# Special Condensed Matter Seminar: "Topological semimetals and chiral transport in inversion asymmetric systems"

The Z2 topological insulators (TIs) are topological phases under time-reversal symmetry. In 2007, we theoretically proposed a universal phase diagram describing a phase transition between 3D TIs and normal insulators (NIs), and we showed that in a TI-NI transition, a Weyl semimetal phase necessarily intervenes between the two phases, when inversion symmetry is broken. In this talk, we show that this scenario holds for materials with any space groups without inversion symmetry. Namely, if the gap of an inversion-asymmetric system is closed by a change of an external parameter, the system runs either into (i) a Weyl semimetal or (ii) a nodal-line semimetal, but no insulator-to-insulator transition happens. This transition is realized for example in tellurium (Te). Tellurium has a unique lattice structure, consisting of helical chains, and therefore lacks inversion and mirror symmetries. According to our ab initio calculation, at high pressure the band gap of Te decreases and finally it runs into a Weyl semimetal phase. We also theoretically propose chiral transport in systems with such helical structures.

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