The research group observed a hybridized excitation of phase and longitudinal fluctuations near quantum critical point of frustrated quantum magnet CsFeCl₃ and revealed its origin.

Fig.1 Inelastic neutron scattering spectra.
The spectra obtained at a chopper spectrometer under (A) 0.0 GPa at 6 K, (B) 0.3 GPa at 2.7 K, and (C) 1.4 GPa at 0.9 K sliced by the energy transfer − wave vector (ħω − q) plane for q = ( − k, 2k, 0). The yellow circles, squares, and red diamonds are the peak positions of the excitations obtained from the constant-q scans using a triple-axis spectrometer. The solid yellow curves are the dispersions calculated by ESW. (D) Calculated neutron cross section by the ESW under 1.4 GPa at 0 K. (E) Calculated neutron cross section in the absence of the cross term under 1.4 GPa at 0 K. The black and red solid curves in (D) and (E) are gapless and gapped modes, respectively.

Research on the dynamical state of matter enhances our basic understanding of properties of matters. Such research is significant because physical properties such as electrical resistance, thermal conductivity, and spin current all govern the performance of devices.

Phase and amplitude fluctuations have been studied separately but researchers have only been able to observe hybridized excitation in the limited material; thermoelectric material. The hybridized excitation had not been experimentally verified for systems such as magnetic materials and superconductors. We can observe excited states in the neutron spectrum, and these excited states intersect when they include either of phase fluctuation or amplitude fluctuation. However, if there are two excited states and include both of them, the two states will start to make mutual concessions.

This study used neutron scattering under pressure to observe hybridized excitation trying to avoid each other. Furthermore, theoretical consideration reveals that the non-collinear magnetic order peculiar to the frustrated quantum magnet strongly hybridizes the phase and amplitude fluctuations near quantum critical point and that these two fluctuations are included in one excitation. As a result, we were able to explain exactly how the dynamical state of matter changes with pressure.

Hybridized excitation near the quantum critical point does not exist only in magnetic materials. It can exist in general in system with spontaneously broken symmetry such as charge density wave system, spin density wave system, and cooled atom system. We expect such hybridized excitation could be verified in various other systems in the future. In addition, the study predicted that the spin thermal conductivity and the speed of the spin wave would increase by crossing the quantum critical point from the pressure change in the dynamical state, which might increase the possibility of flow control of heat and spin.

Associate Professor Takatsugu Masuda of the Institute for Solid State Physics at the University of Tokyo led the research in collaboration with Shizuoka University, Tokyo Institute of Technology, High Energy Accelerator Research Organization (KEK), and Oak Ridge National Laboratory (ORNL) in U.S.

Reference：

• Journal：Science Advances
• Title：Novel Excitations near Quantum Criticality in Geometrically Frustrated Antiferromagnet CsFeCl₃
• Authors：Shohei Hayashida, Masashige Matsumoto, Masato Hagihala, Nobuyuki Kurita, HidekazuTanaka, Shinichi Itoh, Tao Hong, Minoru Soda, Yoshiya Uwatoko, and Takatsugu Masuda*
• DOI：10.1126/sciadv.aaw5639