  We introduce a two-dimensional frustrated Heisenberg antiferromagnet on
interpenetrating honeycomb and triangular lattices. Classically the two
sublattices decouple, and "order from disorder" drives them into a coplanar
state. Applying Friedan's geometric approach to nonlinear sigma models, we show
that the scaling of the spin-stiffnesses corresponds to the Ricci flow of a 4D
metric tensor. At low temperatures, the relative phase between the spins on the
two sublattices is described by a six-state clock model with an emergent
critical phase.
