References of "Silhanek, Alejandro"
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See detailDisrupting the wall accumulation of human sperm cells by artificial corrugation
Guidobaldi, H.A.; Jeyaram, Y.; Condat, C.A. et al

in Biomicrofluidics (2015), 9

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very di erent from that observed in the bulk. Swimming along walls is bene cial for ... [more ▼]

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very di erent from that observed in the bulk. Swimming along walls is bene cial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells 60 um long, strongly confi ned to 25u m shallow chambers. We investigate the nature of the cell trajectories between the con fining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confi rmed by depth resolved confocal microscopy studies. We show that the introduction of arti cial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an e ffect which we hypothesize may be valuable for micro fluidic applications in biomedicine. [less ▲]

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See detailMicrowave-stimulated superconductivity due to presence of vortices
Lara, Antonio; Aliev, Farkhad; Silhanek, Alejandro ULg et al

in Scientific Reports (2015), 5

The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in ... [more ▼]

The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in astronomy. Suprisingly, a sufficiently high power subgap radiation may stimulate superconductivity itself. The possibility of stimulating type II superconductors, in which the radiation may interact also with vortex cores, remains however unclear. Here we report on superconductivity enhanced by GHz radiation in type II superconducting Pb films in the presence of vortices. The stimulation effect is more clearly observed in the upper critical field and less pronounced in the critical temperature. The magnetic field dependence of the vortex related microwave losses in a film with periodic pinning reveals a reduced dissipation of mobile vortices in the stimulated regime due to a reduction of the core size. Results of numerical simulations support the validy of this conclusion. Our findings may have intriguing connections with holographic superconductors in which the possibility of stimulation is under current debate. [less ▲]

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See detailTwo-Photon Luminescence of Gold Nanorods Mediated by Higher Order Plasmon Modes
Verellen, Niels; Denkova, Denitza; De Clercq, Ben et al

in ACS Photonics (2015), 2

Metallic nanorod antennas can be considered as an analogue to classical half-wave dipole antennas, constituting an important tool for manipulating linear and nonlinear lightmatter interactions in ... [more ▼]

Metallic nanorod antennas can be considered as an analogue to classical half-wave dipole antennas, constituting an important tool for manipulating linear and nonlinear lightmatter interactions in nanoscale volumes. Using two-photon luminescence (TPL) scanning laser microscopy, we investigate such optical antennas beyond their fundamental dipole mode. The antenna mode dispersion is extracted from the nonlinear TPL measurement and reveals a TPL process that is dominated by plasmon-induced enhancement of the two photon absorption in the metal. Additionally, a clear signature of the mode parity is observed in the TPL images. TPL maxima are observed outside the antenna boundaries for even parity modes, whereas they are located inside for odd modes. It is concluded that for even modes the two-photon luminescence emission is strongly mediated by retardation of the excitation field, a consequence of their zero net-dipole moment. This selective excitation of different mode parities is highly relevant for nanoscale enhanced nonlinear optics, as well as plasmonic nanosensor applications and tuning of radiative properties of quantum emitters. [less ▲]

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See detailA closer look at the low frequency dynamics of vortex matter
Raes, Bart; de Souza Silva, Clecio C.; Silhanek, Alejandro ULg et al

in Physical Review. B : Condensed Matter (2014), 90

Using scanning susceptibility microscopy, we shed new light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain ... [more ▼]

Using scanning susceptibility microscopy, we shed new light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain the macroscopic ac electromagnetic properties of superconductors. The measurements, carried out on a 2H-NbSe2 single crystal at relatively high temperature T = 6.8 K, show a linear amplitude dependence of the global ac-susceptibility for excitation amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude response, typically attributed to the oscillation of vortices in a potential well defi ned by a single, relaxing, Labusch constant, actually corresponds to strongly non-uniform vortex shaking. This is particularly pronounced in the fi eld-cooled disordered phase, which undergoes a dynamic reorganization above 0.8 Oe as evidenced by the healing of lattice defects and a more uniform oscillation of vortices. These observations are corroborated by molecular dynamics simulations when choosing the microscopic input parameters from the experiments. The theoretical simulations allow us to reconstruct the vortex trajectories providing deeper insight in the thermally induced hopping dynamics and the vortex lattice reordering. [less ▲]

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See detailClassical analogy for the deflection of flux avalanches by a metallic layer
Brisbois, Jérémy ULg; Vanderheyden, Benoît ULg; Colauto, Fabiano et al

in New Journal of Physics (2014), 16(10), 103003

Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably ... [more ▼]

Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably, in some cases the flux is totally excluded from the area covered by the conductive layer. We present a simple classical model that accounts for this behaviour and considers a magnetic monopole approaching a semi-infinite conductive plane. This model suggests that magnetic braking is an important mechanism responsible for avalanche deflection. [less ▲]

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See detailLower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap
Abdel-Hafiez, M.; Pereira, P.J.; Kuzmichev, S.A. et al

in Physical Review. B : Condensed Matter (2014), 90

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field ... [more ▼]

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field Hc1(T) of a Ca0.32Na0.68Fe2As2 (Tc ≈ 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same Tc. We estimated the range of the large gap value L = 6–8 meV (depending on small variation of Tc) and its a k space anisotropy of about 30%, and the small gap Delta ≈ 1.7 ± 0.3 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps. [less ▲]

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See detailGeometrical guidance and trapping transition of human sperm cells
Guidobaldi, Alejandro; Jeyaram, Yogesh; Berdakin, Ivan et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2014), 89

The guidance of human sperm cells under con finement in quasi 2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations ... [more ▼]

The guidance of human sperm cells under con finement in quasi 2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with diluted sperm populations, for which eff ects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identi ed and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specifi c swimming strategy. [less ▲]

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See detailDetermination of the magnetic penetration depth in a superconducting Pb film
Brisbois, Jérémy ULg; Raes, Bart; Van de Vondel, Joris et al

in Journal of Applied Physics (2014), 115(10), 103906

By means of scanning Hall probe microscopy technique we accurately map the magnetic field pattern produced by Meissner screening currents in a thin superconducting Pb stripe. The obtained field profile ... [more ▼]

By means of scanning Hall probe microscopy technique we accurately map the magnetic field pattern produced by Meissner screening currents in a thin superconducting Pb stripe. The obtained field profile allows us to quantitatively estimate the Pearl length Λ without the need of pre-calibrating the Hall sensor. This fact contrasts with the information acquired through the spatial field dependence of an individual flux quantum where the scanning height and the magnetic penetration depth combine in a single inseparable parameter. The derived London penetration depth λL coincides with the values previously reported for bulk Pb once the kinetic suppression of the order parameter is properly taken into account. [less ▲]

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See detailFluxonics
Silhanek, Alejandro ULg

Conference (2014, January)

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See detailControllable morphology of flux avalanches in microstructured superconductors
Motta, M.; Colauto, F.; Vestgarden, J.I. et al

in Physical Review. B : Condensed Matter (2014)

The morphology of abrupt bursts of magnetic flux into superconducting films with engineered periodic pinning centers (antidots) has been investigated. Guided flux avalanches of thermomagnetic origin ... [more ▼]

The morphology of abrupt bursts of magnetic flux into superconducting films with engineered periodic pinning centers (antidots) has been investigated. Guided flux avalanches of thermomagnetic origin develop a tree-like structure, with the main trunk perpendicular to the borders of the sample, while secondary branches follow well-defi ned directions determined by the geometrical details of the underlying periodic pinning landscape. Strikingly, we demonstrate that in a superconductor with relatively weak random pinning, the morphology of such flux avalanches can be fully controlled by proper combinations of lattice symmetry and antidot geometry. Moreover, the resulting flux patterns can be reproduced, to the fi nest details, by simulations based on a phenomenological thermomagnetic model. In turn, this model can be used to predict such complex structures and to estimate physical variables of more di fficult experimental access, such as the local values of temperature and electric fi eld. [less ▲]

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See detailLateral Magnetic Near-Field Imaging of Plasmonic Nanoantennas With Increasing Complexity
Denkova, D.; Verellen, N.; Silhanek, Alejandro ULg et al

in Small : Nano Micro (2014)

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field ... [more ▼]

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near-field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near-field distributions of variously shaped plasmonic nanoantennas by using hollow-pyramid aperture scanning near-field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality. [less ▲]

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See detailCURRENT CROWDING EFFECTS IN NANOSTRUCTURED SUPERCONDUCTORS
Silhanek, Alejandro ULg

Conference (2013, December)

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See detailTemperature dependence of the lower critical field Hc1(T) evidences nodeless superconductivity in FeSe
Abdel-Hafiez, M.; Ge, J.; Vasiliev, A.N. et al

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

We investigate the temperature dependence of the lower critical fi eld Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the ... [more ▼]

We investigate the temperature dependence of the lower critical fi eld Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration fi eld has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change of the Hc1(T) curvature is observed, which is attributed to multiband superconductivity. The London penetration depth Lambda_ab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes Delta_1 = 0.41 +- 0.1meV and Delta_2 = 3.33+- 0.25meV or a single anisotropic s-wave order parameter, the temperature-dependence of the lower critical eld Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless. [less ▲]

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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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