64,299 research outputs found
Acyclic orientations on the Sierpinski gasket
We study the number of acyclic orientations on the generalized
two-dimensional Sierpinski gasket at stage with equal to
two and three, and determine the asymptotic behaviors. We also derive upper
bounds for the asymptotic growth constants for and -dimensional
Sierpinski gasket .Comment: 20 pages, 8 figures and 6 table
Collisions of antiprotons with hydrogen molecular ions
Time-dependent close-coupling calculations of the ionization and excitation
cross section for antiproton collisions with molecular hydrogen ions are
performed in an impact-energy range from 0.5 keV to 10 MeV. The
Born-Oppenheimer and Franck-Condon approximations as well as the impact
parameter method are applied in order to describe the target molecule and the
collision process. It is shown that three perpendicular orientations of the
molecular axis with respect to the trajectory are sufficient to accurately
reproduce the ionization cross section calculated by [Sakimoto, Phys. Rev. A
71, 062704 (2005)] reducing the numerical effort drastically. The
independent-event model is employed to approximate the cross section for double
ionization and H+ production in antiproton collisions with H2.Comment: 12 pages, 5 figures, 4 table
Enhanced visibility of graphene: effect of one-dimensional photonic crystal
We investigate theoretically the light reflectance of a graphene layer
prepared on the top of one-dimensional Si/SiO2 photonic crystal (1DPC). It is
shown that the visibility of the graphene layers is enhanced greatly when 1DPC
is added, and the visibility can be tuned by changing the incident angle and
light wavelengths. This phenomenon is caused by the absorption of the graphene
layer and the enhanced reflectance of the 1DPC.Comment: 4 pages, 4 figures. published, ApplPhysLett_91_18190
Theoretical study of space plasmas Final report, 16 Feb. 1964 - 15 Mar. 1965
Interchange stability of Van Allen belt - Effect of resonant magnetic moment violation on trapped particles - Exact solution of universal instabilit
Theoretical studies of space plasmas Summary report, 3 May 1965 - 1 May 1966
Synchrotron radiation, ionospheric currents, auroral bombardment, and plasma instabilitie
Electrodynamics of Magnetars III: Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere
We consider the details of the QED processes that create electron-positron
pairs in magnetic fields approaching and exceeding 10^{14} G. The formation of
free and bound pairs is addressed, and the importance of positronium
dissociation by thermal X-rays is noted. We calculate the collision cross
section between an X-ray and a gamma ray, and point out a resonance in the
cross section when the gamma ray is close to the threshold for pair conversion.
We also discuss how the pair creation rate in the open-field circuit and the
outer magnetosphere can be strongly enhanced by instabilities near the light
cylinder. When the current has a strong fluctuating component, a cascade
develops. We examine the details of particle heating, and show that a high rate
of pair creation can be sustained close to the star, but only if the spin
period is shorter than several seconds. The dissipation rate in this turbulent
state can easily accommodate the observed radio output of the transient
radio-emitting magnetars, and even their infrared emission. Finally, we outline
how a very high rate of pair creation on the open magnetic field lines can help
to stabilize a static twist in the closed magnetosphere and to regulate the
loss of magnetic helicity by reconnection at the light cylinder.Comment: 25 pages, submitted to the Astrophysical Journa
Magnetic and Thermodynamic Properties of the Collective Paramagnet-Spin Liquid Pyrochlore Tb2Ti2O7
In a recent letter [Phys. Rev. Lett. {\bf 82}, 1012 (1999)] it was found that
the Tb magnetic moments in the TbTiO pyrochlore lattice of
corner-sharing tetrahedra remain in a {\it collective paramagnetic} state down
to 70mK. In this paper we present results from d.c. magnetic susceptibility,
specific heat data, inelastic neutron scattering measurements, and crystal
field calculations that strongly suggest that (1) the Tb ions in
TbTiO possess a moment of approximatively 5, and (2)
the ground state tensor is extremely anisotropic below a temperature of
K, with Ising-like Tb magnetic moments confined to point along
a local cubic direction
dramatically reduces the frustration otherwise present in a Heisenberg
pyrochlore antiferromagnet. The results presented herein underpin the
conceptual difficulty in understanding the microscopic mechanism(s) responsible
for TbTiO failing to develop long-range order at a temperature of
the order of the paramagnetic Curie-Weiss temperature K. We suggest that dipolar interactions and extra perturbative exchange
coupling(s)beyond nearest-neighbors may be responsible for the lack of ordering
of TbTiO.Comment: 8 POSTSCRIPT figures included. Submitted to Physical Review B.
Contact: [email protected]
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
Comment on "Light-front Schwinger model at finite temperature"
In a recent paper by A. Das and X. Zhou [Phys. Rev. D 68, 065017 (2003)] it
is claimed that explicit evaluation of the thermal photon self-energy in the
Schwinger model gives off-shell thermal Green functions that are different in
light-front and conventional quantizations. We show that the claimed difference
originates from an erroneous simplification of the fermion propagator used in
the light-front calculation.Comment: 8 pages, revtex4, added section refuting the massless limit proposed
in hep-th/031102
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