Two-Dimensional van der Waals Polar Metal MoOBr₂

Two-dimensional (2D) van der Waals (vdW) polar metals, which integrate long-range polar order and metallic conductivity, have garnered enormous interest for their extraordinary properties, including non-reciprocal charge transport and nonlinear optical effects. However, intrinsic 2D vdW polar metals remain exceedingly rare, as the dense itinerant electrons strongly screen internal electric fields and destabilize the macroscopic polarization needed for polar order. Herein, we report the discovery of MoOBr₂, the first 2D vdW polar metal stacked by polar atomic layers. Single-crystal X-ray diffraction and atomic-resolution scanning transmission electron microscopy reveal that MoOBr₂ crystallizes in the non-centrosymmetric space group C2, with polarity originating from asymmetric [MoO₂Br₄] octahedral distortions. Angle-resolved photoemission spectroscopy and electrical transport measurements confirm its metallic behavior with a room-temperature resistivity of 5.13 × 10^-5 Ω m and a carrier concentration of 1.34 × 10²⁰ cm^-3. The broken spatial inversion symmetry of MoOBr₂ was further demonstrated by both second-harmonic generation and second-order nonlinear electrical transport measurement, providing the complementary signatures of the polar metallic state. Remarkably, MoOBr₂ exhibits a high thermal conductivity of 219 W m^-1 K^-1, enabling efficient heat dissipation in high-power-density nanoscale devices. This work not only benefits next-generation multifunctional spintronic and nonlinear optoelectronic applications but also offers a rational chemical design strategy for new polar metals.