72 research outputs found
Global axisymmetric Magnetorotational Instability with density gradients
We examine global incompressible axisymmetric perturbations of a
differentially rotating MHD plasma with radial density gradients. It is shown
that the standard magnetorotational instability, (MRI) criterion drawn from the
local dispersion relation is often misleading. If the equilibrium magnetic
field is either purely axial or purely toroidal, the problem reduces to finding
the global radial eigenvalues of an effective potential. The standard Keplerian
profile including the origin is mathematically ill-posed, and thus any solution
will depend strongly on the inner boundary. We find a class of unstable modes
localized by the form of the rotation and density profiles, with reduced
dependence on boundary conditions.Comment: 22 pages, 5 figure
Non-Hermitian chiral phononics through optomechanically-induced squeezing
Imposing chirality on a physical system engenders unconventional energy flow
and responses, such as the Aharonov-Bohm effect and the topological quantum
Hall phase for electrons in a symmetry-breaking magnetic field. Recently, great
interest has arisen in combining that principle with broken Hermiticity to
explore novel topological phases and applications. Here, we report unique
phononic states formed when combining the controlled breaking of time-reversal
symmetry with non-Hermitian dynamics, both induced through time-modulated
radiation pressure forces in small nano-optomechanical networks. We observe
chiral energy flow among mechanical resonators in a synthetic dimension and
Aharonov-Bohm tuning of their hybridised modes. Introducing
particle-non-conserving squeezing interactions, we discover a non-Hermitian
Aharonov-Bohm effect in ring-shaped networks in which mechanical quasiparticles
experience parametric gain. The resulting nontrivial complex mode spectra
indicate flux-tuning of squeezing, exceptional points, instabilities and
unidirectional phononic amplification. This rich new phenomenology points the
way to the exploration of new non-Hermitian topological bosonic phases and
applications in sensing and transport that exploit spatiotemporal symmetry
breaking.Comment: Included Main body and Methods (19 pages, 12 figures), in addition to
the Supplementary Information document (13 pages, 5 figures
Relaxed States in Relativistic Multi-Fluid Plasmas
The evolution equations for a plasma comprising multiple species of charged
fluids with relativistic bulk and thermal motion are derived. It is shown that
a minimal fluid coupling model allows a natural casting of the evolution
equations in terms of generalized vorticity which treats the fluid motion and
electromagnetic fields equally. Equilibria can be found using a variational
principle based on minimizing the total enstrophy subject to energy and
helicity constraints. A subset of these equilibria correspond to minimum
energy. The equations for these states are presented with example solutions
showing the structure of the relaxed states.Comment: 8 pages, 2 figure
Spectrum of Global Magnetorotational Instability in a Narrow Transition Layer
The Global Magnetorotational Instability (MRI) is investigated for a
configuration in which the rotation frequency changes only in a narrow
transition region. If the vertical wavelength of the unstable mode is of the
same order or smaller than the width of this region, the growth rates can
differ significantly from those given by a local analysis. In addition, the
non-axisymmetric spectrum admits overstable modes with a non-trivial dependence
on azimuthal wavelength, a feature missed by the local theory. In the limit of
vanishing transition region width, the Rayleigh-centrifugal instability is
recovered in the axisymmetric case, and the Kelvin-Helmholtz instability in the
non-axisymmetric case.Comment: 21 Pages, 5 figure
Quadrature nonreciprocity: unidirectional bosonic transmission without breaking time-reversal symmetry
Nonreciprocity means that the transmission of a signal depends on its
direction of propagation. Despite vastly different platforms and underlying
working principles, the realisations of nonreciprocal transport in linear,
time-independent systems rely on Aharonov-Bohm interference among several
pathways and require breaking time-reversal symmetry. Here we extend the notion
of nonreciprocity to unidirectional bosonic transport in systems with a
time-reversal symmetric Hamiltonian by exploiting interference between
beamsplitter (excitation preserving) and two-mode-squeezing (excitation
non-preserving) interactions. In contrast to standard nonreciprocity, this
unidirectional transport manifests when the mode quadratures are resolved with
respect to an external reference phase. Hence we dub this phenomenon quadrature
nonreciprocity. First, we experimentally demonstrate it in the minimal system
of two coupled nanomechanical modes orchestrated by optomechanical
interactions. Next, we develop a theoretical framework to characterise the
class of networks exhibiting quadrature nonreciprocity based on features of
their particle-hole graphs. In addition to unidirectionality, these networks
can exhibit an even-odd pairing between collective quadratures, which we
confirm experimentally in a four-mode system, and an exponential end-to-end
gain in the case of arrays of cavities. Our work opens up new avenues for
signal routing and quantum-limited amplification in bosonic systems.Comment: Includes: Main Text (7 pages, 4 figures), Methods & References (5
pages, 1 figure), Supplementary Information (14 pages, 2 figures
Changes in the stool and oropharyngeal microbiome in obsessive-compulsive disorder
