Mogens T. Levinsen

Dynamics of crystal formation in the Greenland NorthGRIP ice core

The North Greenland Icecore Project (NorthGRIP) ice core provides palaeoclimatic information back to about 120 kyr BP. The size distributions of ice crystals in the upper 880 m of the NorthGRIP ice core, which cover a time-span of approximately 5300 years, have been obtained previously. The distributions evolve towards a universal curve, indicating a common underlying physical process in the formation of crystals We identify this process as an interplay between fragmentation of the crystals and diffusion of their grain boundaries. The process is described by a two-parameter differential equation to which we obtain the exact solution. The solution is in excellent agreement with the measured distributions.

Dissipation and ordering in capillary waves at high aspect ratios

We present an experimental study of high-aspect-ratio Faraday waves. We have measured the dispersion relation and the damping rate, together with the critical amplitude for the primary instability for a wide range of frequencies. We find that our results are well explained by the linear theory, if damping from the moving contact line is considered in addition to the bulk damping. Just above the primary instability a seemingly disordered stationary state is observed. We argue that this state is a superposition of normal modes. Approximately 5% above the primary instability this state breaks down in favour of a quasi-crystalline state. This result is discussed, partly in the light of the recent third-order nonlinear theory.

Fractal structure of subharmonic steps in a Josephson junction: An analog computer calculation

Abstract We have studied the ac and dc driven Josephson junction on an analog computer. A multitude of subharmonic steps are observed. At the critical line where hysteresis sets in the steps form a complete devils staircase with a fractal dimension D ∼0.87 and a decay exponent δ ∼ 3 for the 1/ Q substeps.

Stable Single-Bubble Sonoluminescence without the presence of noble gases

We report that in spite of the commonly accepted view that stable Single-Bubble Sonoluminescence (SBSL) can only be achieved in water in the presence of a noble gas or hydrogen, long term stable SBSL can in fact be sustained with only diatomic gases like e.g. nitrogen being present. Compared to that of a stable argon bubble, the emission is much weaker and the spectrum looks much colder. Simulations support that the above quoted view, based on the dissociation hypothesis, is an erroneous inference from this theory.

Alternative method to deduce bubble dynamics in single-bubble sonoluminescence experiments

In this paper we present an experimental approach that allows to deduce the important dynamical parameters of single sonoluminescing bubbles (pressure amplitude, ambient radius, radius-time curve). The technique is based on a few previously confirmed theoretical assumptions and requires the knowledge of quantities such as the amplitude of the electric excitation and the phase of the flashes in the acoustic period. These quantities are easily measurable by a digital oscilloscope, avoiding the cost of the expensive lasers or ultrafast cameras of previous methods. We show the technique in a particular example and compare the results with conventional Mie scattering. We find that within the experimental uncertainties these two techniques provide similar results.

SUBHARMONIC GAP STRUCTURE IN THE CHARACTERISTICS OF YBA2CU3O7-X MICROBRIDGES

Abstract The subharmonic gap structures corresponding to large, 2 Δ 1 = 48 meV, and small, 2 Δ 2 = 10.3 meV, components of the energy gap were observed in the first derivatives of the current-voltage characteristics of YBa 2 Cu 3 O 7 − x epitaxial thin film microbridges. The appearance of the subharmonic gap structures is attributed to Andreev reflection.

Relative diffusion in a chaotic system: Capillary waves in the Faraday experiment

Abstract We present experimental results for the motion of particles moving on capillary ripples. The ripples are formed on the surface of a fluid undergoing vertical oscillations. We find an anomalous relative diffusion: particles initially close separate in time, moving more rapidly apart as their distance increases. This result is surprising, as self-diffusion measurements indicate that single particles essentially move like random walkers.

Fractional Brownian motion of particles in capillary waves

Abstract On the surface of a vertically oscillating fluid, capillary waves with a clearly discernible wavelength λ are formed if the amplitude of the oscillations exceeds a critical value. Particles sprinkled on the fluid surface are experimentally found to move in an almost Brownian motion when measured over distances larger than λ. We extend earlier studies of the diffusivity to length scales ranging from 0.1λ to 10λ. We observe a cross-over in the diffusive motion from a strongly anomalous diffusion below λ, to a motion that is closer to being Brownian above λ. Our observations show that the particle motion is well described by an amplitude-independent fractional Brownian motion, effective at sizes less than λ, convoluted with an amplitude-dependent fractional Brownian motion, effective on all length scales smaller than the system size. At large amplitudes our results are in surprising agreement with diffusivity measurements from upper-ocean studies.

Fractal Structure of Subharmonic Steps in Dissipative Systems Described by a Driven Damped Pendulum Equation

Since it became clear that the return map for the driven damped pendulum equation (1) with I(o) = sino at and below the transition to chaos is a circle map for every irrational rotation number W,1 the connection between the scaling laws for the pendulum system and the structures obtained by iterations of circle maps has been studied extensively. In particular, the universal scaling behavior of the complete devil’s staircase obtained by iterations of circle maps2 like the sine map (2) with a zero slope third order inflection point has been conjectured also to be found at the transition to chaos in every dissipative dynamical system with two competing frequencies.3