2,613 research outputs found
Electron self-energy in A3C60 (A=K, Rb): Effects of t1u plasmon in GW approximation
The electron self-energy of the t1u states in A3C60 (A=K, Rb) is calculated
using the so-called GW approximation. The calculation is performed within a
model which considers the t1u charge carrier plasmon at 0.5 eV and takes into
account scattering of the electrons within the t1u band. A moderate reduction
(35 %) of the t1u band width is obtained.Comment: 4 pages, revtex, 1 figure more information at
http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
TCP over High Speed Variable Capacity Links: A Simulation Study for Bandwidth Allocation
New optical network technologies provide opportunities for fast, controllable bandwidth management. These technologies can now explicitly provide resources to data paths, creating demand driven bandwidth reservation across networks where an applications bandwidth needs can be meet almost exactly. Dynamic synchronous Transfer Mode (DTM) is a gigabit network technology that provides channels with dynamically adjustable capacity. TCP is a reliable end-to-end transport protocol that adapts its rate to the available capacity. Both TCP and the DTM bandwidth can react to changes in the network load, creating a complex system with inter-dependent feedback mechanisms. The contribution of this work is an assessment of a bandwidth allocation scheme for TCP flows on variable capacity technologies. We have created a simulation environment using ns-2 and our results indicate that the allocation of bandwidth maximises TCP throughput for most flows, thus saving valuable capacity when compared to a scheme such as link over-provisioning. We highlight one situation where the allocation scheme might have some deficiencies against the static reservation of resources, and describe its causes. This type of situation warrants further investigation to understand how the algorithm can be modified to achieve performance similar to that of the fixed bandwidth case
Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields
Vertically coupled double quantum rings submitted to a perpendicular magnetic
field are addressed within the local spin-density functional theory. We
describe the structure of quantum ring molecules containing up to 40 electrons
considering different inter-ring distances and intensities of the applied
magnetic field. When the rings are quantum mechanically strongly coupled, only
bonding states are occupied and the addition spectrum of the artificial
molecules resembles that of a single quantum ring, with some small differences
appearing as an effect of the magnetic field. Despite the latter has the
tendency to flatten the spectra, in the strong coupling limit some clear peaks
are still found even when that can be interpretated from the
single-particle energy levels analogously as at zero applied field, namely in
terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring
distance, the occupation of the first antibonding orbitals washes out such
structures and the addition spectra become flatter and irregular. In the weak
coupling regime, numerous isospin oscillations are found as a function of .Comment: 27 pages, 11 figures. To be published in Phys. Rev.
Circumstellar H-alpha from SN 1994D and future Type Ia supernovae: an observational test of progenitor models
Searching for the presence of circumstellar material is currently the only
direct way to discriminate between the different types of possible progenitor
systems for Type Ia supernovae. We have therefore looked for narrow H-alpha in
a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 days
before maximum and only 6.5 days after explosion. We derive an upper limit of
2.0E-16 erg cm^{-2} s^{-1} for an unresolved emission line at the local H II
region velocity. To estimate the limit this puts on wind density, we have made
time-dependent photoionization calculations. Assuming spherical symmetry we
find an upper limit of the mass loss rate which is roughly 1.5E-5 solar masses
per year for a wind speed of 10 km s^{-1}. This limit can exclude only the
highest-mass-loss-rate symbiotic systems as progenitors. We discuss the effect
of asymmetry and assess the relative merits of early optical, radio and X-ray
limits in constraining mass loss from Type Ia progenitors. We find that X-ray
observations can probably provide the most useful limits on the progenitor mass
loss, while high-resolution optical spectroscopy offers our only chance of
actually identifying circumstellar hydrogen.Comment: 7 pages, LaTeX, 3 Postscript figures, mn.sty, psfig.tex; MNRAS, in
pres
Facial first impressions from another angle: How social judgements are influenced by changeable and invariant facial properties
Funding Information: Australian Research Council (ARC) Centre of Excellence in Cognition and its Disorders. Grant Number: CE110001021 ARC Discovery Outstanding Researcher Award. Grant Number: DP130102300Peer reviewedPostprin
Temperature dependent effective mass renormalization in a Coulomb Fermi liquid
We calculate numerically the quasiparticle effective mass (m*)
renormalization as a function of temperature and electron density in two- and
three-dimensional electron systems with long-range Coulomb interaction. In two
dimensions, the leading temperature correction is linear and positive with the
slope being a universal density independent number in the high density limit.
We predict an enhancement of the effective mass at low temperatures and a
non-monotonic temperature dependence at higher temperatures (T/T_F ~ 0.1) with
the peak shifting toward higher temperatures as density decreases. In three
dimensions, we find that the effective mass temperature dependence is nonlinear
and non-universal, and depends on the electron density in a complicated way. At
very high densities, the leading correction is positive, while at lower
densities it changes sign and the effective mass decreases monotonically from
its zero temperature value with increasing temperature
- …