Macroscopic persistent currents in YBa2Cu3O7

Persistent currents in polycrystalline YBa₂Cu₃O₇ rings have been investigated by measuring the spatial distribution of the magnetic flux trapped in field-cooled samples. The results unambiguously show that macroscopic persistent ring currents exist. The critical current density depends very sensitively on the sample quality, reachingj _c 250 A/cm² at 77 K for our best samples.     

Laser-induced ablation dynamics and flight of thin polymer films

We investigated the ejection dynamics of triazene polymer layers in the thickness range of 40 nm to 600 nm upon nanosecond laser ablation at a wavelength of 532 nm. The ablation is due to laser-induced thermal degradation of a small part of the polymer in contact with the silicon substrate. The subsequent dynamics of the flying polymer layer are measured with sub-nanosecond time resolution. The evaluation of the initial velocity for different film thicknesses gives insight into the energy transfer process during the acceleration of the films.     

Direct measurement of entropic forces induced by rigid rods

We present the first direct depletion potential measurements for a single colloidal sphere close to a wall in a suspension of rigid colloidal rods. Since all particle interactions are as good as hard-core-like, the depletion potentials are of entirely entropic origin. We developed a density functional theory approach that accurately accounts for this experimental situation. The depletion potentials calculated for different rod number densities are in very good quantitative agreement with the experimental results.     

Stationary soliton on a charged surface of liquid helium and hydrogen films

A change in the shape of a charged surface of liquid hydrogen and helium — the formation of a solitary wave (a positively charged hump for hydrogen and a negatively charged dimple for helium)-is observed in an electric field exceeding a critical value under conditions of total compensation of the applied field by the surface charge. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 547–552 (10 April 1997)     

Femtosecond and picosecond near-field ablation of gold nanotriangles: nanostructuring and nanomelting

We report on recent insights into the interaction between ultra-fast laser pulses and plasmonic nanoparticles. We discuss femtosecond near-field ablation as a simple but versatile tool for the nanoscale modification of surfaces and the high-resolution measurement of a nanostructure’s near field. Two model systems are presented, illustrating the complexity of near-field distributions. Furthermore, finite difference time domain calculations in combination with absorption spectra provide a deeper insight into the factors influencing the near-field distribution. For the first time, an almost perfect agreement between the measured ablation pattern and experiment has been reached for gold triangles with a side length around 500 nm. Additionally, the results from picosecond laser irradiated plasmonic structures display a new regime of nanoscale laser material processing. We present first results showing nanometre confined melting induced by laser pulses.     

Measurements of the ns-laserpulse induced expansion of (111) silicon below and above the melting threshold on the nanosecond time scale

The surface displacement of 111-silicon wafers of 0.675 mm and 3.05 mm thickness, respectively, during intense ns laser irradiation is determined on the nm-vertical and ns-time scale using an optimized Michelson interferometer. The surface dynamics is observed below as well as above the melting threshold. We show that the obtained surface expansion below the melting threshold is in good agreement with theoretical heat transfer calculations. Additionally, thickness-dependent bending in the samples is illustrated and the acoustic reflections from the backside of the sample are observed. Maximum thermal displacement at the first expansion plateau is around 6 nm before melting occurs. We show that qualitative agreement between experimental results and simulation above the melting threshold can be established for the first time by taking the phase shift change in the reflection for molten silicon into account.     

Nanosecond laser pulse induced vertical movement of thin gold films on silicon determined by a modified Michelson interferometer

The vertical movement of a 40 nm thin Au film on a silicon substrate during intense nanosecond (ns) laser irradiation is determined on the nm vertical and ns time scales using an optimized Michelson interferometer. The balanced setup with two detectors uses the inverse interference signal and accounts for transient reflectivity changes during irradiation. We show that a change in phase shift upon reflection must be taken into account to gain quantitative results. Three distinct fluence regimes can be distinguished, characterized by transient reflectivity behavior, dewetting processes and film detachment. Maximum displacement velocities are determined to be 0.6 m/s and 1.9 m/s below and above the melting threshold of the metal, respectively. Flight velocities of detaching liquid films are found to be between 30 and 70 m/s for many nanoseconds.