Design of a cavity backed spiral antenna with improved pattern symmetry

In this paper we investigate the azimuthal pattern symmetry of an Archimedean spiral antenna which is designed to operate over the frequency range 3-10 GHz. The performance of the spiral in free space is compared with a structure that is backed by a perfect electric conductor with a separation distance of ?/4 at the operating frequencies. The latter arrangement exhibits a higher gain, however it is observed that the radiation patterns are less symmetrical about boresight and this performance degradation increases with frequency. The predicted 3 dB beamwidth difference is shown to vary between 14° (3 GHz) and 51° (10 GHz). An improved antenna design is described which reduces the pattern asymmetry to ˜ 2° at 10 GHz. The reduction in modal contamination is obtained by inserting slots carefully arranged in a radial pattern to disrupt the surface currents that flow on the ground plane of the antenna

Dynamical symmetry breaking with optimal control: Reducing the number of pieces

We analyze the production of defects during the dynamical crossing of a mean-field phase transition with a real order parameter. When the parameter that brings the system across the critical point changes in time according to a power-law schedule, we recover the predictions dictated by the well-known Kibble-Zurek theory. For a fixed duration of the evolution, we show that the average number of defects can be drastically reduced for a very large but finite system, by optimizing the time dependence of the driving using optimal control techniques. Furthermore, the optimized protocol is robust against small fluctuations.

Site symmetry dependence of repulsive interactions between chemisorbed oxygen atoms on Pt{100}-(1x1)

Ab initio total energy calculations using density functional theory with the generalized gradient approximation have been performed for the chemisorption of oxygen atoms on a Pt{100}-(1 x 1) slab. Binding energies for the adsorption of oxygen on different high-symmetry sites are presented. The bridge site is the most stable at a coverage of 0.5 ML, followed by the fourfold hollow site. The atop site is the least stable. This finding is rationalized by analyzing the ''local structures'' formed upon oxygen chemisorption. The binding energies and heats of adsorption at different oxygen coverages show that pairwise repulsive interactions are considerably stronger between oxygen atoms occupying fourfold sites than those occupying bridge sites. Analysis of the partial charge densities associated with Bloch states demonstrates that the O-Pt bond is considerably more localized at the bridge site. These effects cause a sharp drop in the heats of adsorption for oxygen on hollow sites when the coverage is increased from 0.25 to 0.5 ML. Mixing between oxygen p orbitals and Pt d orbitals can be observed over the whole metal d-band energy range.

Nonclassicality and criticality in symmetry protected magnetic phases

Quantum and global discord in a spin-1 Heisenberg chain subject to single-ion anisotropy (uniaxial field) are studied using exact diagonalisation and the density matrix renormalisation group (DMRG). We find that these measures of quantum non-classicality are able to detect the quantum phase transitions confining the symmetry protected Haldane phase and show critical scaling with universal exponents. Moreover, in the case of thermal states, we find that quantum discord can increase with increasing temperature.

Improving antimicrobial release kinetics from hydrophobic polymer implants via ion pairing to combat biomedical-device related infection

Purpose The aim of this study is to improve the drug release properties of antimicrobial agents from hydrophobic biomaterials using using an ion pairing strategy. In so doing antimicrobial agents may be eluted and maintained over a sufficient time period thereby preventing bacterial colonisation and subsequent biofilm formation on medical devices. Methods The model antimicrobial agent was chlorhexidine and the selected fatty acid counter ions were capric acid, myristic acid and stearic acid. The polymethyl methacrylate films were loaded with 2% of fatty acid:antimicrobial agent at the following molar ratios; 0.5:1M, 1:1M and 2:1M and thermally polymerized using azobisisobutyronitrile initiator. Drug release experiments were subsequently performed over a 3-month period and the mass of drug released under sink conditions (pH 7.0, 37oC) quantified using a validated HPLC-UV method. Results In all platforms, a burst of chlorhexidine release was observed over the initial 24-hour period. Similar release kinetics were observed between the formulations during the initial 28 days. However, as time progressed, the chlorhexidine baseline plateaued after 56 days whereas formulations containing the counterions appeared to continuously elute linearly with time. As can be observed in figure 1, the rank order of total chlorhexidine release in the presence of 0.5M fatty acid was myristic acid (40%) > capric acid (35%) > stearic acid (30%)> chlorhexidine baseline (15%). Conclusion The incorporation of fatty acids within the formulation significantly improved chlorhexidine solubility within both the monomer and the polymer and enhanced the drug release kinetics over the period of study. This is attributed to the greater diffusivity of chlorhexidine through PMMA in the presence of fatty acids. In th absence of fatty acids, chlorhexidine release was facilitated by dissolution of surface associated drug particles. This study has illustrated the ability of fatty acids to modulate chlorhexidine release from a model biomaterial through enhanced diffusivity. This strategy may prove advantageous for improved medical devices with enhanced resistance to infection.

New pairing scheme to overhaul BCS theory

In a superconductor pair occupancy probabilities are doubly defined with conflicting values when normal and umklapp scattering coexist with the same destination momentum. To resolve this issue a new pairing scheme is introduced to assert normal–umklapp frustration under such circumstances. Superconductivity then arises solely from residual umklapp scattering to destination momenta not reached by normal scattering. Consequent Tc calculations from first principles for niobium, tantalum, lead and aluminum turn out to be accurate within a few percent. A new perspective is revealed to support Matthias׳ rule. New light is also shed relevant to the future study of metallic hydrogen.

Protocolised weaning from mechanical ventilation: pairing a qualitative review with an effectiveness review

In our systematic review of protocolised weaning from mechanical ventilation (Blackwood 2014) we found significant heterogeneity that could not be explained by subgroup analysis (type of protocol, ICU). We suspected that factors unreported in the trials relating to context and mechanisms of using the weaning protocols contributed to the heterogeneity. Therefore we set out to conduct a Cochrane qualitative evidence-synthesis of ‘sibling studies’ (qualitative studies undertaken alongside the included trials that may have examined these factors) and ‘stand-alone’ qualitative studies reporting barriers and facilitators to successful implementation of weaning protocols. The qualitative review was novel, there were few templates or guidelines which challenged us to consider how best to synthesise and report this evidence. However, the benefits of conducting this review are that not only do we have a template for future qualitative syntheses for the ACE group, but specifically for trials of weaning protocols, we found context-specific evidence concerning if, how and why specific protocols have been effective in the settings in which they were delivered and received.