08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Microwave properties of superconducting SrTiO3 at mK-temperatures(2022) Beydeda, CenkIn this thesis the properties of superconducting Nb-doped SrTiO3 are investigated, more concrete the optical conductivity was obtained as function of temperature, magnetic flux density and frequency. Superconducting Stripline resonators were used to probe the optical properties of Nb:SrTiO3. The optical conductivity of Nb:SrTiO3 reveals features that are typically associated with a dirty single-gap superconductor. At low frequencies the coherence peak predicted by the BCS theory is observed. In the type II superconductor Nb:SrTiO3 two critical magnetic flux densities are observed that correspond to two superconducting bands. The real part of the optical conductivity displays a strong initial increase in dependence of magnetic flux density even at lowest achieved temperature to values multiple times of the DC conductivity. The critical magnetic flux densities and the critical temperatures show a dome-shaped dependence on the Nb-doping.Item Open Access Microwave investigations on SrTiO3-based materials at mK temperatures(2018) Zinßer, MarioStrontiumtitanat (STO) ist ein Halbleiter, der unter einer bestimmten n-Dotierung bei Milli KelvinTemperaturen (mK) einen Phasenübergang in den supraleitenden Zustand aufweist. Es war damit einer der ersten ”supraleitenden Halbleiter” und zugleich der erste Oxid-Supraleiter, der unter anderem die mit dem Nobel-Preis 1987 ausgezeichnete Entdeckung von Hochtemperatur-Supraleitern vorantrieb. Darüber hinaus wurde zum ersten Mal ein sogenanntes dome-förmiges Verhalten als Funktion der Dotierung festgestellt, welches durch eine Reduktion an Sauerstoffatomen oder eine Ersetzung von Titanatomen durch Niobatome (Nb-STO) erzielt wird. Die hierfür benötigten Ladungsträgerdichten und die damit verbunden Energieskalen sind verhältnismäßig klein und schränken damit die Anzahl an möglichen Kopplungsmechanismen der Cooperpaare erheblich ein. Dies ist nicht in Einklang mit der BCS-Theorie zu bringen und bis zum heutigen Tag nicht vollständig verstanden, weshalb STO ein aktuelles Forschungsthema darstellt. An der Grenzschicht zu Lanthan Aluminiumoxid (LAO) weist STO ein zweidimensionales Elektronengas extrem mobiler, freier Ladungsträger auf, das ebenfalls einen supraleitenden Ubergang bei mK- Temperaturen zeigt. Wie auch in STO selbst ist die Supraleitung dort dome-förmig und entstammt mehreren elektronischen Bändern. Diese Arbeit soll unter anderem auch zum Verständnis der Supraleitung an der LAO-STO-Grenzschicht beitragen. Hierfür sollen die elektronischen Bandeigenschaften und mit besonderem Schwerpunkt die effektiven Massen der bei mK-Temperaturen zur Supraleitung beitragenden Elektronen untersucht werden. Zur direkten Messung dieser bietet sich die Methode der Zyklotronresonanz an, bei der durch ein extern angelegtes Magnetfeld Leitungselektronen in definierte Laundauniveaus gequantelt werden und deren Ubergang mit entsprechenden Energien im µeV-Bereich angeregt wird. Eine der Hauptherausforderungen dieser Messmethode liegt in der Notwendigkeit einer höheren Messfrequenz als die Streurate, was nur für sehr reine Metalle im genannten Energiebereich erreicht werden kann. Die hierzu nötige Mikrowellenstrahlung wird mittels koplanarer und streifenleitenden Wellenleitern übertragen und deren Transmissionssignal analysiert. Um ein entsprechendes, der Zyklotronresonanz zuzuschreibendes Messsignal vorzufinden, wurden mehrere Experimente durchgeführt. Eines von diesen beinhaltet Anzeichen eines Zyklotron-Messsignals. Obwohl dieses Ergebnis nicht reproduziert werden konnte und damit dessen Verlässlichkeit in Frage zu stellen ist, gingen Messwerte aus dieser hervor, die effektive Massen zwischen 3 und 4 Elektronenmassen und Streuraten zwischen 9 und 10 GHz ergaben, was mit publizierter Literatur in Einklang steht. Außerdem wurden Messungen der komplexen, optischen Leitfähigkeit innerhalb des supraleitenden domes von Nb-STO durchgeführt. Mit diesen ist es möglich, zwei zur effektive Masse korrelierte Kohärenzlängen zu bestimmen und dadurch das Verhältnis beider Massen der zur Supraleitung beitragenden Elektronen zu bestimmen. Darüber hinaus wurden diese Daten noch genutzt, um einige andere charakteristische Eigenschaften wie die mittlere, freie Wegl¨ange, die Fermigeschwindigkeit von beiden supraleitenden Bändern und die suprafluide Dichte zu bestimmen. Letztere wurde zur Berechnung der London’schen Eindringtiefe und des Ginzburg-Landau-Parameters genutzt, die die Zugehörigkeit zur Klasse der Typ-II-Supraleiter von Nb-STO bestätigen. Des