Experimental observation of bulk nodal lines and electronic surface states in ZrB2

Rui Lou,Shanshan Sun,Shancai Wang,Dawei Shen,Zhonghao Liu,Kai Liu,Zhong-Yi Lu,Zilu Wang,Hechang Lei,Man Li,Zhe Sun,Xuchuan Wu,Yaobo Huang,Pengjie Guo,Chenghe Li,Hong Ding,Qi Wang

doi:10.1038/s41535-018-0121-4

Abstract

Topological nodal-line semimetals are characterized by line-contact bulk band crossings and topological surface states. Breaking certain protecting symmetry turns this system into a Dirac semimetal or Weyl semimetal that hosts zero-dimensional isolated nodal points. Recent advances in band theory predicted a topological nodal-line semimetal state possessing a new type of nodal line in AlB2-type diborides. Here, we report an experimental realization of nodal-line fermions and associated surface states near the Fermi energy in ZrB2 by angle-resolved photoemission spectroscopy combined with first-principles calculations. The Dirac nodal lines in ZrB2 wind into two groups of nodal rings, which are linked together along the Γ-K direction. We further observe a distinct surface state connecting to each nodal line, indicative of the nontrivial topological nature of the bulk nodal lines. Therefore, our results provide convincing experimental evidence of nodal-line semimetal states in ZrB2 both in the bulk and on the surface, suggesting ZrB2 as a remarkable platform for discovering unique phenomena induced by nodal-line fermions.

Highlights

The realization of novel quantum states of matter with nontrivial topology beyond topological insulators has become a significant objective in current condensed-matter physics research.[1,2,3] Very recently, the discovery of topological semimetals has achieved this goal, which ignites extensive work focusing on the exotic topological properties and their underlying connection with the electronic structure.[4]
We investigate the electronic structure of ZrB2, which is predicted to host similar nodal-line configurations and surface states to that of TiB2.26,27 By using angle-resolved photoemission spectroscopy (ARPES) and firstprinciples calculations, we clearly observe two groups of nodal rings embedded in different mirror planes
In order to clarify the nontrivial topology of the bulk nodal lines realized in ZrB2, which are protected by the mirror-reflection symmetries and the combination of spatial-inversion symmetry (P) and time-reversal symmetry (T), i.e., the P·T symmetry, here we illustrate the connection between the nodal lines and the surface states by combining both the bulk and surface observations

Summary

Introduction

The realization of novel quantum states of matter with nontrivial topology beyond topological insulators has become a significant objective in current condensed-matter physics research.[1,2,3] Very recently, the discovery of topological semimetals has achieved this goal, which ignites extensive work focusing on the exotic topological properties and their underlying connection with the electronic structure.[4]. In Dirac and Weyl semimetals, the bulk nodes are discrete in the BZ and their surface projections are connected by surface Fermi arcs.[4] While in nodal-line semimetals, the bulk nodes extend along one-dimensional curves and the corresponding surface states are flat in dispersion according to the previous nodal-line modelings,[7] where the band crossings of a nodal line should occur at zero energy with a constraint chiral symmetry. The flat surface bands are dubbed the drumhead states. The chiral symmetry is not exact in a real crystal, resulting in the nodal line does not generally occur at a constant energy, the associated topological surface states are not flat either.[8,9]

Methods

Results

Conclusion