学术报告    Critical level crossings and the comprehensive ground state phase diagram in the square-lattice spin-1/2 J1-J2 Heisenberg antiferromagnet

We use the DMRG method to calculate several energy eigenvalues of the frustrated S = 1/2 square-lattice J1-J2 Heisenberg model on 2L × L cylinders with L ≤ 10. We identify excited-level crossings versus the coupling ratio g = J2/J1 and study their drifts with the system size L. The lowest singlet-singlet and singlet-triplet crossings converge rapidly (with corrections ∝ L−2) to different g values, and we argue that these correspond to ground-state transitions between the N ?eel antiferromagnet and a gapless spin liquid, at gc1 ≈ 0.45, and between the spin liquid and a valence-bond-solid at gc2 ≈ 0.52. Previous studies of order parameters were not able to positively discriminate between an extended spin liquid phase and a critical point. In the last, we use ground states quantum numbers and their degeneracy to unambiguously nail down the nature of the valence bond solid phase between the gapless spin liquid and the colinear phase.