13,319 research outputs found
Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer
We have studied temperature dependences of electron transport in graphene and
its bilayer and found extremely low electron-phonon scattering rates that set
the fundamental limit on possible charge carrier mobilities at room
temperature. Our measurements have shown that mobilities significantly higher
than 200,000 cm2/Vs are achievable, if extrinsic disorder is eliminated. A
sharp (threshold-like) increase in resistivity observed above approximately
200K is unexpected but can qualitatively be understood within a model of a
rippled graphene sheet in which scattering occurs on intra-ripple flexural
phonons
Moderately Luminous type II Supernovae
Core-collapse Supernovae (CC-SNe) descend from progenitors more massive than
about 8 Msun. Because of the young age of the progenitors, the ejecta may
eventually interact with the circumstellar medium (CSM) via highly energetic
processes detectable in the radio, X-ray, ultraviolet (UV) and, sometimes, in
the optical domains. In this paper we present ultraviolet, optical and near
infrared observations of five type II SNe, namely SNe 2009dd, 2007pk, 2010aj,
1995ad, and 1996W. Together with few other SNe they form a group of moderately
luminous type II events. We collected photometry and spectroscopy with several
telescopes in order to construct well-sampled light curves and spectral
evolutions from the photospheric to the nebular phases. Both photometry and
spectroscopy indicate a degree of heterogeneity in this sample. The light
curves have luminous peak magnitudes (). The ejected
masses of ^56\ni for three SNe span a wide range of values
(MsunM(\ni)Msun), while for a fourth
(SN2010aj) we could determine a stringent upper limit (Msun).
Clues of interaction, such as the presence of high velocity (HV) features of
the Balmer lines, are visible in the photospheric spectra of SNe 2009dd and
1996W. For SN2007pk we observe a spectral transition from a type IIn to a
standard type II SN. Modelling the observations of SNe 2009dd, 2010aj and
1995ad with radiation hydrodynamics codes, we infer kinetic plus thermal
energies of about 0.2-0.5 foe, initial radii of 2-5 cm and
ejected masses of 5.0-9.5 Msun. These values suggest moderate-mass,
super-asymptotic giant branch (SAGB) or red super-giants (RSG) stars as SN
precursors, in analogy with other luminous type IIP SNe 2007od and 2009bw.Comment: 28 pages, 27 fig, accepted by A&A, 3 pages of online material,
abstract abridged. revised significantly with respect to the previous versio
Explosion of a massive, He-rich star at z=0.16
We present spectroscopic and photometric data of the peculiar SN 2001gh,
discovered by the 'Southern inTermediate Redshift ESO Supernova Search'
(STRESS) at a redshift z=0.16. SN 2001gh has relatively high luminosity at
maximum (M_B = -18.55 mag), while the light curve shows a broad peak. An
early-time spectrum shows an almost featureless, blue continuum with a few weak
and shallow P-Cygni lines that we attribute to HeI. HeI lines remain the only
spectral features visible in a subsequent spectrum, obtained one month later. A
remarkable property of SN 2001gh is the lack of significant spectral evolution
over the temporal window of nearly one month separating the two spectra. In
order to explain the properties of SN 2001gh, three powering mechanism are
explored, including radioactive decays of a moderately large amount of 56Ni,
magnetar spin-down, and interaction of SN ejecta with circumstellar medium. We
favour the latter scenario, with a SN Ib wrapped in a dense, circumstellar
shell. The fact that no models provide an excellent fit with observations,
confirms the troublesome interpretation of the nature of SN 2001gh. A rate
estimate for SN 2001gh-like event is also provided, confirming the intrinsic
rarity of these objects.Comment: 11 pages, 8 figures, 3 tables. Accepted by MNRA
Energy distribution in a BTZ black hole spacetime
We evaluate the energy distribution associated with the (2+1)-dimensional
rotating BTZ black hole. The energy-momentum complexes of Landau-Lifshitz and
Weinberg are employed for this computation. Both prescriptions give exactly the
same form of energy distribution. Therefore, these results provide evidence in
support of the claim that, for a given gravitational background, different
energy-momentum complexes can give identical results in three dimensions, as it
is the case in four dimensions.Comment: 16 pages, LaTeX; v2: comments, clarifications and references added,
version to appear in Int.J.Mod.Phys.
