Surface excitations relaxation in the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$

TL;DR

This study uses electron spin resonance to investigate surface excitations in the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$, revealing how surface states influence spin relaxation and ESR line shape evolution at low temperatures.

Contribution

It introduces a microscopic ESR technique to probe surface excitations in a topological Kondo insulator, highlighting their role in spin dynamics and relaxation mechanisms.

Findings

Gd$^{3+}$ ESR line shape transitions from localized to diffusive-like with temperature.

Surface modifications affect microwave penetration and ESR line shape.

Surface or near-surface excitations significantly contribute to spin relaxation.

Abstract

The interplay between non-trivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$ by means of electron spin resonance (ESR) of Gd$^{3+}$ ions at low temperatures. Our analysis reveals that the Gd$^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples (x $\approx$ 0.0002) the Gd$^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the Gd$^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the Gd$^{3+}$ spin dynamics. Our work brings new insights into the importance of non-trivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally.