重费米子扭曲对称三层石墨烯的低温近场光电压显微术
Cryo-Near-Field Photovoltage Microscopy of Heavy-Fermion Twisted Symmetric Trilayer Graphene
AI总结：
本研究利用低温扫描近场光学显微镜在扭曲对称三层石墨烯中观测到重费米子行为，揭示了其塞贝克系数对经典模型的偏离及相互作用强度的空间变化。
摘要：
自最初在魔角扭曲双层石墨烯的平坦狄拉克能带中实验观测到关联绝缘体和超导性以来，寻找解释其强电子相互作用的微观描述的工作便已开始。尽管看似不一致的电子输运和扫描隧道显微术实验表明局域和扩展电子轨道之间存在二分性，但将这两种模式结合起来的确凿实验证据一直备受期待。在此，我们报告了在扭曲对称三层石墨烯（TSTG）的平坦电子能带中进行的局部光热电测量。我们使用一台低温扫描近场光学显微镜，其振荡的原子力显微镜（AFM）针尖受到红外光子辐照，在平面样品中产生纳米级热点，从而生成我们在全局探测的光电流。我们观察到非相互作用Mott形式在低温（10K）下发生破缺，表明电子相互作用的重要性。我们的测量揭示了塞贝克系数的整体负偏移，以及TSTG莫尔超晶格所有正整数填充处局部光电压值的显著峰值，进一步表明其与经典双带半导体塞贝克响应的显著偏离。我们使用相互作用拓扑重费米子模型解释了这些观察结果。此外，我们的数据揭示了相对相互作用强度的空间变化，该变化依赖于测量的局部扭曲角（1.2° - 1.6°）。我们的发现为莫尔石墨烯拓扑平坦能带中的重费米子行为提供了实验证据，并展示了将局部热电测量应用于无序极限下其他强关联材料的途径。
英文摘要：
Ever since the initial experimental observation of correlated insulators and superconductivity in the flat Dirac bands of magic angle twisted bilayer graphene, a search for the microscopic description that explains its strong electronic interactions has begun. While the seemingly disagreeing electronic transport and scanning tunneling microscopy experiments suggest a dichotomy between local and extended electronic orbitals, definitive experimental evidence merging the two patterns together has been much sought after. Here, we report on the local photothermoelectric measurements in the flat electronic bands of twisted symmetric trilayer graphene (TSTG). We use a cryogenic scanning near-field optical microscope with an oscillating atomic force microscopy (AFM) tip irradiated by the infrared photons to create a nanoscopic hot spot in the planar samples, which generates a photocurrent that we probe globally. We observe a breakdown of the non-interacting Mott formalism at low temperatures (10K), signaling the importance of the electronic interactions. Our measurements reveal an overall negative offset of the Seebeck coefficient and significant peaks of the local photovoltage values at all positive integer fillings of the TSTG's moiré superlattice, further indicating a substantial deviation from the classical two-band semiconductor Seebeck response. We explain these observations using the interacting topological heavy-fermion model. In addition, our data reveal a spatial variation of the relative interaction strength dependent on the measured local twist angle (1.2° - 1.6°). Our findings provide experimental evidence of heavy fermion behaviour in the topological flat bands of moiré graphene and epitomize an avenue to apply local thermoelectric measurements to other strongly correlated materials in the disorder-free limit.
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