46,827 research outputs found
Anomalous bulk behaviour in the free parafermion spin chain
We demonstrate using direct numerical diagonalization and extrapolation
methods that boundary conditions have a profound effect on the bulk properties
of a simple model for for which the model hamiltonian is
non-hermitian. For the model reduces to the well known quantum Ising
model in a transverse field. For open boundary conditions the model is
known to be solved exactly in terms of free parafermions. Once the ends of the
open chain are connected by considering the model on a ring, the bulk
properties, including the ground-state energy per site, are seen to differ
dramatically with increasing . Other properties, such as the leading
finite-size corrections to the ground-state energy, the mass gap exponent and
the specific heat exponent, are also seen to be dependent on the boundary
conditions. We speculate that this anomalous bulk behaviour is a topological
effect.Comment: 8 pages, 8 figures, minor change
The effect of small elongations on the electronic and optical signatures in InAs nanocrystal quantum dots
We present a detailed theoretical investigation of the electronic structure and
optical properties of InAs nanocrystals at the transition from spheres to rods. Using
a semiempirical pseudopotential approach, we predict that, despite the qualitative
similarity of both intra- and inter-band optical spectra, for NCs with R > 15 °A even
slight elongations should result in shifts of the order of hundreds of meV in the spacings
between STM peaks measured in the positive bias regime, in the position of the intra-
band absorption peaks associated with transitions in the conduction band and in the
separation between the first and the fifth peak in PLE experiments. Our results show
that, based on the spectroscopic data, it should be possible to discriminate between
spherical and elongated NCs with aspect ratios of length over diameter as small as
1.2. Indeed our results suggest that many nominally spherical experimental samples
contained a large fraction of slightly elongated structures
Direction Detector on an Excitable Field: Field Computation with Coincidence Detection
Living organisms process information without any central control unit and
without any ruling clock. We have been studying a novel computational strategy
that uses a geometrically arranged excitable field, i.e., "field computation."
As an extension of this research, in the present article we report the
construction of a "direction detector" on an excitable field. Using a numerical
simulation, we show that the direction of a input source signal can be detected
by applying the characteristic as a "coincidence detector" embedded on an
excitable field. In addition, we show that this direction detection actually
works in an experiment using an excitable chemical system. These results are
discussed in relation to the future development of "field computation."Comment: 6 pages, 3 figure
Machine Assisted Proof of ARMv7 Instruction Level Isolation Properties
In this paper, we formally verify security properties of the ARMv7 Instruction Set Architecture (ISA) for user mode executions.
To obtain guarantees that arbitrary (and unknown) user processes are able to run isolated from privileged software and other user processes, instruction level noninterference and integrity properties are provided, along with proofs that transitions to privileged modes can only occur in a controlled manner.
This work establishes a main requirement for operating system and hypervisor verification, as demonstrated for the PROSPER separation kernel. The proof is performed in the HOL4 theorem prover, taking the Cambridge model of ARM as basis.
To this end, a proof tool has been developed, which assists the verification of relational state predicates semi-automatically
Fluctuations and stability in front propagation
Propagating fronts arising from bistable reaction-diffusion equations are a
purely deterministic effect. Stochastic reaction-diffusion processes also show
front propagation which coincides with the deterministic effect in the limit of
small fluctuations (usually, large populations). However, for larger
fluctuations propagation can be affected. We give an example, based on the
classic spruce-budworm model, where the direction of wave propagation, i.e.,
the relative stability of two phases, can be reversed by fluctuations.Comment: 5 pages, 5 figure
Spin wave dispersion softening in the ferromagnetic Kondo lattice model for manganites
Spin dynamics is calculated in the ferromagnetic (FM) state of the
generalized Kondo lattice model taking into account strong on-site correlations
between e_g electrons and antiferromagnetic (AFM) exchange among t_{2g} spins.
Our study suggests that competing FM double-exchange and AFM super-exchange
interaction lead to a rather nontrivial spin-wave spectrum. While spin
excitations have a conventional Dq^2 spectrum in the long-wavelength limit,
there is a strong deviation from the spin-wave spectrum of the isotropic
Heisenberg model close to the zone boundary. The relevance of our results to
the experimental data are discussed.Comment: 6 RevTex pages, 3 embedded PostScript figure
Chemotactic Collapse and Mesenchymal Morphogenesis
We study the effect of chemotactic signaling among mesenchymal cells. We show
that the particular physiology of the mesenchymal cells allows one-dimensional
collapse in contrast to the case of bacteria, and that the mesenchymal
morphogenesis represents thus a more complex type of pattern formation than
those found in bacterial colonies. We finally compare our theoretical
predictions with recent in vitro experiments
Use of Donkeys to Guard Sheep and Goats in Texas
Though livestock guarding dogs have received considerable attention in recent years, other animals including donkeys (Equus asinus) are being used to protect sheep and goats from predation by coyotes (Canis latrans). In Texas many ranchers prefer donkeys due to low cost, relatively small maintenance requirements, and compatibility with other predator control methods. This paper describes husbandry practices for use of guard donkeys and relates rancher accounts of their effectiveness in protecting sheep and goats
Radiation-effects Research Facilities
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
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