Two-Photon Absorption Of Di-Alkyl-Amino-Nitro-Stilbene Side-Chain Polymer

The two photon absorption spectrum has been measured in the 780–1600 nm range for a di‐alkyl‐amino‐nitro‐stilbene side‐chain polymer. A single two photon peak centered at 920 nm is observed with a peak two photon coefficient of 5.5 cm/GW. Bleaching via two photon absorption at this wavelength is investigated by measuring a refractive index change after an intensive exposure

Characteristic molecular properties of one-electron double quantum rings under magnetic fields

The molecular states of conduction electrons in laterally coupled quantum rings are investigated theoretically. The states are shown to have a distinct magnetic field dependence, which gives rise to periodic fluctuations of the tunnel splitting and ring angular momentum in the vicinity of the ground state crossings. The origin of these effects can be traced back to the Aharonov-Bohm oscillations of the energy levels, along with the quantum mechanical tunneling between the rings. We propose a setup using double quantum rings which shows that Aharonov-Bohm effects can be observed even if the net magnetic flux trapped by the carriers is zero.Comment: 16 pages (iopart format), 10 figures, accepted in J.Phys.Cond.Mat

Site-specific immobilization of microbes using carbon nanotubes and dielectrophoretic force for microfluidic applications

We developed a microbial immobilization method for successful applications in microfluidic devices. Single-walled nanotubes and Escherichia coli were aligned between two cantilever electrodes by a positive dielectrophoretic force resulting in a film of single-walled nanotubes with attached Escherichia coli. Because this film has a suspended and porous structure, it has a larger reaction area and higher reactant transfer efficiency than film attached to the substrate surface. The cell density of film was easily controlled by varying the cell concentration of the suspension and varying the electric field. The film showed excellent stability of enzyme activity, as demonstrated by measuring continuous reaction and long-term storage times using recombinant Escherichia coli that expressed organophosphorus hydrolase.X1133sciescopu

General approaches for shear-correcting coordinate transformations in Bragg coherent diffraction imaging: Part 2

X-ray Bragg coherent diffraction imaging has been demonstrated as a powerful three-dimensional (3D) microscopy approach for the investigation of sub-micrometer-scale crystalline particles. It is based on the measurement of a series of coherent diffraction intensity patterns that are numerically inverted to retrieve an image of the spatial distribution of relative phase and amplitude of the Bragg structure factor of the scatterer. This 3D information, which is collected through an angular rotation of the sample, is necessarily obtained in a non-orthogonal frame in Fourier space that must be eventually reconciled. To deal with this, the currently favored approach (detailed in Part I) is to perform the entire inversion in conjugate non-orthogonal real and Fourier space frames, and to transform the 3D sample image into an orthogonal frame as a post-processing step for result analysis. In this article, a direct follow-up of Part I, we demonstrate two different transformation strategies that enable the entire inversion procedure of the measured data set to be performed in an orthogonal frame. The new approaches described here build mathematical and numerical frameworks that apply to the cases of evenly and non-evenly sampled data along the direction of sample rotation (the rocking curve). The value of these methods is that they rely on and incorporate significantly more information about the experimental geometry into the design of the phase retrieval Fourier transformation than the strategy presented in Part I. Two important outcomes are 1) that the resulting sample image is correctly interpreted in a shear-free frame, and 2) physically realistic constraints of BCDI phase retrieval that are difficult to implement with current methods are easily incorporated. Computing scripts are also given to aid readers in the implementation of the proposed formalisms

Critical Behaviour of Superfluid 4^4He in Aerogel

We report on Monte Carlo studies of the critical behaviour of superfluid $^4$He in the presence of quenched disorder with long-range fractal correlations. According to the heuristic argument by Harris, uncorrelated disorder is irrelevant when the specific heat critical exponent $\alpha$ is negative, which is the case for the pure $^4$He. However, experiments on helium in aerogel have shown that the superfluid density critical exponent $\zeta$ changes. We hypothesize that this is a cross-over effect due to the fractal nature of aerogel. Modelling the aerogel as an incipient percolating cluster in 3D and weakening the bonds at the fractal sites, we perform XY-model simulations, which demonstrate an increase in $\zeta$ from $0.67 \pm 0.005$ for the pure case to an apparent value of $0.722\pm 0.005$ in the presence of the fractal disorder, provided that the helium correlation length does not exceed the fractal correlation length.Comment: 4 pages, RevTex, 3 postscript figures, LaTeX file and figures have been uuencoded

Phase diagram of a Disordered Boson Hubbard Model in Two Dimensions

We study the zero-temperature phase transition of a two-dimensional disordered boson Hubbard model. The phase diagram of this model is constructed in terms of the disorder strength and the chemical potential. Via quantum Monte Carlo simulations, we find a multicritical line separating the weak-disorder regime, where a random potential is irrelevant, from the strong-disorder regime. In the weak-disorder regime, the Mott-insulator-to-superfluid transition occurs, while, in the strong-disorder regime, the Bose-glass-to-superfluid transition occurs. On the multicritical line, the insulator-to-superfluid transition has the dynamical critical exponent $z=1.35 \pm 0.05$ and the correlation length critical exponent $\nu=0.67 \pm 0.03$, that are different from the values for the transitions off the line. We suggest that the proliferation of the particle-hole pairs screens out the weak disorder effects.Comment: 4 pages, 4 figures, to be published in PR