6,040 research outputs found
Local Volume Effects in the Generalized Pseudopotential Theory
The generalized pseudopotential theory (GPT) is a powerful method for
deriving real-space transferable interatomic potentials. Using a coarse-grained
electronic structure, one can explicitly calculate the pair ion-ion and
multi-ion interactions in simple and transition metals. Whilst successful in
determining bulk properties, in central force metals the GPT fails to describe
crystal defects for which there is a significant local volume change. A
previous paper [PhysRevLett.66.3036 (1991)] found that by allowing the GPT
total energy to depend upon some spatially-averaged local electron density, the
energetics of vacancies and surfaces could be calculated within experimental
ranges. In this paper, we develop the formalism further by explicitly
calculating the forces and stress tensor associated with this total energy. We
call this scheme the adaptive GPT (aGPT) and it is capable of both molecular
dynamics and molecular statics. We apply the aGPT to vacancy formation and
divacancy binding in hcp Mg and also calculate the local electron density
corrections to the bulk elastic constants and phonon dispersion for which there
is refinement over the baseline GPT treatment.Comment: 11 pages, 6 figure
Topological (Sliced) Doping of a 3D Peierls System: Predicted Structure of Doped BaBiO3
At hole concentrations below x=0.4, Ba_(1-x)K_xBiO_3 is non-metallic. At x=0,
pure BaBiO3 is a Peierls insulator. Very dilute holes create bipolaronic point
defects in the Peierls order parameter. Here we find that the Rice-Sneddon
version of Peierls theory predicts that more concentrated holes should form
stacking faults (two-dimensional topological defects, called slices) in the
Peierls order parameter. However, the long-range Coulomb interaction, left out
of the Rice-Sneddon model, destabilizes slices in favor of point bipolarons at
low concentrations, leaving a window near 30% doping where the sliced state is
marginally stable.Comment: 6 pages with 5 embedded postscript figure
Friedel oscillations responsible for stacking fault of adatoms: The case of Mg(0001) and Be(0001)
We perform a first-principles study of Mg adatom and adislands on the
Mg(0001) surface, and Be adatom on Be(0001), to obtain further insights into
the previously reported energetic preference of the fcc faulty stacking of Mg
monomers on Mg(0001). We first provide a viewpoint on how Friedel oscillations
influence ionic relaxation on these surfaces. Our three-dimensional
charge-density analysis demonstrates that Friedel oscillations have maxima
which are more spatially localized than what one-dimensional average density or
two-dimensional cross sectional plots could possibly inform: The well-known
charge-density enhancement around the topmost surface layer of Mg(0001) is
strongly localized at its fcc hollow sites. The charge accumulation at this
site explains the energetically preferred stacking fault of the Mg monomer,
dimer and trimer. Yet, larger islands prefer the normal hcp stacking.
Surprisingly, the mechanism by which the fcc site becomes energetically more
favorable is not that of enhancing the surface-adatom bonds but rather those
between surface atoms. To confirm our conclusions, we analyze the stacking of
Be adatom on Be(0001) - a surface also largely influenced by Friedel
oscillations. We find, in fact, a much stronger effect: The charge enhancement
at the fcc site is even larger and, consequently, the stacking-fault energy
favoring the fcc site is quite large, 44 meV.Comment: Submitted to Physical Review
Topological phases with parafermions: theory and blueprints
We concisely review the recent evolution in the study of parafermions --
exotic emergent excitations that generalize Majorana fermions and similarly
underpin a host of novel phenomena. First we illustrate the intimate connection
between Z_3-symmetric "spin" chains and one-dimensional parafermion lattice
models, highlighting how the latter host a topological phase featuring
protected edge zero modes. We then tour several blueprints for the laboratory
realization of parafermion zero modes -- focusing on quantum
Hall/superconductor hybrids, quantum Hall bilayers, and two-dimensional
topological insulators -- and describe striking experimental fingerprints that
they provide. Finally, we discuss how coupled parafermion arrays in quantum
Hall architectures yield topological phases that potentially furnish hardware
for a universal, intrinsically decoherence-free quantum computer.Comment: 14 pages, 4 figures; slated for Annual Reviews of Condensed Matter
Physic
A stable and accurate control-volume technique based on integrated radial basis function networks for fluid-flow problems
Radial basis function networks (RBFNs) have been widely used in solving partial differential equations as they
are able to provide fast convergence. Integrated RBFNs have the ability to avoid the problem of reduced convergence-rate caused by differentiation. This paper is concerned with the use of integrated RBFNs in the context of control-volume discretisations for the simulation of fluid-flow problems. Special attention is given to (i) the development of a stable high-order upwind scheme for the convection term and (ii) the development of a local high-order approximation scheme for the diffusion term. Benchmark
problems including the lid-driven triangular-cavity flow are
employed to validate the present technique. Accurate results at high values of the Reynolds number are obtained using relatively-coarse grids
Avoiding core's DUE & SDC via acoustic wave detectors and tailored error containment and recovery
The trend of downsizing transistors and operating voltage scaling has made the processor chip more sensitive against radiation phenomena making soft errors an important challenge. New reliability techniques for handling soft errors in the logic and memories that allow meeting the desired failures-in-time (FIT) target are key to keep harnessing the benefits of Moore's law. The failure to scale the soft error rate caused by particle strikes, may soon limit the total number of cores that one may have running at the same time. This paper proposes a light-weight and scalable architecture to eliminate silent data corruption errors (SDC) and detected unrecoverable errors (DUE) of a core. The architecture uses acoustic wave detectors for error detection. We propose to recover by confining the errors in the cache hierarchy, allowing us to deal with the relatively long detection latencies. Our results show that the proposed mechanism protects the whole core (logic, latches and memory arrays) incurring performance overhead as low as 0.60%. © 2014 IEEE.Peer ReviewedPostprint (author's final draft
Metabolite essentiality elucidates robustness of Escherichia coli metabolism
Complex biological systems are very robust to genetic and environmental
changes at all levels of organization. Many biological functions of Escherichia
coli metabolism can be sustained against single-gene or even multiple-gene
mutations by using redundant or alternative pathways. Thus, only a limited
number of genes have been identified to be lethal to the cell. In this regard,
the reaction-centric gene deletion study has a limitation in understanding the
metabolic robustness. Here, we report the use of flux-sum, which is the
summation of all incoming or outgoing fluxes around a particular metabolite
under pseudo-steady state conditions, as a good conserved property for
elucidating such robustness of E. coli from the metabolite point of view. The
functional behavior, as well as the structural and evolutionary properties of
metabolites essential to the cell survival, was investigated by means of a
constraints-based flux analysis under perturbed conditions. The essential
metabolites are capable of maintaining a steady flux-sum even against severe
perturbation by actively redistributing the relevant fluxes. Disrupting the
flux-sum maintenance was found to suppress cell growth. This approach of
analyzing metabolite essentiality provides insight into cellular robustness and
concomitant fragility, which can be used for several applications, including
the development of new drugs for treating pathogens.Comment: Supplements available at
http://stat.kaist.ac.kr/publication/2007/PJKim_pnas_supplement.pd
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