Breaking Permittivity‐Loss Constraint via Surface and Bulk Carrier Engineering in Topological Insulator Bi ₂ Te ₃

Abstract The development of low‐frequency microwave absorbers is facing a critical challenge that conventional impedance‐matching strategies based on λ/4 resonance principles necessitate prohibitively thick of materials, particularly for frequencies below 3 GHz. To address this conflict issue, Mg 2+ ‐substituted topological insulator Bi 2 Te 3 nanosheets are synthesized, leveraging carrier transport modulation to enable low‐frequency absorption. Experimental and theoretical results reveal that Mg 2+ doping systematically regulates carrier types and concentrations in both surface states and bulk states, thereby tailoring the dielectric response. Remarkably, the doped nanosheets achieve an ultrahigh relative permittivity of up to 100 at 2 GHz while maintaining a low loss tangent ( tanδ ɛ < 0.4), overcoming the inherent permittivity‐loss trade‐off in traditional microwave absorbers. This anomalous behavior is attributed to changes in electrical transport caused by Mg doping, affecting both the surface and bulk states. A breakthrough electrical thickness (d/λ) of 0.029 is achieved in the doped material for 99% microwave absorption at 2.6 GHz, representing the thinnest S band absorber based on natural materials reported to date. This work establishes a carrier engineering paradigm to address the longstanding oversize thickness challenge in low‐frequency microwave absorption technologies.