222 research outputs found
Catastrophic vs Gradual Collapse of Thin-Walled Nanocrystalline Ni Hollow Cylinders As Building Blocks of Microlattice Structures
Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation. Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale – nano, micro, and macro levels – is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength
Finite-temperature phase transition in a class of 4-state Potts antiferromagnets
We argue that the 4-state Potts antiferromagnet has a finite-temperature
phase transition on any Eulerian plane triangulation in which one sublattice
consists of vertices of degree 4. We furthermore predict the universality class
of this transition. We then present transfer-matrix and Monte Carlo data
confirming these predictions for the cases of the union-jack and bisected
hexagonal lattices.Comment: LaTeX2e/Revtex4, 5 page
Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia
Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an
exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary.
Sedimentologically, the boundary succession can be divided into three sequences representing successive
episodes of late transgressive to early highstand deposition; four parasequences are recognized in the
sequence corresponding lithostratigraphically to the Manykai Formation. Small shelly fossils are abundant
and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence
of faunal elements, biostratigraphic correlations between the two regions have been controversial because
numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern
sections have first appearances scattered through more than thirty metres of section in the western Anabar.
Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to
two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha
Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern
Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the
unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to
provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented
in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated
horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately
three to six million years, during which essentially all 'basal Tommotian' small shelly fossils
evolved
Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models. V. Further Results for the Square-Lattice Chromatic Polynomial
We derive some new structural results for the transfer matrix of
square-lattice Potts models with free and cylindrical boundary conditions. In
particular, we obtain explicit closed-form expressions for the dominant (at
large |q|) diagonal entry in the transfer matrix, for arbitrary widths m, as
the solution of a special one-dimensional polymer model. We also obtain the
large-q expansion of the bulk and surface (resp. corner) free energies for the
zero-temperature antiferromagnet (= chromatic polynomial) through order q^{-47}
(resp. q^{-46}). Finally, we compute chromatic roots for strips of widths 9 <=
m <= 12 with free boundary conditions and locate roughly the limiting curves.Comment: 111 pages (LaTeX2e). Includes tex file, three sty files, and 19
Postscript figures. Also included are Mathematica files data_CYL.m and
data_FREE.m. Many changes from version 1: new material on series expansions
and their analysis, and several proofs of previously conjectured results.
Final version to be published in J. Stat. Phy
Transfer matrices and partition-function zeros for antiferromagnetic Potts models. VI. Square lattice with special boundary conditions
We study, using transfer-matrix methods, the partition-function zeros of the
square-lattice q-state Potts antiferromagnet at zero temperature (=
square-lattice chromatic polynomial) for the special boundary conditions that
are obtained from an m x n grid with free boundary conditions by adjoining one
new vertex adjacent to all the sites in the leftmost column and a second new
vertex adjacent to all the sites in the rightmost column. We provide numerical
evidence that the partition-function zeros are becoming dense everywhere in the
complex q-plane outside the limiting curve B_\infty(sq) for this model with
ordinary (e.g. free or cylindrical) boundary conditions. Despite this, the
infinite-volume free energy is perfectly analytic in this region.Comment: 114 pages (LaTeX2e). Includes tex file, three sty files, and 23
Postscript figures. Also included are Mathematica files data_Eq.m,
data_Neq.m,and data_Diff.m. Many changes from version 1, including several
proofs of previously conjectured results. Final version to be published in J.
Stat. Phy
Spanning forests and the q-state Potts model in the limit q \to 0
We study the q-state Potts model with nearest-neighbor coupling v=e^{\beta
J}-1 in the limit q,v \to 0 with the ratio w = v/q held fixed. Combinatorially,
this limit gives rise to the generating polynomial of spanning forests;
physically, it provides information about the Potts-model phase diagram in the
neighborhood of (q,v) = (0,0). We have studied this model on the square and
triangular lattices, using a transfer-matrix approach at both real and complex
values of w. For both lattices, we have computed the symbolic transfer matrices
for cylindrical strips of widths 2 \le L \le 10, as well as the limiting curves
of partition-function zeros in the complex w-plane. For real w, we find two
distinct phases separated by a transition point w=w_0, where w_0 = -1/4 (resp.
