This thesis considers various strongly correlated quantum phases in solid state and cold atom spin systems. In the first part we focus on phases emerging in multi-orbital materials. We study even-parity spin-triplet superconductivity originating from Hund's coupling between t2g orbitals and investigate the effect of spin-orbit interaction on spin-triplet and spin-singlet pairing. Various aspects of the pairing state are discussed against the backdrop of the spin-triplet superconductor Sr2RuO4. Motivated by the remarkable phenomena observed in the bilayer compound Sr3Ru2O7, which point to the formation of an electronic nematic phase in the presence of critical fluctuations, we investigate how such a broken symmetry state emerges from electronic interactions. Since the broken x-y symmetry is revealed experimentally by applying a small in-plane magnetic field component, we examine nematic phases in a bilayer system and the role of the in-plane magnetic field using a phenomenological approach. In addition, we propose a microscopic mechanism for nematic phase formation specific to Sr3Ru2O7. The model is based on a realistic multi-orbital band structure and local and nearest neighbour interactions. Considering all t2g-orbital derived bands on an equal footing, we find a nematic quantum critical point and a nearby meta-nematic transition in the phase diagram. This finding harbours important implications for the phenomena observed in Sr3Ru2O7. The second part is devoted to the study of the anisotropic bilinear biquadratic spin-1 Heisenberg model, where the existence of an unusual direct phase transition between a spin-nematic phase and a dimerized valence bond solid phase in the quasi-1D limit was conjectured based on Quantum Monte Carlo simulations. We establish the quasi-1D phase diagram using a large-N Schwinger boson approach and show that the phase transition is largely conventional except possibly at two particular points. We further discuss how to realize and to detect such phases in an optical lattice.

This thesis considers various strongly correlated quantum phases in solid state and cold atom spin systems. In the first part we focus on phases emerging in multi-orbital materials. We study even-parity spin-triplet superconductivity originating from Hund's coupling between t2g orbitals and investigate the effect of spin-orbit interaction on spin-triplet and spin-singlet pairing. Various aspects of the pairing state are discussed against the backdrop of the spin-triplet superconductor Sr2RuO4. Motivated by the remarkable phenomena observed in the bilayer compound Sr3Ru2O7, which point to the formation of an electronic nematic phase in the presence of critical fluctuations, we investigate how such a broken symmetry state emerges from electronic interactions. Since the broken x-y symmetry is revealed experimentally by applying a small in-plane magnetic field component, we examine nematic phases in a bilayer system and the role of the in-plane magnetic field using a phenomenological approach. In addition, we propose a microscopic mechanism for nematic phase formation specific to Sr3Ru2O7. The model is based on a realistic multi-orbital band structure and local and nearest neighbour interactions. Considering all t2g-orbital derived bands on an equal footing, we find a nematic quantum critical point and a nearby meta-nematic transition in the phase diagram. This finding harbours important implications for the phenomena observed in Sr3Ru2O7. The second part is devoted to the study of the anisotropic bilinear biquadratic spin-1 Heisenberg model, where the existence of an unusual direct phase transition between a spin-nematic phase and a dimerized valence bond solid phase in the quasi-1D limit was conjectured based on Quantum Monte Carlo simulations. We establish the quasi-1D phase diagram using a large-N Schwinger boson approach and show that the phase transition is largely conventional except possibly at two particular points. We further discuss how to realize and to detect such phases in an optical lattice.

Magnetic frustration is a phenomenon arising in spin systems when spin interactions cannot all be satisfied at the same time. A typical example of geometric frustration is a triangle with Ising-spins at its vertices and antiferromagnetic interaction. While we can easily anti-align two neighbouring spins, it is not possible for the third one to simultaneously anti-align with both of them. Another flavour of magnetic frustration is the so called exchange frustration, where different spin components interact in an Ising fashion on different bonds. Moreover, frustrated spin systems give rise to exotic states of matter, such as spin liquids, spin ices and nematic phases. As frustrated systems are rarely analytically solvable, numerical techniques are of the utmost importance in this framework. This dissertation is concerned with a specific class of models, namely one- and quasi-one-dimensional spin systems and studies their properties by making use of the density matrix renormalisation group technique. This method has been shown to be extremely powerful and reliable to study chain and ladder models. We consider examples of both geometric and exchange frustration. For the former, we take into consideration one of the prototypical examples of geometric frustration in one dimension: the J1-J2 model with ferromagnetic nearest-neighbour interaction J1<0 and antiferromagnetic next-nearest-neighbour interaction J2>0. Our results show the existence of a Haldane gap supported by a special AKLT-like valence bond solid state in a specific region of the coupling ratio. Furthermore, we consider the effect of dimerisation of the first-neighbour coupling. This dimerisation affects the critical point and the ground state underlying the spin gap. These models are of interest in the context of cuprate chain materials such as LiVCuO4, LiSbCuO4 and PbCuSO4(OH)2. Concerning exchange frustration, we consider the celebrated Kitaev-Heisenberg model: it is an extension of the exactly solvable Kitaev model with an additional Heisenberg interaction. The Kitaev-Heisenberg model is currently the minimal model for candidate Kitaev materials. The extended model is not analytically solvable and numerics are needed to study the properties of the system. While both the original Kitaev and the Kitaev-Heisenberg models live on a honeycomb lattice, we here perform systematic studies of the Kitaev-Heisenberg chain and of the two-legged ladder. While the chain cannot support a Kitaev spin liquid state, it shows nevertheless a rich phase diagram despite being a one-dimensional system. The long-range ordered states of the honeycomb can be understood in terms of coupled chains within the Kitaev-Heisenberg model. Following this reasoning, we turn our attention to the Kitaev-Heisenberg model on a two-legged ladder. Remarkably, the phase diagram of the ladder is extremely similar to that of the honeycomb model and the differences can be explained in terms of the different dimensionalities. In particular, the ladder exhibits a topologically non-trivial phase with no long-range order, i.e., a spin liquid. Finally, we investigate the low-lying excitations of the Kitaev-Heisenberg model for both the chain and the ladder geometry.

