39,495 research outputs found
Vacuum polarization for neutral particles in 2+1 dimensions
In 2+1 dimensions there exists a duality between a charged Dirac particle
coupled minimally to a background vector potential and a neutral one coupled
nonminimally to a background electromagnetic field strength. A constant uniform
background electric current induces in the vacuum of the neutral particle a
fermion current which is proportional to the background one. A background
electromagnetic plane wave induces no current in the vacuum. For constant but
nonuniform background electric charge, known results for charged particles can
be translated to give the induced fermion number. Some new examples with
infinite background electric charge are presented. The induced spin and total
angular momentum are also discussed.Comment: REVTeX, 7 pages, no figur
Effective Potential for Uniform Magnetic Fields through Pauli Interaction
We have calculated the explicit form of the real and imaginary parts of the
effective potential for uniform magnetic fields which interact with spin-1/2
fermions through the Pauli interaction. It is found that the non-vanishing
imaginary part develops for a magnetic field stronger than a critical field,
whose strength is the ratio of the fermion mass to its magnetic moment. This
implies the instability of the uniform magnetic field beyond the critical field
strength to produce fermion pairs with the production rate density
in the
presence of Pauli interaction.Comment: 9 pages with 1 figur
Time evolution, cyclic solutions and geometric phases for general spin in an arbitrarily varying magnetic field
A neutral particle with general spin and magnetic moment moving in an
arbitrarily varying magnetic field is studied. The time evolution operator for
the Schr\"odinger equation can be obtained if one can find a unit vector that
satisfies the equation obeyed by the mean of the spin operator. There exist at
least cyclic solutions in any time interval. Some particular time
interval may exist in which all solutions are cyclic. The nonadiabatic
geometric phase for cyclic solutions generally contains extra terms in addition
to the familiar one that is proportional to the solid angle subtended by the
closed trace of the spin vector.Comment: revtex4, 8 pages, no figur
Low-field magnetotransport in graphene cavity devices
Confinement and edge structures are known to play significant roles in
electronic and transport properties of two-dimensional materials. Here, we
report on low-temperature magnetotransport measurements of lithographically
patterned graphene cavity nanodevices. It is found that the evolution of the
low-field magnetoconductance characteristics with varying carrier density
exhibits different behaviors in graphene cavity and bulk graphene devices. In
the graphene cavity devices, we have observed that intravalley scattering
becomes dominant as the Fermi level gets close to the Dirac point. We associate
this enhanced intravalley scattering to the effect of charge inhomogeneities
and edge disorder in the confined graphene nanostructures. We have also
observed that the dephasing rate of carriers in the cavity devices follows a
parabolic temperature dependence, indicating that the direct Coulomb
interaction scattering mechanism governs the dephasing at low temperatures. Our
results demonstrate the importance of confinement in carrier transport in
graphene nanostructure devices.Comment: 13 pages, 5 figure
Concerning the Motion and Orientation of Flux Transfer Events Produced by Component and Antiparallel Reconnection
We employ the Cooling et al. (2001) model to predict the location, orientation, motion, and signatures of flux transfer events (FTEs) generated at the solstices and equinoxes along extended subsolar component and high ]latitude antiparallel reconnection curves for typical solar wind plasma conditions and various interplanetary magnetic field (IMF) strengths and directions. In general, events generated by the two mechanisms maintain the strikingly different orientations they begin with as they move toward the terminator in opposite pairs of magnetopause quadrants. The curves along which events generated by component reconnection form bow toward the winter cusp. Events generated by antiparallel reconnection form on the equatorial magnetopause during intervals of strongly southward IMF orientation during the equinoxes, form in the winter hemisphere and only reach the dayside equatorial magnetopause during the solstices when the IMF strength is very large and the IMF points strongly southward, never reach the equatorial dayside magnetopause when the IMF has a substantial dawnward or duskward component, and never reach the equatorial flank magnetopause during intervals of northward and dawnward or duskward IMF orientation. Magnetosheath magnetic fields typically have strong components transverse to events generated by component reconnection but only weak components transverse to the axes of events generated by antiparallel reconnection. As a result, much stronger bipolar magnetic field signatures normal to the nominal magnetopause should accompany events generated by component reconnection. The results presented in this paper suggest that events generated by component reconnection predominate on the dayside equatorial and flank magnetopause for most solar wind conditions
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