18,362 research outputs found
Effect of disorder on a Pomeranchuk instability
We study the effect of disorder on the order parameter equation and
transition temperature of a Pomeranchuk-type Fermi-surface instability using
replica mean field theory. We consider the example of a phase transition to a
type Fermi surface distortion, and show that, in the regime
where such a transition is second order, the transition temperature is reduced
by disorder in essentially the same way as that for a d-wave superconductor. We
argue that observing this disorder dependence of metal-to-metal transition
might be a useful indicator of a finite angular momentum Fermi surface
distortion.Comment: 4.1 pages, 3 figs. Version as published in EPL. Added data of
Sr3Ru2O7 to theory curves of Fig.2, and some clarification of derivation of
result
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
Distinct functional domains of neurofibromatosis type 1 regulate immediate versus long-term memory formation
Neurofibromatosis type 1 (NF1) is a dominant genetic disorder that causes tumors of the peripheral nervous system. In addition, >40% of afflicted children have learning difficulties. The NF1 protein contains a highly conserved GTPase-activating protein domain that inhibits Ras activity, and the C-terminal region regulates cAMP levels via G-protein-dependent activation of adenylyl cyclase. Behavioral analysis indicates that learning is disrupted in both Drosophila and mouse NF1 models. Our previous work has shown that defective cAMP signaling leads to the learning phenotype in Drosophila Nf1 mutants. In the present report, our experiments showed that in addition to learning, long-term memory was also abolished in Nf1 mutants. However, altered NF1-regulated Ras activity is responsible for this defect rather than altered cAMP levels. Furthermore, by expressing clinically relevant human NF1 mutations and deletions in Drosophila Nf1-null mutants, we demonstrated that the GAP-related domain of NF1 was necessary and sufficient for long-term memory, whereas the C-terminal domain of NF1 was essential for immediate memory. Thus, we show that two separate functional domains of the same protein can participate independently in the formation of two distinct memory components
The role of atrial natriuretic peptide to attenuate inflammation in a mouse skin wound and individually perfused rat mesenteric microvessels.
We tested the hypothesis that the anti-inflammatory actions of atrial natriuretic peptide (ANP) result from the modulation of leukocyte adhesion to inflamed endothelium and not solely ANP ligation of endothelial receptors to stabilize endothelial barrier function. We measured vascular permeability to albumin and accumulation of fluorescent neutrophils in a full-thickness skin wound on the flank of LysM-EGFP mice 24Â h after formation. Vascular permeability in individually perfused rat mesenteric microvessels was also measured after leukocytes were washed out of the vessel lumen. Thrombin increased albumin permeability and increased the accumulation of neutrophils. The thrombin-induced inflammatory responses were attenuated by pretreating the wound with ANP (30Â min). During pretreatment ANP did not lower permeability, but transiently increased baseline albumin permeability concomitant with the reduction in neutrophil accumulation. ANP did not attenuate acute increases in permeability to histamine and bradykinin in individually perfused rat microvessels. The hypothesis that anti-inflammatory actions of ANP depend solely on endothelial responses that stabilize the endothelial barrier is not supported by our results in either individually perfused microvessels in the absence of circulating leukocytes or the more chronic skin wound model. Our results conform to the alternate hypothesis that ANP modulates the interaction of leukocytes with the inflamed microvascular wall of the 24Â h wound. Taken together with our previous observations that ANP reduces deformability of neutrophils and their strength of attachment, rolling, and transvascular migration, these observations provide the basis for additional investigations of ANP as an anti-inflammatory agent to modulate leukocyte-endothelial cell interactions
Ab-initio calculation of all-optical time-resolved calorimetry of nanosized systems: Evidence of nanosecond-decoupling of electron and phonon temperatures
The thermal dynamics induced by ultrashort laser pulses in nanoscale systems,
i.e. all-optical time-resolved nanocalorimetry is theoretically investigated
from 300 to 1.5 K. We report ab-initio calculations describing the temperature
dependence of the electron-phonon interactions for Cu nanodisks supported on
Si. The electrons and phonons temperatures are found to decouple on the ns time
scale at 10 K, which is two orders of magnitude in excess with respect to that
found for standard low-temperature transport experiments. By accounting for the
