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C. M. Aegerter

Publications and source records attributed to C. M. Aegerter.

13 recordsLinked to original sources
arXiv2012

Coarsening dynamics of three dimensional levitated foams: from wet to dry

We study diamagnetically levitated foams with widely different liquid fractions. Due to the levitation, drainage is effectively suppressed and the dynamics is driven by the coarsening of the foam bubbles. For dry foams, the bubble size increases as the square root of foam age, as expected from a generalized von Neumann law. At higher liquid content, the behavior changes to that of Ostwald ripening where the bubbles grow with the 1/3 power of the age. Using Diffusing Wave Spectroscopy we study the local dynamics in the different regimes and find diffusive behavior for dry foams and kinetic behavior for wet foams.

cond-mat.soft↗
arXiv2012

Direct determination of the transition to localization of light in three dimensions

In the diffusive transport of waves in three dimensional media, there should be a phase transition with increasing disorder to a state where no transport occurs. This transition was first discussed by Anderson in 1958 in the context of the metal insulator transition, but as was realized later it is generic for all waves . However, the quest for the experimental demonstration of "Anderson" or "strong" localization of waves in 3D has been a challenging task. For electrons and cold atoms, the challenge lies in the possibility of bound states in a disordered potential well. Therefore, electromagnetic and acoustic waves have been the prime candidates for the observation of Anderson localization. The main challenge using light lies in the distinction between effects of absorption and localization. Here we present measurements of the time-dependence of the transverse width of the intensity distribution of the transmitted waves, which provides a direct measure of the localization length and is independent of absorption. From this we find direct evidence for a localization transition in three dimensions and determine the corresponding localization lengths.

cond-mat.dis-nn↗
arXiv2008

Experimental investigation of the freely cooling granular gas

Using diamagnetically levitated particles we investigate the dynamics of the freely cooling granular gas. At early times we find good agreement with Haff's law, where the time scale for particle collisions can be determined from independent measurements. At late times, clustering of particles occurs. This can be included in a Haff-like description taking into account the decreasing number of free particles. With this a good description of the data is possible over the whole time range.

cond-mat.stat-mech↗
arXiv2007

Conservation of energy in coherent backscattering of light

Although conservation of energy is fundamental in physics, its principles seem to be violated in the field of wave propagation in turbid media by the energy enhancement of the coherent backscattering cone. In this letter we present experimental data which show that the energy enhancement of the cone is balanced by an energy cutback at all scattering angles. Moreover, we give a complete theoretical description, which is in good agreement with these data. The additional terms needed to enforce energy conservation in this description result from an interference effect between incident and multiply scattered waves, which is reminiscent of the optical theorem in single scattering.

cond-mat.dis-nn↗
arXiv2004

Dynamic roughening of the magnetic flux landscape in YBa$_2$Cu$_3$O$_{7-x}$

We study the magnetic flux landscape in YBa$_2$Cu$_3$O$_{7-x}$ thin films as a two dimensional rough surface. The vortex density in the superconductor forms a self-affine structure in both space and time. This is characterized by a roughness exponent $α= 0.76(3)$ and a growth exponent $β= 0.57(6)$. This is due to the structure and distribution of flux avalanches in the self-organized critical state, which is formed in the superconductor. We also discuss our results in the context of other roughening systems in the presence of quenched disorder.

cond-mat.stat-mech↗
arXiv2004

Self-organized criticality induced by quenched disorder: experiments on flux avalanches in NbH$_x$ films

We present an experimental study of the influence of quenched disorder on the distribution of flux avalanches in type-II superconductors. In the presence of much quenched disorder, the avalanche sizes are power-law distributed and show finite size scaling, as expected from self-organized criticality (SOC). Furthermore, the shape of the avalanches is observed to be fractal. In the absence of quenched disorder, a preferred size of avalanches is observed and avalanches are smooth. These observations indicate that a certain minimum amount of disorder is necessary for SOC behavior. We relate these findings to the appearance or non-appearance of SOC in other experimental systems, particularly piles of sand.

cond-mat.stat-mech↗
arXiv2003

Self-organized criticality in the Bean state in YBa$_2$Cu$_3$O$_{7-x}$ thin films

The penetration of magnetic flux into a thin film of YBa$_2$Cu$_3$O$_{7-x}$ is studied when the external field is ramped slowly. In this case the flux penetrates in bursts or avalanches. The size of these avalanches is distributed according to a power law with an exponent of $τ$ = 1.29(2). The additional observation of finite-size scaling of the avalanche distributions, with an avalanche dimension D = 1.89(3), gives strong indications towards self-organized criticality in this system. Furthermore we determine exponents governing the roughening dynamics of the flux surface using some universal scaling relations. These exponents are compared to those obtained from a standard roughening analysis.

cond-mat.soft↗
arXiv2003

Observation of vortex lattice melting in large untwinned YBa$_2$Cu$_3$O$_{7-x}$ single crystals

We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

cond-mat.supr-con↗
arXiv2003

Transition from Type-I to Type-II Superconducting Behaviour with Temperature observed by $μ$SR and SANS

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi ($>$3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter $κ$ passes through its critical value of $1/\sqrt{2}$ with temperature.

cond-mat.supr-con↗
arXiv2003

Avalanche dynamics, surface roughening and self-organized criticality - experiments on a 3 dimensional pile of rice

We present a two-dimensional system which exhibits features of self-organized criticality. The avalanches which occur on the surface of a pile of rice are found to exhibit finite size scaling in their probability distribution. The critical exponents are $τ$ = 1.21(2) for the avalanche size distribution and $D$ = 1.99(2) for the cut-off size. Furthermore the geometry of the avalanches is studied leading to a fractal dimension of the active sites of $d_B$ = 1.58(2). Using a set of scaling relations, we can calculate the roughness exponent $α= D - d_B$ = 0.41(3) and the dynamic exponent $z = D(2 - τ)$ = 1.56(8). This result is compared with that obtained from a power spectrum analysis of the surface roughness, which yields $α$ = 0.42(3) and $z$ = 1.5(1) in excellent agreement with those obtained from the scaling relations.

cond-mat.soft↗
arXiv2002

A two-dimensional rough surface: Experiments on a pile of rice

Dynamical roughening of interfaces has received much attention in recent years. However, experiments have been restricted to one dimensional (1d) systems. Moreover, theoretical studies of the two dimensional (2d) case have been highly inconclusive. Here we introduce an experimental 2d system, with which the theories can be tested. As is shown, the surface of a 2d pile of rice shows roughening behaviour in both space and time, with a roughness exponent $α_{2d}$ = 0.39(3) and a growth exponent $β_{2d}$ = 0.27(3).

physics.data-an↗
arXiv1999

Flux-Line Lattice Structures in Untwinned YBa2Cu3O

A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O is presented. In fields parallel to the c-axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy, the penetration depth ratio, of 1.18(2) was obtained. The high quality of the data is such that second order diffraction is observed, indicating a well ordered FLL. With the field at 33 degrees to c a field dependent re-orientation of the lattice is observed around 3T.

cond-mat.supr-con↗