Localization in fractal and multifractal media

The propagation of waves in highly inhomogeneous media is a problem of interest in multiple fields including seismology, acoustics and electromagnetism. It is also relevant for technological applications such as the design of sound absorbing materials or the fabrication of optically devices for multi-wavelength operation. A paradigmatic example of a highly inhomogeneous media is one in which the density or stiffness has fractal or multifractal properties. We investigate wave propagation in one dimensional media with these features. We have found that, for weak disorder, localization effects do not arrest wave propagation provided that the box fractal dimension D of the density profile is D < 3/2. This result holds for both fractal and multifractal media providing thus a simple universal characterization for the existence of localization in these systems. Moreover we show that our model verifies the scaling theory of localization and discuss practical applications of our results.Comment: 4 pages, 5 figure

Long-range spin-pairing order and spin defects in quantum spin-1/2 ladders

For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range spin-pairing order can be identified which enables the separation of the space spanned by finite-range (covalent) valence-bond configurations into w+1 subspaces. Since every subspace has an equivalent counter subspace connected by translational symmetry, twofold degeneracy, breaking traslational symmetry is found except for the subspace where the ground state of w=even belongs to. In terms of energy ordering, (non)degeneracy and the discontinuities introduced in the long-range spin-pairing order by topological spin defects, the differences between even and odd ladders are explained in a general and systematic way.Comment: 16 pages, 7 figures, 2 tables. To be publish in The European Physical J.

Neutrino Masses and Mixing: Where We Stand and Where We are Going

In this talk I review our present knowledge on neutrino masses and mixing as well as the expectations from near future experiments.Comment: 19 Pages, 11 figures. Review talk given at the 10th International Conference on Supersymmetry and Unification of Fundamental Interactions, SUSY02 (June 17-23, 2002, DESY, Hamburg

Classical intermittency and quantum Anderson transition

We investigate the quantum properties of 1D quantum systems whose classical counterpart presents intermittency. The spectral correlations are expressed in terms of the eigenvalues of an anomalous diffusion operator by using recent semiclassical techniques. For certain values of the parameters the spectral properties of our model show similarities with those of a disordered system at the Anderson transition. In Hamiltonian systems, intermittency is closely related to the presence of cantori in the classical phase space. We suggest, based on this relation, that our findings may be relevant for the description of the spectral correlations of (non-KAM) Hamiltonians with a classical phase space filled by cantori. Finally we discuss the extension of our results to higher dimensions and their relation to Anderson models with long range hopping.Comment: 4 pages, typos corrected, references adde

Eigenvalues and eigenfunctions of the anharmonic oscillator V(x,y)=x2y2V(x,y)=x^{2}y^{2}

We obtain sufficiently accurate eigenvalues and eigenfunctions for the anharmonic oscillator with potential $V(x,y)=x^{2}y^{2}$ by means of three different methods. Our results strongly suggest that the spectrum of this oscillator is discrete in agreement with early rigorous mathematical proofs and against a recent statement that cast doubts about it