Structural investigation of Nb-based layer sulfides

In this work we have investigated the intercalation of electron-donors between NbS2 slabs in Nb-based layer sulfides. Two series of Sr substituted Nb-based misfit sulfides belonging to the 1.5Q/1H and 1Q/1H series of misfit layer compounds have been synthesised. For large lanthanides (Ln=La, Ce), only the 1Q/1H compounds formed whereas for smaller lanthanides and yttrium, both types of phases can be obtained. The crystal structure of misfit sulfide (Pr0.55Sr0.45S)1.15NbS2 has been refined using the composite approach. In the Q-slab, Pr-atoms are partly replaced by Sr with a random distribution over one cation position. The crystal structure of misfit sulfide [(Sm1/3Sr2/3S)1.5]1.15NbS2 belonging to the 1.5Q/1H series have also been determined. The obtained results suggest a preferred occupancy of the cation positions in the slab where Sr atoms mainly occupy positions on the exterior of the slab while Sm atoms are in the center of the slab. The (La1-xSrxS)1.15NbS2 solid solution (0.1<x<0.9) has also been studied. It was found that the maximum value of Sr substitution is 40-50% and therefore, the minimal value of charge transfer to stabilize this structure type is about 0.6ē per Nb atom. An attempt to synthesize SrxNbS2 (0.1≤x≤0.5) intercalates was made but single phases were not obtained and increasing the temperature from 1000оС to 1100оС leads to the decomposition of these intercalates. Single crystals of Sr0.22Nb1.05S2 and Sr0.23NbS2 were found and their structures were determined. The structures belong to two different types of packings with statistical distribution of Sr between layers. A new superconducting sulfide, "EuNb2S5", was investigated by ED and HREM and its structure model consisting of Nb7S14 and (Eu3S4)2 slabs alternating along the c-axis is suggested. An attempt to suggest a model for the structure of "SrNb2S5" by means of X-ray single crystal diffraction was made. The proposed structure consists of two types of slabs: a Nb7S14 and a [Sr6(NbS4)2S] slab with niobium in tetrahedral coordination. It is shown that "SrNb2S5" and "EuNb2S5" are have similar structures. For the first time, single crystals of the complex sulfide BaNb0.9S3 have also been studied by means of X-ray single crystal diffraction. The single crystal refinement and EDX analysis showed the existence of cation vacancies at the niobium position. BaNb0.9S3 has also been studied by ED and no superstructure was found which implies that and the vacancies are statistically distributed. No improvement of the magnetic properties of the studied compounds was observed in comparison to NbS2.

Synthesis and properties of substituted Hg-based superconductors

This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg1-xMx)Ba2Can-1CunO2n+2+δ). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (Tc) and irreversibility field (Hirr). The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (T, t, p(Hg), p(O2)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa2Ca2Cu3O8+δ (Hg-1223). The formation of solid solutions with the formula (Hg1-xCux)Ba2Ca2Cu3O8+δ (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa2CaCu2O6+δ (Hg-1212) phase. The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF2 as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of Tc vs. a parameter as well as enhanced Tc values (ΔT ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO2) layers), which may be the significant factors influencing this Tc increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high Tc value (≈ 166 K) at P ≈ 23 GPa. The last part describes the investigation of substituted Hg-based superconductors in the series (Hg0.9M0.1)Ba2CuO4+δ {(Hg,M)-1201}, where M = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg0.9Mo0.1)Ba2CuO4+δ showed the most improved irreversibility line position among the (Hg,M)-1201 compounds studied in this series.

