351 research outputs found
Self-excited Oscillations of Charge-Spin Accumulation Due to Single-electron Tunneling
We theoretically study electronic transport through a layer of quantum dots
connecting two metallic leads. By the inclusion of an inductor in series with
the junction, we show that steady electronic transport in such a system may be
unstable with respect to temporal oscillations caused by an interplay between
the Coulomb blockade of tunneling and spin accumulation in the dots. When this
instability occurs, a new stable regime is reached, where the average spin and
charge in the dots oscillate periodically in time. The frequency of these
oscillations is typically of the order of 1GHz for realistic values of the
junction parameters
Revegetation of ski runs in Serbia: Case studies of Mts. Stara Planina and Divčibare
Revegetation is the most sustainable method of soil stabilization at ski runs. In order to establish a stable plant community, it is recommended to use native species. However, non-native species are most often used. In this paper the revegetation of ski runs at two ski resorts is investigated: Divčibare and Stara Planina. Seven species were used for the revegetation of the ski run at the Divčibare ski resort of which six species were native. Six species were used for the revegetation of the Stara Planina ski resort, of which two species were native. It was established that the plant species used in the seed mixtures were suitable for erosion control at the investigated ski resorts
Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber
We study theoretically the generation of photon pairs by spontaneous
four-wave mixing (SFWM) in photonic crystal optical fiber. We show that it is
possible to engineer two-photon states with specific spectral correlation
(``entanglement'') properties suitable for quantum information processing
applications. We focus on the case exhibiting no spectral correlations in the
two-photon component of the state, which we call factorability, and which
allows heralding of single-photon pure-state wave packets without the need for
spectral post filtering. We show that spontaneous four wave mixing exhibits a
remarkable flexibility, permitting a wider class of two-photon states,
including ultra-broadband, highly-anticorrelated states.Comment: 17 pages, 7 figures, submitte
Spin-Polaronic Effects in Electric Shuttling in a Single Molecule Transistor with Magnetic Leads
Current-voltage characteristics of a spintromechanical device, in which
spin-polarized electrons tunnel between magnetic leads with anti-parallel
magnetization through a single level movable quantum dot, are calculated. New
exchange- and electromechanical coupling-induced (spin-polaronic) effects that
determine strongly nonlinear current-voltage characteristics were found. In the
low-voltage regime of electron transport the voltage-dependent and exchange
field-induced displacement of quantum dot towards the source electrode leads to
nonmonotonic behavior of differential conductance that demonstrates the lifting
of spin-polaronic effects by electric field. At high voltages the onset of
electron shuttling results in the drop of current and negative differential
conductance, caused by mechanically-induced increase of tunnel resistivities
and exchange field-induced suppression of spin-flips in magnetic field. The
dependence of these predicted spin effects on the oscillations frequency of the
dot and the strength of electron-electron correlations is discussed.Comment: 8 pages, 4 figure
Coulomb-promoted spintromechanics in magnetic shuttle devices
Exchange forces on the movable dot ("shuttle") in a magnetic shuttle device
depend on the parity of the number of shuttling electrons. The performance of
such a device can therefore be tuned by changing the strength of Coulomb
correlations to block or unblock parity fluctuations. We show that by
increasing the spintro-mechanics of the device crosses over, at ,
from a mechanically stable regime to a regime of spin-induced shuttle
instabilities. This is due to enhanced spin-dependent mechanical forces as
parity fluctuations are reduced by a Coulomb blockade of tunneling and
demonstrates that single-electron manipulation of single-spin controlled
nano-mechanics is possible.Comment: 5 pages, 2 figures and a supplementary information fil
Mid-infrared optical parametric amplifier using silicon nanophotonic waveguides
All-optical signal processing is envisioned as an approach to dramatically
decrease power consumption and speed up performance of next-generation optical
telecommunications networks. Nonlinear optical effects, such as four-wave
mixing (FWM) and parametric gain, have long been explored to realize
all-optical functions in glass fibers. An alternative approach is to employ
nanoscale engineering of silicon waveguides to enhance the optical
nonlinearities by up to five orders of magnitude, enabling integrated
chip-scale all-optical signal processing. Previously, strong two-photon
absorption (TPA) of the telecom-band pump has been a fundamental and
unavoidable obstacle, limiting parametric gain to values on the order of a few
dB. Here we demonstrate a silicon nanophotonic optical parametric amplifier
exhibiting gain as large as 25.4 dB, by operating the pump in the mid-IR near
one-half the band-gap energy (E~0.55eV, lambda~2200nm), at which parasitic
TPA-related absorption vanishes. This gain is high enough to compensate all
insertion losses, resulting in 13 dB net off-chip amplification. Furthermore,
dispersion engineering dramatically increases the gain bandwidth to more than
220 nm, all realized using an ultra-compact 4 mm silicon chip. Beyond its
significant relevance to all-optical signal processing, the broadband
parametric gain also facilitates the simultaneous generation of multiple
on-chip mid-IR sources through cascaded FWM, covering a 500 nm spectral range.
