5,956 research outputs found
Vortex Dynamics in Classical Non--Abelian Spin Models
We discuss the abelian vortex dynamics in the abelian projection approach to
non-abelian spin models. We show numerically that in the three-dimensional
SU(2) spin model in the Maximal Abelian projection the abelian off-diagonal
vortices are not responsible for the phase transition contrary to the diagonal
vortices. A generalization of the abelian projection approach to SU(N) spin
models is briefly discussed.Comment: 7 pages, LaTeX, 1 figure, uses epsf.sty; Introduction is extended and
a few references are added; to be published in JETP Let
Direct Synthesis of Propene Using Supported Bifunctional Nickel Catalysts: Preparation and Potential
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MODELING JET INTERACTIONS WITH THE AMBIENT MEDIUM
Recent high-resolution (see, e.g., [13]) observations of astrophysical jets reveal complex structures apparently caused by ejecta from the central engine as the ejecta interact with the surrounding interstellar material. These observations include time-lapsed âmoviesâ of both AGN and microquasars jets which also show that the jet phenomena are highly time-dependent. Such observations can be used to inform models of the jetâambient-medium interactions. Based on an analysis of these data, we posit that a significant part of the observed phenomena come from the interaction of the ejecta with prior ejecta as well as interstellar material. In this view, astrophysical jets interact with the ambient medium through which they propagate, entraining and accelerating it. We show some elements of the modeling of these jets in this paper, including energy loss and heating via plasma processes, and large scale hydrodynamic and relativistic hydrodynamic simulations
Multiscale Modeling of Astrophysical Jets
We are developing the capability for a multi-scale code to model the energy deposition rate and momentum transfer rate of an astrophysical jet which generates strong plasma turbulence in its interaction with the ambient medium through which it propagates. We start with a highly parallelized version of the VH-1 Hydrodynamics Code (Coella and Wood 1984, and Saxton et al., 2005). We are also considering the PLUTO code (Mignone et al. 2007) to model the jet in the magnetohydrodynamic (MHD) and relativistic, magnetohydrodynamic (RMHD) regimes. Particle-in-Cell approaches are also being used to benchmark a wave-population models of the two-stream instability and associated plasma processes in order to determine energy deposition and momentum transfer rates for these modes of jet-ambient medium interactions. We show some elements of the modeling of these jets in this paper, including energy loss and heating via plasma processes, and large scale hydrodynamic and relativistic hydrodynamic simulations. A preliminary simulation of a jet from the galactic center region is used to lend credence to the jet as the source of the so-called the Fermi Bubble (see, e.g., Su, M. & Finkbeiner, D. P., 2012)*It is with great sorrow that we acknowledge the loss of our colleague and friend of more than thirty years, Dr. John Ural Guillory, to his battle with cancer
Influence of bottom topography on integral constraints in zonal flows with parameterized potential vorticity fluxes
An integral constraint for eddy fluxes of potential vorticity (PV), corresponding to global momentum conservation, is applied to two-layer zonal quasi-geostrophic channel flow. This constraint must be satisfied for any type of parameterization of eddy PV fluxes. Bottom topography strongly influence the integral constraint compared to a flat bottom channel. An analytical solution for the mean flow solution has been found by using asymptotic expansion in a small parameter which is the ratio of the Rossby radius to the meridional extent of the channel. Applying the integral constraint to this solution, one can find restrictions for eddy PV transfer coefficients which relate the eddy fluxes of PV to the mean flow. These restrictions strongly deviate from restrictions for the channel with flat bottom topography
Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning
The disordered flux line lattice in single crystals of the slightly overdoped
aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by
magnetization measurements, small-angle neutron scattering (SANS), and magnetic
force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex
pinning precludes the formation of an ordered Abrikosov lattice. Instead, a
vitreous vortex phase (vortex glass) with a short-range hexagonal order is
observed. Statistical processing of MFM datasets lets us directly measure its
radial and angular distribution functions and extract the radial correlation
length \zeta. In contrast to predictions of the collective pinning model, no
increase in the correlated volume with the applied field is observed. Instead,
we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the
applied magnetic field, where R1 is the radius of the first coordination shell
of the vortex lattice. Such universal scaling of \zeta implies that the vortex
pinning in iron arsenides remains strong even in the absence of static
