9,405 research outputs found
Coincidence Problem in Cyclic Phantom Models of the Universe
We examine cyclic phantom models for the universe, in which the universe is
dominated sequentially by radiation, matter, and a phantom dark energy field,
followed by a standard inflationary phase. Since this cycle repeats endlessly,
the Universe spends a substantial portion of its lifetime in a state for which
the matter and dark energy densities have comparable magnitudes, thus
ameliorating the coincidence problem. We calculate the fraction of time that
the universe spends in such a coincidental state and find that it is nearly the
same as in the case of a phantom model with a future big rip. In the limit
where the dark energy equation of state parameter, w, is close to -1, we show
that the fraction of time, f, for which the ratio of the dark energy density to
the matter density lies between r_1 and r_2, is f = -(1+w) ln [(\sqrt{r_2} +
\sqrt{1+r_2})/(\sqrt{r_1} + \sqrt{1+r_1})].Comment: 4 pages, no figures, discussion and references adde
Contractual Versus Non-Contractual Trade: The Role of Institutions in China
Recent research has demonstrated the importance of institutional quality at the country level for both the volume of trade and the ability to trade in differentiated goods that rely on contract enforcement. This paper takes advantage of cross-provincial variation in institutional quality in China, and export data that distinguishes between foreign and domestic exporters and processing versus ordinary trade, to show that institutional quality is a significant factor in determining Chinese provincial export patterns. Institutions matter more for processing trade, and more for foreign firms, just as we would expect from a greater reliance on contracts in these cases.
Exact Potts Model Partition Functions for Strips of the Honeycomb Lattice
We present exact calculations of the Potts model partition function
for arbitrary and temperature-like variable on -vertex
strip graphs of the honeycomb lattice for a variety of transverse widths
equal to vertices and for arbitrarily great length, with free
longitudinal boundary conditions and free and periodic transverse boundary
conditions. These partition functions have the form
, where
denotes the number of repeated subgraphs in the longitudinal direction. We give
general formulas for for arbitrary . We also present plots of
zeros of the partition function in the plane for various values of and
in the plane for various values of . Explicit results for partition
functions are given in the text for (free) and (cylindrical),
and plots of partition function zeros are given for up to 5 (free) and
(cylindrical). Plots of the internal energy and specific heat per site
for infinite-length strips are also presented.Comment: 39 pages, 34 eps figures, 3 sty file
2,2-Bis[(2-halo-4-aminophenoxy)phenyl]-hexafluoropropane
There are provided the aromatic diamines 2,2-bis-[(2-halo-4-aminophenoxy)-phenyl]hexafluoropropane, where the attached ortho halogen is preferably chlorine, and 4,4'-bis(4-aminophenoxy)biphenyl, as novel monomers for polyimide polymerizations. The former, when reacted with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, provides a polyimide having exceptional high-temperature performance. The latter diamine is a low-cost monomer for polyimide production
High-temperature polyimides prepared from 2,2-bis-[(2-halo-4-aminophenoxy)-phenyl]hexafluoropropane
There are provided the aromatic diamines 2,2-bis-[(2-halo-4-aminophenoxy)-phenyl]hexafluoropropane, where the attached ortho halogen is preferably chlorine, and 4,4'-bis(4-aminophenoxy)biphenyl, as novel monomers for polyimide polymerizations. The former, when reacted with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, provides a polyimide having exceptional high-temperature performance. The latter diamine is a low-cost monomer for polyimide production
Fiber reinforced thermoplastic resin matrix composites
Polyimide polymer composites having a combination of enhanced thermal and mechanical properties even when subjected to service temperatures as high as 700.degree. F. are described. They comprise (a) from 10 to 50 parts by weight of a thermoplastic polyimide resin prepared from 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane and (b) from 90 to 50 parts by weight of continuous reinforcing fibers, the total of (a) and (b) being 100 parts by weight. Composites based on polyimide resin formed from 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane and pyromellitic dianhydride and continuous carbon fibers retained at least about 50% of their room temperature shear strength after exposure to 700.degree. F. for a period of 16 hours in flowing air. Preferably, the thermoplastic polyimide resin is formed in situ in the composite material by thermal imidization of a corresponding amide-acid polymer prepared from 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane. It is also preferred to initially size the continuous reinforcing fibers with up to about one percent by weight of an amide-acid polymer prepared from 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane. In this way imidization at a suitable elevated temperature results in the in-situ formation of a substantially homogeneous thermoplastic matrix of the polyimide resin tightly and intimately bonded to the continuous fibers. The resultant composites tend to have optimum thermo-mechanical properties
Prediction of Nontrivial Band Topology and Superconductivity in MgPb
The interplay of BCS superconductivity and nontrivial band topology is
expected to give rise to opportunities for creating topological
superconductors, achieved through pairing spin-filtered boundary modes via
superconducting proximity effects. The thus-engineered topological
superconductivity can, for example, facilitate the search for Majorana fermion
quasiparticles in condensed matter systems. Here we report a first-principles
study of MgPb and predict that it should be a superconducting topological
material. The band topology of MgPb is identical to that of the archetypal
quantum spin Hall insulator HgTe, while isostructural and isoelectronic
MgSn is topologically trivial; a trivial to topological transition is
predicted for MgSnPb for x~0.77. We propose that
MgPb-MgSn quantum wells should generate robust spin-filtered edge
currents in analogy to HgTe/CdTe quantum wells. In addition, our calculations
predict that MgPb should become superconducting upon electron doping.
Therefore, MgPb is expected to provide a practical material platform for
studying emergent phenomena arising from the interplay of superconductivity and
band topology.Comment: 5 figure
Photoluminescence modification by high-order photonic band with abnormal dispersion in ZnO inverse opal
We measured the angle- and polarization-resolved reflection and
photoluminescence spectra of ZnO inverse opals. Significant enhancement of
spontaneous emission is observed. The enhanced emission not only has good
directionality but also can be linearly polarized. A detailed theoretical
analysis and numerical simulation reveal that such enhancement is caused by the
abnormal dispersion of a high-order photonic band. The frozen mode at a
stationary inflection point of a dispersion curve can strongly modify the
intensity, directionality and polarization of spontaneous emission.Comment: 22 pages, 11 figures, figures modified, references added, more
explanation adde
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