Diagnostic techniques for EHD and MHD interaction

The impact of plasma technologies is growing both in the academic and in the industrial fields. Nowadays, a great interest is focused in plasma applications in aeronautics and astronautics domains. Plasma actuators based on the Magneto-Hydro-Dynamic (MHD) and Electro- Hydro-Dynamic (EHD) interactions are potentially able to suitably modify the fluid-dynamics characteristics around a flying body without utilizing moving parts. This could lead to the control of an aircraft with negligible response time, more reliability and improvements of the performance. In order to study the aforementioned interactions, a series of experiments and a wide number of diagnostic techniques have been utilized. The EHD interaction, realized by means of a Dielectric Barrier Discharge (DBD) actuator, and its impact on the boundary layer have been evaluated by means of two different experiments. In the first one a three phase multi-electrode flat panel actuator is used. Different external flow velocities (from 1 to 20m/s) and different values of the supplied voltage and frequency have been considered. Moreover a change of the phase sequence has been done to verify the influence of the electric field existing between successive phases. Measurements of the induced speed had shown the effect of the supply voltage and the frequency, and the phase order in the momentum transfer phenomenon. Gains in velocity, inside the boundary layer, of about 5m/s have been obtained. Spectroscopic measurements allowed to determine the rotational and the vibrational temperature of the plasma which lie in the range of 320 ÷ 440°K and of 3000 ÷ 3900°K respectively. A deviation from thermodynamic equilibrium had been found. The second EHD experiment is realized on a single electrode pair DBD actuator driven by nano-pulses superimposed to a DC or an AC bias. This new supply system separates the plasma formation mechanism from the acceleration action on the fluid, leading to an higher degree of the control of the process. Both the voltage and the frequency of the nano-pulses and the amplitude and the waveform of the bias have been varied during the experiment. Plasma jets and vortex behavior had been observed by means of fast Schlieren imaging. This allowed a deeper understanding of the EHD interaction process. A velocity increase in the boundary layer of about 2m/s had been measured. Thrust measurements have been performed by means of a scales and compared with experimental data reported in the literature. For similar voltage amplitudes thrust larger than those of the literature, had been observed. Surface charge measurements led to realize a modified DBD actuator able to obtain similar performances when compared with that of other experiments. However in this case a DC bias replacing the AC bias had been used. MHD interaction experiments had been carried out in a hypersonic wind tunnel in argon with a flow of Mach 6. Before the MHD experiments a thermal, fluid-dynamic and plasma characterization of the hypersonic argon plasma flow have been done. The electron temperature and the electron number density had been determined by means of emission spectroscopy and microwave absorption measurements. A deviation from thermodynamic equilibrium had been observed. The electron number density showed to be frozen at the stagnation region condition in the expansion through the nozzle. MHD experiments have been performed using two axial symmetric test bodies. Similar magnetic configurations were used. Permanent magnets inserted into the test body allowed to generate inside the plasma azimuthal currents around the conical shape of the body. These Faraday currents are responsible of the MHD body force which acts against the flow. The MHD interaction process has been observed by means of fast imaging, pressure and electrical measurements. Images showed bright rings due to the Faraday currents heating and exciting the plasma particles. Pressure measurements showed increases of the pressure in the regions where the MHD interaction is large. The pressure is 10 to 15% larger than when the MHD interaction process is silent. Finally by means of electrostatic probes mounted flush on the test body lateral surface Hall fields of about 500V/m had been measured. These results have been used for the validation of a numerical MHD code.

Theoretical and Experimental Study of the Magnetic Separation of Pollutants from Wastewater

This Thesys reports the study of a HGMS (High GradientMagnetic Separation) process for the treatment of industrialwastewaters that considers an assisted chemical-physical pre-treatment for the removal of heavy metals through the bound by adsorption with added iron-oxide particulate matter (hematite). The considered filter, constituted by ferromagnetic stainless steel wool and permanent magnets, is studied with a new approach based on a statistical analysis that requires the study of the trajectories of the particles. Experimental activity on a laboratory device has been carried out in order to test the model.

