8,175 research outputs found
Local dynamics in high-order harmonic generation using Bohmian trajectories
We investigate high-order harmonic generation from a Bohmian-mechanical
perspective, and find that the innermost part of the core, represented by a
single Bohmian trajectory, leads to the main contributions to the high-harmonic
spectra. Using time-frequency analysis, we associate this central Bohmian
trajectory to an ensemble of unbound classical trajectories leaving and
returning to the core, in agreement with the three step model. In the Bohmian
scenario, this physical picture builds up non-locally near the core via the
quantum mechanical phase of the wavefunction. This implies that the flow of the
wavefunction far from the core alters the central Bohmian trajectory. We also
show how this phase degrades in time for the peripheral Bohmian trajectories as
they leave the core region.Comment: 7 pages, 3 figures; the manuscript has been considerably extended and
modified with regard to the previous version
Elucidating the role of hyperfine interactions on organic magnetoresistance using deuterated aluminium tris(8-hydroxyquinoline)
Measurements of the effect of a magnetic field on the light output and
current through an organic light emitting diode made with deuterated aluminium
tris(8-hydroxyquinoline) have shown that hyperfine coupling with protons is not
the cause of the intrinsic organic magnetoresistance. We suggest that
interactions with unpaired electrons in the device may be responsible.Comment: Submitte
Discovery of the Fomalhaut C debris disc
Fomalhaut is one of the most interesting and well studied nearby stars,
hosting at least one planet, a spectacular debris ring, and two distant
low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We
observed both companions with Herschel, and while no disc was detected around
the secondary, TW PsA, we have discovered the second debris disc in the
Fomalhaut system, around LP 876-10. This detection is only the second case of
two debris discs seen in a multiple system, both of which are relatively wide
(3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP
876-10). The disc is cool (24K) and relatively bright, with a fractional
luminosity , and represents the rare
observation of a debris disc around an M dwarf. Further work should attempt to
find if the presence of two discs in the Fomalhaut system is coincidental,
perhaps simply due to the relatively young system age of 440 Myr, or if the
stellar components have dynamically interacted and the system is even more
complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma
Animating Volcanic Processes
Animation is a powerful tool for making scientific processes accessible to non-specialists. Volcanic processes, for example, often require animated visualizations to explain the physics of what occurs beneath the Earth’s surface. We are partnering with geoscience professor Jeffrey Johnson to tell the story of Santiaguito Volcano (Guatemala), where eruptions occur approximately hourly and specialized sensors are used to detect pre-eruptive activity. Specifically, we are working to illustrate subtle slope changes caused by fluctuation in the volcano\u27s pressure, which are detected by devices called tiltmeters.
We use vector animation to create stylized representations of these Earth processes. Our work combines digitally-hand-drawn frames with the use of automatic interpolation to create smooth and graphical animations. Annotations and visual cues are added to draw focus to important sequences. Our goal is to make educational animations more entertaining by incorporating artistic flourishes like characters, a stylish color palette, and a hand-drawn look.
