209 research outputs found
Influence of chopped laser light onto the electronic transport through atomic-sized contacts
This article reports on the influence of laser irradiation onto the
electrical conductance of gold nanocontacts established with the mechanically
controllable breakjunction technique (MCB). We concentrate here on the study of
reversible conductance changes which can be as high as 200%. We investigate the
dependence on the initial conductance of the contacts, the wavelength, the
intensity and position of the laser spot with respect to the sample. Under most
conditions an enhancement of the conductance is observed. We discuss several
physical mechanisms which might contribute to the observed effect including
thermal expansion, rectification and photon-assisted transport. We conclude
that thermal expansion is not the dominating one.Comment: 20 pages with 7 figures; conference contribution on the 9th near
field optics conference 2006 in Lausanne, Switzerland; accepted by the
Journal of Microscop
Character Expansions for the Orthogonal and Symplectic Groups
Formulas for the expansion of arbitrary invariant group functions in terms of
the characters for the Sp(2N), SO(2N+1), and SO(2N) groups are derived using a
combinatorial method. The method is similar to one used by Balantekin to expand
group functions over the characters of the U(N) group. All three expansions
have been checked for all N by using them to calculate the known expansions of
the generating function of the homogeneous symmetric functions. An expansion of
the exponential of the traces of group elements, appearing in the finite-volume
gauge field partition functions, is worked out for the orthogonal and
symplectic groups.Comment: 20 pages, in REVTE
Derivation of determinantal structures for random matrix ensembles in a new way
There are several methods to treat ensembles of random matrices in symmetric
spaces, circular matrices, chiral matrices and others. Orthogonal polynomials
and the supersymmetry method are particular powerful techniques. Here, we
present a new approach to calculate averages over ratios of characteristic
polynomials. At first sight paradoxically, one can coin our approach
"supersymmetry without supersymmetry" because we use structures from
supersymmetry without actually mapping onto superspaces. We address two kinds
of integrals which cover a wide range of applications for random matrix
ensembles. For probability densities factorizing in the eigenvalues we find
determinantal structures in a unifying way. As a new application we derive an
expression for the k-point correlation function of an arbitrary rotation
invariant probability density over the Hermitian matrices in the presence of an
external field.Comment: 36 pages; 2 table
Analyzing symmetry breaking within a chaotic quantum system via Bayesian inference
Bayesian inference is applied to the level fluctuations of two coupled
microwave billiards in order to extract the coupling strength. The coupled
resonators provide a model of a chaotic quantum system containing two coupled
symmetry classes of levels. The number variance is used to quantify the level
fluctuations as a function of the coupling and to construct the conditional
probability distribution of the data. The prior distribution of the coupling
parameter is obtained from an invariance argument on the entropy of the
posterior distribution.Comment: Example from chaotic dynamics. 8 pages, 7 figures. Submitted to PR
Parity Effects in Eigenvalue Correlators, Parametric and Crossover Correlators in Random Matrix Models: Application to Mesoscopic systems
This paper summarizes some work I've been doing on eigenvalue correlators of
Random Matrix Models which show some interesting behaviour. First we consider
matrix models with gaps in there spectrum or density of eigenvalues. The
density-density correlators of these models depend on whether N, where N is the
size of the matrix, takes even or odd values. The fact that this dependence
persists in the large N thermodynamic limit is an unusual property and may have
consequences in the study of one electron effects in mesoscopic systems.
Secondly, we study the parametric and cross correlators of the Harish
Chandra-Itzykson-Zuber matrix model. The analytic expressions determine how the
correlators change as a parameter (e.g. the strength of a perturbation in the
hamiltonian of the chaotic system or external magnetic field on a sample of
material) is varied. The results are relevant for the conductance fluctuations
in disordered mesoscopic systems.Comment: 12 pages, Latex, 2 Figure
Quantum Chaos in the Yang-Mills-Higgs System at Finite Temperature
The quantum chaos in the finite-temperature Yang-Mills-Higgs system is
studied. The energy spectrum of a spatially homogeneous SU(2) Yang-Mills-Higgs
is calculated within thermofield dynamics. Level statistics of the spectra is
studied by plotting nearest-level spacing distribution histograms. It is found
that finite temperature effects lead to a strengthening of chaotic effects,
i.e. spectrum which has Poissonian distribution at zero temperature has
Gaussian distribution at finite-temperature.Comment: 6 pages, 5 figures, Revte
Distribution of the local density of states, reflection coefficient and Wigner delay time in absorbing ergodic systems at the point of chiral symmetry
Employing the chiral Unitary Ensemble of random matrices we calculate the
probability distribution of the local density of states for zero-dimensional
("quantum chaotic") two-sublattice systems at the point of chiral symmetry E=0
and in the presence of uniform absorption. The obtained result can be used to
find the distributions of the reflection coefficent and of the Wigner time
delay for such systems.Comment: 4 pages, 3 figure
Effect of resonances on the transport properties of two-dimensional disordered systems
We study both analytically and numerically how the electronic structure and
the transport properties of a two-dimensional disordered system are modified in
the presence of resonances. The energy dependence of the density of states and
the localization length at different resonance energies and strengths of
coupling between resonances and random states are determined. The results show,
that at energy equals to the resonance energy there is an enhancement in the
density of states. In contrast, the localization length remains unaffected from
the presence of the resonances and is similar to the one of the standard
Anderson model. Finally, we calculate the diffusion constant as a function of
energy and we reveal interesting analogies with experimental results on light
scattering in the presence of Mie resonances.Comment: 4 pages, 4 figures, accepted in Phys. Rev. B (2000
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