Parity Anomalous Semimetal with Minimal Conductivity Induced by an In-Plane Magnetic Field

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 (σ_{xy},σ_{xx}). The first stage culminates in the PAS at the fixed point [(e^{2}/2h),m(e^{2}/h)], where m≈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.