Weiteren legt der Vergleich der Kohärenzlänge mit der mittleren freien Weglänge und die Einordnung von Nb-STO in das Home’sche Gesetz ein dirty limit-Verhalten für Supraleiter nahe. All diese Eigenschaften sind starke Anzeichen für ein Mehrband-Supraleiter mit lediglich einer Energielücke in Nb-STO.Item Open Access Microwave response of the heavy-fermion superconductor CeCu2Si2(2019) Haug, AndréThis thesis covers microwave experiments carried out on CeCu2Si2 as my master's project. Our data cover a range from 200 MHz to 20 GHz and 80 mK up to 25 K. Our measurements indicate that, in spite of the recent findings from specific-heat measurements, CeCu2Si2 is indeed a d-wave rather than an s-wave superconductor. This is supported by things like uncondensed spectral weight, and the general behavior of e. g. superfluid density.Item Open Access Planar microwave resonators on SrTiO3 substrates(2019) Engl, VincentItem Open Access Size and temporal distributions in nanoscale magnetic materials via advanced extraction methods(2015) Höfel, UdoMagnetic nanoparticles possess a multitude of fields of application, for example in biotechnology and utilization as (magnetically) easily separable catalysts. Among the diverse fabrication methods that allow the production of nanoparticles with magnetic properties respectively a chemical composition tailored for a specific task one can find the bottom-up sol-gel dip-coating technique, with which the CoNi nanoparticles embedded in silica were created. Those nanoparticles exhibit a wavelength dependent coercivity if irradiated with laser light. The mentioned behaviour was modeled via an extended, single domain ferromagnetic Stoner-Wohlfarth model implemented in Mathematica. Therein the amount of permitted angles between the nanoparticles easy axis and the external applied field can be limited to arbitrary starting and ending angles. Furthermore, a particle size distribution dependent model for superparamagnetic magnetisation measurements is discussed and its implementation in Mathematica documented. To complement the models programed in Mathematica the extended Jiles-Atherton model that rules out unphysical behaviour and allows the simulation of hystereses was implemented, such that the models available comprise superparamagnetic, single and multi domain ferromagnetic behaviour. Another simple to set up, top-down and „green“ production method is laser ablation sythesis in solution (LASiS). Nitinol (NiTi) nanoparticles created with this technique are of great interest as nitinol shows some exceptional properties, exempli gratia a shape memory, a high resistance to material fatigue and biocompatibility. Thus an extensive (magnetic) characterisation of these nanoparticles is carried out that should ease further production of task specific nanoparticles. Hence the influence of the LASiS liquid on the magnetic properties of the nanoparticles was also analysed. The impact of the laser power on the magnetic behaviour was scrutinized on iron nanoparticles, such that in conclusion some of the „knobs“ that allow tailoring of the properties of the nanoparticles are better understood and therefore grant a more precise parameter choice at the time of fabrication.Item Open Access Dielectric measurements at GHz frequencies(2019) Wendel, LarsAmongst the many dielectric materials that are topic of present research, two examples might be mentioned: SrTiO3 and nano-confined water molecules in single crystals. In SrTiO3 a quantum paraelectric or incipient ferroelectric behavior can be observed [1]. For low temperatures SrTiO3 is in a paraelectric phase, and the dielectric constant as a function of temperature follows a Curie-Weiss behavior. However, a real phase transition is not fully reached, indicated by the absence of a divergence in the permittivity at lowest observable temperatures. Quantum fluctuations then start to come into play and stabilize the frequency of the soft mode which leads to a saturation of the permittivity [2]. The dielectric constant of SrTiO3 is strongly anisotropic and depends vastly on temperature and frequency range and can go up to around 20000 [3, 4]. To confine water molecules in single crystals and observe the (quantum) paraelectric behavior, suitable hosts are needed. Possible candidates are single crystals of beryl or cordierite belonging to the gemstone family, where the water molecules are confined in structural channels [5, 6]. These large open channels run parallel to the crystal c-axis. In beryl the channels consist of six SiO4 tetrahedral rings, while in cordierite they consist of four SiO4 and two AlO4 tetrahedral rings. The