12,843 research outputs found
Two-Dimensional Electrons in a Strong Magnetic Field with Disorder: Divergence of the Localization Length
Electrons on a square lattice with half a flux quantum per plaquette are
considered. An effective description for the current loops is given by a
two-dimensional Dirac theory with random mass. It is shown that the
conductivity and the localization length can be calculated from a product of
Dirac Green's functions with the {\it same} frequency. This implies that the
delocalization of electrons in a magnetic field is due to a critical point in a
phase with a spontaneously broken discrete symmetry. The estimation of the
localization length is performed for a generalized model with fermion
levels using a --expansion and the Schwarz inequality. An argument for the
existence of two Hall transition points is given in terms of percolation
theory.Comment: 10 pages, RevTeX, no figure
On the mechanism of irradiation enhanced exchange bias
By means of layer resolved ion irradiation the mechanisms involved in the
irradiation driven modifications of the exchange bias effect in NiFe/FeMn
bilayers have been investigated. It is shown that not only the locations of the
defects but also the magnetic coupling between both layers during the
irradiation process is of crucial importance. This requires an extension of
current models accounting for defects in exchange bias systems.Comment: 3 pages, 3 figures, revised version, added results from further
structural characterization by TEM, submitted to Europhysics Letter
Integer Quantum Hall Effect for Lattice Fermions
A two-dimensional lattice model for non-interacting fermions in a magnetic
field with half a flux quantum per plaquette and levels per site is
considered. This is a model which exhibits the Integer Quantum Hall Effect
(IQHE) in the presence of disorder. It presents an alternative to the
continuous picture for the IQHE with Landau levels. The large limit can be
solved: two Hall transitions appear and there is an interpolating behavior
between the two Hall plateaux. Although this approach to the IQHE is different
from the traditional one with Landau levels because of different symmetries
(continuous for Landau levels and discrete here), some characteristic features
are reproduced. For instance, the slope of the Hall conductivity is infinite at
the transition points and the electronic states are delocalized only at the
transitions.Comment: 9 pages, Plain-Te
Equivalence of domains for hyperbolic Hubbard-Stratonovich transformations
We settle a long standing issue concerning the traditional derivation of
non-compact non-linear sigma models in the theory of disordered electron
systems: the hyperbolic Hubbard-Stratonovich (HS) transformation of
Pruisken-Schaefer type. Only recently the validity of such transformations was
proved in the case of U(p,q) (non-compact unitary) and O(p,q) (non-compact
orthogonal) symmetry. In this article we give a proof for general non-compact
symmetry groups. Moreover we show that the Pruisken-Schaefer type
transformations are related to other variants of the HS transformation by
deformation of the domain of integration. In particular we clarify the origin
of surprising sign factors which were recently discovered in the case of
orthogonal symmetry.Comment: 30 pages, 3 figure
Polytopality and Cartesian products of graphs
We study the question of polytopality of graphs: when is a given graph the
graph of a polytope? We first review the known necessary conditions for a graph
to be polytopal, and we provide several families of graphs which satisfy all
these conditions, but which nonetheless are not graphs of polytopes. Our main
contribution concerns the polytopality of Cartesian products of non-polytopal
graphs. On the one hand, we show that products of simple polytopes are the only
simple polytopes whose graph is a product. On the other hand, we provide a
general method to construct (non-simple) polytopal products whose factors are
not polytopal.Comment: 21 pages, 10 figure
Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation
Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, capping, and spacer layers is observed.
© 2010 American Institute of PhysicsFCT-PTDC/CTM/100756/2008program PESSOA EGIDE/GRICESFCT-SFRH/BD/45774/2008FCT-SFRH/BD/44635/200
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