22,066 research outputs found
PRISM: a tool for automatic verification of probabilistic systems
Probabilistic model checking is an automatic formal verification technique for analysing quantitative properties of systems which exhibit stochastic behaviour. PRISM is a probabilistic model checking tool which has already been successfully deployed in a wide range of application domains, from real-time communication protocols to biological signalling pathways. The tool has recently undergone a significant amount of development. Major additions include facilities to manually explore models, Monte-Carlo discrete-event simulation techniques for approximate model analysis (including support for distributed simulation) and the ability to compute cost- and reward-based measures, e.g. "the expected energy consumption of the system before the first failure occurs". This paper presents an overview of all the main features of PRISM. More information can be found on the website: www.cs.bham.ac.uk/~dxp/prism
Quasiparticle mirages in the tunneling spectra of d-wave superconductors
We illustrate the importance of many-body effects in the Fourier transformed
local density of states (FT-LDOS) of d-wave superconductors from a model of
electrons coupled to an Einstein mode with energy Omega_0. For bias energies
significantly larger than Omega_0 the quasiparticles have short lifetimes due
to this coupling, and the FT-LDOS is featureless if the electron-impurity
scattering is treated within the Born approximation. In this regime it is
important to include boson exchange for the electron-impurity scattering which
provides a `step down' in energy for the electrons and allows for long
lifetimes. This many-body effect produces qualitatively different results,
namely the presence of peaks in the FT-LDOS which are mirrors of the
quasiparticle interference peaks which occur at bias energies smaller than ~
Omega_0. The experimental observation of these quasiparticle mirages would be
an important step forward in elucidating the role of many-body effects in
FT-LDOS measurements.Comment: revised text with new figures, to be published, Phys Rev
Assume-guarantee verification for probabilistic systems
We present a compositional verification technique for systems that exhibit both probabilistic and nondeterministic behaviour. We adopt an assume- guarantee approach to verification, where both the assumptions made about system components and the guarantees that they provide are regular safety properties, represented by finite automata. Unlike previous proposals for assume-guarantee reasoning about probabilistic systems, our approach does not require that components interact in a fully synchronous fashion. In addition, the compositional verification method is efficient and fully automated, based on a reduction to the problem of multi-objective probabilistic model checking. We present asymmetric and circular assume-guarantee rules, and show how they can be adapted to form quantitative queries, yielding lower and upper bounds on the actual probabilities that a property is satisfied. Our techniques have been implemented and applied to several large case studies, including instances where conventional probabilistic verification is infeasible
The Temperature Evolution of the Spectral Peak in High Temperature Superconductors
Recent photoemission data in the high temperature cuprate superconductor
Bi2212 have been interpreted in terms of a sharp spectral peak with a
temperature independent lifetime, whose weight strongly decreases upon heating.
By a detailed analysis of the data, we are able to extract the temperature
dependence of the electron self-energy, and demonstrate that this intepretation
is misleading. Rather, the spectral peak loses its integrity above Tc due to a
large reduction in the electron lifetime.Comment: 5 pages, revtex, 4 encapsulated postscript figure
Photon Conserving Radiative Transfer around Point Sources in multi-dimensional Numerical Cosmology
Many questions in physical cosmology regarding the thermal and ionization
history of the intergalactic medium are now successfully studied with the help
of cosmological hydrodynamical simulations. Here we present a numerical method
that solves the radiative transfer around point sources within a three
dimensional cartesian grid. The method is energy conserving independently of
resolution: this ensures the correct propagation speeds of ionization fronts.
We describe the details of the algorithm, and compute as first numerical
application the ionized region surrounding a mini-quasar in a cosmological
density field at z=7.Comment: 5 pages, 4 figures, submitted to ApJ
Technique for producing wind-tunnel heat-transfer models
Inexpensive thin skinned wind tunnel models with thermocouples on certain surface areas were fabricated. Thermocouples were designed for measuring aerodynamic heat transfer in wind tunnels
Extraction of the Electron Self-Energy from Angle Resolved Photoemission Data: Application to Bi2212
The self-energy , the fundamental function which
describes the effects of many-body interactions on an electron in a solid, is
usually difficult to obtain directly from experimental data. In this paper, we
show that by making certain reasonable assumptions, the self-energy can be
directly determined from angle resolved photoemission data. We demonstrate this
method on data for the high temperature superconductor
(Bi2212) in the normal, superconducting, and pseudogap phases.Comment: expanded version (6 pages), to be published, Phys Rev B (1 Sept 99
Dynamic nuclear polarization from current-induced electron spin polarization
Current-induced electron spin polarization is shown to produce nuclear
hyperpolarization through dynamic nuclear polarization. Saturated fields of
several millitesla are generated upon the application of electric field over a
timescale of a hundred seconds in InGaAs epilayers and measured using optical
Larmor magnetometry. The dependence on temperature, external magnetic field,
and applied voltage is investigated. We find an asymmetry in which the
saturation nuclear field depends on the relative alignment of the electrically
generated spin polarization and the external magnetic field, which we attribute
to an interplay between various electron spin dynamical processes.Comment: 5 pages, 4 figure
Phenomenology of Photoemission Lineshapes of High Tc Superconductors
We introduce a simple phenomenological form for the self-energy which allows
us to extract important information from angle resolved photoemission data on
the high Tc superconductor Bi2212. First, we find a rapid suppression of the
single particle scattering rate below Tc for all doping levels. Second, we find
that in the overdoped materials the gap Delta at all k-points on the Fermi
surface has significant temperature dependence and vanishes near Tc. In
contrast, in the underdoped samples such behavior is found only at k-points
close to the diagonal. Near (pi,0), Delta is essentially T-independent in the
underdoped samples. The filling-in of the pseudogap with increasing T is
described by a broadening proportional to T-Tc, which is naturally explained by
pairing correlations above Tc.Comment: 4 pages, revtex, 3 encapsulated postscript figure
Odd Parity and Line Nodes in Non-Symmorphic Superconductors
Group theory arguments have been invoked to argue that odd parity order
parameters cannot have line nodes in the presence of spin-orbit coupling. In
this paper we show that these arguments do not hold for certain non-symmorphic
superconductors. Specifically, we demonstrate that when the underlying crystal
has a twofold screw axis, half of the odd parity representations vanish on the
Brillouin zone face perpendicular to this axis. Many unconventional
superconductors have non-symmorphic space groups, and we discuss implications
for several materials, including UPt3, UBe13, Li2Pt3B and Na4Ir3O8.Comment: 4 page
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