  Using modified spin wave (MSW) method, we study the $J_1-J_2$ Heisenberg
model with first and second neighbor antiferromagnetic exchange interactions.
For symmetric $S=1/2$ model, with the same couplings for all the equivalent
neighbors, we find three phase in terms of frustration parameter
${\bar\alpha=J_2/J_1}$: (1) a commensurate collinear ordering with staggered
magnetization (N{\'e}el.I state) for $0\leq{\bar\alpha}\lesssim 0.207$ , (2) a
magnetically gapped disordered state for $0.207\lesssim{\bar\alpha}\lesssim
0.369$, preserving all the symmetries of the Hamiltonian and lattice, hence by
definition is a quantum spin liquid (QSL) state and (3) a commensurate
collinear ordering in which two out of three nearest neighbor magnetizations
are antiparallel and the remaining pair are parallel (N{\'e}el.II state), for
$0.396\lesssim{\bar\alpha}\leq 1$. We also explore the phase diagram of
distorted $J_1-J_2$ model with $S=1/2$. Distortion is introduced as an
inequality of one nearest neighbor coupling with the other two. This yields a
richer phase diagram by the appearance of a new gapped QSL, a gapless QSL and
also a valence bond crystal (VBC) phase in addition to the previously three
phases found for undistorted model.
