756,508 research outputs found
Chiral Symmetry Restoration in the Instanton Liquid at Finite Density
The properties of the QCD partition function at finite chemical potential are
studied within the instanton liquid model. It is shown that the density
dependence of the quark-induced instanton-antiinstanton (I-A) interaction leads
to the formation of topologically neutral I-A pairs ('molecules'), resulting in
a first order chiral phase transition at a critical chemical potential
MeV. At somewhat higher densities ( MeV), the
quark Fermi surface becomes instable with respect to diquark condensation
(Cooper pairs) generating BCS-type energy gaps of order 50 MeV.Comment: 7 pages LaTeX, 4 eps-figures and espcrc1.sty included, to appear in
the Proc. of the 'QCD at Finite Baryon Density'-Workshop (Bielefeld,
27.-30.04.98
Influence of supercoiling on the disruption of dsDNA
We propose that supercoiling energizes double-stranded DNA (dsDNA) so as to
facilitate thermal fluctuations to an unzipped state. We support this with a
model of two elastic rods coupled via forces that represent base pair
interactions. Supercoiling is shown to lead to a spatially localized higher
energy state in a small region of dsDNA consisting of a few base pairs. This
causes the distance between specific base pairs to be extended, enhancing the
thermal probability for their disruption. Our theory permits the development of
an analogy between this unzipping transition and a second order phase
transition, for which the possibility of a new set of critical exponents is
identified
Confluence in UnTyped Higher-Order Theories by means of Critical Pairs
User-defined higher-order rewrite rules are becoming a standard in proof assistants based on intuitionistic type theory. This raises the question of proving that they preserve the properties of beta-reductions for the corresponding type systems. We develop here techniques that reduce confluence proofs to the checking of various forms of critical pairs for higher-order rewrite rules extending beta-reduction on pure lambda-terms. The present paper concentrates on the case where rewrite rules are left-linear and critical pairs can be joined without using beta-rewrite steps. The other two cases will be addressed in forthcoming papers
Exactly solvable model of a quantum spin glass
A mean field spherical model with random couplings between pairs, quartets,
and possibly higher multiplets of spins is considered. It has the same critical
behavior as the Sherrington-Kirkpatrick model. It thus exhibits replica
symmetry breaking. The order parameter function is solved exactly in the whole
low temperature phase. The zero field cooled susceptibility remains finite at
low . Next a quantum version of the system is considered. Whereas the
magnetic properties are not altered qualitatively, the thermodynamics is now
regular at small temperatures.Comment: 4 pages postscript. Revised version, to appear in Phys. Rev. Let
Untyped Confluence in Dependent Type Theories
International audienceWe investigate techniques based on van Oostrom's decreasing diagrams that reduce confluence proofs to the checking of critical pairs in the absence of termination properties, which are useful in dependent type calculi to prove confluence on untyped terms. These techniques are applied to a complex example originating from practice: a faithful encoding, in an extension of LF with rewrite rules on objects and types, of a subset of the calculus of inductive constructions with a cumulative hierarchy of predicative universes above Prop. The rules may be first-order or higher-order, plain or modulo, non-linear on the right or on the left. Variables which occur non-linearly in lefthand sides of rules must take their values in confined types: in our example, the natural numbers. The first-order rules are assumed to be terminating and confluent modulo some theory: in our example, associativity, commutativity and identity. Critical pairs involving higher-order rules must satisfy van Oostrom's decreasing diagram condition wrt their indexes taken as labels
Stability Boundaries for Resonant Migrating Planet Pairs
Convergent migration allows pairs of planet to become trapped into mean
motion resonances. Once in resonance, the planets' eccentricities grow to an
equilibrium value that depends on the ratio of migration time scale to the
eccentricity damping timescale, , with higher values of
equilibrium eccentricity for lower values of . For low equilibrium
eccentricities, . The stability of a planet pair
depends on eccentricity so the system can become unstable before it reaches its
equilibrium eccentricity. Using a resonant overlap criterion that takes into
account the role of first and second order resonances and depends on
eccentricity, we find a function that defines the lowest
value for , as a function of the ratio of total planet mass to stellar mass
() and the period ratio of the resonance defined as ,
that allows two convergently migrating planets to remain stable in resonance at
their equilibrium eccentricities. We scaled the functions for each
resonance of the same order into a single function . The function
for planet pairs in first order resonances is linear with increasing planet
mass and quadratic for pairs in second order resonances with a coefficient
depending on the relative migration rate and strongly on the planet to planet
mass ratio. The linear relation continues until the mass approaches a critical
mass defined by the 2/7 resonance overlap instability law and .
We compared our analytic boundary with an observed sample of resonant two
planet systems. All but one of the first order resonant planet pair systems
found by radial velocity measurements are well inside the stability region
estimated by this model. We calculated for Kepler systems without
well-constrained eccentricities and found only weak constraints on .Comment: 11 pages, 7 figure
Nernst effect as a probe of superconducting fluctuations in disordered thin films
In amorphous superconducting thin films of and ,
a finite Nernst coefficient can be detected in a wide range of temperature and
magnetic field. Due to the negligible contribution of normal quasi-particles,
superconducting fluctuations easily dominate the Nernst response in the entire
range of study. In the vicinity of the critical temperature and in the
zero-field limit, the magnitude of the signal is in quantitative agreement with
what is theoretically expected for the Gaussian fluctuations of the
superconducting order parameter. Even at higher temperatures and finite
magnetic field, the Nernst coefficient is set by the size of superconducting
fluctuations. The Nernst coefficient emerges as a direct probe of the ghost
critical field, the normal-state mirror of the upper critical field. Moreover,
upon leaving the normal state with fluctuating Cooper pairs, we show that the
temperature evolution of the Nernst coefficient is different whether the system
enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting
regime.Comment: Submitted to New. J. Phys. for a focus issue on "Superconductors with
Exotic Symmetries
Nonequilibrium noise and current fluctuations at the superconducting phase transition
We study non-Gaussian out-of-equilibrium current fluctuations in a mesoscopic
NSN circuit at the point of a superconducting phase transition. The setup
consists of a voltage-biased thin film nanobridge superconductor (S) connected
to two normal-metal (N) leads by tunnel junctions. We find that above a
critical temperature fluctuations of the superconducting order parameter
associated with the preformed Cooper pairs mediate inelastic electron
scattering that promotes strong current fluctuations. Though the conductance is
suppressed due to the depletion of the quasiparticle density of states, higher
cumulants of current fluctuations are parametrically enhanced. We identify
experimentally relevant transport regime where excess current noise may reach
or even exceed the level of the thermal noise.Comment: 5 pages, 3 figure
Resonant mode conversion in the waveguides with an unbroken and broken PT-symmetry
We study resonant mode conversion in the PT-symmetric multimode waveguides,
where symmetry breaking manifests itself in sequential destabilization
(appearance of the complex eigenvalues) of the pairs of adjacent guided modes.
We show that the efficient mode conversion is possible even in the presence of
the resonant longitudinal modulation of the complex refractive index. The
distinguishing feature of the resonant mode conversion in the PT-symmetric
structure is a drastic growth of the width of the resonance curve when the
gain/losses coefficient approaches a critical value, at which symmetry breaking
occurs. We found that in the system with broken symmetry the resonant coupling
between exponentially growing mode with stable higher-order one effectively
stabilizes dynamically coupled pair of modes and remarkably diminishes the
average rate of the total power growth.Comment: 4 pages, 6 figure
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