Design of pi-conjugated organic materials for one-dimensional energy transport in nanochannels

Various end-substituted distyrylbenzenes have been synthesized to serve as guest molecules in inclusion compounds to promote efficient energy transport along one-dimensional channels. Their optical and photophysical properties have been characterized at both experimental and theoretical levels. All molecules display a large transition dipole moment between the ground state and lowest excited state and hence a short radiative lifetime (on the order of 1-2 ns). They also exhibit a large spectral overlap between the emission and absorption spectra, which enables efficient energy transport between molecules arranged in a head-to-tail configuration in nanochannels. Hopping rates on the order of 10(12) s(-1) are calculated at a full quantum-chemical level; this is much larger than the radiative lifetimes and opens the way for energy migration over large distances. Changes in the nature of the terminal substituents are found to modulate the optical properties weakly but to impact significantly the energy transfer rates.     

Nanostructured Ti-W mixed-metal oxides: structural and electronic properties

In this article, the structural and electronic properties of Ti-W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV-vis spectroscopy, and density functional calculations. A series of Ti-W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO(2) reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cation distance of W centers. Upon addition of W to the TiO(2) structure, the Ti charge is not strongly modified, while changes in the W-O interaction appear to drive a modest modification of the W d-electron density throughout the Ti-W series. A combination of these geometrical and electronic effects produced Ti K- and W L(I)/L(III)-edge XANES/EXAFS spectra with distinctive features. UV-vis spectra show a nonlinear decrease of the band gap in the Ti-W solid solutions with a characteristic turning point at a W content of ca. 15 atom %. The relationship between local/long-range order and electronic parameters is discussed on the basis of these experimental results.     

Fluorescence lifetime enhancement of organic chromophores attached to gold nanoparticles

We present for the first time experimental evidence of fluorescence lifetime enhancement of organic chromophores attached to metal nanospheres via radiative decay engineering. The hybrid system (HS) was a modified "diconjugated" molecular probe, 4-acetamido-4'-maleimidylstilbene-2,2'-dithiol (AMDT), covalently bound to the surface of 5-nm-diameter Au nanospheres by its two sulfur atoms, at a distance d < 1 nm and with its molecular axis parallel to the surface of the nanoparticle surface. We measured a fluorescence lifetime increase of a factor of 2 at room temperature (tau(AMDT) = (4.32 +/- 0.10) ns and tau(HS) = (8.73 +/- 0.23) ns) and a factor of 3.4 at 4.2 K (tau(AMDT) = (2.64 +/- 0.07) ns and tau(HS) = (7.96 +/- 0.14) ns). We also found that the fluorescence quantum yield of this hybrid system is not reduced, proof of a weak energy transfer between the molecular probe and the nanoparticle. These results demonstrate that a molecular dipole oriented parallel to the metal surface tends to be reduced by the coupling with its image.     

Quasiparticle approach to the modulational instability of drift waves coupling to zonal flows

The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach. Simulations have been conducted in which a particle-in-cell representation is used for the quasiparticles, while a fluid model is employed for the plasma. The interactions have been studied in a plasma edge configuration which has applications in both tokamak physics and magnetopause boundary layer studies. Simulation results show the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients.     

Controllable soliton emission from a Bose-Einstein condensate

We demonstrate, through numerical simulations, the controllable emission of matter-wave bursts from a Bose-Einstein condensate in a shallow optical dipole trap. The process is triggered by spatial variations of the scattering length along the trapping axis. In our approach, the outcoupling mechanisms are atom-atom interactions and thus, the trap remains unaltered. Once emitted, the matter wave forms a robust soliton. We calculate analytically the parameters for the experimental implementation of these matter-wave bursts of solitons.     

Resolving the discrepancy of 135 MeV pd elastic scattering cross sections and relativistic effects

Three precise measurements for elastic pd scattering at 135 MeV/A have been performed with the three different experimental setups. The cross sections are described well by the theoretical predictions based on modern nucleon-nucleon forces combined with three-nucleon forces. Relativistic Faddeev calculations show that relativistic effects are restricted to backward angles. This result supports the two measurements recently reported by RIKEN and contradicts the KVI data.     

Ensemble averages and nonextensivity at the edge of chaos of one-dimensional maps

Ensemble averages of the sensitivity to initial conditions xi(t) and the entropy production per unit of time of a new family of one-dimensional dissipative maps, x(t+1)=1-ae(-1/|x(t)|(z))(z>0), and of the known logisticlike maps, x(t+1)=1-a|x(t)|(z)(z>1), are numerically studied, both for strong (Lyapunov exponent lambda(1)>0) and weak (chaos threshold, i.e., lambda(1)=0) chaotic cases. In all cases we verify the following: (i) both [ln((q)x triple bond (x(1-q)-1)/(1-q); ln((1)x=ln(x] and [S(q) triple bond (1- sigma p(q)(i))/(q-1); S(1)=- sigma p(i)ln(p(i)] linearly increase with time for (and only for) a special value of q, q(av)(sen), and (ii) the slope of and that of coincide, thus interestingly extending the well known Pesin theorem. For strong chaos, q(av)(sen)=1, whereas at the edge of chaos q(av)(sen)(z)<1.     

Surface reconstruction and decahedral structure of bimetallic nanoparticles

We report on energetic surface reconstruction phenomena observed on bimetallic nanoparticle systems of AuPd and AuCu, similar to a resolidification effect observed during the cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in [100] facets on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence so far. We demonstrate experimentally as well as by computational simulations that this new eutectic structure is favored in such nanoalloy systems.     

Stabilized two-dimensional vector solitons

In this Letter, we introduce the concept of stabilized vector solitons as nonlinear waves constructed by the addition of mutually incoherent fractions of Townes solitons that are stabilized under the effect of a periodic modulation of the nonlinearity. We analyze the stability of these new kinds of structures and describe their behavior and formation in Manakov-like interactions. Potential applications of our results in Bose-Einstein condensation and nonlinear optics are also discussed.