Low-energy singlet sector in the spin-1/2 J₁-J₂ Heisenberg model on a square lattice
Aktersky A.Yu., Syromyatnikov A.V.

Поступила в редакцию: 4 Августа 2016

DOI: 10.7868/S0044451016120000

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