water molecules are trapped in the center of the channels during the growth process and can occur in two distinct orientations: Either the electric dipole is perpendicular (type I) or parallel (type II) to the c-axis. For type I paraelectric behavior of the water molecules can be observed. This is due to the prevented hydrogen bonding, while the dipole-dipole interactions are kept which results in incipient ferroelectricity. The dielectric permittivity follows a Curie-Weiss behavior and saturates for low temperatures due to quantum fluctuations [6]. Whereas the permittivity of SrTiO3 ranges up to very large values, the dielectric constant of confined water molecules only goes up to around 20. Measurements were performed up to THz frequencies, where especially the cordierite system lacks information about the microwave regime. The different frequency regimes require separate approaches to probe the sample under study [7–9]. Different high frequency dielectric measurement techniques can access the desired dielectric properties in the GHz range of the bulk material. The methods can be roughly classified in four categories: Transmission and reflection line techniques, free-space methods, open-ended coaxial-probe techniques and resonant techniques [10–14]. Each technique has advantages and limitations regarding sample material and shape and which frequencies are possible to probe. Generally, the measurement methods can be classified if discrete frequencies are measured or the measurement is of broadband type. In broadband approaches the electromagnetic wave passes the material and allows to access frequencies continuously over a wide range. An example is the transmission line method where frequencies from the MHz up to the GHz range can be accessed [15]. However, this method lacks heavily in accuracy due to the non-resonant fixture type. Furthermore, unwanted parasitic losses occur in the microwave regime, strongly 11. Motivation influencing the measurement. Since the accuracy is of major interest, resonant techniques are suited for bulk dielectric measurements [16–18]. The obtained results are of highest accuracy compared to the other mentioned techniques. However, one big disadvantages is the accessibility of only a single frequency. The different resonant approaches are directly fabricated for the sample under test and are therefore able to probe only a single frequency in the spectrum. Especially regarding frequency-dependent dielectric properties this is an enormous weakness. Microwave waveguide resonator utilize the cavity perturbation theory and are able to probe numerous discrete frequencies [19]. Most of all, planar microwave resonators provide a wide field of potential researches [20–24]. It is possible to probe higher harmonics of a fundamental resonance frequency and perform measurements at cryogenic temperatures, while remaining at highly accurate results. A further upside is the simple fabrication of the the planar resonators as well as an fast and straightforward sample preparation of the studied materials. However, planar waveguide resonators are used for studies on dielectric thin films [25], whereas probing bulk dielectrics with this method is a recent research area. In this thesis a new resonant approach for bulk dielectric samples is developed in the microwave regime up to 20 GHz. With simulations utilized, different possible approaches are firstly studied regarding their suitability and functionality. This is investigated with focusing on the challenges which occur by going from thin films under study to bulk dielectrics. The respective approaches are then tested within experiments to check the simulated predictions. Finally, a new method is established by measuring known dielectrics and comparing the results to the literature. This thesis begins with a short guide into fundamental theoretical principles required for characterizing resonators and the analysis of microwave resonator experiments. Afterwards, the state of art of high frequency dielectric measurements is presented including several exemplary methods. Following, simulation and experimental basics and the consistency between them are elucidated. The first approach where the bulk dielectric sample acts as perturbation to the waveguide is then discussed for two different planar resonator structures. The new method with the resonator directly shaped on the sample under study is subsequently analyzed and discussed in detail including model requirements, simulations and experiments on TiO2, MgO and LaAlO3. This work finishes with an outlook for possible future experiments for the established resonant approach.