameter of complex order mixing waves p and s in the presence of a Zeeman field.In the fourth and last chapter I will present a study of the states of edge that may include this type of superconductors.

In dieser Arbeit wird das eindimensionale Kondo-Gitter-Modell untersucht, das die Wechselwirkung eines Gitters lokaler magnetischer Momente mit unkorrelierten Leitungselektronen beschreibt. Mit Hilfe der Methode der kontinuierlichen unitären Transformationen (Flussgleichungen) wird das Modell im Parameterbereich schwacher Wechselwirkungsstärke betrachtet. In diesem Bereich zeigt das Modell so genanntes Luttinger-Flüssigkeitsverhalten. Im Rahmen der Flussgleichungsmethode wird der Hamilton-Operator auf ein effektives Modell abgebildet, in dem Elektronen und Spinmomente vollständig entkoppelt sind. Das Resultat dieses Prozesses ist ein Modell, das ein nichtwechselwirkendes Elektronengas und eine Heisenberg-Spinkette beschreibt. Das Eigenwertproblem der Heisenberg-Kette wird im Rahmen einer Schwinger-Boson-Molekularfeld-Theorie beschrieben. Zur Charakterisierung der Grundzustandseigenschaften des eindimensionalen Kondo-Gitter-Modells wurden verschiedene Erwartungswerte und Korrelationsfunktionen betrachtet. Neben statischen Größen, wie der Ladungskorrelationsfunktion der Elektronen oder der Spinkorrelationsfunktion der lokalen Spinmomente, werden dynamische Größen, wie die elektronische Zustandsdichte oder die dynamischen Spinstrukturfaktoren der Elektronen und der lokalen Spinmomente, berechnet. / The one-dimensional Kondo lattice model is investigated. This model describes the interaction between a lattice of local magnetic moments and uncorrelated conduction electrons. It is studied by means of the continuous unitary transformation's method (flow equations) within the parameter regime of weak interaction strength. Here the model shows so called Luttinger liquid behaviour. Within the framework of the flow equation's method the original Hamiltonian is mapped on an effective model, where electrons and local moments are completely decoupled. The result of this process is a model describing a non-interacting electron gas and a Heisenberg spin chain. The eigenvalue problem of the Heisenberg chain is described within a Schwinger bosons molecular field theory. In order to characterise the ground state properties of the one-dimensional Kondo lattice model different expectation values and correlation functions are investigated. Beside static properties like the charge correlation function of the electrons or the local moment's spin correlation function, dynamic properties are determined, like the electronic density of states or the dynamic spin structure factor of both the electrons and the local moments.

No description available.

In dieser Arbeit wird das eindimensionale Kondo-Gitter-Modell untersucht, das die Wechselwirkung eines Gitters lokaler magnetischer Momente mit unkorrelierten Leitungselektronen beschreibt. Mit Hilfe der Methode der kontinuierlichen unitären Transformationen (Flussgleichungen) wird das Modell im Parameterbereich schwacher Wechselwirkungsstärke betrachtet. In diesem Bereich zeigt das Modell so genanntes Luttinger-Flüssigkeitsverhalten. Im Rahmen der Flussgleichungsmethode wird der Hamilton-Operator auf ein effektives Modell abgebildet, in dem Elektronen und Spinmomente vollständig entkoppelt sind. Das Resultat dieses Prozesses ist ein Modell, das ein nichtwechselwirkendes Elektronengas und eine Heisenberg-Spinkette beschreibt. Das Eigenwertproblem der Heisenberg-Kette wird im Rahmen einer Schwinger-Boson-Molekularfeld-Theorie beschrieben. Zur Charakterisierung der Grundzustandseigenschaften des eindimensionalen Kondo-Gitter-Modells wurden verschiedene Erwartungswerte und Korrelationsfunktionen betrachtet. Neben statischen Größen, wie der Ladungskorrelationsfunktion der Elektronen oder der Spinkorrelationsfunktion der lokalen Spinmomente, werden dynamische Größen, wie die elektronische Zustandsdichte oder die dynamischen Spinstrukturfaktoren der Elektronen und der lokalen Spinmomente, berechnet. / The one-dimensional Kondo lattice model is investigated. This model describes the interaction between a lattice of local magnetic moments and uncorrelated conduction electrons. It is studied by means of the continuous unitary transformation's method (flow equations) within the parameter regime of weak interaction strength. Here the model shows so called Luttinger liquid behaviour. Within the framework of the flow equation's method the original Hamiltonian is mapped on an effective model, where electrons and local moments are completely decoupled. The result of this process is a model describing a non-interacting electron gas and a Heisenberg spin chain. The eigenvalue problem of the Heisenberg chain is described within a Schwinger bosons molecular field theory. In order to characterise the ground state properties of the one-dimensional Kondo lattice model different expectation values and correlation functions are investigated. Beside static properties like the charge correlation function of the electrons or the local moment's spin correlation function, dynamic properties are determined, like the electronic density of states or the dynamic spin structure factor of both the electrons and the local moments.

