Thermally Stable Negative Permittivity and Fano like Resonance in Multi-Doped Manganite

We report an atypical dielectric behavior of a newly synthesized A-site multi-doped La_0.4Bi_0.3Sr_0.2Ba_0.1MnO₃ manganite, which has not been reported previously. The material exhibits a thermally robust negative real permittivity across a broad frequency (10 KHz-100MHz) and temperature range (230K-400K). This is interpreted as result of a collective plasmonic state of free electrons and modeled according to the Drude formalism. Dielectric losses exhibit exceptionally low values and tanδ reaches notable minima (0.0002<tanδ<0.01). A striking feature is a double zero-crossing of real permittivity around 40 MHz which emerges from the interplay between slow interfacial polarization and fast grain response. This previously unreported phenomenon, in single-phase manganites, suggests a Fano-Like resonance modeled according to the Lorentz-Drude formalism. Furthermore, the imaginary permittivity exhibits negative values at high frequencies, a phenomenon rarely observed in natural materials. These combined phenomena, unprecedented in this material class, position La_0.4Bi_0.3Sr_0.2Ba_0.1MnO₃ as a unique functional oxide with high potential for applications in metamaterials, near-zero permittivity structures, and electromagnetic wave modulators.