Surface conductivity on monolayer Pb films formed on Si(111) measured by low temperature scanning tunneling potentiometry

抄録

It has been known that Pb monolayers on Si(111) shows various phases depending on nominal coverages. When the nominal coverage is higher than 1.2ML, it shows superconducting transition [1]. Moreover, it has been known that the behavior of superconducting vortices at steps depends on the coverage. For √3 × √43 reconstructed phases, whose nominal coverage is 1.23ML, vortices are repelled by steps [2], suggesting that superconducting properties are strongly separated by steps. On the other hand, for striped incommensurate (SIC) phases, whose coverage is 1.33ML, vortices are just trapped by steps keeping their original circular shape [3], suggesting weaker separation by steps. These facts indicate that the difference of small amount of coverage makes significant influence on the superconducting states, and thus it is quite important to understand local conductivities of steps and terraces on these surfaces.

Scanning tunneling potentiometry (STP) is a powerful tool to measure the surface conductivity. STP allows us to measure electrochemical potential in nanometer scale during flowing in-plane current. Recently we have developed low temperature STP (LT-STP) systems. We used low conductive Si(111) as a substrate in order to negate the conductivity through the substrate. As electrodes to flow the in-plane current, we deposited Ta pads on the Si(111) substrate before loading into the UHV chamber.

Here in this research, we measured surface conductivity on several coverages of Pb monolayers on Si(111). From the results, we found that the step conductivity is modified by small difference of the coverages. We will show the details of the measurements in our presentation.

References:

[1] T. Zhang, et al., Nature physics 6, 104 (2010).

[2] F. Oguro, M. Haze et al., Physical Review B 103, 085416 (2021).

[3] Y. Sato, M. Haze et al., Physical Review Letters 130, 106002 (2023).