Orientadores: Guillermo Gerardo Cabrera Oyarzun, Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-09-24T18:12:10Z (GMT). No. of bitstreams: 1 Sa_EduardoPeresNovaisde_D.pdf: 1083760 bytes, checksum: ffdca0eae18642d2c13d907af6a959b2 (MD5) Previous issue date: 2001 / Resumo: Neste trabalho, estudamos a rede de Kondo em uma dimensão por bosonização abeliana e a cadeia de Heisenberg pelo ansatz de Bethe. Conectamos o problema da rede de Kondo com o problema Jahn-Teller cooperativo e com o problema de 2 líquidos de Luttinger interagentes. A principal técnica usada foi o grupo de renormalização do gás de Coulomb generalizado, que surge ao integrarmos os graus de liberdade bosônicos. Procuramos explicar o diagrama de fase da cadeia de Ising-Kondo e da rede de Kondo, tanto para acoplamentos ferromagnéticos, como para acoplamentos antiferromagnéticos. Finalmente, estudamos as soluções de Bethe para cadeias de Heisenberg finitas e infinitas / Abstract: In this thesis, we studied the one-dimensional Kondo lattice model with abelian bosonization and the Heisenberg chain with the Bethe ansatz. We connected the Kondo lattice with the cooperative Jahn-Teller and with the 2 Luttinger liquids interacting by hopping. The main technique used was the renormalization group for the generalized Coulomb gas, that is obtained when we integrate the bosonic degrees of freedom. We explained the phase diagrams for the Ising-Kondo chain and the Kondo lattice, for ferromagnetic and anti-ferromagnetic coupling. Finally, we studied the Bethe solutions for the finite and thermodynamic Heisenberg chain / Doutorado / Física / Doutor em Ciências

The aim of the present thesis is to explore the potential of X-ray magnetic circular dichroism(XMCD) experiments on gaining new insights into Kondo and heavy fermion materials. XMCD, which is derived from X-ray absorption spectroscopy (XAS), allows probing magnetic polarization specific to the different elements in a material and to their atomic orbitals. In particular, at the Ce M4,5 edges the method is sensitive to the localized 4f level, which provides the magnetic impurity moment responsible for Kondo physics in Ce compounds. Hence, Ce M4,5 XMCD is ideally suited to investigate local magnetism in the presence of interaction of impurity and conduction electrons in such materials. As a model material, CePt5/Pt(111) surface intermetallics were chosen for the present study. This thin-film material can be prepared by well-defined procedures involving molecular beam epitaxy. Crystalline Ordered samples are obtained by exploiting the single-crystallinity of the Pt(111) substrate. The surface character of thin films ideally matches the probing depth of soft X-ray spectroscopy in the total electron yield mode. The XMCD and XAS experiments, taking into account dependence on temperature, angle of incidence, sample thickness and external magnetic field, revealed the presence of four relevant energy scales that influence the magnetic response: 1. The 4f level in CePt5/Pt(111) is subject to significant crystal field (CF) splitting, which leads to reorganization of the six j = 5/2 sublevels. The hexagonal symmetry of the crystal structure conserves mj as a good quantum number. The proposed CF scheme, which is derived from measurements of the paramagnetic susceptibility by XMCD as well as linear dichroism in XAS, consists of nearly degenerate |1/2> and |3/2> doublets with the |5/2> doublet excited by E5/2 = 15 ... 25 meV. 2. Single impurity Kondo interaction significantly couples the magnetic moments of the impurity and conduction electrons. A signature thereof is the f0 -> f1 contribution to Ce M4,5 XAS, the strength of which can be tuned by control of the sample thickness. This finding is in line with the observation of reduced effective 4f moments as detected by XMCD. 3. Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction induces ferromagnetic correlations on the impurity lattice, which induces a positive Curie-Weiss temperature in the temperature-dependent inverse susceptibility. 