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Lanthanide-based luminescent hybrid materials.

The proximity effect at superconductor-ferromagnet interfaces produces damped oscillatory behavior of the Cooper pair wave function within the ferromagnetic medium. This is analogous to the inhomogeneous superconductivity, predicted long ago by Fulde and Ferrell (P. Fulde and R. A. Ferrell, 1964, ``Superconductivity in a strong spin-exchange field,'' Phys. Rev. 135, A550--A563), and by Larkin and Ovchinnikov (A. I. Larkin and Y. N. Ovchinnikov, 1964, ``Inhomogeneous state of superconductors,'' Zh. Eksp. Teor. Fiz. 47, 1136--1146 [Sov. Phys. JETP 20, 762--769 (1965)]), and sought by condensed-matter experimentalists ever since. This article offers a qualitative analysis of the proximity effect in the presence of an exchange field and then provides a description of the properties of superconductor-ferromagnet heterostructures. Special attention is paid to the striking nonmonotonic dependence of the critical temperature of multilayers and bilayers on the ferromagnetic layer thickness as well as to the conditions under which ``$\ensuremath{\pi}$'' Josephson junctions are realized. Recent progress in the preparation of high-quality hybrid systems has permitted the observation of many interesting experimental effects, which are also discussed. Finally, the author analyzes the phenomenon of domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.

Proximity effects in superconductor-ferromagnet heterostructures

In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with unbiased external input signals, deterministic and random alike, can assist directed motion of particles at submicron scales. In such cases, one speaks of ``Brownian motors.'' In this review the constructive role of Brownian motion is exemplified for various physical and technological setups, which are inspired by the cellular molecular machinery: the working principles and characteristics of stylized devices are discussed to show how fluctuations, either thermal or extrinsic, can be used to control diffusive particle transport. Recent experimental demonstrations of this concept are surveyed with particular attention to transport in artificial, i.e., nonbiological, nanopores, lithographic tracks, and optical traps, where single-particle currents were first measured. Much emphasis is given to two- and three-dimensional devices containing many interacting particles of one or more species; for this class of artificial motors, noise rectification results also from the interplay of particle Brownian motion and geometric constraints. Recently, selective control and optimization of the transport of interacting colloidal particles and magnetic vortices have been successfully achieved, thus leading to the new generation of microfluidic and superconducting devices presented here. The field has recently been enriched with impressive experimental achievements in building artificial Brownian motor devices that even operate within the quantum domain by harvesting quantum Brownian motion. Sundry akin topics include activities aimed at noise-assisted shuttling other degrees of freedom such as charge, spin, or even heat and the assembly of chemical synthetic molecular motors. This review ends with a perspective for future pathways and potential new applications.

/pdf/artificial-brownian-motors-controlling-transport-on-the-1553poaezw.pdf

Artificial Brownian motors: Controlling transport on the nanoscale

The current-carrying capacity and reliability studies of multiwalled carbon nanotubes under high current densities (>109 A/cm2) show that no observable failure in the nanotube structure and no measurable change in the resistance are detected at temperatures up to 250 °C and for time scales up to 2 weeks. Our results suggest that nanotubes are potential candidates as interconnects in future large-scale integrated nanoelectronic devices.

Reliability and current carrying capacity of carbon nanotubes

Hotspot formation is observed in a structured thin superconducting film of Bi2Sr2CaCu2Ox (BSCCO) using the fluorescent thermal imaging technique. The BSCCO film is deposited on SrTiO3 (STO) and has a superconducting transition at 80 K. A film of rare-earth doped polymer film deposited directly on the superconductor is used as thermal sensor down to 4 K.

Observation of hotspot in BSCCO thin film structure by fluorescent thermal imaging

Experimental studies and theoretical modeling of the levitation force between a permanent magnet and superconducting thin film are reported Measurements of the forceFz and magnetic stiffness k z5udFz /dzu as functions of the magnet-superconductor separation z, show several features contrasting all previous levitation force data for bulk superconductors In particular, the Fz(z) curves measured for decreasing and increasing separation form hysteresis loops of nearly symmetrical shape, also displaying a peak in the repulsive force branch Recent theories for flux penetration in thin type-II superconductors in transverse magnetic fields are invoked to explain the results, which were obtained using a cylindrical Nd-Fe-B magnet and a YBa2Cu3O72d circular disk made by laser ablation We derive explicit formulas for both Fz and k z , reproducing quantitatively all the features seen experimentally @S0163-1829~99!02537-0#

Levitation force from high-T c superconducting thin-film disks

Thermomagnetic instability in general, and dendritic flux avalanches in particular, have attracted considerable attention of both scientists and engineers working on superconductor applications. Though being harmful for the performance of many superconducting devices, the avalanches provide a fruitful playground for experimental and theoretical studies of complex dynamics of the vortex matter. In this paper we report on the progress in understanding the mechanisms responsible for the development of the giant magnetic avalanches. We review recent results on magneto-optical imaging of the fingering instability in superconducting films and analyze them on the basis of recent theoretical model that establishes criteria for onset of the dendritic avalanches.

/pdf/dendritic-flux-avalanches-in-superconducting-films-2tkwtqe0oc.pdf

Dendritic flux avalanches in superconducting films

We have observed the occurrence of dendritic flux avalanches in an amorphous film of Mo
 84
Si
 16
. These events are understood to have a thermomagnetic origin and involve the abrupt penetration of bursts of magnetic flux taking place within a limited window of temperatures and magnetic fields. While dc-magnetometry allows one to determine the threshold fields for the occurrence of the thermomagnetic instabilities, magneto-optical imaging reveals the spatial distribution of magnetic flux throughout the sample. Conducting appropriate experiments, typical for this goal, avalanches were confirmed to be a characteristic of this material, ruling out the otherwise admissible possibility of an experimental artifact or a feature related to defects in the film. After the present observation, amorphous MoSi alloys, or a-MoSi, can be included in the gallery of superconducting materials exhibiting flux avalanches when in the form of thin films, a characteristic that must be carefully taken into consideration when one plans to employ films of those materials in applications.

First Observation of Flux Avalanches in a-MoSi Superconducting Thin Films

The time dependence of flux patterns of an untwinned YBa2Cu3O7 ? ? single crystal showing the meandering instability is observed at T = 65 K using magneto-optical imaging. When applying a reversed external field to a remanent state, along the front of penetrating antivortices, flux droplets are formed which can separate from the flux front and move in a spiral-like fashion. It is shown that the vortices along the flux front are in fast motion. These observations are discussed using a newly developed theory. Furthermore, the flux patterns are analyzed in terms of flux creep.

Tom H. Johansen

Papers

Observation of hotspot in BSCCO thin film structure by fluorescent thermal imaging

Levitation force from high-T c superconducting thin-film disks

Dendritic flux avalanches in superconducting films

First Observation of Flux Avalanches in a-MoSi Superconducting Thin Films

Turbulent relaxation in the vortex lattice