Critical Ising spin dynamics near the percolation threshold

Relaxation dynamics of an elastic string in random media

We investigate numerically the relaxation dynamics of an elastic string in two-dimensional random media by thermal fluctuations starting from a flat configuration. Measuring spatial fluctuations of its mean position, we find that the correlation length grows in time asymptotically as xi approximately (ln t)1/chi . This implies that the relaxation dynamics is driven by thermal activations over random energy barriers which scale as EB(l) approximately l;chi with a length scale l . Numerical data strongly suggest that the energy barrier exponent chi is identical to the energy fluctuation exponent chi=1/3 . We also find that there exists a long transient regime, where the correlation length follows a power-law dynamics as xi approximately t1/z with a nonuniversal dynamic exponent z . The origin of the transient scaling behavior is discussed in the context of the relaxation dynamics on finite ramified clusters of disorder.

Domain growth in Ising systems with quenched disorder

We present results from extensive Monte Carlo (MC) simulations of domain growth in ferromagnets and binary mixtures with quenched disorder. These are modeled by the random-bond Ising model and the dilute Ising model with either nonconserved (Glauber) spin-flip kinetics or conserved (Kawasaki) spin-exchange kinetics. In all cases, our MC results are consistent with power-law growth with an exponent theta(T, epsilon) which depends on the quench temperature T and the disorder amplitude epsilon. Such exponents arise naturally when the coarsening domains are trapped by energy barriers that grow logarithmically with the domain size. Our MC results show excellent agreement with the predicted dependence of theta(T, epsilon).

New universality class in spin-one-half Fibonacci Heisenberg chains

Low energy properties of the S=1/2 antiferromagnetic Heisenberg chains with Fibonacci exchange modulation are studied using the real space renormalization group method for strong exchange modulation. Using the analytical solution of the recursion equation, the true asymptotic behavoir is revealed, which was veiled by the finite size effect in the previous numerical works. It is found that the ground state of this model belongs to a new universality class with a logarithmically divergent dynamical exponent which is neither like Fibonacci XY chains nor like XY chains with relevant aperiodicity.

Microscopic description of an Ising spin glass near the percolation threshold

Monte Carlo results using a microscopic model to describe FexZn(1-x)F2 indicate that its spin-glass phase at x=0.25 and zero magnetic field is characterized by the presence of antiferromagnetic fractal domains, separated by random vacancies and strongly correlated in time. The effective local random-field distribution corroborates this glassy behavior, which emerges irrespective of ab initio competing interactions and is a consequence of the fractal domain structure near the percolation threshold, x(p)=0.24. The aging properties of the system are in agreement with predictions of short-range stochastic spin-glass models and with the droplets model for spin glass close to percolation.

Fractals, phase transitions and criticality

Analogies between behaviour in fractal media and at phase transitions suggest deeper connections. These arise from the scale invariance of configurations at continuous phase transitions, making fractal view-points useful there and making scaling techniques necessary in both areas. These concepts, inter-relations, and techniques are illustrated with the percolation problem and the Ising spin system, which provide simple examples of geometrical and thermal transitions respectively. Diluted magnets involve both geometrical and thermal effects and, when prepared at concentrations near the percolation threshold, exhibit the influence of geometrical self-similarity on such varied processes as cooperative thermal behaviour and dynamics. Phase transitions in these systems are briefly discussed. The anomalous dynamical behaviour of self-similar systems is introduced, and illustrated by the critical dynamics of Heisenberg and Ising magnets diluted to the percolation threshold. These involve linear (spin wave), and nonlinear (activated) dynamical processes on the underlying fractal percolation structure, leading in the linear case to the magnetic analogue of ‘fracton’ dynamics and in the case of activated dynamics to a highly singular critical behaviour.