On the percolation regime of the bulk transport of the Bi$_{1.08}$Sn$_{0.02}$Sb$_{0.9}$Te$_2$S topological insulator

The formation of nanosized charge droplets in the bulk of a three-dimensional topological insulator Bi$_{1.08}$Sn$_{0.02}$Sb$_{0.9}$Te$_2$S was evidenced using electron spin resonance. Since electrons and holes are “locked” in these droplets far apart from each other, their participation in bulk conductivity is possible only due to thermal activation or to variable range hopping. Our transport measurements give evidence that at relatively high temperatures conduction follows the activation behavior with the activation energies which due to percolation may be much smaller than the half of the band gap. We also demonstrate that the variable range hoping replaces the activated transport at sufficiently low temperatures.