Correlation effects in disordered metallic photonic crystal slabs

We analyze the influence of correlations on the optical properties of disordered metallic photonic crystal slabs experimentally and theoretically. Different disorder models with different nearest-neighbor correlations are considered. We present a theory that allows us to quantitatively calculate the optical properties of the different samples. We find that different kinds of correlations produce characteristic spectral features such as peak reduction and inhomogeneous broadening. These features are caused by reduced excitation efficiencies and the excitation of multiple resonances.     

Perspectives on biological growth and remodeling

The continuum mechanical treatment of biological growth and remodeling has attracted considerable attention over the past fifteen years. Many aspects of these problems are now well-understood, yet there remain areas in need of significant development from the standpoint of experiments, theory, and computation. In this perspective paper we review the state of the field and highlight open questions, challenges, and avenues for further development.     

Direct and efficient N-heterocyclic carbene-catalyzed hydroxymethylation of aldehydes

The N-heterocyclic carbene-catalyzed coupling of several aldehydes with paraformaldehyde is reported, directly providing the corresponding valuable hydroxymethyl ketones. Results of first mechanistic experiments for this remarkably selective transformation are also provided.     

Structure and thermodynamics of lipid bilayers on polyethylene glycol cushions: fact and fiction of PEG cushioned membranes

In developing well hydrated polymer cushioned membranes, structural studies are often neglected. In this work, neutron and X-ray reflectivity studies reveal that hybrid bilayer/polyethylene glycol (PEG) systems created from mixtures of phospholipids and PEG conjugated lipopolymers do not yield a hydrated cushion beneath the bilayer unless the terminal ends of the lipopolymers are functionalized with reactive end groups and can covalently bind (tether) to the underlying support surface. While reactive PEG tethered systems yielded bilayers with near complete surface coverage, a bimodal distribution of heights with sub-micrometer lateral dimensions was observed consisting of cushioned membrane domains and uncushioned regions in close proximity to the support. The membrane fraction cushioned by the hydrated polymer could be controlled by adjusting the molar ratio of lipopolymer in the bilayer. A general phase diagram based on the free energy of the various configurations is derived that qualitatively predicts the observed behavior and the resulting structure of such systems a priori. As further evidenced by ellipsometry, atomic force and fluorescence microscopy, the tethered system provides a simple means for fabricating small cushioned domains within a membrane.     

Impact of membrane fluidity on steric stabilization by lipopolymers

In this work, the impact of lipid lateral mobility on the steric interaction between membranes containing poly(ethylene glycol) (PEG) functionalized lipids was investigated using the surface force apparatus. The force-distance profiles show the presence of electrostatic and steric repulsion that arise from the presence of negatively charged PEG functionalized lipids. Fluid-phase bilayers have high lateral diffusion relative to gel-phase bilayers; however, a quantitative comparison of the interaction forces between membranes in these two different phase states demonstrates a reduced rate of diffusion in the fluid phase for the PEG-lipids under constrained geometries. Thus, the amount of polymer in the contact zone can be modulated and is reduced with fluid membranes; however, complete exclusion was not achieved. As a result, the steric repulsion afforded by PEG chains or binding affinity of ligated PEG chains can only be modestly tailored by the phase state of the liposome.     

Beyond Directing Groups: Transition‐Metal‐Catalyzed C?H Activation of Simple Arenes

The use of coordinating moieties as directing groups for the functionalization of aromatic C? H bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of C? H activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non‐chelate‐assisted C? H activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.     

Stress concentrations in fractured compact bone simulated with a special class of anisotropic gradient elasticity

A new format of anisotropic gradient elasticity is formulated and implemented to simulate stress concentrations in cortical bone. The higher-order effect of the underlying microstructure in cortical bone is accounted for through the introduction of two length scale parameters and associated strain gradient terms which modify the response of the standard elastic macroscopic continuum: one internal length related to the longitudinal fibres and the other related to the transversal Haversian systems. Thus, anisotropic material behaviour is not only included in the anisotropy of the elastic effective stiffness properties, but also in the anisotropic sources of heterogeneity. The model is validated numerically in tests with bone fractures in the longitudinal and the transversal directions. It was found that the dominant length scale effects are those that coincide with the direction of fracture, as defined by the orientation of a pre-existing crack.     

Algebraic fidelity decay for local perturbations

From a reflection measurement in a rectangular microwave billiard with randomly distributed scatterers the scattering and the ordinary fidelity was studied. The position of one of the scatterers is the perturbation parameter. Such perturbations can be considered as local since wave functions are influenced only locally, in contrast to, e.g., the situation where the fidelity decay is caused by the shift of one billiard wall. Using the random-plane-wave conjecture, an analytic expression for the fidelity decay due to the shift of one scatterer has been obtained, yielding an algebraic 1/t decay for long times. A perfect agreement between experiment and theory has been found, including a predicted scaling behavior concerning the dependence of the fidelity decay on the shift distance. The only free parameter has been determined independently from the variance of the level velocities.     

Enhancement of localization in one-dimensional random potentials with long-range correlations

We experimentally study the effect of enhancement of localization in weak one-dimensional random potentials. Our experimental setup is a single-mode waveguide with 100 tunable scatterers periodically inserted into the waveguide. By measuring the amplitudes of transmitted and reflected waves in the spacing between each pair of scatterers, we observe a strong decrease of the localization length when white-noise scatterers are replaced by a correlated arrangement of scatterers.