Together, these results provide a foundation for the construction of
silicon-based room-temperature mid-IR light sources including tunable
chip-scale parametric oscillators, optical frequency combs, and supercontinuum
generators
Dzyaloshinskii-Moriya Interaction and Spiral Order in Spin-orbit Coupled Optical Lattices
We show that the recent experimental realization of spin-orbit coupling in
ultracold atomic gases can be used to study different types of spin spiral
order and resulting multiferroic effects. Spin-orbit coupling in optical
lattices can give rise to the Dzyaloshinskii-Moriya (DM) spin interaction which
is essential for spin spiral order. By taking into account spin-orbit coupling
and an external Zeeman field, we derive an effective spin model in the Mott
insulator regime at half filling and demonstrate that the DM interaction in
optical lattices can be made extremely strong with realistic experimental
parameters. The rich finite temperature phase diagrams of the effective spin
models for fermions and bosons are obtained via classical Monte Carlo
simulations.Comment: 7 pages, 5 figure
Design, expression and characterization of mutants of fasciculin optimized for interaction with its target, acetylcholinesterase
Predicting mutations that enhance protein–protein affinity remains a challenging task, especially for high-affinity complexes. To test our capability to improve the affinity of such complexes, we studied interaction of acetylcholinesterase with the snake toxin, fasciculin. Using the program ORBIT, we redesigned fasciculin's sequence to enhance its interactions with Torpedo californica acetylcholinesterase. Mutations were predicted in 5 out of 13 interfacial residues on fasciculin, preserving most of the polar inter-molecular contacts seen in the wild-type toxin/enzyme complex. To experimentally characterize fasciculin mutants, we developed an efficient strategy to over-express the toxin in Escherichia coli, followed by refolding to the native conformation. Despite our predictions, a designed quintuple fasciculin mutant displayed reduced affinity for the enzyme. However, removal of a single mutation in the designed sequence produced a quadruple mutant with improved affinity. Moreover, one designed mutation produced 7-fold enhancement in affinity for acetylcholinesterase. This led us to reassess our criteria for enhancing affinity of the toxin for the enzyme. We observed that the change in the predicted inter-molecular energy, rather than in the total energy, correlates well with the change in the experimental free energy of binding, and hence may serve as a criterion for enhancement of affinity in protein–protein complexes
A Functional Proteomic Method for Biomarker Discovery
The sequencing of the human genome holds out the hope for personalized medicine, but it is clear that analysis of DNA or RNA content alone is not sufficient to understand most disease processes. Proteomic strategies that allow unbiased identification of proteins and their post-transcriptional and -translation modifications are an essential complement to genomic strategies. However, the enormity of the proteome and limitations in proteomic methods make it difficult to determine the targets that are particularly relevant to human disease. Methods are therefore needed that allow rational identification of targets based on function and relevance to disease. Screening methodologies such as phage display, SELEX, and small-molecule combinatorial chemistry have been widely used to discover specific ligands for cells or tissues of interest, such as tumors. Those ligands can be used in turn as affinity probes to identify their cognate molecular targets when they are not known in advance. Here we report an easy, robust and generally applicable approach in which phage particles bearing cell- or tissue-specific peptides serve directly as the affinity probes for their molecular targets. For proof of principle, the method successfully identified molecular binding partners, three of them novel, for 15 peptides specific for pancreatic cancer
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