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At least 1,045 records · Page 58Linked to original sources
PubMed2002

Nonequilibrium statistical mechanics of mixed quantum classical ensembles: application to noncontact atomic force microscopy.

Using the nonequilibrium statistical operator method, we suggest a new general method of treating dynamics of a combined system consisting of interacting classical and quantum parts. The method is illustrated on the tip dynamics in the noncontact atomic force microscopy (NC-AFM) where a macroscopic tip interacts with a quantum microscopic system (the surface and the nanotip). The derived general equation of motion for the tip and the Fokker-Planck equation, applicable even at low temperatures, contain memory effects and a friction term which should (at least partially) be responsible for the observed energy dissipation in NC-AFM experiments.

Journal Article↗
PubMed2003

Generating a superposition of spin states in an atomic ensemble.

A method for generating a mesoscopic superposition state of the collective spin variable of a gas of atoms is proposed. The state consists of a superposition of the atomic spins pointing in two slightly different directions. It is obtained by using off resonant light to carry out quantum nondemolition measurements of the spins. The relevant experimental conditions, which require very dense atomic samples, can be realized with presently available techniques. Long-lived atomic superposition states may become useful as an off-line resource for quantum computing with otherwise linear operations.

Journal Article↗
PubMed2004

Aharonov-Bohm signature for neutral polarized excitons in type-II quantum dot ensembles.

The Aharonov-Bohm effect is commonly believed to be a typical feature of the motion of a charged particle interacting with the electromagnetic vector potential. Here we present a magnetophotoluminescence study of type-II InP/GaAs self-assembled quantum dots, revealing the Aharonov-Bohm-type oscillations for neutral excitons when the hole ground state changes its angular momentum from l(h)=0 to l(h)=1, 2, and 3. The hole-ring parameters derived from a simple model are in excellent agreement with the structural parameters for this system.

Journal Article↗
PubMed2004

Low temperature emission spectra of individual single-walled carbon nanotubes: multiplicity of subspecies within single-species nanotube ensembles.

Low-temperature photoluminescence (PL) studies of individual semiconducting single-walled carbon nanotubes reveal ultranarrow peaks (down to 0.25 meV linewidths) that exhibit blinking and spectral wandering. Multiple peaks appear within bands previously assigned to nanotubes of certain chiralities, indicating the existence of numerous subspecies within single-chirality specimens. The sharp PL features show two types of distinctly different shapes (symmetric versus asymmetric) and temperature dependences (weak versus strong), which we attribute to the presence of unintentionally doped nanotubes along with undoped species.

Journal Article↗
PubMed2004

Dynamics of coherent and incoherent spin polarizations in ensembles of quantum dots.

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals approximately 10 times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.

Journal Article↗
PubMed2004

Reproducible sequence generation in random neural ensembles.

Little is known about the conditions that neural circuits have to satisfy to generate reproducible sequences. Evidently, the genetic code cannot control all the details of the complex circuits in the brain. In this Letter, we give the conditions on the connectivity degree that lead to reproducible and robust sequences in a neural population of randomly coupled excitatory and inhibitory neurons. In contrast to the traditional theoretical view we show that the sequences do not need to be learned. In the framework proposed here just the averaged characteristics of the random circuits have to be under genetic control. We found that rhythmic sequences can be generated if random networks are in the vicinity of an excitatory-inhibitory synaptic balance. Reproducible transient sequences, on the other hand, are found far from a synaptic balance.

Action Potentials↗
PubMed2005

Conductance of an ensemble of molecular wires: a statistical analysis.

We report on a first principles analysis of quantum transport through molecular wires made of 4,4'bipyridine and 6-alkanedithiol contacted by Au electrodes. We investigate how charge transport is altered due to small structure changes at the molecule-electrode contacts. These changes include distance between the molecule and the contact, extra metal atoms at the Au surface, binding sites, molecular orientation, and bias voltages. By investigating hundreds of wires we extract a statistical picture on transport properties of the two different molecules. We compare quantitatively with the corresponding experimental data.

Journal Article↗
PubMed2006

Directed spontaneous emission from an extended ensemble of N atoms: timing is everything.

