Novel Excitations near Quantum Criticality in Geometrically Frustrated Antiferromagnet CsFeCl$_{3}$

TL;DR

This study uses neutron scattering to explore how magnetic excitations evolve near quantum criticality in a geometrically frustrated antiferromagnet, revealing hybridized spin fluctuations and novel excitations.

Contribution

It demonstrates the continuous evolution of magnetic modes through a quantum phase transition and uncovers hybridized excitations due to noncollinearity.

Findings

Single mode becomes soft at critical pressure

Modes split into gapless and gapped in ordered phase

Hybridization of phase and amplitude fluctuations observed

Abstract

Investigation of materials that exhibit quantum phase transition provides valuable insights into fundamental problems in physics. We present neutron scattering under pressure in a triangular-lattice antiferromagnet which has a quantum disorder in the low-pressure phase and a noncollinear structure in the high-pressure phase. The neutron spectrum continuously evolves through the critical pressure; a single mode in the disordered state becomes soft with the pressure, and it splits into gapless and gapped modes in the ordered phase. Extended spin-wave theory reveals that the longitudinal and transverse fluctuations of spins are hybridized in the modes because of the noncollinearity, and novel magnetic excitations are formed. We report a new hybridization of the phase and amplitude fluctuations of the order parameter in a spontaneously symmetry-broken state.