2,195 research outputs found
Electric Field Tunable Ultrafast Interlayer Charge Transfer in Graphene/WS<sub>2</sub> Heterostructure
Van der Waals heterostructures composed of two-dimensional materials offer an unprecedented control over their properties and have attracted tremendous research interest in various optoelectronic applications. Here, we study the photoinduced charge transfer in graphene/WS2 heterostructure by time-dependent density functional theory molecular dynamics. Our results show that holes transfer from graphene to WS2 two times faster than electrons, and the occurrence of interlayer charge transfer is found correlated with vibrational modes of graphene and WS2. It is further demonstrated that the carrier dynamics can be efficiently modulated by external electric fields. Detailed analysis confirms that the carrier transfer rate at heterointerface is governed by the coupling between donor and acceptor states, which is the result of the competition between interlayer and intralayer relaxation processes. Our study provides insights into the understanding of ultrafast interlayer charge transfer processes in heterostructures and broadens their future applications in photovoltaic devices
Mass enhancement in narrow band systems
A perturbative study of the Holstein Molecular Crystal Model which accounts
for lattice structure and dimensionality effects is presented. Antiadiabatic
conditions peculiar of narrow band materials and an intermediate to strong
electron-phonon coupling are assumed. The polaron effective mass depends
crucially in all dimensions on the intermolecular coupling strengths which also
affect the size of the lattice deformation associated with the small polaron
formation.Comment: Istituto Nazionale di Fisica della Materia - Dipartimento di
Matematica e Fisica, Istituto Nazionale di Fisica della Materia Universita'
di Camerino, 62032 Camerino, Ital
Elasticity-driven Nanoscale Texturing in Complex Electronic Materials
Finescale probes of many complex electronic materials have revealed a
non-uniform nanoworld of sign-varying textures in strain, charge and
magnetization, forming meandering ribbons, stripe segments or droplets. We
introduce and simulate a Ginzburg-Landau model for a structural transition,
with strains coupling to charge and magnetization. Charge doping acts as a
local stress that deforms surrounding unit cells without generating defects.
This seemingly innocuous constraint of elastic `compatibility', in fact induces
crucial anisotropic long-range forces of unit-cell discrete symmetry, that
interweave opposite-sign competing strains to produce polaronic elasto-magnetic
textures in the composite variables. Simulations with random local doping below
the solid-solid transformation temperature reveal rich multiscale texturing
from induced elastic fields: nanoscale phase separation, mesoscale intrinsic
inhomogeneities, textural cross-coupling to external stress and magnetic field,
and temperature-dependent percolation. We describe how this composite textured
polaron concept can be valuable for doped manganites, cuprates and other
complex electronic materials.Comment: Preprin
Hubble Space Telescope and Ground-Based Optical and Ultraviolet Observations of GRB010222
We report on Hubble Space Telescope WFPC2 optical and STIS near ultraviolet
MAMA observations, and ground-based optical observations of GRB010222, spanning
15 hrs to 71 days. The observations are well-described by a relativistic
blast-wave model with a hard electron-energy distribution, p = 1.57, and a jet
transition at t_j=0.93 days. These values are slightly larger than previously
found as a result of a correction for the contribution from the host galaxy to
the late-time ground-based observations and the larger temporal baseline
provided by the Hubble Space Telescope observations. The host galaxy is found
to contain a very compact core (size <0.25 arcsec) which coincides with the
position of the optical transient. The STIS near ultraviolet MAMA observations
allow for an investigation of the extinction properties along the line of sight
to GRB010222. We find that the far ultraviolet curvature component (c_4) is
rather large. In combination with the low optical extinction A_V =0.11 mag,
when compared to the Hydrogen column inferred from X-ray observations, we
suggest that this is evidence for dust destruction.Comment: ApJ, in pres
Supernova 2012ec: Identification of the progenitor and early monitoring with PESSTO
We present the identification of the progenitor of the Type IIP SN 2012ec in
archival pre-explosion HST WFPC2 and ACS/WFC F814W images. The properties of