Untersuchungen zur Strahldynamik am Mainzer Mikrotron

Diese Arbeit untersucht die longitudinale und transversaleStrahldynamik am Mainzer Mikrotron MAMI. Die gemessenen Abbildungseigenschaften werden mit den Design-Rechnungen verglichen. Dadurch konnte die Strahlqualitaet von MAMI B und das Design der neuen HDSM Mikrotronstufe verbessert werden. Es wurde eine Stoerungsrechnung formuliert, um die 6-DAbbildungsmatrix entlang der Beschleunigungsstrecke zu berechnen. Ausgehend von der linearisierten Hamilton Funktion wird die Transfermatrix M in eine unendliche Summe ueber Matrizen M(n) zerlegt, die jeweils eine n-fache Wechselwirkung des Strahls mit dem Quadrupolanteil des Fuehrungsfeldes darstellen. Dank des tieferen Einblicks in die Auswirkung von Feldfehlern konnte damit das Mikrotron-Modell leicht an die gemessenen Transfermatrizen angepasst werden. Ferner wurde die Identifizierung und Korrektur anti-symmetrischer Feldfehler in den Mikrotron-Dipolen untersucht. Es wurde ein Messverfahren entwickelt, um kleine Feldkomponenten in der Bahnebene von der Groessenordnung 10E-3 zu erkennen. Das vorgeschlagene Verfahren wurde mit Hilfe des Simulationsprogramms TOSCA ausgetestet. Schliesslich wurde die Stabilitaet der Longitudinaloptik verbessert. Dadurch konnte eine hochpraezise Energiestabi-lisierung verwirklicht werden. Bei 855 MeV Strahlenergie wird eine Stabilitaet von etwa 10E-6 erreicht.

Un concetto innovativo di limitatore di corrente di guasto superconduttivo

The thesis topic concerns the limitation of fault current high temperature superconducting (HTS), reported in scientific literature by the acronym HTSFCL (High Temperature Superconducting Fault Current Limiter) or more commonly with SFCL. These devices, at least in their ideal concept, turn on limiting short-circuit current only when the event of failure occurs, and are transparent to the network during normal operating conditions. The thesis is therefore focused on the study of diff�erent types of SFCL and results in the production of a new and original concept of superconducting limiter, called "DC Resistive SFCL". It has designed and patented in the Department of Electrical Engineering University of Bologna. The author and ing. Antonio Morandi (tutor) are the inventors. The objective of the thesis is therefore to propose a type of SFCL which may have the potential to be a viable economic solution as well as technique. The innovative concept of DC Resistive SFCL device, in fact, provides a DC operating conditions for the used superconducting (SC). It allows the use of cryogen-free solutions for cooling system and the exploitation of cheap SC materials (MgB2), both of reality are already commercially existing and indeed precluded by the types of SFCL which provides an AC operating conditions for the used SC material.

Kalorimetrische Tieftemperaturdetektoren für niederenergetische (E 1 MeV/amu) Schwerionen und ihr erster Einsatz in der Beschleuniger-Massenspektrometrie zur Spurenanalyse von 236 U

