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.
Highlights
The concept of topology in condensed matter physics emerged from breakthroughs in the quantum Hall effect [1]; more recently such a concept was generalized to other states of matter, such as topological insulators [2], Dirac and Weyl semimetals [3,4,5], and other exotic phenomena [6,7]
Combining microwave power-dependent electron spin resonance (ESR), focused ion beam (FIB) for cutting trenches on the sample surface, and complementary resistivity measurements, we provide evidence for surface excitations contributing to the ESR relaxation
Using a two-layered model, where we consider that the conducting carriers at the surface are the main contributors to the resistivity at low temperatures [26,27], we show that there is a systematic increase of the skin depth and a decrease of the mean free path as a function of the density of FIB-cut trenches (Table I in Appendix B)
Summary
The concept of topology in condensed matter physics emerged from breakthroughs in the quantum Hall effect [1]; more recently such a concept was generalized to other states of matter, such as topological insulators [2], Dirac and Weyl semimetals [3,4,5], and other exotic phenomena [6,7]. The prototypical material to study this interplay is the Kondo insulator SmB6 [9,13,14]. Extensively studied over the last 40 years, the prediction of a topological insulating ground state in this compound brought back interest in this system [15,16,17,18,19,20,21,22].
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