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Originally the Kondo effect describes a scattering mechanism of electrons in metals that have defects with internal quantum mechanical degrees of freedom. The characteristic dynamic interplay between localized and itinerant states occurs in many different forms. It is now clear that the Kondo effect shapes the properties of various classes of quantum materials and is a key to understanding their unusual properties. In the following we outline a selection of the most important developments in this Kondomania., Comment: 12 pages in preprint form, in German, 9 figures; invited contribution for publication in Physik in unserer Zeit

In this thesis the effect of interfacial Dzyaloshinskii-Moriya-interaction (iDMI) is studied. This effect, which induces a frequency shift of spin waves with fixed wave vector, is investigated for different multilayers. Only spin waves with its wave vector perpendicular to the magnetization are detected. These spin waves are called surface waves. Brillouin light scattering (BLS) technique is used for the wave vector dependent detection of spin wave frequencies. Different setups were build and compared including a new setup, which does not use a focusing lens in front of the sample. This setup exhibits an increased wave vector resolution and simplifies changing between different samples. However, the signal intensity obtained by this setup is too low to measure spin waves under the given circumstances in reasonable time.

Weyl semimetals feature linear band crossings with non-trivial topological properties. These (Weyl) points can have a strong impact on the transport response, when they are located close to the Fermi level. The Weyl points can lead to, among other things, a large anomalous Hall effect. Co3Sn2S2 is a prototypical candidate of this material class and displays intriguing physics, originating from its topological as well as magnetic properties. Many interesting transport responses, including the anomalous Hall effect and the anomalous Nernst effect have been found to be realised in this compound. Previous experiments have been carried out exclusively in (bulk) single crystals of Co3Sn2S2. It is demonstrated in this thesis, that microsturctures with well defined contacts can fabricated from bulk single crystals using focused ion beam cutting. Laser lithography is employed to define the electrical contacts to the micro ribbons. The microstructured devices are used to study the evolution of the magnetoresistance and magnetothermopower with magnetic field and temperature. The magnetoelectric transport response in Co3Sn2S2 is addressed in the first part of this thesis. Co3Sn2S2 exhibits a particularly large anomalous Hall effect in the transverse magnetoresistance. The anomalous Hall conductivity is found to be independent of the longitudinal conductivity, suggesting an intrinsic origin of the anomalous Hall effect. This notion is further corroborated by comparing the experimental results with band structure calculations, using the Berry curvature. Furthermore, the agreement between measurements on the microstructures with measurements on single crystals demonstrates the high quality of the microstructured devices. Additionally to the transverse magnetoresistance also the longitudinal magnetoresistance of Co3Sn2S2 is studied. The magnetoresistance decreases with increasing magnetic field for temperatures above 100 K. Futhermore, the magnetoresistance is found to be highly anisotropic with respect to the direction of the magnetic field. The negative magnetoresistance can be consistently explained invoking the magnon magnetoresistance. This effect is present in the ferromagnetic as well as the paramagnetic phase and is the dominant magnetoresistance effect above 100 K. The remarkably strong magneto-crystalline anisotropy of Co3Sn2S2 along its crystalline c-axis leads to the interesting observation, that the magnon magnetoresistance appears to depend on the projection of the external magnetic field on the magnetization. At low temperatures, magnon modes are frozen out and the magnon magnetoresistance vanishes. However, the anisotropy of the carrier mobilities gives rise to an anisotropic orbital magnetoresistance. Interestingly, no clear signs of crystalline/non-crystalline anisotropic magnetoresistance, depending on the magnetization direction, can be observed. The salient features in the magnetoelectric response can be understood through the comprehensive investigations reported in this thesis. In the second part, the thermal counterparts of the magnetoelectric transport responses are investigated. Again, a surprisingly large transverse (Nernst) effect is found. The Nernst conductivity reaches up to 4 A/(m K) in Co3Sn2S2. Unlike their magnetoelectric counterparts, the Nernst coefficient and the Nernst conductivity exhibit a sign change as a function of temperature. These experimental results strengthen the theoretical framework, that the anomalous Hall and Nernst effect probe different states. Furthermore, an additional contribution at the magnetic phase transition can be identified using the Mott relation. This contribution indicates that the anomalous Nernst conductivity can be enhanced by magnetic fluctuations near the magnetic ordering temperature. It appears to be a generic contribution in ferromagnetic materials and it is so far not accounted for by this particular formulation of the Mott relation. This subtle feature is uncovered when comparing the large magnetoelectric and magnetothermal transport response in the magnetic Weyl semimetal Co3Sn2S2.

In this thesis, the ferromagnetic metal SrRuO3 and the frustrated antiferromagnet Cs3Fe2Br9 are investigated. New high-quality single crystals of SrRuO3 grown with the floating-zone technique are used to study the critical phenomena at the ferromagnetic phase transition. A shape-memory effect is found below TC, which makes the material ferroelastic and thus multiferroic. Spin-orbit coupling yields spin-split bands and a Weyl point close to the Fermi energy acts as a magnetic monopole in momentum space, which causes an intrinsic anomalous Hall effect. Furthermore, the anomalous softening of the magnon gap and the stiffness can be described with the Hall conductivity, which makes the explanation via the Berry-curvature contribution conclusive. The new material Cs3Fe2Br9 consists of face-sharing Fe2Br9 bi-octahedra that are arranged in triangular layers stacked to form a hexagonal structure and orders antiferromagnetically. In contrast to isostructural classical spin systems that form a singlet ground state, the two S = 5/2 moments in each bi-octahedron are oriented parallel along c. The in-plane nearest-neighbor interaction overcompensates the antiferromagnetic dimer coupling. The antiferromagnetic ground state is substantially frustrated. For a magnetic field along the easy-axis, an extremely rich phase diagram with ten phases is found. Two phases host a fractional magnetization and one phase shows a ferroelectric polarization. The results are compared to analytical and numerical predictions for triangular and hexagonal lattices and related materials

In antiferromagnets, the interplay of spin frustration and spin-lattice coupling has been extensively studied as the source of complex spin patterns and exotic magnetism. Here, we demonstrate that, although neglected in the past, the spin-lattice coupling is essential to ferrimagnetic spinels as well. We performed ultrahigh-field magnetization measurements up to 110 T on a Yafet-Kittel ferrimagnetic spinel, MnCr2S4, which was complemented by measurements of magnetostriction and sound velocities up to 60 T. Classical Monte Carlo calculations were performed to identify the complex high-field spin structures. Our minimal model incorporating spin-lattice coupling accounts for the experimental results and corroborates the complete phase diagram, including two new high-field phase transitions at 75 and 85 T.Magnetoelastic coupling induces striking effects: An extremely robust magnetization plateau is embedded between two unconventional spin-asymmetric phases. Ferrimagnetic spinels provide a new platform to study asymmetric and multiferroic phases stabilized by spin-lattice coupling.

We performed temperature-dependent infrared transmittance and reflectance measurements on topological insulators and spin-orbit-entangled Mott insulators. Both material classes exhibit strong spin-orbit coupling which gives rise to topologically protected phases. Potential applications are hindered, due to various deviations of the synthesized crystals from the ideal topological insulator or spin-orbit-entangled Mott insulators, respectively. In this thesis several promising candidates for the realization of either models are investigated. For topological insulators the bulk conductivity poses a problem. We report on thicknessdependent data on thin films of Bi2Te3 which show that the bulk optical conductivity σ1(ω) superimposes the surface conductance. Therefore, we continue the search for signatures of the surface state on compensated (Bi0.57Sb0.43)2Te3, for which we report on a very low σ1(ω), but we find no direct evidence for the topologically protected surface state. The compensation of topological insulators can be performed more sophisticated by partly substituting Te with Se. Thereby, the family of topological insulators Bi2-xSbxTe3-ySey with insulating behavior is created. Nevertheless, compensated topological insulators have a reduced activation energy compared to the intrinsic gap, which is usually attributed to impurity band. Lately, the formation of charge puddles in compensated topological insulators, due to the random distribution of defects, thus reducing the activation energy, was predicted. We report on the experimental verification of charge puddles and reveal the highly non-monotonic temperature dependence of them. Additionally, we describe, in collaboration with the group of Prof. Rosch of the Institute for Theoretical Physics at the University of Cologne, the observations semiquantitatively by Monte Carlo simulations. Thereby, we gain insight into, e.g., the defect density and the level of compensation depending on the composition which allows to choose the composition best suited the each purpose. One of the thrilling phenomena in Kitaev physics is the occurrence of a quantum spin liquid in the ground state. However, finite Heisenberg exchange due to a trigonal crystal field or finite mixing of the t2g and eg states, causes deviations from pure j=1/2 states, which results in a zigzag-ordered ground state instead of the sought-after quantum spin liquid. Due to these deviations even the formation of quasi-molecular orbitals instead of local j = 1=2 states was suggested for RuCl3 and Na2IrO3. We investigate σ1(ω) and identify different excitations, thereby gaining insight into the underlying physics. In α-RuCl3 we reveal the single, double, and triple spin-orbit exciton. Hence, local j=1/2 states are formed in this compound. For Na2IrO3 precise predictions on the optical spectrum depending on the underlying physics allow us to conclude from σ1(ω) that the local j=1/2 scenario is appropriate. Furthermore, we report on signatures of a controversially discussed peak obtained by RIXS, in the optical spectrum including magnon sidebands.

