Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr₃Si₇

Abstract

The new Kondo material YbIr₃Si₇, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed ²⁹Si-NMR on YbIr₃Si₇ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the c axis, a single NMR peak in the paramagnetic state splits into three peaks below Tɴ. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the c-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the c axis antiferromagnetic (AFM-I) phase are rotated to the ab plane with a ferromagnetic component along the c-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field Hм, the nuclear spin-lattice relaxation rate 1/T₁ was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of 1/T₁ versus the c-axis magnetic field suggests that the charge-neutral fermions in YbIr₃Si₇ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.