Zeeman Field-Induced Two-Dimensional Weyl Semimetal Phase in Cadmium Arsenide

We report a topological phase transition in quantum-confined cadmium arsenide (Cd₃As₂) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two dimensional Weyl semimetal (2D WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that are protected by space-time inversion (<i>C</i>₂<i>T</i>) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of <i>h</i>/<i>e</i>² that persist over a range of in-plane magnetic fields. Moreover, applying a small out-of-plane magnetic field, while keeping the in-plane field within the stability range of the 2D WSM phase, gives rise to a well-developed odd integer quantum Hall effect, characteristic of degenerate, massive Weyl fermions. A minimal four-band <i>k</i>-<i>p</i> model of Cd₃As₂ which incorporates first-principles effective <i>g</i> factors qualitatively explains our findings.