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At least 1,261 records · Page 70Linked to original sources
PubMed2002

Delocalizing transition of Bose-Einstein condensates in optical lattices.

A Bose-Einstein condensate trapped in a two-dimensional optical lattice exhibits an abrupt transition manifested by the macroscopic wave function changing character from spatially localized to extended. Resulting from a bifurcation, this irreversible transition takes place as the interwell potential barrier is adiabatically decreased below a critical value. This is in sharp contrast to the corresponding one-dimensional case where such a bifurcation is absent and the extent of a localized mode is continuously tunable. We demonstrate how these phenomena can be experimentally explored.

Journal Article↗
PubMed2003

Fano resonances with discrete breathers.

A theoretical study of linear wave scattering by time-periodic spatially localized excitations (discrete breathers) is presented. A peculiar effect of total reflection occurs due to a Fano resonance when a localized state originating from closed channels resonates with the open channel. For the discrete nonlinear Schrödinger chain, we give an analytical result for the frequency dependence of the transmission coefficient, including the possibility of resonant reflection. We extend the analysis to chains of weakly coupled anharmonic oscillators and discuss the relevance of the effect for electronic transport spectroscopy of mesoscopic systems.

Journal Article↗
PubMed2003

Chemically active substitutional nitrogen impurity in carbon nanotubes.

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 at. %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.

Journal Article↗
PubMed2004

Two-dimensional electron-hole liquid in single Si quantum wells with large electronic and dielectric confinement.

We report a luminescence study of the electronic properties of the 2D electron-hole liquid in crystalline Si quantum wells with SiO2 dielectric barriers. The Fermi-Dirac condensation of e-h pairs into a metallic liquid is strongly enhanced by spatial localization. We present experimental evidence for the formation of liquid nanodroplets, with size increasing with e-h pair density. The quantum confined regime is observed for well width below 15 nm. The data are analyzed in a confinement model that takes account of the band-gap renormalization by 2D many-body effects and the increase of the Coulomb interactions due to the dielectric mismatch between the Si well and the SiO2 barriers.

Journal Article↗
PubMed2004

Matter-wave gap vortices in optical lattices.

We predict the existence of spatially localized nontrivial topological states of a Bose-Einstein condensate with repulsive atomic interactions confined by an optical lattice. These nonlinear localized states, matter-wave gap vortices, carry a vortexlike phase dislocation and exist in the gaps of the matter-wave band-gap spectrum due to the Bragg scattering. We discuss the structure, stability, and formation dynamics of the gap vortices in the case of two-dimensional optical lattices.

Journal Article↗
PubMed2005

Bumps, breathers, and waves in a neural network with spike frequency adaptation.

We introduce a continuum model of neural tissue that includes the effects of spike frequency adaptation (SFA). The basic model is an integral equation for synaptic activity that depends upon nonlocal network connectivity, synaptic response, and the firing rate of a single neuron. We consider a phenomenological model of SFA via a simple state-dependent threshold firing rate function. As without SFA, Mexican-hat connectivity allows for the existence of spatially localized states (bumps). Importantly recent Evans function techniques are used to show that bumps may destabilize leading to the emergence of breathers and traveling waves. Moreover, a similar analysis for traveling pulses leads to the conditions necessary to observe a stable traveling breather. Simulations confirm our theoretical predictions and illustrate the rich behavior of this model.

Action Potentials↗
PubMed2005

Azimuthons: spatially modulated vortex solitons.

We introduce a novel class of spatially localized self-trapped ringlike singular optical beams in nonlinear media, the so-called azimuthons, which appear due to a continuous azimuthal deformation of vortex solitons. We demonstrate that the azimuthons are characterized by two independent integer indices, the topological charge m and the number N of the intensity peaks along the ring. Each soliton family includes azimuthons with negative, positive, and zero angular velocity.

Journal Article↗
PubMed2006

Reaction driven convection around a stably stratified chemical front.

A vertical stratification of a light and hot fluid over a heavy and cold one is expected to be stable with regard to buoyancy-driven convection. Here we show that chemical reactions can trigger convection around chemical fronts even in cases where concentration and heat both contribute to a stable density stratification. The balance between intrinsic thermal and solutal density gradients initiated by a spatially localized reaction zone and double diffusive mechanisms are at the origin of a new convective instability, the mechanism of which is explained by a displaced particle argument. Linear stability analysis of a reaction-diffusion-convection model confirmed by nonlinear simulations delimits the instability region in the parameter space spanned by the thermal and solutal Rayleigh numbers. Experimental systems in which to test our theoretical predictions are proposed.

Journal Article↗
PubMed2006

Mixed Wannier-Bloch functions for electrons and phonons in periodic systems.

We introduce mixed Wannier-Bloch functions for studying electronic and vibrational spectra of periodic systems. These functions carry both spatial localization and limited spectral broadening, thereby combining the advantages of descriptions based on energy eigenstates (Bloch states) and position eigenstates (Wannier states). For the analysis of vibrational modes, a lattice position operator is introduced, analogous to the electronic Berry-phase position operator. Application to vitreous SiO2 demonstrates that mixed Wannier-Bloch functions constitute a powerful tool for tracking fingerprints of short- and medium-range structural order in electronic and vibrational spectra.

