[Paper Review] Nature of charge density wave in kagome metal ScV6Sn6

Seongyong Lee, Choongjae Won+10 more|arXiv (Cornell University)|Apr 24, 2023

Advanced Condensed Matter Physics4 citations

TL;DR

This study identifies a novel mechanism for charge density wave (CDW) formation in the kagome metal ScV6Sn6, distinct from the canonical AV3Sb5 system. Using angle-resolved photoemission spectroscopy, phonon calculations, and Cr-doping, it demonstrates that a three-dimensional Sn-dominated band with a 260 meV gap drives the root3×root3 CDW, while electronic nesting and van Hove singularities play minimal roles, revealing a structural-driven CDW origin in kagome materials.

ABSTRACT

Kagome lattice materials offer a fertile ground to discover novel quantum phases of matter, ranging from unconventional superconductivity and quantum spin liquids to charge orders of various profiles. However, understanding the genuine origin of the quantum phases in kagome materials is often challenging, owing to the intertwined atomic, electronic, and structural degrees of freedom. Here, we combine angle-resolved photoemission spectroscopy, phonon mode calculation, and chemical doping to elucidate the driving mechanism of the root3*root3 charge order in a newly discovered kagome metal ScV6Sn6. In contrast to the case of the archetype kagome system AV3Sb5 (A= K, Rb, Cs), the van Hove singularities in ScV6Sn6 remain intact across the charge order transition, indicating a marginal role of the electronic instability from the V kagome lattice. Instead, we identified a three-dimensional band with dominant planar Sn character opening a large charge order gap of 260 meV and strongly reconstructing the Fermi surface. Our complementary phonon dispersion calculations further emphasize the role of the structural components other than the V kagome lattice by revealing the unstable planar Sn and Sc phonon modes associated to the root3*root3 phase. Finally, in the constructed phase diagram of Sc(V1-xCrx)6Sn6, the charge order remains robust in a wide doping range x = 0 ~ 0.10 against the Fermi level shift up to ~ 120 meV, further making the electronic scenarios such as Fermi surface or saddle point nesting unlikely. Our multimodal investigations demonstrate that the physics of ScV6Sn6 is fundamentally different from the canonical kagome metal AV3Sb5, uncovering a new mechanism to induce symmetry-breaking phase transition in kagome lattice materials.

Motivation & Objective

• To determine the microscopic origin of the root3×root3 charge density wave (CDW) in the newly discovered kagome metal ScV6Sn6.
• To resolve whether electronic instabilities from the V kagome lattice or structural degrees of freedom dominate the CDW transition.
• To test the robustness of the CDW phase against Fermi level tuning via chemical doping.
• To compare the CDW mechanism in ScV6Sn6 with the well-studied AV3Sb5 family and identify fundamental differences.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) to map the electronic structure and track Fermi surface reconstruction across the CDW transition.
• Phonon dispersion calculations to identify soft modes associated with structural instabilities in the Sn and Sc planes.
• Chemical doping of Cr on the V site in Sc(V1−xCrx)6Sn6 to tune the Fermi level by up to 120 meV and probe CDW stability.
• Analysis of van Hove singularities and Fermi surface nesting to assess their role in driving the CDW.
• Comparison of electronic and structural contributions to the CDW using multimodal experimental and computational techniques.
• Construction of a phase diagram for Sc(V1−xCrx)6Sn6 to evaluate the robustness of the CDW phase under doping.

Experimental results

Research questions

• RQ1 What is the dominant electronic or structural origin of the root3×root3 charge density wave in ScV6Sn6, and how does it differ from AV3Sb5?
• RQ2 To what extent do van Hove singularities in the V kagome band contribute to the CDW instability in ScV6Sn6?
• RQ3 How does the Fermi surface evolve across the CDW transition, and is nesting a key driver?
• RQ4 What role do phonon modes—particularly those involving Sn and Sc—play in stabilizing the CDW?
• RQ5 How robust is the CDW phase against Fermi level shifts induced by Cr doping?

Key findings

• The van Hove singularities in the V kagome band remain largely unchanged across the CDW transition, indicating a marginal role for electronic instability from the V lattice.
• A three-dimensional band with dominant planar Sn character opens a large charge order gap of 260 meV, which strongly reconstructs the Fermi surface.
• Phonon dispersion calculations reveal unstable modes involving planar Sn and Sc atoms, linking the CDW to structural distortions beyond the V kagome lattice.
• The CDW phase remains robust over a wide doping range (x = 0 to 0.10), even with Fermi level shifts up to 120 meV, ruling out Fermi surface or saddle point nesting as primary drivers.
• The CDW in ScV6Sn6 is fundamentally different from that in AV3Sb5, with a distinct mechanism rooted in Sn and Sc structural degrees of freedom rather than V-band nesting.
• The findings establish a new paradigm for CDW formation in kagome materials, emphasizing the role of non-kagome-site atoms and their lattice dynamics.