Schwinger boson mean field study of the $J_1$-$J_2$ Heisenberg quantum antiferromagnet on the triangular lattice

TL;DR

This study uses Schwinger boson mean field theory to explore the phase diagram of the $J_1$-$J_2$ Heisenberg model on a triangular lattice, revealing a spin liquid phase and different types of magnetic order.

Contribution

It introduces a $ ext{kappa}$ parameter variation in SBMFT to identify a spin liquid phase and analyze phase transitions in the $J_1$-$J_2$ model.

Findings

A spin liquid phase appears around $ ext{kappa} \, extasciitilde \, 0.6$ between ordered states.

The static structure factor peaks are similar in the spin liquid and ordered phases.

Transitions from the spin liquid to ordered states are continuous or first-order depending on the state.

Abstract

We use Schwinger boson mean field theory (SBMFT) to study the ground state of the spin-$S$ triangular-lattice Heisenberg model with nearest ($J_1$) and next-nearest ($J_2$) neighbor antiferromagnetic interactions. Previous work on the $S=1/2$ model leads us to consider two spin liquid Ans\"{a}tze, one symmetric and one nematic, which upon spinon condensation give magnetically ordered states with 120$^{\circ}$ order and collinear stripe order, respectively. The SBMFT contains the parameter $\kappa$, the expectation value of the number of bosons per site, which in the exact theory equals $2S$. For $\kappa=1$ there is a direct, first-order transition between the ordered states as $J_2/J_1$ increases. Motivated by arguments that in SBMFT, smaller $\kappa$ may be more appropriate for describing the $S=1/2$ case qualitatively, we find that in a $\kappa$ window around 0.6, a region with the (gapped $Z_2$) symmetric spin liquid opens up between the ordered states. As a consequence, the static structure factor has the same peak locations in the spin liquid as in the 120$^{\circ}$ ordered state, and the phase transitions into the 120$^{\circ}$ and collinear stripe ordered states are continuous and first-order, respectively.