Bernd-Uwe Runge

Optical near field effects in surface nanostructuring and laser cleaning

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.

Laser cleaning of silicon wafers: mechanisms and efficiencies

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.

Ultrafast magnetic flux dendrite propagation into thin superconducting films

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.

Dendritic and homogeneous regimes of flux penetration into YBCO films

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.

Magnetic instability in YBa2Cu3O7−δ films

Abstract 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.

Observation of a New Mechanism of Spontaneous Generation of Magnetic Flux in a Superconductor

We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through

MAGNETO-OPTIC INVESTIGATION OF MAGNETIC FLUX PENETRATION ON A NANOSECOND TIMESCALE

{T}_{c}

Investigations of the vortex matter in Bi2Sr2Ca1Cu2O8+δ single crystals

. Values of the spontaneous flux appear random from one cooldown to the next, following 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.

Dynamics of the dendritic flux instability in YBa2Cu3O7 − δ films

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 17Okmls and a distinct later stage were the velocity is lower (= 18 km/s).

Velocity measurements of the dendritic instability in Y Ni2B2C

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.