Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO$_3$

We used inelastic neutron scattering to show that well below its N{e}el temperature, $T_{rm N}$, the two-dimensional (2D) XY nearly-triangular antiferromagnet YMnO$_{3}$ has a prominent {it central peak} associated with 2D antiferromagnetic fluctuations with a characteristic life time of 0.55(5) ps, coexisting with the conventional long-lived spin-waves. Existence of the two time scales suggests competition between the N{e}el phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.