Metallic multilayers for X-band Bragg reflector applications

We present a structural and high frequency (8.72GHz) electrical characterization of sputter deposited Ti/W, Ti/Ru and Mo/Ti metallic multilayers for potential application as acoustic Bragg reflectors. We prove that all metallic multilayers comprised of different acoustic impedance metals such as Ti, W, Mo are promising candidates for Bragg reflector/bottom electrode in full X-band thin film acoustic resonators. Values for high frequency resistivity of the order of $10^{-8} ohm.m$ are measured by use of a contact-free/non-invasive sheet resistance method

Superdirective Antennas of Coupled Helical Elements

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordUsing magnetically-coupled structures formed of subwavelength metallic helices, we demonstrate superdirective end-fire radiation in the low GHz frequency range. Numerical, experimental and analytical results are presented on superdirective dimers that are almost three times smaller compared to previously demonstrated dimers of split-ring-resonators (0.09λ compared with 0.25λ). Optimisation of such structures in terms of their size, directivity, efficiency and operational passband is demonstrated.Engineering and Physical Sciences Research Council (EPSRC

Broadband Artificial Magnetic Conductors Constructed of Magnetically Coupled Elements

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordAn Artificial Magnetic Conductor (AMC) is a type of metamaterial that can be used to enhance the performance in several antenna and microwave design applications. In this work we propose an analytical approach for size and bandwidth optimization of AMCs constructed of magnetically coupled elements Numerical results (obtained using COMSOL) for the optimized structure demonstrate performance close to the theoretical limit.Engineering and Physical Sciences Research Council (EPSRC)Defence Science and Technology Laboratory (Dstl

Near-field electromagnetic coupling between helices

This is the final version. Available on open access from IOP Publishing via the DOI in this recordData availability statement: The data that support the findings of this study are available upon reasonable request from the authors.A fresh approach to the design of metamaterials formed from closely spaced helices is developed based on the retrieved values of electric and magnetic coupling coefficients between adjacent helices. A coupling retrieval method has been implemented to obtain numerical and experimental values for both electric- and magnetic-coupling coefficients between helical elements. Couplings between both right- and left-handed helices in side-by-side and axial arrangements are evaluated providing both positive and negative electric-, as well as magnetic-coupling components. The dependence of coupling strength on separation distance, relative axial rotation, helical pitch and the geometrical arrangement of the helices has been quantified both numerically and experimentally and a geometry giving zero net-coupling between very close helices is illustrated.Engineering and Physical Sciences Research Council (EPSRC)Defence Science and Technology Laboratory (Dstl

Non-local effects in the mean-field disc dynamo. II. Numerical and asymptotic solutions

The thin-disc global asymptotics are discussed for axisymmetric mean-field dynamos with vacuum boundary conditions allowing for non-local terms arising from a finite radial component of the mean magnetic field at the disc surface. This leads to an integro-differential operator in the equation for the radial distribution of the mean magnetic field strength, $Q(r)$ in the disc plane at a distance $r$ from its centre; an asymptotic form of its solution at large distances from the dynamo active region is obtained. Numerical solutions of the integro-differential equation confirm that the non-local effects act similarly to an enhanced magnetic diffusion. This leads to a wider radial distribution of the eigensolution and faster propagation of magnetic fronts, compared to solutions with the radial surface field neglected. Another result of non-local effects is a slowly decaying algebraic tail of the eigenfunctions outside the dynamo active region, $Q(r)\sim r^{-4}$, which is shown to persist in nonlinear solutions where $\alpha$-quenching is included. The non-local nature of the solutions can affect the radial profile of the regular magnetic field in spiral galaxies and accretion discs at large distances from the centre.Comment: Revised version, as accepted; Geophys. Astrophys. Fluid Dyna

Broadband negative-index surface-waves on arrays of capped helices

A novel metasurface comprised of capped helices arranged as a hexagonal array that supports a broadband near-isotropic negative-index microwave surface-wave is designed, manufactured, and experimentally characterized. The surface-mode dispersion is studied both numerically and experimentally with the operational band more than double the bandwidth of structures previously reported in the literature. Hence, it is shown how one may provide a structured surface that supports just a negative-index mode over a wide operational band with no forward wave being simultaneously excited

3D-printed Metasurfaces of Capped Helices Providing Broadband Negative Mode Index

Using structures of subwavelength metallic capped helices with both negative electric and magnetic couplings, we demonstrate a broadband negative mode index metasurface. Numerical and experimental results are presented for the structure with negative dispersion bandwidth of 44% compared to the resonant frequency of individual capped helices. Optimisation of such structures in terms of their size, element geometry and operational passband width is demonstrated

Temperature stability of thin film refractory plasmonic materials

Materials such as W, TiN, and SrRuO3 (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their stability at high operational temperatures. However, investigation of the reproducibility of the optical properties after thermal cycling between room and elevated temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, SrRuO3 and SrNbO3 are investigated to assess their viability for robust refractory plasmonic applications. These results are further compared to the performance of SrMoO3 reported in literature. Films ranging in thickness from 50 to 105 nm are deposited on MgO, SrTiO3 and Si substrates by e-beam evaporation, RF magnetron sputtering and pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 to 1000° C for one hour, to establish the maximum cycling temperature and potential longevity at elevated temperatures for each material. It is found that SrRuO3 retains metallic behaviour after annealing at 800° C, while SrNbO3 undergoes a phase transition resulting in a loss of metallic behaviour after annealing at 400° C. Importantly, the optical properties of TiN and TiON are degraded as a result of oxidation and show a loss of metallic behaviour after annealing at 500° C, while the same is not observed in Au until annealing at 600° C. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions

The Effects of Cholesterol Oxidation on Erythrocyte Plasma Membranes: A Monolayer Study

This is the final version. Available on open access from MDPI via the DOI in this recordData Availability Statement: All data are available from the corresponding authors.Cholesterol plays a key role in the molecular and mesoscopic organisation of lipid membranes and it is expected that changes in its molecular structure (e.g., through environmental factors such as oxidative stress) may affect adversely membrane properties and function. In this study, we present evidence that oxidation of cholesterol has significant effects on the mechanical properties, molecular and mesoscopic organisation and lipid–sterol interactions in condensed monolayers composed of the main species found in the inner leaflet of the erythrocyte membrane. Using a combination of experimental methods (static area compressibility, surface dilatational rheology, fluorescence microscopy, and surface sensitive X-ray techniques) and atomistic molecular dynamics simulations, we show that oxidation of cholesterol to 7-ketocholesterol leads to stiffening of the monolayer (under both static and dynamic conditions), significant changes in the monolayer microdomain organisation, disruption in the van der Waals, electrostatic and hydrophobic interactions between the sterol and the other lipid species, and the lipid membrane hydration. Surface sensitive X-ray techniques reveal that, whilst the molecular packing mode is not significantly affected by cholesterol oxidation in these condensed phases, there are subtle changes in membrane thickness and a significant decrease in the coherence length in monolayers containing 7-ketocholesterol.Engineering and Physical Sciences Research Council (EPSRC