When sizable quantum fluctuations and strong frustration mechanisms act in concert to repel the formation of conventional long-range order in quantum magnets, they can make way for massively entangled spin liquid phases which may imbue the material with extraordinary properties. The search for such curious phases of matter has proceeded for several decades, gaining extra momentum some fifteen years ago when Kitaev proposed an analytically solvable model for a quantum spin liquid with anyonic excitations on the basis of realistic microscopic spin exchange terms [Kitaev, Annals of Physics 321, 2 (2006)]. Attempts to identify different models or materials which harbor quantum spin liquid ground states have kept researchers -- experimentalists and theorists alike -- in suspense ever since. However, the simulation of quantum many-body systems poses a serious challenge even to modern numerical techniques, particularly in the case of frustrated quantum magnetism in three spatial dimensions. Such models evade tractability by many established approaches, leaving a methodological void. In this thesis, we report on recent progress in cutting-edge implementations of the pseudo-fermion functional renormalization group (pf-FRG), which has originally been proposed by Reuther and Wölfle in the context of two-dimensional frustrated quantum magnetism with highly symmetric spin interactions [Reuther and Wölfle, Phys. Rev. B 81, 144410 (2010)]. Reflecting the growing interest in models with SU(2) symmetry-breaking spin exchange terms that has followed the unearthing of Kitaev's honeycomb model, we present a generalized implementation of the pf-FRG which is suited to numerically simulate arbitrary microscopic models with diagonal or off-diagonal two-spin interactions, even in three-dimensional frustrated quantum magnets. We provide insight into the inner workings of the method which has emerged over the course of the last couple of years, arguing that the pf-FRG formalism simultaneously combines aspects of a large-S expansion as well as a large-N expansion on equal footing, thus being able to resolve the subtle interplay between magnetic ordering tendencies and disruptive quantum fluctuations. Moreover, on a case by case basis we explore the stability of quantum spin liquids in paradigmatic models of frustrated quantum magnetism and elucidate the joint action of geometric frustration, exchange frustration, and quantum fluctuations to inhibit the formation of magnetic long-range order. Examples include: (i) a Heisenberg spin model on the three-dimensional diamond lattice where geometric frustration arises from antiferromagnetic next-nearest neighbor interactions. The additional competition with nearest neighbor interactions leads to an unusually large ground state degeneracy already on classical level. The quantum-to-classical transition is studied by systematically varying the spin length and the different roles of quantum fluctuations and thermal fluctuations are discussed. The theory is applied to interpret experiments on the spin-liquid candidate NiRh2O4; (ii) a Heisenberg antiferromagnet on the face centered cubic lattice which is augmented by competing bond-directional interactions, evolving around the concurrent manifestation of geometric frustration and exchange frustration. A situation is identified where both mechanisms collude to give rise to an unusually large degree of frustration over a wide parameter regime. We discuss related experimental findings on the highly frustrated iridate compound Ba2CeIrO6; (iii) a Heisenberg antiferromagnet on the kagome lattice with additional Dzyaloshinskii-Moriya interactions. The robustness of the unperturbed kagome antiferromagnet's spin liquid ground state against low-symmetry Dzyaloshinskii-Moriya interactions is investigated. Implications for the spin-liquid candidate herbertsmithite are discussed.

With quantum information processing in mind, homodyne optical amplification tremendously enhances the sensitivity of spin noise spectroscopy applied to delicate few- and single-spin systems. In realizing the allied experimental techniques, the authors are able to boost the low-frequency spin noise signal of a few remaining impurities in an isotopically enriched Rb vapor above the electronic noise level dictated by the photodetector system. This proof-of-principle experiment advances semiconductor spin research and facilitates higher-order spin noise measurements on semiconductor qubits, such as single (In,Ga)As quantum dots.

Once more, neutron scattering using large high-quality single crystals is proven to be a powerful technique to study magnetism on a microscopic level. Several layered Ruthenate single-crystals could be grown in better quality or larger dimensions than the existing crystals at the starting of this work, enabling the neutron scattering studies [10, 35, 25, 36, 26]. Thus important progress in the research of layered Ruthenates could be achieved: The incommensurable magnetic fluctuations in Sr2RuO4 , which are induced by the Fermi-surface nesting of the quasi one-dimensional bands, were studied in the superconducting and non-superconducting state up to very low energy transfers, far below the expected cooper-pair binding energies. Therefore, it is strongly suggested that the quasi one-dimensional bands are not the active bands driving the superconductivity, giving an important contribution to the question of the symmetry of the superconducting order parameter. The magnon dispersion of Ca2RuO4 was determined throughout the whole first Brillouin zone giving an important contribution to the discussion of the ground state of this compound and revealing the enhanced strength of spin-orbit coupling in that 4d system. The oxygen octahedron rotation of SrRuO3 was determined with high precision very efficiently. Thus, it became possible to shed light on the six-fold twinning of this compound and the changing twinning fraction induced by magnetic fields. A reorientation of the crystal structure in the magnetic phase induced by magnetic fields and a memory effect of the reorientation, which goes in hand with a magnetic shape-memory effect, was discovered in that oxide. The spin density distribution in the unit cell of SrRuO3 could be reconstructed using polarized neutron diffraction data. A spin-density wave has been confirmed in Ca0.5Sr1.5RuO4 . The propagation vector amounts to (0.305,0.305,1) and the spins freeze in like in spin- or cluster- glasses becoming static at about 4K [10].

Different representatives of the group of quasi-one-dimensional spin-chain compounds Ba(1-x)Sr(x)(Co(1-y)M(y))2V2O8 with M= Mn, Cu, Ni, Mg and (Ba,Sr)Mn2V2O8 were synthesized, characterized and their magnetic phase diagrams determined for H||c, H||a and H||[110]. The tetragonal structure of these compounds consists of screw chains of edge-sharing MO6 octahedra, where depending on the transition metal M different spin values are realized. The chains are separated by nonmagnetic Ba or Sr and V ions. Large single crystals were grown via the floating-zone method in a mirror furnace employing the twice scanning method. The single crystal studies of the S=5/2 Heisenberg spin chains BaMn2V2O8 (T_N = 37 K) and SrMn2V2O8 (T_N = 42 K) reveal a weak positive field dependence. BaMn2V2O8 shows a spin-flop transition at H^(perpendicular c} = 300 mT as is observed in the susceptibility and the magnetostriction. Deduced from this data a magnetic structure is suggested similar to the magnetic structure of SrMn2V2O8 which reveals ferromagnetic planes (spins along (+-)a) that are ordered antiferromagnetically to each other along the chain direction c [100]. BaCo2V2O8 is a quasi-one dimensional effective S=1/2 Ising chain where the antiferromagnetic Ising chains along c have a frustrated inter-chain arrangement leading to the presence of two magnetic domains. Below T_N = 5.4 K the system undergoes a structural distortion as confirmed by zero-field high-resolution thermal-expansion. Via magnetoelastic coupling the structural domains couple to the magnetic domains and can be manipulated by uniaxial pressure or magnetic fields along a. The expected distortion of the order of 10^(-5) could hardly be detected in high-resolution single crystal X-ray diffraction. For H||c, the Néel ordered state breaks down at H_c= 3.9 T where an incommensurate (IC) phase is entered in the low-T region [24,110]. A first order transition is confirmed by a hysteresis in the low-T magnetostriction. Thermal expansion yields very broad anomalies in the low-T region of the high-field curves and the IC-to-paramagnetic phase boundary was traced up to 17 T. For H perpendicular c, an in-plane anisotropy was found with a weak field dependence of T_N for H||[110], but a complete suppression of T_N at H^(||a} = 10 T. Kimura et al. developed a model where a magnetic field H||a is accompanied by a staggered transverse field component via a staggered g tensor [96]. The predicted breakdown of the magnetic order for H||a is evidenced in magnetostriction and the collapse of magnetic superstructure peaks in single-crystal neutron diffraction. The field-induced detwinning of the structural/magnetic domains is clearly observed in the different field hysterses of magnetic Bragg peaks being sensitive to different domains. Moreover, the magnetic field induces an increasing intensity for nuclear reflections due to a uniform ferromagnetic component parallel to to the external field. In addition, the presence of forbidden reflections and their field-dependent behavior are explained by the presence of the staggered moments giving a qualitative confirmation of the Kimura model. Different measures (thermal conductivity, heat capacity, neutron diffraction) show additional anomalies around 6 T whose origin remains unknown. The investigation of out-of-chain substitutions in the Ba(1-x)Sr(x)Co2V2O8 series and of in-chain substitutions in the BaCo1.9M0.1V2O8 series (M= Mn, Ni, Cu, Mg) yields a preservation of the orthorhombic distortion (below T_N) and of both the Ising and in-plane anisotropy. Judging from these experimental results the magnetic structure is essentially preserved for all substitutions which was recently confirmed for SrCo2V2O8 by neutron diffraction [100]. Only indirect hints for a transition from the \neel ordered into an IC phase for H||c are given by hysteretic magnetostriction curves in the low-T regime. The Sr substitution yields a slight decline of the Ising anisotropy but an increase of the in-plane anisotropy as is reflected in the magnetic phase diagrams. These changes in the anisotropies can be understood by a systematic increase of the distortion of the CoO6 octahedra with increasing Sr content. Due to the decreasing inter-chain distance with increasing Sr content a change of J^(perp)_(NN) and J^(perp)_(NNN) is suggested in a way that the resulting frustration increases, which in turn results in a weakened effective inter-chain interaction and a weakly decreasing T_N with increasing Sr content. The substitution of, e.g., Cu and Mg yields a drastic reduction of the ordering temperature. Due to the strong suppression of T_N by Mg in contrast to, e.g., Ni the NN intra-chain interaction is suggested to be the dominant coupling mechanism of the finite chain segments.