Condensed matter physics provides us with an opportunity to explore a large variety of systems with diverse properties. Central to the understanding of these systems is a characterization of the nature of their ground states and low energy excitation. Often, such systems show various forms of emergent properties that are absent in the microscopic level. Identification of such emergent phases of condensed matter form an important avenue of research in the field. In this thesis example of such phases and their associated phase transitions have been studied. The work presented here may be broadly divided into two themes: construction of the theoretical framework for understanding materials already studied experimentally, and, trying to provide new theoretical avenues which may be relevant for understanding future experiments. In these studies we shall explore some unconventional phases and phase transitions that may occur in condensed matter systems. A comprehensive understanding of the properties of such unconventional phases and phase transitions is important in the context of the large array of experimentally studied materials that regularly defy conventional wisdom in more than one way. The thesis consists of two distinct parts. In the first part we study three problems in frustrated magnets. The second part consists of studies of the tunnelling spectroscopy of metal-insulator-superconductor junctions in graphene. Studies in frustrated magnets have opened up the possibility of existence of a whole range of phases beyond the already known magnetically ordered ones. Some of these new phases, like the spin nematic or the valence bond solid, display some other conventional order themselves. Others, like the much sort after spin liquid phases displays a whole new kind of order that cannot be captured through the celebrated Landau’s classification of phases on the basis of symmetry breaking and associated order parameters. The phase transitions in these systems are also equally interesting and lead to intriguing possibilities that demand new modes of analysis. In this part of the thesis we shall study the different properties of three magnets with spin-1/2, 1 and 3/2 respectively. We start by providing an introduction to frustrated spin systems in Chapter [1]. The origin of antiferromagnetic interactions in Mott insulators is discussed and the concept of frustration of magnetic interaction is explained. We also point out the causes that may destroy magnetic order in spin systems, particularly the role of quantum fluctuations in presence or absence of magnetic frustration. This is followed with a brief outline of various magnetically ordered and disordered ground states with particular emphasis on the description of the later. We also give a brief outline of various properties of such phases and associated quantum phase transitions particularly noting the influences of quantum interferences encoded in the Berry phase terms. A brief description of the finite temperature properties is also provided. We end an outline of various experimentally relevant compounds that requires comprehensive understanding, some of which have been addressed in this thesis. In Chapter [2] we study the properties of a spin-nematic state in context of the recently discovered spin-1 Mott insulator Nickel Gallium Sulphide (NiGa2S4). This isotropic triangular lattice compound shows no spin ordering till low temperatures. We propose that it may have a particular type of spin-nematic ground state and explain the experimentally observed properties of the compound on the basis of our proposal. Starting from a two band Hubbard model description, relevant for the compound, we derive the Bilinear Biquadratic spin Hamiltonian. We then show, within mean field theory, that this Hamiltonian describes a transition from the spiral state to a ferro-nematic state as a function of the ratio of bilinear and biquadratic couplings. We also study the possible effects of small pinning disorder andmagnetic field and suggest experiments that can possibly distinguish the proposed nematic state from others. In Chapter [3] we explore the effects of the magneto-elastic coupling in the spin-3/2 B-site chromite spinel Cadmium Chromite (CdCr2O4). In this compound the spins form a pyrochlore lattice. Nearest neighbour spins interact antiferromagnetically. Due to frustration the system does not order at low temperatures and instead goes into a classical spin liquid state. Such a cooperative paramagnet is very susceptible to external perturbations which may relieve their frustration. In CdCr2O4, at lower temperatures the magnetic frustration is relieved by distorting the lattice through a first order