The cutoff-dependence of the Casimir force within an inhomogeneous medium

We consider the ground state energy of the electromagnetic field in a piston geometry. In the idealised case, where the piston and the walls of the chamber are taken as perfect mirrors, the Casimir pressure on the piston is finite and independent of the small scale physics of the media that compose the mirrors; the Casimir-energy of the system can be regularised and is cutoff-independent. Yet we find that, when the body of the piston is filled with an inhomogeneous dielectric medium, the Casimir energy is cutoff-dependent, and the value of the pressure is thus inextricably dependent on the detailed behaviour of the mirror and the medium at large wave-vectors. This result is inconsistent with recent proposals for regularising Casimir forces in inhomogeneous media.Comment: 6 pages, 2 figure

Photonic Hall effect and helical Zitterbewegung in a synthetic Weyl system

This is the final version. Available from the publisher via the DOI in this record.Systems supporting Weyl points have gained increasing attention in condensed physics, photonics and acoustics due to their rich physics, such as Fermi arcs and chiral anomalies. Acting as sources or drains of Berry curvature, Weyl points exhibit a singularity of the Berry curvature at their core. It is, therefore, expected that the induced effect of the Berry curvature can be dramatically enhanced in systems supporting Weyl points. In this work, we construct synthetic Weyl points in a photonic crystal that consists of a honeycomb array of coupled rods with slowly varying radii along the direction of propagation. The system possesses photonic Weyl points in the synthetic space of two momenta plus an additional physical parameter with an enhanced Hall effect resulting from the large Berry curvature in the vicinity of the Weyl point. Interestingly, a helical Zitterbewegung (ZB) is observed when the wave packet traverses very close to a Weyl point, which is attributed to the contribution of the non-Abelian Berry connection arising from the near degenerate eigenstates.Royal Society (Charity)National Science Foundation of ChinaEuropean Research Council (ERC

Evidence-Based Professional Development of Science Teachers in Two Countries

The focus of this collaborative research project of King?s College London, and the Weizmann Institute, Israel. project is on investigating the ways in which teachers can demonstrate accomplished teaching in a specific domain of science and on the teacher learning that is generated through continuing professional development programs (CPD) that lead towards such practice. The interest lies in what processes and inputs are required to help secondary school science teachers develop expertise in a specific aspect of science teaching. `It focuses on the design of the CPD programmes and examines the importance of an evidence-based approach through portfolioconstruction in which professional dialogue pathes the way for teacher learning. The set of papers highlight the need to set professional challenge while tailoring CPD to teachers? needs to create the environment in which teachers can advance and transform their practice. The cross-culture perspective added to the richness of the development and enabled the researchers to examine which aspects were fundamental to the design by considering similarities and differences between the domains

Self-induced parametric amplification arising from nonlinear elastic coupling in a micromechanical resonating disk gyroscope

Parametric amplification, resulting from intentionally varying a parameter in a resonator at twice its resonant frequency, has been successfully employed to increase the sensitivity of many micro- and nano-scale sensors. Here, we introduce the concept of self-induced parametric amplification, which arises naturally from nonlinear elastic coupling between the degenerate vibration modes in a micromechanical disk-resonator, and is not externally applied. The device functions as a gyroscope wherein angular rotation is detected from Coriolis coupling of elastic vibration energy from a driven vibration mode into a second degenerate sensing mode. While nonlinear elasticity in silicon resonators is extremely weak, in this high quality-factor device, ppm-level nonlinear elastic effects result in an order-of-magnitude increase in the observed sensitivity to Coriolis force relative to linear theory. Perfect degeneracy of the primary and secondary vibration modes is achieved through electrostatic frequency tuning, which also enables the phase and frequency of the parametric coupling to be varied, and we show that the resulting phase and frequency dependence of the amplification follow the theory of parametric resonance. We expect that this phenomenon will be useful for both fundamental studies of dynamic systems with low dissipation and for increasing signal-to-noise ratio in practical applications such as gyroscopes

Do we understand the unquenched value of fB?

