18,643 research outputs found
Optical properties of graphene antidot lattices
Undoped graphene is semi-metallic and thus not suitable for many electronic
and optoelectronic applications requiring gapped semiconductor materials.
However, a periodic array of holes (antidot lattice) renders graphene
semiconducting with a controllable band gap. Using atomistic modelling, we
demonstrate that this artificial nanomaterial is a dipole-allowed direct gap
semiconductor with a very pronounced optical absorption edge. Hence, optical
infrared spectroscopy should be an ideal probe of the electronic structure. To
address realistic experimental situations, we include effects due to disorder
and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.
The Integral Burst Alert System (IBAS)
We describe the INTEGRAL Burst Alert System (IBAS): the automatic software
for the rapid distribution of the coordinates of the Gamma-Ray Bursts detected
by INTEGRAL. IBAS is implemented as a ground based system, working on the
near-real time telemetry stream. During the first six months of operations, six
GRB have been detected in the field of view of the INTEGRAL instruments and
localized by IBAS. Positions with an accuracy of a few arcminutes are currently
distributed by IBAS to the community for follow-up observations within a few
tens of seconds of the event.Comment: 7 pages, latex, 5 figures, Accepted for publication on A&A Special
Issue on First Science with INTEGRA
A mapping approach to synchronization in the "Zajfman trap": stability conditions and the synchronization mechanism
We present a two particle model to explain the mechanism that stabilizes a
bunch of positively charged ions in an "ion trap resonator" [Pedersen etal,
Phys. Rev. Lett. 87 (2001) 055001]. The model decomposes the motion of the two
ions into two mappings for the free motion in different parts of the trap and
one for a compressing momentum kick. The ions' interaction is modelled by a
time delay, which then changes the balance between adjacent momentum kicks.
Through these mappings we identify the microscopic process that is responsible
for synchronization and give the conditions for that regime.Comment: 12 pages, 9 figures; submitted to Phys Rev
Superselection in the presence of constraints
For systems which contain both superselection structure and constraints, we
study compatibility between constraining and superselection. Specifically, we
start with a generalisation of Doplicher-Roberts superselection theory to the
case of nontrivial centre, and a set of Dirac quantum constraints and find
conditions under which the superselection structures will survive constraining
in some form. This involves an analysis of the restriction and factorisation of
superselection structures. We develop an example for this theory, modelled on
interacting QED.Comment: Latex, 38 page
Entanglement entropy of fermions in any dimension and the Widom conjecture
We show that entanglement entropy of free fermions scales faster then area
law, as opposed to the scaling for the harmonic lattice, for example.
We also suggest and provide evidence in support of an explicit formula for the
entanglement entropy of free fermions in any dimension , as the size of a subsystem
, where is the Fermi surface and
is the boundary of the region in real space. The expression for the constant
is based on a conjecture due to H. Widom. We
prove that a similar expression holds for the particle number fluctuations and
use it to prove a two sided estimates on the entropy .Comment: Final versio
Trees with Given Stability Number and Minimum Number of Stable Sets
We study the structure of trees minimizing their number of stable sets for
given order and stability number . Our main result is that the
edges of a non-trivial extremal tree can be partitioned into stars,
each of size or , so that every vertex is included in at most two
distinct stars, and the centers of these stars form a stable set of the tree.Comment: v2: Referees' comments incorporate
Tur\'an Graphs, Stability Number, and Fibonacci Index
The Fibonacci index of a graph is the number of its stable sets. This
parameter is widely studied and has applications in chemical graph theory. In
this paper, we establish tight upper bounds for the Fibonacci index in terms of
the stability number and the order of general graphs and connected graphs.
Tur\'an graphs frequently appear in extremal graph theory. We show that Tur\'an
graphs and a connected variant of them are also extremal for these particular
problems.Comment: 11 pages, 3 figure
Volume-energy correlations in the slow degrees of freedom of computer-simulated phospholipid membranes
Constant-pressure molecular-dynamics simulations of phospholipid membranes in
the fluid phase reveal strong correlations between equilibrium fluctuations of
volume and energy on the nanosecond time-scale. The existence of strong
volume-energy correlations was previously deduced indirectly by Heimburg from
experiments focusing on the phase transition between the fluid and the ordered
gel phases. The correlations, which are reported here for three different
membranes (DMPC, DMPS-Na, and DMPSH), have volume-energy correlation
coefficients ranging from 0.81 to 0.89. The DMPC membrane was studied at two
temperatures showing that the correlation coefficient increases as the phase
transition is approached
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