4,883 research outputs found
Diagnostics of macroscopic quantum states of Bose-Einstein condensate in double-well potential by nonstationary Josephson effect
We propose a method of diagnostic of a degenerate ground state of Bose
condensate in a double well potential. The method is based on the study of the
one-particle coherent tunneling under switching the time-dependent weak
Josephson coupling between the wells. We obtain a simple expression that allows
to determine the phase of the condensate and the total number of the particles
in the condensate from the relative number of the particles in two wells
measured before the Josephson coupling is switched on and
after it is switched off. The specifics of the application of the method in the
cases of the external and the internal Josephson effect are discussed.Comment: 3 page
Remarks on non-gaussian fluctuations of the inflaton and constancy of \zeta outside the horizon
We point out that the non-gaussianity arising from cubic self interactions of
the inflaton field is proportional to \xi N_e where \xi ~ V"' and N_e is the
number of e-foldings from horizon exit till the end of inflation. For scales of
interest N_e = 60, and for models of inflation such as new inflation, natural
inflation and running mass inflation \xi is large compared to the slow roll
parameter \epsilon ~ V'^{2}. Therefore the contribution from self interactions
should not be outrightly ignored while retaining other terms in the
non-gaussianity parameter f_{NL}. But the N_e dependent term seems to imply the
growth of non-gaussianities outside the horizon. Therefore we briefly discuss
the issue of the constancy of correlations of the curvature perturbation \zeta
outside the horizon. We then calculate the 3-point function of the inflaton
fluctuations using the canonical formalism and further obtain the 3-point
function of \zeta_k. We find that the N_e dependent contribution to f_{NL} from
self interactions of the inflaton field is cancelled by contributions from
other terms associated with non-linearities in cosmological perturbation
theory.Comment: 16 pages, Minor changes, matches the published version. v3: Minor
typo correcte
Quantum versus Semiclassical Description of Selftrapping: Anharmonic Effects
Selftrapping has been traditionally studied on the assumption that
quasiparticles interact with harmonic phonons and that this interaction is
linear in the displacement of the phonon. To complement recent semiclassical
studies of anharmonicity and nonlinearity in this context, we present below a
fully quantum mechanical analysis of a two-site system, where the oscillator is
described by a tunably anharmonic potential, with a square well with infinite
walls and the harmonic potential as its extreme limits, and wherein the
interaction is nonlinear in the oscillator displacement. We find that even
highly anharmonic polarons behave similar to their harmonic counterparts in
that selftrapping is preserved for long times in the limit of strong coupling,
and that the polaronic tunneling time scale depends exponentially on the
polaron binding energy. Further, in agreement, with earlier results related to
harmonic polarons, the semiclassical approximation agrees with the full quantum
result in the massive oscillator limit of small oscillator frequency and strong
quasiparticle-oscillator coupling.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
Comparison of 3D confocal Raman and high energy X-ray diffraction for the measurement of molten sand infiltration in turbine blade coatings
Thermal barrier coatings (TBCs) are used to protect metallic blades subjected to the extreme temperatures found within the turbine sections following the combustion chamber of jet engines. Yttria-stabilized zirconia (YSZ) is the standard high temperature ceramic material used for TBCs. One of the primary deposition methods of these TBCs on the metallic blades is electron-beam physical vapor deposition (EB-PVD), which gives YSZ a characteristic columnar, porous structure. This allows for a higher strain tolerance than that of other deposition methods; however, it also increases the susceptibility of the coating to molten sand and volcanic ash infiltration, referred to as calcium-magnesium-alumino-silicates (CMAS), which greatly reduces the lifetime of TBCs. A variety of techniques are used to analyze and quantify the damage to the coating; two of which will be discussed and compared in this paper. 3D confocal Raman spectroscopy and high-energy x-ray diffraction (XRD) measurements were acquired non-destructively from CMAS-infiltrated samples. The results were compared to show how the similarities and differences in the data collected can reveal a complete picture of the chemical degradation within the TBC due to CMAS. This includes the comparison of phase volume fractions found by each method. Both methods showed that higher temperatures and longer annealing times lead to a greater volume of the monoclinic phase, which is linked to the eventual failure of the coating. 3D confocal Raman provides local phase volume fractions, spatially resolved for the assessing the effects of infiltration on single columns. Meanwhile, XRD provides a more global quantification of phase volume fractions throughout the probed volume. This work highlights the complimentary nature of 3D confocal Raman and XRD for high accuracy determination of degradation mechanisms for improved lifetime predictions of TBCs
How to Influence People with Partial Incentives
We study the power of fractional allocations of resources to maximize
influence in a network. This work extends in a natural way the well-studied
model by Kempe, Kleinberg, and Tardos (2003), where a designer selects a
(small) seed set of nodes in a social network to influence directly, this
influence cascades when other nodes reach certain thresholds of neighbor
influence, and the goal is to maximize the final number of influenced nodes.
