The square lattice J1-J2 Heisenberg antiferromagnet

Full text: There is considerable interest in the nature of the ground state of the Heisenberg antiferromagnet with nearest and next-nearest neighbour interaction (J1-J2 model) as α = J2 /J1 is increased from zero. There is strong evidence for the Neel phase vanishing at a quantum critical point α ≅ 0.38 and the onset of columnar order phase at α ≅ 0.60 . The intermediate region ( 0.38 < α < 0.60) has no magnetic long range order and has been termed a 'spin liquid' phase. Recent work has found evidence for dimerization and/or plaquette order in the spin liquid region. It now appears that the ground state phase diagram is more complex than originally thought. We have obtained evidence for a sequence of 4 quantum critical points at a - 0.34 ≅ 0.38, 0.50, 0.60. The nature of these and the implications for the phase diagram will be discussed