Physical review. B, Condensed matter
Critical and finite-size behavior of the Heisenberg model with face-centered-cubic anisotropy
Lyle D. Roelofs, Charles S. Jackson
Abstract
The disordering transition of half a monolayer of chemisorbed oxygen on Ru(001) can be classified in the universality class of the two-dimensional Heisenberg model with face-centered anisotropy. Theoretical studies of this class suggest fluctuation-induced first-order-transition behavior. Experimental study of the O/Ru(001) system, on the other hand, reveals a continuous phase transition with exponents near those of the three-state Potts model. We have investigated this discrepancy via Monte Carlo simulations of the Heisenberg model using the histogram method to determine transition order, focusing on two possible resolutions of the discrepancy: (1) finite-size rounding might be expected to render the experimental transition apparently continuous even if it is actually first order; or (2) transition order might depend on the details of the interaction energies. Despite its seeming inconsistency with the standard understanding of universality, we have strong evidence for the latter explanation; there appears to be a parameter regime of second-order behavior within standard ferromagnetic Heisenberg symmetry. We delineate this breakdown of universality and speculate as to its mechanism.