27,013 research outputs found
Wave packet approach to transport in mesoscopic systems
Wave packets provide a well established and versatile tool for studying
time-dependent effects in molecular physics. Here, we demonstrate the
application of wave packets to mesoscopic nanodevices at low temperatures. The
electronic transport in the devices is expressed in terms of scattering and
transmission coefficients, which are efficiently obtained by solving an initial
value problem (IVP) using the time-dependent Schroedinger equation. The
formulation as an IVP makes non-trivial device topologies accessible and by
tuning the wave packet parameters one can extract the scattering properties for
a large range of energies.Comment: 12 pages, 4 figure
Numerical study on Anderson transitions in three-dimensional disordered systems in random magnetic fields
The Anderson transitions in a random magnetic field in three dimensions are
investigated numerically. The critical behavior near the transition point is
analyzed in detail by means of the transfer matrix method with high accuracy
for systems both with and without an additional random scalar potential. We
find the critical exponent for the localization length to be with a strong random scalar potential. Without it, the exponent is
smaller but increases with the system sizes and extrapolates to the above value
within the error bars. These results support the conventional classification of
universality classes due to symmetry. Fractal dimensionality of the wave
function at the critical point is also estimated by the equation-of-motion
method.Comment: 9 pages, 3 figures, to appear in Annalen der Physi
Dynamics of a large-spin-boson system in the strong coupling regime
We investigate collective effects of an ensemble of biased two level systems
interacting with a bosonic bath in the strong coupling regime. The two level
systems are described by a large pseudo-spin J. An equation for the expectation
value M(t) of the z-component of the pseudo spin is derived and solved
numerically for an ohmic bath at T=0. In case of a large cut-off frequency of
the spectral function, a Markov approximation is justified and an analytical
solution is presented. We find that M(t) relaxes towards a highly correlated
state with maximum value for large times. However, this relaxation is
extremely slow for most parameter values so as if the system was "frozen in" by
interaction with the bosonic bath.Comment: 4 pages, 2 figures, to be published in proceedings of MB1
Random Network Models and Quantum Phase Transitions in Two Dimensions
An overview of the random network model invented by Chalker and Coddington,
and its generalizations, is provided. After a short introduction into the
physics of the Integer Quantum Hall Effect, which historically has been the
motivation for introducing the network model, the percolation model for
electrons in spatial dimension 2 in a strong perpendicular magnetic field and a
spatially correlated random potential is described. Based on this, the network
model is established, using the concepts of percolating probability amplitude
and tunneling. Its localization properties and its behavior at the critical
point are discussed including a short survey on the statistics of energy levels
and wave function amplitudes. Magneto-transport is reviewed with emphasis on
some new results on conductance distributions. Generalizations are performed by
establishing equivalent Hamiltonians. In particular, the significance of
mappings to the Dirac model and the two dimensional Ising model are discussed.
A description of renormalization group treatments is given. The classification
of two dimensional random systems according to their symmetries is outlined.
This provides access to the complete set of quantum phase transitions like the
thermal Hall transition and the spin quantum Hall transition in two dimension.
The supersymmetric effective field theory for the critical properties of
network models is formulated. The network model is extended to higher
dimensions including remarks on the chiral metal phase at the surface of a
multi-layer quantum Hall system.Comment: 176 pages, final version, references correcte
NNLO contributions to inclusive-jet production in DIS and determination of \alpha_s
We present the first calculation of inclusive jet production in
deep-inelastic scattering with approximate next-to-next-to-leading order
(aNNLO) contributions, obtained from a unified threshold resummation formalism.
The leading coefficients are computed analytically. We show that the aNNLO
contributions reduce the theoretical prediction for jet production in
deep-inelastic scattering, improve the description of the final HERA data in
particular at high photon virtuality Q^2 and increase the central fit value of
the strong coupling constant.Comment: 5 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1310.172
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