In this paper, we reveal composition-tunable topological states in Co-doped Fe(Te,Se). By introducing Co doping, the topological surface state and bulk Dirac semimetal state can be continuously shifted toward the Fermi level, allowing different topological phases to coexist and evolve in one superconducting material. This provides a promising platform for studying topological superconductivity and multiple Majorana modes.

Related achievement titled “Evolution of the Intertwining Correlated Topological Phases in Iron-Based Superconductor Fe(Te,Se)” was published in JACS. https://doi.org/10.1021/jacs.5c19995

In this paper, we find a new excitonic insulator candidate, Ta₂Pd₃Te₅. It has the advantage that the excitonic states do not entangle with CDW or structure transition, unlike the previous excitonic cadidates. Thus it is a promising platform to study the excitonic physics.

Related achievement titled “Spontaneous Gap Opening and Potential Excitonic States in an Ideal Dirac Semimetal:Ta₂Pd₃Te₅” was published on PHYSICAL REVIEW X. https://doi.org/10.1103/PhysRevX.14.011047

We made a two-week trip to the Institute for Solid State Physics, the University of Tokyo for ARPES experiments. The Institute is located in Kashiwa City, Chiba Prefecture, and specializes in fundamental solid state sciences.

During our visit, we conducted experiments primarily in Professor Kondo’s laboratory and Professor Okazaki’s laboratory, utilizing cutting-edge equipments such as high-resolution ARPES with a 7eV laser, and an ARPES system capable of reaching ultra-low temperatures. Additionally, we had the opportunity to visit facilities dedicated to tr-ARPES and spin-resolved ARPES.

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This is one of the papers in a special topic for ARPES studies on quantum materials. We reviewed all the ARPES studies on the topological superconducting states in iron-based superconductors in one chapter. The paper can be found at: https://doi.org/10.1360/SSPMA-2023-0138