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# Past Events

## Failure of Nielsen-Ninomiya Theorem and Fragile Band Topology of Twisted Bilayer Graphene

Speaker: | Dr. Bohm-Jung Yang |

Affiliation: | Department of Physics and Astronomy, Seoul National University, Seoul, Korea Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul, Korea |

Date: | August 22, 2019 (Thursday) |

Time: | 4:00 p.m. |

Venue: | Room 522, 5/F, Chong Yuet Ming Physics Building, HKU |

### Abstract

I am going to talk about novel topological properties of twisted bilayer graphene at magic angle, a representative spinless fermion system with space time inversion I_{ST} symmetry. We show that the Wannier obstruction and the fragile topology of the nearly flat bands in twisted bilayer graphene at magic angle are manifestations of the nontrivial topology of two-dimensional real wave functions characterized by the Euler class. To prove this, we examine the generic band topology of two dimensional real fermions in systems with space-time inversion I_{ST} symmetry. The Euler class is an integer topological invariant classifying real two band systems. We show that a two-band system with a nonzero Euler class cannot have I_{ST}-symmetric Wannier representation. Moreover, a two-band system with the Euler class e_{2} has band crossing points whose total winding number is equal to 2e_{2}. Thus the conventional Nielsen-Ninomiya theorem fails in systems with a nonzero Euler class. We propose that the topological phase transition between two insulators carrying distinct Euler classes can be described in terms of the pair creation and annihilation of vortices accompanied by winding number changes across Dirac strings. When the number of bands is bigger than two, there is a Z_{2} topological invariant classifying the band topology, that is, the second Stiefel Whitney class (w_2). Two bands with an even (odd) Euler class turn into a system with w_2=0 (w_2=1) when additional trivial bands are added. Although the nontrivial second Stiefel-Whitney class remains robust against adding trivial bands, it does not impose Wannier obstruction when the number of bands is bigger than two. However, when the resulting multi-band system with the nontrivial second Stiefel-Whitney class is supplemented by additional chiral symmetry, a nontrivial second-order topology and the associated corner charges are guaranteed.

References:

[1] J. Ahn, D. W. Kim, Y.K.Kim and **B. –J. Yang**, “Band topology and linking structure of nodal line semimetals with Z2 monopole charges”, Phys. Rev. Lett. 121, 106403 (2018).

[2] J. Ahn, S. J. Park and **B. –J. Yang**, “Failure of Nielsen-Ninomiya theorem and fragile topology in systems with space-time inversion symmetry: application to twisted bilayer graphene at magic angle”, Phys. Rev. X, 9, 021013 (2019).

*Coffee and tea will be served 20 minutes prior to the seminar.*

*Anyone interested is welcome to attend.*