# Quantum Phase Transition in the Four-Spin Exchange Antiferromagnet

## TL;DR

This paper investigates the quantum phase transition in a square lattice antiferromagnet with four-spin exchanges, revealing critical fluctuations and mode condensation near the transition point, but leaves the transition order uncertain.

## Contribution

It provides a beyond-mean-field analysis of the transition, highlighting the role of singlet and triplet mode condensation and lattice symmetry restoration.

## Key findings

- Presence of a low-energy singlet mode near criticality
- Simultaneous condensation of singlet and triplet modes
- Indications of lattice symmetry restoration at transition

## Abstract

We study the S=1/2 Heisenberg antiferromagnet on a square lattice with nearest-neighbor and plaquette four-spin exchanges (introduced by A.W. Sandvik, Phys. Rev. Lett. {\bf 98}, 227202 (2007).) This model undergoes a quantum phase transition from a spontaneously dimerized phase to N\'eel order at a critical coupling. We show that as the critical point is approached from the dimerized side, the system exhibits strong fluctuations in the dimer background, reflected in the presence of a low-energy singlet mode, with a simultaneous rise in the triplet quasiparticle density. We find that both singlet and triplet modes of high density condense at the transition, signaling restoration of lattice symmetry. In our approach, which goes beyond mean-field theory in terms of the triplet excitations, the transition appears sharp; however since our method breaks down near the critical point, we argue that we cannot make a definite conclusion regarding the order of the transition.