Uploaded January 2024 | Updated September 2026, 2 weeks ago
Speaker: Giandomenico Palumbo (DIAS)
Abstract: Topological insulating (TI) phases were originally highlighted for their disorder-robust bulk responses, such as the quantized Hall conductivity of 2D Chern insulators. With the discovery of time-reversal- (T-) invariant 2D TIs, focus has since shifted to boundary states as signatures of 2D and 3D TIs and topological crystalline insulators (TCIs). However, in T-invariant 3D TCIs such as bismuth, $\alpha$-BiBr, and MoTe2 -termed higher-order TCIs (HOTIs)- the boundary signatures manifest as 1D hinge states, whose configurations are dependent on sample details, and bulk signatures remain unknown. In this talk, I will introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the bulk topological properties of spinful 3D insulators. I will show that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators, “spin-Weyl” semimetal states with gapless spin spectra, and T-doubled axion insulator states with nontrivial partial axion angles $\theta^{\pm}=\pi$ indicative of a 3D spin-magnetoelectric bulk response.
Speaker: Giandomenico Palumbo (DIAS)
Abstract: Topological insulating (TI) phases were originally highlighted for their disorder-robust bulk responses, such as the quantized Hall conductivity of 2D Chern insulators. With the discovery of time-reversal- (T-) invariant 2D TIs, focus has since shifted to boundary states as signatures of 2D and 3D TIs and topological crystalline insulators (TCIs). However, in T-invariant 3D TCIs such as bismuth, $\alpha$-BiBr, and MoTe2 -termed higher-order TCIs (HOTIs)- the boundary signatures manifest as 1D hinge states, whose configurations are dependent on sample details, and bulk signatures remain unknown. In this talk, I will introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the bulk topological properties of spinful 3D insulators. I will show that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators, “spin-Weyl” semimetal states with gapless spin spectra, and T-doubled axion insulator states with nontrivial partial axion angles $\theta^{\pm}=\pi$ indicative of a 3D spin-magnetoelectric bulk response.