4. Indications for the transition to a coherent heavy fermion state are found in the inverse susceptibility at T ~ 20 K; the ferromagnetic ground state is not observed. The fielddependence of the magnetic moment in the coherent state can be interpreted in terms of a metamagnetic transition. This allows studying basic characteristics of the renormalized band structure of a heavy fermion system by XMCD. The disentanglement of these different contributions to the 4f magnetism not only required extensive Ce M4,5 XAS and XMCD data, but also a thorough structural characterization of the material, a fundamental study of the Ce M4,5 line shape in relation to the degree of 4f hybridization and the development of a model for the paramagnetic susceptibility. The unit cell dimensions and sample morphology of CePt5/Pt(111) intermetallics were studied by low-energy electron diffraction (LEED) and scanning transmission electron microscopy (STEM). These experiments showed that well-defined intermetallic films form on top of the substrate. This lead to introduction of the film thickness t, measured in unit cells (u.c.), as a key feature to characterize the samples. Systematic LEED measurements in the thickness range t ~ 1 ... 15 u.c. allowed identification of six different phases, which could be interpreted as resulting from the same crystal structure with different rotational alignments and lattice constants. An accurate determination of the surface lattice constant at t ~ 3 u.c. could be achieved by interpretation of additional superstructure spots as arising from a well-defined combination of substrate and film lattices. The thicknessdependence of the lateral lattice constant could be explained in terms of lattice relaxation. Confirmation of the CePt5 stoichiometry and structure was performed by use of thicknessdependent XAS and a representative LEED-IV study. The results of this study indicate that the intermetallic films exhibit hexagonal CaCu5 structure over the entire range of thicknesses that were studied. The terminating layer consists purely of Pt with one additional Pt atom per unit cell compared to the bulk structure. The line shape of Ce M4,5 spectra was analyzed with the help of full multiplet calculations. Experimentally, characteristic variations of the line shape were observed with increasing f0 -> f1 contribution. The calculations show that these variations are not due to an admixture of j = 7/2 character to the ground state, as often stated in the literature. As alternatives, this observation can be explained by either considering an additional contribution to the spectrum or by assumption of an asymmetric lifetime profile. The model that was developed for the inverse paramagnetic susceptibility contains the hexagonal crystal field, magnetic coupling of the impurity moments in a mean field scheme and Kondo screening. The latter is included phenomenologically by screening factors for the effective moment. Assumption of doublet-specific screening factors, which means that the degree of Kondo interaction depends on the mj character of the 4f sublevels, allows satisfactory reproduction of the experimental data. / Das Ziel der vorliegenden Arbeit ist die Untersuchung der Frage, welche neuen Einsichten in Kondo- und schwere Fermionen-Materialien mittels Röntgenzirkulardichroismus-Experimenten (XMCD) gewonnen werden können. Die Methode XMCD, die sich aus der Röntgenabsorption (XAS) ableitet, detektiert die magnetischen Polarisation gezielt für die verschiedenen Elemente in einem Material und für deren Atomorbitale. Insbesondere an den Ce M4,5 Absorptionskanten bietet die Methode Zugang zum lokalisierten 4f Niveau und damit zum magnetische Störstellenmoment in Ce-Verbindungen, welches eine Voraussetzung für Kondo-Physik ist. Ce M4,5 XMCD ist daher bestens geeignet, um lokalen Magnetismus in Gegenwart von Wechselwirkung zwischen Störstellen- und Leitungselektronen zu studieren. Als Modellmaterial wurde für diese Arbeit CePt5/Pt(111), eine oberflächennahe intermetallische Verbindung, gewählt. Die Präparation dieses Dünnfilmmaterials mithilfe von Molekularstrahlepitaxie kann bestens kontrolliert werden. Die Ausnutzung des einkristallinen Pt(111) Substrats liefert kristallin geordnete Proben. Der Oberflächencharakter der Filme ist gut auf die Informationstiefe von Absorptionsspektroskopie im weichen Röntgenbereich abgestimmt, wenn im Modus totaler Elektronenausbeute gemessen wird. Die XAS- und XMCD-Experimente unter Variation von Temperatur, Einfallswinkel, Probendicke und Magnetfeld lassen die Gegenwart von vier relevanten Energieskalen für das magnetische Verhalten erkennen: 1. Das 4f Niveau in CePt5/Pt(111) ist einer deutlichen Kristallfeldaufspaltung unterworfen, welche eine Neuordnung der sechs j = 5/2 Unterniveaus bewirkt. Dabei erhält die hexagonale Symmetrie der Kristallstruktur mj als gute Quantenzahl. Basierend auf Messungen der paramagnetischen Suszeptibilität mit XMCD und des Lineardichroismus in XAS wird ein Niveauschema vorgeschlagen, das aus beinahe entarteten |1/2> und |3/2 > Dubletts besteht, das |5/2> Dublett folgt bei einer höheren Energie von E5/2 = 15 ... 