Analysis of the Local Electrochemical Potential of Current Carrying Topological Insulating Surfaces
Final Report Abstract
The projects deals with the analysis of local charge transport at the surface of prototype topological insulator materials. In particular, the Bi2Se3 and Bi2Te3 were analyzed. Thereby, the focus lies on the analysis of thin TI films which were prepared in situ. The analysis was performed using a multi probe scanning tunneling microscopy setup under ultra high vacuum conditions. With this, the prepared thin film samples could be analyzed by means of STM analysis. During the analysis of the topography via STM a transverse current passes through the samples in particular through the surface. This lead to a landscape of the local potential along the surface which is correlated to the microscopic transport properties. The metallic leads to the surface were set by using two individually controllable probes of the 4 point probe STM setup. Now, the potential at the tunneling tip of the measuring STM is adjusted in such a way that the mean DC tunneling current vanishes. Under these conditions, the externally controlled voltage at the tip represents the local voltage, i.e. the local potential, at the surface at the position of the STM tip. If this is automatically performed at each lateral position of the STM tip one gets access to the local potential landscape. Figure shows a sketch of the experimental results. While a battery imprints a lateral current though the Bi2Se3 surface, the local topography and the local potential are measured simultaneously. The 3D representation shows the STM topography exhibiting various 1 QL high Bi2Se3 step edges and terraces. The lateral distribution of the surface potential is superimposed to the data as color information. Here, one has to note that a constant gradient was already subtracted from the potential data. Each Bi2Se3 terrace exhibits now a rather constant potential while at each QL step edge a abrupt voltage drop, i.e. drop of the potential occurs. Here, we found that the conduction electrons propagating near the surface of the Bi2Se3 thin film a scattered a the step edges which are identified as barriers for the charge transport. Similar results were found for the case of domain boundaries within the Bi2Se3 film whereas in the case of domain boundary scattering the impact to the transport is more severe. However, due to the quality of the film scattering in the films bulk e.g. by phonons are still an important factor which is manifested in a constant gradient of the potential along the direction of the transverse current. Our analysis allowed us to visualize the scattering of conduction electrons in real space and allowed us to determine the individual resistivities (conductivities) of nanoscale defects. Similar results were also found for thin films of Bi2Te3. However, the preparation of suitable thin films is more difficult and a in deep analysis of preparation parameters was necessary. In this project we decipher the elementary contributions to the macroscopically observed resistance of TI materials.
