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R. J. Wijngaarden

Publications and source records attributed to R. J. Wijngaarden.

13 recordsLinked to original sources
arXiv2013

Magnetic flux density and the critical field in the intermediate state of type-I superconductors

To address unsolved fundamental problems of the intermediate state (IS), the equilibrium magnetic flux structure and the critical field in a high purity type-I superconductor (indium film) are investigated using magneto-optical imaging with a 3D vector magnet and electrical transport measurements. The least expected observation is that the critical field in the IS can be as small as nearly 40% of the thermodynamic critical field $H_c$. This indicates that the flux density in the \textit{bulk} of normal domains can be \textit{considerably} less than $H_c$, in apparent contradiction with the long established paradigm, stating that the normal phase is unstable below $H_c$. Here we present a novel theoretical model consistently describing this and \textit{all} other properties of the IS. Moreover, our model, based the rigorous thermodynamic treatment of observed laminar flux structure in a tilted field, allows for a \textit{quantitative} determination of the domain-wall parameter and the coherence length, and provides new insight into the properties of all superconductors.

cond-mat.supr-con↗
arXiv2013

Equilibrium intermediate-state patterns in a type-I superconducting slab in an arbitrarily oriented applied magnetic field

The equilibrium topology of superconducting and normal domains in flat type-I superconductors is investigated. Important improvements with respect to previous work are: (1) the energy of the external magnetic field, as deformed by the presence of superconducting domains, is calculated in the same way for three different topologies, and (2) calculations are made for arbitrary orientation of the applied field. A phase diagram is presented for the minimum-energy topology as a function of applied field magnitude and angle. For small (large) applied fields normal (superconducting) tubes are found, while for intermediate fields parallel domains have a lower energy. The range of field magnitudes for which the superconducting-tubes structure is favored shrinks when the field is more in-plane oriented.

cond-mat.supr-con↗
arXiv2009

Mg/Ti multilayers: structural, optical and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming the hydride before Mg. From in-situ measurements of the bilayer thickness L at different hydrogen pressures, we observe large out-of-plane expansions of the Mg and Ti layers upon hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. Upon unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.

cond-mat.mtrl-sci↗
arXiv2009

Halving the Casimir force with conductive oxides

The possibility to modify the strength of the Casimir effect by tailoring the dielectric functions of the interacting surfaces is regarded as a unique opportunity in the development of Micro- and NanoElectroMechanical Systems. In air, however, one expects that, unless noble metals are used, the electrostatic force arising from trapped charges overcomes the Casimir attraction, leaving no room for exploitation of Casimir force engineering at ambient conditions. Here we show that, in the presence of a conductive oxide, the Casimir force can be the dominant interaction even in air, and that the use of conductive oxides allows one to reduce the Casimir force up to a factor of 2 when compared to noble metals.

quant-ph↗
arXiv2009

Magneto-optical imaging of magnetic deflagration in Mn12-Acetate

For the first time, the morphology and dynamics of spin avalanches in Mn12-Acetate crystals using magneto-optical imaging has been explored. We observe an inhomogeneous relaxation of the magnetization, the spins reversing first at one edge of the crystal and a few milliseconds later at the other end. Our data fit well with the theory of magnetic deflagration, demonstrating that very slow deflagration rates can be obtained, which makes new types of experiments possible.

cond-mat.mes-hall↗
arXiv2005

Dynamics of stripe patterns in type-I superconductors subject to a rotating field

The evolution of stripe patterns in type-I superconductors subject to a rotating in-plane magnetic field is investigated magneto-optically. The experimental results reveal a very rich and interesting behavior of the patterns. For small rotation angles, a small parallel displacement of the main part of the stripes and a co-rotation of their very ends is observed. For larger angles, small sideward protrusions develop, which then generate a zigzag instability, ultimately leading to a breaking of stripes into smaller segments. The short segments then start to co-rotate with the applied field although they lag behind by approximately $10^\circ$. Very interestingly, if the rotation is continued, also reconnection of segments into longer stripes takes place. These observations demonstrate the importance of pinning in type-I superconductors.

cond-mat.supr-con↗
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↗
arXiv2004

Dendritic flux penetration in Pb films with a periodic array of antidots

We explore the flux-jump regime in type-II Pb thin films with a periodic array of antidots by means of magneto-optical measurements. A direct visualization of the magnetic flux distribution allows to identify a rich morphology of flux penetration patterns. We determine the phase boundary $H^*(T)$ between dendritic penetration at low temperatures and a smooth flux invasion at high temperatures and fields. For the whole range of fields and temperatures studied, guided vortex motion along the principal axes of the square pinning array is clearly observed. In particular, the branching process of the dendrite expansion is fully governed by the underlying pinning topology. A comparative study between macroscopic techniques and direct local visualization shed light onto the puzzling $T-$ and $H-$independent magnetic response observed at low temperatures and fields. Finally, we find that the distribution of avalanche sizes at low temperatures can be described by a power law with exponent $τ\sim 0.9(1)$.

cond-mat.supr-con↗
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

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↗