Vortex dynamics in the high-temperature superconductors YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 + delta in low- and high-dissipative transport measurements

This thesis investigates phenomena of vortex dynamics in type II superconductors depending on the dimensionality of the flux-line system and the strength of the driving force. In the low dissipative regime of Bi_2Sr_2CaCu_2O_{8+delta} (BSCCO) the influence of oxygen stoichiometry on flux-line tension was examined. An entanglement crossover of the vortex system at low magnetic fields was identified and a comprehensive B-T phase diagram of solid and fluid phases derived.In YBa_2Cu_3O_7 (YBCO) extremely long (>100 mm) high-quality measurement bridges allowed to extend the electric-field window in transport measurements by up to three orders of magnitude. Complementing analyses of the data conclusively produced dynamic exponents of the glass transition z~9 considerably higher than theoretically predicted and previously reported. In high-dissipative measurements a voltage instability appearing in the current-voltage characteristics of type II superconductors was observed for the first time in BSCCO and shown to result from a Larkin-Ovchinnikov flux-flow vortex instability under the influence of quasi-particle heating. However, in an analogous investigation of YBCO the instability was found to appear only in the temperature and magnetic-field regime of the vortex-glass state. Rapid-pulse measurements fully confirmed this correlation of vortex glass and instability in YBCO and revealed a constant rise time (~µs).

Optische Spektroskopie an Fermium (Z=100)

Im Rahmen der vorliegenden Arbeit wurde erstmals Laser-Atomspektroskopie an einem Element durchgeführt, für das bisher keine atomaren Niveaus bekannt waren. Die Experimente wurden am Element Fermium mit der Ordnungszahl Z=100 mit der Resonanzionisationsspektroskopie (RIS) in einer Puffergaszelle durchgeführt. Verwendet wurde das Isotop 255Fm mit einer Halbwertszeit von 20.1 h, das im Hochflusskernreaktor des ORNL, Oak Ridge, USA, hergestellt wurde. Die von einem elektrochemischen Filament in das Argon-Puffergas bei einer Temperatur von 960(20)°C abgedampften Fm-Atome wurden mit Lasern in einem Zweistufenprozess resonant ionisiert. Dazu wurde das Licht eines Excimerlaser gepumpten Farbstofflasers für den ersten Anregungsschritt um die Wellenlänge 400 nm durchgestimmt. Ein Teil des Excimer (XeF) Laser Pumplichtes mit den Wellenlänge 351/353 nm wurde für die nicht-resonante Ionisation verwendet. Die Ionen wurden mit Hilfe elektrischer Felder aus der optischen Zelle extrahiert und nach einem Quadrupol Massenfilter mit einem Channeltron-Detektor massenselektiv nachgewiesen. Trotz der geringen Probenmenge von 2.7 x 10^10 eingesetzten Atomen wurden zwei atomare Resonanzen bei Energien von 25099.8(2) cm-1 und 25111.8(2) cm-1 gefunden und das Sättigungsverhalten dieser Linien gemessen. Es wurde ein theoretisches Modell entwickelt, dass sowohl das spektrale Profil der sättigungsverbreiterten Linien als auch die Sättigungskurven beschreibt. Durch Anpassung an die Messdaten konnten die partiellen Übergangsraten in den 3H6 Grundzustand Aki=3.6(7) x 10^6/s und Aki=3.6(6) x 10^6/s bestimmt werden. Der Vergleich der Niveauenergien und Übergangsraten mit Multikonfigurations Dirac-Fock Rechnungen legt die spektroskopische Klassifizierung der beobachteten Niveaus als 5f12 7s7p 5I6 und 5G6 Terme nahe. Weiterhin wurde ein Übergang bei 25740 cm-1 gefunden, der aufgrund der beobachteten Linienbreite von 1000 GHz als Rydbergzustand Zustand mit der Niveauenergie 51480 cm-1 interpretiert wurde und über einen Zweiphotonen Prozess angeregt werden kann. Basierend auf dieser Annahme wurde die Obergrenze für die Ionisationsenergie IP = 52140 cm-1 = 6.5 eV abgeschätzt. In den Messungen wurden Verschiebungen in den Zeitverteilungsspektren zwischen den mono-atomaren Ionen Fm+ und Cf+ und dem Molekül-Ion UO+ festgestellt und auf Driftzeitunterschiede im elektrischen Feld der gasgefüllten optischen Zelle zurückgeführt. Unter einfachen Modellannahmen wurde daraus auf die relativen Unterschiede Delta_r(Fm+,Cf+)/r(Cf+) ˜ -0.2 % und Delta_r(UO+,Cf+)/r(Cf+) ˜ 20 % in den Ionenradien geschlossen. Über die Bestimmung der Abnahme der Fm-a Aktivität des Filamentes auf der einen Seite und die Messung der Resonanzzählrate auf der anderen Seite, wurde die Nachweiseffizienz der Apparatur zu 4.5(3) x 10^-4 bestimmt. Die Nachweisapparatur wurde mit dem Ziel weiterentwickelt, Laserspektroskopie am Isotop 251Fm durchzuführen, das über die Reaktion 249Cf(a,2n)251Fm direkt in der optischen Zelle erzeugt werden soll. Das Verfahren wurde am chemischen Homolog Erbium getestet. Dabei wurde das Isotop 163Er über die Reaktion 161Dy(a,2n)163Er erzeugt und nach Resonanzionisation nachgewiesen. Die Nachweiseffizienz der Methode wurde zu 1 x 10^-4 bestimmt.

