Non-equilibrium behavior at a liquid-gas critical point

Second-order phase transitions in a non-equilibrium liquid-gas model with reversible mode couplings, i.e., model H for binary-fluid critical dynamics, are studied using dynamic field theory and the renormalization group. The system is driven out of equilibrium either by considering different values for the noise strengths in the Langevin equations describing the evolution of the dynamic variables (effectively placing these at different temperatures), or more generally by allowing for anisotropic noise strengths, i.e., by constraining the dynamics to be at different temperatures in d_par- and d_perp-dimensional subspaces, respectively. In the first, case, we find one infrared-stable and one unstable renormalization group fixed point. At the stable fixed point, detailed balance is dynamically restored, with the two noise strengths becoming asymptotically equal. The ensuing critical behavior is that of the standard equilibrium model H. At the novel unstable fixed point, the temperature ratio for the dynamic variables is renormalized to infinity, resulting in an effective decoupling between the two modes. We compute the critical exponents at this new fixed point to one-loop order. For model H with spatially anisotropic noise, we observe a critical softening only in the d_perp-dimensional sector in wave vector space with lower noise temperature. The ensuing effective two-temperature model H does not have any stable fixed point in any physical dimension, at least to one-loop order. We obtain formal expressions for the novel critical exponents in a double expansion about the upper critical dimension d_c = 4 - d_par and with respect to d_par, i.e., about the equilibrium theory.Comment: 17 pages, revtex, one figure and EPJB style files include

Dipolar Interactions in Superconductor-Ferromagnet Heterostructures

We consider a simple model for a superlattice composed of a thin magnetic film placed between two bulk superconductors. The magnetic film is modelled by a planar but otherwise arbitrary distribution of magnetic dipoles and the superconductors are treated in the London approximation. Due to the linearity of the problem, we are able to compute the magnetic energy of the film in the presence of the superconductors. We show that in the case of small wavenumbers compared to the inverse London penetration depth, the magnetic energy resembles the energy of a distribution of magnetisation in a two dimensional space. Possible experimental applications of these results are discussed.Comment: RevTeX, 29 pages, 3 figures. To be published in Phys. Rev. B. Changes were made (including adding fig. 3), to conform with the referee's report. Reference 54 was also adde

Diffusion-annihilation dynamics in one spatial dimension

We discuss a reaction-diffusion model in one dimension subjected to an external driving force. Each lattice site may be occupied by at most one particle. The particles hop with asymmetric rates (the sum of which is one) to the right or left nearest neighbour site if it is vacant, and annihilate with rate one if it is occupied. We compute the long time behaviour of the space dependent average density in states where the initial density profiles are step functions. We also compute the exact time dependence of the particle density for uncorrelated random initial conditions. The representation of the uncorrelated random initial state and also of the step function profile in terms of free fermions allows the calculation of time-dependent higher order correlation functions. We outline the procedure using a field theoretic approach.Comment: 26 pages, 1 Postscript figure, uses epsf.st

The effects of violating detailed balance on critical dynamics

We present an overview of the effects of detailed-balance violating perturbations on the universal static and dynamic scaling behavior near a critical point. It is demonstrated that the standard critical dynamics universality classes are generally quite robust: In systems with non-conserved order parameter, detailed balance is effectively restored at criticality. This also holds for models with conserved order parameter, and isotropic non-equilibrium perturbations. Genuinely novel features are found only for models with conserved order parameter and spatially anisotropic noise correlations.Comment: 4 pages, revtex, no figure

The Casimir spectrum revisited

We examine the mathematical and physical significance of the spectral density sigma(w) introduced by Ford in Phys. Rev. D38, 528 (1988), defining the contribution of each frequency to the renormalised energy density of a quantum field. Firstly, by considering a simple example, we argue that sigma(w) is well defined, in the sense of being regulator independent, despite an apparently regulator dependent definition. We then suggest that sigma(w) is a spectral distribution, rather than a function, which only produces physically meaningful results when integrated over a sufficiently large range of frequencies and with a high energy smooth enough regulator. Moreover, sigma(w) is seen to be simply the difference between the bare spectral density and the spectral density of the reference background. This interpretation yields a simple `rule of thumb' to writing down a (formal) expression for sigma(w) as shown in an explicit example. Finally, by considering an example in which the sign of the Casimir force varies, we show that the spectrum carries no manifest information about this sign; it can only be inferred by integrating sigma(w).Comment: 10 pages, 4 figure

Strong light-matter interaction in systems described by a modified Dirac equation

The bulk states of some materials, such as topological insulators, are described by a modified Dirac equation. Such systems may have trivial and non-trivial phases. In this paper, we show that in the non-trivial phase a strong light-matter interaction exists in a two-dimensional system, which leads to an optical conductivity at least one order of magnitude larger than that of graphene.JES's work contract is financed in the framework of the Program of Recruitment of Post Doctoral Researchers for the Portuguese Scientific and Technological System, with the Operational Program Human Potential (POPH) of the QREN, participated by the European Social Fund (ESF) and national funds of the Portuguese Ministry of Education and Science (MEC). The authors acknowledge support provided to the current research project by FEDER through the COMPETE Program and by FCT in the framework of the Strategic Project PEST-C/FIS/UI607/2011

