Theoretical seminar | 26 January 2022

Topological states of matter, such as quantum (spin) Hall and especially higher-order states, are attracting increasing attention in photonics due to their robustness to disorder and high localization. However, the range of available models is often limited to complicated lattice geometries or artificially engineered gauge fields. Here, with intuition from a recent 1D realization [1], we exploit the concept of orbital hybridization in photonic systems [2] and find two simple 2D waveguide lattice models where accidental orbital degeneracy leads to higher-order topology. First, we show that s-f hybridized kagome lattice is a novel multipolar higher-order topological insulator (HOTI) which may exhibit fully flat-band dynamics for fine-tuned model parameters. Second, we find an unconventional HOTI phase in s-d hybridized square lattice which reduces to two known HOTI layers [5] for two orbital pseudospins. Our work paves a way towards higher-order topology which is protected by crystalline symmetries only partially, while some protection is provided by the symmetries of on-site orbital modes.

Main paper/arXiv, related to the seminar, and other references

[1] R. S. Savelev and M. A. Gorlach, Phys. Rev. B 102, 161112 (2020).
[2] X. Huang, Y. Lai, Z. Hang et al., Nature Mater. 10, 582–586 (2011).
[3] B.-Y. Xie et al., Phys. Rev. B 98, 205147 (2018).
[4] M. Mazanov and M. A. Gorlach, arXiv:2111.01224 (2021).