Orthorhombic TaAs: A New Topological Phase of the Archetypical Weyl Semimetal

We report on a new, topologically nontrivial phase of TaAs identified in thin TaAs layers grown by molecular beam epitaxy. Structural investigations clearly show a new atom arrangement, confirmed by the presence of 104 reflections in the X-ray diffraction pattern, forbidden for the well-known tetragonal phase. Density functional theory confirms the presence of a new orthorhombic phase and reveals that its formation energy is slightly higher (by ∼0.6 meV per atom) than for the tetragonal phase. The orthorhombic TaAs is a topological Weyl semimetal with 20 Weyl nodes. Weak antilocalization of a topological origin is observed at low temperatures. With the Fermi energy relatively deep in the valence band, no other signatures of the chiral properties are resolved. The demonstration of the new phase, combined with the molecular beam epitaxy capabilities of doping and strain adjustment, opens a new way toward the fine-tuning of Weyl semimetal layers and heterostructures.