Controlling flux flow dissipation by changing flux pinning in superconducting films

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 ▲]

Localization of superconductivity in superconductor–electromagnet hybrids

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 ▲]

Scanning Hall probe microscopy of unconventional vortex patterns in the two-gap MgB2 superconductor

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 ▲]

Morphology of Flux Avalanches in Patterned Superconducting

in Journal of Superconductivity & Novel Magnetism (2012)

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 ▲]

Flux avalanches triggered by AC magnetic fields in superconducting thin films

in Physica C (2012), 479

Flux avalanches are known to occur as a consequence of thermomagnetic instabilities. Some of their fingerprints are jumps in magnetization curves, or a paramagnetic reentrance in AC susceptibility ... [more ▼]

Flux avalanches are known to occur as a consequence of thermomagnetic instabilities. Some of their fingerprints are jumps in magnetization curves, or a paramagnetic reentrance in AC susceptibility measurements. In this work we have studied flux avalanches triggered by an AC field cycle by means of AC susceptibility and residual magnetization after an applied AC field measured as a function of an AC excitation field (h). These measurements allow comparing both results with magneto-optical imaging carried out in similar conditions. The results show a correspondence for the onset of the avalanche activity, as well as between the residual magnetic moment and the mean gray value calculated from the magneto-optical images in the remanent state. [less ▲]

Robustness of the scanning second harmonic generation microscopy technique for characterization of hotspot patterns in plasmonic nanomaterials

in Proceedings of SPIE (2012), 8424

Scanning second harmonic generation (SHG) microscopy is becoming an important tool for characterizing nanopatterned metal surfaces and mapping plasmonic local field enhancements. Here we study G-shaped ... [more ▼]

Scanning second harmonic generation (SHG) microscopy is becoming an important tool for characterizing nanopatterned metal surfaces and mapping plasmonic local field enhancements. Here we study G-shaped and mirror-G-shaped gold nanostructures and test the robustness of the experimental results versus the direction of scanning, the numerical aperture of the objective, the magnification, and the size of the laser spot on the sample. We find that none of these parameters has a significant influence on the experimental results. [less ▲]

Guided vortex motion and vortex ratchets in nanostructured superconductors

in Moshchalkov, V.V.; Woerdenweber, R.; Lang, W. (Eds.) Nanoscience and Engineering in Superconductivity (2010)

In type II superconductors, an external magnetic field can partially penetrate into the superconducting phase in the form of magnetic flux lines or vortices. The repulsive interaction between vortices ... [more ▼]

In type II superconductors, an external magnetic field can partially penetrate into the superconducting phase in the form of magnetic flux lines or vortices. The repulsive interaction between vortices makes them to arrange in a triangular lattice, known as Abrikosov vortex lattice. This periodic vortex distribution is very fragile and can be easily distorted by introducing pinning centers such as local alterations of the superconducting condensate density. The dominant role of the vortex-pinning site interaction not only permits to control the static vortex patterns and to enhance the maximum dissipationless current sustainable by the superconducting material but also allows one to gain control on the dynamics of vortices. Among the ultimate motivations behind the manipulation of the vortex motion are the better performance of superconductor-based devices by reducing the noise in superconducting quantum interference-based systems, development of superconducting terahertz emitters, reversiblemanipulation of local field distribution through flux lenses, or even providing a way to predefine the optical transmission through the system. In this chapter, we discuss two relevant mechanisms used in most envisaged fluxonics devices, namely the guidance of vortices through predefined paths and the rectification of the average vortex motion. The former can be achieved with any sort of confinement potential such as local depletion of the order parameter or local enhancements of the current density. In contrast, rectification effects result from the lack of inversion symmetry of the pinning landscape which tends to favor the vortex flow in one particular direction. We also discuss a new route for further flexibility and tunability of these fluxonics components by introducing ferromagnetic pinning centers interacting with vortices via their magnetic stray field. [less ▲]

Flux pinning properties of holes and blind holes arranged periodically in a superconductor

in Physica C: Superconductivity (2004), 404(1-4), 298-301

We investigate the dynamic response of a vortex lattice in nanostructured superconducting Pb films by means of ac-magnetization measurements. We compare the flux pinning properties of superconducting Pb ... [more ▼]

We investigate the dynamic response of a vortex lattice in nanostructured superconducting Pb films by means of ac-magnetization measurements. We compare the flux pinning properties of superconducting Pb films with arrays of fully perforated holes (antidots) and partially drilled holes (blind holes). Our results show that an array of blind holes, characterized by a thin superconducting bottom layer, gives rise to a less efficient pinning potential than an array of antidots. On top of that, a lower number of flux quanta trapped per pinning site (the saturation number), is observed for the blind hole array. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

Dynamic regimes in films with a periodic array of antidots

in European Physical Journal B -- Condensed Matter (2004), 37(1), 19-24

We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac drive amplitudes h, vortices moving inside the ... [more ▼]

We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac drive amplitudes h, vortices moving inside the pinning potential give rise to a frequency- and h-independent response together with a scarce dissipation. For higher amplitudes, the average distance travelled by vortices surpasses the pinning range and a critical state develops. We found that the boundary h*(H,T) between these regimes smoothly decreases as T increases whereas a step-like behavior is observed as a function of field. We demonstrate that these steps in h*(H) arise from sharp changes in the pinning strength corresponding to different vortex configurations. For a wide set of data at several fields and temperatures in the critical state regime, we show that the scaling laws based on the simple Bean model are satisfied. [less ▲]