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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.

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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

Analysis of the crystal weighing method applied to liquid encapsulated Czochralski growth

/pdf/ray-optics-behavior-of-flu-x-avalanche-propagation-in-4bbnkxfg03.pdf

Ray optics behavior of flu x avalanche propagation in superconducting fi lms

We present high-resolution measurements of the repulsive vertical force and its associated stiffness between a NdBFe magnet and a YBa 2 Cu 3 O 7−δ superconductor in cylindrical geometry. The results are compared with theoretical predictions. The calculations are based on a model in which the superconductor is assumed to be either a sintered granular material or consisting of grains embedded in a nonactive matrix so that only intragranular currents are important. The critical state model is applied to each grain individually and closed form expressions for both vertical force F z and stiffness are obtained in a configuration with cylindrical symmetry. The present model explains all features of the experimental results in a consistent way. A very good quantitative agreement has been obtained using only three adjustable parameters.

The vertical magnetic force and stiffness between a cylindrical magnet and a high-Tc superconductor

The ac losses at 20 and 30 Hz were measured for two disk-shaped YBa2Cu3O7 films in perpendicular peak applied magnetic fields up to ∼0.2 T in liquid nitrogen. One of the films had a significantly higher critical-current density than the other as determined from the loss measurements. Also, it exhibited a more-uniform flux penetration around the circumference of the disk than the other as observed by magneto-optical images of these films in perpendicular dc fields. The results from this film were compared with theoretical predictions of ac losses for disks of thin superconducting films in perpendicular magnetic fields using the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 55, 9355 (1994)], and the Kim critical-current model [D. V. Shantsev et al., Phys. Rev. B 61, 9699 (2000)]. The asymptotic Bean model predictions for low and high fields were in reasonably good agreement with the data. The numerical calculation of the losses following Shantsev et al. was found to give extremely good agreement with the loss data throughout the entire field range of the measurement when the Kim model for the critical-current density was used.

ac losses in circular disks of thin YBa2Cu3O7 films in perpendicular magnetic fields

Stability of the vortex matter -- magnetic flux lines penetrating into the material -- in type-II superconductor films is crucially important for their application. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger an electromagnetic breakdown. In this paper, we review recent theoretical and experimental results on development of the above mentioned thermomagnetic instability. Starting from linear stability analysis for the initial critical-state flux distribution we then discuss a numerical procedure allowing to analyze developed flux avalanches. As an example of this approach we consider ultra-fast dendritic flux avalanches in thin superconducting disks. At the initial stage the flux front corresponding to the dendrite's trunk moves with velocity up to 100~km/s. At later stage the almost constant velocity leads to a specific propagation regime similar to ray optics. We discuss this regime observed in superconducting films coated by normal strips. Finally, we discuss dramatic enhancement of the anisotropy of the flux patterns due to specific dynamics. In this way we demonstrate that the combination of the linear stability analysis with the numerical approach provides an efficient framework for understanding the ultra-fast coupled non-local dynamics of electromagnetic fields and dissipation in superconductor films.

/pdf/nucleation-and-propagation-of-thermomagnetic-avalanches-in-1mytxci3bf.pdf

Tom H. Johansen

Papers

Analysis of the crystal weighing method applied to liquid encapsulated Czochralski growth

Ray optics behavior of flu x avalanche propagation in superconducting fi lms

The vertical magnetic force and stiffness between a cylindrical magnet and a high-Tc superconductor

ac losses in circular disks of thin YBa2Cu3O7 films in perpendicular magnetic fields

Nucleation and propagation of thermomagnetic avalanches in thin-film superconductors