Prof. Yukiko Yamada-Takamura

Japan Advanced Institute of Science and Technology, Japan
Yukiko received Ph.D. in Metallurgy from The University of Tokyo (UT) in 1998. After her JSPS Postdoctoral Fellowship working on ion-assisted growth and characterization of thin films with metastable nitride and oxide phases at UT and also at IPP Garching, she joined the Surface Science Division of IMR, Tohoku University as a research associate where she studied nitride and boride thin film growths using UHV-SPM combined with growth chambers. In 2006, she joined School of Materials Science in JAIST as a tenure-track faculty to start her own research group, where she is a full professor, now. Her current research interest is in 2D materials which can be only realized on substrate surfaces, such as “silicene”.

Talk title: Novel 2D materials stabilized on surfaces
To realize group IV two-dimensional (2D) materials which lack “layered host materials”, one has to grow them epitaxially on single-crystalline substrates. Such novel materials have to be characterized thoroughly by complementary surface characterization methods, and first-principles calculation is indispensable to understand their crystal and electronic structures since they are very different from their bulk counterparts. One of such materials is “silicene”, which is a honeycomb structure made of Si atoms. We have demonstrated, in 2012, by a combination of scanning tunnelling spectroscopy (STM), core-level and angle-resolved photoelectron spectroscopy (ARPES), and first-principles electronic structure calculations, that on epitaxial ZrB₂(0001) films grown on Si(111) substrates, Si atoms from the substrate segregate to the film surface and crystallize in a honeycomb structure [1,2].

In this talk, I would like to introduce a 2D Ge lattice having a “bitriangular” structure, which was synthesized on ZrB₂(0001) film surface using similar method. Our theoretical study on freestanding bitriangular lattice demonstrated that the flat band of a kagome lattice can be embedded in this 2D structure [3]. We have found out by combing STM observation, total reflection high energy positron diffraction, photoelectron spectroscopy, and first-principles calculations, Ge atoms segregate and crystallize into such a bitriangular lattice on the surface of ZrB₂(0001) films grown epitaxially on Ge(111) substrates [4]. The electronic structure measured by ARPES at room temperature reveals “nearly” flat band at Fermi level which is evidence of the metallic nature of this 2D Ge layer. Moreover, at temperature lower than 150 K, the in-plane unit cell of this 2D Ge layer becomes 3 times larger, indicative of a transition to charge density wave phase. The detail of the structural change and the possible mechanism will be discussed.

[1] A. Fleurence, R. Friedlein, T. Ozaki, H. Kawai, Y. Wang, and Y. Yamada-Takamura, Phys. Rev. Lett. 108, 245501 (2012).
[2] Y. Yamada-Takamura and R. Friedlein, Sci. Technol. Adv. Mater. 15, 064404 (2014).
[3] C.-C. Lee, A. Fleurence, Y. Yamada-Takamura, and T. Ozaki, Phys. Rev. B 100, 045150 (2019).
[4] A. Fleurence et al., Phys. Rev. B 95, 201102(R) (2020).