摘要

One or two atomic layers grown on crystal surfaces are recently found to be superconducting: a monolayer of Pb [1,2,3], two atomic layers of In [1,2], Ga [4], and Tl [9], and a monolayer of Tl+Pb alloy [5] on Si(111) surface, a single unit layer of FeSe [6,7] film, Ca-intercalated double-layer graphene [8], and so on. Interesting issues of these ‘atomic-layer superconductors’ may be three fold; (1) Large fluctuation due to two-dimensionality (2D), (2) Influence of substrates, and (3) symmetry breaking.
(1) Large fluctuation due to 2D: According to Mermin-Wagner Theorem, 2D lattices do not have phase transitions due to large fluctuation. In reality, because of the ‘quasi-2D’ nature, superconductivity occurs even in monatomic layers. Large fluctuation induces Aslamazov-Larkin- Maki-Thompson corrections due to the amplitude fluctuation, and Berezinskii-Kosterlitz- Thouless (BKT) transitions and a Bose metal phase [9] due to phase fluctuation.
(2) Influence of substrate: The superconducting transition temperatures TC of most of the known atomic-layer superconductors are lower than those of the bulk materials. One exception is the single unit-layer FeSe film which shows TC higher than 100 K while that of the bulk FeSe crystal is a few K [6,7]. This example indicates possibility to enhance TC by making materials as thin as monolayer thick on suitable substrates.
(3) Symmetry breaking: Since the material surfaces are in a situation of break-down of space-inversion symmetry, spin degeneracy in electronic states can be lifted (Rashba effect) [5]. Superconductivity at surfaces and monolayers are then novel because singlet- and triplet- Coopers can be mixed (parity-broken superconductivity). Actually, scanning tunneling spectra taken from the superconducting (Tl+Pb) monolayer is not reproduced by BCS theory based on s-wave superconductivity. A pseudo-gap is found beyond the upper critical field and at the vortex core in this system. This may be a breakthrough in research exploring new types of superconductivity.
In my talk I will show some experimental data of in situ four-point probe transport measurements and ultra-low temperature scanning tunneling microscopy/spectroscopy on some atomic-layer superconductors. This work is based on collaboration with the groups of A. A. Saranin, A. V. Zotov in Russia, and Y. Hasegawa at ISSP of Univ. Tokyo.

[1] T. Zhang, et al., Nat. Phys. 6, 104 (2010). [2] T. Uchihashi, et al., Phys. Rev. Lett. 107, 207001 (2011). [3] M. Yamada, et al., Phys. Rev. Lett. 110, 237001 (2013).
[4] W.-H. Zhang, et al., Phys. Rev. Let. 114, 107003 (2015). [5] A.V. Matetskiy, et al., Phys. Rev. Lett. 115, 147003 (2015). [6] W.-H. Zhang, et al., Chin. Phys. Lett. 31, 017401(2014). [7] J.-F. Ge, et al., Nat. Materials 14, 285 (2015).  [8] S. Ichinokura, et al., ACS Nano 10, 2761 (2016). [9] S. Ichinokura, et al., 2D Materials 4, 025020 (2017).

报告人简介

He received his DSc. in Physics (Paper Doctor) from The University of Tokyo in 1991 for research on electron holography done at Hitachi Company under supervision of Dr. Akira Tonomura before joining the University of Tokyo. In 1991-1994, he was a research associate in a research group of Professor Shozo Ino at the Department of Physics, University of Tokyo. Since 1994, he promoted to be an associate professor and a Principal Investigator conducting his own group of experimental surface physics in the Department. Since 2009 he is a full professor. He supervised 18 DSc. students and 31 MS students. His interest has been focused on transport of not only electric charges but also spins at crystal surfaces and 2D nanosheets. He has published about 200 original papers and about 20 review articles and book chapters including three books (in Japanese) of single work.

Host: Prof. Canhua Liu