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

The new Kondo material ${\mathrm{YbIr}}_{3}{\mathrm{Si}}_{7}$, 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 $^{29}\mathrm{Si}\text{\ensuremath{-}}\mathrm{NMR}$ on ${\mathrm{YbIr}}_{3}{\mathrm{Si}}_{7}$ 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 ${T}_{\mathrm{N}}$. 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 ${H}_{\mathrm{M}}$, the nuclear spin-lattice relaxation rate $1/{T}_{1}$ was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of $1/{T}_{1}$ versus the $c$-axis magnetic field suggests that the charge-neutral fermions in ${\mathrm{YbIr}}_{3}{\mathrm{Si}}_{7}$ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.