Order by disorder in the classical antiferromagnetic honeycomb lattice

F. A. GÓMEZ ALBARRACÍN

Toulouse

Seminario; Seminar of the Strongly Correlated Electron Sytems Group at LPT (Laboratoire de Physique Théorique) de Toulouse; 2015

Laboratoire de Physique Théorique de Toulouse

(talk) In this talk we present a classical frustrated spin system in the bilayer honeycomb lattice. We first motivate this work by discussing experimental results in  Bi3Mn4O12NO3, a material that has been established as an antiferromagnetic S=3/2 system. Experimental results are consistent with frustration. In this work, we propose that the frustration would be between layers. We study the system at a strongly frustrated point under a magnetic field. In this point, there is a high degeneracy of the ground state and thermal fluctuations (order by disorder) select configurations with soft and zero modes for all values of the magnetic field at low enough temperatures. We comment on the pseudo plateau at m=1/2, which is remarkable since it is not fully collinear, as it is in other classical systems where order by disorder is at play. In this case, in the pseudo plateau the system can be described as two sublattices of "classical dimers": one of polarized spins ("triplets") and one of free opposite pairs of spins ("classical singlets"). We also study the phase transitions with temperature and confirm that there is a Z2 symmetry breaking through the standard Binder cumulant analysis of a suitable order parameter. We also coment briefly on the behaviour of the system at a lower frustrated region, where the ground states  also show a numer of zero modes.