The interplay between topological materials and local symmetry breaking yields diverse topological quantum phenomena. A notable example is the parity-anomalous semimetal (PAS), which hosts a single unpaired gapless Dirac cone with a half-integer quantized Hall conductivity. Here, we realize this phase in a magnetic topological sandwich structure by applying an in-plane magnetic field. This configuration aligns the magnetization of one surface in plane while preserving a partially out-of-plane magnetization on the opposite surface, satisfying the condition for a gapless surface state near the Fermi level on only one surface. Our key evidence is a distinctive two-stage evolution of the conductivity tensor $({\sigma }_{xy},{\sigma }_{xx})$. The first stage culminates in the PAS at the fixed point $[(e^2/2h),m(e^2/h)]$, where $m\approx 0.6$ corresponds to the minimal longitudinal conductivity of a single gapless Dirac cone of fermions on a two-dimensional lattice. This PAS state remains stabilized and is superposed with a gapped band flow in the second stage. This observation demonstrates that this state stabilized by the in-plane field resists localization—in contrast to conventional expectation for two-dimensional electron systems with broken time-reversal symmetry. The dynamic transition from an integer quantized Hall insulator to a half-integer quantized Hall semimetal establishes this material system as a versatile platform for exploring the physics of the parity anomaly.

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• Hall effect
• Quantum transport
• Topological insulators
• Topological phases of matter
• Transport phenomena
• Topological materials