The physics of heavy-fermion materials is governed by the hybridization between itinerant conduction electrons and localized f electrons (c-f hybridization). At low temperatures the Kondo effect leads to the emergence of heavy quasiparticles with a huge effective mass that can be up to 1000 times larger than the bare electron mass, giving the class of heavy fermions their name. The screening of the localized magnetic moments by the conduction electrons due to the Kondo effect competes with the indirect Ruderman-Kittel-Kasuya-Yoshida exchange, which also has its origin in the hybridization interaction and wants to drive the system into a magnetically ordered ground state. Heavy fermions are regarded as dense Kondo lattice systems, in which the balance between the two interactions is of crucial importance for the ground-state properties. Valence fluctuations, Kondo insulating and semiconducting behavior, spin and charge gaps, unconventional superconductivity, quantum criticality and non-Fermi liquid behavior are observed in these materials, with remarkable consequences for magnetic, thermodynamic and electronic properties at low temperatures. ----- This thesis is mainly focused on the class of Kondo insulators, which also comprises Kondo semiconductors and semimetals. When the right conditions are met -- i.e. for a certain number of electrons per unit cell and for certain symmetries of the electronic structure -- the interaction between the conduction and f electrons opens a narrow hybridization gap close to the Fermi level. The (anisotropic) hybridization gap is found to be responsible for many anomalous properties of Kondo insulators and several theoretical studies point out the importance of the crystalline electric field (CEF) ground-state symmetry for the gap formation. ----- Here we investigate the Kondo semiconductor CeNiSn, as well as the CeM2Al10 compound family with M = Ru, Os and Fe. CeNiSn has an orthorhombic crystal structure, is one of the first narrow-gap Kondo semiconductors and has been intensively studied in the past. Many explanations for the low-temperature behavior have been suggested, all of which stress the significance of the 4f CEF ground state, which will be investigated in the framework of this thesis. The CeM2Al10 compounds belong to a fairly new class of cerium Kondo semiconductors. They also crystallize in an orthorhombic structure and order antiferromagnetically in the case of M = Ru and Os with the ordered moments being small and not aligned along the easy magnetic axis. In combination with the unexpectedly high ordering temperatures of T_N = 27K (Ru) and 29K (Os), the magnetic order is of rather unusual character. The physical properties show large anisotropies and presence of c-f hybridization despite the high ordering temperatures. We therefore determine the CEF wave functions in this compound family and quantify the degree of c-f hybridization in order to address speculations about the impact of hybridization on the magnetic order. In addition, on the search for parameters which correlate with ground-state properties in heavy-fermion compounds, we investigate the CEF ground states of the intermetallic substitution series CeRh(1-x)Ir(x)In5 since its phase diagram covers all phases of interest, from antiferromagnetic to superconducting as well as regions of phase coexistence. ----- To shed light on the issues above, namely the CEF ground state and the hybridization strength, respectively, two new experimental approaches are used: polarization-dependent x-ray absorption spectroscopy (XAS) and hard x-ray photoelectron spectroscopy (HAXPES). Commonly, the CEF is investigated by inelastic neutron scattering, however, in Kondo insulators a substantial broadening of the CEF excitations and the existence of spin gaps prevent the access to the CEF. Recently, linearly polarized XAS at the Ce M_(4,5) edge has been proven to be highly useful when it comes to the determination of the 4f ground-state wave function in tetragonal rare earth systems (and also in cubic compounds under the influence of an external magnetic field). In the present thesis the same technique is applied to the above-mentioned materials, demonstrating that linearly polarized XAS can be employed to obtain an unambiguous and reliable picture of the CEF ground state even in Kondo-insulating systems and even for symmetries lower than tetragonal. In addition to that, HAXPES measurements on the CeM2Al10 series are presented. A common technique for studying hybridization effects in rare earths, and their electronic structure in general, is photoelectron spectroscopy in the soft x-ray range (hv

The topic of this Ph.D. thesis is the magneto-optical investigation of the electrically-induced spin transport and of the laser-induced spin Seebeck transport in n-GaAs. This work is part of the research project "Spin Caloritronics in III-V-Semiconductors" of the DFG priority programm "Spin Caloric Transport" (SPP 1538).Measurements are performed by spatially and time-resolved Faraday spectroscopy. Here, a local spin polarisation is generated by ultrafast laser pulses and detected via the magneto-optical Faraday effect with variable time delay. The material system under investigation, i.e. n-GaAs doped to the metal-to-insulator transition at $n = 2 \cdot 10^{16}$ cm$^{-3}$, is optimally suited for spin transport experiments due to long spin dephasing times $\tau_s \geq 100$ ns at low temperatures, which result in large spin transport length of $\sim 10\,\mu$m.The measurement and analysis method used in this work allows to study the temporal evolution of the optically generated spin polarisation by Fourier transformation of spatially resolved magnetic field scans (RSA scans) with very high spatial resolution and to determine the spin drift velocity $v_s$, the spin diffusion constant $D_s$ and the spin dephasing time $\tau_s$.Spin transport in an applied electric field shows Ohmic transport behaviour $v_s = - \mu_s E$ with spin mobility $\mu_s = (1159 \pm 4)$ cm V$^{-1}$ s$^{-1}$ at $T = 10$ K. Spin transport measurements in dependence of the excitation energy and of the excitation density indicate that, under intense excitation at low temperatures, spin drift and diffusion are influenced by a strong electron-hole interaction. The temperature dependence of the electrically-induced spin transport shows the first experimental verification of the generalized Einstein relation for semiconductors at the metal-to-insulator transition at low temperatures.Spin Seebeck transport in n-GaAs is investigated with laser-induced temperature gradients, which are characterized by photoluminescence measurements. While the spin diffusion constant $D_s$ and the spin dephasing time $\tau_s$ seem to be dominantly influenced by the large, optically excited electron and hole density, the spin drift velocity $v_s$ shows the expected behaviour for spin Seebeck transport in laser-induced temperature gradients in all the measurements. Particularly, the simultaneous vanishing of $v_s$ and of the temperature gradient $\frac{dT}{dx}$ at a lattice temperature of $T = 30$ K confirms the conclusion that these experiments show the first evidence of lateral spin Seebeck transport in a non-magnetic semiconductor.

A normal metallic film sandwiched between two insulators may have strong spin-orbit coupling near the metal-insulator interfaces, even if spin-orbit coupling is negligible in the bulk of the film. In this paper, we study two technologically important and deeply interconnected effects that arise from interfacial spin-orbit coupling in metallic films. The first is the spin Hall effect, whereby a charge current in the plane of the film is partially converted into an orthogonal spin current in the same plane. The second is the Edelstein effect, in which a charge current produces an in-plane, transverse spin polarization. At variance with strictly two-dimensional Rashba systems, we find that the spin Hall conductivity has a finite value even if spin-orbit interaction with impurities is neglected and ``vertex corrections'' are properly taken into account. Even more remarkably, such a finite value becomes ``universal'' in a certain configuration. This is a direct consequence of the spatial dependence of spin-orbit coupling on the third dimension, perpendicular to the plane of the film. The nonvanishing spin Hall conductivity has a profound influence on the Edelstein effect, which we show to consist of two terms, the first with the standard form valid in a strictly two-dimensional Rashba system, and a second arising from the presence of the third dimension. Whereas the standard term is proportional to the momentum relaxation time, the new one scales with the spin relaxation time. Our results, although derived in a specific model, should be valid rather generally, whenever a spatially dependent Rashba spin-orbit coupling is present and the electron motion is not strictly two dimensional.

