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Continuous phase transition from Néel state to Z_2 spin liquid state on a square lattice

Recent numerical studies of the $J_1$-$J_2$ model on a square lattice suggest a possible continuous phase transition between the Néel state and a gapped spin-liquid state with Z$_2$ topological order. We show that such a phase transition can be realized through two steps: First bring the Néel state to the U(1) deconfined quantum critical point, which has been studied in the context of Néel -- valence bond solid (VBS) state phase transition. Then condense the spinon pair -- skyrmion/antiskyrmion bound state, which carries both gauge charge and flux of the U(1) gauge field emerging at the deconfined quantum critical point. We also propose a Schwinger boson projective wave function to realize such a Z$_2$ spin liquid state and find that it has a relatively low variational energy ($-0.4893J_1$/site) for the $J_1$-$J_2$ model at $J_2=0.5J_1$. The spin liquid state we obtain breaks the fourfold rotational symmetry of the square lattice and therefore is a nematic spin liquid state. This direct continuous phase transition from the Néel state to a spin liquid state may be realized in the $J_1$-$J_2$ model, or the anisotropic $J_{1x}$-$J_{1y}$-$J_2$ model.