The constituent particles of matter can arrange themselves in various ways, giving rise to emergent phenomena that can be surprisingly rich and often cannot be understood by studying only the individual constituents. Discovering and understanding the emergence of such phenomena in quantum materials—especially those in which multiple degrees of freedom or energy scales are delicately balanced—is of fundamental interest to condensed-matter research 1 , 2 . Here we report on the surprising observation of emergent ferroelectricity in graphene-based moiré heterostructures. Ferroelectric materials show electrically switchable electric dipoles, which are usually formed by spatial separation between the average centres of positive and negative charge within the unit cell. On this basis, it is difficult to imagine graphene—a material composed of only carbon atoms—exhibiting ferroelectricity 3 . However, in this work we realize switchable ferroelectricity in Bernal-stacked bilayer graphene sandwiched between two hexagonal boron nitride layers. By introducing a moiré superlattice potential (via aligning bilayer graphene with the top and/or bottom boron nitride crystals), we observe prominent and robust hysteretic behaviour of the graphene resistance with an externally applied out-of-plane displacement field. Our systematic transport measurements reveal a rich and striking response as a function of displacement field and electron filling, and beyond the framework of conventional ferroelectrics. We further directly probe the ferroelectric polarization through a non-local monolayer graphene sensor. Our results suggest an unconventional, odd-parity electronic ordering in the bilayer graphene/boron nitride moiré system. This emergent moiré ferroelectricity may enable ultrafast, programmable and atomically thin carbon-based memory devices. Electronic ferroelectricity is observed in a graphene-based moiré heterostructure, which is explained using a spontaneous interlayer charge-transfer model driven by layer-specific on-site Coulomb repulsion.

中文翻译：

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莫尔异质结构中的非常规铁电性

物质的组成粒子可以以各种方式排列自己，从而产生出奇丰富的涌现现象，通常仅通过研究单个组成部分是无法理解的。发现和理解量子材料中这种现象的出现——尤其是那些多自由度或能量尺度微妙平衡的材料——是凝聚态研究的根本利益 1, 2 。在这里，我们报告了对基于石墨烯的莫尔异质结构中出现的铁电性的惊人观察。铁电材料表现出可电切换的电偶极子，通常由单元电池内正负电荷的平均中心之间的空间分离形成。以这个为基础，很难想象石墨烯——一种仅由碳原子组成的材料——表现出铁电性 3 。然而，在这项工作中，我们在夹在两个六方氮化硼层之间的 Bernal 堆叠双层石墨烯中实现了可切换的铁电性。通过引入莫尔超晶格势（通过将双层石墨烯与顶部和/或底部氮化硼晶体对齐），我们观察到石墨烯电阻具有显着且稳健的滞后行为，并具有外部施加的面外位移场。我们的系统传输测量揭示了作为位移场和电子填充的函数的丰富而惊人的响应，并且超出了传统铁电体的框架。我们通过非局部单层石墨烯传感器进一步直接探测铁电极化。我们的结果表明双层石墨烯/氮化硼莫尔系统中存在非常规的奇偶校验电子排序。这种新兴的莫尔铁电体可以实现超快、可编程和原子级薄的碳基存储设备。在基于石墨烯的莫尔异质结构中观察到电子铁电性，这可以使用由层特定的现场库仑排斥驱动的自发层间电荷转移模型来解释。