Spintronic devices hold great potential for future applilcations in information technology. In this work, the design and fabrication of a lateral spin valve for investigating the spin diffusion length of paramagnetic Fe60Al40 will be discussed. Samples were prepared by defining polycrystalline circuits using electron beam lithography, thin film deposition, and lift-off processing. The developed sample layout allows for both local and non-local measurement routines. Cobalt was employed as ferromagnetic polarizing material, while aluminium holds for reference measurements of the spin diffusion length. The determination of the so far unknown spin diffusion length of paramagnetic Fe60Al40 shall then be performed by detecting resistance changes at different temperatures and for different polarizer distances. Highly sensitive electric transport measurements were carried out in a so-called probe station, which allows for achieving very low temperatures via liquid He cooling. Additionally, paramagnetic ferromagnetic heterostructures were investigated with respect to possible giant magneto resistance effects.

Considering metallic films at room temperature, we present the first theoretical study of the spin Nernst and thermal Edelstein effects which takes into account dynamical spin-orbit coupling, i.e., direct spin-orbit coupling with the vibrating lattice (phonons) and impurities. This gives rise to two novel processes, namely a dynamical Elliott-Yafet spin relaxation and a dynamical side-jump mechanism. Both are the high-temperature counterparts of the well-known $T = 0$ Elliott-Yafet and side-jump, central to the current understanding of the spin Hall, spin Nernst and Edelstein effects at low $T$. We consider the experimentally relevant regime $T > T_D$, with $T_D$ the Debye temperature, as the latter is lower than room temperature in transition metals such as Pt, Au and Ta typically employed in spin injection/extraction experiments. We show that the interplay between intrinsic (Bychkov-Rashba type) and extrinsic (dynamical) spin-orbit coupling yields a nonlinear $T$- dependence of the spin Nernst and spin Hall conductivities., 9 pages, 4 figures

Within the scope of this thesis different transition-metal oxides with open d shells are investigated by means of spectroscopic ellipsometry in the energy range from 0.75eV to 5.5eV for temperatures ranging from 15K to 490K. The focus is on spin and orbital degrees of freedom and their impact on the optical spectra. The multipeak structures observed in the optical conductivity show a pronounced dependence on both temperature and polarization. We analyze the spectra in terms of multiplets which form the upper Hubbard band. Our optical analysis of the multi-orbital Mott-Hubbard insulators YVO3, GdVO3, and CeVO3 yields a consistent description of the observed absorption bands in terms of 3d3 excited states which constitute the upper Hubbard band and thereby solves the discrepancies of the optical spectra of YVO3 reported in the literature. The temperature and polarization dependence of the optical spectra reflects the complex spin and orbital ordering phase diagram of RVO3 (R=Y, rare earth ion). A comparison of our data with theoretical predictions based on either rigid orbital order or strong orbital fluctuations leads us to the conclusion that orbital fluctuations cannot be strong in RVO3. The line shape and temperature dependence of a feature observed in the optical conductivity at around 2eV gives evidence for an excitonic Mott-Hubbard resonance, i.e. not a truly bound state below the gap but a resonance within the absorption band, and demonstrates the important role played by the kinetic energy for exciton formation in orbitally ordered Mott-Hubbard insulators. Due to the layered structure of the correlated insulator LaSrFeO4 the optical spectra strongly depend on polarization. This anisotropy in combination with their different spectral weights offers an efficient tool to disentangle Mott-Hubbard excitations, corresponding to an electron transfer between neighboring Fe(3+) sites, and charge-transfer excitations, corresponding to an electron transfer from the oxygen 2p band to the Fe 3d band. We arrive at a consistent peak assignment and find that the lowest dipole-allowed excitation, which contributes to the in-plane optical conductivity only, is of Mott-Hubbard type. This result is rather unexpected at first sight as the 3d5 electron configuration of Fe(3+) is particularly stable due to the intra-atomic Hund exchange. We argue that the Fe 3d - O 2p hybridization and particularly the large splitting of the eg level originating from the tetragonal structure justify our result. The temperature dependence of the Mott-Hubbard excitations is only weak. This finding is in accordance with the fact that variations of nearest-neighbor spin-spin and orbital-orbital correlations are not strong below room temperature in LaSrFeO4 with a Néel temperature of 366K. A further concern of this work lies on the correlated insulators LaCoO3 and EuCoO3 with Co(3+) 3d6 electron configuration, which have attracted a lot of interest because of the spin-state degree of freedom. The low-spin state, the intermediate-spin state, and the high-spin state, lie energetically close in these pseudocubic perovskites. It is well established that a thermal population of the high-spin state takes place from the low-spin ground state at T>25K in LaCoO3. This so-called spin-state transition is shifted to much higher temperatures in the sister compound EuCoO3. In addition, we present optical data of the single-layered perovskites La(2-x)SrxCoO4 (x=0, 0.33, 0.45, 0.5, 0.9) and La1.5Ca0.5CoO4, which contain both Co(3+) 3d6 and Co(2+) 3d7 ions. In agreement with claims of the literature for a doping-induced spin-state transition from a Co(3+) low-spin state being realized in La(2-x)Sr(x)CoO4 for doping concentrations x

This thesis deals with thin films of the ferromagnetic semiconductor EuO. These are prepared by molecular beam epitaxy under ultra high vacuum conditions and their properties are investigated in-situ by means of low energy electron diffraction, magneto-optical Kerr effect (MOKE), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The experiments are carried out in a low temperature STM apparatus with a base temperature of 5.3K. The development of a highly sensitive MOKE instrument is in the technical focus of this work. EuO is grown in (100) orientation on a ferromagnetic Ni(100) single crystal as well as on single crystalline graphene on Ir(111). An antiferromagnetic coupling between the magnetic moments of the Ni(100) and the EuO(100) film turns out. The 3.3nm thin stoichiometric films of EuO on graphene show a Curie temperature Tc=75K, which is increased with respect to films of the same thickness grown on other substrates and even with respect to bulk EuO. Point defects in EuO films are analyzed with help of atomically resolved STM images. The defect concentration can be quantified using highly-resolved STM topography images. The mobility of the oxygen vacancies is shown by a series of topography STM images and a mobility of the vacancies along the EuO directions is found. Investigations of the electronic structure of the EuO(100) surface are performed by STM and STS, which require a high sample quality, i.e, a topographically very well ordered and flat surface with a minimum of defects. The ultra thin films of EuO on graphene described in thesis are of such high quality. A local n-doping and an upward band bending is found at the oxygen vacancy sites. Tunneling spectroscopy maps of the ferromagnetic EuO(100) surface show standing wave patterns, which are the characteristic fingerprint of the surface state in EuO(100). The energy vs. momentum space of the surface state is explored by evaluating the surface state scattering patterns. A dispersion relation determined this way of the surface state is free electron like, with an effective mass of the mass of a free electron. The same measurements are performed in the paramagnetic phase of EuO at 81K. Thereby the surface state is shifted in energy. The shift indicates indirectly the spin degeneracy of the surface state in the paramagnetic phase, which is lifted in the ferromagnetic phase of EuO. This behavior is analogous to the well known spin splitting of the EuO conduction band. The spin splitting of the surface state (measured with this work) amounts to 0.66eV.

In this thesis ground state properties of S = 1 spin chains with exchange and single-ion anisotropy are analysed. Following earlier publications two cases are mainly considered: A fixed ratio of the two anisotropies as well as a fixed and strong exchange anisotropy. The method primarily employed is the density matrix renormalization group (DMRG) for a finite chain length and in the thermodynamic limit (iDMRG). The phase diagrams of the quantum spin chain and the corresponding classical model are compared for the first time. Thereby the similarity of the phase diagrams, the correspondence of the phases and the striking differences are elucidated. The various phases of the quantum model include antiferromagnetic, ferromagnetic, spin liquid and supersolid phases. Their properties are discussed in detail. Analysing the correlation functions and using a correspondence to Luttinger liquids one can distinguish commensurate and incommensurate regions of the spin liquid phase. For different parameters the spin liquid phase may be subdivided into regions of ferroquadrupolar and spin-density-wave ordering. Some of the quantum phase transitions between the various phases are studied. Particularly the transition between the spin liquid and the supersolid phase is identified to be in the two-dimensional Ising universality class.

In der vorliegenden Arbeit werden inhomogene Ladungs- und Spinverteilungen, sog. Streifenstrukturen, als Grundzustandseigenschaften von dotieterten Einband-Hubbard-Modellen untersucht. Experimentell wurden Streifenstrukturen in dotierten Kuprat- Supraleitern nachgewiesen. Das Hubbard-Modell wird anhand von Gutzwiller-Wellenfunktionen untersucht. Der ursprügliche Gutzwiller-Ansatz wurde dabei um zwei äußere Variationsfelder erweitert, welche die Ausprägung von Ladungs- und Spininhomgenitäten bewerkstelligen. Um methodische Probleme in 2D-Rechnungen zu umgehen, führen wir unsere Berechnungen in einer Dimension unter Verwendung von Superzellen durch. Systeme mit ausschließlich Nächstem-Nachbar-Hopping zeigen bei Dotierungen 0, In the present work we study inhomogenous charge and spin distributions, so called stripes, as groundstate properties of doped one band Hubbard models. There is experimental evidence for stripes in doped cuprate-superconductors. Thereby, the Hubbard model is investigated by using Gutzwiller-type wave functions. In order to allow for inhomogenous charge and spin distributions, the original Gutzwiller Ansatz has been extended by two atomic fields per sites which are treated as further variational parameters. To avoid numerical problems as seen in two dimensional supercell calculations, we have performed calculations with one dimensional supercells. Systems with only nearest neighbour hopping show stripes with a characteristic periodicity n=1/d for a given doping level 0

Electrical injection and detection of spin-polarized electrons is demonstrated for the first time in a single wafer, all-semiconductor, GaAs-based lateral spintronic device, employing p+-(Ga,Mn)As/n+-GaAs ferromagnetic Esaki diodes as spin aligning contacts. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunnelling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the relatively high value of 50%., 11 pages, 3 figures, sent to PRL

The present paper studies the dependance of F-centre diffusion on the Ca++ impurity concentration in KCl crystals in the temperature range 300 - 400°C. An expression is obtained for the dependance of the F-centre diffusion coefficient on impurity concentration.

