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.

Nous étudions les effets des corrélations Kondo sur le déphasage de transmission d'une boîte quantique. Ce travail est motivé par les expériences d'interférométrie quantique réalisées ces dernières années permettant d'accéder au déphasage subi par un électron au passage à travers une boîte quantique dans le régime soit Kondo soit blocage de Coulomb. Notre analyse est basée sur la théorie de la diffusion et les calculs de Bethe ansatz dans un modèle d'Anderson. Dans l'état non magnétique et à basses températures, on trouve que le déphasage est égal à l'occupation de la boîte en unités de Π. Cette prédiction est en excellent accord avec les données expérimentales. Nous proposons ensuite une interprétation des « sauts de phase » observés quand la tension de grille varie, soit « incomplets » soit « complets » suivant que le système est dans le régime Kondo ou blocage de Coulomb. Finalement nous étudions la densité spectrale et le déphasage à tension de polarisation et température finies, dans une approximation de type NCA du modèle d'Anderson exprimé dans la représentation de bosons esclaves en utilisant le formalisme de Keldysh pour décrire la situation hors équilibre. Un accord qualitatif est trouvé avec les résultats expérimentaux.

No description available.

Abstract The softening effects of ferromagnetic magnon on some ferromagnetic semiconductors and colossal magnetoresistance manganites have attracted much attention. Such effect can be calculated from the single orbital ferromagnetic Kondo lattice model in proper conducting carrier numbers utilizing the equation of motion method with one magnon excitation and Random Phase approximations. However, if we take into account the Coulomb repulsion and use the Gutzwiller projection method to transfer this repulsion force to conducting bandwidth modulation, the softening effect disappear. This paper describes qualitively the effect of softening on properties of different colossal manganites.

Stuttgart, Univ., Diss., 2000.

Zugl.: Köln, Univ., Diss., 2009

Zugl.: Dresden, Techn. Univ., Diss., 2009

Made available in DSpace on 2016-12-12T20:15:53Z (GMT). No. of bitstreams: 1 capa e sumario.pdf: 56826 bytes, checksum: 4bd9d172c8abd2474a434fbae3b75124 (MD5) Previous issue date: 2008-10-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O modelo da rede de Kondo descreve uma rede de spins localizados Si que interagem com os elétrons de condução via um acoplamento de troca local J. Assumindo um acoplamento ferromagnético J > 0 da regra de Hund, o modelo pode ser usado para descrever alguns materiais magnetocalóricos tais como Gd(SixGe1−x)4, La(Fe1−xSix)13, e LaCa1−xMnxO3, que são importantes para refrigeração magnética próximo a temperatura ambiente. Os momentos localizados são descritos no Hamiltoniano modelo por operadores de spin, e os elétrons de condução por operadores fermiônicos. Para estudar o efeito magnetocalórico, um campo magnético uniforme externo e adicionado através do termo Zeeman. Pela média dos graus de liberdade dos operadores fermiônicos, obtém-se um acoplamento de troca indireto Jij entre spins dos sítios i e j, que corresponde a interação RKKY. O valor médio {Szi} auto-consistente é calculado no Hamiltoniano efetivo de Heisenberg na aproximação de fase aleatória (RPA). Os elétrons de condução para um dado valor de {Szi} é obtido das correspondentes funções de Green através do método da equação de movimento. A pressão e a dependência da dopagem com a temperatura de Curie são levadas em conta no cálculo de Jij