Nuclear charge radius determination of 7,10 Be and the one-neutron halo nucleus 11 Be

The only nuclear model independent method for the determination of nuclear charge radii of short-lived radioactive isotopes is the measurement of the isotope shift. For light elements (Z < 10) extremely high accuracy in experiment and theory is required and was only reached for He and Li so far. The nuclear charge radii of the lightest elements are of great interest because they have isotopes which exhibit so-called halo nuclei. Those nuclei are characterized by a a very exotic nuclear structure: They have a compact core and an area of less dense nuclear matter that extends far from this core. Examples for halo nuclei are 6^He, 8^He, 11^Li and 11^Be that is investigated in this thesis. Furthermore these isotopes are of interest because up to now only for such systems with a few nucleons the nuclear structure can be calculated ab-initio. In the Institut für Kernchemie at the Johannes Gutenberg-Universität Mainz two approaches with different accuracy were developed. The goal of these approaches was the measurement of the isotope shifts between (7,10,11)^Be^+ and 9^Be^+ in the D1 line. The …first approach is laser spectroscopy on laser cooled Be^+ ions that are trapped in a linear Paul trap. The accessible accuracy should be in the order of some 100 kHz. In this thesis two types of linear Paul traps were developed for this purpose. Moreover, the peripheral experimental setup was simulated and constructed. It allows the efficient deceleration of fast ions with an initial energy of 60 keV down to some eV and an effcient transport into the ion trap. For one of the Paul traps the ion trapping could already be demonstrated, while the optical detection of captured 9^Be^+ ions could not be completed, because the development work was delayed by the second approach. The second approach uses the technique of collinear laser spectroscopy that was already applied in the last 30 years for measuring isotope shifts of plenty of heavier isotopes. For light elements (Z < 10), it was so far not possible to reach the accuracy that is required to extract information about nuclear charge radii. The combination of collinear laser spectroscopy with the most modern methods of frequency metrology …finally permitted the …first-time determination of the nuclear charge radii of (7,10)^Be and the one neutron halo nucleus 11^Be at the COLLAPS experiment at ISOLDE/ CERN. In the course of the work reported in this thesis it was possible to measure the absolute transition frequencies and the isotope shifts in the D1 line for the Be isotopes mentioned above with an accuracy of better than 2 MHz. Combination with the most recent calculations of the mass effect allowed the extraction of the nuclear charge radii of (7,10,11)^Be with an relative accuracy better than 1%. The nuclear charge radius decreases from 7^Be continuously to 10^Be and increases again for 11^Be. This result is compared with predictions of ab-initio nuclear models which reproduce the observed trend. Particularly the "Greens Function Monte Carlo" and the "Fermionic Molecular Dynamic" model show very good agreement.