Heavy-fermion systems (HFS, several lanthanide or actinide compounds) exhibit a linear coefficient of the low-temperature specific heat that is much larger than in usual metals, and this enhancement is usually attributed to quasiparticles with a very large effective mass. The essential features of HFS are based on the interplay of localised, strongly correlated f electrons with broad conduction bands as described by the periodic Anderson model (PAM). We investigate the temperature dependence of the transport properties of HFS (e.g. resistivity, thermoelectric power) in the linear-response regime (using Kubo formulae and the Jonson-Mahan theorem).In the dynamical mean-field theory (DMFT), the PAM is mapped on an effective single-impurity model. As impurity solvers we use both Wilson's numerical renormalization group and the modified perturbation theory. In addition, we explore the influence of substitutional disorder on the transport properties of HFS by an extension of the DMFT to a coherent-potential approximation for disordered strongly-correlated electron systems (PAM with random potentials). The typical experimental results are reproduced, in particular the temperature and concentration dependence of the resistivity and the thermoelectric power and their absolute magnitude for both metallic HFS and Kondo insulators.Finally, we examine four different estimates for possible characteristic low-temperature scales of the PAM without disorder. The results indicate that there is only one relevant scale; we get a one-parameter scaling of thermodynamic and some transport properties with a strongly occupancy-dependent scaling function.Also published by Mensch-und-Buch-Verlag Berlin 2008, ISBN 978-3-86664-504-2.See also corresponding articles:DOI 10.1103/PhysRevB.74.195119DOI 10.1103/PhysRevB.77.115125

This work provides a detailed investigation of the magnetism of homonuclear and exchange-coupled transition metal compounds as well as lanthanide and actinide systems. The measurement of the magnetic susceptibility was performed by means of a SQUID magnetometer in the temperature range between 1.7,K and 300,K under consideration of an optimized calibration algorithm for molecular compounds. The computer program CONDON enables the modelling of magnetic properties which are controlled by single-ion effects (interelectronic repulsion, spin-orbit coupling, ligand-field effect) and interatomic exchange interactions (Heisenberg and molecular-field model). The electronic perturbations are considered simultaneously by CONDON using of the full basis of microstates, whereby the magnetically relevant parameters are optimized. In summary, the computer program CONDON can be regarded as an excellent tool for the clarification of the magnetic properties of a plethora of metal complexes. This thesis demonstrates, that CONDON not only elucidates the magnetic behaviour of the compounds, but also predicts the appropriate measurement conditions for temperature and applied field strength.

Es wird eine Abschätzung der Operatornorm des Vektors der Riesz-Transformationen auf Heisenberg-Typ-Gruppen gegeben.

We consider the production of spatially separated entangled electrons in transport between a superconductor and ferromagnets: Spin singlet Cooper pairs are filtered into different leads by spin dependent tunneling. We find the full counting statistics depending on the magnetizations and identify the probabilities for productions of entangled electrons taking into account the superconducting proximity effect. Positive contribution to cross correlations are due to coherent transport of two electrons from the superconductor and negative contribution caused by Fermi statistics. We finally show the conditions for a violation of a Bell inequality as experimental test of entanglement.

The basic element for innovative sensors and memory devices is the magnetic tunnel junction. It consists of two ferromagnetic layers. These are separated by a thin insulating alumina layer. The resistance of the insulator (the tunneling barrier) depends on the relative orientation of the magnetization of the adjacent ferromagnetic layers. The change in resistance is called tunneling magnetoresistance (TMR) effect. In a simple approximation the TMR effect depends on the spin polarization of the ferromagnetic layers. Using half-metals - these are fully spin polarized - one can theoretically expect an infinite TMR effect. The preparation of junctions (this includes the deposition of the layers, oxidation of an aluminum layer to the insulating alumina barrier and tempering of the stack) with ideal layers and interfaces is often impossible. Thus, in actual tunnel junctions the TMR effect scales with the quality of the electrodes, the barrier and the interface between these layers. This thesis deals with the optimization of tunnel junctions with half-metallic electrodes, e.g. the ferromagnetic oxide magnetite (Fe3O4) and the Heusler alloys Co2MnSi and Co2FeSi. The main topic is a systematic characterization of the magnetic, chemical and electronic structure of the interface between electrode and insulator., Grundbaustein für neuartige Sensoren und Datenspeicher ist das magnetische Tunnelelement. Dieses besteht im Wesentlichen aus zwei ferromagnetischen Elektroden, die durch eine dünne Aluminiumoxid-Isolatorschicht voneinander getrennt sind. Der Widerstand des Isolators (der so genannten Tunnelbarriere) hängt dabei von der relativen Orientierung der Magnetisierung der beiden Ferromagnete ab. Die Widerstandsänderung wird Magnetowiderstandseffekt genannt. In erster Näherung hängt die Effektgröße von der Spinpolarisation der ferromagnetischen Elektroden ab. Werden Halbmetalle, d.h. vollständig spinpolarisierte Elektroden, verwendet, kann im Idealfall eine unendlich große Widerstandsänderung realisiert werden. Bei der Probenpräparation, die die Schichtabscheidung, die Oxidation einer Al-Schicht zur isolierenden Aluminiumoxid-Barriere sowie das Tempern der Probe umfasst, kann es zu Abweichungen vom idealen Schichtsystem mit scharfen Grenzflächen kommen. Im realen System hängt der Magnetowiderstandseffekt daher essentiell von der Qualität der Elektroden, der Barriere und deren Grenzflächen ab. Diese Arbeit befasst sich mit der Optimierung von Tunnelelementen mit halbmetallischen Elektroden, wie das ferromagnetische Eisenoxid Magnetit (Fe3O4) und die Heusler-Legierungen Co2MnSi und Co2FeSi. Der Fokus liegt dabei auf einer systematischen Charakterisierung der magnetischen, chemischen und elektronischen Eigenschaften der Barrieren/Elektroden-Grenzfläche.

Motivated by recent experiments on cuprate antiferromagnets two-dimensional Ising and anisotropic Heisenberg models have been studied. Ising models with mobile and pinned defects are shown to describe the formation and thermal destruction of defect stripes. Modelling a specific "telephone number compound", a realistic Heisenberg model with randomly distributed, static defects reproduces nicely the experimental data on an anomaly in the susceptibility as a function of the external field. Moreover basic aspects of the phase diagrams of the XXZ antiferromagnet on a square lattice and variants have been analysed, using ground state considerations and Monte Carlo techniques. In particular the role of biconical structures and fluctuations in the classical version has been emphasised. For the quantum variant, the previously proposed scenario of a first-order transition between the antiferromagnetic and spin-flop phases has been scrutinised.

