Electrical resistance of individual defects at a topological insulator surface

In a collaboration between three Peter Grünberg Instituts the resistance of individual defects at a topoligical insulator surface was investigated. The work was recently published in a Nature Communications article. Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone.

Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering with great detail.

Here, we use scanning tunneling potentiometry to analyze the resistance of different kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator thin film. The largest localized voltage drop we find to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analyzed by means of a resistor network model. The effect resulting from the voids is found to be small compared to the other defects.

Image (a) shows an overlay of topography as terrain and potential distribution as color code. The topography is dominated by quintuple layer steps while in the potential we observe an overall linear voltage slope on the terraces and additional voltage jumps located along lines at the sample surface, e.g. the one highlighted by the yellow arrow. Scan size: 300 nm. (b) Topography showing two quintuple layer steps at the sample surface. The section indicated by the solid white line is shown in (d). Scale bar: 20 nm. (c) Corresponding potential map with subtracted linear background. Sharp voltage drops are located at the position of topographic steps and along the dotted line which we explain as a domain boundary in the topological insulator film. The corresponding potential section indicated by the solid white line is shown in (d): Black line graph: Height profile from (b). Red line graph: Potential section from (c). More information can be found in: Nat. Commun. 8 (2017) 15704.

Figure (a) shows an STM image of a typical void in the topological insulator thin film surface. Scale bar: 5 nm. (b) Corresponding potential map showing a dipole shaped feature centered at the defect. The lobes of the dipole are aligned with the macroscopic current direction. (c) Resistor network model mask with indicated schematic of the resistors. (d) Calculated potential distribution around the defect resulting from the resistor network model shown in (c), after background subtraction. (e) Sections indicated in (a)-(d). Solid black line: Experimental height profile section from (a). Solid red line: Experimental potential section from (b). Dashed black line: Section of the model system shown in (c). Dotted blue line: Calculated potential section from (d). More information can be found in Nat. Commun. 8 (2017) 15704.