â€‹PRESENTER: Prof. Shuichi Murakami

**Material Science and Engineering Graduate Seminar**

**Theoretical Approaches for Topological Materials**

ABSTRACT:

Topological materials have been attracting much interest in condensed matter physics. The first and well-known example of topological systems is the quantum Hall system. Nonetheless, because it requires a strong electric field, possibilities for its applications are limited. Meanwhile, theoretical proposals of the spin Hall effect [1,2] have triggered new possibilities for topological systems [4]. Prior to these proposals, Hall effects and topological phases were thought to be possible only when the time-reversal symmetry is broken, i.e. when a magnetic field or magnetism is present in the system. In contrast, the spin Hall effect is realized in nonmagnetic metals and semiconductors, and the topological insulators are found in various narrow-gap semiconductors. Furthermore, from the concept of topological insulators, a new phase called topological semimetals has been proposed, and now there are many materials found. In my presentation, we show recent developments in the field of topological materials from our point of view [3-9]. In partic-ular, we discuss topological phase transitions [3,5,6], which idea is utilized for search of topological mate-rials.

References

[1] S. Murakami, N. Nagaosa, and S.-C. Zhang, Science 301, (2003) 1348.

[2] J. Sinova, D. Culcer, Q. Niu, N. A. Sinitsyn, T. Jungwirth, and A. H. MacDonald, Phys. Rev. Lett. 92 (2004) 126603.

[3] S. Murakami, New J. Phys. 9, 356 (2007).

[4] S. Murakami, Phys. Rev. Lett. 97, 236805(2006).

[5] S. Murakami, M. Hirayama, R. Okugawa, S. Ishibashi, T. Miyake, Sci. Adv. 3, e1602680 (2017).

[6] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, and T. Miyake, Phys. Rev. Lett. 114, 206401 (2015).

[7] M. Hirayama, S. Matsuishi, H. Hosono, and S. Murakami, Phys. Rev. X 8, 031067 (2018).

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

BIOGRAPHY:

Shuichi Murakami obtained his Ph. D.(1999) degree from the University of Tokyo. He moved to Tokyo Institute of Technology as an associate professor in 2007 and became a professor in the Department of Physics and in Materials Research Center for Element Strategy in 2012. He has been working on theories of spintronics and topological phenomena in condensed matter physics. He received various prizes, including Sir Martin Wood Prize in 2010, IBM Japan Science Prize in 2011, JSPS Prize in 2012, and APS Fellow in 2017.