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物理学院“博约学术论坛”系列报告(第237期)

发布日期:2019年12月17日

题目: Some exotic topological states in Magnetic groups

报告人:徐刚  教授(华中科技大学)

时间:2019年12月18日(周三)下午16:00

地点:新葡萄京娱乐场8455中心教学楼501

摘要:   In Magnetic groups (MGs), the time reversal symmetry is broken, but its combinations with some crystalline symmetries, named as crystalline time reversal symmetry (CTRS), are preserved, and can protect the novel topological phase against the local perturbations. In this talk, I will introduce three works on the exotic topological quantum states in magnetic groups. 1. Antiferromagnetic (AFM) Dirac semimetals (DSMs) 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 MGs. 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. 2. 4’/m’-respecting crisscross AFM model in 2D and 3D, both belonging to the Z2 classification and exhibiting interesting magnetic high-order topological insulating (HOTI) phases. The topologically nontrivial phase in the 2D model is characterized by the fractional charge localized around the corners and the quantized charge quadrupole moment. Moreover, our 2D model also exhibits the quantized magnetic quadrupole moment, which is a unique feature compared with previous studies. The 3D system stacked from layers of the 2D model possesses the HOTI phase holding chiral 1D metallic states on the hinge, which corresponds to the Wannier center flow between the valence and conduction bands. 3. New type of topological superconductor (TSC) phase in the 1-dimensional (1D) MGs. In the 1D system generated by  or  symmetry, such CTRS could lead to the BDI class of TSC with  topological invariant, rather than the conventional D- or DIII-class (Z2 invariant). More interestingly, the TSC phase in the MGs generated by and  are totally beyond the Altland and Zirnbauer symmetry classes, but belong to a new type of helical (helical  for system) classification, in which the robust Majorana zero modes are protected by the unitary chiral symmetry. The minimal tight binding BdG Hamiltonians satisfying or  MGs are also constructed to illustrate the novel TSC phase.

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