Novel Excitations near Quantum Criticality in Geometrically Frustrated Antiferromagnet CsFeCl${}_3$

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.

Supplementary material comes first and main text is followed. 12 pages, 3 figures and 1 table for supplementary material. 13 pages and 4 figures for main text