Macroscopic Degeneracy and Emergent Frustration in a Honeycomb Lattice   Magnet
  Using a hybrid method based on fermionic diagonalization and classical Monte Carlo, we investigate the interplay between itinerant and localized spins, with competing double- and super-exchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized groundstate is stable that has macroscopic degeneracy. The presence of these states on a non-frustrated honeycomb lattice highlights a novel phenomenon in this itinerant electron system: emergent geometrical frustration and degeneracy. 
J\"orn W.F. Venderbos, Maria Daghofer, Jeroen van den Brink and   Sanjeev Kumar
cond-mat.str-el