Superconductivity at Surfaces studied by Scanning Tunneling Microscopy

Yukio Hasegawa^1*

¹The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Japan

* Presenter:Yukio Hasegawa, email:hasegawa@issp.u-tokyo.ac.jp

Superconductivity that emerges in metallic surface states is one of the ultimately thin two-dimensional (2D) superconductors. One of the advantages, if compared with other 2D superconductors, is that atomically well-ordered structures can be easily formed in macroscopic dimensions because of the thermal stability through the self-organized structural reconstruction. Basic properties such as atomic structure and electronic states are well characterized by standard surface science techniques including scanning tunneling microscopy (STM), and can be modify in a controlled manner through the deposition and adsorption of additional materials.
One ubiquitous feature of the 2D atomically-thin electronic systems is the natural presence of atomic steps on its substrate. Atomic steps are considered to strongly affect electron transport as they decouple neighboring surface terraces [1]. We have demonstrated that the steps of the √3x√3-In/Si(111) surface superconductor behave as a Josephson junction and hold elongated vortices called Josephson vortices along the steps [2]. On striped incommensurate (SIC) phase of Pb/Si(111) the steps are found to block the propagation of the superconducting proximity effect and enhance it when they are located within the coherence length [3].
In two-dimensional superconductors usual orbital pair breaking of the superconductivity by in-plane magnetic field can be suppressed, allowing the Zeeman pair breaking to determine the critical magnetic field. There is however no protection against perpendicular magnetic fields. Using STM, we found that in narrow terraces of the Pb/Si(111) surface whose width is less than the coherence length superconductivity is protected against perpendicular magnetic fields. It is presumably due to the suppression of orbital pair breaking by the step confinement. Since the density and the coupling strength of the steps can be controlled, our study opens a way to design 2D superconductors that maintain the pair correlation under magnetic field in all directions.

[1] M. Hamada and Y. Hasegawa, Phys. Rev. B 99, 125402 1-5 (2019)
[2] S. Yoshizawa, H. Kim, T. Kawakami, Y. Nagai, T. Nakayama, Xiao Hu, Y. Hasegawa, T. Uchihashi, Phys. Rev. Lett. 113, 247004 1-5 (2014).
[3] H. Kim, S.-Z. Lin, M. J. Graf, Y. Miyata, Y. Nagai, T. Kato, and Y. Hasegawa, Phys. Rev. Lett.,117, 116802 1-5 (2016)

Keywords: scanning tunneling microscopy, surface superconductivity, monolayer superconductivity