physics behind our results we suggest an alternative route for overhauling the
present knowledge of the electron-phonon decoupling mechanism in nanoscale
systems by replacing the mK temperature requirements of conventional
experiments with experiments in the time-domain.Comment: 5 pages, 3 figures. Accepted on Physical Review B
Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579
M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei
which have an estimated mass for the central black hole, detected hard X-ray
emission, and detected optical/UV emission. In contrast to the canonical ``big
blue bump,'' both have optical/UV spectra which decrease with increasing
frequency in a plot. Barring significant reddening by dust and/or
large errors in the black hole mass estimates, the optical/UV spectra of these
systems require that the inner edge of a geometrically thin, optically thick,
accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray
radiation can be explained by an optically thin, two temperature,
advection-dominated accretion flow at smaller radii.Comment: emulateapj.sty, to appear in ApJ Letter
The Effects of Diffuse Ionized Gas and Spatial Resolution on Metallicity Gradients: TYPHOON Two-Dimensional Spectrophotometry of M83
We present a systematic study of the diffuse ionized gas (DIG) in M83 and its
effects on the measurement of metallicity gradients at varying resolution
scales. Using spectrophotometric data cubes of M83 obtained at the 2.5m duPont
telescope at Las Campanas Observatory as part of the TYPHOON program, we
separate the HII regions from the DIG using the [SII]/H ratio, HIIphot
(HII finding algorithm) and the H surface brightness. We find that the
contribution to the overall H luminosity is approximately equal for the
HII and DIG regions. The data is then rebinned to simulate low-resolution
observations at varying resolution scales from 41 pc up to 1005 pc. Metallicity
gradients are measured using five different metallicity diagnostics at each
resolution. We find that all metallicity diagnostics used are affected by the
inclusion of DIG to varying degrees. We discuss the reasons of why the
metallicity gradients are significantly affected by DIG using the HII dominance
and emission line ratio radial profiles. We find that applying the
[SII]/H cut will provide a closer estimate of the true metallicity
gradient up to a resolution of 1005 pc for all metallicity diagnostics used in
this study.Comment: 26 pages, 12 figures + Appendix/Supplementary Material, accepted for
publication by MNRA
Starburst-AGN mixing: TYPHOON observations of NGC 1365, NGC 1068, and the effect of spatial resolution on the AGN fraction
We demonstrate a robust method of resolving the star-formation and AGN
contributions to emission lines using two very well known AGN systems: NGC
1365, and NGC 1068, using the high spatial resolution data from the
TYPHOON/PrISM survey. We expand the previous method of calculating the AGN
fraction by using theoretical-based model grids rather than empirical points.
The high spatial resolution of the TYPHOON/PrISM observations show evidence of
both star formation and AGN activity occurring in the nuclei of the two
galaxies. We rebin the data to the lower resolutions, typically found in other
integral field spectroscopy surveys such as SAMI, MaNGA, and CALIFA. The
results show that when rebinned from the native resolution of TYPHOON (< 200
pc/pixel) to 1 kpc/pixel, the effects include a roughly 3 kpc increase in the
radius of measured AGN activity, and a factor of 2 to 7 increase in the
detection of low surface brightness features such as shocks. All of this
information is critical, because information on certain physical processes may
be lost at varying resolutions. We make recommendations for analysing data at
current IFU survey resolutions.Comment: 30 pages, 28 figures, accepted for publication by MNRA
A RIAM/lamellipodin-talin-integrin complex forms the tip of sticky fingers that guide cell migration.
The leading edge of migrating cells contains rapidly translocating activated integrins associated with growing actin filaments that form 'sticky fingers' to sense extracellular matrix and guide cell migration. Here we utilized indirect bimolecular fluorescence complementation to visualize a molecular complex containing a Mig-10/RIAM/lamellipodin (MRL) protein (Rap1-GTP-interacting adaptor molecule (RIAM) or lamellipodin), talin and activated integrins in living cells. This complex localizes at the tips of growing actin filaments in lamellipodial and filopodial protrusions, thus corresponding to the tips of the 'sticky fingers.' Formation of the complex requires talin to form a bridge between the MRL protein and the integrins. Moreover, disruption of the MRL protein-integrin-talin (MIT) complex markedly impairs cell protrusion. These data reveal the molecular basis of the formation of 'sticky fingers' at the leading edge of migrating cells and show that an MIT complex drives these protrusions
- …