Although the etiology of obsessive-compulsive disorder (OCD) is largely unknown, it is accepted that OCD is a complex disorder. There is a known bi-directional interaction between the gut microbiome and brain activity. Several authors have reported associations between changes in gut microbiota and neuropsychiatric disorders, including depression or autism. Furthermore, a pediatric-onset neuropsychiatric OCD-related syndrome occurs after streptococcal infection, which might indicate that exposure to certain microbes could be involved in OCD susceptibility. However, only one study has investigated the microbiome of OCD patients to date. We performed 16S ribosomal RNA gene-based metagenomic sequencing to analyze the stool and oropharyngeal microbiome composition of 32 OCD cases and 32 age and gender matched controls. We estimated different Ξ±- and Ξ²-diversity measures and performed LEfSe and Wilcoxon tests to assess differences in bacterial distribution. OCD stool samples showed a trend towards lower bacterial Ξ±-diversity, as well as an increase of the relative abundance of Rikenellaceae, particularly of the genus Alistipes, and lower relative abundance of Prevotellaceae, and two genera within the Lachnospiraceae: Agathobacer and Coprococcus. However, we did not observe a different Bacteroidetes to Firmicutes ratio between OCD cases and controls. Analysis of the oropharyngeal microbiome composition showed a lower Fusobacteria to Actinobacteria ratio in OCD cases. In conclusion, we observed an imbalance in the gut and oropharyngeal microbiomes of OCD cases, including, in stool, an increase of bacteria from the Rikenellaceae family, associated with gut inflammation, and a decrease of bacteria from the Coprococcus genus, associated with DOPAC synthesis
Inferring stabilizing mutations from protein phylogenies : application to influenza hemagglutinin
One selection pressure shaping sequence evolution is the requirement that a protein fold with sufficient stability to perform its biological functions. We present a conceptual framework that explains how this requirement causes the probability that a particular amino acid mutation is fixed during evolution to depend on its effect on protein stability. We mathematically formalize this framework to develop a Bayesian approach for inferring the stability effects of individual mutations from homologous protein sequences of known phylogeny. This approach is able to predict published experimentally measured mutational stability effects (ΞΞG values) with an accuracy that exceeds both a state-of-the-art physicochemical modeling program and the sequence-based consensus approach. As a further test, we use our phylogenetic inference approach to predict stabilizing mutations to influenza hemagglutinin. We introduce these mutations into a temperature-sensitive influenza virus with a defect in its hemagglutinin gene and experimentally demonstrate that some of the mutations allow the virus to grow at higher temperatures. Our work therefore describes a powerful new approach for predicting stabilizing mutations that can be successfully applied even to large, complex proteins such as hemagglutinin. This approach also makes a mathematical link between phylogenetics and experimentally measurable protein properties, potentially paving the way for more accurate analyses of molecular evolution
Ground-breaking Exoplanet Science with the ANDES spectrograph at the ELT
In the past decade the study of exoplanet atmospheres at high-spectral
resolution, via transmission/emission spectroscopy and cross-correlation
techniques for atomic/molecular mapping, has become a powerful and consolidated
methodology. The current limitation is the signal-to-noise ratio during a
planetary transit. This limitation will be overcome by ANDES, an optical and
near-infrared high-resolution spectrograph for the ELT. ANDES will be a
powerful transformational instrument for exoplanet science. It will enable the
study of giant planet atmospheres, allowing not only an exquisite determination
of atmospheric composition, but also the study of isotopic compositions,
dynamics and weather patterns, mapping the planetary atmospheres and probing
atmospheric formation and evolution models. The unprecedented angular
resolution of ANDES, will also allow us to explore the initial conditions in
which planets form in proto-planetary disks. The main science case of ANDES,
however, is the study of small, rocky exoplanet atmospheres, including the
potential for biomarker detections, and the ability to reach this science case
is driving its instrumental design. Here we discuss our simulations and the
observing strategies to achieve this specific science goal. Since ANDES will be
operational at the same time as NASA's JWST and ESA's ARIEL missions, it will
provide enormous synergies in the characterization of planetary atmospheres at
high and low spectral resolution. Moreover, ANDES will be able to probe for the
first time the atmospheres of several giant and small planets in reflected
light. In particular, we show how ANDES will be able to unlock the reflected
light atmospheric signal of a golden sample of nearby non-transiting habitable
zone earth-sized planets within a few tenths of nights, a scientific objective
that no other currently approved astronomical facility will be able to reach.Comment: 66 pages (103 with references) 20 figures. Submitted to Experimental
Astronom
The Science Performance of JWST as Characterized in Commissioning
This paper characterizes the actual science performance of the James Webb
Space Telescope (JWST), as determined from the six month commissioning period.
We summarize the performance of the spacecraft, telescope, science instruments,
and ground system, with an emphasis on differences from pre-launch
expectations. Commissioning has made clear that JWST is fully capable of
achieving the discoveries for which it was built. Moreover, almost across the
board, the science performance of JWST is better than expected; in most cases,
JWST will go deeper faster than expected. The telescope and instrument suite
have demonstrated the sensitivity, stability, image quality, and spectral range
that are necessary to transform our understanding of the cosmos through
observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures;
https://iopscience.iop.org/article/10.1088/1538-3873/acb29
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