magnetoelastic transition. Thus the compound presents a case where the relevant perturbation to the frustrated spin interactions is provided by spin-phonon coupling. An effect of such perturbations on a cooperative paramagnet is of general interest and all aspects of this are not understood presently. We take the initial step of characterizing the spin-phonon interaction in detail. Based on recent sound velocity experiments, we construct a microscopic theory for the sound velocity renormalization due to the spin-phonon coupling and explain the recent experimental data obtained by S. Zherlitsyn et al. using our theory we can explain the dependence of the sound velocity on temperature as well as magnetic field. We also construct a Landau theory to explain (qualitatively) the behaviour of sound velocity across the magneto-structural transition. Further, we discuss the effects due to the small Dzyaloshinskii-Moriya interaction that may be present in these compounds. In Chapter [4] we study the possibility of a direct second order quantum phase transition from spiral to dimer phase in two dimensional antiferromagnets. Such transitions between phases with incompatible symmetries are forbidden within conventional Landau Ginzburg-Wilson paradigm of critical phenomena. Early works showed that when the spiral is destroyed by long wavelength fluctuations a fractionalized Z2 spin liquid is obtained. In this work we show an alternative way–the quantum destruction of the spiral magnet. We argue that, when the defects of the spiral phase proliferate and condense, their associated Berry phase automatically leads to dimerization. We apply our theory to study concrete lattice models where such transitions may be observed. This transition is an example of a Landau forbidden deconfined quantum phase transition. The proposed critical theory is naturally written in terms of fractional degrees of freedom which emerge right at the critical point. These fractional particles interact with each other through emergent gauge fields and are deconfined right at the critical point (but are confined in either of the two adjoining phases). We argue, based on existing results, that the monopoles of the gauge field are dangerously irrelevant right at the critical point rendering the later noncompact. The critical point is characterized by an emergent global U (1) conservation law that is absent in the microscopic model, a typical feature of a deconfined quantum critical point. The resultant field theory belongs to the class of anisotropic NCCP3 class which may be studied numerically in future to understand its critical properties. In modern condensed matter physics the emergence of new and novel phases of matter have often been associated with the presence of strong correlations. Indeed, strongly correlated systems seem to harbour in them the potential to realize some of the most unconventional and exotic emergent phases of matter. However in graphene, which is a single layer of graphite, the emergence of novel properties, as present experiments suggest, is due to its unique band structure and not a fallout of intricate correlation effects. Band structure studies of graphene suggest that the material is a zero gap semiconductor with the low energy excitations resembling massless Dirac quasi-particles. The consequence of this is immediate and interesting. It has lead to the possibility of exploring the physics of relativistic fermions in two spatial dimensions and much of this has been studied with great vigour in the last five years. In our studies, presented in Chapter [5], we explore one of the many consequence of this emergent Dirac structure of the low energy quasi-particles, namely the properties of metal-insulator-superconductor junctions of graphene. The twin effect of Klein tunneling of Dirac fermions (and associated transmission resonances) and Andreev reflection (both specular and retro) sets them aside from their conventional counterparts. The graphene normal metal-insulator-superconductor (NIS) junctions show strikingly different properties like oscillations in the sub-gap tunneling conductance as a function of both barrier strength and width. We make a detailed study of this for arbitrary barrier strengths and widths with and without Fermi-surface mismatch between the normal and the superconducting sides. The amplitude of these oscillations are maximum for aligned Fermi surface and vanishes for large Fermi surface mismatch. We provide an understanding for this unconventional behaviour of graphene NIS junctions. We also suggest experimental tests for our theory. Such experimental verification will reveal one more remarkable emergent property in a condensed matter system.