[pt] Nesta tese foram estudados três sistemas nanoscópicos compostos de pontos quânticos (PQs). No primeiro deles foi analisada a denominada nuvem Kondo, ou a extensão da blindagem que os spins da banda de condução fazem do spin de uma impureza magnética embebida em uma matriz metálica e representada, no nosso caso, por um PQ. As propriedades da nuvem Kondo foram obtidas através da manifestação da ressonância Kondo na densidade de estados local nos sítios da matriz metálica e também através das correlações de spin entre o spin do elétron no PQ e os spins da banda de condução. Foi possível encontrar uma concordância entre as extensões da nuvem Kondo obtidas com ambos métodos. O segundo sistema estudado consiste em uma estrutura de três PQs alinhados e com o PQ central acoplado a dois contatos metálicos. Foi analisada a operação deste sistema como uma porta lógica quântica cujo funcionamento depende do estado de carga do PQ central. Foi feito um estudo dependente do tempo das propriedades do sistema e, em particular, da correlação dos spins dos PQs laterais. Mostramos que o efeito Kondo, refletido na condutância do sistema, pode ser uma ferramenta fundamental para conhecer o estado da porta quântica. Os primeiros dois sistemas foram tratados usando o método dos Bósons Escravos na aproximação de campo médio. Finalmente, foi estudado o transporte termoelétrico em um sistema de dois PQs quando um deles está acoplado a contatos metálicos unidimensionais. O sistema foi analisado no regime de resposta linear e não linear a um potencial externo no regime de bloqueio de Coulomb. Mostramos que a presença de ressonâncias Fano e de uma singularidade de Van-Hove na densidade de estados dos contatos unidimensionais perto do nível de Fermi são ingredientes fundamentais para o aumento da eficiência termoelétrica do dispositivo. O problema de muitos corpos foi resolvido na aproximação de Hubbard III que permite um estudo correto das propriedades de transporte deste sistema para T maior que TK, onde TK é a temperatura Kondo. / [en] In this thesis, were studied three nanoscopic quantum dot (QD) systems. First, the so-called Kondo cloud was analyzed, the extension of the conduction band spin screening of a magnetic impurity embedded in a metallic matrix and represented, in our case, by a QD. The Kondo cloud properties were obtained studying the way in which the local density of states of the metallic matrix sites reflects the Kondo resonance and also through the spin-spin correlations between the QD and the conduction band spins. It was possible to find a good agreement between the Kondo cloud extensions obtained using both methods. The second system consists of three aligned QDs with the central QD connected to two metallic leads. The operation of this system as a quantum gate was studied, which depends on the central QD charge. A time dependent study of the system properties and, in particular, of the lateral QDs spin correlation was developed. We showed that the Kondo effect, reflected in the conductance, could be a fundamental tool to measure the information contained in the quantum gate state. The first two systems were treated using the Slave Bosons Mean Field Approximation method. Finally, we studied the thermoelectric transport of a two QD system when one of them is connected to two onedimensional leads. The system was analyzed in the linear and nonlinear response to an external applied potential, always in the Coulomb blockade regime. It was found that the presence of Fano resonances and a Van-Hove singularity in the one-dimensional lead density of states near the Fermi level are fundamental ingredients to enhance thermoelectric efficiency. The many-body problem was treated in the Hubbard III approximation, which is a correct approach to study the transport properties for T greater than TK, where TK is the Kondo temperature.

Orientador: Flavio Cesar Guimarães Gandra / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-07-25T14:14:43Z (GMT). No. of bitstreams: 1 MedinaNeto_Antonio_D.pdf: 3096313 bytes, checksum: 6225278625a9c2336fa97379dac95eff (MD5) Previous issue date: 1998 / Resumo: Neste trabalho apresentamos e discutimos a influência da diluição magnética na competição entre interação RKKY e efeito Kondo nas séries (La1-xCex)3A1, (Ce1-zYz)3Al, (La1-xCex)Pd2Al3 e (La1-xCex)Ni. Através do estudo das propriedades estruturais, magnéticas, térmicas e de transporte destas séries, mostramos que a mudança do estado fundamental destes compostos, pode ser explicada considerando a variação do produto do parâmetro de troca (J), entre o elétron 4f do Ce e os elétrons de condução, e a densidade de estados no nível de Fermi (n(eF)), provocada pela variação do volume da célula unitária. Verificamos que, isoladamente, cada série de compostos obedece um diagrama tipo Doniach. Para os compostos que apresentam ordenamento magnético (Ce3Al e CePd2Al3), observamos que a relação g ~ 1/TK não é obedecida (para Jn(eF) < (Jn(eF))c), e a interação entre os elétrons que constituem o pico ressonante de Kondo e o campo médio local, é a responsável pela redução em y. No estudo do sistema (La1-xCex)Ni, mostramos que a Ressonância Paramagnética Eletrônica é uma técnica útil na determinação da concentração crítica para a transição do regime de impureza de Kondo para rede de Kondo. Esta transição pode ser claramente observada como uma descontinuidade no alargamento térmico (d(DH)/dT) e um máximo para geff para a concentração crítica / Abstract: In this work we study the influence of the magnetic dilution on the competition between the RKKY interaction and Kondo effect for the series of compounds (La1-xCex)3Al, (Ce1-zYz)3Al, (La1-xCex)Pd2Al3 and (La1-xCex)Ni. Measurements of the structural, magnetic, calorimetric and transport proprieties, show that the changes of the ground state these systems can be explained by the variation of the product Jn(eF) (J is exchange parameter between 4fCe and conduction electrons and n( eF ) is density of state of conduction electrons ), induced by the unit cell volume variation. We verify that each system obey the Doniach diagram. For the systems that present magnetic ordering (Ce3Al e CePd2Al3) we found that the relation g ~1/TK does not hold (for Jn(eF)< (Jn(eF))c) as a consequence of the interaction between the electrons of the Kondo resonant peak with the local field. In the study of the system (La1-xCex)Ni, we show that Electron Spin Resonance (ESR) is a useful technique for the determination of the critical concentration (Xc), where the system changes from Kondo single impurity to Kondo lattice regime. The ESR results show a discontinuity on the thermal broadening and a maximum in the g-value at x=xc / Doutorado / Física / Doutor em Ciências