Simulating a two-dimensional frustrated spin system with fermionic resonating-valence-bond states

The frustrated Heisenberg J1-J2 model on a square lattice is numerically investigated by variational Monte Carlo simulations. We propose an antiferromagnetic fermion resonating-valence-bond (AF-fRVB) state that has the ability to examine the entire phase diagram in the J1-J2 model. Two phase transition points, the second order around J2/J1=0.45 and the first order around J2/J1=0.6, can be extracted more clearly than the conventional bosonic RVB state. At the maximally frustrated point (J2/J1=0.5), the AF-fRVB state shows the variational ground-state energy in the thermodynamic limit very close to the one estimated by the projected entangled pair state at the largest bond dimension available. On the other hand, in the frustrated regime 0.4 J2/J1≤0.5, AF-fRVB states with exts2 (using the terminology in the field of iron-based superconductors) and dxy pairing symmetries are degenerate in the thermodynamic limit, implying the existence of gapless Dirac excitations in the spinon spectrum.