The interplay of non-trivial topology and Kondo physics has stimulated a flourish of experimental reports in the mixed valence SmB₆ compound, some of which appear to be in contradiction. The origin of such discrepancies may lie on the fragility of the Kondo insulating phase. We locally explore Al-flux grown Sm_1-xGd_xB₆ single crystals (0 ≤ x ≤ 0.02) with electron spin resonance (ESR) and bulk measurements [1]. Although the Gd³⁺ ESR spectra in highly dilute systems (x ≈ 0.0004) show a cubic insulating environment, the increase of Gd³⁺ concentration changes the ESR response to a metallic ESR lineshape. Remarkably, all the samples still show insulating dc electrical resistivity. Our results hint that the Kondo insulating state is destroyed locally around impurities before a global percolation occurs. These results not only explain the discrepancy between dc and ac conductivity [2,3], but also point to a scenario to explain the presence of quantum oscillations in magnetization in absence of oscillations in electrical resistivity [4].

[1] J. C. Souza et al., Phys. Rev. Research 2(4):043181 (2020).
[2] N. J. Laurita et al. Phys. Rev. B 94(16):165154 (2016).
[3] Y. S. Eo et al. Proc. of Nat. Acad. Soc. 116(26): 12638-12641 (2019).
[4] Y. S. Eo et al. Phys. Rev. B, 101(15):155109 (2020).