Starke elektronische Korrelationen und die daraus resultierenden vielfältigen Phänomenen sind Gegenstand der modernen Festkörperphysik. Solche Korrelationen finden sich in den verschiedensten Systemen vom Isolator über die Halbleiter bis hin zu Metallen. In dieser Arbeit werden die durch Korrelationen hervorgerufenen Phänomene in zwei niederdimensionalen Übergangsmetalloxiden und zwei intermetallischen Verbindungen mithilfe der HF-ESR untersucht. Die Elektronenspin-Resonanz (ESR) nutzt als lokale Messmethode den Spin der Elektronen als Sonde, um die magnetischen Eigenschaften im Umfeld des Elektrons und die Wechselwirkungen (WW) mit anderen Elektronen zu erforschen. Mit stärker werdenden Elektron-Elektron (EE)-Korrelationen kommt es (unter anderem) zu einer Verbreiterung der Resonanz, sodass, um die Resonanz zu beobachten, höhere Frequenzen bzw. größere Felder als in kommerziellen ESR-Spektrometern erreichbar, nötig sind. Mit der in dieser Arbeit genutzten Hochfeld/Hochfrequenz-Elektronenspin-Resonanz (HF-ESR) mit einem frei durchstimmbaren Frequenzbereich von $\nu=\vu{20- 700}{GHz}$ kann speziellen Fragestellungen nachgegangen werden, bei denen die Anregungsenergien im Bereich von $h\nu$ liegen oder Resonanz-Effekte bei hohen Felder beobachtet werden sollen. CaCu$_2$O$_3$ zeigt die gleiche Kristallstruktur wie \chem{SrCu_2O_3}, einem Lehrbuchbeispiel für eine 2-beinige Spin\textfrac{1}{2}-Leiter mit einem nichtmagnetischen Grundzustand und einer großen Spinlücke zum ersten angeregten Zustand. \chem{CaCu_2O_3} zeigt dagegen überraschenderweise einen antiferromagnetischen (AFM) Grundzustand mit einer relativ hohen Übergangstemperatur. Um der Ursache der AFM-Ordnung auf den Grund zu gehen, wurde eine kombinierte Studie der Magnetisierung und der HF-ESR an einer Reihe von Zn-dotierten \chem{CaCu_2O_3} durchgeführt. Im Gegensatz zum Sr-Material sind die \chem{Cu_2O_3}-Leiter-Ebenen durch einen geringeren Sprossenwinkel leicht gewellt, desweiteren zeigt \chem{CaCu_2O_3} eine nichtstöchiometrische Zusammensetzung \chem{Ca_{1- x} Cu_{2+x}O_3}, mit einem Überschuss von Cu von $x\sim 0.16$ im nichtmagnetischen \chem{Cu^{1+}}-Zustand, welches auf Ca-Plätzen sitzt. Wir werden zeigen, dass (i) die Extra-Spins im undotierten Material \emph{nicht} in den Ketten sitzen, sondern auf regelmäßigen Zwischengitterpositionen. Sie rekrutieren sich aus dem überschüssigen \chem{Cu^{1+}}, dessen Position in der Nähe einer O-Fehlstelle instabil wird, sich verschiebt und den Zustand in ein magnetischen \chem{Cu^{2+}} ändert, (ii) dass durch die Position der Extra-Spins eine Kopplung übernächster Spin-Leitern zustande kommt, welche die Frustration der Spin-Leitern aufhebt und einen AFM-Grundzustand mit solch hoher Übergangstemperatur erlaubt und (iii) dass diese Position der Extra-Spins die zusätzliche schwache kommensurable Spinstruktur erklären kann, die im AFM- Zustand neben der inkommensurablen Spinstruktur der Leiter-Spins beobachtet wurde. Das einfach geschichtete Manganat \textbf{LaSrMnO$_4$} ist ein zweidimensionaler Vertreter der Übergangsmetalloxide. In diesem Material gibt es starke Korrelationen zwischen dem orbitalen und dem magnetischen Freiheitsgrad, sodass die AFM-Ordnung unterhalb von $T_N\sim\vu{125}{K}$ mit einer ferro-orbitalen Ordnung der \chem{Mn^{3+}} $3d$-Orbitale einhergeht. Mithilfe der HF-ESR konnte die temperaturabhängige Mischung der $3d$-Orbitale direkt bestimmt und damit die Theorie der ferro-orbitalen Ordnung quantitativ bestätigt werden. Im AFM geordneten Zustand, unterhalb von $T_\text{stat}\sim\vu{40}{K}&amp;lt;T_N$ wurde eine starke feldabhängige Reduktion der Mikrowellen-Transmission beobachtet, deren Frequenzabhängigkeit ein direkter Hinweis auf ferromagnetische (FM) Polaronen ist, die durch die WW von zusätzlichen Ladungsträgern mit den AFM-geordneten Grundspins entstehen. GdNi$_2$B$_2$C Die intermetallische Verbindungen der Nickelborkarbide $R\chem{Ni_2B_2C}$ ($R$ - Seltene Erdmetalle) zogen seit der Entdeckung von Supraleitung in einigen dieser Verbindungen große Aufmerksamkeit auf sich. Sie zeigen hochkomplexe magnetische Phasendiagramme mit einem Wechselspiel zwischen Supraleitung und der damit konkurrierenden AFM-Ordnung mit unterschiedlichsten Spinstrukturen. Ein Grund für diese Komplexität ist die starke magnetische Anisotropie, die durch die Aufspaltung des $J$-Multipletts der $f$-Orbitale der $R$ im Kristallfeld hervorgerufen wird. Das nicht supraleitende \chem{GdNi_2B_2C} erhielt als Modell-System viel Aufmerksamkeit, da \chem{Gd^{3+}} mit einer halbgefüllten $4f$-Schale keine magnetische Anisotropie zeigen