4,791 research outputs found
Thermodynamically Stable One-Component Metallic Quasicrystals
Classical density-functional theory is employed to study finite-temperature
trends in the relative stabilities of one-component quasicrystals interacting
via effective metallic pair potentials derived from pseudopotential theory.
Comparing the free energies of several periodic crystals and rational
approximant models of quasicrystals over a range of pseudopotential parameters,
thermodynamically stable quasicrystals are predicted for parameters approaching
the limits of mechanical stability of the crystalline structures. The results
support and significantly extend conclusions of previous ground-state
lattice-sum studies.Comment: REVTeX, 13 pages + 2 figures, to appear, Europhys. Let
Self-Assembly of Monatomic Complex Crystals and Quasicrystals with a Double-Well Interaction Potential
For the study of crystal formation and dynamics we introduce a simple
two-dimensional monatomic model system with a parametrized interaction
potential. We find in molecular dynamics simulations that a surprising variety
of crystals, a decagonal and a dodecagonal quasicrystal are self-assembled. In
the case of the quasicrystals the particles reorder by phason flips at elevated
temperatures. During annealing the entropically stabilized decagonal
quasicrystal undergoes a reversible phase transition at 65% of the melting
temperature into an approximant, which is monitored by the rotation of the de
Bruijn surface in hyperspace.Comment: 4 pages, 6 figures. Physical Review Letters, in Press (April 2007
The Structure of Barium in the hcp Phase Under High Pressure
Recent experimental results on two hcp phases of barium under high pressure
show interesting variation of the lattice parameters. They are here interpreted
in terms of electronic structure calculation by using the LMTO method and
generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II
the dramatic drop in c/a is an instability analogous to that in the group II
metals but with the transfer of s to d electrons playing a crucial role in Ba.
Meanwhile in phase V, the instability decrease a lot due to the core repulsion
at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx,
71.15LaComment: 29 pages, 8 figure
Enhanced entrainability of genetic oscillators by period mismatch
Biological oscillators coordinate individual cellular components so that they
function coherently and collectively. They are typically composed of multiple
feedback loops, and period mismatch is unavoidable in biological
implementations. We investigated the advantageous effect of this period
mismatch in terms of a synchronization response to external stimuli.
Specifically, we considered two fundamental models of genetic circuits: smooth-
and relaxation oscillators. Using phase reduction and Floquet multipliers, we
numerically analyzed their entrainability under different coupling strengths
and period ratios. We found that a period mismatch induces better entrainment
in both types of oscillator; the enhancement occurs in the vicinity of the
bifurcation on their limit cycles. In the smooth oscillator, the optimal period
ratio for the enhancement coincides with the experimentally observed ratio,
which suggests biological exploitation of the period mismatch. Although the
origin of multiple feedback loops is often explained as a passive mechanism to
ensure robustness against perturbation, we study the active benefits of the
period mismatch, which include increasing the efficiency of the genetic
oscillators. Our findings show a qualitatively different perspective for both
the inherent advantages of multiple loops and their essentiality.Comment: 28 pages, 13 figure
Systemic epidermal nevus with involvement of the oral mucosa due to FGFR3 mutation
Our results show that activating FGFR3 mutations can also affect the oral mucosa and that extracutaneous manifestations of EN syndrome can be subtle. We highlight the theoretical risk of the patient having an offspring with thanatophoric dysplasia as gonadal mosaicism for the R248C mutation cannot be excluded
Thomomys bottae pocket gophers of the central Rio Grande Valley, New Mexico: local differentiation, gene flow, and historical biogeography
Representatives of two strongly differentiated geographic units within Thomomys bottae come in contact along the Rio Grande south of Albuquerque, New Mexico. The two forms share an average genic similarity of only 69%, and differ in karyotype by as much as 17 pairs of uniarmed autosomes. This high level of genic and chromosomal differentiation might suggest extremely limited introgression, or perhaps none at all. However, diagnostic alleles at several strongly differentiated loci were detected in contact zone populations of the opposite group, indicating that some gene flow does oc\u3c;ur. Suitable habitat and available land area are limited in the zone of contact, and these factors, in combination with the structure of local breeding populations of pocket gophers, contribute to restriction of gene flow through the contact zone. The measured width of the contact zone corresponds reasonably well with predictions derived from a neutral diffusion cline model under current estimates of gene flow rates and time of secondary contact
Black Students Get an Edge in Reading
The purpose of this article is not so much to report that minority children are having trouble in learning to read all kinds of children are for that matter·· but to point out that with the proper kind of instruction black children can do as well as other children. Consider briefly the findings of a few research studies documenting contentions about the reading acquisition problems that numbers of black children have
Super-resolution imaging and estimation of protein copy numbers at single synapses with DNA-PAINT
In the brain, the strength of each individual synapse is defined by the complement of proteins present or the "local proteome." Activity-dependent changes in synaptic strength are the result of changes in this local proteome and posttranslational protein modifications. Although most synaptic proteins have been identified, we still know little about protein copy numbers in individual synapses and variations between synapses. We use DNA-point accumulation for imaging in nanoscale topography as a single-molecule super-resolution imaging technique to visualize and quantify protein copy numbers in single synapses. The imaging technique provides near-molecular spatial resolution, is unaffected by photobleaching, enables imaging of large field of views, and provides quantitative molecular information. We demonstrate these benefits by accessing copy numbers of surface AMPA-type receptors at single synapses of rat hippocampal neurons along dendritic segments
Multipole nonlinearity of metamaterials
We report on the linear and nonlinear optical response of metamaterials
evoked by first and second order multipoles. The analytical ground on which our
approach bases permits for new insights into the functionality of
metamaterials. For the sake of clarity we focus here on a key geometry, namely
the split-ring resonator, although the introduced formalism can be applied to
arbitrary structures. We derive the equations that describe linear and
nonlinear light propagation where special emphasis is put on second harmonic
generation. This contribution basically aims at stretching versatile and
existing concepts to describe light propagation in nonlinear media towards the
realm of metamaterials.Comment: 7 pages, 3 figure
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