1,336 research outputs found
Bound states in the continuum driven by AC fields
We report the formation of bound states in the continuum driven by AC fields.
This system consists of a quantum ring connected to two leads. An AC side-gate
voltage controls the interference pattern of the electrons passing through the
system. We model the system by two sites in parallel connected to two
semi-infinite lattices. The energy of these sites change harmonically with
time. We obtain the transmission probability and the local density of states at
the ring sites as a function of the parameters that define the system. The
transmission probability displays a Fano profile when the energy of the
incoming electron matches the driving frequency. Correspondingly, the local
density of states presents a narrow peak that approaches a Dirac delta function
in the weak coupling limit. We attribute these features to the presence of
bound states in the continuum.Comment: 5 pages, 3 figure
Localisation and finite-size effects in graphene flakes
We show that electron states in disordered graphene, with an onsite potential that induces inter-valley scattering, are localised for all energies at disorder as small as of the band width of clean graphene. We clarify that, in order for this Anderson-type localisation to be manifested, graphene flakes of size or larger are needed. For smaller samples, due to the surprisingly large extent of the electronic wave functions, a regime of apparently extended (or even critical) states is identified. Our results complement earlier studies of macroscopically large samples and can explain the divergence of results for finite-size graphene flakes
Hydrogen atom as an eigenvalue problem in 3D spaces of constant curvature and minimal length
An old result of A.F. Stevenson [Phys. Rev.} 59, 842 (1941)] concerning the
Kepler-Coulomb quantum problem on the three-dimensional (3D) hypersphere is
considered from the perspective of the radial Schr\"odinger equations on 3D
spaces of any (either positive, zero or negative) constant curvature. Further
to Stevenson, we show in detail how to get the hypergeometric wavefunction for
the hydrogen atom case. Finally, we make a comparison between the ``space
curvature" effects and minimal length effects for the hydrogen spectrumComment: 6 pages, v
Momentum-resolved evolution of the Kondo lattice into 'hidden-order' in URu2Si2
We study, using high-resolution angle-resolved photoemission spectroscopy,
the evolution of the electronic structure in URu2Si2 at the Gamma, Z and X
high-symmetry points from the high-temperature Kondo-screened regime to the
low-temperature `hidden-order' (HO) state. At all temperatures and symmetry
points, we find structures resulting from the interaction between heavy and
light bands, related to the Kondo lattice formation. At the X point, we
directly measure a hybridization gap of 11 meV already open at temperatures
above the ordered phase. Strikingly, we find that while the HO induces
pronounced changes at Gamma and Z, the hybridization gap at X does not change,
indicating that the hidden-order parameter is anisotropic. Furthermore, at the
Gamma and Z points, we observe the opening of a gap in momentum in the HO
state, and show that the associated electronic structure results from the
hybridization of a light electron band with the Kondo-lattice bands
characterizing the paramagnetic state.Comment: Updated published version. Mansucript + Supplemental Material (8
pages, 9 figures). Submitted 16 September 201
Further ALMA observations and detailed modeling of the Red Rectangle
We present new high-quality ALMA observations of the Red Rectangle (a well
known post-AGB object) in C17O J=6-5 and H13CN J=4-3 line emission and results
from a new reduction of already published 13CO J=3-2 data. A detailed model
fitting of all the molecular line data, including previous maps and single-dish
spectra, was performed using a sophisticated code. These observations and the
corresponding modeling allowed us to deepen the analysis of the nebular
properties. We also stress the uncertainties in the model fitting.
We confirm the presence of a rotating equatorial disk and an outflow, which
is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 Mo,
and that of the CO-rich outflow is ~ 10 times smaller. High temperatures of ~
100 K are derived for most components. From comparison of the mass values, we
roughly estimate the lifetime of the rotating disk, which is found to be of
about 10000 yr. Taking data of a few other post-AGB composite nebulae into
account, we find that the lifetimes of disks around post-AGB stars typically
range between 5000 and more than 20000 yr. The angular momentum of the disk is
found to be high, ~ 9 Mo AU km/s, which is comparable to that of the stellar
system at present. Our observations of H13CN show a particularly wide velocity
dispersion and indicate that this molecule is only abundant in the inner
Keplerian disk, at ~ 60 AU from the stellar system. We suggest that HCN is
formed in a dense photodissociation region (PDR) due to the UV excess known to
be produced by the stellar system, following chemical mechanisms that are well
established for interstellar medium PDRs and disks orbiting young stars. We
further suggest that this UV excess could lead to the efficient formation and
excitation of PAHs and other C-bearing macromolecules, whose emission is very
intense in the optical counterpart.Comment: Astronomy & Astrohysics, in press; 17 pages, 18 figures, 1 tabl
ALMA observations of the Red Rectangle, a preliminary analysis
We aim to study equatorial disks in rotation and axial outflows in post-AGB
objects, as to disclose the formation and shaping mechanisms in planetary
nebulae. So far, both disks and outflows had not been observed simultaneously.
We have obtained high-quality ALMA observations of 12CO and 13CO J=3-2 and
12CO J=6-5 line emission in the Red Rectangle, the only post-AGB/protoplanetary
object in which a disk in rotation has been mapped up to date.
These observations provide an unprecedented description of the complex
structure of this source. Together with an equatorial disk in rotation, we find
a low-velocity outflow that occupies more or less the region placed between the
disk and the optical X-shaped nebula. From our observations and preliminary
modeling of the data, we confirm the previously known properties of the disk
and obtain a first description of the structure, dynamics, and physical
conditions of the outflow.Comment: 5 pages, 5 figure
Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH231.8+4.2
We present continuum and molecular line emission ALMA observations of OH
231.8+4.2, a well studied bipolar nebula around an asymptotic giant branch
(AGB) star. The high angular resolution (~0.2-0.3 arcsec) and sensitivity of
our ALMA maps provide the most detailed and accurate description of the overall
nebular structure and kinematics of this object to date. We have identified a
number of outflow components previously unknown. Species studied in this work
include 12CO, 13CO, CS, SO, SO2, OCS, SiO, SiS, H3O+, Na37Cl, and CH3OH. The
molecules Na37Cl and CH3OH are first detections in OH 231.8+4.2, with CH3OH
being also a first detection in an AGB star. Our ALMA maps bring to light the
totally unexpected position of the mass-losing AGB star (QX Pup) relative to
the large-scale outflow. QX Pup is enshrouded within a compact (<60 AU) parcel
of dust and gas (clump S) in expansion (V~5-7 km/s) that is displaced by
0.6arcsec to the south of the dense equatorial region (or waist) where the
bipolar lobes join. Our SiO maps disclose a compact bipolar outflow that
emerges from QX Pup's vicinity. This outflow is oriented similarly to the
large-scale nebula but the expansion velocities are about ten times lower (~35
km/s). We deduce short kinematical ages for the SiO outflow, ranging from
~50-80 yr, in regions within ~150 AU, to ~400-500 yr at the lobe tips (~3500
AU). Adjacent to the SiO outflow, we identify a small-scale hourglass-shaped
structure (mini-hourglass) that is probably made of compressed ambient material
formed as the SiO outflow penetrates the dense, central regions of the nebula.
The lobes and the equatorial waist of the mini-hourglass are both radially
expanding with a constant velocity gradient. The mini-waist is characterized by
extremely low velocities, down to ~1 km/s at ~150 AU, which tentatively suggest
the presence of a stable structure. (abridged
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