sollte. Die vorgestellte ESR-Studie an \chem{GdNi_2B_2C} wird jedoch zeigen, dass dieser vermeintlich reine Spinmagnet eine ungewöhnlich starke magnetische Anisotropie besitzt, die sich auf die hochkomplexe Bandstruktur zurückführen lässt. Das Einbeziehen dieser Resultate in die Modellierung des Systems wird helfen, die Abweichungen zwischen Modell und Realität zu erklären. YbRh$_2$Si$_2$ In diesem schwere-Fermionen-System, indem die magnetischen Yb ($4f$) ein regelmäßiges Kondo-Gitter aufbauen, konkurrieren die EE-WW und die Ruderman-Kittel-Kasuya-Yosida-(RKKY)-WW miteinander, sodass in diesem Material durch die Veränderung eines angelegten Magnetfelds $B$ und der Temperatur $T$ der Zustand von einer AFM-Ordnung, zu einem (paramagnetischen) Schweres-Fermion- (LFL) bzw. Nicht-LFL-Verhalten (NFL) eingestellt werden kann. Unterhalb der Kondo-Temperatur führt eine starke Hybridisierung von $4f$-Elektronen mit Leitungselektronen zu einer deutlichen Verbreiterung der ansonsten atomar-scharfen $4f$-Zustände, sodass die Entwicklung einer schmalen Elektronen-Spin-Resonanz im Kondo-Zustand von \chem{YbRh_2Si_2} sehr überraschend war. Da die bisher veröffentlichten ESR-Messungen vollständig im NFL-Bereich lagen, werden in dieser Arbeit HF-ESR-Daten vorgestellt, die einen tieferen Einblick in die Physik dieser Resonanz erlauben, da sie einen $B-T$-Bereich abdecken, in dem ein Übergang zum LFL-Bereich stattfindet. Die gemessenen $B$- und $T$-Abhängigkeiten der ESR-Parameter im NFL- und im LFL-Bereich weisen darauf hin, dass das Resonanz-Phänomen in \chem{YbRh_2Si_2} als Resonanz schwerer Fermionen betrachtet werden muss. / Strong electronic correlation and the resultant phenomena are object of interest in the modern solid state physics. Such correlation can be found in totally different systems from insulators and semiconductors to metals. This thesis presents HF-ESR studies of such phenomena in two low dimensional transition metal oxides and two intermetallic compounds. In ESR the electron spin is used as a local probe to measure the interaction between electrons and the magnetic properties nearby. With increasing electron-electron (EE) interaction the resonance becomes broader, so higher frequencies and higher magnetic fields as usual in commercial available ESR devices are needed to study strong EE interactions. With the used HF-ESR device with a frequency range $\nu=\vu{20-700}{GHz}$ special questions can be investigated where the excitation energies are in the order of $h\nu$ or the resonance effects in high magnetic fields can be explored. \textbf{CaCu$_2$O$_3$} have the same crystal structure as \chem{SrCu_2O_3}, a textbook example for a 2-leg spin-\textfrac{1}{2}-ladder with a nonmagnetic groundstate and a spin gap separating the excited state. Surprisingly \chem{CaCu_2O_3} shows an antiferromagnetic (afm) ground state with a relatively high transition temperature. To get a deeper insight in the unexpected afm ordering a combined magnetization and HF-ESR study was performed on a set of Zn-doped \chem{CaCu_2O_3} samples. Contrary to the Sr-compound in \chem{CaCu_2O_3} the \chem{Cu_2O_3}-ladder-layers are buckled due to a reduced rung angle. Furthermore it is a nonstoichiometric compound \chem{Ca_{1- x} Cu_{2+x}O_{3- \delta}}, with an excess of Cu in the order of $x\sim 0.16$ which is in the nonmagnetic \chem{Cu^{1+}}-state, sitting close to Ca-sites and a deficiency of oxygen $\delta\sim 0.07$. With this study one can show that (i) in the undoped compound the extra-spins, responsible for the magnetic Curie-Weiss-behavior, do not sit in the chains, they are sitting on low-symmetry interstitial sites. They recruit themselves from excess \chem{Cu^{1+}}, where the position becomes unstable close to a O-vacancy so they shift to a interstitial site and become \chem{Cu^{2+}}, (ii) the interstitial site of the extra-spins couple n.n. ladders inside a layer with a direct afm exchange path which lifts the frustration of the spin-ladders so that a afm order with such a high ordering temperature can happen and (iii) the regular interstitial site of the extra-spins explains the weak commensurate spin structure additionally found to the incommensurate spin structure of the ladder-spins in the afm ordered state The single layered manganate \textbf{LaSrMnO$_4$} is a two dimensional member of the transition metal oxides. In this compound a strong correlation between the orbital and magnetic degree of freedom can be found, so that the afm ordering below $T_N\sim\vu{125}{K}$ comes along with a ferro-orbital ordering of the \chem{Mn^{3+}} $3d$-Orbitals. With