References of "Silhanek, Alejandro"
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See detailUltra-narrow superconducting junctions: electromigration to shed light on quantum point contacts
Baumans, Xavier ULg; Cerbu, Dorin; Adami, Obaïd-Allah ULg et al

Conference (2016, September 06)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter [1]-[3] as a function of the wire width. Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or λ) have been explored both experimentally and theoretically in depth, the superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much less attention. The lack of experimental results is in part due to the difficulty of sample fabrication, at dimensions beyond the limit reached by conventional lithographic techniques. A promising direction consists of controlling the local displacement of atom by an electron wind, a process known as electromigration (EM) [4] . This effect relies on the combination of local temperature rise and substantial current crowding at nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small electronic devices, it can be used in a controllable way to further decrease locally the cross section of the nanowire towards single atomic contacts. In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~ 150 nm 2 . In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature [5] . We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields [5] which can be attributed to the suppression of superconductivity in the contact leads [6] . Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal the strong potential of the proposed fabrication method to explore various fascinating superconducting phenomena in atomic-size constrictions. [less ▲]

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See detailPhase Slips In Tunable Width Point Contacts Created By Electromigration
Silhanek, Alejandro ULg

Conference (2016, September 05)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. In this presentation, we explore in-situ controlled electromigration (EM) to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailImpurity scattering effects on the superconducting properties and the tetragonal-to-orthorhombic phase transition in FeSe
Abdel-Hafiez, Mahmoud; Pu, Y. J.; Brisbois, Jérémy ULg et al

in Physical Review B (2016), 93

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on ... [more ▼]

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on the magneto-optical imaging, thermodynamic and transport properties, as well as in situ angle-resolved photoemission spectroscopy (ARPES) studies, on the impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site is found to decrease the superconducting transition temperature. The upper critical field and specific heat all indicate a possible multiband superconductivity with strong coupling in the Co-doped system. A remarkable feature in FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, a signature of a crossover from a semiconductinglike behavior to metallic behavior. A structural tetragonal-to-orthorhombic phase transition Ts (a consequence of the electronic nematicity) is suppressed by either physical or chemical pressures. Due to the reconstruction of the Fermi surface at Ts, specific heat anomalies at Ts present ΔCp/Ts≈γn, the Sommerfield coefficient at low temperature. This reflects additional electronic instability in the FeSe(1−x)Sx system. ARPES data between 180 and 282 K indicates the existence of a chemical potential shift with increasing thermal excitations, resulting in a change of the Fermi-surface topology and exhibiting a semimetal behavior. We found that the temperature-induced Lifshitz transition is much higher than the temperature for the nematic order. [less ▲]

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See detailImprinting superconducting vortex footsteps in a magnetic layer
Brisbois, Jérémy ULg; Motta, Maycon; Avila Osses, Jonathan ULg et al

in Scientific Reports (2016), 6

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit ... [more ▼]

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories. [less ▲]

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See detailUltra-narrow superconducting junctions fabricated by controlled electromigration
Silhanek, Alejandro ULg

Conference (2016, April 12)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or λ) have been explored both experimentally and theoretically in depth, the superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much less attention. The lack of experimental results is in part due to the difficulty of sample fabrication, at dimensions beyond the limit reached by conventional lithographic techniques. A promising direction consists of controlling the local displacement of atoms by an electron wind, a process known as electromigration (EM). This effect relies on the combination of local temperature rise and substantial current crowding at nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small electronic devices, it can be used in a controllable way to further decrease locally the cross section of the nanowire towards single atomic contacts. In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~ 150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal the strong potential of the proposed fabrication method to explore various fascinating superconducting phenomena in atomic-size constrictions. [less ▲]

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See detailThermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration
Silhanek, Alejandro ULg

Conference (2016, April)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. In this presentation, we explore in-situ controlled electromigration (EM) to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailMagnetic flux penetration in Nb superconducting films with lithographically defined micro-indentations
Brisbois, Jérémy ULg; Adami, Obaïd-Allah ULg; Avila Osses, Jonathan ULg et al

in Physical Review B (2016), 93(5), 054521

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ... [more ▼]

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines (d-lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent n. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities. [less ▲]

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See detailThermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration
Baumans, Xavier ULg; Cerbu, Dorin; Adami, Obaïd-Allah ULg et al

in Nature Communications (2016), 7

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to ... [more ▼]

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to understand the detrimental fluctuations of the superconducting order parameter as the wire width shrinks. In this paper, we use controlled electromigration to narrow down aluminium nanoconstrictions. We demonstrate that a transition from thermally assisted phase slips to quantum phase slips takes place when the cross section becomes less than 150 nm2 . In the regime dominated by quantum phase slips the nanowire loses its capacity to carry current without dissipation, even at the lowest possible temperature. We also show that the constrictions exhibit a negative magnetoresistance at low-magnetic fields, which can be attributed to the suppression of superconductivity in the contact leads. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic-size contacts. [less ▲]

