Downtown Waterville Feasibility Study Waterville, Maine

The Purpose and Need for this project is to: “Revitalize the Downtown to improve the aesthetics, support existing businesses and encourage economic growth, improve pedestrian and bicycle accommodations and provide adequate parking while maintaining vehicular capacity in the overall area.” Contributions and assistance in the completion of the study were provided by the City of Waterville, Colby College, the Maine Department of Transportation, and the General Public

Nucleation of superconductivity and vortex matter in superconductor-ferromagnet hybrids

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature superconductors (S) and ferromagnets (F), are reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H-ext, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affects the nucleation of superconductivity, leading to an exotic dependence of the critical temperature T-c on H-ext. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram T-H-ext various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this 'soft' S/F hybrid requires rearrangements of both superconducting and ferromagnetic characteristics, as compared with 'hard' S/F structures.status: publishe

Scanning Hall probe microscopy of vortex patterns in a superconducting microsquare

We present direct observation of vortices confined to a superconducting 4×4 µm2 square Pb film at 4.2 K with a high resolution scanning Hall probe microscope. Integrals of the magnetic field penetrating the square as a function of the applied field show that an applied-field dependence of vorticity L agrees with calculations based on the Ginzburg-Landau (GL) equation for a square sample geometry. From asymmetric field distribution of the confined magnetic flux, the locations of vortices on the square are inferred for vorticities L=1–5. The obtained vortex patterns are analogous with the calculations in the framework of the full GL equation for temperatures considerably lower than the superconducting transition temperature Tc.status: publishe

Pinning centers produced by magnetic microstructures

We investigate the flux pinning and dynamic properties of superconducting vortices in an Al film with an array of magnetic bars deposited on top. The dimensions of each bar are chosen in such a way that they host a single magnetic domain. These micromagnets are distributed periodically in a rectangular array with 0.5 mu m separation parallel to the longest side of the bars and displaced laterally by a distance w. We show that, for w > Lambda, where Lambda is the effective field penetration depth, the pinning strength is almost independent of w whereas the critical temperature at zero field, T-c(0), decreases as similar to w(-1). For w < Lambda the opposite behavior is observed, i.e. T-c(0) seems to saturate to a nearly w-independent value and the transition from large to small w is accompanied by a large suppression of the critical current j(c) together with a clear change in the shape of the current-voltage characteristics. In particular, the substantial weakening of the pinning potential for w < Lambda gives rise to an unexpected flux flow response. The field evolution of this regime allows us to determine whether the magnetic bars induce vortex-antivortex pairs in the system. The present findings suggest that practical application of magnetic pinning centers are restricted to low field values.status: publishe

Effect of reversed magnetic domains on superconductivity in Pb/BaFe12O19 hybrids

In this letter, the effect of reversed magnetic domains of BaFe12O19 on superconductivity is investigated in Pb/BaFe12O19 hybrids. The critical field of the Pb film is increased by about 5 kOe due to the compensation of the applied field by the stray field above the reversed domains. Being related to smaller critical fields of Pb, at,fields near the saturation field of BaFe12O19, the superconductivity can only exist above the reversed domains even at low temperatures. As a consequence of the pure reversed domain superconductivity, magnetic-field-induced superconductivity is observed in a broad temperature range. (c) 2006 American Institute. of Physics.status: publishe

Magnetic flux patterns in superconductors deposited on a lattice of magnetic dots: A magneto-optical imaging study

We investigate the flux penetration in Pb films, of different shapes, deposited on top of a periodic array of Co/Pt dots with perpendicular anisotropy by means of magnetization and magneto-optical measurements. A clear dependence of the critical current density on the magnetic state of the dots and their polarity with respect to the direction of the applied magnetic field is observed by both techniques. The magnetic state of the dots changes the flux penetration from smooth to channelling. Additionally, in the fully magnetized state, an anisotropic current distribution is observed in circular-shaped samples. The flux penetration is dominated by avalanches only for configurations which correspond to a high critical current, irrespective of its origin, be it low temperature, magnetization state of the dots, or angle between the lattice of dots and the edge of the sample

Plasmons Reveal the Direction of Magnetization in Nickel Nanostructures

We have applied the surface sensitive nonlinear optical technique of magnetization induced second harmonic generation. (MSHG) to plasmonic, magnetic nanostructures made of Ni. We show that surface plasmon contributions to the MSHG signal can reveal the direction of the magnetization. Both the plasmonic and the magnetic nonlinear optical responses can be tuned; our results indicate novel ways to combine nanophotonics, nanoelectronics, and nanomagnetics and suggest the possibility for large magneto-chiral effects in metamaterials