Non-Additive Coupling of Stacking Registry and Buckling Orientation in Bilayer Plumbene: Structural Screening and Buckling-Controlled Optoelectronic Response

Using first-principles calculations, we examine four representative initial configurations of bilayer plumbene (AA-bb, AA-bt, AB-bb, and AB-bt) with different stacking registries and relative buckling orientations. After structural relaxation, phonon dispersions show no imaginary frequencies along the sampled high-symmetry path for AA-bb and AA-bt, whereas AB-bb and AB-bt exhibit instabilities at finite wave vectors. We therefore focus the electronic and optical analyses on the two AA-derived structures. Both are metallic with electronic states near the Fermi level dominated by Pb-p orbitals. The in-plane Drude weight of AA-bt is approximately 6.11 times that of AA-bb, and the two configurations also exhibit distinct features in their dielectric response and two-dimensional absorptance. Charge-density differences reveal local electronic redistribution, while the small layer-resolved Bader population differences remain sensitive to the numerical grid. These results highlight the role of relaxed interlayer geometry in shaping the electronic and optical responses of freestanding bilayer plumbene within the independent-particle approximation.