240 research outputs found
Kinetic Heterogeneities in a Highly Supercooled Liquid
We study a highly supercooled two-dimensional fluid mixture via molecular
dynamics simulation. We follow bond breakage events among particle pairs, which
occur on the scale of the relaxation time . Large scale
heterogeneities analogous to the critical fluctuations in Ising systems are
found in the spatial distribution of bonds which are broken in a time interval
with a width of order . The structure factor of the broken
bond density is well approximated by the Ornstein-Zernike form. The correlation
length is of order at the lowest temperature studied,
being the particle size. The weakly bonded regions thus identified evolve in
time with strong spatial correlations.Comment: 3 pages, 6 figure
Genetic architecture of delayed senescence, biomass, and grain yield under drought stress in cowpea
CowpeaThe stay-green phenomenon is a key plant trait with wide usage in managing crop production under limited water conditions. This trait enhances delayed senescence, biomass, and grain yield under drought stress. In this study we sought to identify QTLs in cowpea (Vigna unguiculata) consistent across experiments conducted in Burkina Faso, Nigeria, Senegal, and the United States of America under limited water conditions. A panel of 383 diverse cowpea accessions and a
recombinant inbred line population (RIL) were SNP genotyped using an Illumina 1536 GoldenGate assay. Phenotypic data from thirteen experiments conducted across the four countries were used to identify SNP-trait associations based on linkage disequilibrium association mapping, with bi-parental QTL mapping as a complementary strategy. We identified seven loci, five of which exhibited evidence suggesting pleiotropic effects (stay-green) between delayed senescence, biomass, and grain yield. Further, we provide evidence suggesting the existence of positive pleiotropy in cowpea based on positively correlated mean phenotypic values (0.34, r ,0.87) and allele effects (0.07, r ,0.86) for delayed senescence and grain yield across three African environments. Three of the five putative stay-green QTLs, Dro-1, 3, and 7 were identified in both RILs and diverse germplasm with resolutions of 3.2 cM or less for each of the three loci, suggesting that these may be valuable targets for marker-assisted breeding in cowpea. Also, the co-location of early vegetative delayed senescence with biomass and grain yield QTLs suggests the possibility of using delayed senescence at the seedling stage as a rapid screening tool for post-flowering drought tolerance in cowpea breeding. BLAST analysis using EST sequences harboring SNPs with the highest associations provided a genomic context for loci identified in this study in closely related common bean (Phaseolus vulgaris) and soybean (Glycine max) reference genomes
Heterogeneous Diffusion in Highly Supercooled Liquids
The diffusivity of tagged particles is demonstrated to be very heterogeneous
on time scales comparable to or shorter than the relaxation time
( the stress relaxation time) in a highly supercooled
liquid via 3D molecular dynamics simulation. The particle motions in the
relatively active regions dominantly contribute to the mean square
displacement, giving rise to a diffusion constant systematically larger than
the Einstein-Stokes value. The van Hove self-correlation function is
shown to have a long distance tail which can be scaled in terms of
for t \ls 3\tau_{\alpha}. Its presence indicates heterogeneous diffusion in
the active regions. However, the diffusion process eventually becomes
homogeneous on time scales longer than the life time of the heterogeneity
structure ().Comment: 4 pages, 5 figure
Evidence for High-frequency Phonon Mediated S-wave Superconductivity : 11B-NMR Study of Al-doped MgB2
We report B-NMR study on Al-doped MgB that addresses a possible
mechanism for a high superconducting (SC) transition temperature () of
K in recently discovered MgB. The result of nuclear spin lattice
relaxation rate in the SC state revealed that the size in the SC gap is
not changed by substituting Al for Mg. The reduction on by Al-doping is
shown to be due to the decrease of . According to the McMillan
equation, the experimental relation between and the relative change in
allowed us to estimate a characteristic phonon frequency K and an electron-phonon coupling constant . These
results suggest that the high- superconductivity in MgB is mediated by
the strong electron-phonon coupling with high-frequency phonons.Comment: 6pages, 3figure
Specific heat and electronic states of superconducting boron-doped silicon carbide
The discoveries of superconductivity in the heavily-boron doped
semiconductors diamond (C:B) in 2004 and silicon (Si:B) in 2006 have renewed
the interest in the physics of the superconducting state of doped
semiconductors. Recently, we discovered superconductivity in the closely
related ''mixed'' system heavily boron-doped silcon carbide (SiC:B).
Interestingly, the latter compound is a type-I superconductor whereas the two
aforementioned materials are type-II. In this paper we present an extensive
analysis of our recent specific-heat study, as well as the band structure and
expected Fermi surfaces. We observe an apparent quadratic temperature
dependence of the electronic specific heat in the superconducting state.
Possible reasons are a nodal gap structure or a residual density of states due
to non-superconducting parts of the sample. The basic superconducting
parameters are estimated in a Ginzburg-Landau framework. We compare and discuss
our results with those reported for C:B and Si:B. Finally, we comment on
possible origins of the difference in the superconductivity of SiC:B compared
to the two ''parent'' materials C:B and Si:B.Comment: 9 pages, 7 figures, 2 tables, submitted to Phys. Rev.
Dynamics of Highly Supercooled Liquids:Heterogeneity, Rheology, and Diffusion
Highly supercooled liquids with soft-core potentials are studied via
molecular dynamics simulations in two and three dimensions in quiescent and
sheared conditions.We may define bonds between neighboring particle pairs
unambiguously owing to the sharpness of the first peak of the pair correlation
functions. Upon structural rearrangements, they break collectively in the form
of clusters whose sizes grow with lowering the temperature . The bond life
time , which depends on and the shear rate \gdot, is on the order
of the usual structural or relaxation time in weak
shear \gdot \tau_{\alpha} \ll 1, while it decreases as 1/\gdot in strong
shear \gdot\tau_{\alpha} \gg 1 due to shear-induced cage breakage.
Accumulated broken bonds in a time interval () closely
resemble the critical fluctuations of Ising spin systems. For example, their
structure factor is well fitted to the Ornstein-Zernike form, which yields the
correlation length representing the maximum size of the clusters composed
of broken bonds. We also find a dynamical scaling relation, , valid for any and \gdot with in two dimensions and
in three dimensions. The viscosity is of order for any and
\gdot, so marked shear-thinning behavior emerges. The shear stress is close
to a limiting stress in a wide shear region. We also examine motion of tagged
particles in shear in three dimensions. The diffusion constant is found to be
of order with for any and \gdot, so
it is much enhanced in strong shear compared with its value at zero shear. This
indicates breakdown of the Einstein-Stokes relation in accord with experiments.
Some possible experiments are also proposed.Comment: 20pages (including figures
Growing Correlation Length on Cooling Below the Onset of Caging in a Simulated Glass-Forming Liquid
We present a calculation of a fourth-order, time-dependent density
correlation function that measures higher-order spatiotemporall correlations of
the density of a liquid. From molecular dynamics simulations of a glass-forming
Lennard-Jones liquid, we find that the characteristic length scale of this
function has a maximum as a function of time which increases steadily beyond
the characteristic length of the static pair correlation function in the
temperature range approaching the mode coupling temperature from above
Electronic structure of MgB: X-ray emission and absorption studies
Measurements of x-ray emission and absorption spectra of the constituents of
MgB are presented. The results obtained are in good agreement with
calculated x-ray spectra, with dipole matrix elements taken into account. The
comparison of x-ray emission spectra of graphite, AlB, and MgB in the
binding energy scale supports the idea of charge transfer from to
bands, which creates holes at the top of the bonding bands and
drives the high-TComment: final version as published in PR
- …