In this work, magnetic semiconductors were investigated. The work on magnetic semiconductors is motivated by the great interest in the field of spinelectronics in recent years. Of particular interest are magnetic semiconductors which are ferromagnetic with Curie-temperatures well above room temperature. As ferromagnetic compound semiconductor, EuS was investigated as thin film system. Particular attention was paid to the influence of the substrate temperature during growth on the sample properties. Changes in the growth temperature lead to a change in stoichiometry of the films with Eu rich compositions for low groth temperatures. Hence, the carrier concentrations increase for decreasing growth temperatures. Therefore, the Curie-temperatures decrease and the resistances increase with increasing growth temperatures. The increase in the Curie-temperature as well as the characteristic transport properties are caused by the strong f-d interaction between conduction band and localized magnetic moments. The samples grown and investigated here show an anomalous Hall effect. This has been observed for the first time in EuS, bulk samples do not show this effect. As diluted magnetic semiconductors (DMS), GaN films magnetically doped with Mn or Gd were investigated. In both material systems, ferromagnetism far above room temperature was demonstrated.While GaGdN shows a homogeneous magnetism, all ferromagnetic GaMnN samples show small clusters (phase separations). In addition, measurements of the optical absorption and the magnetic circular dichroism (MCD) were performed on the GaMnN samples. In the optical measurements, the Mn was identified as deep acceptor. At low temperatures, both the magneto-absorption and the MCD show a Zeeman-shift of an absorption band for all doping levels. The exchange energies between valence band and localized magnetic moments can be extracted from fits of the Brillouin function to the Zeeman shifts. This yields values between 1.4eV and 1.7eV for the lowest doping levels and between 1.7eV and 2.1eV for the highest doped sample. This fits into the general trend observed in other DMS, where the exchange constants show a pronounced dependence on the lattice constant. At low temperatures, no ferromagnetic behavior was found in the magneto-optical experiments. The signal is dominated by the localized Mn spin system. At room temperature, the MCD shows a ferromagnetic signature. At elevated temperatures, the signal from the Mn spin system is small enough that the influence of the clusters on the band structure of the host lattice becomes visible. Contrary to the GaMnN samples, the GaGdN films show a homogeneous magnetism. Because of the strong localization of the magnetic 4f states of the Gd and the high resistance of the films, direct-, double- and superexchange as well as carrier mediated ferromagnetism can be ruled out as exchange mechanisms. A possible exchange path is an intra-atomic f-d exchange between the localized magnetic 4f and the band states analog to the Euchalcogenides. This is consistent with the increase of the Curie-temperature upon increase of free carriers by Si co-doping. The magnetically doped GaN thin film systems investigated here exhibit different behavior. The exchange mechanism proposed for GaGdN leads to a homogeneous ferromagnetism and is similar to the magnetic exchange in the Eu chalcogenides. The interplay between this intra-atomic f-d exchange and the magnetic end electric properties was demonstrated for the EuS samples. The behavior of the GaMnN samples is contrary to the aforementioned samples. In the ferromagnetic samples, the extrinsic and intrinsic magnetic properties were separated and the magnitude of the exchange between localized magnetic states and the valence band was determined. The samples investigated here do not show the homogeneous carrier mediated ferromagnetism theoretically predicted.

In this work the electronic structures, densities of states, chemical bonding, magnetic exchange Parameters and Curie temperatures of binary and ternary ferromagnetic alloys are analyzed. The electronic structure of ferromagnetic MnAl has been calculated using density-functional techniques (TB-LMTO-ASA, FPLAPW) and quantum chemically analyzed by means of the crystal orbital Hamilton population analysis. The crystal structure of the ferromagnetic tetragonal MnAl may be understood to originate from the structure of nonmagnetic cubic MnAl with a CsCl motif through a two-step process. While the nonmagnetic cubic structure is stable against a structural deformation, antibonding Mn--Mn interactions at the Fermi level lead to spin polarization and the onset of magnetism, i.e., a symmetry reduction taking place solely in the electronic degrees of freedom, by that emptying antibonding Mn-Mn states. Residual antibonding Al--Al states can only be removed by a subsequent, energetically smaller structural deformation towards the tetragonal system. As a final result, homonuclear bonding is strengthened and heteronuclear bonding is weakened. Corresponding DFT calculations of the electronic structure as well as the calculation of the chemical bonding and the magnetic exchange interactions have been performed on the basis of LDA and GGA for a series of ferromagnetic full Heusler alloys of general formula Co2MnZ (Z=Ga, Si, Ge, Sn), Rh2MnZ (Z=Ge, Sn, Pb), Ni2MnZ (Z=Ga, In, Sn),Pd2MnZ (Z=Sn, Sb) and Cu2MnZ (Z=Al, In, Sn). There is general agreement between experimental and theoretical moments by either the LDA or the GGA, and the amount of local exchange is characteristic for the atomic nature of the transition metal and Mn atoms. The connection between the electronic spectra and the magnetic interactions have been studied. Correlations between the chemical bondings in Heusler alloys derived from COHP analysis and magnetic phenomena are obvious, and different mechanisms leading to spin polarization and ferromagnetism are derived. Different mechanisms contributing to the exchange coupling are revealed. The band dependence of the exchange parameters, their dependence on volume and valence electron concentration have been thoroughly analyzed within the Green function technique. Finite temperature effects (Curie temperatures) are analysed using the mean-field description, and a surprisingly simple relationship between structural properties and the Curie temperature is found. The alloy variations of the Curie temperatures calculated are in good agreement with experimental data. The analysis demonstrates that the exchange parameter Jij dependence on the Z atom may be described within a rigid band approximation, having straightforward implications for the influence of the atomic volume of Z, thereby allowing semi-quantitative predictions. The magnetic exchange parameters and also Curie temperatures decrease along the row Cu, Ni, Rh, Pd, in agreement with the degree of d localisation for the transition metal. The X-Mn interactions are very important for systems with sizable magnetic moments on the transition metal (Co, Rh and Ni), making the magnetic short-range order effects stronger in these materials. The X-Mn interactions are limited by first neighbors while Mn-Mn interactions are quite long ranged. Non-spin-polarized COHP bonding analyses evidence antibonding metal-metal interactions as a prerequisite for becoming ferromagnetic, and these show up for the 3d-metal-Mn and also 4d-metal-Mn combinations but not for the wider Mn-Mn interactions. As an exception to this rule, the Cu2MnZ class of compounds evidences, on the opposite, large antibonding Mn-Mn interactions at the Fermi level; different mechanisms for spin polarization are obvious.

A study about the transversal and longitudinal properties of photonic crystals and their effects on the dynamics of the microscopic semiconductor coherences within the scope of a selfconsistent semiclassical theory A study about the dynamics of coherent spin- and charge-currents in semiconductor quantum wells for a three band model using a kp-bandstructure and including relaxation processes due to the interaction with LO-phonons, Eine Studie zu den transversalen und longitudinalen Eigenschaften photonischer Kristallstrukturen und deren Auswirkung auf die Dynamik der mikroskopischen Halbleiter-Kohärenzen im Rahmen einer selbstkonsistenten semiklassischen Theorie Eine Studie zur Dynamik von kohärenten Spin- und Ladungsströmen in Halbleiter-Quantenfilmen für ein Drei-Band-Modell mit einer kp-Bandstruktur und auf der Wechselwirkung mit Phononen basierenden Relaxationsprozessen

The Falicov-Kimball Model (FKM) is one of the simplest two-band models for correlated electrons in solid state theory. It describes itinerant c-electrons which interact with localized f-electrons by short-range Coulomb repulsion :i:U:/i:. Since its introduction in 1969 it has been used to study metal-insulator transitions, mixed-valence phenomena, crystallization etc. During the past few years the possibility of electronic ferroelectricity (EFE) has been suggested and controversely discussed.:p:This work extends the FKM by an f-f hopping :i:t:/i::sub:f:/sub:, giving the f-band a realistic finite bandwidth. Our aim is to study phase transitions and to create a ground state phase diagram in the weak-coupling limit. Using self-consistent Hartree-Fock calculations, we examine charge density waves (CDW) and electronic ferroelectricity (EFE) simultaneously.:p:As a result, we find a rich phase diagram for :i:t:/i::sub:f:/sub: and the f-level energy :i:E:/i::sub:f:/sub:. Near the symmetric case (:i:E:/i::sub:f:/sub:=0) the system builds a stable CDW phase with an insulating intraband gap in both c- and f-densities of states. For higher values of :i:E:/i::sub:f:/sub: it undergoes a first order phase transitionleading to an (anti-)ferroelectric phase depending on the sign of :i:t:/i::sub:f:/sub:. The intraband gap persists as an effective hybridization gap, giving rise to an interpretation as excitonic insulator phase. If one raises the f-level energy further the ferroelectronic phase vanishes and is replaced by a band insulator (for high values of :i:U:/i:) or, alternatively, by a semi-metal phase (for low values of :i:U:/i:). The (anti-)ferroelectric phase transition is of second order. No coexistence of CDW and excitonic insulator phases is found in the numerical Hartree-Fock treatment.

