Evidence of Electronic Phase Separation in the Strongly Correlated Semiconductor YbB12

We investigate high-quality single-domain crystals of YbB₁₂ using the precise x-ray diffraction technique in combination with the low-temperature polarized THz–infrared spectroscopy and accurate magnetotransport measurements. It is shown for the first time that this archetypal strongly correlated system with a metal-insulator transition to a mysterious dielectric ground state with a metal Fermi surface [Science 362, 65 (2018) and ibid 362, 32 (2018)] is actually a heterogeneous compound in the regime of electronic phase separation. Changes in the configuration of the discovered dynamic charge stripes are proposed upon cooling. As a result, a conclusion is drawn in favor of a crossover between different patterns of the filamentary electronic structure penetrating the semiconducting matrix of YbB₁₂. We argue that the discovery of stripes in YbB₁₂ is fundamental, elucidating the nature of exotic dielectric state in Kondo insulators.