81 research outputs found
Landau Theory of Barocaloric Plastic Crystals
We present a simple Landau phenomenology for plastic-to-crystal phase
transitions and use the resulting model to calculate barocaloric effects in
plastic crystals that are driven by hydrostatic pressure. The essential
ingredients of the model are (i) a multipole-moment order parameter that
describes the orientational ordering of the constituent molecules, (ii)
coupling between such order parameter and elastic strains, and (iii) the
thermal expansion of the solid. The model captures main features of
plastic-to-crystal phase transitions, namely large volume and entropy changes
at the transition, and strong dependence of the transition temperature with
pressure. Using solid C under GPa as case example, we show that
calculated peak isothermal entropy changes of and peak adiabatic entropy changes of agree well
with experimental values.Comment: 17 pages, 3 figure
Band structure of hydrogenated Si nanosheets and nanotubes
The band structure of fully hydrogenated Si nanosheets and nanotubes are
elucidated by the use of an empirical tight-binding model. The hydrogenated Si
sheet is a semiconductor with indirect band gap of about 2.2 eV. The symmetries
of the wave functions allow us to explain the origin of the gap. We predict
that, for certain chiralities, hydrogenated Si nanotubes represent a new type
of semiconductor, one with co-existing direct and indirect gaps of exactly the
same magnitude. This behavior is different from the Hamada rule established for
non-hydrogenated carbon and silicon nanotubes. Comparison to an ab initio
calculation is made.Comment: 9 pages, 4 figures, to appear in J. Phys.: Condens. Matte
Electronic structure of silicon-based nanostructures
We have developed an unifying tight-binding Hamiltonian that can account for
the electronic properties of recently proposed Si-based nanostructures, namely,
Si graphene-like sheets and Si nanotubes. We considered the and
models up to first- and second-nearest neighbors, respectively. Our
results show that the Si graphene-like sheets considered here are metals or
zero-gap semiconductors, and that the corresponding Si nanotubes follow the
so-called Hamada's rule [Phys. Rev. Lett. {\bf 68}, 1579 1992]. Comparison to a
recent {\it ab initio} calculation is made.Comment: 12 pages, 6 Figure
A variational method in the problem of screening an external charge in strongly correlated metals
We describe a variational calculation for the problem of screening of a point
charge in a layered correlated metal for dopings close to the Mott transition
where the screening is non-linear due to the proximity to the incompressible
insulating state. We find that external charge can induce locally
incompressible regions and that the non-linear dependence of the screening on
density can induce overscreening in the nearest nearby layers while preserving
overall charge neutrality.Comment: 7 pages, 1 figure, final version as publishe
Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites
Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term structural negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near T=0 K yielding the most prominent displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. The effects of compositional disorder on NTE and structural phase stability in perovskites are discussed
Structure factor of a relaxor ferroelectric
We study a minimal model for a relaxor ferroelectric including dipolar interactions, and short-range harmonic and anharmonic forces for the critical modes as in the theory of pure ferroelectrics together with quenched disorder coupled linearly to the critical modes. We present the simplest approximate solution of the model necessary to obtain the principal features of the correlation functions. Specifically, we calculate and compare the structure factor measured by neutron scattering in different characteristic regimes of temperature in the relaxor PbMg1/3Nb2/3O3.U.S. Department of Energy, Office of Basic Energy Sciences under contract no. DE-AC02-06CH11357Universidad de Costa Rica. Vicerrectoría de Investigación ( project no. 816-B5-220 )UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA
Large isotropic negative thermal expansion above a structural quantum phase transition
Perovskite structured materials contain myriad tunable ordered phases of electronic and magnetic origin with proven technological importance and strong promise for a variety of energy solutions. An always-contributing influence beneath these cooperative and competing interactions is the lattice, whose physics may be obscured in complex perovskites by the many coupled degrees of freedom, which makes these systems interesting. Here, we report signatures of an approach to a quantum phase transition very near the ground state of the nonmagnetic, ionic insulating, simple cubic perovskite material ScF3, and show that its physical properties are strongly effected as much as 100 K above the putative transition. Spatial and temporal correlations in the high-symmetry cubic phase determined using energy- and momentum-resolved inelastic x-ray scattering as well as x-ray diffraction reveal that soft mode, central peak, and thermal expansion phenomena are all strongly influenced by the transition.National Science Foundation Award No. DMR-1506825US Department of Energy, Office of Basic Energy Sciences under Contract No. DE- AC02-06CH11357Yale Prize Postdoctoral FellowshipNSF Grant No. DMR-0115852Universidad de Costa Rica. Vicerrectoría de Investigación Projecto No. 816-B5-220UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA
Brucella abortus Uses a Stealthy Strategy to Avoid Activation of the Innate Immune System during the Onset of Infection
To unravel the strategy by which Brucella abortus establishes chronic infections, we explored its early interaction with innate immunity.
Methodology/Principal Findings
Brucella did not induce proinflammatory responses as demonstrated by the absence of leukocyte recruitment, humoral or cellular blood changes in mice. Brucella hampered neutrophil (PMN) function and PMN depletion did not influence the course of infection. Brucella barely induced proinflammatory cytokines and consumed complement, and was strongly resistant to bactericidal peptides, PMN extracts and serum. Brucella LPS (BrLPS), NH-polysaccharides, cyclic glucans, outer membrane fragments or disrupted bacterial cells displayed low biological activity in mice and cells. The lack of proinflammatory responses was not due to conspicuous inhibitory mechanisms mediated by the invading Brucella or its products. When activated 24 h post-infection macrophages did not kill Brucella, indicating that the replication niche was not fusiogenic with lysosomes. Brucella intracellular replication did not interrupt the cell cycle or caused cytotoxicity in WT, TLR4 and TLR2 knockout cells. TNF-α-induction was TLR4- and TLR2-dependent for live but not for killed B. abortus. However, intracellular replication in TLR4, TLR2 and TLR4/2 knockout cells was not altered and the infection course and anti-Brucella immunity development upon BrLPS injection was unaffected in TLR4 mutant mice.
Conclusion/Significance
We propose that Brucella has developed a stealth strategy through PAMPs reduction, modification and hiding, ensuring by this manner low stimulatory activity and toxicity for cells. This strategy allows Brucella to reach its replication niche before activation of antimicrobial mechanisms by adaptive immunity. This model is consistent with clinical profiles observed in humans and natural hosts at the onset of infection and could be valid for those intracellular pathogens phylogenetically related to Brucella that also cause long lasting infections
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