A collection of static atoms is fixed in a crystal at a low temperature and prepared by a pulse of incident radiation of wave vector . The atoms are well described by an entangled Dicke-like state, in which each atom carries a characteristic phase factor exp(ik0.r(j)), where is the atomic position in the crystal. It is shown that a single photon absorbed by the N atoms will be followed by spontaneous emission in the same direction. Furthermore, phase matched emission is found when one photon is absorbed by N atoms followed by two-photon down-conversion.

Journal Article↗
PubMed2006

Field squeeze operators in optical cavities with atomic ensembles.

We propose a method of generating unitarily single and two-mode field squeezing in an optical cavity with an atomic cloud. Through a suitable laser system, we are able to engineer a squeeze field operator decoupled from the atomic degrees of freedom, yielding a large squeeze parameter that is scaled up by the number of atoms, and realizing degenerate and nondegenerate parametric amplification. By means of the input-output theory we show that ideal squeezed states and perfect squeezing could be approached at the output. The scheme is robust to decoherence processes.

Journal Article↗
PubMed2006

Crossover from conserving to lossy transport in circular random-matrix ensembles.

In a quantum dot with three leads, the transmission matrix t12 between two of these leads is a truncation of a unitary scattering matrix S, which we treat as random. As the number of channels in the third lead is increased, the constraints from the symmetry of S become less stringent and t12 becomes closer to a matrix of complex Gaussian random numbers with no constraints. We consider the distribution of the singular values of t12, which is related to a number of physical quantities.

Journal Article↗
PubMed2006

Coherent kinetic control over crystal orientation in macroscopic ensembles of polymer nanorods and nanotubes.

We show that the crystal orientation in polymer nanotubes and nanorods inside porous templates is controlled by the kinetics of nucleation and growth under 2D confinement. Two clear limiting cases are identified: In separated nanostructures, any crystal orientation allowing the growth of lamellar crystals along the pores appears statistically. If a bulklike surface film connects the nanostructures, macroscopic arrays with uniform crystal orientation are obtained, in which the dominant growth direction of the crystals is aligned with the long axes of the pores of the template.

Crystallization↗
PubMed1989

A sunflower helianthinin gene upstream sequence ensemble contains an enhancer and sites of nuclear protein interaction.

Genes encoding helianthinin, the major seed protein in sunflower, are highly regulated. We have identified putative cis-acting and trans-acting elements that may function in the control of helianthinin expression. A 404-base pair DNA fragment on the sunflower helianthinin gene HaG3D, located 322 base pairs from the transcriptional start site, enhanced beta-glucuronidase expression in transgenic tobacco embryos. Sequences within this fragment were found to bind nuclear proteins present in both sunflower embryo and hypocotyl nuclear extracts. The binding site was localized by phenanthroline-copper ion footprinting experiments to A/T-rich sequences located from -705 to -654. Binding competition experiments revealed that these sunflower proteins also bind to upstream promoter sequences from another helianthinin gene (HaG3A) and two other plant embryo-specific genes, carrot DcG3 and French bean phaseolin. However, sequences of the cauliflower mosaic virus 35S promoter/enhancer complex failed to compete for its binding. Phenanthroline-copper ion footprinting experiments showed that the binding sites for the sunflower proteins in HaG3A (-1463 to -1514 and -702 to -653) and in phaseolin (-671 to -627) are also very A/T-rich, have similar sizes, and are located at similar distances from their respective promoters.

2S Albumins, Plant↗
PubMed2005

Sensitivity of the folding/unfolding transition state ensemble of chymotrypsin inhibitor 2 to changes in temperature and solvent.

To better characterize the transition state for folding/unfolding and its sensitivity to environmental changes, we have run multiple molecular dynamics simulations of chymotrypsin inhibitor 2 (CI2) under varying solvent conditions and temperature. The transition state structures agree well with experiment, and are similar under all of the conditions investigated here. Increasing the temperature leads to some movement in the position of the transition state along several reaction coordinates, as measured by changes in properties of the transition state structures. These structural changes are in the direction of a more native-like transition state as denaturation conditions become more severe, as expected for a Hammond effect. These structural changes are not, however, reflected in the global structure as measured by the total number of contacts or the average S-values. These results suggest that the small changes in average Phi-values with temperature seen by experiment may be due to an increase in the sensitivity of the transition state to mutation rather than a change in the average structure of the transition state. A simple analysis of the rates of unfolding indicates that the free energy barrier to unfolding decreases with increasing temperature, but even in our very high temperature simulations there is a small free energy barrier.

