23 research outputs found
Topological properties of quantum periodic Hamiltonians
We consider periodic quantum Hamiltonians on the torus phase space
(Harper-like Hamiltonians). We calculate the topological Chern index which
characterizes each spectral band in the generic case. This calculation is made
by a semi-classical approach with use of quasi-modes. As a result, the Chern
index is equal to the homotopy of the path of these quasi-modes on phase space
as the Floquet parameter (\theta) of the band is varied. It is quite
interesting that the Chern indices, defined as topological quantum numbers, can
be expressed from simple properties of the classical trajectories.Comment: 27 pages, 14 figure
Scaling law in the Standard Map critical function. Interpolating hamiltonian and frequency map analysis
We study the behaviour of the Standard map critical function in a
neighbourhood of a fixed resonance, that is the scaling law at the fixed
resonance. We prove that for the fundamental resonance the scaling law is
linear. We show numerical evidence that for the other resonances , , and and relatively prime, the scaling law follows a
power--law with exponent .Comment: AMS-LaTeX2e, 29 pages with 8 figures, submitted to Nonlinearit
On recurrence and ergodicity for geodesic flows on noncompact periodic polygonal surfaces
We study the recurrence and ergodicity for the billiard on noncompact
polygonal surfaces with a free, cocompact action of or . In the
-periodic case, we establish criteria for recurrence. In the more difficult
-periodic case, we establish some general results. For a particular
family of -periodic polygonal surfaces, known in the physics literature
as the wind-tree model, assuming certain restrictions of geometric nature, we
obtain the ergodic decomposition of directional billiard dynamics for a dense,
countable set of directions. This is a consequence of our results on the
ergodicity of \ZZ-valued cocycles over irrational rotations.Comment: 48 pages, 12 figure
Zitterbewegung and semiclassical observables for the Dirac equation
In a semiclassical context we investigate the Zitterbewegung of relativistic
particles with spin 1/2 moving in external fields. It is shown that the
analogue of Zitterbewegung for general observables can be removed to arbitrary
order in \hbar by projecting to dynamically almost invariant subspaces of the
quantum mechanical Hilbert space which are associated with particles and
anti-particles. This not only allows to identify observables with a
semiclassical meaning, but also to recover combined classical dynamics for the
translational and spin degrees of freedom. Finally, we discuss properties of
eigenspinors of a Dirac-Hamiltonian when these are projected to the almost
invariant subspaces, including the phenomenon of quantum ergodicity
Linear stability in billiards with potential
A general formula for the linearized Poincar\'e map of a billiard with a
potential is derived. The stability of periodic orbits is given by the trace of
a product of matrices describing the piecewise free motion between reflections
and the contributions from the reflections alone. For the case without
potential this gives well known formulas. Four billiards with potentials for
which the free motion is integrable are treated as examples: The linear
gravitational potential, the constant magnetic field, the harmonic potential,
and a billiard in a rotating frame of reference, imitating the restricted three
body problem. The linear stability of periodic orbits with period one and two
is analyzed with the help of stability diagrams, showing the essential
parameter dependence of the residue of the periodic orbits for these examples.Comment: 22 pages, LaTex, 4 Figure
Perturbation analysis of weakly discrete kinks
We present a perturbation theory of kink solutions of discrete Klein-Gordon
chains. The unperturbed solutions correspond to the kinks of the adjoint
partial differential equation. The perturbation theory is based on a
reformulation of the discrete chain problem into a partial differential
equation with spatially modulated mass density. The first order corrections to
the kink solutions are obtained analytically and are shown to agree with exact
numerical results. We discuss the problem of calculating the Peierls-Nabarro
barrier.Comment: 13 pages, 6 figures, REVTE
Quantum chaos in a deformable billiard: Applications to quantum dots
We perform a detailed numerical study of energy-level and wavefunction
statistics of a deformable quantum billiard focusing on properties relevant to
semiconductor quantum dots. We consider the family of Robnik billiards
generated by simple conformal maps of the unit disk; the shape of this family
of billiards may be varied continuously at fixed area by tuning the parameters
of the map. The classical dynamics of these billiards is well-understood and
this allows us to study the quantum properties of subfamilies which span the
transition from integrability to chaos as well as families at approximately
constant degree of chaoticity (Kolmogorov entropy). In the regime of hard chaos
we find that the statistical properties of interest are well-described by
random-matrix theory and completely insensitive to the particular shape of the
dot. However in the nearly-integrable regime non-universal behavior is found.
Specifically, the level-width distribution is well-described by the predicted
distribution both in the presence and absence of magnetic flux when
the system is fully chaotic; however it departs substantially from this
behavior in the mixed regime. The chaotic behavior corroborates the previously
predicted behavior of the peak-height distribution for deformed quantum dots.
We also investigate the energy-level correlation functions which are found to
agree well with the behavior calculated for quasi-zero-dimensional disordered
systems.Comment: 25 pages (revtex 3.0). 16 figures are available by mail or fax upon
request at [email protected]
Visual Explorations of Dynamics: the Standard Map
The Macintosh application \textit{StdMap} allows easy exploration of many of
the phenomena of area-preserving mappings. This tutorial explains some of these
phenomena and presents a number of simple experiments centered on the use of
this program.Comment: Corrections in a couple of equations, and updated to the latest
version of StdMa