Animation permits creative freedom to incorporate artistic touches to scientific materials. The Santiaguito project is expected to be the first of several volcano STEM stories that could be produced using the same stylization. The project is especially gratifying in that it is able to bring together the departments of science and art; we believe collaboration between different disciplines is a fantastic way to engage with what exists outside our everyday lives
Time-dependent quantum Monte Carlo: preparation of the ground state
We study one-dimensional (1D) and two-dimensional (2D) Helium atoms using a
new time-dependent quantum Monte Carlo (TDQMC) method. The TDQMC method employs
random walkers, with a separate guiding wave attached to each walker. The
ground state is calculated by a self-consistent solution of complex-time
Schroedinger equations for the guiding waves and of equations for the velocity
fields of the walkers. Our results show that the many-body wavefunction and the
ground state energy of the model atoms are very close to those predicted by the
standard diffusion quantum Monte Carlo method. The obtained ground state can
further be used to examine correlated time-dependent processes which include,
for example, interaction of atoms and molecules with external electromagnetic
fields.Comment: 9 pages, 5 figure
A target repurposing approach identifies N-myristoyltransferase as a new candidate drug target in filarial nematodes
Myristoylation is a lipid modification involving the addition of a 14-carbon unsaturated fatty acid, myristic acid, to the N-terminal glycine of a subset of proteins, a modification that promotes their binding to cell membranes for varied biological functions. The process is catalyzed by myristoyl-CoA:protein N-myristoyltransferase (NMT), an enzyme which has been validated as a drug target in human cancers, and for infectious diseases caused by fungi, viruses and protozoan parasites. We purified Caenorhabditis elegans and Brugia malayi NMTs as active recombinant proteins and carried out kinetic analyses with their essential fatty acid donor, myristoyl-CoA and peptide substrates. Biochemical and structural analyses both revealed that the nematode enzymes are canonical NMTs, sharing a high degree of conservation with protozoan NMT enzymes. Inhibitory compounds that target NMT in protozoan species inhibited the nematode NMTs with IC50 values of 2.5-10 nM, and were active against B. malayi microfilariae and adult worms at 12.5 µM and 50 µM respectively, and C. elegans (25 µM) in culture. RNA interference and gene deletion in C. elegans further showed that NMT is essential for nematode viability. The effects observed are likely due to disruption of the function of several downstream target proteins. Potential substrates of NMT in B. malayi are predicted using bioinformatic analysis. Our genetic and chemical studies highlight the importance of myristoylation in the synthesis of functional proteins in nematodes and have shown for the first time that NMT is required for viability in parasitic nematodes. These results suggest that targeting NMT could be a valid approach for the development of chemotherapeutic agents against nematode diseases including filariasis
Bohmian trajectories and the Path Integral Paradigm. Complexified Lagrangian Mechanics
David Bohm shown that the Schr{\"o}dinger equation, that is a "visiting card"
of quantum mechanics, can be decomposed onto two equations for real functions -
action and probability density. The first equation is the Hamilton-Jacobi (HJ)
equation, a "visiting card" of classical mechanics, to be modified by the
Bohmian quantum potential. And the second is the continuity equation. The
latter can be transformed to the entropy balance equation. The Bohmian quantum
potential is transformed to two Bohmian quantum correctors. The first corrector
modifies kinetic energy term of the HJ equation, and the second one modifies
potential energy term. Unification of the quantum HJ equation and the entropy
balance equation gives complexified HJ equation containing complex kinetic and
potential terms. Imaginary parts of these terms have order of smallness about
the Planck constant. The Bohmian quantum corrector is indispensable term
modifying the Feynman's path integral by expanding coordinates and momenta to
imaginary sector.Comment: 14 pages, 3 figures, 46 references, 48 equation
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
Hot exozodiacal dust resolved around Vega with IOTA/IONIC
Although debris discs have been detected around a significant number of
main-sequence stars, only a few of them are known to harbour hot dust in their
inner part where terrestrial planets may have formed. Thanks to infrared
interferometric observations, it is possible to obtain a direct measurement of
these regions, which are of prime importance for preparing future exo-Earth
characterisation missions. In this context, we have resolved the exozodiacal
dust disc around Vega with the help of infrared stellar interferometry and
estimated the integrated H-band flux originating from the first few AUs of the
debris disc. Using precise H-band interferometric measurements obtained with
the 3-telescope IOTA/IONIC interferometer (Mount Hopkins, Arizona), thorough
modelling of both interferometric data (squared visibility and closure phase)
and spectral energy distribution was performed to constrain the nature of the
near-infrared excess emission. The most straightforward scenario consists in a
compact dust disc producing a thermal emission that is largely dominated by
small grains located between 0.1 and 0.3 AU from Vega and accounting for 1.23
+/- 0.45% of the near-infrared stellar flux for our best-fit model. This flux
ratio is shown to vary slightly with the geometry of the model used to fit our
interferometric data (variations within +/-0.19%). Initially revealed by K-band
CHARA/FLUOR observations, the presence of hot exozodiacal dust in the vicinity
of Vega is confirmed by our H-band IOTA/IONIC measurements at the 3-sigma
level. Whereas the origin of the dust is still uncertain, its presence and the
possible connection with the outer disc suggest that the Vega system is
currently undergoing major dynamical perturbations.Comment: 10 pages, 9 figures, accepted for publication in A&
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