Denser-than-crystalline disordered polydisperse ellipse packings

By employing SWAP Monte Carlo moves during slow Lubachevsky-Stillinger-like particle growth, we generate well-thermalized disordered jammed ellipse packings for a wide range of particle aspect ratios $\alpha$ and maximum/minimum particle-diameter ratios $\lambda$. For $\lambda >{\lambda }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}(\alpha )$, the packings are denser than the perfect monodisperse ellipse crystal (${\phi }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}^{\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{o}}=\pi \mathrm{/}2\sqrt{3}\simeq 0.9069$) despite lacking any obvious long-range structural order; we find that ${\lambda }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}(\alpha )$ remains below $2.88$ over the range $1\le \alpha \le 5$, and drops below $1.65$ for $\alpha \simeq 1.2$. These disordered $\phi >{\phi }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}^{\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{o}}$ packings are the first of their kind reported for anisotropic particles. Multiscale analyses reveal that their characteristic structural motifs are strongly $\alpha$- and $\lambda$-dependent, but none of the examined metrics change sharply as the $\lambda ={\lambda }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}(\alpha )$ boundary is crossed. For example, we find that packings with $\alpha \simeq 1.2$ and $\lambda \simeq {\lambda }_{\mathrm{x}\mathrm{t}\mathrm{a}\mathrm{l}}(\alpha )$ exhibit a fractionated-hexatic-plastic-polycrystalline structure that (like that of the recently-reported ideal disk glasses) can be exactly-isostatic, triangulated, and effectively-hyperuniform with respect to packing-fraction fluctuations. In contrast, even-denser packings with larger $\alpha$ and $\lambda$ have none of these properties.