Probing the Quantized Berry Phases in NbSe₂ Using Scanning Tunneling Microscopy

Topologically trivial insulators can be divided into two main categories: atomic insulators (AI) and obstructed atomic insulators (OAI) [1]. While both can be described by exponentially localized Wannier orbitals, the key difference is that in an OAI, the center of charge of these orbitals is located at empty sites within the unit cell, rather than on atoms. Despite many theoretical predictions, the unambiguous experimental identification of an OAI phase has remained elusive. In this presentation, we provide the first direct experimental evidence of an OAI phase in 1H-NbSe₂. We devise a method to extract the inter-orbital correlation functions from the local spectral function, using the orbital wave functions obtained from ab initio methods. The local spectral function can be probed through scanning tunneling microscopy. We apply our method to new experimental data and extract the inter-orbital correlation functions for the atomic band of 1H-NbSe₂ that crosses the Fermi level. Consequently, we show that this atomic band realizes an optimally compact OAI, marking the first unambiguous experimental identification of this phase.

[1] Y. Xu, et al., Phys. Rev. B 109, 165139 (2024)