A Transport Study on the Surface Hybridization in Ultrathin 3D Topological Insulators

The quantum spin Hall (QSH) effect exhibits helical edge channels allowing for a quantized conductance that can be utilized in the development of quantum computing [1]. These QSH states are expected to be present in the hybridization gap that opens at the Dirac point in ultrathin 3D TIs when the opposite topological surface states hybridize [2]. We have contacted ultrathin colloidal nanoplatelets (NPLs) of the topological insulator bismuth-selenide with niobium leads to form a topological Josephson junction. In these NPLs, one-dimensional edge states have been measured with scanning tunnelling spectroscopy [2]. In our study, we show showcase tunability of the chemical potential and measure a dominant ballistic contribution of the supercurrent in the junction, which is a signature of topological conduction. Furthermore, we can deplete the NPLs to such extent that the supercurrent is destroyed. Most interestingly, we reveal the presence of a hybridization gap for NPLs of 5 nm thickness. NPLs of other thicknesses are studied to probe the thickness dependent hybridization. Similarly, we reveal the presence of such a hybridization gap in 6 nm (Bi_1-xSb_x)₂Te₃ films deposited with molecular beam epitaxy. In this material, the chemical potential is intrinsically close to the Dirac point allowing us to probe possible hybridization effects in a transport study without requiring high gate voltages. In fact, when we tune the chemical potential through the Dirac point, we find an insulating phase for low bias and temperature, which is interpreted as a hybridization gap. We study films with other thicknesses to further investigate the possible presence of QSH states in this system.



[1] Hasan, M. Z. et al., Rev. Mod. Phys. 82, 3045–3067 (2010).

[2] Asmar, M. M. et al, Phys. Rev. B 97, 075419 (2018)

[3] Moes, J. R. et al. Nano Lett. 24, 5110-5116 (2024)