Peptides↗
PubMed2006

NMR characterizations of an amyloidogenic conformational ensemble of the PI3K SH3 domain.

Amyloid formation is associated with structural changes of native polypeptides to monomeric intermediate states and their self-assembly into insoluble aggregates. Characterizations of the amyloidogenic intermediate state are, therefore, of great importance in understanding the early stage of amyloidogenesis. Here, we present NMR investigations of the structural and dynamic properties of the acid-unfolded amyloidogenic intermediate state of the phosphatidylinositol 3-kinase (PI3K) SH3 domain--a model peptide. The monomeric amyloidogenic state of the SH3 domain studied at pH 2.0 (35 degrees C) was shown to be substantially disordered with no secondary structural preferences. (15)N NMR relaxation experiments indicated that the unfolded polypeptide is highly flexible on a subnanosecond timescale when observed under the amyloidogenic condition (pH 2.0, 35 degrees C). However, more restricted motions were detected in residues located primarily in the beta-strands as well as in a loop in the native fold. In addition, nonnative long-range interactions were observed between the residues with the reduced flexibility by paramagnetic relaxation enhancement (PRE) experiments. These indicate that the acid-unfolded state of the SH3 domain adopts a partly folded conformation through nonnative long-range contacts between the dynamically restricted residues at the amyloid-forming condition.

Amino Acid Sequence↗
PubMed2004

An ethics ensemble: abortion, Thomson, Finnis and the case of the violin-player.

This paper considers approaches to the ethics of abortion and putative links to "rights" debates. In particular, it revisits two papers on the topic from the early 1970's, written by Judith Jarvis Thomson and John Finnis respectively. Consideration of the discourse produced by these papers to some extent reveals the strategic importance of linguistic and conceptual organisation--the rhetorical forces underlying claims to disinterested, analytical standing. In particular, the paper reviews the practical ethics analogy proposed by Thomson; the deployment of "Hohfeldian" rights analysis by John Finnis, the link to the abortion debate and the broader implications of such deployment in its relation to current "rights" discourse.

Abortion, Induced↗
PubMed2004

Practicality of performing medical procedures in chemical protective ensembles.

OBJECTIVE: To determine whether certain life saving medical procedures can be successfully performed while wearing different levels of personal protective equipment (PPE), and whether these procedures can be performed in a clinically useful time frame. METHODS: We assessed the capability of eight medical personnel to perform airway maintenance and antidote administration procedures on manikins, in all four described levels of PPE. The levels are: Level A--a fully encapsulated chemically resistant suit; Level B--a chemically resistant suit, gloves and boots with a full-faced positive pressure supplied air respirator; Level C--a chemically resistant splash suit, boots and gloves with an air-purifying positive or negative pressure respirator; Level D--a work uniform. Time in seconds to inflate the lungs of the manikin with bag-valve-mask, laryngeal mask airway (LMA) and endotracheal tube (ETT) were determined, as was the time to secure LMAs and ETTs with either tape or linen ties. Time to insert a cannula in a manikin was also determined. RESULTS: There was a significant difference in time taken to perform procedures in differing levels of personal protective equipment (F21,72 = 1.75, P = 0.04). Significant differences were found in: time to lung inflation using an endotracheal tube (A vs. C mean difference and standard error 75.6 +/- 23.9 s, P = 0.03; A vs. D mean difference and standard error 78.6 +/- 23.9 s, P = 0.03); time to insert a cannula (A vs. D mean difference and standard error 63.6 +/- 11.1 s, P < 0.001; C vs. D mean difference and standard error 40.0 +/- 11.1 s, P = 0.01). CONCLUSIONS: A significantly greater time to complete procedures was documented in Level A PPE (fully encapsulated suits) compared with Levels C and D. There was however, no significant difference in times between Level B and Level C. The common practice of equipping hospital and medical staff with only Level C protection should be re-evaluated.

Antidotes↗