References of "Silhanek, Alejandro"
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See detailCurrent crowding effects in nanostructured superconductors
Silhanek, Alejandro ULg

Conference (2013, September)

When current stream lines are forced to depart from their rectilinear trajectory due to the presence of an obstacle, they conglomerate at the sharp bends encountered on their path. The consequent current ... [more ▼]

When current stream lines are forced to depart from their rectilinear trajectory due to the presence of an obstacle, they conglomerate at the sharp bends encountered on their path. The consequent current crowding is present in normal metals as well as in superconductors and has been recently recognized as an important factor limiting the performance of superconducting single-photon detectors, leading to vortex motion rectification, and being a source of unwanted ratchet signal. In this talk we will show that in nanostructured superconductors at low temperatures, current crowding can also trigger abrupt flux avalanches developing well defined geometrical patterns. [less ▲]

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See detailDirect observation of the depairing current density in single-crystalline Ba0.5K0.5Fe2As2 microbridge with nanoscale thickness
Li, Jun; Yuan, Jie; Yuan, Ya-Hua et al

in Applied Physics Letters (2013), 103

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18 nm. The Jc of the microbridge with thickness down to 91 nm is ... [more ▼]

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18 nm. The Jc of the microbridge with thickness down to 91 nm is 10.8 MA/cm2 at 35 K, and reaches 944.4 MA/cm2 by extrapolating Jc(T) to T = 0 K using a two-gap s-wave Ginzburg-Landau model, well in accordance with the depairing current limit. The temperature, magnetic field, and angular-dependence of Jc(T,H,q) indicated weaker field dependence and weakly anisotropic factor of 1.15 (1 T) and 1.26 (5 T), which also yielded the validity of the anisotropic Ginzburg-Landau scaling. [less ▲]

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See detailNanostripe length dependence of plasmon-induced material deformations
Valev, V.K.; Libaers, W.; Zywietz, U. et al

in Optics Letters (2013), 38

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations—or “nanobumps”—are created. We have studied the dependence of these nanobumps on the length of ... [more ▼]

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations—or “nanobumps”—are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents. [less ▲]

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See detailRendering dark modes bright by using asymmetric split ring resonators
Jeyaram, Y.; Verellen, N.; Zheng, X. et al

in Optics Express (2013), 21

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite ... [more ▼]

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite several higher order modes of both even (l = 2) and odd (l = 3, 5) order, which are otherwise inaccessible for a normally incident plane wave in symmetric structures. Experimentally we observe that the even mode resonances of asymmetric resonators have a quality factor 5.8 times higher than the higher order odd resonances. [less ▲]

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See detailVortex ratchet induced by controlled edge roughness
Cerbu, D.; Gladilin, V.N.; Cuppens, J. et al

in New Journal of Physics (2013), 15

We demonstrate theoretically and experimentally the generation of rectified mean vortex displacement resulting from a controlled difference between the surface barriers at the opposite borders of a ... [more ▼]

We demonstrate theoretically and experimentally the generation of rectified mean vortex displacement resulting from a controlled difference between the surface barriers at the opposite borders of a superconducting strip. Our investigation focuses on Al superconducting strips where, in one of the two sample borders, a saw tooth-like array of micro-indentations has been imprinted. The origin of the vortex ratchet effect is based on the fact that (i) the onset of vortex motion is mainly governed by the entrance/nucleation of vortices and (ii) the current lines bunching produced by the indentations facilitates the entrance/nucleation of vortices. Only for one current direction the indentations are positioned at the side of vortex entry and the onset of the resistive regime is lowered compared to the opposite current direction. This investigation points to the relevance of ubiquitous border effects typically neglected when interpreting vortex ratchet measurements on samples with arrays of local asymmetric pinning sites. [less ▲]

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See detailEnhanced pinning in superconducting thin films with graded pinning landscapes
Motta, M.; Colauto, F.; Ortiz, W.A. et al

in Applied Physics Letters (2013), 102

A graded distribution of antidots in superconducting a-Mo79Ge21 thin films has been investigated by magnetization and magneto-optical imaging measurements. The pinning landscape has maximum density at the ... [more ▼]

A graded distribution of antidots in superconducting a-Mo79Ge21 thin films has been investigated by magnetization and magneto-optical imaging measurements. The pinning landscape has maximum density at the sample border, decreasing linearly towards the center. Its overall performance is noticeably superior than that for a sample with uniformly distributed antidots: For high temperatures and low fields, the critical current is enhanced, whereas the region of thermomagnetic instabilities in the field-temperature diagram is significantly suppressed. These findings confirm the relevance of graded landscapes on the enhancement of pinning efficiency, as recently predicted by Misko and Nori [Phys. Rev. B 85, 184506 (2012)]. [less ▲]

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See detailInfluence of swimming strategy on microorganism separation by asymmetric obstacles
Berdakin, I; Jeyaram, Y.; Moshchalkov, V.V. et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2013), 87

It has been shown that a nanoliter chamber separated by a wall of asymmetric obstacles can lead to an inhomogeneous distribution of self-propelled microorganisms. Although it is well established that this ... [more ▼]

