Understanding electron transport in materials is key to investigating their electronic properties. However, most of the transport measurements have been performed at macroscopic scales, and thus the development of microscopic methods to measure transport is pivotal. Scanning tunneling potentiometry (STP), based on scanning tunneling microscopy (STM), reveals transport properties in nanometer spatial resolution as schematically shown in Fig. 1.

Our group has been developing STP [1,2], which enables us to visualize spatial variation of electrochemical potential across a sample surface. Assuming constant current density, the gradient of the electrochemical potential corresponds to resistance. Combined with STM, one can correlate potential features to atomic-scale surface structures and local density of states.

Until now, STP measurements have been mostly limited to room temperature. However, many intriguing physical phenomena, such as superconductivity and electron localization, emerge only at low temperatures. Recently, we have successfully developed an STP system that operates at low temperatures (LT-STP) by optimizing all setups for the low temperature system [3, 4].

Figures 2(a) and (b) show the topographic image and the corresponding STP image taken at low temperatures, respectively. The images are obtained on a monolayer Pb film formed on a Si(111) substrate. The coverage is approximately 1.3 monolayers (1.3 Pb atoms per 1 Si unit cell). Since we performed the sample fabrication and measurements in the same chamber, we obtained extremely clean surfaces. Figure 2(c) shows the cross-sectional profiles taken along the blue and red dashed lines in Fig. 2(a) and (b), respectively. We found that the STP signal has no abrupt change at the step (indicated by the black arrow), indicating that electric current flows smoothly across atomic steps [3].

On the other hand, our further experiments revealed that by reducing the coverage to 1.0 monolayers, the steps act as a barrier to the transport [4]. Our findings suggest that the small difference of the coverage and structures affect their transport properties. They also open new possibilities for studying quantum effects in clean, low-temperature environments.

References

• [1] M. Hamada and Y. Hasegawa, Jpn. J. Appl. Phys. 51 125202 (2012).
• [2] M. Hamada and Y. Hasegawa, Phys. Rev. B 99, 125402 (2019).
• [3] M. Hamada, M. Haze, J. Okazaki, and Y. Hasegawa, Phys. Rev. Applied 23, 064051 (2025).
• [4] M. Haze, J. Okazaki, M. Hamada, and Y. Hasegawa, submitted to Appl. Surf. Sci.

Authors

• M. Haze M. Hamada, J. Okazaki, and  Y. Hasegawa