18,211 research outputs found
Composite Fermions and quantum Hall systems: Role of the Coulomb pseudopotential
The mean field composite Fermion (CF) picture successfully predicts angular
momenta of multiplets forming the lowest energy band in fractional quantum Hall
(FQH) systems. This success cannot be attributed to a cancellation between
Coulomb and Chern-Simons interactions beyond the mean field, because these
interactions have totally different energy scales. Rather, it results from the
behavior of the Coulomb pseudopotential V(L) (pair energy as a function of pair
angular momentum) in the lowest Landau level (LL). The class of short range
repulsive pseudopotentials is defined that lead to short range Laughlin like
correlations in many body systems and to which the CF model can be applied.
These Laughlin correlations are described quantitatively using the formalism of
fractional parentage. The discussion is illustrated with an analysis of the
energy spectra obtained in numerical diagonalization of up to eleven electrons
in the lowest and excited LL's. The qualitative difference in the behavior of
V(L) is shown to sometimes invalidate the mean field CF picture when applied to
higher LL's. For example, the nu=7/3 state is not a Laughlin nu=1/3 state in
the first excited LL. The analysis of the involved pseudopotentials also
explains the success or failure of the CF picture when applied to other systems
of charged Fermions with Coulomb repulsion, such as the Laughlin quasiparticles
in the FQH hierarchy or charged excitons in an electron-hole plasma.Comment: 27 pages, 23 figures, revised version (significant changes in text
and figures), submitted to Phil. Mag.
Path integral evaluation of equilibrium isotope effects
A general and rigorous methodology to compute the quantum equilibrium isotope
effect is described. Unlike standard approaches, ours does not assume
separability of rotational and vibrational motions and does not make the
harmonic approximation for vibrations or rigid rotor approximation for the
rotations. In particular, zero point energy and anharmonicity effects are
described correctly quantum mechanically. The approach is based on the
thermodynamic integration with respect to the mass of isotopes and on the
Feynman path integral representation of the partition function. An efficient
estimator for the derivative of free energy is used whose statistical error is
independent of the number of imaginary time slices in the path integral,
speeding up calculations by a factor of 60 at 500 K. We describe the
implementation of the methodology in the molecular dynamics package Amber 10.
The method is tested on three [1,5] sigmatropic hydrogen shift reactions.
Because of the computational expense, we use ab initio potentials to evaluate
the equilibrium isotope effects within the harmonic approximation, and then the
path integral method together with semiempirical potentials to evaluate the
anharmonicity corrections. Our calculations show that the anharmonicity effects
amount up to 30% of the symmetry reduced reaction free energy. The numerical
results are compared with recent experiments of Doering and coworkers,
confirming the accuracy of the most recent measurement on
2,4,6,7,9-pentamethyl-5-(5,5-H)methylene-11,11a-dihydro-12H-naphthacene
as well as concerns about compromised accuracy, due to side reactions, of
another measurement on
2-methyl-10-(10,10-H)methylenebicyclo[4.4.0]dec-1-ene.Comment: 14 pages, 8 figures, 6 table
Theoretical study of resonant x-ray emission spectroscopy of Mn films on Ag
We report a theoretical study on resonant x-ray emission spectra (RXES) in
the whole energy region of the Mn white lines for three prototypical
Mn/Ag(001) systems: (i) a Mn impurity in Ag, (ii) an adsorbed Mn monolayer on
Ag, and (iii) a thick Mn film. The calculated RXES spectra depend strongly on
the excitation energy. At excitation, the spectra of all three systems
are dominated by the elastic peak. For excitation energies around , and
between and , however, most of the spectral weight comes from
inelastic x-ray scattering. The line shape of these inelastic ``satellite''
structures changes considerably between the three considered Mn/Ag systems, a
fact that may be attributed to changes in the bonding nature of the Mn-
orbitals. The system-dependence of the RXES spectrum is thus found to be much
stronger than that of the corresponding absorption spectrum. Our results
suggest that RXES in the Mn region may be used as a sensitive probe
of the local environment of Mn atoms.Comment: 9 pages, 11 figure
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
Near-UV Observations of CS29497-030: New Constraints on Neutron-Capture Nucleosynthesis Processes
Employing spectra obtained with the new Keck I HIRES near-UV sensitive
detector, we have performed a comprehensive chemical composition analysis of
the binary blue metal-poor star CS29497-030. Abundances for 29 elements and
upper limits for an additional seven have been derived, concentrating on
elements largely produced via neutron-capture nucleosynthesis. Included in our
analysis are the two elements that define the termination point of the slow
neutron-capture process, lead and bismuth. We determine an extremely high value
of [Pb/Fe] = +3.65 +/- 0.07 (sigma = 0.13) from three features, supporting the
single-feature result obtained in previous studies. We also detect Bi for the
first time in a metal-poor star. Our derived Bi/Pb ratio is in accord with
those predicted from the most recent FRANEC calculations of the slow
neutron-capture process in low-mass AGB stars. We find that the neutron-capture
elemental abundances of CS29497-030 are best explained by an AGB model that
also includes very significant amounts of pre-enrichment of rapid
neutron-capture process material in the protostellar cloud out of which the
CS29497-030 binary system formed. Thus, CS29497-030 is both an ``r+s'' and
``extrinsic AGB'' star. Furthermore, we find that the mass of the AGB model can
be further constrained by the abundance of the light odd-element [Na/Fe] which
is sensitive to the neutron excess.Comment: 7 pages = 4 + 2 colour encapsulated postscript figures + 1 table; to
appear in ApJ Letters; additional jpeg figure available at
ftp://www.astro.caltech.edu/users/iii/cs2949703
Itâs the way he tells them (and who is listening):menâs dominance is positively correlated with their preference for jokes told by dominant-sounding men
While much research has explored humorous exchange in relation to mate choice, recent perspectives have emphasized the importance of humor for monitoring interest within social partnerships more generally. Indeed, given that similarity is thought to be important in the maintenance of social partnerships, we may expect humor appreciation to vary according to the degree of similarity between humor producers and recipients. In the current study we report evidence for such variation that is specific to menâs judgments of other menâs humor. Here we manipulated voice pitch in a set of âone-linerâ jokes to create low-pitched and high-pitched versions of men and women telling jokes. A composite measure of menâs own dominance was positively correlated with their preference for jokes told by other men with lowered voice pitch (a vocal cue to dominance). A follow-up study demonstrated that self-reported dominance was positively related to menâs choice of low-pitch men as friends when judging humorous audio clips but not when judging neutral control audio clips, suggesting that humor may be important in mediating the effect of dominance on friendship choice. These studies indicate systematic variation in humor appreciation related to friendship choices which may function to promote cohesion within male partnerships based on status
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
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