  We show that a weak periodic drive removes the accidental degeneracy in the
ground state of the XY antiferromagnet on the triangular lattice in a uniform
static magnetic field. The underlying mechanism involves adding a small
periodically modulated component to the magnetic field, which influences
finite-frequency modes to generate an effective potential for the accidental
pseudo-Goldstone mode. This selection can be arranged to compete with the
degeneracy lifting via the thermal order by disorder mechanism. This yields a
non-equilibrium phase transition as the relative strength of the two mechanisms
is tuned by varying temperature. This proposal may be amenable to experimental
realization, in particular as applying the field is noninvasive, and no complex
bath engineering is required.
