Metal-Insulator Transition Driven by the Interplay of Vacancies and Charge Orders in Square-Net Materials...

Engineering narrow-bandgap semiconductors remains a pivotal challenge for next-generation electronic and energy devices. Charge density wave (CDW) systems offer a promising platform for bandgap engineering. However, most 2D and 3D CDW systems remain metallic despite exhibiting Fermi surface nesting. Here, a doping-dependent metal-insulator transition (MIT) with tunable bandgaps is reported in square-net materials GdSb_xTe_2-x-δ and a cooperative interaction between CDWs and vacancies that drives the MIT is discovered. Angle-resolved photoemission spectroscopy (ARPES) reveals the MIT in the low Sb-content regime of GdSb_xTe_2-x-δ, with a maximum energy gap of Δ ≈ 98 meV at x = 0.16, corroborated by electrical transport measurements. Following the MIT, X-ray diffraction reveals a doping-dependent shift of the CDW wavevector toward a commensurate structure with q = 0.25 a*, concurrent with the appearance of Te vacancies in the square-net layers. Density functional theory (DFT) calculations attribute the gap formation to the ordered Te vacancies modulated by the 4×1×1 CDW superstructure, which suppresses the electronic states near the Fermi level. Contrasting with the partial gap scenarios in conventional CDW systems, this synergy between the CDW and the vacancy stabilizes the insulating phase, offering a distinct avenue for narrow bandgap engineering in electronic materials.