Impedance measurements performed on a weakly frustrated triangular array of Josephson junctions over a wide range of frequencies and at temperatures such that vortex pinning is irrelevant reveal that vortex dynamics in an ideal two-dimensional (2D) superconductor does not obey Drude's classical prediction for a 2D Coulomb gas of free and independent vortex charges. An analysis in terms of a complex vortex dielectric constant implies that the vortex mobility vanishes logarithmically in the limit of small frequencies, thereby pointing to anomalous vortex diffusion.PACS numbers: 74.60. Ge, 74.25.Nf, The concept of vortex is essential to understand the physics of two-dimensional (2D) superfluids [1], In neutral superfluids and, under appropriate conditions, in charged superfluids the interaction between two vortices depends logarithmically on their separation, a feature leading to a natural description of the vortex medium in terms of a 2D Coulomb gas analog [2]. Detailed insight into the physics of 2D superfluids emerging from this picture is provided by studies of their response to a timedependent perturbation. The Andronikashvili torsional oscillator has proven to be quite successful to investigate vortex dynamics near the Kosterlitz-Thouless (KT) transition of liquid-helium films [3]. A corresponding probe for 2D superconductors relies on a two-coil mutual inductance technique [4] which allows us to extract the dynamical properties of the vortices from measurements of the sample's complex sheet impedance [5,6]. However, the experiments performed so far on liquid-helium films could not systematically explore the response as a function of frequency, while the investigations carried out on superconducting films [7] were almost unvariably affected by pinning effects masking the intrinsic 2D Coulomb-gas properties of the vortex medium. In this Letter, we report a study of the complex dielectric constant sico) of a dilute system of vortices created by a small perpendicular magnetic field H in an almost pinning-free triangular array of Josephson junctions. Our data, taken over a wide range of driving angular frequencies co, reveal novel and unexpected aspects of the dynamics of vortex excitations in an ideal, i.e., pinning-free, 2D superconductor.Compared to superconducting films, Josephson junction arrays (JJA) prepared with modern microfabrication techniques provide nearly ideal systems in which vortex pinning due to ever present disorder can be kept at extremely low levels. Moreover, in triangular arrays intrinsic pinning effects resulting from the periodic nature of the system are much weaker than in other lattice structures and become totally irrelevant at temperatures T appreciably lower than the zero-field KT transition temperature TKT [8]. Thus, if T is not too far below TKT and H corresponds to small values (<0.05) of the frustration parameter /, defined as the number of flux quanta per elementary triangular cell, one would expect the vortex medium in a triangular JJA to behave as a 2D Coulomb gas of free (i.e....

Poly(ethylene‐co‐vinyl acetate) (EVA) is the primary polymer used for encapsulation of photovoltaic (PV) modules. Its degree of cross‐linking (gel content) is taken as a major quality reference. The EVA gel content is normally measured by Soxhlet extraction and more recently also by Differential Scanning Calorimetry (DSC). The DSC method is proven here to be fast and effective but is, as the Soxhlet extraction method, destructive to the PV module. With the aim of developing a fast and non‐destructive method to determine the gel content, a number of analytical techniques are presented. The most promising method is ultraviolet/visible/near‐infrared (UV/Vis/NIR) optical transmission. The measured diffuse transmission reflects the EVA crystallite size, which is related to the EVA gel content. This opens the possibility to apply an in‐line analysis of every PV module immediately after the lamination step and could significantly contribute to the process quality control that is needed in future high‐throughput production lines of PV modules. Copyright © 2011 John Wiley & Sons, Ltd.

We review our recent progress on the fabrication of near-infrared photodetectors based on intersubband transitions in AlN/GaN superlattice structures. Such devices were first demonstrated in 2003, and have since then seen a quite substantial development both in terms of detector responsivity and high speed operation. Nowadays, the most impressive results include characterization up to 3 GHz using a directly modulated semiconductor laser and up to 13.3 GHz using an ultra-short pulse solid state laser.

considerations, we will concentrate on the fabrication and characterization of modulators, switches, photodetectors, and light emitters. At the end of this paper, an outlook to future trends and developments in this emerging field will be given.

We report on a quantum cascade detector based on nearly strain compensated InGaAs/ InAlAs pseudomorphically grown on InP substrate and detecting light at short wavelengths around 4 m. The background limited infrared performance ͑BLIP͒ condition is met at a temperature of 108 K with a high detectivity of D BLIP * = 1.2ϫ 10 11 Jones.