Unconventional temperature evolution of quantum oscillations in Sn-doped Bi1.1Sb0.9Te2S topological insulator

Among various topological insulators, Sn-doped Bi1.1Sb0.9Te2S stands out for its exceptional properties. It has a wide energy gap and typically exhibits a well-isolated Dirac point and a Fermi level positioned within the gap. Here, we investigate high-quality samples that display metallic-like low-temperature resistivity attributed to surface states, pronounced quantum oscillations observable even at 40 K, and a Fermi level located approximately 100 meV above the Dirac point. In this work, we focus on the quantum oscillations that show an unusual effect: a strong temperature dependence of the oscillation frequency, which decreases by around 10% between 2 and 40 K. This reduction significantly exceeds the known effects of the Sommerfeld and topological corrections for Dirac quasi-particles, which could account for only one-eighth of the observed change. We attribute the observed effect to the temperature-induced renormalization of the bulk bandgap size due to electron–phonon interactions, which in turn affect the position of the surface Dirac point within the gap. Furthermore, we propose that in this compound, surface quantum oscillations can serve as a precise tool for investigating the low-temperature evolution of the bulk bandgap size.