Despite extensive study from both practical and theoretical viewpoints, this
model limits the designer to a binary choice for each node, with no way to
apply intermediate levels of influence. This model captures some settings
precisely, e.g. exposure to an idea or pathogen, but it fails to capture very
relevant concerns in others, for example, a manufacturer promoting a new
product by distributing five "20% off" coupons instead of giving away one free
product.
While fractional versions of problems tend to be easier to solve than
integral versions, for influence maximization, we show that the two versions
have essentially the same computational complexity. On the other hand, the two
versions can have vastly different solutions: the added flexibility of
fractional allocation can lead to significantly improved influence. Our main
theoretical contribution is to show how to adapt the major positive results
from the integral case to the fractional case. Specifically, Mossel and Roch
(2006) used the submodularity of influence to obtain their integral results; we
introduce a new notion of continuous submodularity, and use this to obtain
matching fractional results. We conclude that we can achieve the same greedy
-approximation for the fractional case as the integral case.
In practice, we find that the fractional model performs substantially better
than the integral model, according to simulations on real-world social network
data
Desugaring JML Method Specifications
JML, which stands for ``Java Modeling Language,\u27\u27 is a behavioral interface specification language (BISL) designed to specify Java modules. JML features a great deal of syntactic sugar that is designed to make method specifications more expressive. This paper presents a desugaring process that boils down all of the syntactic sugars in JML method specifications into a much simpler form. This desugaring will help one understand the meaning of these sugars, for example for use in program verification. It may also help manipulation of JML method specifications by tools
Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems
We present results of a reconnaissance for stellar companions to all 131
radial-velocity-detected candidate extrasolar planetary systems known as of
July 1, 2005. CPM companions were investigated using the multi-epoch DSS
images, and confirmed by matching the trigonometric parallax distances of the
primaries to companion distances estimated photometrically. We also attempt to
confirm or refute companions listed in the Washington Double Star Catalog, the
Catalogs of Nearby Stars, in Hipparcos results, and in Duquennoy & Mayor
(1991).
Our findings indicate that a lower limit of 30 (23%) of the 131 exoplanet
systems have stellar companions. We report new stellar companions to HD 38529
and HD 188015, and a new candidate companion to HD 169830. We confirm many
previously reported stellar companions, including six stars in five systems
that are recognized for the first time as companions to exoplanet hosts. We
have found evidence that 20 entries in the Washington Double Star Catalog are
not gravitationally bound companions. At least three, and possibly five, of the
exoplanet systems reside in triple star systems. Three exoplanet systems have
potentially close-in stellar companions ~ 20 AU away from the primary. Finally,
two of the exoplanet systems contain white dwarf companions. This comprehensive
assessment of exoplanet systems indicates that solar systems are found in a
variety of stellar multiplicity environments - singles, binaries, and triples;
and that planets survive the post-main-sequence evolution of companion stars.Comment: 52 pages, 7 figures, Accepted for publication in Ap
- …