Observation of floating surface state in obstructed atomic insulator candidate NiP$_2$

TL;DR

This study reports the discovery of a floating surface state in NiP$_2$, an obstructed atomic insulator, using spectroscopy and theoretical calculations, revealing a unique surface state originating from obstructed Wannier charge centers.

Contribution

It provides the first experimental observation of a floating surface state in an obstructed atomic insulator, linking it to obstructed Wannier charge centers and surface reconstruction effects.

Findings

Floating surface state observed on NiP$_2$ (100) surface.

Surface state is isolated from bulk states and originates from obstructed Wannier centers.

Surface reconstruction splits the obstructed surface state.

Abstract

Obstructed atomic insulator is recently proposed as an unconventional material, in which electric charge centers localized at sites away from the atoms. A half-filling surface state would emerge at specific interfaces cutting through these charge centers and avoid intersecting any atoms. In this article, we utilized angle-resolved photoemission spectroscopy and density functional theory calculations to study one of the obstructed atomic insulator candidates, NiP$_2$. A floating surface state with large effective mass that is isolated from all bulk states is resolved on the (100) cleavage plane, distinct from previously reported surface states in obstructed atomic insulators that are merged into bulk bands. Density functional theory calculation results elucidate that this floating surface state is originated from the obstructed Wannier charge centers, albeit underwent surface reconstruction that splits the half-filled obstructed surface state. Our findings not only shed lights on the spectroscopy study of obstructed atomic insulators and obstructed surface states, but also provide possible route for development of new catalysts.