Im Rahmen der vorliegenden Arbeit wurden zum ersten Mal kalorimetrische Tieftemperatur-Detektoren in der Beschleuniger-Massenspektrometrie (Accelerator Mass Spectrometry AMS), einer Standard-Methode zur Bestimmung kleinster Isotopenverhältnisse, eingesetzt, um das Isotopenverhältnis von 236U zu 238U zu bestimmen. Das Uran-Isotop 236U entsteht in der Neutroneneinfang-Reaktion 235U(n,gamma)236U und kann daher als Monitor-Nuklid für Neutronenflüsse verwendet werden. Die Detektoren bestehen aus einem Saphir-Absorber, auf den ein supraleitender Aluminium-Film aufgedampft ist, der als Thermistor dient. Ein energetisches Schwerion deponiert seine kinetische Energie als Wärme im Absorber, dessen Temperaturänderung durch die Widerstandsänderung des Supraleiters nachgewiesen wird. Mit solchen Detektoren konnte in vorhergehenden Experimenten bei GSI in einem Energiebereich von E = 5 - 300 MeV/amu für eine Vielzahl von Ionen von Neon bis Uran eine relative Energieauflösung von (1 - 4) E-3 erreicht werden. Der für die Beschleuniger-Massenspektrometrie typische Energiebereich liegt bei E = 0.1 - 1 MeV/amu. Im ersten Schritt wurde daher die systematische Untersuchung der Detektoreigenschaften auf diesen Energiebereich ausgedehnt. Diese Untersuchungen sowie die AMS-Messungen wurden am Tandem-Beschleuniger VERA des Instituts für Isotopenforschung und Kernphysik der Universität Wien durchgeführt. In einem Energiebereich von 10 - 60 MeV konnte für verschiedene Ionen (13C, 197Au, 238U) zunächst eine relative Energieauflösung von DeltaE/E = 7 E-3 erreicht werden. Dies übertrifft die Auflösung konventioneller Ionisations-Detektoren um ca. eine Größenordnung. Durch eine Verbesserung thermischer und elektronischer Rauschbeiträge konnte in einem zweiten Experiment für Uran der Energie 17 MeV die Auflösung auf DeltaE/E = 4.6 E-3 verbessert werden. Die Energie-Response des Detektors war linear über den gesamten beobachteten Energiebereich und unabhängig von der Ionenmasse; bis auf ein Niveau von 0.1 % wurde kein Pulshöhendefekt beobachtet. Diese Ergebnisse zeigen, daß solche Detektoren ein wertvolles Werkzeug in der Schwerionenphysik im Bereich relativ niedriger Ionenenergien darstellen. Mit der erreichten Energieauflösung war es möglich, für mehrere Proben aus natürlichem Uran das Isotopenverhältnis 236U/238U zu bestimmen: Um einen Material-Standard für Uran in der AMS zu etablieren, wurde das Isotopenverhältnis 236U/238U für zwei Proben aus der Mine ''K.u.K. Joachimsthal'' möglichst präzise bestimmt. Die Ergebnisse in der vorliegenden Arbeit stimmen gut mit früheren Messungen überein, die mit einem konventionellen Detektorsystem durchgeführt wurden. Sowohl der statistische als auch der systematische Fehler konnten deutlich reduziert werden. Für eine weitere Probe, extrahiert aus dem Wasser einer Uran-haltigen Quelle in Bad Gastein, wurde ein Isotopenverhältnis von 6.1 E-12 gemessen. Dies stellt das kleinste bislang für 236U/238U gemessene Isotopenverhältnis dar und bedeutet eine Steigerung der Sensitivität um eine Größenordnung. Die erreichte Energieauflösung ermöglicht es außerdem, die Detektoren zur direkten Massenidentifikation von schweren Ionen mittels einer kombinierten Energie-Flugzeit-Messung einzusetzen. In ersten Test-Messungen im Rahmen der vorliegenden Arbeit wurde eine Massenauflösung von DeltaM/M = (8.5 - 11.0) E-3 erreicht. In einem ersten Test für den Einsatz dieser Detektoren zum Nachweis sog. ''superschwerer Elemente (Z >= 112)'' erlaubte der große dynamische Bereich, die Reaktionsprodukte und ihre nachfolgenden Alpha-Zerfälle mit hoher Energieauflösung simultan und zeitaufgelöst nachzuweisen.

Präparation, Charakterisierung und elektronische Eigenschaften von dünnen Schichten des Hochtemperatursupraleiters HgReBa 2 Ca n-1 Cu n O y

