Graphene, Advanced 2D Materials & Semiconductors

Biography

Biography: Junji Tominaga

Abstract

Multiferroics is a keyword for future electronics. However, it is still difficult to induce large electrical and magnetic properties at room temperature. It relies on that electric dipole moments are usually related to p-electrons while the magnetic moments are to d-electrons. If a large magnetic moment is induced from p-electrons, giant multiferroics would be possible. A combination of a topological insulator and a ferroelectric insulator may open a new era to realize such the multifunctionality. Topological insulators, such as Bi₂Te₃ and Sb₂Te₃, usually satisfy both spatial inversion and time reversal symmetry. The topological surface bands are mainly made of the band inversion of 6p- or 5p-electrons of Te, Sb, and Bi. On the other hand, GeTe is known as a ferroelectric material, which has a large spin-orbit coupling (SOC) compared with other oxide ferroelectric materials. Due to the large SOC, it shows a large Rashba-like spin split band. It is noted here that the existence of an electric dipole moment breaks the spatial inversion symmetry. If thin films of Sb₂Te₃ and GeTe are piled up alternatively, what happens on the band structure as the bulk film? Actually, both layers can share a lattice plane using (0001) and (111) through a van der Waals force. As the bulk film, the spatial inversion symmetry is broken. Therefore, plural Dirac cones appear apart from the Γ-point in the k-space, resulting in a Weyl semimetal. Weyl semimetals are magnetic sensitive because spin bands are lifted from the band degeneration. In the presentation, we show several experimental results of the Weyl semimetal from superlattices consisted of GeTe and Sb₂Te₃ sublayers at room temperature.