Observation of Dirac-like energy band and ring-torus Fermi surface in noncentrosymmetric pnictide CaAgAs

Recently, hexagonal pnictides CaAgX (X= P and As) were predict to be a candidate of topological line-node semimetal with a single line node under negligible spin-orbit coupling [1]. First-principles band structure calculations suggested inverted band structure and very small spin-orbit gap for CaAgP, whereas the spin-orbit gap for CaAgAs is ~ 0.1 eV. In this presentation, we report bulk-sensitive soft x-ray ARPES measurement of CaAgAs [2]. We have established the bulk valence-band structure in 3D Brillouin zone, and observed Dirac-like energy band and ring-torus Fermi surface associated with the line node due to an intersection of bulk Ag 5s and As 4p bands at the mirror plane of the crystal. Intriguingly, we found that no other bands cross the Fermi level, thereby the low-energy excitations are solely characterized by the Dirac-like band in this material. The present result strongly suggests that CaAgX family provides an excellent platform to study interplay among line node, mirror symmetry, and exotic topological properties.
[1] A. Yamakage et al., J. Phys. Soc. Jpn. 85, 013708 (2016).
[2] D. Takane et al., arXiv:1708.06874.