Layer-Number-Dependent Metal-Insulator Transition in Topological Semimetal Nb₃SiTe₆

Topologically nontrivial materials are distinguished by their inherent robustness against symmetry-preserving perturbations. However, the resilience of the topologically protected degeneracies against electronic instabilities remains largely unexplored. Here, by employing transport measurements, we demonstrate a layer-number-dependent metal-insulator transition in the topological semimetal Nb₃SiTe₆, which has a layered structure and exhibits Fermi-liquid-like metallicity with a hole carrier density of ∼3.3 × 10²¹ cm^-3 down to the bilayers. An unexpected insulating ground state only emerges in the monolayer and can be effectively tuned by electrostatic gating. The charge neutrality point coincides with a sign change of the Hall signal. Such ambipolar gating behavior contradicts the degeneracy protection under the glide-mirror symmetry in the single-particle picture. Combined with theoretical calculations, we suggest that electronic instabilities could offer a plausible mechanism for gap opening at the symmetry-protected nodal lines. Our findings provide crucial insights for understanding the rich dimension-related metal-insulator transitions in topological materials.