w_0 = -0.1753 \pm 0.0002) for the square (resp. triangular) lattice. For w >
w_0 we find a non-critical disordered phase, while for w < w_0 our results are
compatible with a massless Berker-Kadanoff phase with conformal charge c = -2
and leading thermal scaling dimension x_{T,1} = 2 (marginal operator). At w =
w_0 we find a "first-order critical point": the first derivative of the free
energy is discontinuous at w_0, while the correlation length diverges as w
\downarrow w_0 (and is infinite at w = w_0). The critical behavior at w = w_0
seems to be the same for both lattices and it differs from that of the
Berker-Kadanoff phase: our results suggest that the conformal charge is c = -1,
the leading thermal scaling dimension is x_{T,1} = 0, and the critical
exponents are \nu = 1/d = 1/2 and \alpha = 1.Comment: 131 pages (LaTeX2e). Includes tex file, three sty files, and 65
Postscript figures. Also included are Mathematica files forests_sq_2-9P.m and
forests_tri_2-9P.m. Final journal versio
A Critical Examination of the X-Wind Model for Chondrule and Calcium-rich, Aluminum-rich Inclusion Formation and Radionuclide Production
Meteoritic data, especially regarding chondrules and calcium-rich,
aluminum-rich inclusions (CAIs), and isotopic evidence for short-lived
radionuclides (SLRs) in the solar nebula, potentially can constrain how
planetary systems form. Intepretation of these data demands an astrophysical
model, and the "X-wind" model of Shu et al. (1996) and collaborators has been
advanced to explain the origin of chondrules, CAIs and SLRs. It posits that
chondrules and CAIs were thermally processed < 0.1 AU from the protostar, then
flung by a magnetocentrifugal outflow to the 2-3 AU region to be incorporated
into chondrites. Here we critically examine key assumptions and predictions of
the X-wind model. We find a number of internal inconsistencies: theory and
observation show no solid material exists at 0.1 AU; particles at 0.1 AU cannot
escape being accreted into the star; particles at 0.1 AU will collide at speeds
high enough to destroy them; thermal sputtering will prevent growth of
particles; and launching of particles in magnetocentrifugal outflows is not
modeled, and may not be possible. We also identify a number of incorrect
predictions of the X-wind model: the oxygen fugacity where CAIs form is orders
of magnitude too oxidizing; chondrule cooling rates are orders of magnitude
lower than those experienced by barred olivine chondrules; chondrule-matrix
complementarity is not predicted; and the SLRs are not produced in their
observed proportions. We conclude that the X-wind model is not relevant to
chondrule and CAI formation and SLR production. We discuss more plausible
models for chondrule and CAI formation and SLR production.Comment: Accepted for publication in The Astrophysical Journa
The Impossibility of a Perfectly Competitive Labor Market
Using the institutional theory of transaction cost, I demonstrate that the assumptions of the competitive labor market model are internally contradictory and lead to the conclusion that on purely theoretical grounds a perfectly competitive labor market is a logical impossibility. By extension, the familiar diagram of wage determination by supply and demand is also a logical impossibility and the neoclassical labor demand curve is not a well-defined construct. The reason is that the perfectly competitive market model presumes zero transaction cost and with zero transaction cost all labor is hired as independent contractors, implying multi-person firms, the employment relationship, and labor market disappear. With positive transaction cost, on the other hand, employment contracts are incomplete and the labor supply curve to the firm is upward sloping, again causing the labor demand curve to be ill-defined. As a result, theory suggests that wage rates are always and everywhere an amalgam of an administered and bargained price. Working Paper 06-0
Pleural Resident Macrophages and Pleural IRA B Cells Promote Efficient Immunity Against Pneumonia by Inducing Early Pleural Space Inflammation
Airway infection is a major cause of mortality worldwide. The identification of new mechanisms aiding in effective host immune response is therefore required. Here, we show that the specific depletion of the pleural immune cell compartment during bacterial pneumonia resulted in a reduced pulmonary immune response and increased mortality in mice. Bacterial airway infection provoked early pleural space (PS) inflammation characterized by innate response activator (IRA) B cell development and pleural large resident macrophage (LRM) necroptosis, the repopulation of LRMs being driven by cellular proliferation in situ. Necroptotic LRMs amplified PS inflammation by stimulating pleural Mincle-expressing macrophages whereas IRA B cells contributed partially to GM-CSF-induced PS inflammation. Upon pulmonary infection, the induction of PS inflammation resulted in reduced bacterial burden whereas the specific depletion of pleural resident macrophages led to increased mortality and bacterial burden and reduced pulmonary immunity. Moreover, mice in which B cells were unable to produce GM-CSF exhibited reduced CD103+ dendritic cells and reduced CD4+ T cell numbers in the draining lymph node. Altogether, our results describe a previously unrecognized mechanism of pleural space inflammation necessary for effective protection against bacterial airway infection
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