Role of steps on superconductivity of monolayer 2D metals

For two-dimensional (2D) metals, defects play a significant and critical role in the transport properties. We investigated the transport properties microscopically using scanning tunneling potentiometry and the macroscopic four-probe method. Then, combined with structural and spectroscopic information obtained by scanning tunneling microscopy/spectroscopy (STM/S), we have revealed the roles of surface steps in their transport properties, including superconductivity [1-3].
2D superconductors undergo transition into insulators by the application of magnetic fields and/or the introduction of disorder, even at zero temperature. As a quantum phase transition, the superconductor-insulator transition (SIT) has been investigated extensively, and recent studies on highly crystalline 2D superconductors that include surface-reconstructed structures by depositing monolayer metals on semiconducting substrates, various unique quantum phases such as quantum metallic phase and quantum Griffiths phase have been reported. However, microscopic understanding of these curious phases is not sufficient because most of the experimental investigations are performed by transport methods, which are fundamentally macroscopic and provide spatially averaged information. In this presentation, we report on transition and quantum phases microscopically using STM. We investigated the superconductivity of the Pb striped-incommensurate (SIC) phase, a stable monolayer superconducting phase of the Pb/Si(111) systems and compared the results with those obtained by electron transport measurements. Because the ultra-thin Pb superconductor was formed on various vicinal substrates, a high density of steps, which work as a disorder and Josephson coupling, was introduced into the superconductor, and the density can be well-controlled by adjusting the tilt angle. Through the observation of vortices under an out-of-plane magnetic field, we investigated the effect of high-density steps on superconductivity.

Reference:
[1] S. Yoshizawa, et al. Phys. Rev. Lett. 113, 247004 (2014).
[2] F. Oguro, et al., Phys. Rev. B 103, 085416 (2021).
[3] Y. Sato, et al, Phys. Rev. Lett. 130, 106002 (2023)