标题： 离散对称 flock 的稳定性：夹层状态、行进区域和运动诱导的钉扎 显示英文标题

标题： Stability of discrete-symmetry flocks: sandwich state, traveling domains and motility-induced pinning

摘要： 极性 flock 在离散活性系统中通常被认为具有鲁棒性，但最近的研究揭示了它们在施加的和自发的波动下的脆弱性。 在这里，我们重新研究四态活性 Potts 模型（APM），并表明其全局有序相在广泛的参数空间中是亚稳态的。 小的反向传播液滴通过诱导持续的夹层状态来破坏 flock 相，其中液滴引起的相反极性车道仍然嵌入在原始 flock 中，尤其是在低噪声下，受空间各向异性的影响。 相比之下，当引入小的横向传播液滴时，它们可以由于与主导流动方向正交的对齐以及增强的持久性而触发完全的相变。 在低扩散和强自推进条件下，这样的横向液滴也会自发出现，将 flock 分裂成多个运动区域，并产生短程有序（SRO）区域。 我们进一步在小扩散和低温条件下识别出一种运动性诱导的钉扎（MIP）转变，当相反极性的粒子相互作用、翻转状态、跳跃并钉扎界面时发生。 我们的综合相图，包括完全反转、夹层共存、条带带、SRO 和 MIP，描绘了热波动、自推进强度和扩散如何在离散活性物质系统中支配 flock 的稳定性。 显示英文摘要

摘要： Polar flocks in discrete active systems are often assumed to be robust, yet recent studies reveal their fragility under both imposed and spontaneous fluctuations. Here, we revisit the four-state active Potts model (APM) and show that its globally ordered phase is metastable across a broad swath of parameter space. Small counter-propagating droplets disrupt the flocking phase by inducing a persistent sandwich state, where the droplet-induced opposite-polarity lane remains embedded within the original flock, particularly pronounced at low noise, influenced by spatial anisotropy. In contrast, small transversely propagating droplets, when introduced into the flock, can trigger complete phase reversal due to their alignment orthogonal to the dominant flow and their enhanced persistence. At low diffusion and strong self-propulsion, such transverse droplets also emerge spontaneously, fragmenting the flock into multiple traveling domains and giving rise to a short-range order (SRO) regime. We further identify a motility-induced pinning (MIP) transition in small diffusion and low-temperature regimes when particles of opposite polarity interact, flip their state, hop, and pin an interface. Our comprehensive phase diagrams, encompassing full reversal, sandwich coexistence, stripe bands, SRO, and MIP, delineate how thermal fluctuations, self-propulsion strength, and diffusion govern flock stability in discrete active matter systems.