Josephson Effect in the Micron and Submicron YBCO Constrictions Fabricated Using the Femtosecond Laser Technique

A femtosecond laser was used successfully to fabricate planar micron and submicron-sized constrictiontype Josephson junctions on YBa₂Cu₃O_7?x thin films. A simple program using G-code (control systems) programming language was written to control the movement of the sample stage during the etching process. The constriction’s geometry was investigated using both atomic force microscopy (AFM) and scanning electron microscopy (SEM). Electrical transport measurements were performed at different temperatures. Shapiro steps were observed and analyzed. The micron-sized constriction shows a linear relationship for the measured critical current against the temperature which is consistent with the behavior of an S–s’–Stype Josephson junction where “S” stands for a bulk superconductive material that is untouched by the laser and “s”’ is superconducting material whose critical temperature is lower than the value of “S” In the case of the narrower submicronsized constriction, the measured critical current dependence with temperature shows an exponential decay, which is consistent with the behavior of the long S–N–Stype Josephson junction where “N” stands for a normal material. A model is proposed to describe the observed behavior by considering the effect of sample heating during the constriction’s fabrication.     

In vitro cell-biological performance and structural characterization of selective laser sintered and plasma surface functionalized polycaprolactone scaffolds for bone regeneration

In the present study a structural characterization and in vitro cell-biological evaluation was performed on polycaprolactone (PCL) scaffolds that were produced by the additive manufacturing technique selective laser sintering (SLS), followed by a plasma-based surface modification technique, either non-thermal oxygen plasma or double protein coating, to functionalize the PCL scaffold surfaces. In the first part of this study pore morphology by means of 2D optical microscopy, surface chemistry by means of hydrophilicity measurement and X-ray photoelectron spectroscopy, strut surface roughness by means of 3D micro-computed tomography (CT) imaging and scaffold mechanical properties by means of compression testing were evaluated before and after the surface modifications. The results showed that both surface modifications increased the PCL scaffold hydrophilicity without altering the morphological and mechanical properties. In the second part of this study the in vitro cell proliferation and differentiation of human osteoprogenitor cells, over 14 days of culture in osteogenic and growth medium were investigated. The O₂ plasma modification gave rise to a significant lower in vitro cell proliferation compared to the untreated and double protein coated scaffolds. Furthermore the double protein coating increased in vitro cell metabolic activity and cell differentiation compared to the untreated and O₂ plasma PCL scaffolds when OM was used.     

Multiple-beam Interferometers: Analogy with Aberrated Optical Systems and Adjustment Tolerances Estimation

Abstract

In this paper we study multiple interference produced by an interferometer when illuminated by an extended quasimonochromatic source. We define a relative modulation factor, which arises from grouping the interference terms for the same frequency. This relative modulation factor measures the departure from ideal behaviour of the interferometer. We also show that for some cases it is possible to associate a single aberrated optical system to each frequency, just as can be done for the single interference case, as was shown in a previous work; this allows an extension of the formalism presented therein. As an example, we calculate, for the case of a double-grating interferometer, the tolerance for the spatial extension of the source, where the modulation factor is close to unity.     

Wavefront Aberration Function

Abstract

In this paper we obtain equations for the two aperture second derivatives of the wavefront aberration function in terms of the position of the local sagittal and tangential foci. The derivatives presented here are valid for meridional rays. The local foci positions for any tangential aperture zone are calculated by tracing one ray that corresponds to that zone and using Coddington's equations. The second derivative equations can be incorporated in the usual procedure of aberration computation, increasing from two to four the number of equations available for each tangential ray traced.     

A study on the rate of degradation of the bioabsorbable polymer polyglycolic acid (PGA)

As part of a study to characterise bioabsorbable scaffolds for tissue engineering an investigation has been conducted into the rate of degradation of polyglycolic acid (PGA). This is one of the most commonly used bioabsorbable materials and has been used in sutures since the 60s and more recently in cell scaffolds, drug delivery devices and bone fixation pins. This study looks at the influence that surface-to-volume ratio i.e. thickness of material, has on degradation. By degrading various thicknesses of PGA in a buffer saline solution over 24 days and testing their properties at regular intervals, a knowledge of how surface-to-volume ratio affects degradation was developed. Properties such as weight loss, crystallinity, molecular weight and structural integrity were measured. Results showed that rate of mass loss was dependent on sample thickness but crystallinity, melting point and molecular weight were independent of thickness.     

Variation of structural and superconducting properties with initial stoichiometry variation in (Bi,Pb)-Sr-Ca-Cu-O bulk superconductor

The effect of the initial stoichiometry variation (variation in Bi, Pb, Sr, Ca and Cu) on various structural and superconducting properties of Pb-doped Bi-based superconductor has been studied. The sample having the initial stoichiometry of (Bi_1.8Pb_0.4)Sr₂Ca_2.2Cu₃O₁₀ was found to give best structural and superconducting properties. The most important factors affectingJ _c in the samples were the type of impurity phases present in the sample, their amount and distribution in the superconducting matrix. Cu-rich stoichiometry enhanced the formation of 110 K phase, but large amounts of undesirable impurities present in the samples decreasedJ _c. The formation of 110 K phase and texturing of (001) planes in a sample depended on the period of heat treatment and stoichiometry.