25 meV. 2. Einzelstörstellen-Kondowechselwirkung bewirkt eine signifikante Kopplung der magnetischen Momente von Störstellen und Leitungszuständen. Dies ist anhand der f0 -> f1-Anteile in Ce M4,5 XAS ersichtlich, deren Stärke über die Probendicke kontrolliert werden kann. In Übereinstimmung damit werden reduzierte effektive 4f-Momente mit XMCD beobachtet. 3. Ruderman-Kittel-Kasuya-Yosida (RKKY) Wechselwirkung erzeugt ferromagnetische Kopplung auf dem Störstellengitter, was zu einer positiven Curie-Weiss-Temperatur in der temperaturabhängigen inversen Suszeptibilität führt. 4. Die inverse Suszeptibilität erlaubt Rückschlüsse auf einen Übergang in den kohärenten schwere-Fermionen-Zustand bei T ~ 20 K. Ein ferromagnetischer Zustand wurde nicht beobachtet. Die Magnetfeldabhängigkeit des magnetischen Moments in diesem Bereich kann im Sinne eines metamagnetischen Übergangs interpretiert werden. Dies eröffnet die Möglichkeit, grundlegende Charakteristika der renormalisierten Bandstruktur eines schweren Fermionen-Systems mittels XMCD zu erforschen. Die Entschlüsselung dieser unterschiedlichen Beiträge zum 4f-Magnetismus erforderte nicht nur umfangreiche Ce M4,5 XAS und XMCD Experimente, sondern auch eine gründliche strukturelle Charakterisierung des Materials, eine grundlegende Studie der spektralen Linienform in Abhängigkeit vom Grad der 4f-Hybridisierung sowie die Entwicklung eines Modells für die paramagnetische Suszeptibilität. Die Abmessungen der Einheitszelle sowie die Filmmorphologie wurden mit niederenergetischer Elektronenbeugung (LEED) und Rastertransmissionselektronenmikroskopie (STEM) analysiert, wobei sich zeigte, dass die intermetallische Verbindung wohldefinierte Filme auf dem Substrat bildet. Daher wird die Filmdicke t, gemessen in Einheitszellen (u.c.), als Hauptmerkmal zur Charakterisierung der Proben eingeführt. Mittels systematische LEED-Messungen im Dickenbereich t ~ 1 ... 15 u.c. wurden sechs verschiedene Phasen identifiziert, welche auf eine gemeinsame Kristallstruktur mit unterschiedlichem Drehwinkel zum Substrat und unterschiedlichen Gitterkonstanten zurückgeführt werden. Bei einer Dicke von t ~ 3 u.c. konnte eine genaue Bestimmung der Oberflächengitterkonstante durchgeführt werden, indem zusätzliche Überstrukturreflexe als Ergebnis von kombinierter Streuung an Substrat- und Filmgitter gedeutet wurden. Die Dickenabhängigkeit der Gitterkonstante kann als abnehmende Gitterverspannung erklärt werden. Die angenommene Stöchiometrie CePt5 und die zugehörige Kristallstruktur konnten mithilfe von dickenabhängigen XAS-Experimenten und einer repräsentativen LEED-IV Studie bestätigt werden. Die Ergebnisse der Letzteren weisen darauf hin dass die Filme im gesamten untersuchten Dickenbereich die CaCu5-Struktur aufweisen. Die Oberflächenabschlusslage besteht rein aus Pt und besitzt im Vergleich zum Volumengitter ein zusätzliches Pt-Atom pro Einheitszelle. Die Linienform von Ce M4,5 Spektren wurde mittels Gesamtmultiplett-Rechnungen analysiert. Die experimentellen Daten zeigen charakteristische Variationen der Linienform in Abhängigkeit des f0 -> f1-Gewichts. Mit Rechnungen konnte gezeigt werden, dass diese Variationen nicht durch eine signifikante Beimischung von j = 7/2-Charakter zum Grundzustand erklärt werden können, was in der Literatur häufig angeführt wird. Zwei Alternativen wurden als mögliche Erklärungen für die Beobachtungen entwickelt: Die Linienformänderungen können entweder durch einen zusätzlichen Beitrag zum Spektrum oder durch asymmetrische Linienprofile erzeugt werden. Das Modell für die inverse paramagnetische Suszeptibilität beinhaltet das hexagonale Kristallfeld, magnetische Kopplung auf dem Strörstellengitter in Molekularfeldnäherung und Kondoabschirmung. Letztere ist phänomenologisch durch Abschirmfaktoren für das effektive Moment berücksichtigt. Eine zufriedenstellende Widergabe der experimentellen Daten konnte durch die Einführung von Dublett-spezifischen Abschirmfaktoren erreicht werden, was darauf hindeutet, dass das Ausmaß der Kondowechselwirkung vom mj-Charakter der 4f Unterniveaus abhängt.