2,132 research outputs found
Phase diagram of the three-dimensional Hubbard model at half filling
We investigate the phase diagram of the three-dimensional Hubbard model at
half filling using quantum Monte Carlo (QMC) simulations. The antiferromagnetic
Neel temperature T_N is determined from the specific heat maximum in
combination with finite-size scaling of the magnetic structure factor. Our
results interpolate smoothly between the asymptotic solutions for weak and
strong coupling, respectively, in contrast to previous QMC simulations. The
location of the metal-insulator transition in the paramagnetic phase above T_N
is determined using the electronic compressibility as criterion.Comment: 6 pages, 6 figures, to be published in Eur. Phys. J. B (2000
A Phylogenetic Analysis of the Genus Fragaria (Strawberry) Using Intron-Containing Sequence from the ADH-1 Gene
The genus Fragaria encompasses species at ploidy levels ranging from diploid to decaploid. The cultivated strawberry, Fragaria×ananassa, and its two immediate progenitors, F. chiloensis and F. virginiana, are octoploids. To elucidate the ancestries of these octoploid species, we performed a phylogenetic analysis using intron-containing sequences of the nuclear ADH-1 gene from 39 germplasm accessions representing nineteen Fragaria species and one outgroup species, Dasiphora fruticosa. All trees from Maximum Parsimony and Maximum Likelihood analyses showed two major clades, Clade A and Clade B. Each of the sampled octoploids contributed alleles to both major clades. All octoploid-derived alleles in Clade A clustered with alleles of diploid F. vesca, with the exception of one octoploid allele that clustered with the alleles of diploid F. mandshurica. All octoploid-derived alleles in clade B clustered with the alleles of only one diploid species, F. iinumae. When gaps encoded as binary characters were included in the Maximum Parsimony analysis, tree resolution was improved with the addition of six nodes, and the bootstrap support was generally higher, rising above the 50% threshold for an additional nine branches. These results, coupled with the congruence of the sequence data and the coded gap data, validate and encourage the employment of sequence sets containing gaps for phylogenetic analysis. Our phylogenetic conclusions, based upon sequence data from the ADH-1 gene located on F. vesca linkage group II, complement and generally agree with those obtained from analyses of protein-encoding genes GBSSI-2 and DHAR located on F. vesca linkage groups V and VII, respectively, but differ from a previous study that utilized rDNA sequences and did not detect the ancestral role of F. iinumae
Double beta decay versus cosmology: Majorana CP phases and nuclear matrix elements
We discuss the relation between the absolute neutrino mass scale, the
effective mass measured in neutrinoless double beta decay, and the Majorana CP
phases. Emphasis is placed on estimating the upper bound on the nuclear matrix
element entering calculations of the double beta decay half life. Consequently,
one of the Majorana CP phases can be constrained when combining the claimed
evidence for neutrinoless double beta decay with the neutrino mass bound from
cosmology.Comment: 11 pages, 3 figure
Laboratory and field measurements of enantiomeric monoterpene emissions as a function of chemotype, light and temperature
Plants emit significant amounts of monoterpenes into the
earth's atmosphere, where they react rapidly to form a multitude of gas phase
species and particles. Many monoterpenes exist in mirror-image forms or
enantiomers. In this study the enantiomeric monoterpene profile for several
representative plants (<i>Quercus ilex L., Rosmarinus officinalis L.</i>,
and <i>Pinus halepensis Mill.</i>) was investigated as a function of
chemotype, light and temperature both in the laboratory and in the field.
Analysis of enantiomeric monoterpenes from 19 <i>Quercus ilex</i>
individuals from Southern France and Spain revealed four regiospecific
chemotypes (genetically fixed emission patterns). In agreement with previous
work, only <i>Quercus ilex</i> emissions increased strongly with light.
However, for all three plant species no consistent enantiomeric variation was
observed as a function of light, and the enantiomeric ratio of α-pinene was found to vary by less than 20% from 100 and
1000 μmol m<sup>−2</sup> s<sup>−1</sup> PAR (photosynthetically active
radiation). The rate of monoterpene emission increased with temperature from
all three plant species, but little variation in the enantiomeric
distribution of α-pinene was observed with temperature. There was
more enantiomeric variability between individuals of the same species than
could be induced by either light or temperature. Field measurements of
α-pinene enantiomer mixing ratios in the air, taken at a
<i>Quercus ilex</i> forest in Southern France, and several other previously
reported field enantiomeric ratio diel cycle profiles are compared. All show
smoothly varying diel cycles (some positive and some negative) even over
changing wind directions. This is surprising in comparison with variations of
enantiomeric emission patterns shown by individuals of the same species
Conditions for detecting CP violation via neutrinoless double beta decay
Neutrinoless double beta decay data together with information on the absolute
neutrino masses obtained from the future KATRIN experiment and/or astrophysical
measurements give a chance to find CP violation in the lepton sector with
Majorana neutrinos. We derive and discuss necessary conditions which make
discovery of such CP violation possible for the future neutrino oscillation and
mass measurements data.Comment: 15 pages, 4 figures, RevTe
Saturation of Cs2 Photoassociation in an Optical Dipole Trap
We present studies of strong coupling in single-photon photoassociation of
cesium dimers using an optical dipole trap. A thermodynamic model of the trap
depletion dynamics is employed to extract absolute rate coefficents. From the
dependence of the rate coefficient on the photoassociation laser intensity, we
observe saturation of the photoassociation scattering probability at the
unitarity limit in quantitative agreement with the theoretical model by Bohn
and Julienne [Phys. Rev. A, 60, 414 (1999)]. Also the corresponding power
broadening of the resonance width is measured. We could not observe an
intensity dependent light shift in contrast to findings for lithium and
rubidium, which is attributed to the absence of a p or d-wave shape resonance
in cesium
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