HF-ESR we have measured the temperature dependent mixing of the $3d$-orbitals and proved quantitatively the theory of ferro-orbital ordering. In the afm ordered state below $T_\text{stat}\sim\vu{40}{K}&amp;lt;T_N$ a strong field dependent decrease of the microwave transmission was observed. The frequency dependence of this phenomena could be explained by ferromagnetic polarons resulting from the interaction of additional charge carriers with the afm ordered spins. \textbf{GdNi$_2$B$_2$C} The intermetallic borocarbides $R\chem{Ni_2B_2C}$ ($R$ - rare earth metal) attract much attention due to the mutual interaction of superconductivity and afm ordering with complex phase diagrams. One reason for this complexity is the strong magnetic anisotropy coming from the splitting of the $J$-multiplets of the $R$'s $f$-orbitals in the crystal field. The nonsuperconducting \chem{GdNi_2B_2C} was widely explored because \chem{Gd^{3+}} with a half filled $4f$-shell should show no anisotropic behavior. The HF-ESR study on this system showed, that the assumed pure spin magnet have a uncommonly strong anisotropy which could be ascribed to a highly complex band structure. Involving this new information will help to adjust the model to the reality. YbRh$_2$Si$_2$ In this heavy fermion system where the magnetic Yb ($4f$) built up a regular Kondo-lattice here is a competition between electron-electron- and the Ruderman-Kittel-Kasuya-Yosida-(RKKY) interaction. Thats why in this compound a afm ordered state, a (paramagnetic) heavy fermion (LFL) and a non-Fermi-liquid behavior can be established by changing the magnetic field $B$ and/or the temperature $T$. Below the Kondo-temperature $T^*$ a strong hybridization between the conduction electrons and the $4f$-electrons leads to a strong broadening of the otherwise atomic sharp $4f$-states. Thats why the observation of a small electron spin resonance below $T^*$ was very surprising. Because the yet published ESR-measurements are fully in the NFL-state, we performed HF-ESR measurements to study a $B-T$ area where a NFL-LFL crossover appears to get a deeper inside of the physics behind this resonance. The behavior of the measured $T$- and $B$-dependences indicate that this resonance phenomena in \chem{YbRh_2Si_2} is a resonance of heavy fermions.

Starke elektronische Korrelationen und die daraus resultierenden vielfältigen Phänomenen sind Gegenstand der modernen Festkörperphysik. Solche Korrelationen finden sich in den verschiedensten Systemen vom Isolator über die Halbleiter bis hin zu Metallen. In dieser Arbeit werden die durch Korrelationen hervorgerufenen Phänomene in zwei niederdimensionalen Übergangsmetalloxiden und zwei intermetallischen Verbindungen mithilfe der HF-ESR untersucht. Die Elektronenspin-Resonanz (ESR) nutzt als lokale Messmethode den Spin der Elektronen als Sonde, um die magnetischen Eigenschaften im Umfeld des Elektrons und die Wechselwirkungen (WW) mit anderen Elektronen zu erforschen. Mit stärker werdenden Elektron-Elektron (EE)-Korrelationen kommt es (unter anderem) zu einer Verbreiterung der Resonanz, sodass, um die Resonanz zu beobachten, höhere Frequenzen bzw. größere Felder als in kommerziellen ESR-Spektrometern erreichbar, nötig sind. Mit der in dieser Arbeit genutzten Hochfeld/Hochfrequenz-Elektronenspin-Resonanz (HF-ESR) mit einem frei durchstimmbaren Frequenzbereich von $\nu=\vu{20- 700}{GHz}$ kann speziellen Fragestellungen nachgegangen werden, bei denen die Anregungsenergien im Bereich von $h\nu$ liegen oder Resonanz-Effekte bei hohen Felder beobachtet werden sollen. CaCu$_2$O$_3$ zeigt die gleiche Kristallstruktur wie \chem{SrCu_2O_3}, einem Lehrbuchbeispiel für eine 2-beinige Spin\textfrac{1}{2}-Leiter mit einem nichtmagnetischen Grundzustand und einer großen Spinlücke zum ersten angeregten Zustand. \chem{CaCu_2O_3} zeigt dagegen überraschenderweise einen antiferromagnetischen (AFM) Grundzustand mit einer relativ hohen Übergangstemperatur. Um der Ursache der AFM-Ordnung auf den Grund zu gehen, wurde eine kombinierte Studie der Magnetisierung und der HF-ESR an einer Reihe von Zn-dotierten \chem{CaCu_2O_3} durchgeführt. Im Gegensatz zum Sr-Material sind die \chem{Cu_2O_3}-Leiter-Ebenen durch einen geringeren Sprossenwinkel leicht gewellt, desweiteren zeigt \chem{CaCu_2O_3} eine nichtstöchiometrische Zusammensetzung \chem{Ca_{1- x} Cu_{2+x}O_3}, mit einem Überschuss von Cu von $x\sim 0.16$ im nichtmagnetischen \chem{Cu^{1+}}-Zustand, welches auf Ca-Plätzen sitzt. Wir werden zeigen, dass (i) die Extra-Spins im undotierten Material \emph{nicht} in den Ketten sitzen, sondern auf