Prof. M. E. Zhitomirsky (CEA-Grenoble, France and ISSP, Univ. Tokyo)
"Classical kagome antiferromagnet in two and three dimensions: new insights"
Motivated by recent experimental realizations of large-S kagome antiferromagnets in hydronium jarosite and in Y0.5Ca0.5BaCo4O7 we reinvestigate properties of the nearest-neighbor classical Heisenberg spin models on two-dimensional kagome and three-dimensional hyperkagome lattices. We argue that for both models the symmetry breaking states at low temperatures are described by non-zero octupole moments or third-rank tensor order parameters. The analytical arguments are supported by large-scale Monte Carlo simulations. The three-dimensional model exhibits a first-order transition into the phase with short-range spin correlations, which nevertheless possesses a long-range order of octupole moments. We also consider the phase diagram of the two-dimensional model in external magnetic field and find a variety of Kosterlitz-Thouless transitions driven by fractional vortices.