Project number		Stand-alone Projects  P16574
Title		Critical pheneomena in random systems
Principal investigator		FOLK Reinhard
Approval date		06.05.2003
University / Research institution		Institut für Theoretische Physik, Universität Linz
Scientific field(s)
Keywords		critical phenomena, random systems, renormalization group theory, critical dynamics
Homepage		http://www.tphys.jku.at/group/folk/folk.html

Critical phenomena in pure systems are in principal more or less well understood. Theory has reached a high quantitative level. Comparison with computer simulations and experiment verifies results obtained with nowadays standard methods.
The main field of research goes to more complicated systems with multicritical behavior and the study of crossover phenomena. However there is a class of systems which are less understood, namely impure systems. One class of such systems are those with quenched (so to say fixed) impurities. These impurities may be defects, substitutional molecules, vacancies or other irregularities in the pure system.
The main question with respect to critical phenomena are: what is the effect of the presence of impurities? Is the phase transition of the pure system changed? Does it remain second order and if so does it belong to the same universality class? Concerning the asymptotic critical behavior this question has been answered by the so called Harris criterium which states that in pure systems with non diverging specific heat, the asymptotic critical behavior is unchanged (characterized by the properties around the stable fixed point of the pure system).
But this is not the whole story. Usually the experimental accessible region is not the asymptotic region but a region showing some crossover behavior, either from the background region to asymptotics or from some region around an unstable fixed point to the asymptotics.
This holds for both statics and dynamics. Therefore a main topic in our project will be the study of a (non-trivial) asymptotic critical behavior in disordered systems for the cases when it differs from that of the corresponding pure system as well as the detailed analysis of the effective critical behavior of random site-disorder and random anisotropy systems. In dynamics besides the relaxational model a coupling to the energy density will be considered. The calculations for this model will be a complicated task as already the pure model constitutes a quite non trivial dynamic model for which the correct field theoretic functions have been found only recently (in the FWF project PHY15247).