Ziel dieser Arbeit war die Pr"{a}paration, Charakterisierung und Untersuchung der elektronischen Eigenschaften von d"{u}nnen Schichten des Hochtemperatursupraleiters HgReBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{y}$, die mittels gepulster Laser-Deposition hergestellt wurden. Die HgRe1212-Filme zeigen in der AC-Suszeptibilit"{a}t einen scharfen "{U}bergang in die supraleitende Phase bei 124 K mit einer "{U}bergangsbreite von 2 K. Die resistiven "{U}berg"{a}nge der Proben wurden mit zunehmender St"{a}rke des externen Magnetfeldes breiter. Aus der Steigung der Arrheniusplots konnte die Aktivierungsenergie f"{u}r verschiedene Feldst"{a}rken bestimmt werden. Weiterhin wurde die Winkelabh"{a}ngigkeit des Depinning-Feldes $B_{dp}(theta)$ der Filme gemessen. Hieraus wurde ein Anisotropiewert von $gamma$ = 7.7 bei 105 K ermittelt. Dies ist relevant, um den f"{u}r Anwendungen wichtigen Bereich im $T$-$B$-$theta$-Phasenraum des Materials absch"{a}tzen zu k"{o}nnen. Die kritische Stromdichte $J_{c}$ der d"{u}nnen Filme aus HgRe-1212 wurde mit Hilfe eines SQUID-Magnetometers gemessen. Die entsprechenden $M$-$H$ Kurven bzw. das magnetische Moment dieser Filme wurde f"{u}r einen weiten Temperatur- und Feldbereich mit einem magnetischen Feld senkrecht zum Film aufgenommen. F"{u}r einen HgRe-1212-Film konnte bei 5 K eine kritische Stromdichte von 1.2 x 10$^{7}$ A/cm$^{2}$ und etwa 2 x 10$^{6}$ A/cm$^{2}$ bei 77 K ermittelt werden. Es wurde die Magnetfeld- und die Temperaturabh"{a}ngigkeit des Hall-Effekts im normalleitenden und im Mischzustand in Magnetfeldern senkrecht zur $ab$-Ebene bis zu 12 T gemessen. Oberhalb der kritischen Temperatur $T_{c}$ steigt der longitudinale spezifische Widerstand $rho_{xx}$ linear mit der Temperatur, w"{a}hrend der spezifische Hall-Widerstand $rho_{yx}$ sich umgekehrt proportional zur Temperatur "{a}ndert. In der N"{a}he von $T_{c}$ und in Feldern kleiner als 3 T wurde eine doppelte Vorzeichen"{a}nderung des spezifischen Hall-Widerstandes beobachtet. Der Hall-Winkel im Normalzustand, cot $theta_{H}= alpha T^{2} + beta$, folgt einer universellen $textit{T }^{2}$-Abh"{a}ngigkeit in allen magnetischen Feldern. In der N"{a}he des Nullwiderstand-Zustandes h"{a}ngt der spezifische Hall-Widerstand $rho_{yx}$ "{u}ber ein Potenzgesetz mit dem longitudinalen Widerstand $rho_{xx}$ zusammen. Das Skalenverhalten zwischen $rho_{yx}$ und $rho_{xx}$ weist eine starke Feld-Abh"{a}ngigkeit auf. Der Skalenexponent $beta$ in der Gleichung $rho_{yx}$ =A $rho_{xx}^{beta}$ steigt von 1.0 bis 1.7, w"{a}hrend das Feld von 1.0 bis 12 T zunimmt.

Extended arm polyphenylene dendrimers

In order to eliminate the de Gennes packing problem, which usually limits the attainable size of dendrimers, a new branching unit containing para-tetraphenylene ethynyl arms has been synthesized and utilized in the preparation of dendrimers of the Müllen type. The divergent principle of synthesis, based on the Dilthey reaction, could be carried up to sixth generation which contains 2776 benzene rings and possesses a diameter in the 27 nm range ("exploded dendrimer"). Monodispersity and dimensions of this and the lower generation species have been studied by MALDI-TOF MS (including the very recent superconducting tunnel junction detector), by size-exclusion chromatography, dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Interesting features, apart from the huge dimension, are the low density and high porosity of these giant molecules which cause extensive aggregation in the gas phase, flattening on solid support (AFM) and the ready incorporation of guest molecules in the condensed phase. Since the synthesis of the para-tetraphenylene arms is quite elaborate, similar dendrimers containing para-terphenylene arms have been prepared; they are accessible more economically ("semi-exploded dendrimers"). It has been shown that they in several aspects mimic the features of the "exploded dendrimers". In order to take advantage of the presence of large internal cavities in this dendrimer type, dendrons containing -C≡C- triple bonds have also been incorporated. Surprisingly, they are readily hydrogenated under the condition of heterogeneous catalysis (Pd/C) which demonstrates the large size of the cavities. As revealed by a quartz microbalance study the post-hydrogenation dendrimers are less prone to incorporate guest molecules than before hydrogenation. Obviously, the more flexible nature of the former reduces porosity, it also leads to significant shrinkage. An interesting perspective is the use of homogeneous hydrogenation catalysts of variable size with the aim of determining the dimension of internal free space.

