Magnon crystallization in the kagome lattice antiferromagnet*

We present numerical evidence for the crystallization of magnons

below the saturation field at non-zero temperatures for the highly frustrated spin-half

kagome Heisenberg antiferromagnet.

This phenomenon can be traced back to the existence of independent localized magnons or equivalently

flat-band multi-magnon states. We present a loop-gas description of these localized magnons

and a phase diagram of this transition, thus providing

information for which magnetic fields and temperatures magnon crystallization

can be observed experimentally.

The emergence of a finite-temperature continuous transition to a

magnon-crystal is expected to be generic for spin models in dimension

D>1 where flat-band multi-magnon ground states break translational

symmetry.