A new picture of the Lifshitz critical behavior

New field theoretic renormalization group methods are developed to describe in a unified fashion the critical exponents of an m-fold Lifshitz point at the two-loop order in the anisotropic (m not equal to d) and isotropic (m=d close to 8) situations. The general theory is illustrated for the N-vector phi^4 model describing a d-dimensional system. A new regularization and renormalization procedure is presented for both types of Lifshitz behavior. The anisotropic cases are formulated with two independent renormalization group transformations. The description of the isotropic behavior requires only one type of renormalization group transformation. We point out the conceptual advantages implicit in this picture and show how this framework is related to other previous renormalization group treatments for the Lifshitz problem. The Feynman diagrams of arbitrary loop-order can be performed analytically provided these integrals are considered to be homogeneous functions of the external momenta scales. The anisotropic universality class (N,d,m) reduces easily to the Ising-like (N,d) when m=0. We show that the isotropic universality class (N,m) when m is close to 8 cannot be obtained from the anisotropic one in the limit d --> m near 8. The exponents for the uniaxial case d=3, N=m=1 are in good agreement with recent Monte Carlo simulations for the ANNNI model.Comment: 48 pages, no figures, two typos fixe

Susceptibility amplitude ratio for generic competing systems

We calculate the susceptibility amplitude ratio near a generic higher character Lifshitz point up to one-loop order. We employ a renormalization group treatment with $L$ independent scaling transformations associated to the various inequivalent subspaces in the anisotropic case in order to compute the ratio above and below the critical temperature and demonstrate its universality. Furthermore, the isotropic results with only one type of competition axes have also been shown to be universal. We describe how the simpler situations of $m$-axial Lifshitz points as well as ordinary (noncompeting) systems can be retrieved from the present framework.Comment: 20 pages, no figure

Is there a common water-activity limit for the three domains of life?

Archaea and Bacteria constitute a majority of life systems on Earth but have long been considered inferior to Eukarya in terms of solute tolerance. Whereas the most halophilic prokaryotes are known for an ability to multiply at saturated NaCl (water activity (a w) 0.755) some xerophilic fungi can germinate, usually at high-sugar concentrations, at values as low as 0.650-0.605 a w. Here, we present evidence that halophilic prokayotes can grow down to water activities of <0.755 for Halanaerobium lacusrosei (0.748), Halobacterium strain 004.1 (0.728), Halobacterium sp. NRC-1 and Halococcus morrhuae (0.717), Haloquadratum walsbyi (0.709), Halococcus salifodinae (0.693), Halobacterium noricense (0.687), Natrinema pallidum (0.681) and haloarchaeal strains GN-2 and GN-5 (0.635 a w). Furthermore, extrapolation of growth curves (prone to giving conservative estimates) indicated theoretical minima down to 0.611 a w for extreme, obligately halophilic Archaea and Bacteria. These were compared with minima for the most solute-tolerant Bacteria in high-sugar (or other non-saline) media (Mycobacterium spp., Tetragenococcus halophilus, Saccharibacter floricola, Staphylococcus aureus and so on) and eukaryotic microbes in saline (Wallemia spp., Basipetospora halophila, Dunaliella spp. and so on) and high-sugar substrates (for example, Xeromyces bisporus, Zygosaccharomyces rouxii, Aspergillus and Eurotium spp.). We also manipulated the balance of chaotropic and kosmotropic stressors for the extreme, xerophilic fungi Aspergillus penicilloides and X. bisporus and, via this approach, their established water-activity limits for mycelial growth (∼0.65) were reduced to 0.640. Furthermore, extrapolations indicated theoretical limits of 0.632 and 0.636 a w for A. penicilloides and X. bisporus, respectively. Collectively, these findings suggest that there is a common water-activity limit that is determined by physicochemical constraints for the three domains of life

Liquid single crystal elastomer/conducting polymer bilayer composite actuator: Modelling and experiments

In order to integrate electroconductive properties in a Liquid Single Crystal Elastomer (LSCE) and to test direct actuation of the LSCE by Joule heating, we present a new bi-layered all-organic composite actuator based on the coupling of a nematic LSCE with a conductive polymer. The bending actuator is fabricated by depositing a thin conductive polymer layer of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) over the surface of a polysiloxane-based monodomain nematic LSCE film. Mechanical properties of PEDOT:PSS, better matched with LSCE ones compared with metals or inorganic nanoparticles used in other approaches, allowed us to develop an all-organic reliable millimetre-scale actuating composite. The thermally induced elongation/compression of the LSCE over 30% is exploited for the fabrication of bending actuators with curvature up to κ = 0.64 mm-1. The LSCE and the composite material are characterized as regards their thermo-mechanical and electrical properties. A model is introduced to describe bending of the composite as a function of the thermo-mechanical properties of the LSCE, and the model is assessed by comparing the model results with the experimental findings. Bending actuation via direct Joule heating of the composite is also assessed by supplying the necessary current (50 mA at 1.3 V) through wires connected to the composite. These results open new possibilities for the application of LCEs in the micro and soft robotics fields, as well as in the biomedical field. © 2013 The Royal Society of Chemistry

Reversible heat-induced microwrinkling of PEDOT:PSS nanofilm surface over a monodomain liquid crystal elastomer

New bilayered composite systems with tunable and temperature-dependent formation of periodical wrinkles on the surface are the object of this report. The samples were prepared by spin-coating deposition of a thin film of the conducting polymer poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on the surface of standard monodomain liquid crystal elastomer (LCE) films. Several bilayered materials were prepared by changing the thickness of PEDOT:PSS nanofilms. Basic characterization showed very good stability and adhesion between the two components also after performing multiple heat cycles around nematic-to-isotropic transition temperature of the LCE. Interestingly, formation of uniaxially aligned microwrinkles was observed, with most of the wrinkles aligned along perpendicular direction with respect to the nematic director, due to reversible elongation/compression of the LCE during thermal cycles. © 2013 Copyright Taylor and Francis Group, LLC

13 C NMR and EPR of carbon nanofoam

The 13C NMR spectra of a carbon nanofoam sample, exhibiting a peak in the ZFC magnetic susceptibility, show a single 13C line at room temperature and a two peak structure at 100 K. The splitting and the NMR shifts are however much too small to be associated with true ferromagnetism as for instance observed in TDAE-C60 below TC. The X-band EPR spectra show the presence of a least three different paramagnetic clusters with ga = 2.0016, gb = 2.0031, gc = 2.0036 and very different electronic spin-lattice relaxation times. In some samples the CW EPR spectra show an additional rather broad and strongly temperature dependent line shifted to lower fields, which is characteristic of ferromagnetic systems. The results thus show an inhomogeneous super-paramagnetic or spin-glass like nature of magnetism in cluster assembled carbon nanofoam rather than homogeneous ferromagnetism