Criteria for infinite avalanches in the zero-temperature nonequilibrium random-field Ising model on a Bethe lattice

Approach to the lower critical dimension of the φ^{4} theory in the derivative expansion of the functional renormalization group

We revisit the approach to the lower critical dimension d_{lc} in the Ising-like φ^{4} theory within the functional renormalization group by studying the lowest approximation levels in the derivative expansion of the effective average action. Our goal is to assess how the latter, which provides a generic approximation scheme valid across dimensions and found to be accurate in d≥2, is able to capture the long-distance physics associated with the expected proliferation of localized excitations near d_{lc}. We show that the convergence of the fixed-point effective potential is nonuniform in the field when d→d_{lc} with the emergence of a boundary layer around the minimum of the potential. This allows us to make analytical predictions for the value of the lower critical dimension d_{lc} and for the behavior of the critical temperature as d→d_{lc}, which are both found in fair agreement with the known results. This confirms the versatility of the theoretical approach.

Hysteresis and return point memory in the random-field Blume-Capel model

We study the zero-temperature steady-state of the random-field Blume-Capel model with spin-flip Glauber dynamics on a random regular graph. The magnetization m as a function of the external field H is observed to have double hysteresis loops with a return point memory. We also solve the model on a Bethe lattice in the approximation that the spin relaxation dynamics is abelian and find good agreement between simulations on random regular graphs and Bethe lattice calculations for negative values of H.

Critical hysteresis on dilute triangular lattice

Critical hysteresis in the zero-temperature random-field Ising model on a two-dimensional triangular lattice was studied earlier with site dilution on one sublattice. It was reported that criticality vanishes if less than one-third of the sublattice is occupied. This appears at variance with recently obtained exact solutions of the model on dilute Bethe lattices and prompts us to revisit the problem using an alternate numerical method. Contrary to our speculation that criticality may not be exactly zero below one-third dilution, the present study indicates it is nearly zero if approximately less than two-thirds of the sublattice is occupied. This suggests that hysteresis on dilute periodic lattices is qualitatively different from that on dilute Bethe lattices. Possible reasons are discussed briefly.

Hysteresis in the Ising model with Glauber dynamics

We use Glauber dynamics to study time and temperature dependence of hysteresis in the pure (without quenched disorder) Ising model on cubic, square, honeycomb lattices as well as random graphs. Results are discussed in the context of more extensive studies of hysteresis in the random field Ising model.