Visualizing "Fermi arcs" and their properties in the Weyl semimetal TaAs

One of the hallmarks of Weyl semi-metals is the existence of unusual topological surface states known as "Fermi arcs". The formation of these states is guaranteed by the existence of bulk Weyl points with opposite chirality. Tantalum Arsenide (TaAs), a member of the newly discovered family of Weyl semi-metals, harbors a host of non-topological (“trivial”) surface states overlapping in energy with the 12 predicted topological "Fermi arcs". This overlap poses a major challenge in identifying the signatures of the arcs. We use scanning tunneling microscopy to address this challenge. In my talk I will show how we harness the inherently distinct spatial structure of trivial and Fermi arc states to isolate the Fermi arcs and to visualize them by quasi particle interference (QPI) measurements [1]. We do so in four distinct ways, each of which highlights a different aspect of their unusual nature - we reveal their relatively isotropic scattering signature, their energy dispersion and its relation to the bulk Weyl points, their deep bulk penetration relative to that of non-topological surface states and their weak coupling to the atomic structure. The latter is obtained by accounting for the spatial structure of the Bloch wave function and its effect on the scattering properties of the electrons off lattice defects. I will discuss the role of the Bloch wave function structure in understanding QPI measurements and show that it provides a novel analysis tool for the spectroscopic characterization of electronic wave functions using scanning tunneling microscopy.