We use the coupled cluster method (CCM) to study the zero-temperature ground-state (GS) properties of a spin-12 J1-J2 Heisenberg antiferromagnet on a triangular lattice with competing nearest-neighbor and next-nearest-neighbor exchange couplings J1>0 and J2≡κJ1>0, respectively, in the window 0≤κ<1. The classical version of the model has a single GS phase transition at κcl=18 in this window from a phase with 3-sublattice antiferromagnetic (AFM) 120 Néel order for κ<κcl to an infinitely degenerate family of 4-sublattice AFM Néel phases for κ>κcl. This classical accidental degeneracy is lifted by quantum fluctuations, which favor a 2-sublattice AFM striped phase. For the quantum model we work directly in the thermodynamic limit of an infinite number of spins, with no consequent need for any finite-size scaling analysis of our results. We perform high-order CCM calculations within a well-controlled hierarchy of approximations, which we show how to extrapolate to the exact limit. In this way we find results for the case κ=0 of the spin-12 model for the GS energy per spin, E/N=-0.5521(2)J1, and the GS magnetic order parameter, M=0.198(5) (in units where the classical value is Mcl=12), which are among the best available. For the spin-12 J1-J2 model we find that the classical transition at κ=κcl is split into two quantum phase transitions at κ1c=0.060(10) and κ2c=0.165(5). The two quasiclassical AFM states (viz., the 120 Néel state and the striped state) are found to be the stable GS phases in the regime κ<κ1c and κ>κ2c, respectively, while in the intermediate regimes κ1c<κ<κ2c the stable GS phase has no evident long-range magnetic order.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Jan 22 2015|