2,093 research outputs found
Andreev scattering in nanoscopic junctions at high magnetic fields
We report on the measurement of multiple Andreev resonances at atomic size
point contacts between two superconducting nanostructures of Pb under magnetic
fields higher than the bulk critical field, where superconductivity is
restricted to a mesoscopic region near the contact. The small number of
conduction channels in this type of contacts permits a quantitative comparison
with theory through the whole field range. We discuss in detail the physical
properties of our structure, in which the normal bulk electrodes induce a
proximity effect into the mesoscopic superconducting part.Comment: 4 page
Space-time in light of Karolyhazy uncertainty relation
General relativity and quantum mechanics provide a natural explanation for
the existence of dark energy with its observed value and predict its dynamics.
Dark energy proves to be necessary for the existence of space-time itself and
determines the rate of its stability.Comment: 5 pages, Two misprints are correcte
Time-inconsistent Planning: Simple Motivation Is Hard to Find
With the introduction of the graph-theoretic time-inconsistent planning model
due to Kleinberg and Oren, it has been possible to investigate the
computational complexity of how a task designer best can support a
present-biased agent in completing the task. In this paper, we study the
complexity of finding a choice reduction for the agent; that is, how to remove
edges and vertices from the task graph such that a present-biased agent will
remain motivated to reach his target even for a limited reward. While this
problem is NP-complete in general, this is not necessarily true for instances
which occur in practice, or for solutions which are of interest to task
designers. For instance, a task designer may desire to find the best task graph
which is not too complicated.
We therefore investigate the problem of finding simple motivating subgraphs.
These are structures where the agent will modify his plan at most times
along the way. We quantify this simplicity in the time-inconsistency model as a
structural parameter: The number of branching vertices (vertices with
out-degree at least ) in a minimal motivating subgraph.
Our results are as follows: We give a linear algorithm for finding an optimal
motivating path, i.e. when . On the negative side, we show that finding a
simple motivating subgraph is NP-complete even if we allow only a single
branching vertex --- revealing that simple motivating subgraphs are indeed hard
to find. However, we give a pseudo-polynomial algorithm for the case when
is fixed and edge weights are rationals, which might be a reasonable assumption
in practice.Comment: An extended abstract of this paper is accepted at AAAI 202
Influence of the confinement geometry on surface superconductivity
The nucleation field for surface superconductivity, , depends on the
geometrical shape of the mesoscopic superconducting sample and is substantially
enhanced with decreasing sample size. As an example we studied circular,
square, triangular and wedge shaped disks. For the wedge the nucleation field
diverges as with decreasing angle () of
the wedge, where is the bulk upper critical field.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev.
Electron locking in semiconductor superlattices
We describe a novel state of electrons and phonons arising in semiconductor
superlattices (SSL) due to strong electron-phonon interactions. These states
are characterized by a localization of phonons and a self-trapping or locking
of electrons in one or several quantum wells due to an additional,
deformational potential arising around these locking wells in SSL. The effect
is enhanced in a longitudinal magnetic field.
Using the tight-binding and adiabatic approximations the whole energy
spectrum of the self-trapped states is found and accurate, analytic expressions
are included for strong electron-phonon coupling. Finally, we discuss possible
experiments which may detect these predicted self-trapped states.Comment: 8 pages, 2 figures. Please note that the published article has the
title 'Electron locking in layered structures by a longitudinal magnetic
field
Characteristic Potentials for Mesoscopic Rings Threaded by an Aharonov-Bohm Flux
Electro-static potentials for samples with the topology of a ring and
penetrated by an Aharonov-Bohm flux are discussed. The sensitivity of the
electron-density distribution to small variations in the flux generates an
effective electro-static potential which is itself a periodic function of flux.
We investigate a simple model in which the flux sensitive potential leads to a
persistent current which is enhanced compared to that of a loop of
non-interacting electrons. For sample geometries with contacts the sensitivity
of the electro-static potential to flux leads to a flux-induced capacitance.
This capacitance gives the variation in charge due to an increment in flux. The
flux-induced capacitance is contrasted with the electro-chemical capacitance
which gives the variation in charge due to an increment in an electro-chemical
potential. The discussion is formulated in terms of characteristic functions
which give the variation of the electro-static potential in the interior of the
conductor due to an increment in the external control parameters (flux,
electro-chemical potentials). Paper submitted to the 16th Nordic Semiconductor
Meeting, Laugarvatan, Iceland, June 12-15, 1994. The proceedings will be
published in Physica Scripta.Comment: 23 pages + 4 figures, revtex, IBM-RC1955
Spanning directed trees with many leaves
The {\sc Directed Maximum Leaf Out-Branching} problem is to find an
out-branching (i.e. a rooted oriented spanning tree) in a given digraph with
the maximum number of leaves. In this paper, we obtain two combinatorial
results on the number of leaves in out-branchings. We show that
- every strongly connected -vertex digraph with minimum in-degree at
least 3 has an out-branching with at least leaves;
- if a strongly connected digraph does not contain an out-branching with
leaves, then the pathwidth of its underlying graph UG() is .
Moreover, if the digraph is acyclic, the pathwidth is at most .
The last result implies that it can be decided in time whether a strongly connected digraph on vertices has an
out-branching with at least leaves. On acyclic digraphs the running time of
our algorithm is
Similarity reduction of the modified Yajima-Oikawa equation
We study a similarity reduction of the modified Yajima-Oikawa hierarchy. The
hierarchy is associated with a non-standard Heisenberg subalgebra in the affine
Lie algebra of type A_2^{(1)}. The system of equations for self-similar
solutions is presented as a Hamiltonian system of degree of freedom two, and
admits a group of B\"acklund transformations isomorphic to the affine Weyl
group of type A_2^{(1)}. We show that the system is equivalent to a
two-parameter family of the fifth Painlev\'e equation.Comment: latex2e file, 18 pages, no figures; (v2)Introduction is modified.
Some typos are correcte
On the angular distribution of extensive air showers
Angular distributions of extensive air showers with different number of
charged particles in the range 2.5x10^5--4x10^7 are derived using the
experimental data obtained with the EAS MSU array. Possible approximations of
the obtained distributions with different empiric functions available in
literature, are analysed. It is shown that the exponential function provides
the best approximation of the angular distributions in the sense of the
chi-squared criterion.Comment: 5 pages including 1 figur
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