18,311 research outputs found
Suppression of weak-localization (and enhancement of noise) by tunnelling in semiclassical chaotic transport
We add simple tunnelling effects and ray-splitting into the recent
trajectory-based semiclassical theory of quantum chaotic transport. We use this
to derive the weak-localization correction to conductance and the shot-noise
for a quantum chaotic cavity (billiard) coupled to leads via
tunnel-barriers. We derive results for arbitrary tunnelling rates and arbitrary
(positive) Ehrenfest time, . For all Ehrenfest times, we show
that the shot-noise is enhanced by the tunnelling, while the weak-localization
is suppressed. In the opaque barrier limit (small tunnelling rates with large
lead widths, such that Drude conductance remains finite), the weak-localization
goes to zero linearly with the tunnelling rate, while the Fano factor of the
shot-noise remains finite but becomes independent of the Ehrenfest time. The
crossover from RMT behaviour () to classical behaviour
() goes exponentially with the ratio of the Ehrenfest time
to the paired-paths survival time. The paired-paths survival time varies
between the dwell time (in the transparent barrier limit) and half the dwell
time (in the opaque barrier limit). Finally our method enables us to see the
physical origin of the suppression of weak-localization; it is due to the fact
that tunnel-barriers ``smear'' the coherent-backscattering peak over reflection
and transmission modes.Comment: 20 pages (version3: fixed error in sect. VC - results unchanged) -
Contents: Tunnelling in semiclassics (3pages), Weak-localization (5pages),
Shot-noise (5pages
Density-functional investigation of rhombohedral stacks of graphene: topological surface states, nonlinear dielectric response, and bulk limit
A DFT-based investigation of rhombohedral (ABC)-type graphene stacks in
finite static electric fields is presented. Electronic band structures and
field-induced charge densities are compared with related literature data as
well as with own results on (AB) stacks. It is found, that the undoped
AB-bilayer has a tiny Fermi line consisting of one electron pocket around the
K-point and one hole pocket on the line K-. In contrast to (AB) stacks,
the breaking of translational symmetry by the surface of finite (ABC) stacks
produces a gap in the bulk-like states for slabs up to a yet unknown critical
thickness , while ideal (ABC) bulk (-graphite)
is a semi-metal. Unlike in (AB) stacks, the ground state of (ABC) stacks is
shown to be topologically non-trivial in the absence of external electric
field. Consequently, surface states crossing the Fermi level must unavoidably
exist in the case of (ABC)-type stacking, which is not the case in (AB)-type
stacks. These surface states in conjunction with the mentioned gap in the
bulk-like states have two major implications. First, electronic transport
parallel to the slab is confined to a surface region up to the critical layer
number . Related implications are expected for stacking domain
walls and grain boundaries. Second, the electronic properties of (ABC) stacks
are highly tunable by an external electric field. In particular, the dielectric
response is found to be strongly nonlinear and can e.g. be used to discriminate
slabs with different layer numbers. Thus, (ABC) stacks rather than (AB) stacks
with more than two layers should be of potential interest for applications
relying on the tunability by an electric field.Comment: 36 pages, 17 figure
High-Order Coupled Cluster Calculations Via Parallel Processing: An Illustration For CaVO
The coupled cluster method (CCM) is a method of quantum many-body theory that
may provide accurate results for the ground-state properties of lattice quantum
spin systems even in the presence of strong frustration and for lattices of
arbitrary spatial dimensionality. Here we present a significant extension of
the method by introducing a new approach that allows an efficient
parallelization of computer codes that carry out ``high-order'' CCM
calculations. We find that we are able to extend such CCM calculations by an
order of magnitude higher than ever before utilized in a high-order CCM
calculation for an antiferromagnet. Furthermore, we use only a relatively
modest number of processors, namely, eight. Such very high-order CCM
calculations are possible {\it only} by using such a parallelized approach. An
illustration of the new approach is presented for the ground-state properties
of a highly frustrated two-dimensional magnetic material, CaVO. Our
best results for the ground-state energy and sublattice magnetization for the
pure nearest-neighbor model are given by and ,
respectively, and we predict that there is no N\'eel ordering in the region
. These results are shown to be in excellent agreement
with the best results of other approximate methods.Comment: 4 page
Semiclassical Approach to Orbital Magnetism of Interacting Diffusive Quantum Systems
We study interaction effects on the orbital magnetism of diffusive mesoscopic
quantum systems. By combining many-body perturbation theory with semiclassical
techniques, we show that the interaction contribution to the ensemble averaged
quantum thermodynamic potential can be reduced to an essentially classical
operator. We compute the magnetic response of disordered rings and dots for
diffusive classical dynamics. Our semiclassical approach reproduces the results
of previous diagrammatic quantum calculations.Comment: 8 pages, revtex, includes 1 postscript fi
Chaos and Interacting Electrons in Ballistic Quantum Dots
We show that the classical dynamics of independent particles can determine
the quantum properties of interacting electrons in the ballistic regime. This
connection is established using diagrammatic perturbation theory and
semiclassical finite-temperature Green functions. Specifically, the orbital
magnetism is greatly enhanced over the Landau susceptibility by the combined
effects of interactions and finite size. The presence of families of periodic
orbits in regular systems makes their susceptibility parametrically larger than
that of chaotic systems, a difference which emerges from correlation terms.Comment: 4 pages, revtex, includes 3 postscript fig
The Walnut, California, earthquakes of July-August, 1959
A swarm of minor earthquakes began on July 29, 1959, near 34° 00′ N, 117° 48′ W. Records at Pasadena show P and S waves reflected from the Moho. A portable instrument recorded some of these at a point about 6 km. from the epicenter. The characteristic false S - P of about one second at short distances was recorded
The density of states of chaotic Andreev billiards
Quantum cavities or dots have markedly different properties depending on
whether their classical counterparts are chaotic or not. Connecting a
superconductor to such a cavity leads to notable proximity effects,
particularly the appearance, predicted by random matrix theory, of a hard gap
in the excitation spectrum of quantum chaotic systems. Andreev billiards are
interesting examples of such structures built with superconductors connected to
a ballistic normal metal billiard since each time an electron hits the
superconducting part it is retroreflected as a hole (and vice-versa). Using a
semiclassical framework for systems with chaotic dynamics, we show how this
reflection, along with the interference due to subtle correlations between the
classical paths of electrons and holes inside the system, are ultimately
responsible for the gap formation. The treatment can be extended to include the
effects of a symmetry breaking magnetic field in the normal part of the
billiard or an Andreev billiard connected to two phase shifted superconductors.