Journal Article↗
PubMed2006

Wannier-stark ladders in one-dimensional elastic systems.

The optical analogues of Bloch oscillations and their associated Wannier-Stark ladders have been recently analyzed. In this Letter we propose an elastic realization of these ladders, employing for this purpose the torsional vibrations of specially designed one-dimensional elastic systems. We have measured, for the first time, the ladder wave amplitudes, which are not directly accessible either in the quantum-mechanical or optical cases. The wave amplitudes are spatially localized and coincide rather well with theoretically predicted amplitudes. The rods we analyze can be used to localize different frequencies in different parts of the elastic systems and vice versa.

Journal Article↗
PubMed2006

Quantifying desorption of saturated hydrocarbons from silicon with quantum calculations and scanning tunneling microscopy.

Electron stimulated desorption of cyclopentene from the Si(100)-(2 x 1) surface is studied experimentally with cryogenic UHV STM and theoretically with transport, electronic structure, and dynamical calculations. Unexpectedly for a saturated hydrocarbon on silicon, desorption is observed at bias magnitudes as low as 2.5 V, albeit the desorption yields are a factor of 500 to 1000 lower than previously reported for unsaturated molecules on silicon. The low threshold voltage for desorption is attributed to hybridization of the molecule with the silicon surface, which results in low-lying ionic resonances within 2-3 eV of the Fermi level. These resonances are long-lived, spatially localized, and displaced in equilibrium with respect to the neutral state. This study highlights the importance of nuclear dynamics in silicon-based molecular electronics and suggests new guidelines for the control of such dynamics.

Journal Article↗
PubMed2005

Progressive inhibition by water deficit of cell wall extensibility and growth along the elongation zone of maize roots is related to increased lignin metabolism and progressive stelar accumulation of wall phenolics.

Water deficit caused by addition of polyethylene glycol 6000 at -0.5 MPa water potential to well-aerated nutrient solution for 48 h inhibited the elongation of maize (Zea mays) seedling primary roots. Segmental growth rates in the root elongation zone were maintained 0 to 3 mm behind the tip, but in comparison with well-watered control roots, progressive growth inhibition was initiated by water deficit as expanding cells crossed the region 3 to 9 mm behind the tip. The mechanical extensibility of the cell walls was also progressively inhibited. We investigated the possible involvement in root growth inhibition by water deficit of alterations in metabolism and accumulation of wall-linked phenolic substances. Water deficit increased expression in the root elongation zone of transcripts of two genes involved in lignin biosynthesis, cinnamoyl-CoA reductase 1 and 2, after only 1 h, i.e. before decreases in wall extensibility. Further increases in transcript expression and increased lignin staining were detected after 48 h. Progressive stress-induced increases in wall-linked phenolics at 3 to 6 and 6 to 9 mm behind the root tip were detected by comparing Fourier transform infrared spectra and UV-fluorescence images of isolated cell walls from water deficit and control roots. Increased UV fluorescence and lignin staining colocated to vascular tissues in the stele. Longitudinal bisection of the elongation zone resulted in inward curvature, suggesting that inner, stelar tissues were also rate limiting for root growth. We suggest that spatially localized changes in wall-phenolic metabolism are involved in the progressive inhibition of wall extensibility and root growth and may facilitate root acclimation to drying environments.

Blotting, Northern↗
PubMed1997

Sucrose-to-Starch Metabolism in Tomato Fruit Undergoing Transient Starch Accumulation.

Immature green tomato (Lycopersicon esculentum) fruits undergo a period of transient starch accumulation characterized by developmental changes in the activities of key enzymes in the sucrose (Suc)-to-starch metabolic pathway. Activities of Suc synthase, fructokinase, ADP-glucose (Glc) pyrophosphorylase, and soluble and insoluble starch synthases decline dramatically in parallel to the decrease in starch levels in the developing fruit. Comparison of "maximal" in vitro activities of the enzymes in the Suc-to-starch pathway suggests that these same enzymes are limiting to the rate of starch accumulation. In contrast, activities of invertase, UDP-Glc pyrophosphorylase, nucleoside diphosphate kinase, phosphoglucoisomerase, and phosphoglucomutase do not exhibit dramatic decreases in activity and appear to be in excess of starch accumulation rates. Starch accumulation is spatially localized in the inner and radial pericarp and columella, whereas the outer pericarp and seed locule contain little starch. The seed locule is characterized by lower activities of Suc synthase, UDP-Glc pyrophosphorylase, phosphoglucomutase, ADP-Glc pyrophosphorylase, and soluble and insoluble starch synthases. The outer pericarp exhibits comparatively lower activities of ADP-Glc pyrophosphorylase and insoluble starch synthase only. These data are discussed in terms of the developmental and tissue-specific coordinated control of Suc-to-starch metabolism.