I review our qualitative understanding of the increase in the value of the B meson decay constant (fB), when dynamical fermions are included in lattice QCD calculations.Comment: 4 pages. Talk at UK Phenomenology Workshop on Heavy Flavour and CP Violation, Durham, 17 - 22 September 2000 (Minor typo fixed

Supine posture changes lung volumes and increases ventilation heterogeneity in cystic fibrosis

INTRODUCTION: Lung Clearance Index (LCI) is recognised as an early marker of cystic fibrosis (CF) lung disease. The effect of posture on LCI however is important when considering longitudinal measurements from infancy and when comparing LCI to imaging studies. METHODS: 35 children with CF and 28 healthy controls (HC) were assessed. Multiple breath washout (MBW) was performed both sitting and supine in triplicate and analysed for LCI, Scond, Sacin, and lung volumes. These values were also corrected for the Fowler dead-space to create 'alveolar' indices. RESULTS: From sitting to supine there was a significant increase in LCI and a significant decrease in FRC for both CF and HC (p<0.01). LCI, when adjusted to estimate 'alveolar' LCI (LCIalv), increased the magnitude of change with posture for both LCIalv and FRCalv in both groups, with a greater effect of change in lung volume in HC compared with children with CF. The % change in LCIalv for all subjects correlated significantly with lung volume % changes, most notably tidal volume/functional residual capacity (Vtalv/FRCalv (r = 0.54,p<0.001)). CONCLUSION: There is a significant increase in LCI from sitting to supine, which we believe to be in part due to changes in lung volume and also increasing ventilation heterogeneity related to posture. This may have implications in longitudinal measurements from infancy to older childhood and for studies comparing supine imaging methods to LCI

Nucleon axial charge from quenched lattice QCD with domain wall fermions

We present a quenched lattice calculation of the nucleon isovector vector and axial-vector charges gV and gA. The chiral symmetry of domain wall fermions makes the calculation of the nucleon axial charge particularly easy since the Ward-Takahashi identity requires the vector and axial-vector currents to have the same renormalization, up to lattice spacing errors of order O(a^2). The DBW2 gauge action provides enhancement of the good chiral symmetry properties of domain wall fermions at larger lattice spacing than the conventional Wilson gauge action. Taking advantage of these methods and performing a high statistics simulation, we find a significant finite volume effect between the nucleon axial charges calculated on lattices with (1.2 fm)^3 and (2.4 fm)^3 volumes (with lattice spacing, a, of about 0.15 fm). On the large volume we find gA = 1.212 +/- 0.027(statistical error) +/- 0.024(normalization error). The quoted systematic error is the dominant (known) one, corresponding to current renormalization. We discuss other possible remaining sources of error. This theoretical first principles calculation, which does not yet include isospin breaking effects, yields a value of gA only a little bit below the experimental one, 1.2670 +/- 0.0030.Comment: 38 pages, 12 figures, 9 tables, Revtex. Version accepted for publication in Physical Review

Scattering Theory and PT\mathcal{P}\mathcal{T}-Symmetry

We outline a global approach to scattering theory in one dimension that allows for the description of a large class of scattering systems and their $\mathcal{P}$-, $\mathcal{T}$-, and $\mathcal{P}\mathcal{T}$-symmetries. In particular, we review various relevant concepts such as Jost solutions, transfer and scattering matrices, reciprocity principle, unidirectional reflection and invisibility, and spectral singularities. We discuss in some detail the mathematical conditions that imply or forbid reciprocal transmission, reciprocal reflection, and the presence of spectral singularities and their time-reversal. We also derive generalized unitarity relations for time-reversal-invariant and $\mathcal{P}\mathcal{T}$-symmetric scattering systems, and explore the consequences of breaking them. The results reported here apply to the scattering systems defined by a real or complex local potential as well as those determined by energy-dependent potentials, nonlocal potentials, and general point interactions.Comment: Slightly expanded revised version, 38 page