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Mit Hilfe von Messfunktionen konnen aus direkt bestimmten Grosen andere Grosen berechnet werden. Diese weisen ebenfalls eine Verteilung auf, die uber eine analytische oder (meist einfachere) numerische Berechnung aus den Verteilungen der Eingangsgrosen bestimmt werden kann. Das numerische Verfahren wird als Monte-Carlo-Simulation bezeichnet. Eine andere Moglichkeit stellt die lineare Naherung des Modells dar, mit der direkt aus den Standardunsicherheiten der Eingangsgrosen die Standardunsicherheit der Ausgangsgrose berechnet werden kann. Bei Messfunktionen mit mehreren Eingangsgrosen stellt die Korrelation der Eingangsgrosen einen wichtigen zusatzlichen Faktor dar, der bei der Kombination der Unsicherheiten mit eingehen muss.

Fur einen vermuteten oder auf Basis einer Theorie begrundeten funktionalen Zusammenhang zwischen den unabhangigen Variablen und der abhangigen Variable, bei der bestimmte Parameter einer Messfunktion zunachst noch unbekannt sind, lassen sich nach der Methode der kleinsten Abweichungsquadrate (engl. least squares fit) die Parameter bestimmen, fur welche die Funktion die Messdaten am besten beschreibt. Der Graph der Funktion passt dann gut zu den Datenpunkten. Die Parameter konnen bei geeigneter Wahl der Funktion eine physikalische Bedeutung haben und das eigentliche Resultat eines Experiments darstellen. Auser bei linearen Zusammenhangen ist die Ermittlung der Parameter i. d. R. nicht analytisch, sondern ausschlieslich numerisch moglich. Falls die Messdaten unterschiedliche Unsicherheiten aufweisen, konnen diese zusatzlich in Form einer sogenannten Gewichtung der Daten berucksichtigt werden.

Eine wichtige Anwendung von Unsicherheiten liegt im Vergleich von Messwerten. Vergleiche ohne Betrachtung der Unsicherheit sind vollig wertlos. Fur diesen Zweck setzt man in der Regel die Differenz der Ergebnisse in Beziehung zur Unsicherheit der zwei Ergebnisse. Liegen die Werte im Vergleich zu ihren Unsicherheiten weit auseinander, so werden sie als verschieden, andernfalls als vertraglich bewertet. Dies kann uber eine Betrachtung des Uberlapps der zwei Unsicherheitsintervalle erfolgen oder uber einen sogenannten Signifikanztest. Letzterer gibt als zusatzliche Information an, mit welcher Wahrscheinlichkeit die Bewertung, dass zwei Ergebnisse als unterschiedlich zu bewerten sind, zutreffend ist.

Bei der Angabe von Messergebnissen ist es wichtig, die richtige Zahl von Ziffern anzugeben, um weder eine zu grose Genauigkeit noch eine zu grose Messunsicherheit zu suggerieren. Die Zahl der sogenannten signifikanten Ziffern ergibt sich aus der Messunsicherheit und dem Wert selbst. Es gibt keine verbindlichen internationalen Regeln, aber sehr gebrauchlich ist bei Endergebnissen die Angabe von zwei signifikanten Ziffern der Unsicherheit. Messunsicherheiten werden dabei immer aufgerundet. Die Messwerte werden dann so gerundet, dass die Wertigkeit der letzten Ziffer von Wert und Unsicherheit ubereinstimmt. Die Ziffernzahl des Wertes selbst kann deutlich groser sein als die der Unsicherheit. Es gibt verschiedene ubliche Schreibweisen fur die Angabe von Messwert und Messunsicherheit. Wichtig ist, dass Verwechslungen mit anderen Angaben moglichst vermieden werden. In wissenschaftlichen Texten findet sich haufig eine kompakte Schreibweise, bei der die Ziffern der Messunsicherheit in runden Klammern direkt an die Ziffern des Messwertes angehangt werden.

Dendritic flux penetration is a well known phenomenon in high temperature superconductors. We investigated the conditions under which this instability can be observed in different materials. We found three qualitatively different behaviors: dendrite development above a critical field and up to a certain temperature, above a critical field with no measurable temperature dependence, or no dendrite development at all.

We have studied the dynamics of magnetic-flux avalanches in superconducting YBa2Cu3O7 − δ films by means of a fast magneto-optic pump-probe technique. Two regimes of propagation are found: in regime I, directly after the nucleation of the avalanches by a femtosecond laser pulse, the velocity v of the flux dendrites depends strongly on the magnetic field, and values of v up to 180 km/s are observed. Within some ten nanoseconds after nucleation regime II is reached, where the propagation velocity has dropped by one order of magnitude and is nearly independent of sample temperature and magnetic field. Comparison with previous experiments on flux jumps in superconductors shows good qualitative agreement.

