The description of the magnetic properties of interacting many-particle systems has been one of the most important goals of physics. The problem is to derive the magnetic properties of such systems from quantum mechanical principles. It is well understood that the magnetization in an atom described by quantum numbers, spin (S), orbital (L), and total angular momentum (J) of its electrons. A set of guidelines, known as Hund's rules, discovered by Friedrich Hermann Hunds help us to determine the quantum numbers for the ground states of free atoms. The question ``to which extent are Hund's rules applicable on different systems such as molecules and solids?'' is still on the agenda. The main problem is that of finding the ground state of the considered system. Density functional theory (DFT) methods apparently are the most widely spread self-consistent methods to investigate the ground state properties. This is due to their high computational efficiency and very good accuracy. In the framework of DFT, usually the total energy is decomposed into kinetic energy, Coulomb energy, and a term called the exchange-correlation energy. Taking into account the relativistic kinetic energy leads to direct and indirect relativistic effects on the electronic structure of a solid. The most pronounced direct effect (although not the biggest in magnitude) is the spin-orbit splitting of band states. A well-known indirect relativistic effect is the change of screening of valence electrons from the nuclear charge by inner-shell electrons. One can ask that how relativistic effects come into play in ordinary density functional theory. Of course ordinary density functional theory does not include those effect. Four-current density functional theory (CDFT), the quantum electrodynamic version of the Hohenberg-Kohn theory is a powerful tool to treat relativistic effects. Although it is principally designed for systems in strong magnetic fields, CDFT can also be applied in situations where currents are present without external magnetic fields. As already pointed out by Rajagopal and Callaway (1973), the most natural way to incorporate magnetism into DFT is the generalization to CDFT. These authors, however, treated its most simple approximation, the spin density functional theory (SDFT), which keeps the spin current only and neglects completely correlation effects of orbital currents. By using the Kohn-Sham-Dirac (KSD) equation, spin-orbit coupling is introduced kinematically. The part of the orbital magnetism that is a consequence of Hund's second rule coupling is absent in this theory and there is not any more a one-to-one mapping of spin densities onto external fields. In solids, in particular in metals, the importance of Hund's second rule coupling (orbital polarization) and Hund's third rule (spin-orbit coupling) is usually interchanged in comparison to atoms. Thus, in applications of the relativistic CDFT to solids, the usual way has been to keep the spin-orbit coupling in the KSD equation (an extension to ordinary Kohn-Sham (KS) equation) and to neglect the orbital contribution to the total current density and approximate exchange-correlation energy functional with spin density only. This scheme includes a spontaneous exchange and correlation spin polarization. Orbital polarization, on the other hand, comes into play not as a correlation effect but also as an effect due to the interplay of spin polarization and spin-orbit coupling: In the presence of both couplings, time reversal symmetry is broken and a non-zero orbital current density may occur. Application of this