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See detailProbing the low-frequency vortex dynamics in a nanostructured superconducting strip
de Souza Silva, Clecio C.; Raes, Bart; Brisbois, Jérémy ULg et al

in Physical Review B (2016)

We investigate by scanning susceptibility microscopy the response of a thin Pb strip, with a square array of submicron antidots, to a low-frequency ac magnetic field applied perpendicularly to the film ... [more ▼]

We investigate by scanning susceptibility microscopy the response of a thin Pb strip, with a square array of submicron antidots, to a low-frequency ac magnetic field applied perpendicularly to the film plane. By mapping the local permeability of the sample within the field range where vortices trapped by the antidots and interstitial vortices coexist, we observed two distinct dynamical regimes occurring at different temperatures. At a temperature just below the superconducting transition, T/Tc = 0.96, the sample response is essentially dominated by the motion of highly mobile interstitial vortices. However, at a slightly lower temperature, T/Tc = 0.93, the interstitial vortices freeze up leading to a strong reduction of the ac screening length. We propose a simple model for the vortex response in this system which fits well to the experimental data. Our analysis suggests that the observed switching to the high mobility regime stems from a resonant effect, where the period of the ac excitation is just large enough to allow interstitial vortices to thermally hop through the weak pinning landscape produced by random material defects. This argument is further supported by the observation of a pronounced enhancement of the out-of-phase response at the crossover between both dynamical regimes. [less ▲]

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See detailOrbitally induced hierarchy of exchange interactions in zigzag antiferromagnetic state of honeycomb silver delafossite Ag3Co2SbO6
Zvereva, E.A.; Stratan, M.I.; Ushakov, A.V. et al

in Dalton Transactions (2016)

We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data ... [more ▼]

We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long range order at low temperatures with Néel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed a field-induced (spin-flop type) transition below TN in moderate magnetic fields. The GGA+U calculations show the importance of the orbital degrees of freedom, which maintain a hierarchy of exchange interaction in the system. The strongest antiferromagnetic exchange coupling was found in the shortest Co-Co pairs and is due to direct and superexchange interaction between the half-filled xz+yz orbitals pointing directly to each other. The other four out of six nearest neighbor exchanges within the cobalt hexagon are suppressed, since for these bonds active half-filled orbitals turned out to be parallel and do not overlap. The electron spin resonance (ESR) spectra reveal a broad absorption line attributed to Co2+ ion in octahedral coordination with average effective g-factor g=2.40±0.05 at room temperature and show strong divergence of ESR parameters below ~ 150 K, which implies an extended region of short-range correlations. Based on the results of magnetic and thermodynamic studies in applied fields, we propose the magnetic phase diagram for the new honeycomb-lattice delafossite. [less ▲]

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See detailCurrent crowding in nanostructured superconductors
Silhanek, Alejandro ULg

Scientific conference (2015, November 11)

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See detailThe onset, evolution and magnetic braking of vortex lattice instabilities in nanostructured superconducting films
Adami, Obaïd-Allah ULg; Jelic, Zeljko ULg; Xue, Cun et al

in Physical Review. B : Condensed Matter (2015), 92

In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow ... [more ▼]

In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow regime to a more dissipative state at suficiently high vortex velocities. Typically this transition manifests itself as a large voltage jump at a particular current density, so-called instability current density J, which is smaller than the depairing current. By tuning the effective pinning strength in Al fi lms, using an artifi cial periodic pinning array of triangular holes, we show that a unique and well defi ned instability current density exists if the pinning is strong, whereas a series of multiple voltage transitions appear in the relatively weaker pinning regime. This behavior is consistent with time-dependent Ginzburg-Landau simulations, where the multiple-step transition can be unambiguously attributed to the progressive development of vortex chains and subsequently phase-slip lines. In addition, we explore experimentally the magnetic braking e ffects, caused by a thick Cu layer deposited on top of the superconductor, on the instabilities and the vortex ratchet effect. [less ▲]

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See detailZigzag antiferromagnetic quantum ground state in monoclinic honeycomb lattice antimonates A3Ni2SbO6 (A=Li, Na)
Zvereva, E.A.; Stratan, M.I.; Ovchenkov, Y.A. et al

in Physical Review. B: Condensed Matter and Materials Physics (2015), 92

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb-lattice monoclinic compounds A3Ni2SbO6 (A=Li, Na ... [more ▼]

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb-lattice monoclinic compounds A3Ni2SbO6 (A=Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic (AFM) long range order at low temperatures with Néel temperatures ~ 14 and 16 K for Li3Ni2SbO6 and Na3Ni2SbO6, respectively. The effective magnetic moments of 4.3 Bohr magnetons/f.u. (Li3Ni2SbO6) and 4.4 Bohr magnetons/f.u. (Na3Ni2SbO6) indicate that Ni2+ is in a high-spin configuration (S=1). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature ~ 8 K (Li3Ni2SbO6) and ~12 K (Na3Ni2SbO6) pointing to the presence of non-negligible ferromagnetic interactions, although the system orders AFM at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below TN that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni2+ ions are present in the honeycomb planes supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations we propose magnetic phase diagrams for the two compounds. [less ▲]

