Low-temperature infrared spectroscopy of the strongly correlated semiconductor Tm 0.19 Yb 0.81 B 12 with dynamic charge stripes.

Tm 1- x Yb x B 12 dodecaborides represent model objects for the studies of quantum critical behavior, metal-insulator transitions (MITs) and complex charge-spin-orbital-phonon coupling phenomena. In spite of intensive investigations, the mechanism of semiconducting ground state formation both in YbB 12 and in the Yb-based strongly correlated electron systems remains a subject of active debates. We have performed first systematic measurements of temperature-dependent spectra of infrared conductivity of Tm 0.19 Yb 81 B 12 at frequencies 40-35 000 cm -1 and in the temperature range 10-300 K. Analysis of the temperature evolution of the observed absorption resonances is performed allowing to associate these with the cooperative dynamic Jahn-Teller instability of the boron sub-lattice. This ferrodistortive effect of B 12 -complexes induces the rattling modes of the rare earth ions leading to emergence of both the intra-gap mixed-type collective excitations and the dynamic charge stripes. We estimate the temperature-dependent effective mass of charge carriers and propose the scenario of transformation of the many-body states in the multiple relaxation channels. We attribute the MIT to the localization of electrons at the vibrationally coupled Yb-Yb pairs, which is accompanied by the electronic phase separation and formation of the nanoscale filamentary structure of electron density (stripes) in Tm 1- x Yb x B 12 compounds.