Critical properties of XY model on two-layer triangular lattice

We investigate phase transitions of the XY model on a two-layer triangular lattice when interactions are competing, by using Monte Carlo simulations. For the system with a ferromagnetic interaction on one of two layers and an antiferromagnetic interaction on the other layer, we find two different types of critical phenomena with two phase-transition temperatures; (1) each of phase transitions due to chirality and (2) the Kosterlitz-Thouless-type phase transition and a chirality transition. We clarify the nature of these phase transitions by using finite-size scaling analyses.