Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Laser nanosources based on planar photonic crystals as new platforms for nanophotonic devices

Two-dimensional photonic crystal lasers have been fabricated on III–V semiconductor slabs. Tuning of the spontaneous emission in micro and nanocavities has been achieved by accurate control of the slab thickness. Different structures, some of them of new application to photonic crystal lasers, have been fabricated like the Suzuki-phase or the coupled-cavity ring-like resonators. Laser emission has been obtained by pulsed optical pumping. Optical characterization of the lasing modes have been performed showing one or more laser peaks centred around 1.55 μm. Far field characterization of the emission pattern has been realized showing different patterns depending on the geometrical shape of the structures. These kinds of devices may be used as efficient nanolaser sources for optical communications or optical sensors.     

Synthesis of a Naphthalimide Functionalized Calix[4]arene; A Host Type Fluorophore for Inclusion Compounds in Organic Medium

This article reports the synthesis and the properties of 5,11,17,23-tetra(t-butyl)-25,27-bis-(ethoxy-1,8-naphthalimide)-26,28-hidroxy-calix[4]arene and the formation of an inclusion compound in organic medium. This functionalized calix[4]arene was conceived as the association of a potential host species with a good fluorophore for optical sensoring purposes. Calix[4]-NI as we will call it, maintains its ‘cone-pinched’ configuration and exhibits typical naphthalimide fluorescence bands in non-polar solvents. Its ability to interact with guest species via hydrogen bonding in its endo-hydrophilic cavity to form inclusion compounds was verified with absorption and fluorescence measurements using N-ethanol-1,8-naphthalimide as guest species, which was projected to fit exactly the host cavity and to interact with its naphthalimide π electrons. For this reason, it was possible to follow the formation of the inclusion compound with electronic spectroscopy.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Coarsening Dynamics in a Simplified DNLS Model

We investigate the coarsening evolution occurring in a simplified stochastic model of the Discrete NonLinear Schrödinger (DNLS) equation in the so-called negative-temperature region. We provide an explanation of the coarsening exponent n=1/3, by invoking an analogy with a suitable exclusion process. In spite of the equivalence with the exponent observed in other known universality classes, this model is certainly different, in that it refers to a dynamics with two conservation laws.