ZhETF, Vol. 150,
p. 1191 (December 2016)
(English translation - JETP,
Vol. 123, No. 6,
available online at www.springer.com
Low-energy singlet sector in the spin-1/2 J1-J2 Heisenberg model on a square lattice
Aktersky A.Yu., Syromyatnikov A.V.
Received: August 4, 2016
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.