Ising transition in the two-dimensional quantum J1-J2 Heisenberg model

We study the thermodynamics of the spin-S two-dimensional quantum Heisenberg antiferromagnet on the square lattice with nearest (J1) and next-nearest (J2) neighbor couplings in its collinear phase (J(2)/J(1)>0.5), using the pure-quantum self-consistent harmonic approximation. Our results show the persistence of a finite-temperature Ising phase transition for every value of the spin, provided that the ratio J(2)/J(1) is greater than a critical value corresponding to the onset of collinear long-range order at zero temperature. We also calculate the spin and temperature dependence of the collinear susceptibility and correlation length, and we discuss our results in light of the experiments on Li2VOSiO4 and related compounds.     

Experimental confirmation of matched bends

Modal conversion between the fundamental mode and the first-order (leaky) mode has been experimentally investigated in high-index-contrast integrated-optical bent waveguides. The matched bend condition has been experimentally confirmed on cascaded bends, and the effect of both matched and unmatched bends on Y branches has been investigated. Good agreement between theoretical predictions and experimental results for both monomode and multimode waveguides is achieved. It is demonstrated that, in bent waveguides, the modal conversion between the fundamental mode and the first (leaky) mode can be accurately controlled to avoid additional losses and transfer function impairments.     

Controlled transmission in the forbidden photonic bandgap via transient nonlinear states

Using three-dimensional time-domain numerical simulations of the nonlinear dispersive Maxwell equations for a defect microcavity in a photonic crystal wire, we show that the transmission through the bandgap can be all-optically modulated via the generation of transient states associated with the nonlinear splitting of the defect mode. Analytical results based on time-domain coupled-mode theory are derived as well.     

Modification of surface properties of electrospun polyamide nanofibers by means of a perfluorinated acridine

Polyamide 6 (PA6) nanofibers were prepared from formic acid solutions by using electrospinning technique. The fibers were smooth, defects free and with diameters smaller than 200 nm. Small amounts of a perfluorinated acridine were added as dopant to the feed solution to modify the wettability of the fibers. The effect of doping on the contact angle values is well apparent. The contact angle values go from 50° of pure PA6 to 120° when 6% of acridine is added. A comparison between fibers and films of pure and doped polyamide 6 was carried out in order to determine the effect of morphology on wettability. Thermal annealing near the T_g of the polymer promoted the segregation of the molecules to the surface, reaching contact angles of 131° with smaller amounts (4%) of acridine. The surface segregation was also promoted by time aging.     

Mixing of electronic states in pentacene adsorption on copper

By combining experimental and theoretical approaches, we study the adsorption of pentacene on copper as a model for the coupling between aromatic molecules and metal surfaces. Our results for the interface electronic structure are not compatible with a purely physisorption picture, which is conventionally employed for such systems. Nay, we demonstrate electronic mixing between molecular orbitals and metal electronic states.     

Uniqueness of balanced metrics on holomorphic vector bundles

Let E→M$E\to M$ be a holomorphic vector bundle over a compact Kähler manifold (M,ω)$(M,\omega )$. We prove that if E$E$ admits a ω$\omega$-balanced metric (in X. Wang’s terminology (Wang, 2005 [3])) then it is unique. This result together with Biliotti and Ghigi (2008) [14] implies the existence and uniqueness of ω$\omega$-balanced metrics of certain direct sums of irreducible homogeneous vector bundles over rational homogeneous varieties. We finally apply our result to show the rigidity of ω$\omega$-balanced Kähler maps into Grassmannians.     

Surface termination of BaTiO3 (001) single crystals: A combined electron spectroscopic and theoretical study

The surface termination of oxide surfaces is of crucial importance for the growth of a second material like metals or other oxides. In this study we have investigated the surface of a BaTiO₃ (001) single crystal during sample preparation by various electron spectroscopic methods. It is shown by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) that during sputtering a Ba rich overlayer is formed, in which the Ba²⁺ ions are under coordinated. Below this layer, an oxygen deficient BaTiO₃ layer is found. During annealing, we observe the reformation of the crystalline structure. UP and MIE spectra provide clear evidence of a BaO terminated surface. X-ray photoelectron diffraction studies support this result, comparing recorded polar angle scans with calculated intensity modulations using multiple scattering cluster models.     

Public understanding of science and the perception of nanotechnology: the roles of interest in science,methodological knowledge,epistemological beliefs,and beliefs about science

In this article, we report data from an online questionnaire study with 587 respondents, representative for the adult U.S. population in terms of age, gender, and level of education. The aim of this study was to assess how interest in science and knowledge as well as beliefs about science are associated with risk and benefit perceptions of nanotechnology. The findings suggest that the U.S. public is still rather unfamiliar with nanotechnology. Those who have some knowledge mainly have gotten it from TV and the Internet. The content of current media reports is perceived as fairly positive. Knowledge of scientific methods is unrelated to benefit and risk perceptions, at least when other predictors are controlled. In contrast, positive beliefs about science (e.g., its impact on economy or health) and more sophisticated epistemological beliefs about the nature of scientific knowledge are moderately linked to more positive perceptions of nanotechnology. The only exception is the perception of scientific uncertainty: This is associated with less positive evaluations. Finally, higher engagement with science is associated with higher risk perceptions. These findings show that laypersons who are engaged with science and who are aware of the inherent uncertainty of scientific evidence might perceive nanotechnology in a somewhat more differentiated way, contrary to how it is portrayed in the media today.     

Colloidal nanocrystal ZnO- and TiO<Subscript>2</Subscript>-modified electrodes sensitized with chlorophyll a and carotenoids: a photoelectrochemical study

Heterostructures formed of films of organic-capped ZnO and TiO₂ nanocrystals (both with the size of ca. 6 nm) and photosynthetic pigments were prepared and characterized. The surface of optically transparent electrodes (Indium Tin Oxide) was modified with nanocrystals and prepared by colloidal synthetic routes. The nanostructured electrodes were sensitized by a mixture of chlorophyll a and carotenoids. The characterization of the hybrid structures, carried out by means of steady-state optical measurements, demonstrated such class of dyes able to extend the photoresponse of the large band-gap semiconductors. The charge-transfer processes between the components of the heterojunction were investigated, and photoelectrochemical measurements taken on the sensitized ZnO and TiO₂ nanocrystals electrodes elucidated the photoactivity of the heterojunctions as a function of the dyes and of the red–ox mediator used in solution. The effect of methyl viologen as different red–ox mediator was also evaluated in order to show its effect on the heterojunction photoactivity. The overall results contributed to describe the photoelectrochemical potential of the investigated heterojunctions, highlighting a higher response of the dye-sensitized ZnO nanocrystals, and then provided the TiO₂-modified counterparts.     

ToF-SIMS and XPS study of ancient papers

The surface composition of 18th century papers was investigated by means of ToF-SIMS and XPS. The aim of the present study was to explore the possibility of using these surface sensitive methods to obtain information which can help to determine the manufacturing process, provenance and state of conservation of ancient papers. The ToF-SIMS results indicate that the analyzed papers were sized by gelatin and that alum was added as hardening agent. The paper sheets produced in near geographical areas but in different paper mills exhibit a similar surface composition and morphology of the fibers as shown by the ToF-SIMS measurements. The ToF-SIMS and the XPS results indicate that a significant fraction of the cellulose fibers is not covered by the gelatin layer. This was observed for the ancient papers and for a modern handmade paper manufactured according to the old recipes.