Two-Dimensional van der Waals Polar Metal MoOBr₂

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Two-dimensional (2D) van der Waals (vdW) polar metals are rare due to strong electron screening that destabilizes polar order.
• These materials are sought after for unique properties like non-reciprocal charge transport and nonlinear optical effects.

Purpose of the Study:

• To report the discovery of a novel 2D vdW polar metal.
• To characterize its structural, electronic, and physical properties.
• To explore its potential applications in next-generation devices.

Main Methods:

• Single-crystal X-ray diffraction and atomic-resolution scanning transmission electron microscopy for structural analysis.
• Angle-resolved photoemission spectroscopy and electrical transport measurements for electronic properties.
• Second-harmonic generation and nonlinear electrical transport measurements to confirm broken inversion symmetry.

Main Results:

• Discovery of MoOBr2, the first 2D vdW polar metal composed of polar atomic layers.
• Crystallization in space group C2 with polarity from distorted [MoO2Br4] octahedra.
• Confirmed metallic behavior (resistivity 5.13 × 10⁻⁵ Ω m, carrier concentration 1.34 × 10²⁰ cm⁻³).
• Demonstrated broken spatial inversion symmetry via nonlinear optical and electrical transport measurements.
• Observed high thermal conductivity (219 W m⁻¹ K⁻¹).

Conclusions:

• MoOBr2 is the first intrinsic 2D vdW polar metal, exhibiting metallic conductivity and polar order.
• Its unique properties, including broken inversion symmetry and high thermal conductivity, make it promising for spintronic and nonlinear optoelectronic applications.
• Presents a new chemical design strategy for discovering novel polar metals.