Plasmons in strongly correlated systems are attracting considerable attention due to their unconventional behavior caused by electronic correlation effects. Recently, flat plasmons with nearly dispersionless frequency-wave vector relations have drawn significant interest because of their intriguing physical origin and promising applications. However, these flat plasmons exist primarily in low-dimensional materials with limited wave vector magnitudes (q < ~0.7 Å⁻¹). Here, we show that long-lived flat plasmons can propagate up to ~1.2 Å⁻¹ in α-Ti₂O₃, a strongly correlated three-dimensional Mott-insulator, with an ultra-small energy fluctuation (<40 meV). The strong correlation effect renormalizes the electronic bands near Fermi level with a small bandwidth, which is responsible for the flat plasmons in α-Ti₂O₃. Moreover, these flat plasmons are not affected by Landau damping over a wide range of wave vectors (q < ~1.2 Å⁻¹) due to symmetry constrains on the electron wavefunctions. Our work provides a strategy for exploring flat plasmons in strongly correlated systems, which in turn may give rise to novel plasmonic devices in which flat and long-lived plasmons are desirable.

强相关系统中的等离激元由于其由电子相关效应引起的非常规行为而引起了相当大的关注。最近，具有几乎无色散频率波矢量关系的平面等离子体由于其有趣的物理起源和有前途的应用而引起了极大的兴趣。然而，这些平面等离子体主要存在于波矢量幅度有限的低维材料中（q  < ~0.7 Å ^-1）。在这里，我们展示了长寿命的平面等离子体可以在 α-Ti ₂ O ₃中传播高达 ~1.2 Å ^-1，一种强相关的三维莫特绝缘体，具有超小的能量波动（<40 meV）。强相关效应使费米能级附近的电子能带重新归一化，带宽很小，这是α-Ti ₂ O ₃中平坦等离子体的原因。此外，由于电子波函数的对称性约束，这些平面等离子激元不受朗道阻尼在宽波矢量 ( q  < ~1.2 Å ^{-1 ) 范围内的影响。}我们的工作提供了一种在强相关系统中探索平面等离子体的策略，这反过来可能会产生新的等离子体装置，其中需要平面和长寿命的等离子体。