188 research outputs found
The quasi-free-standing nature of graphene on H-saturated SiC(0001)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
Mid-infrared spectroscopy of carbon stars in the Small Magellanic Cloud
We have observed a sample of 36 objects in the Small Magellanic Cloud (SMC)
with the Infrared Spectrometer on the Spitzer Space Telescope. Nineteen of
these sources are carbon stars. An examination of the near- and mid-infrared
photometry shows that the carbon-rich and oxygen-rich dust sources follow two
easily separated sequences. A comparison of the spectra of the 19 carbon stars
in the SMC to spectra from the Infrared Space Observatory (ISO) of carbon stars
in the Galaxy reveals significant differences. The absorption bands at 7.5 um
and 13.7 um due to C2H2 are stronger in the SMC sample, and the SiC dust
emission feature at 11.3 um is weaker. Our measurements of the MgS dust
emission feature at 26-30 um are less conclusive, but this feature appears to
be weaker in the SMC sample as well. All of these results are consistent with
the lower metallicity in the SMC. The lower abundance of SiC grains in the SMC
may result in less efficient carbon-rich dust production, which could explain
the excess C2H2 gas seen in the spectra. The sources in the SMC with the
strongest SiC dust emission tend to have redder infrared colors than the other
sources in the sample, which implies more amorphous carbon, and they also tend
to show stronger MgS dust emission. The weakest SiC emission features tend to
be shifted to the blue; these spectra may arise from low-density shells with
large SiC grains.Comment: Accepted by ApJ 21 March, 2006, 13 pages (emulateapJ), 12 figure
Cold Dust in Three Massive Evolved Stars in the LMC
Massive evolved stars can produce large amounts of dust, and far-infrared
(IR) data are essential for determining the contribution of cold dust to the
total dust mass. Using Herschel, we search for cold dust in three very dusty
massive evolved stars in the Large Magellanic Cloud: R71 is a Luminous Blue
Variable, HD36402 is a Wolf-Rayet triple system, and IRAS05280-6910 is a red
supergiant. We model the spectral energy distributions using radiative transfer
codes and find that these three stars have mass-loss rates up to 10^-3 solar
masses/year, suggesting that high-mass stars are important contributors to the
life-cycle of dust. We found far-IR excesses in two objects, but these excesses
appear to be associated with ISM and star-forming regions. Cold dust (T < 100
K) may thus not be an important contributor to the dust masses of evolved
stars.Comment: accepted to A&A as part of the Herschel first results special issu
Ice chemistry in embedded young stellar objects in the Large Magellanic Cloud
We present spectroscopic observations of a sample of 15 embedded young
stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations
were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the
SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS
spectral range: the bending mode of CO_2 ice at 15.2 micron and the ice band
between 5 and 7 micron that includes contributions from the bending mode of
water ice at 6 micron amongst other ice species. The 5-7 micron band is
difficult to identify in our LMC sample due to the conspicuous presence of PAH
emission superimposed onto the ice spectra. We identify water ice in the
spectra of two sources; the spectrum of one of those sources also exhibits the
6.8 micron ice feature attributed to ammonium and methanol. We model the CO_2
band in detail, using the combination of laboratory ice profiles available in
the literature. We find that a significant fraction (> 50%) of CO_2 ice is
locked in a water-rich component, consistent with what is observed for Galactic
sources. The majority of the sources in the LMC also require a pure-CO_2
contribution to the ice profile, evidence of thermal processing. There is a
suggestion that CO_2 production might be enhanced in the LMC, but the size of
the available sample precludes firmer conclusions. We place our results in the
context of the star formation environment in the LMC.Comment: Minor corrections to Table 2. Accepted for publication in ApJ, 66
pages, 9 figures (some in color), 4 table
Intrinsic Terahertz Plasmons and Magnetoplasmons in Large Scale Monolayer Graphene
We show that in graphene epitaxially grown on SiC the Drude absorption is
transformed into a strong terahertz plasmonic peak due to natural nanoscale
inhomogeneities, such as substrate terraces and wrinkles. The excitation of the
plasmon modifies dramatically the magneto-optical response and in particular
the Faraday rotation. This makes graphene a unique playground for
plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2
orders of magnitude smaller than in conventional plasmonic materials such as
noble metals.Comment: to appear in Nano Letter
The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud
Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium.
Aims: This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).
Methods: The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2-1 line.
Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC.
Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models
Symmetry Breaking in Few Layer Graphene Films
Recently, it was demonstrated that the quasiparticle dynamics, the
layer-dependent charge and potential, and the c-axis screening coefficient
could be extracted from measurements of the spectral function of few layer
graphene films grown epitaxially on SiC using angle-resolved photoemission
spectroscopy (ARPES). In this article we review these findings, and present
detailed methodology for extracting such parameters from ARPES. We also present
detailed arguments against the possibility of an energy gap at the Dirac
crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007
Regensburg Submitted to New Journal of Physic
Murine Gamma Herpesvirus 68 Hijacks MAVS and IKKβ to Abrogate NFκB Activation and Antiviral Cytokine Production
Upon viral infection, mitochondrial antiviral signaling (MAVS) protein serves as a key adaptor to promote cytokine production. We report here that murine gamma herpesvirus 68 (γHV68), a model virus for oncogenic human gamma herpesviruses, subverts cytokine production via the MAVS adaptor. During early infection, γHV68 hijacks MAVS and IKKβ to induce the site-specific phosphorylation of RelA, a crucial subunit of the transcriptionally active NFκB dimer, which primes RelA for the proteasome-mediated degradation. As such, γHV68 efficiently abrogated NFκB activation and cytokine gene expression. Conversely, uncoupling RelA degradation from γHV68 infection promoted NFκB activation and elevated cytokine production. Loss of MAVS increased cytokine production and immune cell infiltration in the lungs of γHV68-infected mice. Moreover, exogenous expression of the phosphorylation- and degradation-resistant RelA variant restored γHV68-induced cytokine production. Our findings uncover an intricate strategy whereby signaling via the upstream MAVS adaptor is intercepted by a pathogen to nullify the immediate downstream effector, RelA, of the innate immune pathway
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