12,012 research outputs found
Crossing of Phantom Divide in Gravity
An explicit model of gravity with realizing a crossing of the phantom
divide is reconstructed. In particular, it is shown that the Big Rip
singularity may appear in the reconstructed model of gravity. Such a Big
Rip singularity could be avoided by adding term or non-singular viable
theory to the model because phantom behavior becomes transient.Comment: 9 pages, 1 figure, to be published in the proceedings of the
International Workshop on Dark Matter, Dark Energy and Matter-antimatter
Asymmetry in Special Issue of Modern Physics Letters A, Department of
Physics, National Tsing Hua University, Hsinchu, Taiwan, 20th - 21st
November, 200
Low-lying even parity meson resonances and spin-flavor symmetry
A study is presented of the wave meson-meson interactions involving
members of the nonet and of the octet. The starting point is an
SU(6) spin-flavor extension of the SU(3) flavor Weinberg-Tomozawa Lagrangian.
SU(6) symmetry breaking terms are then included to account for the physical
meson masses and decay constants, while preserving partial conservation of the
axial current in the light pseudoscalar sector. Next, the matrix amplitudes
are obtained by solving the Bethe Salpeter equation in coupled-channel with the
kernel built from the above interactions. The poles found on the first and
second Riemann sheets of the amplitudes are identified with their possible
Particle Data Group (PDG) counterparts. It is shown that most of the low-lying
even parity PDG meson resonances, specially in the and sectors,
can be classified according to multiplets of the spin-flavor symmetry group
SU(6). The , and some resonances cannot be
accommodated within this SU(6) scheme and thus they would be clear candidates
to be glueballs or hybrids. Finally, we predict the existence of five exotic
resonances ( and/or ) with masses in the range 1.4--1.6 GeV,
which would complete the , , and multiplets of
SU(3)SU(2).Comment: 43 pages, 2 figures, 61 tables. Improved discussion of Section II. To
appear in Physical Review
Exploration of H2 binding to the [NiFe]-hydrogenase active site with multiconfigurational density functional theory
The combination of density functional theory (DFT) with a
multiconfigurational wave function is an efficient way to include dynamical
correlation in calculations with multiconfiguration self-consistent field wave
functions. These methods can potentially be employed to elucidate reaction
mechanisms in bio-inorganic chemistry, where many other methods become either
too computationally expensive or too inaccurate. In this paper, a complete
active space (CAS) short-range DFT (CAS-srDFT) hybrid was employed to
investigate a bio-inorganic system, namely H2 binding to the active site of
[NiFe] hydrogenase. This system was previously investigated with
coupled-cluster (CC) and multiconfigurational methods in form of
cumulant-approximated second-order perturbation theory, based on the density
matrix renormalization group (DMRG). We find that it is more favorable for H2
to bind to Ni than to Fe, in agreement with previous CC and DMRG calculations.
The accuracy of CAS-srDFT is comparable to both CC and DMRG, despite that much
smaller active spaces were employed. This enhanced efficiency at smaller active
spaces shows that CAS-srDFT can become a useful method for bio-inorganic
chemistry.Comment: 22 page
STRUCTURES, PROPERTIES AND FUNCTIONALITIES OF MAGNETIC DOMAIN WALLS IN THIN FILMS, NANOWIRES AND ATOMIC CHAINS: MICROMAGNETIC AND AB INITIO STUDIES
Structures, properties and functionalities of magnetic domain walls in thin film, nanowires and atomic chains are studied by micromagnetic simulations and ab initio calculations in this dissertation. For magnetic domain walls in thin films, we computationally investigated the dynamics of one-dimensional domain wall line in ultrathin ferromagnetic film, and the exponent α = 1.24 ± 0.05 is obtained in the creep regime near depinning force, indicating the washboard potential model is supported by our simulations. Furthermore, the roughness, creep, depinning and flow of domain wall line with commonly existed substructures driven by magnetic field are also studied. Our simulation results demonstrate that substructures will decrease the roughness exponent ζ, increase the critical depinning force, and reduce the effective creep energy barrier. Current induced domain-wall substructure motion is also studied, which is found quite different from current induced domain wall motion.
For magnetic domain walls in nanowires, field and current induced domain wall motion is studied, and some relevant spintronic devices are proposed based on micromagnetic simulations. Novel nanometer transverse-domain-wall-based logic elements, 360° domain wall generator and shift register are proposed. When spinpolarized current is applied, the critical current for domain wall depinning can be substantially reduced and conveniently tuned by controlling domain wall number in the pile-up at pinning site, in analogy to dislocation pile-up responsible for Hall-Petch effect in mechanical strength. Furthermore, threshold currents for domain wall depinning and transportation through circular geometry in planar nanowire induced by spin transfer torques and spin-orbit torques are theoretically calculated. In addition, magnetic vortex racetrack memory which combines both conceptions of magnetic vortex domain walls and racetrack is also proposed using micromagnetic simulations.
For magnetic domain walls in Ni atomic chains, a truly magnetic domain wall structure and the single domain switching process are investigated by both ab initio studies and spin dynamics simulations. Spin moment softening effect caused by the hybridization effect between two spin channels is considered. The atomic domain wall as narrow as 4 atom-distance with slight spin moment softening effect indicates a relatively evident ballistic magnetoresistance effect, and the large EB indicates the strong stability of single domain state
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