##### Description

__Nicholas Kioussis__

^{1}Jinwoong Kim

^{1}1, California State University, Northridge, Northridge, California, United States

Topology in various guises plays a central role in modern condensed matter physics. Although the original applications of topological ideas to band structures in semiconductors relied on the existence of a fully gapped bulk spectrum, more recently it has been recognized that protected surface states can arise even in *gapless* systems. The prototypical example of a gapless topological phase is a Weyl semi-metal showing linear dispersion around nodes termed as Weyl points, as the three-dimensional analog of graphene. Surface Fermi arcs are the most prominent manifestation of the topological nature of Weyl semi-metals. I will present predictions of the emergence of Weyl semimetal phase in two distinct cases:

(1)The topological crystalline insulator, Pb_{1−x} Sn_{x}Te exhibits topological phase transition upon the band inversion strength which can be tailored by the substitutional mixing ratio, strain, thermal expansion, ferroelectric displacement, and/or material thickness via quantum confinement effect. The SnTe building block of the compound is also known to exhibit a ferroelectric transition at low temperatures which leads to inversion symmetry breakdown. Using ab-initio-tight-binding calculations we have explored the parameter space associated with both band inversion and ferroelectric displacement. The calculated topological phase diagram shows the emergence of a Weyl semimetal phase which can be tuned with an external magnetic field.^{1}

(2)The interfacial phase-change memory (iPCM) GeTe/Sb2Te3 continues to attract a great deal of interest not only because they are promising candidates for the next generation non-volatile random-access memories but also for their fascinating topological properties. Depending on the atomic-layer-stacking sequence of the GeTe block the iPCM can be either in the ``SET'' (Ge-Te-Ge-Te) or ``RESET'' (Te-Ge-Ge-Te) states where the former exhibits a ferroelectric polarization and an electric conductivity which is two orders of magnitude higher than that of the RESET state. *Ab initio* electronic structure calculations reveal that the ferroelectric polarization in the "SET" phase which breaks the inversion symmetry results in the emergence of a Weyl semimetal phase with a large electric conductivity due to the gapless Weyl nodes.

^{1}T. Liang, S. Kushwaha, J. Kim, Q. Gibson, J. Lin, N. Kioussis, R. J. Cava, and N. P. Ong, A pressure-induced topological phase with large Berry curvature in Pb _{1−x}Sn_{x}Te, Science Advances (2017), Vol. 3, no. 5, e1602510, DOI: 10.1126/sciadv.1602510