# [Seminar] "Topology and transport in inversion asymmetric crystals" by Prof. Shuichi Murakami

### Date

Monday, July 30, 2018 - 15:30 to 16:30

C700 - Lab3

### Description

Shuichi Murakami

Professor, Department of Physics, Tokyo Institute of Technology
Professor, Materials Research Center for Element Strategy, Tokyo Institute of Technology,

Abstract of Talk

When a crystal lacks spatial inversion symmetry, it sometimes allows novel band structures and transport properties which are absent in inversion-symmetric crystals. For example, Weyl semimetals [1], which have Dirac cones in the band structure near the Fermi energy, are allowed in inversion asymmetric crystals, and they actually emerge in a universal way. We showed that a Weyl semimetal phase always appears in a transition between topological and ordinary insulators for any inversion-asymmetric crystals [1]. Moreover, if the gap of an inversion-asymmetric system is closed by a change of an external parameter, the system runs either into a Weyl semimetal phase or a nodal-line semimetal [2]. It is realized e.g. in tellurium (Te). Tellurium consists of helical chains, lacking inversion symmetry. At high pressure the band gap of Te closes and it runs into a Weyl semimetal phase, as shown by our ab initio calculation [3].
In such inversion asymmetric crystals, we propose that the current induces an orbital magnetization, and we call this effect an orbital Edelstein effect. For example, in tellurium where the crystal has a helical chains, a current along the helical axis induces an orbital magnetization [4-5], as is analogous to classical solenoids. Moreover, a similar effect appears for phonons. Each phonon eigenmode has angular momentum due to rotational motions of the nuclei, but their sum is zero in equilibrium. Meanwhile a heat current in the Te crystal induces a nonzero total angular momentum [6], which we call a phonon thermal Edelstein effect.

[1] S. Murakami, New J. Phys. 9, 356 (2007).
[2] S. Murakami, M. Hirayama, R. Okugawa, S. Ishibashi, T. Miyake, Sci. Adv. 3, e1602680 (2017).
[3] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, and T. Miyake, Phys. Rev. Lett. 114, 206401 (2015).
[4] T. Yoda, T. Yokoyama, and S. Murakami, Sci. Rep. 5, 12024 (2015).
[5] T. Yoda, T. Yokoyama, and S. Murakami, Nano Lett. 18, 916 (2018).
[6] M. Hamada, E. Minamitani, M. Hirayama, S. Murakami, preprint (2018).

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