Observation of a nodal chain with Dirac surface states in TiB2

TL;DR

This paper reports the experimental discovery of a nodal-chain topological semimetal in TiB2, featuring Dirac surface states and complex nodal line configurations confirmed by angle-resolved photoemission spectroscopy.

Contribution

It provides the first observation of a nodal-chain structure with Dirac surface states in TiB2, expanding understanding of topological semimetals.

Findings

Nodal rings observed in orthogonal Brillouin zone planes

Nodal rings connect to form a nodal-chain structure

Surface states with Dirac-cone-like band structures identified

Abstract

Topological nodal-line semimetals (TNLSMs) are characterized by symmetry-protected band crossings extending along one-dimensional lines in momentum space. The nodal lines exhibit a variety of possible configurations, such as nodal ring, nodal link, nodal chain, and nodal knot. Here, using angle-resolved photoemission spectroscopy, we observe nodal rings in the orthogonal kz = 0 and kx = 0 planes of the Brillouin zone in TiB2. The nodal rings connect with each other on the intersecting line {\Gamma}-K of the orthogonal planes, forming a remarkable nodal-chain structure. Furthermore, we observe surface states (SSs) on the (001) cleaved surface, which are consistent with the calculated SSs considering the contribution from both Ti and B terminations. The calculated SSs have novel Dirac-cone-like band structures, which are distinct from the usual drumhead SSs with a single flat band proposed in other TNLSMs.