scheme to 3d and 4f magnets yields orbital moments that are smaller than related experimental values by typically a factor of two. Orbital magnetism in a solid is strongly influenced by the ligand field, originating from the structural environment and geometry of the solid. The orbital moments in a solid with cubic symmetry are expected to be quenched if spin-orbit coupling is neglected. However, spin-orbit coupling induces orbital moments, accordingly. The relativistic nature of the spin-orbit coupling requires orbital magnetism to be treated within QED, and the treatment of QED in solids is possible in the frame of current density functional theory. The kinematic spin-orbit coupling is accounted for in many DFT calculations of magnetic systems within the LSDA. However, a strong deviation of the LSDA orbital moments from experiment is found in such approaches. To avoid such deviations, orbital polarization corrections would be desirable. In this Thesis, those corrections have been investigated in the framework of CDFT. After a short review for CDFT in Chapter 2, in Chapter 3, an "ad hoc" OP correction term (OPB) suggested by Brooks and Eriksson is given. This correction in some cases gives quite reasonable corrections to orbital moments of magnetic materials. Another OP correction (OPE), which has been introduced recently, was derived from the CDFT in the non-relativistic limit. Unfortunately, the program can only incompletely be carried through, as there are reasonable but uncontrolled approximations to be made in two steps of the derivation. Nevertheless, the result is quite close to the "ad hoc"ansatz. The calculated OPE energies for 3d and 4f free ions are in qualitative agreement with OPB energies. In Chapter 4, both corrections are implemented in the FPLO scheme to calculate orbital moments in solids. We found that both OPB and OPE corrections implemented in FPLO method, yield reasonably well the orbital magnetic moments of bcc Fe, hcp Co and fcc Ni compared with experiment. In Chapter 5, the effect of spin-orbit coupling and orbital polarization corrections on the spin and orbital magnetism of full-Heusler alloys is investigated by means of local spin density calculations. It is demonstrated, that OP corrections are needed to explain the experimental orbital moments. Model calculations employing one ligand field parameter yield the correct order of magnitude of the orbital moments, but do not account for its quantitative composition dependence. The spin-orbit coupling reduces the degree of spin polarization of the density of states at Fermi level by a few percent. We have shown that the orbital polarization corrections do not change significantly the spin polarization degree at the Fermi level. We also provide arguments that Co2FeSi might not be a half-metal as suggested by recent experiments. In Chapter 6, to understand recent XMCD data for Co impurities in gold, the electronic structure of Co impurities inside gold has been calculated in the framework of local spin density approximation. The orbital and spin magnetic moment have been evaluated. In agreement with experimental findings, the orbital moment is enhanced with respect to Co metal. On the other hand, internal relaxations are found to reduce the orbital moment considerably, whereas the spin moment is less affected. Both OPB and OPE yield a large orbital moment for Co impurities. However, those calculated orbital moments are almost by a factor of two larger than the experimental values. We also found that the orbital magnetic moment of Co may strongly depend on pressure.

