Vortex-antivortex lattice in superfluid films

In a recent experiment by Chen, Roesler, and Mochel, a new transition was found in the superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ film on a hydrogen surface, with a transition temperature below the Kosterlitz-Thouless transition temperature. We suggest that the intermediate phase between these two transitions is a lattice form by the superfluid vortices and antivortices. The transverse phonon mode of this lattice is identified with the ‘‘extra mode’’ detected in the third sound measurement.