Therefore we are able to see how these effects can remold and eventually
suppress the gap. Furthermore the semiclassical framework is able to cover the
effect of a finite Ehrenfest time which also causes the gap to shrink. However
for intermediate values this leads to the appearance of a second hard gap - a
clear signature of the Ehrenfest time.Comment: Refereed version. 23 pages, 19 figure
Unilateral and bilateral corticotomies for correction of maxillary transverse discrepancies
Surgically-assisted rapid maxillary expansion in adults has been proved effective in overcoming the strong resistance of the maxillary complex after growth is completed, particularly after the second decade of life. The aim of this study was to describe the dental and the skeletal expansion and relapse, as well as the amount of tipping of the two maxillary bones and first permanent molars, during a rapid maxillary expansion procedure combined with unilateral and bilateral corticotomies. The sample consisted of four adult patients, two presenting with bilateral and two with unilateral cross-bite. Records were taken before and after rapid maxillary expansion, at the end of retention and at least 12 months post-retention. In the cases of bilateral cross-bite the same amount of skeletal expansion was observed on both sides. The angular changes measured at the upper first molars indicated important tipping on both sides, which tended to relapse moderately during the retention and post-retention period. Following unilateral surgery, the operated side showed more than twice the amount of skeletal expansion than the non-operated side. The angular changes presented twice as much tipping and relapse on the operated side. The results of this study demonstrate that unilateral cross-bites in adults can be corrected with unilateral corticotomy and rapid maxillary expansion using the contralateral non-operated side as anchorage. Stability appeared satisfactory in all case
Universality in chaotic quantum transport: The concordance between random matrix and semiclassical theories
Electronic transport through chaotic quantum dots exhibits universal, system
independent, properties, consistent with random matrix theory. The quantum
transport can also be rooted, via the semiclassical approximation, in sums over
the classical scattering trajectories. Correlations between such trajectories
can be organized diagrammatically and have been shown to yield universal
answers for some observables. Here, we develop the general combinatorial
treatment of the semiclassical diagrams, through a connection to factorizations
of permutations. We show agreement between the semiclassical and random matrix
approaches to the moments of the transmission eigenvalues. The result is valid
for all moments to all orders of the expansion in inverse channel number for
all three main symmetry classes (with and without time reversal symmetry and
spin-orbit interaction) and extends to nonlinear statistics. This finally
explains the applicability of random matrix theory to chaotic quantum transport
in terms of the underlying dynamics as well as providing semiclassical access
to the probability density of the transmission eigenvalues.Comment: Refereed version. 5 pages, 4 figure
Resistance Breeding in Apple at Dresden-Pillnitz
Resistance breeding in apple has a long tradition at the Institute of Fruit Breeding now Julius Kuehn-institute in Dresden-Pillnitz. The breeding was aimed at the production of multiple resistance cultivars to allow a more sustainable and environmentally friendly production of apple. In the last decades a series of resistant cultivars (Re®-cultivars) bred in Dresden-Pillnitz has been released, ‘Recolor’ and ‘Rekarda’ in 2006. The main topic in the resistance breeding programme was scab resistance and the donor of scab resistance in most cultivars was Malus x floribunda 821. Due to the development of strains that are able to overcome resistance genes inherited by M. x floribunda 821 and due to the fact that single resistance genes can be broken easily, pyramiding of resistance genes is necessary. Besides scab, fire blight and powdery mildew are the main disease for which a pyramiding of genes is aspired in Pillnitz. Biotechnical approaches are necessary for the early detection of pyramided resistance genes in breeding clones. This paper will give an overview of the resistance breeding of apple in Pillnitz and the methods used
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