17,573 research outputs found
Electron states of mono- and bilayer graphene on SiC probed by STM
We present a scanning tunneling microscopy (STM) study of a
gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis
of atomic scale images, we identify two different kinds of terraces, which we
unambiguously attribute to mono- and bilayer graphene capping a C-rich
interface. At low temperature, both terraces show
quantum interferences generated by static impurities. Such interferences are a
fingerprint of -like states close to the Fermi level. We conclude that the
metallic states of the first graphene layer are almost unperturbed by the
underlying interface, in agreement with recent photoemission experiments (A.
Bostwick et al., Nature Physics 3, 36 (2007))Comment: 4 pages, 3 figures submitte
A New Constraint on the Escape Fraction in Distant Galaxies Using Gamma-ray Burst Afterglow Spectroscopy
We describe a new method to measure the escape fraction fesc of ionizing
radiation from distant star-forming galaxies using the afterglow spectra of
long-duration gamma-ray bursts (GRBs). Optical spectra of GRB afterglows allow
us to evaluate the optical depth of the host ISM, according to the neutral
hydrogen column density N(HI) observed along the sightlines toward the
star-forming regions where the GRBs are found. Different from previous effort
in searching for faint, transmitted Lyman continuum photons, our method is not
subject to background subtraction uncertainties and does not require prior
knowledge of either the spectral shape of the host galaxy population or the IGM
Lya forest absorption along these GRB sightlines. Because most GRBs occur in
sub-L_* galaxies, our study also offers the first constraint on fesc for
distant low-mass galaxies that dominate the cosmic luminosity density. We have
compiled a sample of 27 GRBs at redshift z>2 for which the underlying N(HI) in
the host ISM are known. These GRBs together offer a statistical sampling of the
integrated optical depth to ionizing photons along random sightlines from
star-forming regions in the host galaxies, and allow us to estimate the mean
escape fraction averaged over different viewing angles. We find
=0.02\pm 0.02 and place a 95% c.l. upper limit <= 0.075 for these
hosts. We discuss possible biases of our approach and implications of the
result. Finally, we propose to extend this technique for measuring at
z~0.2 using spectra of core-collapse supernovae.Comment: Five journal pages, including one figure; ApJL in pres
Field dependence of magnetization reversal by spin transfer
We analyse the effect of the applied field (Happl) on the current-driven
magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe
two different types of transition between the parallel (P) and antiparallel
(AP) magnetic configurations of the Co layers. If Happl is weaker than a rather
small threshold value, the transitions between P and AP are irreversible and
relatively sharp. For Happl exceding the threshold value, the same transitions
are progressive and reversible. We show that the criteria for the stability of
the P and AP states and the experimentally observed behavior can be precisely
accounted for by introducing the current-induced torque of the spin transfer
models in a Landau-Lifschitz-Gilbert equation. This approach also provides a
good description for the field dependence of the critical currents
Magnetoplasmons in quasi-neutral epitaxial graphene nanoribbons
We present infrared transmission spectroscopy study of the inter-Landau-level
excitations in quasi-neutral epitaxial graphene nanoribbon arrays. We observed
a substantial deviation in energy of the transition
from the characteristic square root magnetic-field dependence of
two-dimensional graphene. This deviation arises from the formation of
upper-hybrid mode between the Landau level transition and the plasmon
resonance. In the quantum regime the hybrid mode exhibits a distinct dispersion
relation, markedly different from that expected for conventional
two-dimensional systems and highly doped graphene
GRBs as Cosmological Probes - Cosmic Chemical Evolution
Long-duration gamma-ray bursts (GRBs) are associated with the death of
metal-poor massive stars. Even though they are highly transient events very
hard to localize, they are so bright that they can be detected in the most
difficult environments. GRB observations are unveiling a surprising view of the
chemical state of the distant universe (redshifts z > 2). Contrary to what is
expected for a high-z metal-poor star, the neutral interstellar medium (ISM)
around GRBs is not metal poor (metallicities vary from ~1/10 solar at z = 6.3
to about solar at z = 2) and is enriched with dust (90-99% of iron is in solid
form). If these metallicities are combined with those measured in the warm ISM
of GRB host galaxies at z < 1, a redshift evolution is observed. Such an
evolution predicts that the stellar masses of the hosts are in the range M* =
10^(8.6-9.8) Msun. This prediction makes use of the mass-metallicity relation
(and its redshift evolution) observed in normal star-forming galaxies.
Independent measurements coming from the optical-NIR photometry of GRB hosts
indicate the same range of stellar masses, with a typical value similar to that
of the Large Magellanic Cloud. This newly detected population of
intermediate-mass galaxies is very hard to find at high redshift using
conventional astronomy. However, it offers a compelling and relatively
inexpensive opportunity to explore galaxy formation and cosmic chemical
evolution beyond known borders, from the primordial universe to the present.Comment: Review article to be published in New Journal of Physics
(http://www.njp.org), Focus Issue on Gamma Ray Burst
The Role of Final State Interactions in Quasielastic Fe Reactions at large
A relativistic finite nucleus calculation using a Dirac optical potential is
used to investigate the importance of final state interactions [FSI] at large
momentum transfers in inclusive quasielastic electronuclear reactions. The
optical potential is derived from first-order multiple scattering theory and
then is used to calculate the FSI in a nonspectral Green's function doorway
approach. At intermediate momentum transfers excellent predictions of the
quasielastic Fe experimental data for the longitudinal response
function are obtained. In comparisons with recent measurements at ~GeV/c the theoretical calculations of give good agreement for
the quasielastic peak shape and amplitude, but place the position of the peak
at an energy transfer of about ~MeV higher than the data.Comment: 13 pages typeset using revtex 3.0 with 6 postscript figures in
accompanying uuencoded file; submitted to Phys. Rev.
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