9,320 research outputs found
Elliptic fibrations associated with the Einstein spacetimes
Given a conformally nonflat Einstein spacetime we define a fibration over
it. The fibres of this fibration are elliptic curves (2-dimensional tori) or
their degenerate counterparts. Their topology depends on the algebraic type of
the Weyl tensor of the Einstein metric. The fibration is a double branched
cover of the bundle of null direction over the spacetime and is equipped with
six linearly independent 1-forms which satisfy certain relatively simple system
of equations.Comment: 15 pages, Late
Contribution to the myrmecofauna (Formicidae, Hymenoptera) of Mt. Kopaonik (Serbia)
The myrmecofauna of Serbia is still insufficiently investigated. In the past ants were not a challenge for the entomologists of Serbia, only a few of which paid attention to ants and registered some ant species among other insect species. Živojinović (1950) registered 18 species of ants in the forest region of Majdanpek (Serbia), although he considered some valid species (Lasius alienus, L. brunneus, Camponotus herculeanus, and Formica pratensis) as subspecies. Vogrin (1955) elaborated Hymenoptera Aculeata of Yugoslavia, and registered 16 species of ants in Serbia (Srem). Gradojević (1963) listed 11 species of ants of Deliblatska Peščara (Deliblato Sands) (Vojvodina, Serbia)
Two atoms in an anisotropic harmonic trap
We consider the system of two interacting atoms confined in axially symmetric
harmonic trap. Within the pseudopotential approximation, we solve the
Schroedinger equation exactly, discussing the limits of quasi-one and
quasi-two-dimensional geometries. Finally, we discuss the application of an
energy-dependent pseudopotential, which allows to extend the validity of our
results to the case of tight traps and large scattering lengths.Comment: RevTeX 4 pages, 2 figure
Orbital and interlayer Skyrmions crystals in bilayer graphene
A graphene bilayer in a transverse magnetic field has a set of Landau levels
with energies where
is the effective cyclotron frequency and
All Landau levels but N=0 are four times degenerate counting spin and valley
degrees of freedom. The Landau level N=0 has an extra degeneracy due to the
fact that orbitals and both have zero kinetic energies. At integer
filling factors, Coulomb interactions produce a set of broken-symmetry states
with partial or full alignement in space of the valley and orbital pseudospins.
These quantum Hall pseudo-ferromagnetic states support topological charged
excitations in the form of orbital and valley Skyrmions. Away from integer
fillings, these topological excitations can condense to form a rich variety of
Skyrme crystals with interesting properties. We study in this paper different
crystal phases that occur when an electric field is applied between the layers.
We show that orbital Skyrmions, in analogy with spin Skyrmions, have a texture
of electrical dipoles that can be controlled by an in-plane electric field.
Moreover, the modulation of electronic density in the crystalline phases are
experimentally accessible through a measurement of their local density of
statesComment: 18 pages with 13 figure
On the algebraic classification of spacetimes
We briefly overview the Petrov classification in four dimensions and its
generalization to higher dimensions.Comment: Submitted to Journal of Physics, conference series, proceedings of
4th meeting on constrained dynamics and quantum gravity, 12-16 September
2005, Sardinia, Ital
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