219,501 research outputs found
Nonuniversal finite-size scaling in anisotropic systems
We study the bulk and finite-size critical behavior of the O symmetric
theory with spatially anisotropic interactions of non-cubic symmetry
in dimensions. In such systems of a given universality class,
two-scale factor universality is absent in bulk correlation functions, and
finite-size scaling functions including the Privman-Fisher scaling form of the
free energy, the Binder cumulant ratio and the Casimir amplitude are shown to
be nonuniversal. In particular it is shown that, for anisotropic confined
systems, isotropy cannot be restored by an anisotropic scale transformation.Comment: 8 pages, 1 figure, accepted for publication in Phys. Rev. E and
modifications of tex
Finite-Size Effects in the Field Theory Above the Upper Critical Dimension
We demonstrate that the standard O(n) symmetric field theory does
not correctly describe the leading finite-size effects near the critical point
of spin systems on a -dimensional lattice with . We show that these
finite-size effects require a description in terms of a lattice Hamiltonian.
For and explicit results are given for the susceptibility
and for the Binder cumulant. They imply that recent analyses of Monte-Carlo
results for the five-dimensional Ising model are not conclusive.Comment: 4 pages, latex, 1 figur
Exceeding the solar cell Shockley-Queisser limit via thermal up-conversion of low-energy photons
Maximum efficiency of ideal single-junction photovoltaic (PV) cells is
limited to 33% (for one sun illumination) by intrinsic losses such as band edge
thermalization, radiative recombination, and inability to absorb below-bandgap
photons. This intrinsic thermodynamic limit, named after Shockley and Queisser
(S-Q), can be exceeded by utilizing low-energy photons either via their
electronic up-conversion or via thermophotovoltaic (TPV) conversion process.
However, electronic up-conversion systems have extremely low efficiencies, and
practical temperature considerations limit the operation of TPV converters to
the narrow-gap PV cells. Here we develop a conceptual design of a hybrid TPV
platform, which exploits thermal up-conversion of low-energy photons and is
compatible with conventional silicon PV cells by using spectral and directional
selectivity of the up-converter. The hybrid platform offers
sunlight-to-electricity conversion efficiency exceeding that imposed by the S-Q
limit on the corresponding PV cells across a broad range of bandgap energies,
under low optical concentration (1-300 suns), operating temperatures in the
range 900-1700K, and in simple flat panel designs. We demonstrate maximum
conversion efficiency of 73% under illumination by non-concentrated sunlight. A
detailed analysis of non-ideal hybrid platforms that allows for up to 15% of
absorption/re-emission losses yields limiting efficiency value of 45% for Si PV
cells.Comment: 28 pages, 9 figure
Non-universal critical Casimir force in confined He near the superfluid transition
We present the results of a one-loop calculation of the effect of a van der
Waals type interaction potential on the critical
Casimir force and specific heat of confined He near the superfluid
transition. We consider a He film of thickness . In the region (correlation length) we find that the van der Waals interaction
causes a leading non-universal non-scaling contribution of to the critical temperature dependence of the Casimir force
above that dominates the universal scaling contribution predicted by earlier theories. For the specific heat we find subleading
non-scaling contributions of and .Comment: 2 pages, submitted to LT23 Proceedings on June 14, 2002, accepted for
publication in Physica B on September 12, 200
Interleukin 2 transcription factors as molecular targets of cAMP inhibition: delayed inhibition kinetics and combinatorial transcription roles
Elevation of cAMP can cause gene-specific inhibition of interleukin 2 (IL-2) expression. To investigate the mechanism of this effect, we have combined electrophoretic mobility shift assays and in vivo genomic footprinting to assess both the availability of putative IL-2 transcription factors in forskolin-treated cells and the functional capacity of these factors to engage their sites in vivo. All observed effects of forskolin depended upon protein kinase A, for they were blocked by introduction of a dominant negative mutant subunit of protein kinase A. In the EL4.E1 cell line, we report specific inhibitory effects of cAMP elevation both on NF-κB/Rel family factors binding at -200 bp, and on a novel, biochemically distinct "TGGGC" factor binding at -225 bp with respect to the IL-2 transcriptional start site. Neither NF-AT nor AP-1 binding activities are detectably inhibited in gel mobility shift assays. Elevation of cAMP inhibits NF-κB activity with delayed kinetics in association with a delayed inhibition of IL-2 RNA accumulation. Activation of cells in the presence of forskolin prevents the maintenance of stable protein-DNA interactions in vivo, not only at the NF-κB and TGGGC sites of the IL-2 enhancer, but also at the NF-AT, AP-1, and other sites. This result, and similar results in cyclosporin A-treated cells, imply that individual IL-2 transcription factors cannot stably bind their target sequences in vivo without coengagement of all other distinct factors at neighboring sites. It is proposed that nonhierarchical, cooperative enhancement of binding is a structural basis of combinatorial transcription factor action at the IL-2 locus
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