30,174 research outputs found
The branch processes of vortex filaments and Hopf Invariant Constraint on Scroll Wave
In this paper, by making use of Duan's topological current theory, the
evolution of the vortex filaments in excitable media is discussed in detail.
The vortex filaments are found generating or annihilating at the limit points
and encountering, splitting, or merging at the bifurcation points of a complex
function . It is also shown that the Hopf invariant of knotted
scroll wave filaments is preserved in the branch processes (splitting, merging,
or encountering) during the evolution of these knotted scroll wave filaments.
Furthermore, it also revealed that the "exclusion principle" in some chemical
media is just the special case of the Hopf invariant constraint, and during the
branch processes the "exclusion principle" is also protected by topology.Comment: 9 pages, 5 figure
Surface phase separation in nanosized charge-ordered manganites
Recent experiments showed that the robust charge-ordering in manganites can
be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism
was evidenced in both nanoparticles and nanowires of charge-ordered manganites.
To explain these observations, a phenomenological model based on surface phase
separation is proposed. The relaxation of superexchange interaction on the
surface layer allows formation of a ferromagnetic shell, whose thickness
increases with decreasing grain size. Possible exchange bias and softening of
the ferromagnetic transition in nanosized charge-ordered manganites are
predicted.Comment: 4 pages, 3 figure
Hints of Standard Model Higgs Boson at the LHC and Light Dark Matter Searches
The most recent results of searches at the LHC for the Higgs boson h have
turned up possible hints of such a particle with mass m_h about 125 GeV
consistent with standard model (SM) expectations. This has many potential
implications for the SM and beyond. We consider some of them in the contexts of
a simple Higgs-portal dark matter (DM) model, the SM plus a real gauge-singlet
scalar field D as the DM candidate, and a couple of its variations. In the
simplest model with one Higgs doublet and three or four generations of
fermions, for D mass m_D DD tends to have a
substantial branching ratio. If future LHC data confirm the preliminary Higgs
indications, m_D will have to exceed m_h/2. To keep the DM lighter than m_h/2,
one will need to extend the model and also satisfy constraints from DM direct
searches. The latter can be accommodated if the model provides sizable isospin
violation in the DM-nucleon interactions. We explore this in a
two-Higgs-doublet model combined with the scalar field D. This model can offer
a 125-GeV SM-like Higgs and a light DM candidate having isospin-violating
interactions with nucleons at roughly the required level, albeit with some
degree of fine-tuning.Comment: 17 pages, 4 figures, slightly revised, main conclusions unchanged,
references added, matches published versio
Dynamic Potential-Ph Diagrams Application to Electrocatalysts for Water Oxidation
The construction and use of "dynamic potential-pH diagrams" (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO(2), Co(3)O(4), Co(3)O(4) electrodeposited in a phosphate medium (Co-Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e. g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.Ministry of Education, University and Research PRIN 2008PF9TWZ, 2008N7CYL5Universita degli Studi di MilanoNational Science Foundation CHE-0808927Robert A. Welch Foundation F-0021Center for Electrochemistr
Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations
The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia
Improved superlensing in two-dimensional photonic crystals with a basis
We study propagation of light in square and hexagonal two-dimensional
photonic crystals. We show, that slabs of these crystals focus light with
subwavelength resolution. We propose a systematic way to increase this
resolution, at an essentially fixed frequency, by employing a hierarchy of
crystals of the same structure, and the same lattice constant, but with an
increasingly complex basis.Comment: 16 Pages, 5 Figure
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