Influence of artificial pinning on vortex lattice instability in superconducting films

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 ▲]

Imaging the Statics and Dynamics of Superconducting Vortices and Antivortices Induced by Magnetic Microdisks

in Physical Review X (2011), 1(2), 021004

If a magnet of microscopic dimensions is brought in close proximity to a superconductor, the quantized nature of their interaction due to the creation of flux quanta in the superconducting system becomes ... [more ▼]

If a magnet of microscopic dimensions is brought in close proximity to a superconductor, the quantized nature of their interaction due to the creation of flux quanta in the superconducting system becomes noticeable. Herein, we directly image, via scanning Hall microscopy, the vortex-antivortex pairs in a superconducting film created by micromagnets. The number of antivortices at equilibrium conditions can be changed either by tuning the magnetic moment of the magnets or by annihilation with externally induced vortices. We demonstrate that small ac field excitations shake the antivortices sitting next to the micromagnets whereas no sizable motion is observed for the vortices sitting on top of the magnets, clearly revealing the different mobility of these two vortex species. A metastable state, which is obtained by applying a field after the system has been cooled down below the superconducting transition, shows a complex graded distribution of coexisting vortices and antivortices forming an intertwined critical state. [less ▲]

Hotspot Decorations Map Plasmonic Patterns with the Resolution of Scanning Probe Techniques

in Physical Review Letters (2011), 106(22),

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be ... [more ▼]

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved. [less ▲]

Linearly polarized second harmonic generation microscopy reveals chirality (vol 18, pg 8286, 2010)

in Optics Express (2011), 19(10), 9242-9244

An erratum is presented to clarify both the direction of light polarization in our experiments and the unit cell geometry of the samples. Additionally, the spectral dependence of the second harmonic ... [more ▼]

An erratum is presented to clarify both the direction of light polarization in our experiments and the unit cell geometry of the samples. Additionally, the spectral dependence of the second harmonic generation response in the G-shaped nanostructures made of gold is corrected. These changes do not affect our previous conclusions. (C) 2011 Optical Society of America [less ▲]

Vortex Core Deformation and Stepper-Motor Ratchet Behavior in a Superconducting Aluminum Film Containing an Array of Holes

in Physical Review Letters (2011), 106(13),

We investigated experimentally the frequency dependence of a superconducting vortex ratchet effect by means of electrical transport measurements and modeled it theoretically using the time-dependent ... [more ▼]

We investigated experimentally the frequency dependence of a superconducting vortex ratchet effect by means of electrical transport measurements and modeled it theoretically using the time-dependent Ginzburg-Landau formalism. We demonstrate that the high frequency vortex behavior can be described as a discrete motion of a particle in a periodic potential, i.e., the so-called stepper-motor behavior. Strikingly, in the more conventional low frequency response a transition takes place from an Abrikosov vortex rectifier to a phase slip line rectifier. This transition is characterized by a strong increase in the rectified voltage and the appearance of a pronounced hysteretic behavior. [less ▲]

Field polarity dependent nucleation of superconductivity in quasi-one-dimensional magnetic templates

in Physica C: Superconductivity (2010), 470(19), 860-862

We investigate the nucleation of superconductivity in an Al/Al(2)O(3)/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 ... [more ▼]

We investigate the nucleation of superconductivity in an Al/Al(2)O(3)/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 mu m thick Py-film form stripes of magnetic domains with alternating out-of-plane stray field. After applying a strong out of plane magnetic field H the superconductor/normal phase boundary becomes asymmetric with respect to H = 0. This lack of field polarity symmetry results from the unbalanced size distribution of domains with opposite polarity. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

Pinning effects on the vortex critical velocity in type-II superconducting thin films

in Physica C: Superconductivity (2010), 470(19), 904-906

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field ... [more ▼]

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

Guided nucleation of superconductivity on a graded magnetic substrate

in Applied Physics Letters (2010), 96(3),

We demonstrate the controlled spatial nucleation of superconductivity in a thin film deposited on periodic arrays of ferromagnetic dots with gradually increasing diameter. The perpendicular magnetization ... [more ▼]

We demonstrate the controlled spatial nucleation of superconductivity in a thin film deposited on periodic arrays of ferromagnetic dots with gradually increasing diameter. The perpendicular magnetization of the dots induces vortex-antivortex molecules in the sample, with the number of (anti)vortices increasing with magnet size. The resulting gradient of antivortex density between the dots predetermines local nucleation of superconductivity in the sample as a function of the applied external field and temperature. In addition, the compensation between the applied magnetic field and the antivortices results in an unprecedented enhancement of the critical temperature. [less ▲]

A Study of Chirality in Gold Nanostructures with Second Harmonic Generation

in Acta Physica Polonica A (2009), 116(4), 498-500

We present the second harmonic generation - circular dichroism response from C-shaped gold nanostructures for P-polarized incoming light. By eliminating the contributions of anisotropy to the signal, an ... [more ▼]

We present the second harmonic generation - circular dichroism response from C-shaped gold nanostructures for P-polarized incoming light. By eliminating the contributions of anisotropy to the signal, an absolute measure of the second harmonic generation - circular dichroism is presented. The system of planar nanostructures is an excellent model for studying the correlation between optical properties and chiral morphology at the nanoscale. Samples with both enantiomorphs of the chiral pattern were prepared and, demonstrating the unambiguous presence of chirality, the second harmonic generation - circular dichroism response from two such samples was fitted simultaneously with a single set of fitting parameters. [less ▲]

Magnetically controlled superconducting weak links

in Applied Physics Letters (2009), 95(3),

We have designed and characterized a magnetic template which can be switched between chains of parallel and antiparallel field distribution by applying an in-plane magnetic field. The parallel field ... [more ▼]

We have designed and characterized a magnetic template which can be switched between chains of parallel and antiparallel field distribution by applying an in-plane magnetic field. The parallel field profile creates highly mobile vortex channels in a superconducting film deposited on top, reproducing the behavior of a weak link as evidenced by the presence of Shapiro steps in the current-voltage characteristics under rf excitation. The Josephson coupling can be fully suppressed by changing the field distribution to the antiparallel state. As a result, a reversible ON/OFF switch for magnetically induced weak links has been demonstrated. [less ▲]