image: As shown in a unit cell of TaAs crystal in the left, (001) surfaces are Fermi arc surfaces owing to the projection of chiral Weyl nodes. Crystal planes of (110) perpendicular to (001) host projection of chiral Weyl points coincide are considered as non-Fermi arc surfaces. Actually, there are also untypical crystal faces, such as the selected ones of (112) and (114) have been never investigated. Here is a 3D topographic STM image containing the one-atom thick steps on the (112) surface in the center. Interestingly, robust and uniform one-dimensional edge states can be observed in the dI/dV mapping with a width over ~ 1 nm in the right. view more
Credit: ©Science China Press
This study is led by Prof. Rui-Rui Du and Prof. Ji Feng (International Center for Quantum Materials in Peking University). Experimental works using a Unisoku-1300 low-temperature scanning tunneling microscopy/spectra system were conducted by Dr. Xiaohu Zheng on high quality TaAs crystals. During measuring the cleaved atomic surface with Miller index (112), which is rarely experimentally investigated before, they discovered a prominent 1D electronic edge states near the Fermi level at the step edge. The 1D edge states can be distinguished from trivial states with a particular width dispersing from the edge to the surface. In order to make clear the origination of the exotic 1D edge states, they conducted measurements on several crystal faces with different indices in TaAs crystal. “It is really a hard task to get the atomic-flat surfaces with different crystal indices, but we succeed and finished the measurements on (110), (112), (114) and (001), and performed STM/STS on the step edges on these crystal faces” Dr. Zheng says. They find the exotic 1D edge states can be observed at the step edges on surfaces such as (110) and (112), but not on (114) and (001).
The calculations performed by Dr. Qiangqiang Gu give a deep understanding of the experiment phenomena. They classified the different crystal face into the Fermi arc allowed surface (AAS) and arc forbidden surface (AFS) according to the symmetry. It discovered although the surface of (110) is a AFS and can’t guarantee the existence of Fermi arcs, the atomic ledge on the AAS (001) surface leads to 1D Fermi arc edge states at the step as a consequence. As the surface index slightly increases from (110) to the (112), although, the surface and the step ledge can host the chiral Weyl nodes simultaneously, the ledge takes a higher weight than the surface, and the 1D Fermi arcs can still be observed at the step. As the surface index increasing and getting closer to (001), the 2D Fermi arc surface states will dominate while the 1D edge states submerging into the Fermi arc surface states, as illustrated in the experiment and calculation results on the (114) crystal face.
The experiment and calculation results in this work draw a clear picture to illustrate the evolution of Fermi arcs on the boundary of a 3D Weyl semimetal, and give a definite answer that how small a surface can keep the topological information (Fermi arc) still intact, specifically, 1D Fermi arc state can exist at the step edge. It is critical in understanding topological boundary-bulk correspondence and the exotic topological properties in gapless topological matters, and in exploring the Fermi arc states in quantum devices.
See the article:
Observation of 1D Fermi arc states in Weyl semimetal TaAs
https://doi.org/10.1093/nsr/nwab191
Journal
National Science Review