5,989 research outputs found
Heterogeneity and Disorder: Contributions of Rolf Landauer
Rolf Landauer made important contributions to many branches of science.
Within the broad area of transport in disordered media, he wrote seminal papers
on electrical conduction in macroscopically inhomogeneous materials, as well as
fundamental analyses of electron transport in quantum mechanical systems with
disorder on the atomic scale. We review here some of these contributions. We
also briefly describe some main events in his personal and scientific life.Comment: 10 pages, 3 figures; presented on the occasion when Rolf Landauer was
awarded, posthumously, the inaugural ETOPIM Medal at the ETOPIM 8 Conference,
which took place during 7--12 June, 2009 in Rethymnon, Cret
Universal conductance reduction in a quantum wire
Even a single point defect in a quantum wire causes a conductance reduction.
In this paper we prove (without any approximations) that for any point impurity
this conductance reduction in all the sub-bands is exactly 2e^2/h. Moreover, it
is shown that in the case of a surface defect, not only is the conductance
minimum independent of the defect characteristics, but the transmission matrix
also converges to universal (defect-independent) values. We also discuss
particle confinement between two arbitrarily weak point defects.Comment: 4 pages, 4 figures (Revtex
TV synchronization system features stability and noise immunity
Horizontal jitter in the video presentation in television systems is prevented by using an additional sync level. This circuitry uses simultaneous signals at both sync and porch frequencies, providing a sync identification from which a coincidence circuit can generate pulses having the required stability and noise immunity
Means for generating a sync signal in an FM communication system Patent
Circuitry for generating sync signals in FM communication systems including video informatio
Exploring the operation of a tiny heat engine
We model a tiny heat engine as a Brownian particle that moves in a viscous
medium in a sawtooth potential (with or without load) assisted by placed hot and cold heat baths along its path. We find closed
form expression for the steady state current as a function of the model
parameters. This enables us to deal with the energetics of the model and
evaluate either its efficiency or its coefficient of performance depending upon
whether the model functions either as a heat engine or as a refrigerator,
respectively. We also study the way current changes with changes in parameters
of interest. When we plot the phase diagrams showing the way the model
operates, we not only find regions where it functions as a heat engine and as a
refrigerator but we also identify a region where the model functions as neither
of the two.Comment: 8 pages and 13 figure
Conductance and polarization in quantum junctions
We revisit the expression for the conductance of a general nanostructure -- such as a quantum point contact -- as obtained from the linear response theory. We show that the conductance represents the strength of the Drude singularity in the conductivity . Using the equation of continuity for electric charge we obtain a formula for conductance in terms of polarization of the system. This identification can be used for direct calculation of the conductance for systems of interest even at the {\it ab-initio} level. In particular, we show that one can evaluate the conductance from calculations for a finite system without the need for special ``transport'' boundary conditions
Energy-Entanglement Relation for Quantum Energy Teleportation
Protocols of quantum energy teleportation (QET), while retaining causality
and local energy conservation, enable the transportation of energy from a
subsystem of a many-body quantum system to a distant subsystem by local
operations and classical communication through ground-state entanglement. We
prove two energy-entanglement inequalities for a minimal QET model. These
relations help us to gain a profound understanding of entanglement itself as a
physical resource by relating entanglement to energy as an evident physical
resource.Comment: 16 pages, 1 figure, to appear in Physics Letter
The Landauer Resistance and Band Spectra for the Counting Quantum Turing Machine
The generalized counting quantum Turing machine (GCQTM) is a machine which,
for any N, enumerates the first integers in succession as binary
strings. The generalization consists of associating a potential with read-1
steps only. The Landauer Resistance (LR) and band spectra were determined for
the tight binding Hamiltonians associated with the GCQTM for energies both
above and below the potential height. For parameters and potentials in the
electron region, the LR fluctuates rapidly between very high and very low
values as a function of momentum. The rapidity and extent of the fluctuations
increases rapidly with increasing N. For N=18, the largest value considered,
the LR shows good transmission probability as a function of momentum with
numerous holes of very high LR values present. This is true for energies above
and below the potential height. It is suggested that the main features of the
LR can be explained by coherent superposition of the component waves reflected
from or transmitted through the potentials in the distribution. If
this explanation is correct, it provides a dramatic illustration of the effects
of quantum nonlocality.Comment: 19 pages Latex, elsart.sty file included, 12 postscript figures,
Submitted to PhysComp96 for publication in Physica-
Escaping the Trap of too Precise Topic Queries
At the very center of digital mathematics libraries lie controlled
vocabularies which qualify the {\it topic} of the documents. These topics are
used when submitting a document to a digital mathematics library and to perform
searches in a library. The latter are refined by the use of these topics as
they allow a precise classification of the mathematics area this document
addresses. However, there is a major risk that users employ too precise topics
to specify their queries: they may be employing a topic that is only "close-by"
but missing to match the right resource. We call this the {\it topic trap}.
Indeed, since 2009, this issue has appeared frequently on the i2geo.net
platform. Other mathematics portals experience the same phenomenon. An approach
to solve this issue is to introduce tolerance in the way queries are understood
by the user. In particular, the approach of including fuzzy matches but this
introduces noise which may prevent the user of understanding the function of
the search engine.
In this paper, we propose a way to escape the topic trap by employing the
navigation between related topics and the count of search results for each
topic. This supports the user in that search for close-by topics is a click
away from a previous search. This approach was realized with the i2geo search
engine and is described in detail where the relation of being {\it related} is
computed by employing textual analysis of the definitions of the concepts
fetched from the Wikipedia encyclopedia.Comment: 12 pages, Conference on Intelligent Computer Mathematics 2013 Bath,
U
Spectrum of Electrons in Graphene as an Alternant Macromolecule and Its Specific Features in Quantum Conductance
An exact description of electrons based on the tight-binding model of
graphene as an alternant, plane macromolecule is presented. The model molecule
can contain an arbitrary number of benzene rings and has armchair- and
zigzag-shaped edges. This suggests an instructive alternative to the most
commonly used approach, where the reference is made to the honeycomb lattice
periodic in its A and B sublattices. Several advantages of the macromolecule
model are demonstrated. The newly derived analytical relations detail our
understanding of electron nature in achiral graphene ribbons and carbon
tubes and classify these structures as quantum wires.Comment: 13 pages 8 figures, revised in line with referee's comment
- …