1,268 research outputs found

### The generic task toolset: High level languages for the construction of planning and problem solving systems

The current generation of languages for the construction of knowledge-based systems as being at too low a level of abstraction is criticized, and the need for higher level languages for building problem solving systems is advanced. A notion of generic information processing tasks in knowledge-based problem solving is introduced. A toolset which can be used to build expert systems in a way that enhances intelligibility and productivity in knowledge acquistion and system construction is described. The power of these ideas is illustrated by paying special attention to a high level language called DSPL. A description is given of how it was used in the construction of a system called MPA, which assists with planning in the domain of offensive counter air missions

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### A Critical Point for Science?

Lecture to Cambridge University Physics Society, March 5th., 2008.Slides accompanying the lecture 'A Critical Point for Science?' at http://sms.cam.ac.uk/media/749894 (which should be opened in a separate window). A web search should be able to identify the references listed in the last slide

### Coupled superconductors and beyond

This paper describes the events leading to the discovery of coupled
superconductors, the author's move in the 1970s to a perspective where mind
plays a role comparable to matter, and the remarkable hostility sometimes
encountered by those who venture into unconventional areas.Comment: Invited paper for special issue of Low Temperature Physics/Fizika
Nizkikh Temperatur devoted to "Quantum Coherent Effects in Superconductors
and New Materials". 6pp. v5: open-access published versio

### Localization of Gauge Fields and Monopole Tunnelling

We study the dynamical localization of a massless gauge field on a
lower-dimensional surface (2-brane). In flat space, the necessary and
sufficient condition for this phenomenon is the existence of confinement in the
bulk. The resulting configuration is equivalent to a dual Josephson junction.
This duality leads to an interesting puzzle, as it implies that a localized
massless theory, even in the Abelian case, must become confining at
exponentially large distances. Through the use of topological arguments we
clarify the physics behind this large-distance confinement and identify the
instantons of the brane world-volume theory that are responsible for its
appearance. We show that they correspond to the (condensed) bulk magnetic
charges (monopoles), that occasionally tunnel through the brane and induce weak
confinement of the brane theory. We consider the possible generalization of
this effect to higher dimensions and discuss phenomenological bounds on the
confinement of electric charges at exponentially large distances within our
Universe.Comment: 11 pages, 3 figures, improvements in the presentation, version to
appear in Physical Review

### Oscillatory decay of a two-component Bose-Einstein condensate

We study the decay of a two-component Bose-Einstein condensate with negative
effective interaction energy. With a decreasing atom number due to losses, the
atom-atom interaction becomes less important and the system undergoes a
transition from a bistable Josephson regime to the monostable Rabi regime,
displaying oscillations in phase and number. We study the equations of motion
and derive an analytical expression for the oscillation amplitude. A quantum
trajectory simulation reveals that the classical description fails for low
emission rates, as expected from analytical considerations. Observation of the
proposed effect will provide evidence for negative effective interaction.Comment: 4 pages, 3 figue

### Universal point contact resistance between thin-film superconductors

A system comprising two superconducting thin films connected by a point
contact is considered. The contact resistance is calculated as a function of
temperature and film geometry, and is found to vanish rapidly with temperature,
according to a universal, nearly activated form, becoming strictly zero only at
zero temperature. At the lowest temperatures, the activation barrier is set
primarily by the superfluid stiffness in the films, and displays only a weak
(i.e., logarithmic) temperature dependence. The Josephson effect is thus
destroyed, albeit only weakly, as a consequence of the power-law-correlated
superconducting fluctuations present in the films below the
Berezinskii-Kosterlitz-Thouless transition temperature. The behavior of the
resistance is discussed, both in various limiting regimes and as it crosses
over between these regimes. Details are presented of a minimal model of the
films and the contact, and of the calculation of the resistance. A formulation
in terms of quantum phase-slip events is employed, which is natural and
effective in the limit of a good contact. However, it is also shown to be
effective even when the contact is poor and is, indeed, indispensable, as the
system always behaves as if it were in the good-contact limit at low enough
temperature. A simple mechanical analogy is introduced to provide some
heuristic understanding of the nearly-activated temperature dependence of the
resistance. Prospects for experimental tests of the predicted behavior are
discussed, and numerical estimates relevant to anticipated experimental
settings are provided.Comment: 29 pages (single column format), 7 figure

### Phase Modulated Thermal Conductance of Josephson Weak Links

We present a theory for quasiparticle heat transport through superconducting
weak links. The thermal conductance depends on the phase difference ($\phi$) of
the superconducting leads. Branch conversion processes, low-energy Andreev
bound states near the contact and the suppression of the local density of
states near the gap edge are related to phase-sensitive transport processes.
Theoretical results for the influence of junction transparency, temperature and
disorder, on the phase modulation of the conductance are reported. For
high-transmission weak links, $D\to 1$, the formation of an Andreev bound state
at $\epsilon_{\text{\tiny b}}=\Delta\cos(\phi/2)$ leads to suppression of the
density of states for the continuum excitations that transport heat, and thus,
to a reduction in the conductance for $\phi\simeq\pi$. For low-transmission
($D\ll 1$) barriers resonant scattering at energies
$\epsilon\simeq(1+D/2)\Delta$ leads to an increase in the thermal conductance
as $T$ drops below $T_c$ (for phase differences near $\phi=\pi$).Comment: 4 pages, 3 figures Expanded discussion of boundary conditions for
Ricatti amplitude

### Macroscopic Symmetry Group Describes Josephson Tunneling in Twinned Crystals

A macroscopic symmetry group describing the superconducting state of an
orthorhombically twinned crystal of YBCO is introduced. This macroscopic
symmetry group is different for different symmetries of twin boundaries.
Josephson tunneling experiments performed on twinned crystals of YBCO determine
this macroscopic symmetry group and hence determine the twin boundary symmetry
(but do not experimentally determine whether the microscopic order parameter is
primarily d- or s-wave). A consequence of the odd-symmetry twin boundaries in
YBCO is the stability of vortices containing one half an elementary flux
quantum at the intersection of a twin boundary and certain grain boundaries.Comment: 6 pages, to be published in the Proceedings of the MOS96 Conference
in the Journal of Low Temperature Physic

### Probing Pseudogap by Josephson Tunneling

We propose here an experiment aimed to determine whether there are
superconducting pairing fluctuations in the pseudogap regime of the high-$T_c$
materials. In the experimental setup, two samples above $T_c$ are brought into
contact at a single point and the differential AC conductivity in the presence
of a constant applied bias voltage between the samples, $V$, should be
measured. We argue the the pairing fluctuations will produce randomly
fluctuating Josephson current with zero mean, however the current-current
correlator will have a characteristic frequency given by Josephson frequency
$\omega_J = 2 e V /\hbar$. We predict that the differential AC conductivity
should have a peak at the Josephson frequency with the width determined by the
phase fluctuations time.Comment: 4 pages, 2 eps figure

### Pion Propagation near the QCD Chiral Phase Transition

We point out that, in analogy with spin waves in antiferromagnets, all
parameters describing the real-time propagation of soft pions at temperatures
below the QCD chiral phase transition can be expressed in terms of static
correlators. This allows, in principle, the determination of the soft pion
dispersion relation on the lattice. Using scaling and universality arguments,
we determine the critical behavior of the parameters of pion propagation. We
predict that when the critical temperature is approached from below, the pole
mass of the pion drops despite the growth of the pion screening mass. This fact
is attributed to the decrease of the pion velocity near the phase transition.Comment: 8 pages (single column), RevTeX; added references, version to be
published in PR

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