magnetism. This result is consistent with all the real- and reciprocal-space
vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published
to date and is thus sample-independent. The failure of the collective pinning
model suggests that the vortices remain in the single-vortex pinning limit even
in high magnetic fields up to 9 T.Comment: 11 pages, 6 figure
Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning
The disordered flux line lattice in single crystals of the slightly overdoped
aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by
magnetization measurements, small-angle neutron scattering (SANS), and magnetic
force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex
pinning precludes the formation of an ordered Abrikosov lattice. Instead, a
vitreous vortex phase (vortex glass) with a short-range hexagonal order is
observed. Statistical processing of MFM datasets lets us directly measure its
radial and angular distribution functions and extract the radial correlation
length \zeta. In contrast to predictions of the collective pinning model, no
increase in the correlated volume with the applied field is observed. Instead,
we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the
applied magnetic field, where R1 is the radius of the first coordination shell
of the vortex lattice. Such universal scaling of \zeta implies that the vortex
pinning in iron arsenides remains strong even in the absence of static
magnetism. This result is consistent with all the real- and reciprocal-space
vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published
to date and is thus sample-independent. The failure of the collective pinning
model suggests that the vortices remain in the single-vortex pinning limit even
in high magnetic fields up to 9 T.Comment: 11 pages, 6 figure
The Length of an SLE - Monte Carlo Studies
The scaling limits of a variety of critical two-dimensional lattice models
are equal to the Schramm-Loewner evolution (SLE) for a suitable value of the
parameter kappa. These lattice models have a natural parametrization of their
random curves given by the length of the curve. This parametrization (with
suitable scaling) should provide a natural parametrization for the curves in
the scaling limit. We conjecture that this parametrization is also given by a
type of fractal variation along the curve, and present Monte Carlo simulations
to support this conjecture. Then we show by simulations that if this fractal
variation is used to parametrize the SLE, then the parametrized curves have the
same distribution as the curves in the scaling limit of the lattice models with
their natural parametrization.Comment: 18 pages, 10 figures. Version 2 replaced the use of "nu" for the
"growth exponent" by 1/d_H, where d_H is the Hausdorff dimension. Various
minor errors were also correcte
Magnetic Breakdown in the electron-doped cuprate superconductor NdCeCuO: the reconstructed Fermi surface survives in the strongly overdoped regime
We report on semiclassical angle-dependent magnetoresistance oscillations
(AMRO) and the Shubnikov-de Haas effect in the electron-overdoped cuprate
superconductor NdCeCuO. Our data provide convincing evidence
for magnetic breakdown in the system. This shows that a reconstructed
multiply-connected Fermi surface persists, at least at strong magnetic fields,
up to the highest doping level of the superconducting regime. Our results
suggest an intimate relation between translational symmetry breaking and the
superconducting pairing in the electron-doped cuprate superconductors.Comment: 5 pages, 4 figures, submitted to PR
Seasonal Ice Cycle at the Mars Phoenix Landing Site: 2. Postlanding CRISM and Ground Observations
The combination of ground observations from the Mars Phoenix Lander and orbital data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provided a detailed view of the formation of late summer surface water ice at the landing site and surrounding regions. CRISM observations of the landing site during and immediately after Phoenix operations were analyzed to track the seasonal and diurnal ice cycles during the late spring to late summer, and a nonlinear mixing model was used to estimate grain sizes and relative abundances of water ice and dust. The surface around the Phoenix landing site was ice-free from late spring through midsummer, although transient patches of mobile ices were observed in an 85 m diameter crater to the northeast of the landing site. At the âŒ10 km diameter Heimdal Crater, located âŒ10 km east of the landing site, permanent patches of water ice were observed to brighten during the late spring and darken during the summer, possibly as fine-grained water ice that was cold trapped onto the ice during late spring sintered into larger grains or finally sublimated, exposing larger-grained ice. CRISM spectra first show evidence of widespread ice during the night at solar longitude (Ls) ⌠109°, âŒ9 sols before Phoenixâs Surface Stereo Imager detected it. CRISM spectra first show evidence of afternoon surface ice and water ice clouds after Ls ⌠155°, after Phoenix operations ended
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