Methodische und technische Weiterentwicklung der 3-He-MRT im Hinblick auf erweiterte lungendiagnostische Anwendungsmöglichkeiten

Die vorliegende Arbeit untersucht die Möglichkeiten und Limitierungen der MR-Lungenbildgebung mit hyperpolarisiertem 3-He bei gegenüber üblichen Magnetfeldstärken reduzierten magnetischen Führungsfeldern. Dabei werden insbesondere auch die funktionellen Bildgebungstechniken (dynamische Bildgebung, diffusionsgewichtete Bildgebung und Bestimmung des Sauerstoff-Partialdrucks) berücksichtigt. Experimentell geschieht dies durch in vivo Messungen an einem 0.1 T-Ganzkörpertomographen. Zur systematischen Untersuchung der MR-Bildgebung unter dem Einfluss diffundierender Kontrastmittel werden analytische Simulationen, Monte-Carlo-Studien und Referenzexperimente durchgeführt. Hier wird das Augenmerk besonders auf den Einfluss von Diffusions- und Suszeptibilitätsartefakten auf die morphologische und die diffusionsgewichtete Bildgebung gerichtet. Die Entwicklung und der Vergleich verschiedener Konzepte zur Erzeugung von MR-Führungsmagnetfeldern führt zur Erfindung eines neuartigen Prinzips zur weiträumigen Homogenisierung von Magnetfeldern. Die Umsetzung dieses Prinzips erfolgt in Form eines besonders kompakten Transportbehälters für kernspinpolarisierte Edelgase. Die Arbeit beinhaltet eine ausführliche Diskussion der MR-Bildgebungstechnik in Theorie und Praxis, um die Anknüpfungspunkte an die angestellten Untersuchungen herauszuarbeiten. Teile dieser Studien wurden von der Europäischen Raumfahrtorganisation ESA finanziert (Contract No.15308/01/NL/PA).

13C NMR of a single molecule magnet: analysis of pseudocontact shifts and residual dipolar couplings

Paramagnetic triple decker complexes of lanthanides are promising Single Molecule Magnets (SMMs), with many potential uses. Some of them show preferable relaxation behavior, which enables the recording of well resolved NMR spectra. These axially symmetric complexes are also strongly magnetically anisotropic, and this property can be described with the axial component of the magnetic susceptibility tensor, χa. For triple decker complexes with phthalocyanine based ligands, the Fermi˗contact contribution is small. Hence, together with the axial symmetry, the experimental chemical shifts in 1H and 13C NMR spectra can be modeled easily by considering pseudocontact and orbital shifts alone. This results in the determination of the χa value, which is also responsible for molecular alignment and consequently for the observation of residual dipolar couplings (RDCs). A detailed analysis of the experimental 1H-13C and 1H-1H couplings revealed that contributions from RDCs (positive and negative) and from dynamic frequency shifts (negative for all observed couplings) have to be considered. Whilst the pseudocontact shifts depend on the average positions of 1H and 13C nuclei relative to the lanthanide ions, the RDCs are related to the mobility of nuclei they correspond to. This phenomenon allows for the measurement of the internal mobility of the various groups in the SMMs.

Electronic transport in the heavy fermion superconductors UPd 2 Al 3 and UNi 2 Al 3

