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

We report a topological phase transition in quantum-confined cadmium arsenide (${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$) 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 (${C}_{2}T$) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of $h/{e}^{2}$ 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 $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$, which incorporates first-principles effective $g$ factors, qualitatively explains our findings.