Non-Reciprocal MEMS Periodic Structure

In recent years, active periodic structures with in-time modulated parameters have drawn ever-increasing attention due to their peculiar (and sometimes exotic) wave propagation properties. Although many experimental works have shown the efficacy of time-modulation strategies, the benchmarks proposed until now have been mostly proof-of-concept demonstrators, with little attention to the feasibility of the solution for practical purposes. In this work, we propose a micro electro-mechanical system (MEMS) periodic structure with modulated electromechanical stiffness featuring non-reciprocal band-gaps that are frequency bands where elastic waves are allowed to travel only in one direction. To this aim, we derive a simplified analytical lumped-parameter model, which is then verified through numerical simulations of both the lumped-parameter system and the high-fidelity multiphysics finite element model including electrostatic effects. We envision that this system, which can easily be manufactured through standard MEMS production processes, may be used as a directional filter in MEMS devices such as insulators and circulators

Isovector response function of hot nuclear matter with Skyrme interactions

We investigate the role of the effective nucleon-nucleon interaction in the description of giant dipole resonances in hot nuclei. For this purpose we calculate the response function of hot nuclear matter to a small isovector external perturbation using various effective Skyrme interactions. We find that for Skyrme forces with an effective mass close to unity an undamped zero sound mode occurs at zero temperature. This mode gives rise in finite nuclei (calculated via the Steinwedel-Jenssen model) to a resonance whose energy agrees with the observed value. We find that zero sound disappears at a temperature of a few MeV, leaving only a broad peak in the dipole strength. For Skyrme forces with a small value of the effective mass (0.4- 0.5), there is no zero sound at zero temperature but only a weak peak located too high in energy. The strength distribution in this case is nearly independent of temperature and shows small collective effects. The relevance of these results for the saturation of photon multiplicities observed in recent experiments is pointed out.Comment: 33 pages Email: [email protected]

U(5) Nambu-Jona-Lasinio model with flavor dependent coupling constants: pseudoscalar and scalar mesons masses

By considering the background field method we calculate one-loop polarization corrections to the coupling constant of the flavor-U(5) Nambu-Jona-Lasinio (NJL) model with degenerate up and down quarks. They break flavor and chiral symmetries and they can be written as $G_{ij}^\Gamma (\bar{\psi} \lambda_i \Gamma \psi) ( \bar{\psi} \lambda_j \Gamma \psi)$, for the scalar and pseudoscalar channels ($\Gamma=I , i \gamma_5$) and $i,j = 0.1,... N_f^2-1$. Their contributions to different observables are computed: quark-antiquark scalar condensates, masses of quark-antiquark meson states (pseudoscalar and scalar) and pseudoscalar meson weak decay constants. The non-covariant three dimensional regularization scheme is employed according to which a three-dimensional momentum cutoff has an unique interpretation for light and heavy quarks. Besides that, flavor dependence of cutoffs is implemented in an unambiguous way.Their values ($\Lambda_f$, f=u,s,c,b) , however, are found to be very close to each other, i.e. the best results are obtained for nearly flavor-independent cutoffs. The NJL-gap equations are found to overestimate the heavy quark condensates at the usual mean field level usually adopted for model. The polarization tensor is calculated entirely in the adjoint representation what may lead to mixing terms. A quite surprisingly good description of all the pseudoscalar -- including the pseudoscalar $\eta$'s -- and most of the scalar meson masses -- is obtained within 5$\%$ to 10$\%$. However, the usual problems to describe the correct mass hierarchy of some light scalar mesons still remains. In spite of the good description of the meson masses, the pseudoscalar meson weak decay constant cannot be described by the NJL model (with relativistic heavy quark propagators) without further interactions or effects.Comment: 20 pages. Abstract above was reduced to cope with filling-form, improved tex

Towards Safer Helmets: Characterisation, Modelling and Monitoring

Bike and ski helmets are mainly made up of two layers: the external shell and the foam liner. The foam liner, typically made of expanded polystyrene (EPS) or polypropylene (EPP), is asked to provide energy absorption in case of impacts. Standard helmet design requires the foam to maximize this energy absorption, thus achieving large deformations (up to 25% in compression) while maintaining the stress level below a threshold value. To optimize the helmet construction in terms of foam composition, structure and density, reliable numerical models are required, which in turn need to be fed with accurate experimental data. A characterisation of several foams was performed, including EPS and EPP having varying densities, under tensile and compressive stress states at varying strain rates. Typical mechanical parameters (elastic moduli and plateau stress in compression, Poisson's ratio) were compared with literature data and applicability of existing models to experimental results was discussed. A marked strain rate dependence – very important for impact applications – was accurately described using the Nagy phenomenological model. The foam microstructure was investigated using scanning electron microscopy (SEM) to assess structural changes before and after compression. The aforementioned mechanical features were then adopted in a rate-dependent constitutive law for crushable foams, to model the shock attenuation properties of helmets and validate the approach against available data. Finally, a microelectromechanical system based in-helmet wireless micro monitoring system was developed and inserted in a helmet prototype. The system is capable of acquiring impact load spectra, providing valuable information to investigate generic impacts with varying angles and energy. In particular, it can monitor the effect of repeated micro-impacts on the residual energy absorption characteristics of the foam