Topological Materials Design Research Unit

Principal Investigator

PI Name

Motoaki Hirayama

Degree

Ph.D.

Title

Unit Leader

Brief Resume

2013	Ph.D., Department of Applied Physics, University of Tokyo
2013	Postdoctoral Researcher, Nanosystem Research Institute, AIST,
2015	Project Assistant Professor, Department of Physics, Tokyo Institute of Technology
2017	Research Scientist, First-Principles Materials Science Research Team, RIKEN Center for Emergent Matter Science
2020	Unit Leader, Topological Materials Design Research Unit, RIKEN Center for Emergent Matter Science (-present)
2020	Project Associate Professor, Quantum-Phase Electronics Center, The University of Tokyo (-present)

Outline

In our unit, we explore novel materials and their properties using first-principles calculations, which are numerical methods for realistic materials. In particular, we focus on the topological properties of the electronic band structure and search for topological materials with non-trivial properties. We investigate the properties and applications of these unique electronic states.　We also include superconducting states and propose the emergence of Majorana fermions. In addition, we develop ab initio methods to treat correlation effects and design a wide range of materials including strongly correlated systems and magnetic systems. We design materials across a wide range of fields, including materials in the chemical and materials fields, such as electrides.

Research Fields

Physics, Materials Science

Keywords

First-principles calculations
Theoretical materials design
Topological materials
Majorana fermions and parafermions
Spin-orbit interaction

Results

Electrides as a New Platform of Topological Materials

Our unit propose e lectrides as a new platform of topological materials. Electrides are a group of materials in which electron e- exists in the interstitial region and stabilizes the structure as an anion. Electrides are being studied in the field of catalysis because of their small work function. For example, in the layered material Sc₂C (Fig. (a)), electrons enter the cavities between the layers and exhibit insulating properties as shown in Fig. (b). The charge density of [Sc₂C]²⁺2e^– extends to interlayer positions that are significantly displaced from the Sc₂C layer due to the anionic electrons 2e^–, resulting in a non-trivial system with a quantized large polarization. Reflecting the bulk topology, a topologically-protected metallic state appears on the Sc₂C surface (Fig. (c)). The metallic surface state originates from the interstitial electron, and therefore floats above the Sc₂C surface (Fig. (d)). This electron cloud has a small work function, making it possible to use Sc₂C as a topological substrate for high-density electron doping. For example, one electron per Mo site can be doped for MoS₂ (Fig. (e)). We have discovered a variety of topological electrides including relativistic systems, which will lead to the development of topological properties across scientific fields.

(a) Crystal structure of the topological electride [Sc₂C]²⁺2e^–, (b) band structure of Sc₂C, (c) band structure of the Sc₂C (111) surface, (d) floating topological surface state, (e) topological carrier doped to MoS₂

Members

Motoaki Hirayama	Unit Leader	motoaki.hirayama[at]riken.jp

Publications

T. Yu, R. Arita, and M. Hirayama

Interstitial-Electron-Induced Topological Molecular Crystals

Adv. Phys. Res. 2, 2200041 (2022)
M. Hirayama, T. Tadano, Y. Nomura, and R. Arita

Materials design of dynamically stable d⁹ layered nickelates

Phys. Rev. B 101, 75107 (2020)
M. Hirayama, S. Matsuishi, H. Hosono, and S. Murakami

Electrides as a New Platform of Topological Materials

Phys. Rev. X 8, 031067 (2018)
M. Hirayama, R. Okugawa, T. Miyake, and S. Murakami

Topological Dirac nodal lines and surface charges in fcc alkaline earth metals

Nat. Commun. 8, 14022 (2017)
M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, and T. Miyake

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium

Phys. Rev. Lett. 114, 206401 (2015)