Discovery of 2D topological insulator (TI) in HgTe/CdTe and 3D TI in Bi1-xSbx initiated the search for new types of topological materials and their device applications. TIs are characterized by the gapless metallic states at the boundary despite insulating bulk. In 3D TIs, Such a boundary state is recognized as a linearly dispersive Dirac-cone energy band at the surface with a helical spin texture (Fig. 1). Recently, a new type of topological material with a Dirac-cone energy band in bulk, termed topological semimetal (TSM), is attracting particular attention. Dirac semimetal is a prototypical TSM, and is characterized by the spin-degenerate bulk 3D Dirac-cone band, and shows outstanding physical properties distinct from the TIs such as extremely high mobility and non-saturating linear magneto-resistance. When spin degeneracy of Dirac semimetal is lifted by breaking space-inversion symmetry or time-reversal symmetry, Weyl semimetal can be realized (Fig. 1). In Weyl semimetals, bulk electrons behave as massless Weyl particles, and there exist Fermi-arc surface states connecting surface projection of Weyl-node pairs. While Dirac and Weyl semimetals are characterized by the band crossing at a discrete point in k space, there exist another type of TSM called line-node semimetal whose Dirac points extend one dimensionally in k space. Such a nodal line is typically protected by specific symmetry of the crystal such as mirror reflection symmetry and nonsymmorphic symmetry.

In this talk, we will present novel electronic states of some exotic topological materials such as TSMs and topological superconductors, studied by angle-resolved photoemission spectroscopy (ARPES). We will focus on the following subjects.

1. Discovery of new chiral fermions in CoSi [1]
2. Nodal line in MgB2-related compound [2]
3. Hourglass fermions in layered material Ta3SiTe6 [3]
4. Nodal loop protected by mirror symmetry in CaAgAs [4]
5. Topological superconductivity in a hybrid of Pb and TlBiSe2 [5].

Takafumi Sato is Professor of World Premiere Institute-Advanced Institute for Materials Research, Tohoku University, Japan. He received his Doctor of Science from Tohoku University, Japan in 2002. His professional experience while at Tohoku University is as follows: Assistant Professor from 2002-2009, Institute of Excellence in Higher Education at Tohku from 2009-2010, Associate Professor from 2010-2017 and Professor from 2017-2019.

He was recognized for “The Young Scientists’ Prize”, Commendation for Science and Technology by MEXT in 2007 and the “Highly Cited Researcher from Thompson Reuters in 2014. His current research is on the study electronic structure of novel functional materials by using ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). His current research targets are the following: search for exotic quasiparticles in topological materials; origin of superconductivity in high-temperature superconductors; spin-dependent electronic structure in strongly spin-orbit-coupled systems; novel electronic states of atomic-layer materials; many-body interactions in strongly correlated electron systems; development of a new ultrahigh-resolution spin-resolved ARPES system.

1. D. Takane et al., Phys. Rev. Lett. 122, 076402 (2019).
2. D. Takane et al., Phys. Rev. B 98, 041105(R) (2018).
3. T. Sato et al., Phys. Rev. B 98, 121111(R) (2018).
4. D. Takane et al., npj Quantum Materials, 3, 1 (2018). [5] C. X. Trang et al., submitted.

Researchers should cite this work as follows:

• Takafumi Sato (2020), "Electronic States of Novel Topological Materials Studied by ARPES," https://nanohub.org/resources/33833.

  BibTex | EndNote

1. angle-resolved photoemission spectrocopy (ARPES)
2. k-space
3. Spectroscopy
4. spintronics
5. topological materials and matter