The description of the magnetic properties of interacting many-particle systems has been one of the most important goals of physics. The problem is to derive the magnetic properties of such systems from quantum mechanical principles. It is well understood that the magnetization in an atom described by quantum numbers, spin (S), orbital (L), and total angular momentum (J) of its electrons. A set of guidelines, known as Hund's rules, discovered by Friedrich Hermann Hunds help us to determine the quantum numbers for the ground states of free atoms. The question ``to which extent are Hund's rules applicable on different systems such as molecules and solids?'' is still on the agenda. The main problem is that of finding the ground state of the considered system. Density functional theory (DFT) methods apparently are the most widely spread self-consistent methods to investigate the ground state properties. This is due to their high computational efficiency and very good accuracy. In the framework of DFT, usually the total energy is decomposed into kinetic energy, Coulomb energy, and a term called the exchange-correlation energy. Taking into account the relativistic kinetic energy leads to direct and indirect relativistic effects on the electronic structure of a solid. The most pronounced direct effect (although not the biggest in magnitude) is the spin-orbit splitting of band states. A well-known indirect relativistic effect is the change of screening of valence electrons from the nuclear charge by inner-shell electrons. One can ask that how relativistic effects come into play in ordinary density functional theory. Of course ordinary density functional theory does not include those effect. Four-current density functional theory (CDFT), the quantum electrodynamic version of the Hohenberg-Kohn theory is a powerful tool to treat relativistic effects. Although it is principally designed for systems in strong magnetic fields, CDFT can also be applied in situations where currents are present without external magnetic fields. As already pointed out by Rajagopal and Callaway (1973), the most natural way to incorporate magnetism into DFT is the generalization to CDFT. These authors, however, treated its most simple approximation, the spin density functional theory (SDFT), which keeps the spin current only and neglects completely correlation effects of orbital currents. By using the Kohn-Sham-Dirac (KSD) equation, spin-orbit coupling is introduced kinematically. The part of the orbital magnetism that is a consequence of Hund's second rule coupling is absent in this theory and there is not any more a one-to-one mapping of spin densities onto external fields. In solids, in particular in metals, the importance of Hund's second rule coupling (orbital polarization) and Hund's third rule (spin-orbit coupling) is usually interchanged in comparison to atoms. Thus, in applications of the relativistic CDFT to solids, the usual way has been to keep the spin-orbit coupling in the KSD equation (an extension to ordinary Kohn-Sham (KS) equation) and to neglect the orbital contribution to the total current density and approximate exchange-correlation energy functional with spin density only. This scheme includes a spontaneous exchange and correlation spin polarization. Orbital polarization, on the other hand, comes into play not as a correlation effect but also as an effect due to the interplay of spin polarization and spin-orbit coupling: In the presence of both couplings, time reversal symmetry is broken and a non-zero orbital current density may occur. Application of this scheme to 3d and 4f magnets yields orbital moments that are smaller than related experimental values by typically a factor of two. Orbital magnetism in a solid is strongly influenced by the ligand field, originating from the structural environment and geometry of the solid. The orbital moments in a solid with cubic symmetry are expected to be quenched if spin-orbit coupling is neglected. However, spin-orbit coupling induces orbital moments, accordingly. The relativistic nature of the spin-orbit coupling requires orbital magnetism to be treated within QED, and the treatment of QED in solids is possible in the frame of current density functional theory. The kinematic spin-orbit coupling is accounted for in many DFT calculations of magnetic systems within the LSDA. However, a strong deviation of the LSDA orbital moments from experiment is found in such approaches. To avoid such deviations, orbital polarization corrections would be desirable. In this Thesis, those corrections have been investigated in the framework of CDFT. After a short review for CDFT in Chapter 2, in Chapter 3, an "ad hoc" OP correction term (OPB) suggested by Brooks and Eriksson is given. This correction in some cases gives quite reasonable corrections to orbital moments of magnetic materials. Another OP correction (OPE), which has been introduced recently, was derived from the CDFT in the non-relativistic limit. Unfortunately, the program can only incompletely be carried through, as there are reasonable but uncontrolled approximations to be made in two steps of the derivation. Nevertheless, the result is quite close to the "ad hoc"ansatz. The calculated OPE energies for 3d and 4f free ions are in qualitative agreement with OPB energies. In Chapter 4, both corrections are implemented in the FPLO scheme to calculate orbital moments in solids. We found that both OPB and OPE corrections implemented in FPLO method, yield reasonably well the orbital magnetic moments of bcc Fe, hcp Co and fcc Ni compared with experiment. In Chapter 5, the effect of spin-orbit coupling and orbital polarization corrections on the spin and orbital magnetism of full-Heusler alloys is investigated by means of local spin density calculations. It is demonstrated, that OP corrections are needed to explain the experimental orbital moments. Model calculations employing one ligand field parameter yield the correct order of magnitude of the orbital moments, but do not account for its quantitative composition dependence. The spin-orbit coupling reduces the degree of spin polarization of the density of states at Fermi level by a few percent. We have shown that the orbital polarization corrections do not change significantly the spin polarization degree at the Fermi level. We also provide arguments that Co2FeSi might not be a half-metal as suggested by recent experiments. In Chapter 6, to understand recent XMCD data for Co impurities in gold, the electronic structure of Co impurities inside gold has been calculated in the framework of local spin density approximation. The orbital and spin magnetic moment have been evaluated. In agreement with experimental findings, the orbital moment is enhanced with respect to Co metal. On the other hand, internal relaxations are found to reduce the orbital moment considerably, whereas the spin moment is less affected. Both OPB and OPE yield a large orbital moment for Co impurities. However, those calculated orbital moments are almost by a factor of two larger than the experimental values. We also found that the orbital magnetic moment of Co may strongly depend on pressure.

