30,143 research outputs found
Perspectives on Pfaffians of Heterotic World-sheet Instantons
To fix the bundle moduli of a heterotic compactification one has to
understand the Pfaffian one-loop prefactor of the classical instanton
contribution. For compactifications on elliptically fibered Calabi-Yau spaces X
this can be made explicit for spectral bundles and world-sheet instantons
supported on rational base curves b: one can express the Pfaffian in a closed
algebraic form as a polynomial, or it may be understood as a theta-function
expression. We elucidate the connection between these two points of view via
the respective perception of the relevant spectral curve, related to its
extrinsic geometry in the ambient space (the elliptic surface in X over b) or
to its intrinsic geometry as abstract Riemann surface. We identify, within a
conceptual description, general vanishing loci of the Pfaffian, and derive
bounds on the vanishing order, relevant to solutions of W=dW=0.Comment: 40 pages; minor changes, discussion section 1.1 adde
Experimental measurements of expanding storable-propellant products simulated by combustion of gaseous reactants
Gaseous reactant combustion simulation of dimethylhydrazine and hydrazine fuel system for nonequilibrium expansion studie
Combinatorial Games with a Pass: A dynamical systems approach
By treating combinatorial games as dynamical systems, we are able to address
a longstanding open question in combinatorial game theory, namely, how the
introduction of a "pass" move into a game affects its behavior. We consider two
well known combinatorial games, 3-pile Nim and 3-row Chomp. In the case of Nim,
we observe that the introduction of the pass dramatically alters the game's
underlying structure, rendering it considerably more complex, while for Chomp,
the pass move is found to have relatively minimal impact. We show how these
results can be understood by recasting these games as dynamical systems
describable by dynamical recursion relations. From these recursion relations we
are able to identify underlying structural connections between these "games
with passes" and a recently introduced class of "generic (perturbed) games."
This connection, together with a (non-rigorous) numerical stability analysis,
allows one to understand and predict the effect of a pass on a game.Comment: 39 pages, 13 figures, published versio
Flame zone of a composite propellant expanded by a laser source
Technique scales flame structure linearly with gas kinetic mean free path, which increases two to three orders of magnitude as pressure decreases like amount. Kinetic and transport time scales expand in proportion so that regression rates for laser-induced flames are two to three orders of magnitude slower
Non-equilibrium Magnetization Dynamics in the Fe_8 Single-Molecule Magnet Induced by High-Intensity Microwave Radiation
Resonant microwave radiation applied to a single crystal of the molecular
magnet Fe_8 induces dramatic changes in the sample's magnetization. Transitions
between excited states are found even though at the nominal system temperature
these levels have negligible population. We find evidence that the sample heats
significantly when the resonance condition is met. In addition, heating is
observed after a short pulse of intense radiation has been turned off,
indicating that the spin system is out of equilibrium with the lattice.Comment: Version to appear in Europhysics Letters. Minor changes and updated
reference
Extended Hodge Theory for Fibred Cusp Manifolds
For a particular class of pseudo manifolds, we show that the intersection
cohomology groups for any perversity may be naturally represented by extended
weighted harmonic forms for a complete metric on the regular stratum with
respect to some weight determined by the perversity. Extended weighted
harmonic forms are harmonic forms that are almost in the given weighted
space for the metric in question, but not quite. This result is akin to the
representation of absolute and relative cohomology groups for a manifold with
boundary by extended harmonic forms on the associated manifold with cylindrical
ends. As in that setting, in the unweighted case, the boundary values of
the extended harmonic forms define a Lagrangian splitting of the boundary space
in the long exact sequence relating upper and lower middle perversity
intersection cohomology groups.Comment: 26 page
Experimental Upper Bound on Superradiance Emission from Mn12 Acetate
We used a Josephson junction as a radiation detector to look for evidence of
the emission of electromagnetic radiation during magnetization avalanches in a
crystal assembly of Mn_12-Acetate. The crystal assembly exhibits avalanches at
several magnetic fields in the temperature range from 1.8 to 2.6 K with
durations of the order of 1 ms. Although a recent study shows evidence of
electromagnetic radiation bursts during these avalanches [J. Tejada, et al.,
Appl. Phys. Lett. {\bf 84}, 2373 (2004)], we were unable to detect any
significant radiation at well-defined frequencies. A control experiment with
external radiation pulses allows us to determine that the energy released as
radiation during an avalanche is less than 1 part in 10^4 of the total energy
released. In addition, our avalanche data indicates that the magnetization
reversal process does not occur uniformly throughout the sample.Comment: 4 RevTeX pages, 3 eps figure
Observation of Strong Coulomb Blockade in Resistively Isolated Tunnel Junctions
We report measurements of the Coulomb-blockade current in resistively
isolated (R_{Isol} >> h/e^{2}) tunnel junctions for the temperature range 60mK
R_{Isol}\gg h/e^{2})$ tunnel junctions for the temperature range 60mK < T <
230mK where the charging energy E_{c} is much greater than the thermal energy.
A zero-bias resistance R_{0} of up to 10^{4}R_{T} (the tunnel resistance of the
bare junction) is obtained. For eV << E_{c}, the I-V curves for a given
R_{Isol} scale as a function of V/T, with I \propto V^{\alpha (R_{Isol})} over
a range of V. The data agree well with numerical calculations of the tunneling
rate that include environmental effects.Comment: 13 pages, 3 eps figure
Three Applications of Instanton Numbers
We use instanton numbers to: (i) stratify moduli of vector bundles, (ii)
calculate relative homology of moduli spaces and (iii) distinguish curve
singularities.Comment: To appear in Communications in Mathematical Physic
- …