It has been shown that a nanoliter chamber separated by a wall of asymmetric obstacles can lead to an inhomogeneous distribution of self-propelled microorganisms. Although it is well established that this rectification effect arises from the interaction between the swimmers and the noncentrosymmetric pillars, here we demonstrate numerically that its efficiency is strongly dependent on the detailed dynamics of the individual microorganism. In particular, for the case of run-and-tumble dynamics, the distribution of run lengths, the rotational diffusion, and the partial preservation of run orientation memory through a tumble are important factors when computing the rectification efficiency. In addition, we optimize the geometrical dimensions of the asymmetric pillars in order to maximize the swimmer concentration and we illustrate how it can be used for sorting by swimming strategy in a long array of parallel obstacles. [less ▲]

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See detailLocal mapping of dissipative vortex motion
Silhanek, Alejandro ULg

Conference (2013, May)

We have recently introduced an experimental technique which combines the spatial resolution of scanning Hall microscopy and sensitivity of ac-susceptibility measurements as a tool to probe the vortex ... [more ▼]

We have recently introduced an experimental technique which combines the spatial resolution of scanning Hall microscopy and sensitivity of ac-susceptibility measurements as a tool to probe the vortex dynamics in superconducting materials. This technique has proven to be particularly adequate to prospect the degree of mobility of each individual vortex, map the pinning landscape, visualize the intra-valley to inter-valley vortex hopping, and reveal the different mobility of vortices and antivortices created by magnetic dots. Furthermore, we have shown that using the phase sensitive character of the local ac-susceptibility technique it is possible to investigate the dissipative response of a single vortex. This technique opens new and exciting possibilities to locally study loss mechanisms in a variety of interesting magnetic systems such as magnetic domain walls. [less ▲]

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See detailCurrent crowding effects in superconducting corner-shaped Al microstrips
Adami, Obaïd-Allah ULg; Cerbu, Dorin; Cabosart, Damien et al

in Applied Physics Letters (2013)

The superconducting critical current of corner-shaped Al superconducting microstrips has been investigated. We demonstrate that the sharp turns lead to asymmetric vortex dynamics, allowing for easier ... [more ▼]

The superconducting critical current of corner-shaped Al superconducting microstrips has been investigated. We demonstrate that the sharp turns lead to asymmetric vortex dynamics, allowing for easier penetration from the inner concave angle than from the outer convex angle. This effect is evidenced by a rectification of the voltage signal otherwise absent in straight superconducting strips. At low magnetic fields, an enhancement of the critical current with increasing magnetic field is observed for a particular combination of field and current polarity, confirming a theoretically predicted competing interplay of superconducting screening currents and applied currents at the inner side of the turn. [less ▲]

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See detailMapping Magnetic Near-Field Distributions of Plasmonic Nanoantennas
Denkova, D; Verellen, N; Silhanek, Alejandro ULg et al

in ACS Nano (2013)

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning nearfield optical microscopy (SNOM). By means of full-field ... [more ▼]

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning nearfield optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution. [less ▲]

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See detailMorphology of Flux Avalanches in Patterned Superconducting Films
Zadorosny, R; Colauto, F; Motta, M et al

in Journal of Superconductivity & Novel Magnetism (2013), 26

It is well known that under certain circumstances, magnetic fields applied perpendicularly to the plane of superconducting films can trigger flux avalanches. In such cases the penetration has a tree-like ... [more ▼]

It is well known that under certain circumstances, magnetic fields applied perpendicularly to the plane of superconducting films can trigger flux avalanches. In such cases the penetration has a tree-like profile. However, in samples where a regular array of antidots, ADs, is present, the avalanches follow the rows of ADs as if they were guiding lines for the abrupt penetration. In this work we used the magneto-optical imaging technique to study the morphology of flux avalanches in two Nb films with a square lattice of square ADs, each one with a different lateral size, and a plain film for reference. We show that the morphology of the flux avalanches is greatly influenced by the size of the interstitial region. [less ▲]

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See detailFirst vortex entry into a perpendicularly magnetized superconducting thin film
Gutierrrez, Joffre; Raes, Bart; Van de Vondel, Joris et al

in Physical Review. B : Condensed Matter (2013), 88

In type-II superconductors, the flux-free state (Meissner state) may be invaded by vortices bearing quantized flux once H is above the lower critical field Hc1. However, the actual first flux penetration ... [more ▼]

In type-II superconductors, the flux-free state (Meissner state) may be invaded by vortices bearing quantized flux once H is above the lower critical field Hc1. However, the actual first flux penetration does not occur at Hc1 due to the presence of a surface barrier and the fact that the Meissner state may also exist as a metastable state up to a larger (superheating) field. In this work we determine the field for the first vortex penetration in superconductors, by directly imaging the first vortex threading a superconducting Pb film with antidots. We find that the first vortex penetration occurs when the surface superconducting currents reach the depairing current, locally breaking superconductivity and allowing a vortex to nucleate. [less ▲]