Mit dieser Arbeit wird ein Beitrag zur Weiterentwicklung der akustischen lasermesstechnischen Verfahren zur Ermittlung von mechanischen und elastischen Eigenschaften von mikromechanischen Mehrschichtsystemen geleistet. Zu diesen Eigenschaften zählen das E-Modul, die Dichte, die Dicke sowie die Poissonzahl. Die meisten akustischen lasermesstechnischen Verfahren basieren auf der optischen Erzeugung von breitbandigen akustischen Wellen in einem Schichtsystem und der Ermittlung der Geschwindigkeit dieser Wellen durch eine Zweipunkte-Messung. Durch die spektrale Analyse des Wellenzuges an den zwei Messpunkten wird die Dispersionskurve als Relation zwischen Geschwindigkeit und Frequenz der Welle ermittelt. Ausgehend von geeigneten Modellen des Schichtsystems werden die mechanischen und elastischen Eigenschaften des Schichtsystems so lange variiert, bis eine Übereinstimmung zwischen modellierter und gemessener Dispersionskurve stattfindet. In der vorliegenden Arbeit wurde die optische Erzeugung der akustischen Wellen schmalbandig realisiert. Dadurch wurde die Zweipunkte-Messung durch eine Einpunktmessung ersetzt und damit die Ungenauigkeit der Wegmessung eliminiert. Außerdem wird die spektrale Analyse des Wellenzuges auf eine einfachere FFT-Berechnung reduziert. Bei der Modellierung wurde ein bestehendes Randelementenmodell auf eine unbegrenzte Zahl von Schichten innerhalb vom Schichtsystem erweitert. In diesem erweiterten Modell ist es möglich, beliebige Kombinationen von Eigenschaften unterschiedlicher Schichten gleichzeitig zu ermitteln. Außerdem können Mehrschichtsysteme mit beliebiger kristalliner Orientierung der Schichten untersucht werden. Um die Grenzen des entwickelten Verfahrens zu zeigen, wurden verschiedene Mehrschichtsysteme untersucht. Darunter ist z.B. ein Schichtsystem mit einer hexagonal angeordneten AIN-Schicht. Weiterhin wurde ein Schichtsystem mit einem sehr niedrigen messbaren Dispersionseffekt von weniger als 1% (polykristalline Siliziumschicht auf einem Siliziumsubstrat) untersucht. Außerdem wurde ein Schichtsystem mit einer Silizium-Germanium-Mischschicht untersucht und aus den Parametern E-Modul, Dichte und Poissonzahl die Germaniumkonzentration in der Schicht ermittelt. / This thesis represents a further development of the existing laser acoustic techniques for the measurement of mechanical and elastic properties of micromechanical multi layer systems. Most of the existing laser acoustic techniques are based on the optothermal generation of broadband surface acoustic waves in a layer system. To measure the velocity of the generated waves, a two points measurement of the wave train is typically performed. From the analysis of the spectrum of the wave trains at the two points, a dispersion curve (velocity of the surface acoustic wave against frequency) can be determined experimentally for the layer system. The dispersion relation is also determined through an appropriate model for the layer system. When the mechanical and elastic properties of the layer system are changed in the model, until the modeled and the experimental dispersion curves match in a least square sense, then the mechanical and elastic properties are found. In the present thesis, the optothermal generation of the surface acoustic waves is performed in a narrowband setup. A second point for the measurement of the acoustic wave train is not needed. The use of only one measurement point instead of two ameliorates the accuracy of the measurement (no distance measurement between two points is needed). Further, no spectrum analysis of the wave train is replaced with a simpler FFT. From the frequency of the wave train and the given wavelength from the generation mask, the velocity for each frequency can be determined easily. For the modeling of the dispersion curve, an existing BEM model is expanded to consider an unlimited number of layers. In this new model, any combination of properties can be determined for layers and substrates with any cristalline orientation given. The developed method is used to determine the properties of many challenging multi layer systems.One layer system has an AIN layer with a hexagonal structure. Another layer system has a very low dispersion effect of less than 1% velocity difference over the frequency range (polycristalline silicon on a silicon substrate). Another layer system contains a Silicon-Germanium layer. From the measured properties Young's modules, density and Poisson ration of this layer, the germanium concentration is determined.

