[论文解读] Reversible Non-Volatile Electronic Switching in a Near Room Temperature van der Waals Ferromagnet

Han Wu, Lei Chen+38 more|arXiv (Cornell University)|Jul 6, 2023

Topological Materials and Phenomena被引用 4

一句话总结

本研究通过控制铁位空位有序性，利用热退火和淬火技术，在接近室温的范德华铁磁体Fe5−δGeTe2中实现了两种不同晶体相之间的可逆、非易失性电子开关。该开关引发了一种类拓扑相变：无序相具有由对称性保护的节点线，而有序相则表现出平带结构并破坏了反演对称性，从而产生对拓扑和自旋电子学应用至关重要的显著电子结构差异。

ABSTRACT

The ability to reversibly toggle between two distinct states in a non-volatile method is important for information storage applications. Such devices have been realized for phase-change materials, which utilizes local heating methods to toggle between a crystalline and an amorphous state with distinct electrical properties. To expand such kind of switching between two topologically distinct phases requires non-volatile switching between two crystalline phases with distinct symmetries. Here we report the observation of reversible and non-volatile switching between two stable and closely-related crystal structures with remarkably distinct electronic structures in the near room temperature van der Waals ferromagnet Fe$_{5-δ}$GeTe$_2$. From a combination of characterization techniques we show that the switching is enabled by the ordering and disordering of an Fe site vacancy that results in distinct crystalline symmetries of the two phases that can be controlled by a thermal annealing and quenching method. Furthermore, from symmetry analysis as well as first principle calculations, we provide understanding of the key distinction in the observed electronic structures of the two phases: topological nodal lines compatible with the preserved global inversion symmetry in the site-disordered phase, and flat bands resulting from quantum destructive interference on a bipartite crystaline lattice formed by the presence of the site order as well as the lifting of the topological degeneracy due to the broken inversion symmetry in the site-ordered phase. Our work not only reveals a rich variety of quantum phases emergent in the metallic van der Waals ferromagnets due to the presence of site ordering, but also demonstrates the potential of these highly tunable two-dimensional magnets for memory and spintronics applications.

研究动机与目标

• 在二维范德华铁磁体中实现两种电子态之间的非易失性、可逆开关。
• 理解铁位空位有序性在稳定具有不同对称性的两种不同晶体相中的作用。
• 探讨空位有序相与无序相之间的电子结构与拓扑特性差异。
• 展示一种无需外场的热驱动开关机制，适用于低功耗电子与自旋电子学器件。
• 确立空位介导的对称性破缺作为在量子材料中设计奇异拓扑相的一般途径。

提出的方法

• 通过助熔法生长高质量的Fe5−δGeTe2单晶，并经机械剥离制备器件。
• 利用热退火与淬火循环，可逆地调控铁位空位的有序与无序状态。
• 采用角分辨光电子能谱（ARPES）测绘两种相的电子能带结构。
• 应用X射线衍射、透射电子显微镜（TEM）与扫描透射显微镜（STEM）确认结构与对称性差异。
• 进行群论对称性分析并结合第一性原理计算，阐明拓扑节点线与平带的起源。
• 利用二次谐波生成（SHG）与光学显微镜探测结构对称性破缺与相变行为。

Figure 1: Phase-tuning in Fe 5-δ GeTe 2 . a , Crystal structure of Fe 5 GeTe 2 with atomic sites labeled. Fe(1) and Ge are modeled as split sites, marked by half-filled circles to represent 50% occupancy. b-d , Schematic and procedures for tuning global symmetry via sublattice ordering. The two quan

实验结果

研究问题

• RQ1 是否可以在无外部电场或电流的情况下，实现二维范德华铁磁体中两种不同电子态之间的可逆、非易失性开关？
• RQ2 铁位空位有序性如何影响Fe5−δGeTe2的晶体对称性与电子结构？
• RQ3 无序相中拓扑节点线的起源是什么？有序相中平带的成因又是什么？
• RQ4 对称性破缺与量子干涉效应如何调控两种相的电子性质？
• RQ5 空位有序转变是否可作为通用机制，用于在量子材料中实现不同的拓扑相？

主要发现

• 通过热退火与淬火，在Fe5−δGeTe2中实现了两种稳定晶体相之间的可逆、非易失性开关，且开关过程发生在室温以上。
• 空位无序相表现出由保持全局反演对称性保护的拓扑节点线，经对称性分析与ARPES实验验证。
• 空位有序相由于在双子晶格上的量子破坏性干涉而呈现平带结构，且由于反演对称性破缺导致拓扑简并度被解除。
• 第一性原理计算证实，两种相具有截然不同的电子结构，其中有序相表现出增强的电子关联效应。
• 该开关机制为非易失性，且无需外加电场、应变或压力，因而具有高能效，适用于器件集成。
• 空位有序转变提供了一条通用路径，可用于在具有本征位点无序性的其他量子材料中工程化拓扑与关联电子相。

Figure 2: Topological nodal lines in the slow-cooled site-disordered phase. a , Schematic showing Dirac cones at K and K’ on a 2D projected BZ plane. b , Fermi surface mapping with LV polarized photons. The red line outlines the 2D BZ boundary and the white dashed line marks the electron pocket surr