Nonequilibrium critical behavior in unidirectionally coupled stochastic processes

Phase transitions from an active into an absorbing, inactive state are generically described by the critical exponents of directed percolation (DP), with upper critical dimension d(c)=4. In the framework of single-species reaction-diffusion systems, this universality class is realized by the combined processes A-->A+A, A+A-->A, and A-->0. We study a hierarchy of such DP processes for particle species A,B,..., unidirectionally coupled via the reactions A-->B, ...(with rates mu(AB),...). When the DP critical points at all levels coincide, multicritical behavior emerges, with density exponents beta(i) which are markedly reduced at each hierarchy level i> or =2. This scenario can be understood on the basis of the mean-field rate equations, which yield beta(i)=1/2(i-1) at the multicritical point. Using field-theoretic renormalization-group techniques in d=4-epsilon dimensions, we identify a new crossover exponent phi, and compute phi=1+O(epsilon(2)) in the multicritical regime (for small mu(AB)) of the second hierarchy level. In the active phase, we calculate the fluctuation correction to the density exponent on the second hierarchy level, beta(2)=1/2-epsilon/8+O(epsilon(2)). Outside the multicritical region, we discuss the crossover to ordinary DP behavior, with the density exponent beta(1)=1-epsilon/6+O(epsilon(2)). Monte Carlo simulations are then employed to confirm the crossover scenario, and to determine the values for the new scaling exponents in dimensions d< or =3, including the critical initial slip exponent. Our theory is connected to specific classes of growth processes and to certain cellular automata, and the above ideas are also applied to unidirectionally coupled pair annihilation processes. We also discuss some technical as well as conceptual problems of the loop expansion, and suggest some possible interpretations of these difficulties.     

The influence of strongly reducing conditions on strong metal-support interactions in Cu/ZnO catalysts used for methanol synthesis

A systematic series of binary and ternary copper catalysts was investigated using the methanol synthesis reaction at atmospheric pressure. Strong metal-support interactions between copper and zinc oxide induced by strongly reducing conditions were probed by the adsorption of carbon monoxide, which was monitored qualitatively and quantitatively by a combination of microcalorimetry, temperature-programmed desorption experiments and Fourier transform infrared spectroscopy. For the zinc oxide-containing catalysts, the pretreatment in flowing carbon monoxide at 493 K resulted in a severe decoration of the copper metal particles with ZnOx adspecies, whereas after methanol synthesis at 493 K the state of the copper was essentially identical to that seen after hydrogen reduction. Copper was always found to be present in its zero-valent state.     

Monte Carlo study of the spin-1 Blume–Capel Ising film

Using Monte Carlo simulations with the Metropolis algorithm, we have studied the influence of crystal-field interaction on the critical behavior of magnetic spin-1 Ising film on a cubic lattice structure. The phase diagrams in the (_kBTc/J,R=_Js/J)$(k_{\mathrm{B}}{T}_{\mathrm{c}}/J,R=J_{\mathrm{s}}/J)$ plane are obtained for different values of the crystal-field interaction. We found that the special point _Rsp(_Rc)$R_{\mathrm{sp}}(R_{\mathrm{c}})$, at which the critical temperature is independent of the film thickness N, is independent of the crystal-field interaction and that the system may exhibit a tricritical behavior.     

Significance of sample dimensions and other parameters for characterizing drawing process of polymer films

Summary Uniaxial drawing of polymer samples by tensile testing, machines usually is characterized by drawing temperature, velocity and ratio. These data in general do not succeed in describing the drawing process completely, therefore different and/or additional parameters are necessary for characterization of drawing process.Striplike specimens of polyamide 6 and polyethylene terephthalate films were cut and drawn to a constant draw ratio. Especially sample dimensions, drawing velocity, drawing temperature and ambient medium were varied. Local draw ratio, birefringence and yield stresses were investigated.

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Zusammenfassung Die Herstellung uniaxial verstreckter Proben mit Hilfe einer Zugpriifmaschine wird üblicherweise durch Angabe von Verstrecktemperatur, relativer Verstreckgeschwindigkeit und Verstreckgrad beschrieben. Es wird gezeigt, dag diese Angaben i. a. nicht ausreichen, um den Verstreckvorgang zu beschreiben, dag hingegen andere und/oder zusätzliche Verstreckparameter eine vollständige Charakterisierung des Verstreckprozesses eher ermöglichen. Hierzu werden aus Polyamid- und Polyäthylenterephthalat-Folien geschnittene Polymerbänder unter Variation von Probendimensionen, absoluter Verstreckgeschwindigkeit, Verstrecktemperatur und Umgebungsmedium uniaxial auf den gleichen relativen Verstreckgrad gedehnt und auf ihre lokale Dehnung und ihre Doppelbrechung hin untersucht.

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With 11 figures and 1 table