The oxygen content in YBCO thin films can be changed locally by scanning a focused laser beam across the film surface at defined oxygen partial pressure. In contrast to annealing experiments in a furnace where the oxygen concentration remains homogeneous in the direction of the film layer, the local heating with a laser beam leads also to a lateral oxygen diffusion in the thin film. By investigating the laser written structures a deeper understanding of the patterning process itself and the oxygen diffusion in YBCO thin films could be obtained. For the detection of laser written structures and natural defects in HTSC thin films the magneto-optical method is highly qualified. It allows to visualize the magnetic screening behaviour with a high lateral resolution in the range of micrometers. In the past this technique was used to investigate the magnetic flux distribution in smaller samples at temperatures far below the boiling point of liquid nitrogen. In this work the magnetic resolution of the method was optimized so that much weaker magnetic field contrasts at 77 K can still be detected. In particular the presented apparatus allows the characterization of double-sided 3” HTSC Wafers even under conditions where cooling with liquid helium or closed-cycle refrigerators is not available.

We suggest a new theoretical approach describing the velocity of magnetic flux dendrite penetration into thin superconducting films. The key assumptions for this approach are based upon experimental observations. We treat a dendrite tip motion as a propagating flux jump instability. Two different regimes of dendrite propagation are found. A fast initial stage is followed by a slow stage, which sets in as soon as a dendrite enters into the vortex-free region. We find that the dendrite velocity is inversely proportional to the sample thickness. The theoretical results and experimental data obtained by a magneto-optic pump-probe technique are compared and excellent agreement between the calculations and measurements is found., Comment: 4 pages, 4 figures

We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through $T_c$. The sign of the spontaneous flux changes randomly from one cooldown to the next, and follows a Gaussian distribution. The width of the distribution increases with the size of the temperature gradient in the sample. Our observations appear inconsistent with the well known mechanisms of flux generation. The dependence on the temperature gradient suggests that the flux may be generated through an instability of the thermoelectric superconducting-normal quasiparticle counterflow.

We have studied the dynamics of iron-garnet films using a pump-probe technique and were able to demonstrate that the response time of such a film is sufficiently short to investigate sub-nanosecond phenomena. Furthermore, we studied the penetration velocity of the dendritic instability in thin YBCO films. We found two regimes: An early stage with a penetration velocity of up to 170 k m/s and a distinct later stage were the velocity is lower (≈ 18 km/s).

We report on experiments on the underlying physical mechanisms in the Dry-(DLC) and Steam Laser Cleaning (SLC) process. Using a frequency doubled, Q-switched Nd:YAG laser (FWHMequals8 ns), we removed polystyrene (PS) particles with diameters from 110-2000 nm from industrial silicon wafers by the DLC process. The experiments have been carried out both in ambient conditions as well as in high vacuum (10-6mbar) and the cleaned areas have been characterized by atomic force microscopy for damage inspection. Besides the determining the cleaning thresholds in laser fluence for a large interval of particle sizes we could show that particle removal in DLC is due to a combination of at least three effects: thermal substrate expansion, local substrate ablation due to field enhancement at the particle and explosive evaporation of absorbed humidity from the air. Which effect dominates the process is subject to the boundary conditions. For our laser parameters no damage free DLC was possible, i.e. whenever a particle was removed by DLC we damaged the substrate by local field enhancement. In our SLC experiments we determined the amount of superheating of a liquid layer adjacent to surfaces with controlled roughness that is necessary, in good agreement with theoretical predictions. Rough surfaces exhibited only a much smaller superheating.

We present a method for directly imaging the undisturbed near field of a particle resting on a surface. A comparison with numerical computations shows good agreement with the results of our experiments. These results have important consequences for laser-assisted particle removal where field enhancement may cause local surface damage and is one of the physical key processes in this cleaning method. On the other hand, the application of near fields at particles allows structuring of surfaces with structure dimensions in the order of 100 nm and even below.

We report transverse-field muon spin rotation studies of the magnetic field distribution n(B) in the vortex state of the high temperature superconductor Bi2Sr2Ca1Cu2O8+δ (Bi-2212) and present data on three sets of overdoped, nearly optimized and underdoped single crystals which to our best knowledge provide the first evidence that a two-stage melting transition of the vortex matter occurs under equilibrium conditions. In addition, we compare the TF-μSR results with those from DC-magnetization measurements and magneto-optic experiments on the same crystals.

Dendritic flux patterns in superconducting YBCO films are studied on a nanosecond time-scale. It is found that dendrites only develop for certain values of the external field and temperature.

Using a magneto-optic technique we have investigated the magnetic flux propagation into and out of superconducting thin YBa 2 Cu 3 O 7− δ films. After field cooling below T c , the external magnetic field B ext perpendicular to the film is changed which gives rise to shielding currents in the sample. The current distribution is disturbed momentarily by heating with a focused laser pulse near the sample edge. This triggers a magnetic instability, in which a magnetic flux avalanche starts to propagate.