Enhancement of charge-neutral fermionic excitation near spin-flop transition\\ in magnetic Kondo material YbIr$_3$Si$_7$

TL;DR

This study investigates charge-neutral fermionic excitations in the Kondo insulator YbIr$_3$Si$_7$ using NMR, revealing their relation to magnetic transitions and providing microscopic insights into their origin.

Contribution

The paper provides the first microscopic NMR evidence linking charge-neutral fermions to magnetic transitions in YbIr$_3$Si$_7$, a Kondo insulator.

Findings

Charge-neutral fermions are confirmed via $1/T_1$ measurements.

Spectral changes indicate a spin-flop transition affecting fermions.

Charge-neutral fermions are closely related to magnetic properties.

Abstract

The new Kondo material YbIr$_3$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 $^{29}$Si-NMR on YbIr$_3$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 $T_{\rm N}$. 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 spin-flop transition, at which the magnetic moments oriented along the $c$ axis (AF-I phase) are rotated to the $ab$ plane with ferromagnetic component along the $c$-axis (AF-II phase). In the vicinity of the spin-flop magnetic field $H_{\rm M}$, 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 $1/T_1$ vs. the $c$-axis magnetic field suggests that the charge-neutral fermions in YbIr$_3$Si$_7$ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.