Order by disorder state selection in classical spin systems: examples in low dimensional systems with competing interactions

F. GÓMEZ ALBARRACÍN

Trieste

Otro; School in Computational Condensed Matter Physics; 2015

International Centre of Theoretical Physics

Competing interactions can give rise to different types of interesting phenomena and exotic phases. Here, we focus on the classical order by disorder (OBD) mechanism: the selection of a state (or a group of states) in the manifold of degenerate states due to thermal fluctuations. We show three examples of classical spins systems where we found evidence of this mechanism. First, the bilayer honeycomb lattice, where OBD is present along all the magnetization curve at a highly frustrated point. Through Monte Carlo simulations (classical Monte Carlo with the overrelaxationalgorithm) and adequate parameters we describe the different phases. In particular,at lower temperatures the evidence of OBD is supported by the specific heat curve and calculations of the quadratic order fluctuations of the ground states. Second, the honeycomb lattice at a highly frustrated point J_1=2J_2. This system has been extensively studied previously, and a magnetization pseudoplateau was found at M=1/2. We show that OBD is the selection mechanism at this particular point in the magnetization curve. We also show preliminary studies of what happens to thisselection when disorder is introduced through non magnetic impurities. Third, the classical spin chain with long range dipolar interactions and antiferromagnetic exchange nearest neighbour coupling. We show that this system is degenerate for a specific value of the antiferromagnetic coupling, and that at this point there is an additional continuous degeneracy. We perform Monte Carlo simulations and find that at low temperatures a state is selected. We propose that this is due to OBD, and investigate this possibility through analytical calculations of the fluctuations