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

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).