2d frustrated Ising model with four phases

In this paper we consider a 2d random Ising system on a square lattice with nearest neighbour interactions. The disorder is short range correlated and asymmetry between the vertical and the horizontal direction is admitted. More precisely, the vertical bonds are supposed to be non random while the horizontal bonds alternate: one row of all non random horizontal bonds is followed by one row where they are independent dichotomic random variables. We solve the model using an approximate approach that replace the quenched average with an annealed average under the constraint that the number of frustrated plaquettes is keep fixed and equals that of the true system. The surprising fact is that for some choices of the parameters of the model there are three second order phase transitions separating four different phases: antiferromagnetic, glassy-like, ferromagnetic and paramagnetic.Comment: 17 pages, Plain TeX, uses Harvmac.tex, 4 ps figures, submitted to Physical Review

Non-Mean-Field Behavior of Realistic Spin Glasses

We provide rigorous proofs which show that the main features of the Parisi solution of the Sherrington-Kirkpatrick spin glass are not valid for more realistic spin glass models in any dimension and at any temperature.Comment: LaTeX file, 8 page

Complex Random Energy Model: Zeros and Fluctuations

The partition function of the random energy model at inverse temperature $\beta$ is a sum of random exponentials $Z_N(\beta)=\sum_{k=1}^N \exp(\beta \sqrt{n} X_k)$, where $X_1,X_2,...$ are independent real standard normal random variables (= random energies), and $n=\log N$. We study the large $N$ limit of the partition function viewed as an analytic function of the complex variable $\beta$. We identify the asymptotic structure of complex zeros of the partition function confirming and extending predictions made in the theoretical physics literature. We prove limit theorems for the random partition function at complex $\beta$, both on the logarithmic scale and on the level of limiting distributions. Our results cover also the case of the sums of independent identically distributed random exponentials with any given correlations between the real and imaginary parts of the random exponent.Comment: 31 pages, 1 figur

Regularity Properties and Pathologies of Position-Space Renormalization-Group Transformations

We reconsider the conceptual foundations of the renormalization-group (RG) formalism, and prove some rigorous theorems on the regularity properties and possible pathologies of the RG map. Regarding regularity, we show that the RG map, defined on a suitable space of interactions (= formal Hamiltonians), is always single-valued and Lipschitz continuous on its domain of definition. This rules out a recently proposed scenario for the RG description of first-order phase transitions. On the pathological side, we make rigorous some arguments of Griffiths, Pearce and Israel, and prove in several cases that the renormalized measure is not a Gibbs measure for any reasonable interaction. This means that the RG map is ill-defined, and that the conventional RG description of first-order phase transitions is not universally valid. For decimation or Kadanoff transformations applied to the Ising model in dimension $d \ge 3$, these pathologies occur in a full neighborhood $\{ \beta > \beta_0 ,\, |h| < \epsilon(\beta) \}$ of the low-temperature part of the first-order phase-transition surface. For block-averaging transformations applied to the Ising model in dimension $d \ge 2$, the pathologies occur at low temperatures for arbitrary magnetic-field strength. Pathologies may also occur in the critical region for Ising models in dimension $d \ge 4$. We discuss in detail the distinction between Gibbsian and non-Gibbsian measures, and give a rather complete catalogue of the known examples. Finally, we discuss the heuristic and numerical evidence on RG pathologies in the light of our rigorous theorems.Comment: 273 pages including 14 figures, Postscript, See also ftp.scri.fsu.edu:hep-lat/papers/9210/9210032.ps.

In situ DRIFTS study of surface species formed over sodium promoted Pt/Al2O3 catalysts during the reduction of NO by C3H6”

Summarization: The nature and the relative population of adsorbed species formed on the surface of un-promoted and sodium promoted Pt catalysts supported on γ-Al2O3 was investigated by using in situ DRIFT Spectroscopy, during the NO reduction by propene. Under steady-state reaction conditions, the surface of the un-promoted Pt/γ-Al2O3 catalyst is mainly covered by adsorbed hydrocarbon fragments, formates, acetates and cyanide species. In contrast, the surface of Na-promoted Pt(Na)/γ- Al2O3 catalysts is predominantly covered by NOx ad-species, carbonyls, cyanides and isocyanates, whereas no hydrocarbons fragments were detected under reaction conditions. The present results demonstrate that the strongly enhanced catalytic performance of Na-modified Pt(Na)/Al2O3 catalysts, can be attributed to the pronounced effect of alkalis on the hydrocarbon and NO chemisorption bonds, which in turn prevents self-poisoning of catalyst surface by strongly bonded hydrocarbon fragments and carboxylates.Παρουσιάστηκε στο: Protection and Restoration of the Environment I

Spectroscopic evidence for the mode of action of alkali promoters in Pt-catalyzed de-NOx chemistry

Summarization: The interaction of NO with sodium-dosed Pt(Na)/γ-Al2O3 catalysts was studied by means of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). With increasing sodium loading, pronounced and progressive red shifts of the Nsingle bondO stretching frequency associated with molecular NO adsorbed on the Pt component were observed. This decrease in ν(NO) correlates with enhancement of NO dissociation at higher temperatures on Na-promoted Pt catalysts under conditions where clean Pt is almost ineffective. These spectroscopic observations provide a clear and consistent explanation for the recently reported very strong promotion by alkalis of the performance of supported Pt catalysts in de-NOx chemistryPresented on: Applied Catalysis B: Environmenta