Quantum phase transitions in the two-dimensional J1-J2 model -: art. no. 104420

We analyze the phase diagram of the frustrated Heisenberg antiferromagnet, the J(1)-J(2) model, in two dimensions. Two quantum phase transitions in the model are already known: the second-order transition from the Neel state to the spin liquid state at (J(2)/J(1))(c2) approximate to 0.38, and the first-order transition from the spin liquid state to the collinear state at (J(2)/J(1))(c4) approximate to 0.60. We have found evidence for two second-order phase transitions: the transition from the spin columnar dimerized state to the state with plaquette-type modulation at (J(2)/J(1)])(c3) = 0.50 +/- 0.02, and the transition from the simple Neel state to the Neel state with spin columnar dimerization at (J(2)/J(1))(c1) = 0.34 +/- 0.04. We also present an independent calculation of (J(2)/J(1))(c2) approximate to 0.38 using another approach.