Universality classes

I am delighted to express my appreciation to the attendees of more than 80 short courses from 1971 through 1995 at government laboratories, companies, and open locations, " ey helped shape this second edition by their questions and comments, as did the more than twenty university classes I taught over the years. 

Another interesting version of the MM model considers a variable excluded-volume interaction between same species particles [92]. In the absence of interactions the system is mapped on the standard MM model which has a first-order IPT between A- and B-saturated phases. On increasing the strength of the interaction the first-order transition line, observed for weak interactions, terminates at a tricritical point where two second-order transitions meet. These transitions, which separate the A-saturated, reactive, and B-saturated phases, belong to the same universality class as directed percolation, as follows from the value of critical exponents calculated by means of time-dependent Monte Carlo simulations and series expansions [92]. 

E. V. Albano. On the universality classes of the discontinuous irreversible phase transitions of a multicomponent reaction system. J Phys A (Math Gen) 27 3751-3758, 1994. 

The phase separation process at late times t is usually governed by a law of the type R t) oc f, where R t) is the characteristic domain size at time t, and n an exponent which depends on the universality class of the model and on the conservation laws in the dynamics. At the presence of amphiphiles, however, the situation is somewhat complicated by the fact that the amphiphiles aggregate at the interfaces and reduce the interfacial tension during the coarsening process, i.e., the interfacial tension depends on the time. This leads to a pronounced slowing down at late times. In order to quantify this effect, Laradji et al. [217,222] have proposed the scaling ansatz... 

In fact, the variable x /Gi controls the "crossover" from one "universality class" " to the other. I.e., there exists a crossover scaling description where data for various Gi (i.e., various N) can be collapsed on a master curve Evidence for this crossover scaling has been seen both in experiments and in Monte Carlo simulations for the bond fluctuation model of symmetric polymer mixtures, e.g Fig. 1. One expects a scaling of the form... 

Many more such relationships can be derived in a similar manner (see [ma85] or [stan71]). For our purposes here, it will suffice to merely take note of the fact that certain relationships among the critical exponents do exist and are in fact commonly exploited. Indeed, we shall soon sec that certain estimates of critical behavior in probabilistic CA system are predicated on the assumptions that (1) certain rules fall into in the same universality class as directed percolation, and (2) the same relationships known to exist among critical exponents in directed percolation must also hold true for PC A (see section 7.2). 

Fig. 10.8. The ordering operator distribution for the three-dimensional Ising universality class (continuous line - data are courtesy of N.B. Wilding). Points are for a homopolymer of chain length r = 200 on a 50 x 50 x 50 simple cubic lattice of coordination number z = 26 [48], The nonuniversal constant A and the critical value of the ordering operator Mc were chosen so that the data have zero mean and unit variance. Reprinted by permission from [6], 2000 IOP Publishing Ltd...

Taking the experimentally measured mass spectrum of hadrons up to 2.5 GeV from the Particle Data Group, Pascalutsa (2003) could show that the hadron level-spacing distribution is remarkably well described by the Wigner surmise for / = 1 (see Fig. 6). This indicates that the fluctuation properties of the hadron spectrum fall into the GOE universality class, and hence hadrons exhibit the quantum chaos phenomenon. One then should be able to describe the statistical properties of hadron spectra using RMT with random Hamiltonians from GOE that are characterized by good time-reversal and rotational symmetry. 

The second virial coefficient is not a universal quantity but depends on the primary chemical structure and the resulting topology of their architecture. It also depends on the conformation of the macromolecules in solution. However, once these individual (i.e., non-universal) characteristics are known, the data can be used as scaling parameters for the description of semidilute solutions. Such scaling has been very successful in the past with flexible linear chains [4, 18]. It also leads for branched macromolecules to a number of universality classes which are related to the various topological classes [9-11,19]. These conclusions will be outlined in the section on semidilute solutions. 

Figures 6-9 illustrate the use of these finite size scaling relations for the square lattice gas with repulsion between both nearest and next nearest neighbors. In Fig. 6 the raw data of Fig. 5 are replotted in scaled form, as suggested by Eq. (37). Note that neither = TJcc) nor the critical exponents are known in beforehand - the phase transition of the (2x1) phase falls in the universality class of the XY model with uniaxial anisotropy which has nonuniversal exponents depending on R. Clearly, it is desirable to estimate without being biased by the choice of the critical exponents. This is possible...

It is not clear whether in the centered rectangular lattice gas of section 3.2 such a Kosterlitz-Thouless transition occurs, or whether the disordered phase extends, though being incommensurate, down to the commensurate (3x1) phase (then this transition is believed to belong to a new chiral universality class ), or whether there is another disorder line for (3 x 1) correlations. However, Kosterlitz-Thouless type transitions have been found for various two-dimensional models the XY ferromagnet , the Coulomb gas . ... 

Thus, there is a continuous variation in the dynamical exponent for 1 < a < 3, while the attachment-detachment universality class holds for a < 1 and the step-edge universality class holds for a > 3. 

Comparing this with Eq. (21) demonstrates that the dynamics of the first step is in the step-edge universality class, and (from Eq. (19))... 

We note that other systems not resembling the simple diatomic molecules considered here may follow a different relationship [86]. There may be other classes of reactions, dehydrogenation or —C bond breaking that may follow other similar relationships and thus form another universality class. We also note that there are exceptions to the relations, most notably for H2 dissociation on near-surface alloys [87]. These deviations from the rules are still describable within the d band model, though [87]. 

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