10,186 research outputs found
Deformed Carroll particle from 2+1 gravity
We consider a point particle coupled to 2+1 gravity, with de Sitter gauge
group SO(3,1). We observe that there are two contraction limits of the gauge
group: one resulting in the Poincare group, and the second with the gauge group
having the form AN(2) \ltimes \an(2)^*. The former case was thoroughly
discussed in the literature, while the latter leads to the deformed particle
action with de Sitter momentum space, like in the case of kappa-Poincare
particle. However, the construction forces the mass shell constraint to have
the form p_0^2 = m^2, so that the effective particle action describes the
deformed Carroll particle.Comment: 10 page
Coherent states for the q-deformed quantum mechanics on a circle
The q-deformed coherent states for a quantum particle on a circle are
introduced and their properties investigated.Comment: 11 pages, 2 PostScript figure
On the definition of velocity in doubly special relativity theories
We discuss the definition of particle velocity in doubly relativity theories.
The general formula relating velocity and four-momentum of particle is given.Comment: 7 page
Quantum mechanics on a circle: Husimi phase space distributions and semiclassical coherent state propagators
We discuss some basic tools for an analysis of one-dimensionalquantum systems
defined on a cyclic coordinate space. The basic features of the generalized
coherent states, the complexifier coherent states are reviewed. These states
are then used to define the corresponding (quasi)densities in phase space. The
properties of these generalized Husimi distributions are discussed, in
particular their zeros.Furthermore, the use of the complexifier coherent states
for a semiclassical analysis is demonstrated by deriving a semiclassical
coherent state propagator in phase space.Comment: 29 page
Discrete Accidental Symmetry for a Particle in a Constant Magnetic Field on a Torus
A classical particle in a constant magnetic field undergoes cyclotron motion
on a circular orbit. At the quantum level, the fact that all classical orbits
are closed gives rise to degeneracies in the spectrum. It is well-known that
the spectrum of a charged particle in a constant magnetic field consists of
infinitely degenerate Landau levels. Just as for the and
potentials, one thus expects some hidden accidental symmetry, in this case with
infinite-dimensional representations. Indeed, the position of the center of the
cyclotron circle plays the role of a Runge-Lenz vector. After identifying the
corresponding accidental symmetry algebra, we re-analyze the system in a finite
periodic volume. Interestingly, similar to the quantum mechanical breaking of
CP invariance due to the -vacuum angle in non-Abelian gauge theories,
quantum effects due to two self-adjoint extension parameters and
explicitly break the continuous translation invariance of the
classical theory. This reduces the symmetry to a discrete magnetic translation
group and leads to finite degeneracy. Similar to a particle moving on a cone, a
particle in a constant magnetic field shows a very peculiar realization of
accidental symmetry in quantum mechanics.Comment: 25 pages, 2 figure
Electronic band structure and exchange coupling constants in ACr2X4 spinels
We present the results of band structure calculations for ACr2X4 (A=Zn, Cd,
Hg and X=O, S, Se) spinels. Effective exchange coupling constants between Cr
spins are determined by fitting the energy of spin spirals to a classical
Heisenberg model. The calculations reproduce the change of the sign of the
dominant nearest-neighbor exchange interaction J1 from antiferromagnetic in
oxides to ferromagnetic in sulfides and selenides. It is verified that the
ferromagnetic contribution to J1 is due to indirect hopping between Cr t2g and
eg states via X p states. Antiferromagnetic coupling between 3-rd Cr neighbors
is found to be important in all the ACr2X4 spinels studied, whereas other
interactions are much weaker. The results are compared to predictions based on
the Goodenough-Kanamori rules of superexchange.Comment: 15 pages, 10 figures, 3 table
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