Computational Quantum Matter Research Team

Principal Investigator

PI Name

Seiji Yunoki

Degree

D.Eng.

Title

Team Leader

Brief Resume

1996	D.Eng., Nagoya University
1996	Postdoctoral Researcher, National High Magnetic Field Laboratory, USA
1999	Postdoctoral Researcher, Materials Science Center, Groningen University, Netherlands
2001	Postdoctoral Researcher, International School for Advanced Studies, Italy
2006	Long-Term Researcher and Research Assistant Professor, Oak Ridge National Laboratory and University of Tennessee, USA
2008	Associate Chief Scientist, Computational Condensed Matter Physics Laboratory, RIKEN
2010	Team Leader, Computational Materials Science Research Team, RIKEN Advance Institute for Computational Science
2013	Team Leader, Computational Quantum Matter Research Team, RIKEN Center for Emergent Matter Science (-present)
2017	Chief Scientist, Computational Condensed Matter Physics Laboratory, RIKEN (-present)
2018	Team Leader, Computational Materials Science Research Team, RIKEN Center for Computational Science (-present)

Outline

Electrons in solids are in motion within the energy band reflecting the lattice structure of each material. The Coulomb interaction, electron-lattice interaction, and spin-orbit interaction have nontrivial effects on the motion of electrons and induce various interesting phenomena. Our aim is to elucidate the emergent quantum phenomena induced by cooperation or competition of these interactions, using state-of-the-art computational methods for condensed matter physics. Our current focus is on various functional transition metal oxides, topological materials, and heterostructures made of these materials. Our research will lead not only to clarify the mechanism of quantum phenomena in existent materials but also to propose novel materials.

Research Fields

Physics, Materials Science

Keywords

Strongly correlated electron system
Magnetism
Superconductivity
Computational condensed matter physics

Results

Mechanism of novel insulating state and possible superconductivity induced by a spin-orbit coupling

Electrons in solids are moving around, affected by the Coulomb interaction, electron-lattice interaction, and spin-orbit interaction, which induces different behaviors characteristic of each material. Recently, the study for the spin-orbit coupling has greatly progressed and attracted much attention. In the 5d transition metal oxide Sr₂IrO₄, the spin and orbital degrees of freedom are strongly entangled due to the large spin-orbit coupling and the novel quantum state is formed. Sr₂IrO₄ is also expected to be a possible superconducting material with a great deal of similarities to the parent compound of cuprate high-temperature superconductivity

We have studied the detailed electronic properties of a 3-orbital Hubbard model for Sr₂IrO₄ with several computational methods. Our calculations have clearly shown that the ground state of this material is an effective total angular momentum J_eff=1/2 antiferromagnetic insulator, where J eff is “pseudospin”,formed by spin and orbital degrees of freedom due to the strong spin-orbit coupling (x-AFI in Fig. (a)). We have also proposed that the d_x2-y2-wave “pseudospin singlet” superconductivity (SC in Fig. (b)) is induced by electron doping into the J_eff=1/2 antiferromagnetic insulator Sr₂IrO₄.

Ground state phase diagram of 3-orbital Hubbard model with a spin-orbit coupling. (a) Electron density n=5, (b) n>5.

Members

Seiji Yunoki	Team Leader	yunoki[at]riken.jp
Sadamichi Maekawa	Senior Research Scientist
Kazuhiro Seki	Research Scientist
Shohei Miyakoshi	Postdoctoral Researcher

Publications

B. H. Kim, K. Seki, T. Shirakawa, and S. Yunoki

Topological property of t_2g⁵ system with a honeycomb lattice structure

Phys. Rev. B 99, 155135 (2019)
T. Kaneko, T. Shirakawa, S. Sorella, and S. Yunoki

Photoinduced eta Pairing in the Hubbard Model

Phys. Rev. Lett. 122, 077002 (2019)
T. Sato, T. Shirakawa, and S. Yunoki

Spin-orbital entangled excitonic insulator with quadrupole order

Phys. Rev. B 99, 075117 (2019)
K. Seki, T. Shirakawa, and S. Yunoki

Variational cluster approach to thermodynamic properties of interacting fermions at finite temperatures: A case study of the two-dimensional single-band Hubbard model at half filling

Phys. Rev. B 98, 205114 (2018)
S. Sorella, K. Seki, O. O. Brovko, T. Shirakawa, S. Miyakoshi, S. Yunoki, and E. Tosatti

Correlation-Driven Dimerization and Topological Gap Opening in Isotropically Strained Graphene

Phys. Rev. Lett. 121, 066402 (2018)