Neutron Scattering Study on Pressure-induced Quantum Phase Transition in Quantum Antiferromagnet CsFeCl₃

Abstract

The pressure-induced quantum phase transition in the quantum magnet CsFeCl₃ has been studied by means of neutron scattering technique under pressures. Neutron diffraction evidences a pressure-induced magnetic long-range order and identifies a realization of the 120º structure with a magnetic propagation vector k_mag = (1/3,1/3,0). The evaluated critical exponent of the order parameter suggests that CsFeCl₃ belongs to the universality class of U(1)×Z₂ symmetry. Inelastic neutron scattering demonstrates softening of the magnetic excitation by applying pressures. In the ordered phase, an excitation with a minimum energy of 0.6 meV is observed in addition to a gapless excitation. Calculation of the spin spectrum reveals that the excitation having the minimum energy of 0.6 meV corresponds to the non-trivial mixed mode of the transverse and longitudinal fluctuations.