SN 2013df, a double-peaked IIb supernova from a compact progenitor and an extended H envelope
Optical observations of the type IIb SN 2013df from a few days to about 250
days after explosion are presented. These observations are complemented with UV
photometry taken by \textit{SWIFT} up to 60 days post-explosion. The
double-peak optical light curve is similar to those of SNe 1993J and 2011fu
although with different decline and rise rates. From the modelling of the
bolometric light curve, we have estimated that the total mass of synthesised
Ni in the explosion is M, while the ejecta mass is
M and the explosion energy erg. In
addition, we have estimated a lower limit to the progenitor radius ranging from
. The spectral evolution indicates that SN 2013df had a
hydrogen envelope similar to SN 1993J ( M). The line
profiles in nebular spectra suggest that the explosion was asymmetric with the
presence of clumps in the ejecta, while the [O\,{\sc i}]
, luminosities, may indicate that the progenitor
of SN 2013df was a relatively low mass star ( M).Comment: 18 pages, 11 figures, 9 tables, accepted for publication in MNRA
Probing the Electronic Structure of Bilayer Graphene by Raman Scattering
The electronic structure of bilayer graphene is investigated from a resonant
Raman study using different laser excitation energies. The values of the
parameters of the Slonczewski-Weiss-McClure model for graphite are measured
experimentally and some of them differ significantly from those reported
previously for graphite, specially that associated with the difference of the
effective mass of electrons and holes. The splitting of the two TO phonon
branches in bilayer graphene is also obtained from the experimental data. Our
results have implications for bilayer graphene electronic devices.Comment: 4 pages, 4 figure
Energy and Momentum Distributions of a (2+1)-dimensional black hole background
Using Einstein, Landau-Lifshitz, Papapetrou and Weinberg energy-momentum
complexes we explicitly evaluate the energy and momentum distributions
associated with a non-static and circularly symmetric three-dimensional
spacetime. The gravitational background under study is an exact solution of the
Einstein's equations in the presence of a cosmological constant and a null
fluid. It can be regarded as the three-dimensional analogue of the Vaidya
metric and represents a non-static spinless (2+1)-dimensional black hole with
an outflux of null radiation. All four above-mentioned prescriptions give
exactly the same energy and momentum distributions for the specific black hole
background. Therefore, the results obtained here provide evidence in support of
the claim that for a given gravitational background, different energy-momentum
complexes can give identical results in three dimensions. Furthermore, in the
limit of zero cosmological constant the results presented here reproduce the
results obtained by Virbhadra who utilized the Landau-Lifshitz energy-momentum
complex for the same (2+1)-dimensional black hole background in the absence of
a cosmological constant.Comment: 19 pages, LaTeX, v3: references added, to appear in Int.J.Mod.Phys.
The Type IIP SN 2007od in UGC 12846: from a bright maximum to dust formation in the nebular phase
Ultraviolet (UV), optical and near infrared (NIR) observations of the type
IIP supernova (SN) 2007od are presented, covering from the maximum light to the
late phase, allowing to investigate in detail different physical phenomena in
the expanding ejecta. These data turn this object into one of the most peculiar
IIP ever studied. The early light curve of SN 2007od is similar to that of a
bright IIPs with a short plateau, a bright peak (MV = -18 mag), but a very
faint optical light curve at late time. However, with the inclusion of mid
infrared (MIR) observations during the radioactive decay we have estimate a
M(56Ni) ~ 2\times10^-2 M\odot. Modeling the bolometric light curve, ejecta
expansion velocities and black-body temperature, we estimate a total ejected
mass was 5 - 7.5 M\odot with a kinetic energy of at least 0.5 \times 10^51 erg.
The early spectra reveal a boxy H{\alpha} profile and high velocities features
of the Balmer series that suggest interaction between the ejecta and a close
circum-stellar matter (CSM). SN 2007od may be, therefore, an intermediate case
between a Type IIn SN and a typical Type IIP SN. Also late spectra show a clear
evidence of CSM and the presence of dust formed inside the ejecta. The episodes
of mass loss short before explosion, the bright plateau, along with the
relatively small amount of 56Ni and the faint [O I] observed in the nebular
spectra are consistent with a super-asympthotic giant branch (super-AGB)
progenitor (M~9.7 - 11 M\odot).Comment: V2, some test added and three figures changed from the first version.
21 pages, 18 figures, accepted for publication in MNRAS on May 24, 201
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