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

The interplay of non-trivial topology and strong electronic correlations has stimulated a flourish of experimental reports in the mixed valence SmB$_{6}$, however there is an increasing desire to find more microscopic techniques to complement results of more traditional techniques [1,2]. In this work, we locally explore the Gd$^{3+}$ environment [3] in Sm$_{1-x}$Gd$_{x}$B$_{6}$ (x $\sim$ 0.0002) by means of electron spin resonance (ESR) [4]. Our analysis reveals an anomalous evolution of the Gd$^{3+}$ ESR line shape, which goes from localized-like at higher temperatures to a diffusive-like at lower ones. Using a focused ion beam, which is a systematic way to manipulate the surface conductivity [5], we show that the localized-like character of the Gd$^{3+}$ ESR spectrum is recovered upon increasing the skin depth of the sample. Our study unveils a compelling evidence of a relaxation process influenced by the presence of surface and/or near-surface excitations, opening exciting routes to be explored both theoretically and experimentally.

[1] P. F. S. Rosa and Z. Fisk, in Bulk and Surface Properties of SmB$^{6}$, in Rare-Earth Borides, edited by D. S. Inosov (Jenny Stanford Publishing, Singapore, 2021).
[2] L. Li et al. Nat. Rev. Phys. 2, 463 (2020)
[3] J. C. Souza et al. Phys. Rev. Research 2, 043181 (2020).
[4] J. C. Souza et al. Phys. Rev. Research 3, 033016 (2021).
[5] M. V. Ale Crivillero et al. Phys. Rev. Research 3, 023162 (2021).