This work addresses the electronical properties of the superconductors UPd2Al3 and UNi2Al3 on the basis of thin film experiments. These isotructural compounds are ideal candiates to study the interplay of magnetism and superconductivity due to the differences of their magnetically ordered states, as well as the experimental evidence for a magnetic pairing mechanism in UPd2Al3. Epitaxial thin film samples of UPd2Al3 and UNi2Al3 were prepared using UHV Molecular Beam Epitaxy (MBE). For UPd2Al3, the change of the growth direction from the intrinsic (001) to epitaxial (100) was predicted and sucessfully demonstrated using LaAlO3 substrates cut in (110) direction. With optimized deposition process parameters for UPd2Al3 (100) on LaAlO3 (110) superconducting samples with critical temperatures up to Tc = 1.75K were obtained. UPd2Al3-AlOx-Ag mesa junctions with superconducting base electrode were prepared and shown to be in the tunneling regime. However, no signatures of a superconducting density of states were observed in the tunneling spectra. The resistive superconducting transition was probed for a possible dependence on the current direction. In contrast to UNi2Al3, the existence of such feature was excluded in UPd2Al3 (100) thin films. The second focus of this work is the dependence of the resisitive transition in UNi2Al3 (100) thin films on the current direction. The experimental fact that the resisitive transition occurs at slightly higher temperatures for I║a than for I║c can be explained within a model of two weakly coupled superconducting bands. Evidence is presented for the key assumption of the two-band model, namely that transport in and out of the ab-plane is generated on different, weakly coupled parts of the Fermi surface. Main indications are the angle dependence of the superconducting transition and the dependence of the upper critical field Bc2 on current and field orientation. Additionally, several possible alternative explanations for the directional splitting of the transition are excluded in this work. An origin due to scattering on crystal defects or impurities is ruled out, likewise a relation to ohmic heating or vortex dynamics. The shift of the transition temperature as function of the current density was found to behave as predicted by the Ginzburg-Landau theory for critical current depairing, which plays a significant role in the two-band model. In conclusion, the directional splitting of the resisitive transition has to be regarded an intrinsic and unique property of UNi2Al3 up to now. Therefore, UNi2Al3 is proposed as a role model for weakly coupled multiband superconductivity. Magnetoresistance in the normalconducting state was measured for UPd2Al3 and UNi2Al3. For UNi2Al3, a negative contribution was observed close to the antiferromagnetic ordering temperature TN only for I║a, which can be associated to reduced spin-disorder scattering. In agreement with previous results it is concluded that the magnetic moments have to be attributed to the same part of the Fermi surface which generates transport in the ab-plane.

Unconventional superconductivity of the heavy fermion compound UNi 2 Al 3

The heavy fermion compound UNi2Al3 exhibits the coexistence of superconductivity and magnetic order at low temperatures, stimulating speculations about possible exotic Cooper-pairing interaction in this superconductor. However, the preparation of good quality bulk single crystals of UNi2Al3 has proven to be a non-trivial task due to metallurgical problems, which result in the formation of an UAl2 impurity phase and hence a strongly reduced sample purity. The present work concentrates on the preparation, characterization and electronic properties investigation of UNi2Al3 single crystalline thin film samples. The preparation of thin films was accomplished in a molecular beam epitaxy (MBE) system. (100)-oriented epitaxial thin films of UNi2Al3 were grown on single crystalline YAlO3 substrates cut in (010)- or (112)-direction. The high crystallographic quality of the samples was proved by several characterisation methods, such as X-ray analysis, RHEED and TEM. To study the magnetic structure of epitaxial thin films resonant magnetic x-ray scattering was employed. The magnetic order of thin the film samples, the formation of magnetic domains with different moment directions, and the magnetic correlation length were discussed. The electronic properties of the UNi2Al3 thin films in the normal and superconducting states were investigated by means of transport measurements. A pronounced anisotropy of the temperature dependent resistivity ρ(T) was observed. Moreover, it was found that the temperature of the resistive superconducting transition depends on the current direction, providing evidence for multiband superconductivity in UNi2Al3. The initial slope of the upper critical field H′c2(T) of the thin film samples suggests an unconventional spin-singlet superconducting state, as opposed to bulk single crystal data. To probe the superconducting gap of UNi2Al3 directly by means of tunnelling spectroscopy many planar junctions of different design employing different techniques were prepared. Despite the tunneling regime of the junctions, no features of the superconducting density of state of UNi2Al3 were ever observed. It is assumed that the absence of UNi2Al3 gap features in the tunneling spectra was caused by imperfections of the tunnelling contacts. The superconductivity of UNi2Al3 was probably suppressed just in a degraded surface layer, resulting in tunneling into non superconducting UNi2Al3. However, alternative explanations such as intrinsic pair breaking effects at the interface to the barrier are also possible.

Unconventional correlated systems

This work comprises three different types of unconventional correlated systems.rnChapters 3-5 of this work are about the open shell compounds Rb4O6 and Cs4O6. These mixed valent compounds contain oxygen in two different modifications: the closed-shell peroxide anion is nonmagnetic, whereas the hyperoxide anion contains an unpaired electrons in an antibonding pi*-orbital. Due to this electron magnetic ordering is rendered possible. In contrast to theoretical predictions, which suggested half-metallic ferromagnetism for Rb4O6,rndominating antiferromagnetic interactions were found in the experiment. Besidesrna symmetry reduction due to the mixed valency, strong electronic correlations of this highly molecular system determine its properties; it is a magnetically frustrated insulator. The corresponding Cs4O6 was found to show similar properties.rnChapters 6-9 of this work are about intermetallic Heusler superconductors. rnAll of these superconductors were rationally designed using the van Hove scenario as a working recipe. A saddle point in the energy dispersion curve of a solid leads to a van Hove singularity in the density of states. In the Ni-based and Pd-based Heusler superconductors presented in this work this sort of a valence instability occurs at the high-symmetry L point and coincides or nearly coincides with the Fermi level. The compounds escape the high density of states at the Fermi energy through a transition into the correlated superconducting state.rnChapter 10 of this work is about the tetragonally distorted ferrimagnetic DO22 phase of Mn3Ga. This hard-magnetic modification is technologically useful for spin torque transfer applications. The phase exhibits two different crystallographic sites that are occupied by Mn atoms and can thus be written as Mn2MnGa. The competition between the mainly itinerant moments of the Mn atoms at the Wyckoff position 4d and the localized moments of the Mn atoms at the Wyckoff position 2b leads to magnetic correlations. The antiferromagnetic orientation of these moments determines the compound to exhibit a resulting magnetic moment of approximately 1 µB per formula unit in a partially compensated ferrimagnetic configuration.

Sviluppo e caratterizzazione di bulk superconduttori in MgB2 per sistemi di levitazione

Superconduttori bulk in MgB2, ottenuti con tecnologia Mg-RLI brevettata da Edison Spa, sono stati oggetto di un'approfondita analisi in termini di forze di levitazione. Questo studio è stato preliminare per la progettazione di un innovativo sistema di levitazione lineare. I risultati ottenuti sperimentalmente sono stati validati attraverso modelli numerici sviluppati ad hoc. I campioni oggetto dello studio sono tre bulk in MgB2 rappresentativi delle tipiche forme usate nelle applicazioni reali: un disco, un cilindro, una piastra. I bulk sono stati misurati con un sistema di misura per le forze di levitazione realizzato a tale scopo. Un protocollo sperimentale è stato seguito per la caratterizzazione di base, sia in condizioni Field Cooling sia Zero Field Cooling, al quale sono state affiancate prove specifiche come la possibilità di mantenere inalterate le proprietà superconduttive attraverso la giunzione di più campioni con la tecnologia Mg-RLI. Un modello numerico è stato sviluppato per convalidare i risultati sperimentali e per studiare l'elettrodinamica della levitazione. Diverse configurazioni di rotori magnetici sono state accoppiate con un cilindro in MgB2 con lo scopo di valutare la soluzione ottimale; questo tema è stato apporofondito attraverso lo sviluppo di un software di simulazione che può tenere conto sia del numero di magneti sia della presenza di anelli in materiale magneti intercalati fra di essi. Studi analoghi sono stati portati avanti su una piastra di MgB2 per simulare il comportamento di una geometria piana. Un sistema di raffreddamento innovativo basato sull'azoto solido è stato studiato per poterlo accoppiare con un sistema di levitazione. Il criostato progettato è costituito da due dewar, uno dentro l'altro; quello interno ha lo scopo di raffreddare l'MgB2 mentre quello esterno di limitare delle perdite verso l'esterno. Il criopattino così ottenuto è accoppiato in condizioni FC ad una rotaia formata da magneti permanenti in NdFeB.