Dynamic Congruence vs. Progressing Bisimulation for CCS

Weak Observational Congruence (woc) defined on CCS agents is not a bisimulation since it does not require two states reached by bisimilar computations of woc agents to be still woc, e.g. \alpha.\tau.\beta.nil and \alpha.\beta.nil are woc but \tau.\beta.nil and \beta.nil are not. This fact prevent us from characterizing CCS semantics (when \tau is considered invisible) as a final algebra, since the semantic function would induce an equivalence over the agents that is both a congruence and a bisimulation. In the paper we introduce a new behavioural equivalence for CCS agents, which is the coarsest among those bisimulations which are also congruences. We call it Dynamic Observational Congruence because it expresses a natural notion of equivalence for concurrent systems required to simulate each other in the presence of dynamic, i.e. run time, (re)configurations. We provide an algebraic characterization of Dynamic Congruence in terms of a universal property of finality. Furthermore we introduce Progressing Bisimulation, which forces processes to simulate each other performing explicit steps. We provide an algebraic characterization of it in terms of finality, two logical characterizations via modal logic in the style of HML and a complete axiomatization for finite agents (consisting of the axioms for Strong Observational Congruence and of two of the three Milner's $\tau$-laws). Finally, we prove that Dynamic Congruence and Progressing Bisimulation coincide for CCS agents

Dependence of transport coefficients of Yb(Rh1−x_{1-x}Cox_x)2_2Si2_2 intermetallics on temperature and cobalt concentration

Dependence of transport coefficients of the Yb(Rh$_{1-x}$Co$_x$)$_2$Si$_2$ series of alloys on temperature and cobalt concentration is explained by an asymmetric Anderson model which takes into account the exchange scattering of conduction electrons on ytterbium ions and the splitting of 4$f$-states by the crystalline electric field (CEF). The substitution of rhodium by cobalt is described as an increase of chemical pressure which reduces the exchange coupling and the CEF splitting. The scaling analysis and numerical NCA solution of the model show that the effective degeneracy of the 4$f$-state at a given temperature depends on the relative magnitude of the Kondo scale and the CEF splitting. Thus, we find that dependence of the thermopower, $S(T)$, on temperature and cobalt concentration can be understood as an interplay of quantum fluctuations, driven by the Kondo effect, and thermal fluctuations, which favor a uniform occupation of the CEF states. The theoretical model captures all the qualitative features of the experimental data and it explains the evolution of the shape of $S(T)$ with the increase of cobalt concentration.Comment: 8 pages, 4 figure