26 research outputs found
Energy transfer from an individual quantum dot to a carbon nanotube
A detailed understanding of energy transduction is crucial for achieving
precise control of energy flow in complex, integrated systems. In this context,
carbon nanotubes (CNTs) are intriguing model systems due to their rich,
chirality-dependent electronic and optical properties. Here, we study the
quenching of fluorescence from isolated quantum dots (QDs) upon approach of
individual CNTs attached to atomic force microscope probes. Precision
measurements of many different CNT/QD pairs reveal behavior consistent with
resonant energy transfer between QD and CNT excitons via a Fohrster-like
dipole-dipole coupling. The data reveal large variations in energy transfer
length scales even though peak efficiencies are narrowly distributed around
96%. This saturation of efficiency is maintained even when energy transfer must
compete with elevated intrinsic non-radiative relaxation rates during QD aging.
These observations suggest that excitons can be created at different locations
along the CNT length, thereby resulting in self-limiting behavior.Comment: 8 pages, 8 figures, with supplementary informatio
Photoassociative Frequency Shift in a Quantum Degenerate Gas
We observe a light-induced frequency shift in single-photon photoassociative
spectra of magnetically trapped, quantum degenerate 7Li. The shift is a
manifestation of the coupling between the threshold continuum scattering states
and discrete bound levels in the excited-state molecular potential induced by
the photoassociation laser. The frequency shift is observed to be linear in the
laser intensity with a measured proportionality constant that is in good
agreement with theoretical predictions. The frequency shift has important
implications for a scheme to alter the interactions between atoms in a
Bose-Einstein condensate using photoassociation resonances.Comment: 3 figure
Fluorescence Near-Field Microscopy of DNA at Sub-10 nm Resolution
We demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with ~sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by ~15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA
Tip-Enhanced Fluorescence Microscopy at 10 Nanometer Resolution
We demonstrate unambiguously that the field enhancement near the apex of a laser-illuminated silicon tip decays according to a power law that is moderated by a single parameter characterizing the tip sharpness. Oscillating the probe in intermittent contact with a semiconductor nanocrystal strongly modulates the fluorescence excitation rate, providing robust optical contrast and enabling excellent background rejection. Laterally encoded demodulation yields images with <10 nm spatial resolution, consistent with independent measurements of tip sharpness
Direct observation of growth and collapse of a Bose-Einstein condensate with attractive interactions
The dynamical behavior of Bose-Einstein condensation (BEC) in a gas with
attractive interactions is striking. Quantum theory predicts that BEC of a
spatially homogeneous gas with attractive interactions is precluded by a
conventional phase transition into either a liquid or solid. When confined to a
trap, however, such a condensate can form provided that its occupation number
does not exceed a limiting value. The stability limit is determined by a
balance between self-attraction and a repulsion arising from position-momentum
uncertainty under conditions of spatial confinement. Near the stability limit,
self-attraction can overwhelm the repulsion, causing the condensate to
collapse. Growth of the condensate, therefore, is punctuated by intermittent
collapses, which are triggered either by macroscopic quantum tunneling or
thermal fluctuation. Previous observation of growth and collapse has been
hampered by the stochastic nature of these mechanisms. Here we reduce the
stochasticity by controlling the initial number of condensate atoms using a
two-photon transition to a diatomic molecular state. This enables us to obtain
the first direct observation of the growth of a condensate with attractive
interactions and its subsequent collapse.Comment: 10 PDF pages, 5 figures (2 color), 19 references, to appear in Nature
Dec. 7 200
Efect of magnetic Gd impurities on superconductivity in MoGe films with different thickness and morphology
We studied the effect of magnetic doping with Gd atoms on the superconducting
properties of amorphous Mo70Ge30 films. We observed that in uniform films
deposited on amorphous Ge, the pair-breaking strength per impurity strongly
decreases with film thickness initially and saturates at a finite value in
films with thickness below the spin-orbit scattering length. The variation is
likely caused by surface induced magnetic anisotropy and is consistent with the
fermionic mechanism of superconductivity suppression. In thin films deposited
on SiN the pair-breaking strength becomes zero. Possible reasons for this
anomalous response are discussed. The morphological distinctions between the
films of the two types were identified using atomic force microscopy with a
carbon nanotube tip
Analysis of Biological Features Associated with Meiotic Recombination Hot and Cold Spots in Saccharomyces cerevisiae
Meiotic recombination is not distributed uniformly throughout the genome. There are regions of high and low recombination rates called hot and cold spots, respectively. The recombination rate parallels the frequency of DNA double-strand breaks (DSBs) that initiate meiotic recombination. The aim is to identify biological features associated with DSB frequency. We constructed vectors representing various chromatin and sequence-based features for 1179 DSB hot spots and 1028 DSB cold spots. Using a feature selection approach, we have identified five features that distinguish hot from cold spots in Saccharomyces cerevisiae with high accuracy, namely the histone marks H3K4me3, H3K14ac, H3K36me3, and H3K79me3; and GC content. Previous studies have associated H3K4me3, H3K36me3, and GC content with areas of mitotic recombination. H3K14ac and H3K79me3 are novel predictions and thus represent good candidates for further experimental study. We also show nucleosome occupancy maps produced using next generation sequencing exhibit a bias at DSB hot spots and this bias is strong enough to obscure biologically relevant information. A computational approach using feature selection can productively be used to identify promising biological associations. H3K14ac and H3K79me3 are novel predictions of chromatin marks associated with meiotic DSBs. Next generation sequencing can exhibit a bias that is strong enough to lead to incorrect conclusions. Care must be taken when interpreting high throughput sequencing data where systematic biases have been documented