Journal Article↗
PubMed1999

Geometrical effects in diffraction analysis.

The use of X-ray and neutron scattering as a tool to study phase transitions is well established. As techniques improve and experiments are made under successively higher resolution, the need to consider the role of both the distribution of diffracting length scales and the incident-beam coherence volume is emphasized. The interplay of diffracting length scales and the beam coherence volume no longer permits calculation of diffraction profiles in terms of the sample intensity response convolved with an instrumental resolution function. Rather, the probe and sample now enter the calculation on an equal footing at the level of the scattering amplitudes. Under these conditions, it is found that the summation of coherent scattering amplitudes leads to characteristic profiles in wave-vector and, in the case of resonant X-ray scattering, energy space. In this latter case, in the vicinity of strong absorption edges, as used for example in resonant magnetic X-ray diffraction, the energy dependence of diffraction profiles may uniquely allow spatial localization of the scattering volume below the sample surface. This observation may considerably augment the range and power of resonant X-ray scattering.

Journal Article↗
PubMed2001

C- and D-weighted ultrasonic imaging using the translating apertures algorithm.

Conventional ultrasonic imaging systems depict tissue backscatter, that is, the ultrasonic energy reflected directly back toward the transmitter. Although diagnostically useful, these systems fail to exploit the information available in components of the sound field scattered in other directions. This paper describes a new method of imaging this angular scatter. First, the translating apertures algorithm (TAA) is used to acquire data at two scattering angles. Then, these data are processed to yield an image of the common scattering with angle and the differential scattering with angle. This paper explores the potential of these common-weighted (c-weighted) and difference-weighted (d-weighted) images using theory and simulations. In addition, it describes and analyzes the performance of the TAA when it is applied using multiple receive elements. Analysis is presented that shows that, in Rayleigh scattering environments, c- and d-weighted images depict compressibility and density variations, respectively. A simulated image and accompanying analysis are presented that show the potential of these techniques to improve soft tissue contrast and to increase the detectability of microcalcifications. A comparison with previous angular scatter measurement techniques shows that use of the TAA significantly reduces statistical variability in measured angular scatter profiles. Spatially localized, statistically reliable angular scatter measurements will enable a broad range of angular scatter imaging techniques. C- and d-weighted imaging may ultimately be applied clinically to identify calcification in atherosclerotic plaques and breast tumors.

Algorithms↗
PubMed2004

Multichannel magnetic stimulation system design considering mutual couplings among the stimulation coils.

We introduce some simulation and experiment results of the multichannel magnetic stimulator development that has been carried out as an initial attempt to realize a multichannel functional magnetic stimulator. For efficient functional magnetic stimulations, precise spatial localization of stimulation sites without any movements of the stimulation coils is very important. We have found that the mutual coupling effect among the adjacent stimulation coils in the coil array has to be considered in the determination of the charge voltages in some coil array configurations. Experimental results obtained with a 4-channel magnetic stimulator are presented.

Computer Simulation↗
PubMed2006

Using hidden scale for salient object detection.

This paper describes a method for detecting salient regions in remote-sensed images, based on scale and contrast interaction. We consider the focus on salient structures as the first stage of an object detection/recognition algorithm, where the salient regions are those likely to contain objects of interest. Salient objects are modeled as spatially localized and contrasted structures with any kind of shape or size. Their detection exploits a probabilistic mixture model that takes two series of multiscale features as input, one that is more sensitive to contrast information, and one that is able to select scale. The model combines them to classify each pixel in salient/nonsalient class, giving a binary segmentation of the image. The few parameters are learned with an EM-type algorithm.

Algorithms↗
PubMed2006

Efficient transmission of compressed data for remote volume visualization.

One of the goals of telemedicine is to enable remote visualization and browsing of medical volumes. There is a need to employ scalable compression schemes and efficient client-server models to obtain interactivity and an enhanced viewing experience. First, we present a scheme that uses JPEG2000 and JPIP (JPEG2000 Interactive Protocol) to transmit data in a multi-resolution and progressive fashion. The server exploits the spatial locality offered by the wavelet transform and packet indexing information to transmit, in so far as possible, compressed volume data relevant to the clients query. Once the client identifies its volume of interest (VOI), the volume is refined progressively within the VOI from an initial lossy to a final lossless representation. Contextual background information can also be made available having quality fading away from the VOI. Second, we present a prioritization that enables the client to progressively visualize scene content from a compressed file. In our specific example, the client is able to make requests to progressively receive data corresponding to any tissue type. The server is now capable of reordering the same compressed data file on the fly to serve data packets prioritized as per the client's request. Lastly, we describe the effect of compression parameters on compression ratio, decoding times and interactivity. We also present suggestions for optimizing JPEG2000 for remote volume visualization and volume browsing applications. The resulting system is ideally suited for client-server applications with the server maintaining the compressed volume data, to be browsed by a client with a low bandwidth constraint.

Algorithms↗