Honeycomb antiferromagnet with a triply degenerate dimer ground state

TL;DR

This paper introduces a honeycomb lattice antiferromagnetic spin-1/2 model combining Heisenberg and a novel exactly solvable multiple spin interaction, revealing a triply degenerate dimer ground state and a quantum phase transition to Ne9el order.

Contribution

The work presents a new exactly solvable multiple spin interaction term on the honeycomb lattice and analyzes its competition with the Heisenberg interaction, uncovering a degenerate ground state and phase transition.

Findings

Exact threefold degenerate dimer ground state without Heisenberg interaction

Continuous quantum phase transition from dimer to Ne9el order

Extension of the ground state degeneracy to general spin-S cases

Abstract

We present an antiferromagnetic quantum spin-1/2 model on honeycomb lattice. It has two parts, one of which is the usual nearest-neighbor Heisenberg model. The other part is a certain multiple spin interaction term, introduced by us, which is exactly solvable for the ground state. Without the Heisenberg part, the model has an exact threefold degenerate dimer ground state. This exact ground state is also noted to exist for the general spin-S case. For the spin-1/2 case, we further carry out the triplon analysis in the ground state, to study the competition between the Heisenberg and the multiple spin interactions. This approximate calculation exhibits a continuous quantum phase transition from the dimer order to N\'eel order.