In this work the optical excitations in the one-dimensional extended Peierls-Hubbard model are studied. There are two competing interactions in this model, namely the Coulomb interaction and the Peierls interaction. The latter causes a dimerised lattice structure. To calculate the optical excitations perturbatively, one needs to identify the dominant interaction of the model. In the field-theoretical limit using different methods, such as a semiclassical approach, mean-field theory and the renormalisation group, one finds that it is the Peierls-interaction which dominates the system and therefore defines the starting point for perturbation theory. Rayleigh-Schrödinger perturbation theory is carried out up to second order in the weak Coulomb interaction for the ground-state energy and the one-particle gap. The singlet and triplet exciton gaps, i.e. charge and spin gap, are calculated using Wannier perturbation theory, two-step perturbation theory and Downfolding perturbation theory. All results are compared to results obtained by using the density-matrix renormalisation group approach (DMRG)., In dieser Arbeit werden die optischen Anregungen des eindimensionalen erweiterten Peierls-Hubbard-Modells anhand verschiedener störungstheoretischer Ansätze untersucht. Dieses Modell beschreibt quasi-eindimensionale, dimerisierte Materialien, wie beispielsweise pi-konjugierte Polymere, deren optische Anregungen zu zahlreichen Anwendungen in der Technik führen. Im erweiterten Peierls-Hubbard-Modell konkurrieren Coulomb-Wechselwirkung und Peierls-Wechselwirkung miteinander, deren Ursachen, nämlich die Elektron-Elektron-Wechselwirkung bzw. Elektron-Ion-Wechselwirkung, grundverschieden sind. Unabhängig voneinander verursachen beide Störungen eine Lücke im Ladungssektor: das System ist ein Isolator. Um die optischen Eigenschaften dieses Modells störungstheoretisch berechnen zu können, muß zunächst die dominante Wechselwirkung identifiziert werden: diese liefert dann den Ausgangspunkt der Störungstheorie. Im Rahmen einer feldtheoretischen Analyse wird das Wechselspiel der beiden Wechselwirkungen betrachtet. Dabei werden die Grenzfälle des Mott-Hubbard-Regimes und des Ladungsdichtewelle-Regimes unterschieden. Es stellt sich heraus, daß die Peierls-Wechselwirkung die dominante Störung darstellt. Damit ist das Peierls-Modell der Ausgangspunkt der Störungstheorie des erweiterten Peierls-Hubbard-Modells; es wird eine Entwicklung in der Coulomb-Wechselwirkung vorgenommen. Die Grundzustandsenergie und die Einteilchenlücke des Modells werden anhand der Rayleigh-Schrödinger-Störungstheorie zweiter Ordnung berechnet. Die exzitonischen Anregungsenergien bzw. Ladungs- und Spinlücke werden anhand verschiedener störungstheoretischer Ansätze bestimmt, da die Rayleigh-Schrödinger-Störungstheorie den exzitonischen Charakter der optischen Anregungen nicht berücksichtigen kann. Dabei handelt es sich um die Wannier-Störungstheorie, die Two-Step-Störungstheorie und die Downfolding-Störungstheorie, wobei die störungstheoretische Entwicklung jeweils bis zur zweiten Ordnung in der Coulomb-Wechselwirkung durchgeführt wird. Die störungstheoretischen Resultate werden mit den zugehörigen numerisch exakten Resultaten der Dichtematrix-Renormierungsgruppe verglichen. Dabei liefert die Wannier-Störungstheorie das beste Ergebnis.

A longstanding theoretical prediction is the orientation of spins by an electrical current flowing through low-dimensional carrier systems of sufficiently low crystallographic symmetry. Here we show by means of terahertz transmission experiments through two-dimensional hole systems a growing spin orientation with an increasing current at room temperature.

We present a theoretical study of a spin field-effect transistor realized in a quantum well formed in a p--doped ferromagnetic-semiconductor- nonmagnetic-semiconductor-ferromagnetic-semiconductor hybrid structure. Based on an envelope-function approach for the hole bands in the various regions of the transistor, we derive the complete theory of coherent transport through the device, which includes both heavy- and light-hole subbands, proper modeling of the mode matching at interfaces, integration over injection angles, Rashba spin precession, interference effects due to multiple reflections, and gate-voltage dependences. Numerical results for the device current as a function of externally tunable parameters are in excellent agreement with approximate analytical formulae., 9 pages, 11 figures

(de)Active optical fibers, fiber lasers, rare-earth-doping, vapor-phase deposition, Ill., graph. Darst

This is a theoretical investigation of the magnetic, structural and dynamical properties of the ferromagnetic Heusler compound Ni-Mn-Ga. Its physical properties have been studied by means of the Density Functional Theory. The challenge of this work is to explain in terms of the electronic structure and phonon spectra the entire variety of different crystal structures of Ni-Mn-Ga and their stability. It was shown that this crystal exibits anomalies both in the electronic structure and the phonons. Important for Ni-Mn-Ga features include modulation shuffling of the structure, phonon softening, phonon acoustic-optical coupling, electron-phonon coupling and the charge transfer. A combination of these properties allowed to draw a solid physical view on the nature of the Heusler alloys and the alloy Ni-Mn-Ga.

The one-dimensional spin-½ Heisenberg model was solved, in principle, by H. Bethe in 1931 using an ad hoc trial wavefunction, now famously known as the Bethe ansatz. Decades went by before the power and scope of this method of exact analysis became widely known for applications to spectrum and thermodynamics of a select class of completely integrable model systems. Until recently, the most vexing exception to immense progress in the further development of the Bethe ansatz has been the absence of a practical method to use the exactly known and readily available Bethe wave functions for the explicit calculation of transition matrix elements. The knowledge of such transition rates is of paramount importance for an understanding of dynamic correlation functions in relation to the underlying quasiparticles and for the interpretation of experimental probes of quantum fluctuations in quasi-one-dimensional magnetic compounds. It was most remarkable, therefore, when Kitanine, Maillet, and Terras succeeded in reducing matrix elements between Bethe wave functions for local spin operators to determinantal expressions. Here these expressions are used to calculate dynamic spin structure factors of the Heisenberg antiferromagnet (XXX model) and the one-dimensional spin-½ XXZ model in the planar regime (|Δ| < 1), both with periodic boundary conditions and an external magnetic field: Determinantal representations are derived for the zero-temperature spin fluctuations parallel and perpendicular to the external field. The singular nature of the Bethe ansatz equations for the case Δ = 0 (XX model) is discussed and the XX limit is performed in the determinantal expressions for the transition matrix elements. As an application, lineshapes for dynamic structure factors relevant for experiments are calculated. The predominant excitations are identified within the framework of quasiparticles excited from a corresponding vacuum.

It is shown that a homogeneous nonequlibrium spin polarization in semiconductor heterostructures results in an electric current. The microscopic origin of the effect is an inherent asymmetry of spin-flip scattering in systems with lifted spin degeneracy caused by k-linear terms in the Hamiltonian.

It is shown that absorption of circularly polarized infrared radiation due to intraband (Drude-like) transitions in n-type bulk semiconductors and due to intra-subband or inter-subband transitions in quantum well (QW) structures results in a monopolar spin orientation of free electrons. Spin polarization in zinc-blende-structure based QWs is demonstrated by the observation of the spin-galvanic and the circular photogalvanic effects. The monopolar spin orientation in n-type materials is shown to be possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.

(de)AFM, SPM, scanning probe microscopy, field emission electron source, AFM probes, Ill., graph. Darst

It is shown that absorption of circularly polarized infrared radiation due to intraband (Drude-like) transitions in n-type bulk semiconductors and due to intra-subband or inter-subband transitions in quantum well (QW) structures results in a monopolar spin orientation of free electrons. Spin polarization in zinc-blende-structure based QWs is demonstrated by the observation of the spin-galvanic and the circular photogalvanic effects. The monopolar spin orientation in n-type materials is shown to be possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.

In this thesis magnetic transitions accompanying the correlation-driven metal-insulator transition in the Hubbard Model with frustration are discussed. Spin systems are called frustrated if there are couplings between sites that hamper the transition to an antiferromagnetically ordered ground state. In the Hubbard Model this can be achieved by a proper choicee of the amplitudes between nearest and next nearest neighbors, generally called t and t'. In the case t'=0 one obtains an antiferromagnetically ordered ground state for every on-site interaction U. For small values of U magnetic symmetry breaking is found together with a metal-insulator transition. In the paramagnetic phase the system changes gradually with increasing U from the metallic to the insulating state. All magnetic transitions are of second order. In the case of finite t' the phase diagram is more complicated and, depending on the degree of frustration, one can find, in addition, first order transitions or more phases, e.g. an antiferromagnetic metal. The main focus of this thesis is on the perturbatively not accessible regions of intermediate coupling. The self-consistency equations for the Bethe lattice with infinite connectivity with finite t' and with total translational invariance are derived. Also, a new method to calculate the corresponding Green and correlation functions is found and solved by means of well-known Monte Carlo and dynamical mean field (DMFT) techniques. It is also discussed how thermodynamical variables depending on U and temperature are influenced by frustration and how the order of the magnetic transition could be changed from second to first order. From the present DMFT results it is concluded that for t'/t=0.5 and intermediate coupling a first order transition from the paramagnetic phase to antiferromagnetism occurs. For t'/t=0.75, even at higher coupling, no antiferromagnetic ordering was found.

The circuit theory of mesoscopic transport provides a unified framework of spin-dependent or superconductivity-related phenomena. We extend this theory to hybrid systems of normal metals, ferromagnets and superconductors. Our main results is an expression of the current through an arbitrary contact between two general isotropic nodes. In certain cases (weak ferromagnet and magnetic tunnel junction) we derive transparent and simple results for transport properties.

The work of the group is centred on the interplay between the electronic and molecular structure of transition metal complexes. Current projects address the role of Jahn-Teller coupling in determining the structural and magnetic properties of complexes with anisotropic ligands such as water and imidazole. We seek to demonstrate that in order to obtain a satisfactory understanding of metal–water and metal–imidazole interactions, which are central to the functionality of the majority of biological systems, both the electronic and vibrational co-ordinates of the system must be considered.