Astrozyten erfüllen unverzichtbare Aufgaben im ZNS. Sie sorgen im Normalfall unter anderem für eine ausgewogene K+/H2O-Clearence, regulieren den Gefäßdurchmesser, bilden die Blut-/Hirnschranke, betreiben “Transmitter-Recycling” und modulieren die interneuronale Signalweitergabe durch prä- und postsynaptische Mechanismen. Die Funktionen und Einflüsse dieser zentralnervösen Gliazellen unter pathologischen Bedingungen im ZNS sind bei weitem nicht so gut untersucht, aber ebenso vielfältig. Eine ganz entscheidende Frage stellt sowohl unter physiologischen wie auch pathologischen Bedingungen das Vorliegen eines Überlappungsfaktors des von benachbarten Astrozyten okkupierten Areals dar. Betrüge ein solcher Faktor ! 1, könnten mehrere Gliazellen das gleiche Areal auch unter pathologischen Bedingungen durch ihre vielfältigen Funktionen unterstützen. Dahingegen würde das Ausbleiben eines Überlappungsgrades ! 1 bedeuten, dass bestimmte Gebiete im Neuropil anfälliger gegen Noxen oder degenerative Veränderungen wären. Um diesen Überlappungsgrad zu untersuchen, wurden Hirnschnitte von Labormäusen mittels einer geeigneten Methodenkombination untersucht. Dabei wurde das durchschnittliche Volumen der Astrozyten im motorischen Kortex durch Golgi- Färbung, sowie deren Zellzahl pro Volumeneinheit durch immunhistochemische Färbungen untersucht und mittels konfokaler Laserscanning-Mikroskopie dokumentiert. Aus diesen Parametern ließ sich ferner der durchschnittliche Überlappungsfaktor im beschriebenen Areal berechnen. Im Interesse dieser Arbeit standen dabei neben dem Unterschied im Überlappungsfaktor der Astrozyten zwischen Wildtyp- und GFAP-/- VIM-/- Knockout- Mäusen, als Beispiel für ein genetisch bedingtes Fehlen dieser Intermediärfilamente, auch der Einfluss des fortschreitenden Lebensalters, so dass für beide Genotypen sowohl junge- als auch alte Tiere untersucht wurden. Folgende Ergebnisse lassen sich zusammenfassen: 1. Das Vorhandensein der Intermediärfilamente GFAP und Vimentin scheint keinen Einfluss auf das Volumen der Astrozyten im motorischen Kortex zu haben. 2. Das Lebensalter der V ersuchstiere steht mit dem V olumen der Astrozyten signifikant in Zusammenhang. Das von Astrozytenfortsätzen der knapp zwei Jahre alten Tiere okkupierte Volumen betrug mit durchschnittlich ca. 61.000 !m3 gut das Doppelte des Volumens in jungen Mäusen (ca. 28.000 !m3). 3. Die Zellzahl der Astrozyten im motorischen Kortex wird offenbar weder vom Lebensalter, noch vom Vorhandensein der Intermediärfilamente GFAP und Vimentin signifikant beeinflusst. 4. Der Überlappungsfaktor der Astrozyten im motorischen Kortex lag bei den jungen Kontroll-Tieren bei 0,87 und bei den jungen DKO-Tieren bei 0,96. 5. Der Überlappungsfaktor der Astrozyten im motorischen Kortex lag bei den alten Kontroll-Tieren bei 2,22 und bei den alten DKO-Tieren bei 2,10. Die Ergebnisse zeigen, dass das Fehlen der Intermediärfilamente GFAP und Vimentin keinen Einfluss auf den Überlappungsgrad der Astrozyten im motorischen Kortex hat. Die Ursache für phänotypisch manifeste Erkrankungen, wie z.B. der Alexander Krankheit, welche durch ein fehlerhaft exprimiertes GFAP in Astrozyten hervorgerufen wird, ist demnach in anderen Mechanismen zu suchen. Großen Einfluss auf den Überlappungsfaktor der Astrozyten hatte dagegen das Lebensalter der Versuchstiere, was sich mit neueren Erkenntnissen zur Funktion der Astrozyten im Hinblick auf Lernvorgänge, aber auch auf degenerative Prozesse, in Zusammenhang bringen lässt.

Zugl.: Karlsruhe, Univ., Diss., 2005

Chemnitz, Techn. Univ., Diss., 2007. / Dt.-Titel: Physikalische Eigenschaften von bleifreiem Lot in flüssigem und festem Zustand.

Universiẗat, Diss., 2001--München.

Universiẗat, Diss., 1997--Saarbrücken. / Erscheinungsjahr an der Haupttitelstelle: 1997.

Universiẗat, Diss., 2005--Karlsruhe.

Festungsschleifungen haben in der Geschichte der abendländischen Städte eine große Tragweite: Sie sind der Endpunkt jeglicher Stadtbefestigungen. Welche Veränderungen der Gesellschaft dazu führten, weshalb und an welchem Punkt der Entwicklung man aufhörte, Städte zu vefestigen, wird beispielhaft an der Stadt Gießen mit Hilfe der "Dichten Beschreibung" nach Clifford Geertz nachvollzogen.