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See detailQuantifying the sorting efficiency of self-propelled run-and-tumble swimmers by geometrical ratchets
Berdakin; Silhanek, Alejandro ULg; Moyano Cortéz, H. N. et al

in Central European Journal of Physics (2013)

Suitable asymmetric microstructures can be used to control the direction of motion in microorganism populations. This rectification process makes it possible to accumulate swimmers in a region of space or ... [more ▼]

Suitable asymmetric microstructures can be used to control the direction of motion in microorganism populations. This rectification process makes it possible to accumulate swimmers in a region of space or to sort different swimmers. Here we study numerically how the separation process depends on the specific motility strategies of the microorganisms involved. Crucial properties such as the separation effi ciency and the separation time for two bacterial strains are precisely defined and evaluated. In particular, the sorting of two bacterial populations inoculated in a box consisting of a series of chambers separated by columns of asymmetric obstacles is investigated. We show how the sorting e fficiency is enhanced by these obstacles and conclude that this kind of sorting can be effi ciently used even when the involved populations differ only in one aspect of their swimming strategy. [less ▲]

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See detailLocal mapping of dissipative vortex motion
Raes, B; Van de Vondel, J; Silhanek, Alejandro ULg et al

in Physical Review. B : Condensed Matter (2012), 86

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved ... [more ▼]

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combining the phase sensitive character of ac susceptibility, allowing us to distinguish between the inductive and dissipative responses, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at time scales comparable to the period of the driving force (i.e., 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states. [less ▲]

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See detailControlling flux flow dissipation by changing flux pinning in superconducting films
Grimaldi, G; Leo, A; Nigro, A et al

in Applied Physics Letters (2012), 100

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting ... [more ▼]

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state. [less ▲]

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See detailInfluence of artificial pinning on vortex lattice instability in superconducting films
Silhanek, Alejandro ULg; Leo, A.; Grimaldi, G. et al

in New Journal of Physics (2012), 14

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an ... [more ▼]

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys. 34 409) and all subsequent refinements of this model which ignore the presence of pinning centers. [less ▲]

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See detailLocalization of superconductivity in superconductor–electromagnet hybrids
Ataklti, G.W.; Aladyshkin, A.Yu.; Gillijns, W. et al

in Superconductor Science and Technology (2012), 25

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located ... [more ▼]

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located underneath the strip. The inhomogeneous magnetic field, induced by the Nb wire, produces a spatial modulation of the critical temperature Tc, leading to a controllable localization of the superconducting order parameter (OP) wavefunction. We demonstrate that close to the phase boundary Tc(Bext) the localized OP solution can be displaced reversibly by either applying an external perpendicular magnetic field Bext or by changing the amplitude of the inhomogeneous field. [less ▲]

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See detailChirality in nonlinear-optical response of planar G-shaped nanostructures
Mamonov, E.A.; Murzina, T.V.; Kolmychek, I.A. et al

in Optics Express (2012), 20(8), 8518

Chirality effects in optical second harmonic generation (SHG) are studied in periodic planar arrays of gold G-shaped nanostructures. We show that G-shaped structures of different handedness demonstrate ... [more ▼]

Chirality effects in optical second harmonic generation (SHG) are studied in periodic planar arrays of gold G-shaped nanostructures. We show that G-shaped structures of different handedness demonstrate different SHG efficiency for the left and right circular polarizations, as well as the opposite directions of the SHG polarization plane rotation. The observed effects are interpreted as the appearance of chirality in the SHG response which allows clear distinguishing of two enantiomers. [less ▲]

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See detailScanning Hall probe microscopy of unconventional vortex patterns in the two-gap MgB2 superconductor
Gutierrez, J.; Raes, B.; Silhanek, Alejandro ULg et al

in Physical Review. B : Condensed Matter (2012), 85

The low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB2 single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large ... [more ▼]

The low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB2 single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large areas have been imaged with single-vortex resolution while changing systematically the thermodynamic parameters for field and temperature. The obtained patterns have been studied and compared with those of a reference 2H-NbSe2 single crystal. We found that the observed vortex patterns in MgB2 (e.g., stripes, clusters) appear due to competing vortex-vortex interactions as suggested by the theory of type-1.5 superconductivity. [less ▲]

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See detailVolumetric method of moments and conceptual multilevel building blocks for nanotopologies
Zheng, X.; Valev, V.K.; Verellen, N. et al

in IEEE Photonics Journal (2012), 4(1), 267-282

Based on the relationship between charge dimensionality and singular field behavior, it is proven that in a volumetric description of a volume current carrying topology, half rooftops of different binary ... [more ▼]

Based on the relationship between charge dimensionality and singular field behavior, it is proven that in a volumetric description of a volume current carrying topology, half rooftops of different binary hierarchical level are allowed without introducing numerical difficulties. This opens the possibility to use a very efficient multi-level hierarchical meshing scheme in a Volumetric Method of Moments (MoM) algorithm. The new meshing scheme is validated by numerical calculations and experiments. It paves the way towards a much more efficient use of MoM in the description of arbitrarily shaped nano-structures at IR and optical frequencies. [less ▲]

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