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See detailStroboscopic phenomena in superconductors with dynamic pinning landscape
Jelic, Zeljko ULg; Milosevic, Milorad; Van de Vondel, Joris et al

in Scientific Reports (2015), 5

Introducing artificial pinning centers is a well established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain ... [more ▼]

Introducing artificial pinning centers is a well established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain without dissipation. In case of spatially periodic pinning, a clear enhancement of the superconducting critical current arises when commensurability between the vortex configurations and the pinning landscape occurs. With recent achievements in (ultrafast) optics and nanoengineered plasmonics it has become possible to exploit the interaction of light with superconductivity, and create not only spatially periodic imprints on the superconducting condensate, but also temporally periodic ones. Here we show that in the latter case, temporal matching phenomena develop, caused by stroboscopic commensurability between the characteristic frequency of the vortex motion under applied current and the frequency of the dynamic pinning. The matching resonances persist in a broad parameter space, including magnetic field, driving current, or material purity, giving rise to unusual features such as externally variable resistance/impedance and Shapiro steps in current-voltage characteristics. All features are tunable by the frequency of the dynamic pinning landscape. These findings open further exploration avenues for using flashing, spatially engineered, and/or mobile excitations on superconductors, permitting us to achieve advanced functionalities. [less ▲]

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See detailMagnetic flux penetration in Nb superconducting films with lithographically defined micro-indentations
Silhanek, Alejandro ULg

Conference (2015, September 12)

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined micro-indentations. We demonstrate that discontinuity lines ... [more ▼]

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined micro-indentations. We demonstrate that discontinuity lines (d-lines) caused by the abrupt bending of current stream lines around the indentations, follow a nearly parabolic shape with concavity depending on the exact form and size of the indentation. We observe a progressive widening of the parabolic d-lines with increasing temperature whose origin may be linked to the weakening of the non-linearity of the current-voltage characteristics. These findings are corroborated and backed up by rigorous numerical simulations and analytical calculations. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border severely modifies the flux front topology. Unlike what has been predicted in the literature, indentations do not serve as nucleation spots for triggering flux avalanches, but instead help to release the flux pressure and avoid magnetothermal instabilities. [less ▲]

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See detailSpeed limit to the Abrikosov lattice in mesoscopic superconductors
Grimaldi, Gaia; Leo, Antonio; Sabatino, P. et al

in Physical Review. B : Condensed Matter (2015), 92

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material Mo3Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current driven ... [more ▼]

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material Mo3Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current driven switching to the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a limiting maximum velocity as a function of the applied magnetic fi eld. Based on time dependent Ginzburg-Landau simulations we argue that the observed behavior is owed to the high velocity vortex dynamics confi ned on a mesoscopic scale. We build up a general phase diagram which includes all possible dynamic confi gurations of Abrikosov lattice in a mesoscopic superconductor. [less ▲]

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See detailElectric current crowding in nanostructured conductors
Silhanek, Alejandro ULg

Conference (2015, May 24)

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 paths. 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 paths. The consequent current crowding is present in normal metals as well as in superconductors and has been, since long ago, identified as a key player in electromigration phenomena leading to failure in integrated circuit interconnects while might also be at the origin of local ablation in planar plasmonic structures. More recently, current crowding has been recognized as an important factor limiting the performance of superconducting single-photon detectors, leading to flux quanta motion rectification, or being a source of unwanted ratchet signal and depletion of surface barriers for magnetic flux penetration. In this talk, we will also show that in nanostructured superconductors at low temperatures, current crowding can trigger abrupt flux avalanches developing well defined geometrical patterns. Our experimental findings are backed up to the finest details, by simulations based on a phenomenological thermomagnetic model which, in turn, can be used to predict such complex structures and, given its reliability, allows one to estimate physical variables of more difficult experimental access, such as the local values of temperature and electric field. [less ▲]

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See detailClassical analogy for the deflection of flux avalanches by a metallic layer
Silhanek, Alejandro ULg

Conference (2015, May 17)

Motivated by the experimental observation of the exclusion of magnetic flux avalanches in a superconducting sample partially covered by a conducting capping layer, we have investigated the simpli fied ... [more ▼]

Motivated by the experimental observation of the exclusion of magnetic flux avalanches in a superconducting sample partially covered by a conducting capping layer, we have investigated the simpli fied case of the interaction of a magnetic charge (monopole and dipole) with a semi-in finite conducting plane. We have found that early theoretical descriptions for the damping enhancement due to the metallic sheet needed a correction at large vortex velocities where a decrease of the damping coe fficient is expected. We also demonstrate that vortex trajectories are strongly modi fied when penetrating into the area covered by the metallic sheet and may even be fully diverted from that area thus providing a qualitative explanation for the bending of the trajectories of flux avalanches. Our fi ndings may be extended to study the damping of Larkin-Ovchinnikov vortex instabilities and phase-slip lines in current driven systems. [less ▲]

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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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