Spin stiffness calculation in anisotropic XY model with Ring exchange interaction

We present the spin wave theory of XY model with anisotropic nearest neighbour (NN) interactions $J(J^{\prime})$ along the $x(y)$ directions, next nearest (NNN) neighbour interaction $J_D$ and the ring exchange interaction $K$ on the square lattice. We calculate the thermodynamic quantities: Zero temperature spin stiffness, internal energy, specific heat and the magnetization. Using the diagonalized Hamiltonian, we show that no soft modes develop when $\eta + \lambda >0$, where $\eta = J^{\prime}/J$ and $\lambda = K/J$. We further show that anisotropy ($\eta =2$) decreases the spin stiffness by 5.7% of its isotropic ($\eta =1$) maximum value for some values of $\lambda$ and $\delta = J_D/J$. A similar reduction shows up in the magnetization. The plot of the stiffness against $\eta (\lambda)$ reaches a maximum at $\eta=0(\lambda =0)$ for specific values of $\delta$ and decreases rapidly as it approaches $\eta + \lambda + 1=0$. In general, the supersolid phase transition suggested earlier\cite{H} will occur in the regime $\eta + \lambda +1 <0$.