Phase diagram of generalized fully frustrated XY model in two dimensions
  It is shown that the phase diagram of the two-dimensional generalized fully-frustrated XY model on a square lattice contains a crossing of the chirality transition and the Kosterlitz-Thouless (KT) transition, as well as a stable phase characterized by a finite helicity modulus $\Upsilon$ and an unbroken chirality symmetry. The crossing point itself is consistent with a critical point without any jump in $\Upsilon$, with the size ($L$) scaling $% \Upsilon\sim L^{-0.63}$ and the critical index $\nu\approx0.77$. The KT transition line remains continuous beyond the crossing but eventually turns into a first-order line. The results are established using Monte-Carlo simulations of the staggered magnetization, helicity modulus, and the fourth-order helicity modulus. 
Petter Minnhagen, Beom Jun Kim, Sebastian Bernhardsson, and Gerardo   Cristofano