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Marchevsky M.,Lawrence Berkeley National Laboratory | Defeo M.P.,Syracuse University | Metlushko V.,University of Illinois at Chicago | Higgins M.J.,Princeton High School | Bhattacharya S.,Tata Institute of Fundamental Research
Journal of Superconductivity and Novel Magnetism | Year: 2013

Stochastic fluxon dynamics and spatial ordering are studied in a superconducting Nb film patterned with a periodic (3×3 micron) square array of holes. First, imaging of the film is performed using Scanning Hall Probe Microscope following application of the random noise magnetic drive. We observe formation of the fluxon stripe patterns oriented preferentially along the principal axes of the array. Next, ac magnetic field is applied as superposition of harmonic and random noise components and the local magnetic response is probed with the micron-sized Hall sensor at a single array site. We find that hopping of the fluxons becomes synchronized with the harmonic driving at certain amplitude of the random noise; the latter is found to be dependent upon the harmonic drive frequency. We attribute our observations to the phenomenon of stochastic resonance that is ubiquitous in non-linear multi-level systems with dynamic threshold and discuss a possibility of flux manipulation using stochastically-resonant periodic driving. © Springer Science+Business Media New York (outside the USA) 2013.

Lim J.-Y.,Hongik University | Park S.-Y.,Hongik University | Lee S.-W.,Hongik University | Yun J.-H.,Princeton High School | And 2 more authors.
Journal of Nanoscience and Nanotechnology | Year: 2016

The lamellar-forming 1-dimensional photonic films were prepared by three different molecular weight, but nearly symmetric poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) block copolymers. Propylene glycol monomethyl ether acetate (PGMEA) was used as solvent to spin-cast the films. The films were submerged into ethanol to swell P2VP block. As the molecular weight of the block copolymer increases, the film shows red-shifted color, because of longer periodic distance. In addition, the color of the films in ethanol was tuned by blending the PS-b-P2VP block copolymers. Copyright © 2016 American Scientific Publishers All rights reserved.

Menon G.I.,Chennai Mathematical Institute | Menon G.I.,National University of Singapore | Ravikumar G.,Bhabha Atomic Research Center | Higgins M.J.,Princeton High School | Bhattacharya S.,Tata Institute of Fundamental Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

An analytic formula describing the suppression of the equilibrium melting line by quenched point pinning disorder is compared to data from ac susceptibility and magnetization measurements in the mixed phase of the layered dichalcogenide low-T c superconductor 2H-NbSe 2. This material exhibits a sharp peak effect in the critical current j c close to the upper critical field H c2(T). Arguing that the disorder-suppressed melting line in this system is to be identified with the locus of peak positions of the critical current as magnetic field and temperature are varied, we demonstrate that this formula provides a remarkably accurate fit to the experimental data over three orders of magnitude in magnetic field. © 2012 American Physical Society.

Lim J.Y.,Hongik University | Yun J.H.,Princeton High School | Jang D.B.,Boston Trinity Academy | Shin D.M.,Hongik University
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

One dimensional structures often limit viewing angle as well as reflection color changes. We report a simple strategy to fabricate the ellipsoidal block copolymer nanoparticles, which have a wider viewing angle. The photonic crystals were made with the polystyrene-block-poly-2-vinyl pyridine(PS-b-P2VP) dispersed in chloroform. When the PS-b-P2VP was sonicated in polyvinyl alcohol(PVA), it became white emulsion. The photonic crystals were formed by self-assembly of block copolymers. We observed particle structure variations according to the difference in molecular weight, which were 52k-57k, 75k-66.5k, and 102k-97k. The swelling and nature of light scattering from structured particles changed by the addition of ethanol. The structure of the photonic crystals was investigated by the Transmission Electron Microscope and the color was measured by color-difference meter. The particles with low molecular weight showed the short wavelength reflection. © 2015 SPIE.

Marchevsky M.,Syracuse University | Marchevsky M.,Lawrence Berkeley National Laboratory | Higgins M.J.,Princeton High School | Bhattacharya S.,Tata Institute of Fundamental Research | Fratello V.J.,Integrated Photonics Inc.
Superconductor Science and Technology | Year: 2011

In a superconducting film deposited on ferromagnetic substrate with perpendicular magnetic anisotropy, vortex matter is confined by the magnetic potential landscape. Using scanning Hall microscopy we visualize flux accumulation and removal in a superconductor-ferrimagnet (S/F) bilayer prepared by rf sputtering of thin niobium film on bismuth-doped rare-earth iron garnet. Penetration of the perpendicular magnetic field in the S/F bilayer follows magnetic domain boundaries and is laterally guided by the garnet magnetization component along the field direction. Upon field removal, localization of the remnant flux at the disclination points of the labyrinthine domain pattern is observed. Our experiments show evidence for strong vortex pinning due the special topology of the domain pattern. Ac magnetic imaging of the transport current distribution in the bilayer reveals complex flow paths commensurate with the magnetic domain boundaries. Topological magnetic pinning can be a useful tool for enhancement and control of critical current in thin film superconductors. © 2011 IOP Publishing Ltd.

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