regelmäßigen Zwischengitterpositionen. Sie rekrutieren sich aus dem überschüssigen \chem{Cu^{1+}}, dessen Position in der Nähe einer O-Fehlstelle instabil wird, sich verschiebt und den Zustand in ein magnetischen \chem{Cu^{2+}} ändert, (ii) dass durch die Position der Extra-Spins eine Kopplung übernächster Spin-Leitern zustande kommt, welche die Frustration der Spin-Leitern aufhebt und einen AFM-Grundzustand mit solch hoher Übergangstemperatur erlaubt und (iii) dass diese Position der Extra-Spins die zusätzliche schwache kommensurable Spinstruktur erklären kann, die im AFM- Zustand neben der inkommensurablen Spinstruktur der Leiter-Spins beobachtet wurde. Das einfach geschichtete Manganat \textbf{LaSrMnO$_4$} ist ein zweidimensionaler Vertreter der Übergangsmetalloxide. In diesem Material gibt es starke Korrelationen zwischen dem orbitalen und dem magnetischen Freiheitsgrad, sodass die AFM-Ordnung unterhalb von $T_N\sim\vu{125}{K}$ mit einer ferro-orbitalen Ordnung der \chem{Mn^{3+}} $3d$-Orbitale einhergeht. Mithilfe der HF-ESR konnte die temperaturabhängige Mischung der $3d$-Orbitale direkt bestimmt und damit die Theorie der ferro-orbitalen Ordnung quantitativ bestätigt werden. Im AFM geordneten Zustand, unterhalb von $T_\text{stat}\sim\vu{40}{K}&amp;lt;T_N$ wurde eine starke feldabhängige Reduktion der Mikrowellen-Transmission beobachtet, deren Frequenzabhängigkeit ein direkter Hinweis auf ferromagnetische (FM) Polaronen ist, die durch die WW von zusätzlichen Ladungsträgern mit den AFM-geordneten Grundspins entstehen. GdNi$_2$B$_2$C Die intermetallische Verbindungen der Nickelborkarbide $R\chem{Ni_2B_2C}$ ($R$ - Seltene Erdmetalle) zogen seit der Entdeckung von Supraleitung in einigen dieser Verbindungen große Aufmerksamkeit auf sich. Sie zeigen hochkomplexe magnetische Phasendiagramme mit einem Wechselspiel zwischen Supraleitung und der damit konkurrierenden AFM-Ordnung mit unterschiedlichsten Spinstrukturen. Ein Grund für diese Komplexität ist die starke magnetische Anisotropie, die durch die Aufspaltung des $J$-Multipletts der $f$-Orbitale der $R$ im Kristallfeld hervorgerufen wird. Das nicht supraleitende \chem{GdNi_2B_2C} erhielt als Modell-System viel Aufmerksamkeit, da \chem{Gd^{3+}} mit einer halbgefüllten $4f$-Schale keine magnetische Anisotropie zeigen sollte. Die vorgestellte ESR-Studie an \chem{GdNi_2B_2C} wird jedoch zeigen, dass dieser vermeintlich reine Spinmagnet eine ungewöhnlich starke magnetische Anisotropie besitzt, die sich auf die hochkomplexe Bandstruktur zurückführen lässt. Das Einbeziehen dieser Resultate in die Modellierung des Systems wird helfen, die Abweichungen zwischen Modell und Realität zu erklären. YbRh$_2$Si$_2$ In diesem schwere-Fermionen-System, indem die magnetischen Yb ($4f$) ein regelmäßiges Kondo-Gitter aufbauen, konkurrieren die EE-WW und die Ruderman-Kittel-Kasuya-Yosida-(RKKY)-WW miteinander, sodass in diesem Material durch die Veränderung eines angelegten Magnetfelds $B$ und der Temperatur $T$ der Zustand von einer AFM-Ordnung, zu einem (paramagnetischen) Schweres-Fermion- (LFL) bzw. Nicht-LFL-Verhalten (NFL) eingestellt werden kann. Unterhalb der Kondo-Temperatur führt eine starke Hybridisierung von $4f$-Elektronen mit Leitungselektronen zu einer deutlichen Verbreiterung der ansonsten atomar-scharfen $4f$-Zustände, sodass die Entwicklung einer schmalen Elektronen-Spin-Resonanz im Kondo-Zustand von \chem{YbRh_2Si_2} sehr überraschend war. Da die bisher veröffentlichten ESR-Messungen vollständig im NFL-Bereich lagen, werden in dieser Arbeit HF-ESR-Daten vorgestellt, die einen tieferen Einblick in die Physik dieser Resonanz erlauben, da sie einen $B-T$-Bereich abdecken, in dem ein Übergang zum LFL-Bereich stattfindet. Die gemessenen $B$- und $T$-Abhängigkeiten der ESR-Parameter im NFL- und im LFL-Bereich weisen darauf hin, dass das Resonanz-Phänomen in \chem{YbRh_2Si_2} als Resonanz schwerer Fermionen betrachtet werden muss. / Strong electronic correlation and the resultant phenomena are object of interest in the modern solid state physics. Such correlation can be found in totally different systems from insulators and semiconductors to metals. This thesis presents HF-ESR studies of such phenomena in two low dimensional transition metal oxides and two intermetallic compounds. In ESR the electron spin is used as a local probe to measure the interaction between electrons and the magnetic properties nearby. With increasing electron-electron (EE) interaction the resonance becomes broader, so higher frequencies and higher magnetic fields as usual in commercial available ESR devices are needed