[论文解读] Nanoscale visualization and spectral fingerprints of the charge order in ScV6Sn6 distinct from other kagome metals

Siyu Cheng, Zheng Ren+24 more|arXiv (Cornell University)|Feb 23, 2023

Topological Materials and Phenomena被引用 14

一句话总结

论文使用扫描隧道显微镜/光谱学和角分辨光电子能谱来表征 ScV6Sn6 的体相电荷密度波，揭示一个较小的约 20 meV 的部分能隙以及仅在表面局部化的调制，未显示转动对称性破缺，这将其与其他 kagome CDW 区别开来。

ABSTRACT

Charge density waves (CDWs) have been tied to a number of unusual phenomena in kagome metals, including rotation symmetry breaking, time-reversal symmetry breaking and superconductivity. The majority of the experiments thus far have focused on the CDW states in AV3Sb5 and FeGe, characterized by the 2a0 by 2a0 period. Recently, a bulk CDW phase (T* ~ 92 K) with a different wave length and orientation has been reported in ScV6Sn6, as the first realization of a CDW state in the broad RM6X6 structure. Here, using a combination of scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal the microscopic structure and the spectroscopic signatures of this charge ordering phase in ScV6Sn6. Differential conductance dI/dV spectra show a partial gap opening in the density-of-states of about 20 meV at the Fermi level. This is much smaller than the spectral gaps observed in AV3Sb5 and FeGe despite the comparable T* temperatures in these systems, suggesting substantially weaker coupling strength in ScV6Sn6. Surprisingly, despite the three-dimensional bulk nature of the charge order, we find that the charge modulation is only observed on the kagome termination. Temperature-dependent band structure evolution suggests a modulation of the surface states as a consequence of the emergent charge order, with an abrupt spectral weight shift below T* consistent with the first-order phase transition. The similarity of the electronic band structures of ScV6Sn6 and TbV6Sn6 (where charge ordering is absent), together with the first-principle calculations, suggests that charge ordering in ScV6Sn6 may not be primarily electronically driven. Interestingly, in contrast to the CDW state of cousin AV3Sb5, we find no evidence supporting rotation symmetry breaking. Our results reveal a distinctive nature of the charge ordering phase in ScV6Sn6 in comparison to other kagome metals.

研究动机与目标

• 在 RM6X6 kagome 家族中，超越 AV3Sb5 与 FeGe，激发对 ScV6Sn6 电荷有序的研究。
• 表征 ScV6Sn6 CDW 的微观结构及其光谱特征。
• 将 ScV6Sn6 CDW 与相关 kagome 材料进行比较，以评估驱动机制与对称性特性。

提出的方法

• 结合扫描隧道显微镜/光谱（STM/STS）以在纳米尺度可视化实空间的电荷调制。
• 使用微分电导 dI/dV 测量来探针局部态密度并识别部分能隙特征。
• 应用角分辨光电子能谱（ARPES）来跟踪温度依赖的带结构演化。
• 进行第一性原理计算，以比较有无电荷有序的电子结构。

实验结果

研究问题

• RQ1 斯V6Sn6 电荷有序相的微观结构是什么？
• RQ2 电荷有序的光谱特征是什么，部分能隙随温度如何演变？
• RQ3 与其他 kagome 金属的 CDW 相比，ScV6Sn6 的对称性和驱动机制有何异同？

主要发现

• 在 dI/dV 谱中观察到费米能级附近约 20 meV 的部分能隙开启。
• 尽管 CDW 具有三维体相性质，但电荷调制仅在 kagome 区域的终面被探测到。
• 温度依赖的带结构演化显示表面态调制以及在 T* 以下的突然光谱权重转移，指示一阶相变。
• ScV6Sn6 的带结构类似于 TbV6Sn6，其电荷有序不存在，表明电荷有序可能并非主要由电子驱动。
• 未发现转动对称性破缺的证据，将 ScV6Sn6 与 AV3Sb5 CDW 行为区分开。