the progenitor are further constrained by non-detections in pre-explosion WFPC2
F450W and F606W images. We report a series of early photometric and
spectroscopic observations of SN 2012ec. The r'-band light curve shows a
plateau with M(r')=-17.0. The early spectrum is similar to the Type IIP SN
1999em, with the expansion velocity measured at Halpha absorption minimum of
-11,700 km/s (at 1 day post-discovery). The photometric and spectroscopic
evolution of SN 2012ec shows it to be a Type IIP SN, discovered only a few days
post-explosion (<6d). We derive a luminosity for the progenitor, in comparison
with MARCS model SEDs, of log L/Lsun = 5.15+/-0.19, from which we infer an
initial mass range of 14-22Msun. This is the first SN with an identified
progenitor to be followed by the Public ESO Spectroscopic Survey of Transient
Objects (PESSTO).Comment: 6 pages, 3 figures, MNRAS accepte
On the Progenitors of Core-Collapse Supernovae
Theory holds that a star born with an initial mass between about 8 and 140
times the mass of the Sun will end its life through the catastrophic
gravitational collapse of its iron core to a neutron star or black hole. This
core collapse process is thought to usually be accompanied by the ejection of
the star's envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the
properties of their progenitor stars directly measured through the examination
of high-resolution images taken prior to the explosion. Here I review what has
been learned from these studies and briefly examine the potential impact on
stellar evolution theory, the existence of "failed supernovae", and our
understanding of the core-collapse explosion mechanism.Comment: 7 Pages, invited review accepted for publication by Astrophysics and
Space Science (special HEDLA 2010 issue
On the progenitor of SN 2005gl and the nature of Type IIn supernovae
We present a study of the type IIn supernova (SN) 2005gl, in the relatively
nearby (d~66 Mpc) galaxy NGC 266. Photometry and spectroscopy of the SN
indicate it is a typical member of its class. Pre-explosion Hubble Space
Telescope (HST) imaging of the location of the SN, along with a precise
localization of this event using the Laser-Guide-Star assisted Adaptive Optics
(LGS-AO) system at Keck Observatory, are combined to identify a luminous
(M_V=-10.3) point source as the possible progenitor of SN 2005gl. If the source
is indeed a single star, it was likely a member of the class of luminous blue
variable stars (LBVs). This finding leads us to consider the possible general
association of SNe IIn with LBV progenitors. We find this is indeed supported
by observations of other SNe, and the known properties of LBV stars. For
example, we argue that should the prototypical Galactic LBV eta Carina explode
in a phase similar to its current state, it will likely produce a type IIn SN.
We discuss our findings in the context of current ideas about the evolution of
massive stars, and review the census of SNe with identified progenitors. We
introduce the concept of the progenitor-SN map as a convenient means to discuss
the present status and future prospects of direct searches for SN progenitors.
We conclude that this field has matured considerably in recent years, and the
transition from anecdotal information about rare single events to robust
associations of progenitor classes with specific SN types has already begun.Comment: Submitted to ApJ. Comments welcom
The sub-energetic GRB 031203 as a cosmic analogue to GRB 980425
Over the six years since the discovery of the gamma-ray burst GRB 980425,
associated with the nearby (distance, ~40 Mpc) supernova 1998bw, astronomers
have fiercely debated the nature of this event. Relative to bursts located at
cosmological distances, (redshift, z~1), GRB 980425 was under-luminous in
gamma-rays by three orders of magnitude. Radio calorimetry showed the explosion
was sub-energetic by a factor of 10. Here, we report observations of the radio
and X-ray afterglow of the recent z=0.105 GRB 031203 and demonstrate that it
too is sub-energetic. Our result, when taken together with the low gamma-ray
luminosity, suggest that GRB 031203 is the first cosmic analogue to GRB 980425.
We find no evidence that this event was a highly collimated explosion viewed
off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically
sub-energetic gamma-ray burst. Such sub-energetic events have faint afterglows.
Intensive follow-up of faint bursts with smooth gamma-ray light curves (common
to both GRBs 031203 and 980425) may enable us to reveal their expected large
population.Comment: To Appear in Nature, August 5, 200
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