[论文解读] Electronic ratchet effect in a moiré system: signatures of excitonic ferroelectricity

Zhi-Ren Zheng, Xueqiao Wang+18 more|arXiv (Cornell University)|Jun 6, 2023

Advanced Memory and Neural Computing被引用 10

一句话总结

tldr: 展示在层对比的石墨烯–氮化硼莫尔蕾体系中的电子棘轮效应，显示由激子驱动的（激子性）铁电性，可编程的残留极化和多重存储状态。

ABSTRACT

Electronic ferroelectricity represents a new paradigm where spontaneous symmetry breaking driven by electronic correlations, in contrast to traditional lattice-driven ferroelectricity, leads to the formation of electric dipoles. Despite the potential application advantages arising from its electronic nature, switchable electronic ferroelectricity remains exceedingly rare. Here, we report the discovery of an electronic ratchet effect that manifests itself as switchable electronic ferroelectricity in a layer-contrasting graphene-boron nitride moiré heterostructure. Our engineered layer-asymmetric moiré potential landscapes result in layer-polarized localized and itinerant electronic subsystems. At particular fillings of the localized subsystem, we find a ratcheting injection of itinerant carriers in a non-volatile manner, leading to a highly unusual ferroelectric response. Strikingly, the remnant polarization can be stabilized at multiple (quasi-continuous) states with behavior markedly distinct from known ferroelectrics. Our experimental observations, simulations, and theoretical analysis suggest that dipolar excitons are the driving force and elementary ferroelectric units in our system. This signifies a new type of electronic ferroelectricity where the formation of dipolar excitons with aligned moments generates a macroscopic polarization and leads to an electronically-driven ferroelectric response, which we term excitonic ferroelectricity. Such new ferroelectrics, driven by quantum objects like dipolar excitons, could pave the way to innovative quantum analog memory and synaptic devices.

研究动机与目标

• 旨在研究由电子相关性而非晶格位移驱动的电子铁电性。
• 探索层不对称的莫尔潜在势如何在系统中形成并存的流动电子与局部化电子子系统。
• 确定偶极激子是否能作为生成可切换极化的基本铁电单元。

提出的方法

• 在大不匹配的情况下将BLG夹在上下BN之间以产生层对比的莫尔潜在势。
• 通过四探针电阻图对顶栈和底栈门电压的函数进行表征以识别LSAS和棘轮化区域。
• 从霍尔测量和总电荷密度中提取游离电子密度与局部化电子密度以区分n_I和n_L。
• 用拟合的Preisach-like滞后元模型分析极化动态以再现P-D回路。
• 提出并量化激子形成作为偶极铁电单元的机制，并与能量尺度（间隙Δ、结合能E_B）进行比较。
• 使用仿真和第一性原理洞见来估计层间激子结合能及局部态间隙。

实验结果

研究问题

• RQ1 BLG/BN中的层对比莫尔势是否会创建共存的游离电子与局部化电子子系统，从而实现电子铁电性？
• RQ2 激子偶极是否可在莫尔系统中作为驱动可切换铁电极化的基本单元？
• RQ3 残留极化的性质及其通过位移场和栅极电压的可调性如何？
• RQ4 游离与局部化子系统的耦合如何产生棘轮式、近连续的记忆行为？

主要发现

• 在D = 0时仍存在残留极化，表现为非易失性、可切换的n_H，指示铁电行为。
• 观察到级联模式，其中LSAS与棘轮化区域交替出现，揭示了与游离载流子共存的层极化局部态。
• 激子形成作为驱动铁电单元的证据，产生固定宽度的滞后以及近连续的一组记忆状态。
• 证明一个可连续放大、铁电回线面积随D场范围变化的棘轮效应。
• 估计层间激子结合能相当大（约100 meV），与观测到的能量间隙和局部化相符。
• 一个拟合的表观模型（Preisach-like）能够捕捉滞后现象以及残留极化对最大施加位移场的三阶段依赖。