[論文レビュー] 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被引用数 14

ひとこと要約

本論文は scanning tunneling microscopy/spectroscopy と angle-resolved photoemission spectroscopy を用いて ScV6Sn6 における bulk 電荷密度波を特徴付け、約20 meV の小さな部分ギャップと表面局在型の変調を明らかにし、回転対称性崩れがないことを示しており、他の kagome CDW とは区別される。

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.

研究の動機と目的

• Motivate the investigation of charge ordering in ScV6Sn6 within the RM6X6 kagome family beyond AV3Sb5 and FeGe.
• Characterize the microscopic structure of the ScV6Sn6 CDW and its spectroscopic signatures.
• Compare the ScV6Sn6 CDW with related kagome materials to assess driving mechanisms and symmetry properties.

提案手法

• Combine scanning tunneling microscopy/spectroscopy (STM/STS) to visualize real-space charge modulation at nanoscale.
• Use differential conductance dI/dV measurements to probe local density of states and identify partial gap features.
• Apply angle-resolved photoemission spectroscopy (ARPES) to track temperature-dependent band structure evolution.
• Perform first-principles calculations to compare electronic structures with and without charge ordering.

実験結果

リサーチクエスチョン

• RQ1 What is the microscopic structure of the charge-ordered phase in ScV6Sn6?
• RQ2 What is the spectroscopic signature of the charge order and how does the partial gap evolve with temperature?
• RQ3 How does ScV6Sn6 CDW compare to CDWs in other kagome metals in terms of symmetry and driving mechanisms?

主な発見

• A partial gap opening of about 20 meV at the Fermi level is observed in dI/dV spectra.
• Charge modulation is detected only on the kagome termination despite the 3D bulk nature of the CDW.
• Temperature-dependent band structure evolution shows a surface-state modulation and an abrupt spectral weight shift below T* indicative of a first-order transition.
• ScV6Sn6 band structures resemble TbV6Sn6 where charge ordering is absent, suggesting charge order may not be primarily electronically driven.
• No evidence of rotation symmetry breaking is found, distinguishing ScV6Sn6 from AV3Sb5 CDW behavior.