Electronic doping of graphene by deposited transition metal atoms

We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionisation potential of the metal clusters in terms of the chemical interaction between graphene and the clusters. We are also able, by modelling the metallic clusters as perfect conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.Comment: 16 pages, 10 figure

Adhesion of dental sealants to enamel with self-etching adhesives in salivar contamination conditions: Influence of the light curing protocol

Purpose: To evaluate the influence of the light curing protocol in the shear bond strength of a sealant to enamel treated with two self-etching adhesive systems, in salivary contamination conditions. Materials and Methods: The dental sealant (Delton, Dentsply) was applied, after saliva contamination, onto the vestibular enamel of sixty human incisors treated with Xeno III (Dentsply) or Prompt-L-Pop (3M/Espe). These two groups were further divided into two subgroups (n = 15) according to the curing time: 1) the adhesive system was cured with the sealant (co-polymerization), and 2) adhesive and sealant were light cured independently (independent polymerization). After the adhesive procedures, specimens were stored in water (37 °C–24 h) and thermal-cycled. Shear bond strength tests were done in an universal testing machine. Data was analyzed with two-way ANOVA. Results: There were no statistical differences (p = 0.267) between the adhesive systems tested. The co-polymerization groups (33.3 ± 9.4 MPa) yielded statistically higher shear bond strength values than the independent polymerization groups (28.2 ± 4.7 MPa). Even with saliva contamination, the self-etching adhesive systems used yielded high shear bond strength values. Conclusions: In the conditions tested, the co-polymerization of the adhesive systems with the sealant led to higher bond strength values to enamel than the independent polymerization.Objectivos: Avaliar a influência do momento da fotopolimerização de dois sistemas adesivos auto-condicionantes, nos valores de resistência adesiva de um selante de fissuras ao esmalte, em condições de contaminação salivar. Materiais e Métodos: O selante de fissuras (Delton, Dentsply) foi aplicado, após contaminação salivar, sobre o esmalte vestibular de sessenta incisivos humanos condicionados com Xeno III (Dentsply) ou Prompt-L-Pop (3M/Espe). Estes dois grupos foram posteriormente divididos em dois subgrupos (n = 15) de acordo com o momento da fotopolimerização do sistema adesivo: 1) o sistema adesivo foi fotopolimerizado conjuntamente com o selante (co-polymerization), e 2) sistema adesivo e selante foram fotopolimerizados independentemente (independent polymerization). Os espécimes foram armazenados em água (37 °C–24 h) e, posteriormente, submetidos a termociclagem. Os ensaios de resistência adesiva foram realizados numa máquina de testes universal. Os resultados foram submetidos a análise estatística ANOVA duas vias. Resultados: Não foram registadas diferenças estatisticamente significativas (p = 0,267) entre os sistemas adesivos usados. Os grupos co-polimerizados obtiveram valores de resistência adesiva mais elevados (33,3 ± 9,4 MPa) relativamente aos grupos em que a polimerização foi realizada de forma independente (28,2 ± 4,7 MPa). Mesmo na presença de contaminação salivar, os sistemas adesivos auto-condicionantes testados obtiveram valores de resistência adesiva elevados. Conclusões: Nas condições testadas, a co-polimerização do sistema adesivo com o selante originou valores de resistência adesiva ao esmalte superiores relativamente à polimerização independente destes dois materiais.info:eu-repo/semantics/publishedVersio

Renormalization of nanoparticle polarizability in the vicinity of a graphene-covered interface

We study the electromagnetic properties of a metamaterial consisting of polarizable (nano)particles and a single graphene sheet placed at the interface between two dielectrics. We show that the particle's polarizability is renormalized because of the electromagnetic coupling to surface plasmons supported by graphene, which results in a dispersive behavior, different for the polarizability components corresponding to the induced dipole moment, parallel and perpendicular to the graphene sheet. In particular, this effect is predicted to take place for a metallic particle whose bare polarizability in the terahertz (THz) region is practically equal to the cube of its radius (times $4\pi \varepsilon _0$). This opens the possibility to excite surface plasmons in graphene and enhance its absorption in the THz range by simply using a monolayer of metallic particles randomly deposited on top of it, as we show by explicit calculations.Financial support from the Portuguese Foundation for Science and Technology (FCT) through Projects PTDC-FIS-113199-2009 and PEst-C/FIS/UI0607/2013 is acknowledged. We also acknowledge support from the European Commission under Graphene Flagship (Contract No. CNECT-ICT-604391). J.E.S.'s work contract is financed in the framework of the Program of Recruitment of Post Doctoral Researchers for the Portuguese Scientific and Technological System, within the Operational Program Human Potential (POPH) of the QREN, participated by the European Social Fund (ESF) and national funds of the Portuguese Ministry of Education and Science (MEC)