该研究团队在能量精细调控的超短激光脉冲控制下，深入研究了磁铁矿晶体在金属至绝缘体转变附近的不同隐藏状态演化过程。通过超快电子衍射技术直接监测磁铁矿的晶体结构变化，他们发现：在近红外光（800 nm）的激发下，原本的三极化子电荷/轨道有序模式被破坏，转而形成由立方金属相和单斜绝缘相构成的相分离状态；而在可见光（400 nm）照射下，光掺杂引发的电荷转移过程被激活，这进一步稳定了电荷密度波波动，促进了三极化子的长程有序，从而增强了单斜绝缘相。
这项研究结果表明，磁铁矿的结构可以通过完全不同的亚稳态隐藏相演化，这些亚稳态隐藏相可以在初始激发弛豫后很长一段时间内达到，这为控制物质涌现特性的方法奠定了基础。
据悉，在非平衡条件下发生的相变可以通过平衡绝热条件无法达到的高能中间状态演变。由于这些隐藏相的微妙性质，它们的直接观察极具挑战性，需要同时在亚皮秒和亚皮米尺度上对物质进行可视化。
附：英文原文
Title: Ultrafast generation of hidden phases via energy-tuned electronic photoexcitation in magnetite
Author: Truc, B., Usai, P., Pennacchio, F., Berruto, G., Claude, R., Madan, I., Sala, V., LaGrange, T., Vanacore, G. M., Benhabib, S., Carbone, F.
Issue&Volume: 2024-6-20
Abstract: Phase transitions occurring in nonequilibrium conditions can evolve through high-energy intermediate states inaccessible via equilibrium adiabatic conditions. Because of the subtle nature of such hidden phases, their direct observation is extremely challenging and requires simultaneous visualization of matter at subpicoseconds and subpicometer scales. Here, we show that a magnetite crystal in the vicinity of its metal-to-insulator transition evolves through different hidden states when controlled via energy-tuned ultrashort laser pulses. By directly monitoring magnetite’s crystal structure with ultrafast electron diffraction, we found that upon near-infrared (800 nm) excitation, the trimeron charge/orbital ordering pattern is destroyed in favor of a phase-separated state made of cubic-metallic and monoclinic-insulating regions. On the contrary, visible light (400 nm) activates a photodoping charge transfer process that further promotes the long-range order of the trimerons by stabilizing the charge density wave fluctuations, leading to the reinforcement of the monoclinic insulating phase. Our results demonstrate that magnetite’s structure can evolve through completely different metastable hidden phases that can be reached long after the initial excitation has relaxed, breaking ground for a protocol to control emergent properties of matter.
DOI: 10.1073/pnas.2316438121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2316438121