2D Weyl-Semimetal States Achieved by a Thickness-Dependent Crossover and Topological Phase Transition in Bi0.96Sb0.04 Thin Films

Despite theoretical expectations for 2D Weyl semimetals (WSMs), realizing stable 2D topological semimetal states experimentally is currently a great challenge. Here, 2D WSM states achieved by a thickness-dependent topological phase transition from 3D Dirac semimetal to 2D WSM in molecular-beam-epitaxy-grown Bi0.96Sb0.04 thin films are reported. 2D weak anti-localization (WAL) and chiral anomaly arise in the Bi0.96Sb0.04 films for thicknesses below approximate to 10 nm, supporting 2D Weyl semimetallic transport in the films. This is particularly evident from magnetoresistance (MR) measurements which show cusp structures at around B = 0, indicating WAL, and negative MR, typical of chiral anomaly, only for layers with thicknesses below approximate to 10 nm. The temperature dependencies of the dephasing length for various thicknesses are consistent with those of the MR. Analysis based on second harmonic generation, terahertz emission, Seebeck/Hall effects, Raman scattering, X-ray diffraction, and X-ray photoemission demonstrates that the Dirac- to Weyl-semimetal phase transition for films thinner than approximate to 10 nm is induced by inversion-symmetry breaking due to the lattice-mismatch strain between the Bi0.96Sb0.04 film and substrate. The realization of 2D WSMs is particularly significant for applications in high-speed electronics, spintronics, and quantum computations due to their high mobility, chiral spin, and topologically-protected quantum qubits.