Based on a special variant of plaquette expansion, an operator is constructed whose eigenvalues give the low-energy singlet spectrum of spin-1 2 Heisenberg antiferromagnet on square lattice with nearest-and frustrating next-nearest-neighbor exchange couplings J1 and J2. It is well known that a non-magnetic phase arises in this model at 0.4 J2/J1 0.6 sandwiched by two Néel ordered phases. In agreement with previous results, we observe a first-order quantum phase transition (QPT) at J2 ≈ 0.64J1 from the non-magnetic phase to the Néel one. Large gap ( 0.4J1) is found in the singlet spectrum at J2 < 0.64J1 that excludes a gapless spin-liquid state at 0.4 J2/J1 0.6 and the deconfined quantum criticality scenario for the QPT to another Néel phase. We observe a first-order QPT at J2 ≈ 0.55J1 presumably between two non-magnetic phases.
Based on a special variant of plaquette expansion, an operator is constructed whose eigenvalues give the low-energy singlet spectrum of spin-Heisenberg antiferromagnet on square lattice with nearest-and frustrating next-nearest-neighbor exchange couplings J1 and J2. It is well known that a non-magnetic phase arises in this model at 0.4 J2/J1 0.6 sandwiched by two Néel ordered phases. In agreement with previous results, we observe a first-order quantum phase transition (QPT) at J2 ≈ 0.64J1 from the non-magnetic phase to the Néel one. Large gap ( 0.4J1) is found in the singlet spectrum at J2 < 0.64J1 that excludes a gapless spin-liquid state at 0.4 J2/J1 0.6 and the deconfined quantum criticality scenario for the QPT to another Néel phase. We observe a first-order QPT at J2 ≈ 0.55J1 presumably between two non-magnetic phases.
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