Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point

TL;DR

This study reports the first direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near a quantum critical point, demonstrating its stability due to an anisotropic energy gap.

Contribution

The paper provides the first experimental evidence of a stable Higgs amplitude mode in a 2D quantum antiferromagnet using neutron scattering and bond-operator theory.

Findings

Higgs amplitude mode observed in a 2D quantum antiferromagnet

Mode is stabilized by an anisotropic energy gap

Mode exhibits an infinite lifetime due to suppressed decay

Abstract

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed matter physics. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu$-$Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics, is prone to decay into a pair of low-energy Nambu$-$Goldstone modes in low dimensions. Especially, observation of a Higgs amplitude mode in two dimensions is an outstanding experimental challenge. Here, using the inelastic neutron scattering and applying the bond-operator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional S=1/2 quantum antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$ near a quantum critical point in two dimensions. Owing to an anisotropic energy gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.