報告題目：Magnetic topological semimetals
報 告 人：徐剛 教授（華中科技大學）
In a 3D solid, the low energy excitation of the linearly dispersive crossing bands satisfies the Dirac (Weyl) equation. Such band crossing is named as Dirac (Weyl) node (DN or WN), and such solid is known as the topological (Dirac or Weyl) semimetal (DSM or WSM), which exhibit remarkable features, such as Fermi arcs, magnetic monopoles and Weyl anomaly. In this talk, I will introduce three works on magnetic topological semimetals. 1. Ferromagnetic HgCr2Se4, the only known of double-Weyl semimetal, where the quantum anomalous Hall effect can be achieved in its quantum-well structure. 2. The long-pursuing ideal WSM realized in the non-collinear magnetic GdSI. We demonstrate that fruitful topological phases can be realized in a specific honeycomb lattice, including the ideal Weyl semimetal, double-Weyl semimetal, 3D strong topological insulator, nodal-line semimetal, and a novel semimetal consisting of both Weyl nodes and nodal-lines. 3. Anti-ferromagnetic (AFM) DSM realized in the interlayer AFM EuCd2As2. In this work, we generalize the concept of DSM to the magnetic space groups (MSGs), and define a new category of DSM in type IV MSGs. Many exotic topological states, such as the triple point semimetal and the AFM topological insulator holding of the half-quantum Hall effect can be derived from such AFM DSMs by breaking certain symmetry, providing an ideal platform to study topological phase transitions.