In this PhD thesis the scanning tunneling microscope is used to perform topographic and spectroscopic investigations an various fullerenes (C6o, CeCCC6o , CeeC5o , CeeC44, Ce(UC36 , LaC«C44 , La(UC6o , YÜC6o, and C59N) . C60 and the endohedral doped fullerenes have been produced in a laser vaporization source and haue been mass-selected prior to soft-landing onto a highly oriented pyrolytic graphite . (C59 N)2 -powder has been evaporated in situ. After localising the deposited fullerenes onto the substrate surface using topographic measurements, spectroscopic investigations has been performed locally an the individual fullerenes . The results of these investigations demonstrated a dramatic change of the electronic structure depending an the doping and geometric structure of the fullerenes. Specifically the gap of all the endohedral doped fullerenes is smaller than the gap of pure C6o . In detail, all cerium-doped fullerenes possess semiconducting properties wich an increasing gap for decreasing cage size. No gap has been detected for LaC«C6o und YÜC6o . Thus, these fullerenes exhibit metallic behaviour. For endohedral doped fullerenes there is only a weak interaction with the substrate. The interaction effect an the electronic structure is negligible in comparsion wich the effect of endohedral doping. The gap of the heterofullerene C59N is large in comparison with the gap of the endohedral doped fullerenes but less than the gap of pure C6o . Furthermore, there is a strong interaction between the heterofullerene and the graphit substrate

The exchange bias anisotropy i.e. the shift of a hysteresis loop of a ferromagnet along the magnetic field axis occurs due to the exchange coupling of the ferromagnet to an antiferromagnet. Its microscopic origin is still under discussion. Available experimental data and the theoretical models for this effect are summarized. The experimentally employed methods of Brillouin light scattering, molecular beam epitaxy, and SQUID-magnetometry are presented. The influence of non-magnetic defects in the antiferromagnet ("Dilution of the antiferromagnet") on the exchange bias field is investigated in the systems Co/Co1-yO and Co/Co1-xMgxO. In both cases an increase of the exchange bias field of up to a factor of 3 is found. Monte-Carlo simulations from Dr. U. Nowak and Prof. Dr. K.D. Usadel (Univ.-Duisburg) qualitatively confirm the experimental results. These investigations result in a new model for exchange bias that is consistent with many of the experimental observations: Disorder in the antiferromagnet enables the formation of antiferromagnetic domains, which then cause a spin excess in the antiferromagnet at the ferromagnetic/antiferromagnetic interface which causes exchange bias. Furthermore, the differences in the exchange bias field determined with Brillouin light scattering and magnetometry, the dependence of the ferromagnetic magnon mode width enhancement, and the role of the applied magnetic field during cooling of the exchange bias system below the Néel temperature is investigated.

Following the investigations of two-dimensional superconducting cuprates and the search for related transition metal oxides a fascinating field of copper oxide compounds, vanadates, manganates and nikelates opened up. These compounds show effects of strong electronic correlations and in particular magnetism in low dimensions. Magnetism in low dimensions has a history reaching back to the origin of quantum mechanics. The suppression of long-range order sets the stage for an enormous complexity of possible ground states and many-body states. The ground state is dominated by strong quantum fluctuations and pronounced spin-spin correlations. Raman light scattering has been used to examine low dimensional quantum spin systems. An important aspect is the examination of magnetic bound states, a universal property of these systems. These states are due to strong triplet triplet interactions. In theoretical models frustration and spin phonon coupling are discussed to be the reason for the strong triplet triplet interaction. The influence of the later on the excitation spectrum has been nearly unresolved. Therefore, an important aspect of this work has been to determine the influence of spin phonon coupling: It has been possible to show that spin phonon coupling posses a strong influence on the spin as well as the phonon system in real systems.

In der vorliegenden Arbeit wird der Einfluß der reduzierten Translationssymmetrie auf die magnetischen Eigenschaften in dünnen Filmen und an Oberflächen auf der Basis des stark korrelierten Hubbard-Modells untersucht. Zunächst wird die Möglichkeit von spontanem Ferromagnetismus im Hubbard-Modell für translationssymmetrische Systeme diskutiert.Verschiedene Näherungsmethoden zur Lösung des Vielteilchenproblems des Hubbard-Modells werden detailliert beschrieben und mit Ergeb nissen von Quanten-Monte-Carlo-Rechnungen verglichen. Die Konsistenz mit exakten Resultaten über die grobe Struktur der Ein-Teilchen-Spektraldichte im Limes starker Coulomb- Wechselwirkung zwischen den Elektronen erweist sich als essentiell wichtig für eine qualitativ korrekte Beschreibung von spontanem Ferromagnetismus. Das Temperaturverhalten in der ferromagnetischen Phase wird anhand von Magnetisierungs kurven sowie mit Hilfe des spinabhängigen Quasiteilchenspekt rums ausführlich diskutiert. Ein genaues Verständnis der Physik des Volumensystems liefert die Basis für den Übergang zu Systemen mit reduzierter Translationssymmetrie. Es wird eine Methode vorgestellt mit der sich approximative Theorien für das translationssymmetrische Hubbard-Modell auf die Behandlung von Filmsystemen verallgemeinern lassen. Die magnetischen Eigenschaften dünner Hubbard-Filme werden mit Hilfe der lagenabhängigen Magnetisierung als Funktion der Temperatur sowie der Filmdicke diskutiert. Die Abhängigkeit der Curie-Temperatur von der Filmdicke wird untersucht. Insbesondere wird auf die Frage nach der magnetischen Stabilität an der Oberfläche eingegangen. In stark korrelierten Elektronensystemen ist für endliche Temperaturen die magnetische Stabilität an der Oberfläche reduziert im Vergleich zu den inneren Lagen, obwohl auf der Basis des bekannten Stoner-Bildes für Bandmagnetismus genau der gegenteilige Trend zu erwarten wäre. Es wird gezeigt,daß sich dieses Verhalten anhand einfacher Argumente versteh en läßt. Die magnetischen Eigenschaften der Hubbard-Filme lassen sich im Detail mit Hilfe der lokalen Quasiteilchenzus tandsdichte sowie der wellenvektorabhängigen Spektraldichte analysieren. Die elektronische Struktur zeigt eine ausgeprägte Spin-, Lagen- und Temperaturabhängigkeit. In einem weiteren Teil der Arbeit wird der temperaturgetrieb ene Reorientierungsübergang der Magnetisierungsrichtung in dünnen metallischen Filmen untersucht. Dazu müssen die die Hubbard-Filme um anisotrope Beiträge der Dipol-Wechselwir kung und der Spin-Bahn-Wechsel wirkung erweitert werden. Das Wechselspiel von Dipol- und Spin-Bahn-Anisotropie führt unter gewissen Bedingungen zu einem Reorientierungsübergang als Funktion der Temperatur. Im Rahmen des hier vorgestellten Zugangs lassen sich sowohl Reorientierungsüber gänge von einer senkrechten in eine parallele Position ("Fe-artig") als auch Reorientierungsübergänge von einer parallelen in eine senkrechte Position ("Ni-artig") der Magnetisierung qualitativ korrekt beschreiben. In this work the influence of the reduced translational symmetry on the magnetic properties of thin itinerant-electr on films and surfaces is investigated within the strongly correlated Hubbard model. Firstly, the possibility of spontaneous ferromagnetism in the Hubbard model is discussed for the case of systems with full translational symmetry. Different approximation schemes for the solution of the many -body problem of the Hubbard model are introduced and discussed in detail. It is found that it is vital for a reasonable description of spontaneous ferromagnetism to be consistent with exact results concerning the general shape of the single-electron spectral density in the limit of strong Coulomb interaction between the electrons. The temperature dependence of the ferromagnetic solutions is discussed in detail by use of the magnetization curves as well as the spin-dependent quasiparticle spectrum. For the investigation of thin films and surfaces the approximation schemes for the bulk system have to be generalized to deal with the reduced translational symmetry. The magnetic behavior of thin Hubbard films is investigated by use of the layer-dependent magnetization as a function of temperature as well as the thickness of the film. The Curie-temperature is calculated as a function of the film thickness. Further, the magnetic stability at the surface is discussed in detail. Here it is found that for strong Coulomb interaction the magnetic stability at finite temperatures is reduced at the surface compared to the inner layers. This observation clearly contradicts the well-known Stoner picture of bandmagnetism and can be explained in terms of general arguments which are based on exact results in the limit of strong Coulomb interaction. The magnetic behavior of the Hubbard films can be analyzed in detail by inspecting the local quasiparticle density of states as well as the wave vector dependent spectral density. The electronic structure is found to be strongly spin-, layer-, and temperature- dependent. The last part of this work is concerned about the temperature-driven reorientation transition in thin metallic films. For the description of the magnetic anisotropy in thin films the dipole interaction as well as the spin-orbit interaction have to be included in the model. By calculating the temperature-dependence of the magnetic anisotropy energy it is found that both types of temperature-driven reorientation transitions, from out-of-plane to in-plane ("Fe-type") and from in-plane to out-of-plane ("Ni-type") magnetization are possible within the generalized Hubbard films.