{"abstract":"Based on a special variant of plaquette expansion, an operator is constructed\nwhose eigenvalues give the low-energy singlet spectrum of spin-$\\frac12$\nHeisenberg antiferromagnet on square lattice with nearest- and frustrating\nnext-nearest-neighbor exchange couplings $J_1$ and $J_2$. It is well known that\na non-magnetic phase arises in this model at $0.4\\lesssim J_2/J_1\\lesssim 0.6$\nsandwiched by two N\\'eel ordered phases. In agreement with previous results, we\nobserve a first-order quantum phase transition (QPT) at $J_2\\approx 0.64J_1$\nfrom the non-magnetic phase to the N\\'eel one. Large gap ($\\gtrsim0.4J_1$) is\nfound in the singlet spectrum at $J_2<0.64J_1$ that excludes a gapless\nspin-liquid state at $0.4\\lesssim J_2/J_1\\lesssim 0.6$ and the deconfined\nquantum criticality scenario for the QPT to another N\\'eel phase. We observe a\nfirst-order QPT at $J_2\\approx 0.55J_1$ presumably between two non-magnetic\nphases.","author":[{"family":"Aktersky"},{"family":"Syromyatnikov"}],"id":"unknown","issued":{"date-parts":[[2016,5,17]]},"language":"en","title":"Low-energy singlet sector in spin-1/2 J_1--J_2 Heisenberg\n model on square lattice","type":"article"}