to study strong EE interactions. With the used HF-ESR device with a frequency range $\nu=\vu{20-700}{GHz}$ special questions can be investigated where the excitation energies are in the order of $h\nu$ or the resonance effects in high magnetic fields can be explored. \textbf{CaCu$_2$O$_3$} have the same crystal structure as \chem{SrCu_2O_3}, a textbook example for a 2-leg spin-\textfrac{1}{2}-ladder with a nonmagnetic groundstate and a spin gap separating the excited state. Surprisingly \chem{CaCu_2O_3} shows an antiferromagnetic (afm) ground state with a relatively high transition temperature. To get a deeper insight in the unexpected afm ordering a combined magnetization and HF-ESR study was performed on a set of Zn-doped \chem{CaCu_2O_3} samples. Contrary to the Sr-compound in \chem{CaCu_2O_3} the \chem{Cu_2O_3}-ladder-layers are buckled due to a reduced rung angle. Furthermore it is a nonstoichiometric compound \chem{Ca_{1- x} Cu_{2+x}O_{3- \delta}}, with an excess of Cu in the order of $x\sim 0.16$ which is in the nonmagnetic \chem{Cu^{1+}}-state, sitting close to Ca-sites and a deficiency of oxygen $\delta\sim 0.07$. With this study one can show that (i) in the undoped compound the extra-spins, responsible for the magnetic Curie-Weiss-behavior, do not sit in the chains, they are sitting on low-symmetry interstitial sites. They recruit themselves from excess \chem{Cu^{1+}}, where the position becomes unstable close to a O-vacancy so they shift to a interstitial site and become \chem{Cu^{2+}}, (ii) the interstitial site of the extra-spins couple n.n. ladders inside a layer with a direct afm exchange path which lifts the frustration of the spin-ladders so that a afm order with such a high ordering temperature can happen and (iii) the regular interstitial site of the extra-spins explains the weak commensurate spin structure additionally found to the incommensurate spin structure of the ladder-spins in the afm ordered state The single layered manganate \textbf{LaSrMnO$_4$} is a two dimensional member of the transition metal oxides. In this compound a strong correlation between the orbital and magnetic degree of freedom can be found, so that the afm ordering below $T_N\sim\vu{125}{K}$ comes along with a ferro-orbital ordering of the \chem{Mn^{3+}} $3d$-Orbitals. With HF-ESR we have measured the temperature dependent mixing of the $3d$-orbitals and proved quantitatively the theory of ferro-orbital ordering. In the afm ordered state below $T_\text{stat}\sim\vu{40}{K}&amp;lt;T_N$ a strong field dependent decrease of the microwave transmission was observed. The frequency dependence of this phenomena could be explained by ferromagnetic polarons resulting from the interaction of additional charge carriers with the afm ordered spins. \textbf{GdNi$_2$B$_2$C} The intermetallic borocarbides $R\chem{Ni_2B_2C}$ ($R$ - rare earth metal) attract much attention due to the mutual interaction of superconductivity and afm ordering with complex phase diagrams. One reason for this complexity is the strong magnetic anisotropy coming from the splitting of the $J$-multiplets of the $R$'s $f$-orbitals in the crystal field. The nonsuperconducting \chem{GdNi_2B_2C} was widely explored because \chem{Gd^{3+}} with a half filled $4f$-shell should show no anisotropic behavior. The HF-ESR study on this system showed, that the assumed pure spin magnet have a uncommonly strong anisotropy which could be ascribed to a highly complex band structure. Involving this new information will help to adjust the model to the reality. YbRh$_2$Si$_2$ In this heavy fermion system where the magnetic Yb ($4f$) built up a regular Kondo-lattice here is a competition between electron-electron- and the Ruderman-Kittel-Kasuya-Yosida-(RKKY) interaction. Thats why in this compound a afm ordered state, a (paramagnetic) heavy fermion (LFL) and a non-Fermi-liquid behavior can be established by changing the magnetic field $B$ and/or the temperature $T$. Below the Kondo-temperature $T^*$ a strong hybridization between the conduction electrons and the $4f$-electrons leads to a strong broadening of the otherwise atomic sharp $4f$-states. Thats why the observation of a small electron spin resonance below $T^*$ was very surprising. Because the yet published ESR-measurements are fully in the NFL-state, we performed HF-ESR measurements to study a $B-T$ area where a NFL-LFL crossover appears to get a deeper inside of the physics behind this resonance. The behavior of the measured $T$- and $B$-dependences indicate that this resonance phenomena in \chem{YbRh_2Si_2} is a resonance of heavy fermions.