Polarons are prevalent in condensed matter systems with strong electron-phonon coupling. The adiabaticity of the polaron relates to its transport properties and spatial extent. To date, only adiabatic small polaron formation has been measured following photoexcitation. The lattice reorganization energy is large enough that the first electron–optical phonon scattering event creates a small polaron without requiring substantial carrier thermalization. We measure that frustrating the iron-centered octahedra in the rare-earth orthoferrite ErFeO₃ leads to antiadiabatic polaron formation. Coherent charge hopping between neighboring Fe³⁺─Fe²⁺ sites is measured with transient extreme ultraviolet spectroscopy and lasts several picoseconds before the polaron forms. The resulting small polaron formation time is an order of magnitude longer than previous measurements and indicates a shallow potential well, even in the excited state. The results emphasize the importance of considering dynamic electron-electron correlations, not just electron-phonon–induced lattice changes, for small polarons for transport, catalysis, and photoexcited applications.

中文翻译：

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相干电荷跳跃通过抗绝热形成机制抑制 ErFeO3 中的光激发小极化子

极化子在具有强电子-声子耦合的凝聚态物质系统中普遍存在。极化子的绝热性与其输运特性和空间范围有关。迄今为止，仅测量了光激发后绝热小极化子的形成。晶格重组能足够大，以至于第一次电子光声子散射事件无需大量载流子热化即可产生小极化子。我们测量到抑制稀土正铁氧体 ErFeO ₃中以铁为中心的八面体会导致抗绝热极化子的形成。使用瞬态极紫外光谱测量相邻 Fe ³⁺ ─Fe ²⁺位点之间的相干电荷跳跃，并且在极化子形成之前持续几皮秒。由此产生的小极化子形成时间比之前的测量长一个数量级，并且表明即使在激发态下也是浅势阱。结果强调了对于传输、催化和光激发应用中的小极化子而言，考虑动态电子-电子相关性的重要性，而不仅仅是电子-声子引起的晶格变化。