Transport properties of YBa 2 Cu 3 O 7, PrBa 2 Cu 3 O 7-superlattices

The understanding of the coupling between superconducting YBa2Cu3O7 (YBCO) layers decoupled by non superconducting PrBa2Cu3O7 (PBCO) layers in c-axis oriented superlattices was the aim of this thesis. For this purpose two conceptually different kind of transport experiments have been performed. rnrnIn the first type of transport experiments the current is flowing parallel to the layers. Here the coupling is probed indirectly using magnetic vortex lines, which are penetrating the superlattice. Movement of the vortex segments in neighbouring YBCO layers is more or less coherent depending on the thickness of both the superconducting and non superconducting layers. This in-plane transport was measured either by sending an external current through bridges patterned in the superlattice or by an induced internal current. rnThe vortex-creep activation energy U was determined by analysis of the in-plane resistive transition in an external magnetic field B oriented along the c-axis. The activation energies for two series of superlattices were investigated. In one series the thickness of the YBCO layers was constant (nY=4 unit cells) and the number of the PBCO unit cells was varied, while in the other the number of PBCO layers was constant (nP=4) and nY varied. The correlation length of the vortex system was determined to be 80 nm along the c-axis direction. It was found that even a single PBCO unit cell in a superlattice effectively cuts the flux lines into shorter weakly coupled segments, and the coupling of the vortex systems in neighbouring layers is negligible already for a thickness of four unit cells of the PBCO layers. A characteristic variation of the activation energy for the two series of superlattices was found, where U0 is proportional to the YBCO thickness. A change in the variation of U0 with the current I in the specimen was observed, which can be explained in terms of a crossover in the vortex creep process, generated by the transport current. At low I values the dislocations mediated (plastic) vortex creep leads to thermally assisted flux-flow behaviour, whereas at high current the dc transport measurements are dominated by elastic (collective) creep.rnThe analysis of standard dc magnetization relaxation data obtained for a series superlattices revealed the occurrence of a crossover from elastic (collective) vortex creep at low temperature to plastic vortex creep at high T. The crossover is generated by the T dependent macroscopic currents induced in the sample. The existence of this creep crossover suggests that, compared with the well known Maley technique, the use of the normalized vortex creep activation energy is a better solution for the determination of vortex creep parameters.rnrnThe second type of transport experiments was to measure directly a possible Josephson coupling between superconducting CuO2 double planes in the superlattices by investigation of the transport properties perpendicular to the superconducting planes. Here three different experiments have been performed. The first one was to pattern mesa structures photolithographically as in previous works. The second used three-dimensional nanostructures cut by a focused ion beam. For the these two experiments insufficient patterning capabilities prevented an observation of the Josephson effect in the current voltage curves. rnA third experiment used a-axis and (110) oriented YBCO films, where in-plane patterning can in principle be sufficient to measure transport perpendicular to the superconducting planes. Therefore the deposition of films with this unusual growth orientation was optimized and investigated. The structural and microstructural evolution of c-axis to a-axis orientation was monitored using x-ray diffraction, scanning electron microscopy and magnetization measurements. Films with full a-axis alignment parallel to the substrate normal could be achieved on (100)SrTiO3. Due to the symmetry of the substrate the c-axis direction in-plane is twofold. Transferring the deposition conditions to films grown on (110)SrTiO3 allowed the growth of (110) oriented YBCO films with a unique in-plane c-axis orientation. While these films were of high quality by crystallographic and macroscopic visual inspection, electron microscopy revealed a coherent crack pattern on a nanoscale. Therefore the actual current path in the sample was not determined by the macroscopic patterning which prohibited investigations of the in-plane anisotropy in this case.rn