[논문 리뷰] Emergence of a new band and the Lifshitz transition in kagome metal ScV$_6$Sn$_6$ with charge density wave

Seoung‐Hun Kang, Haoxiang Li외 11명|arXiv (Cornell University)|2023. 02. 27.

Topological Materials and Phenomena인용 수 20

한 줄 요약

본 논문은 ARPES, STM, 및 DFT를 이용해 ScV6Sn6를 연구하고, CDW 형성과 관련된 온도 의존적 새로운 표면 밴드와 Lifshitz 전이를 보고하며, 페르미 준위 갭은 관찰되지 않는다.

ABSTRACT

Topological kagome systems have been a topic of great interest in condensed matter physics due totheir unique electronic properties. The vanadium-based kagome materials are particularly intrigu-ing since they exhibit exotic phenomena such as charge density wave (CDW) and unconventionalsuperconductivity. The origin of these electronic instabilities is not fully understood, and the re-cent discovery of a charge density wave in ScV6Sn6provides a new avenue for investigation. In thiswork, we investigate the electronic structure of the novel kagome metal ScV6Sn6using angle resolvedphotoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and first-principlesdensity functional theory calculations. Our analysis reveals for the first time the temperature-dependent band changes of ScV6Sn6and identifies a new band that exhibits a strong signatureof a structure with CDW below the critical temperature. Further analysis revealed that this newband is due to the surface kagome layer of the CDW structure. In addition, a Lifshitz transition isidentified in the ARPES spectra that is related to the saddle point moving across the Fermi levelat the critical temperature for the CDW formation. This result shows the CDW behavior may alsobe related to nesting of the saddle point, similar to related materials. However, no energy gap is observed at the Fermi level and thus the CDW is not a typical Fermi surface nesting scenario. These results provide new insights into the underlying physics of the CDW in the kagome materials and could have implications for the development of materials with new functionality.

연구 동기 및 목표

• CDW 전이에 걸쳐 ScV6Sn6의 전자 구조를 조사한다.
• CDW가 밴드 구조, 페르미 표면, 그리고 가능한 네스팅 시나리오에 어떤 영향을 주는지 식별한다.
• CDW 상태에서 나타나는 새로운 밴드의 기원을 결정한다.
• CDW 특징이 페르미 표면을 갭으로 만드는지, 아니면 표면 상태에서 기인하는지 평가한다.

제안 방법

• 밴드 구조와 페르미 표면을 매핑하기 위한 각도분해 광전자분광학(ARPES).
• 주변 존재하는 상태를 탐지하기 위한 주사 터널링 현미경/분광학(STM/S)으로 국부 밀도 상태와 표면 엔딩 효과를 조사하기 위해 STM/S(주사 터널링 현미경/분광학)를 사용한다.
• 스핀-궤도 결합을 포함한 퍼스트-principles 밀도범함 이론(DFT) 계산으로 벌크 및 표면 전자 구조를 모델링한다.
• 고대칭 방향에서 ARPES와의 비교를 위해 밴드 구조의 언폴딩(언폴딩)을 수행한다.
• V d 오비탈(특히 dz^2 성분)에 초점을 맞춘 오비탈 해석을 수행한다.

실험 결과

연구 질문

• RQ1 Does ScV6Sn6 exhibit a CDW-induced modification of its electronic structure detectable by ARPES and STM?
• RQ2 Is there a Lifshitz transition associated with the CDW formation in ScV6Sn6?
• RQ3 What is the origin of the observed new band appearing in the CDW state—bulk or surface states?
• RQ4 Does the CDW in ScV6Sn6 open a gap at the Fermi level, indicating a Peierls-like nesting scenario?
• RQ5 How does the surface termination influence the observed electronic structure in the CDW state?

주요 결과

• A temperature-dependent band shift across the CDW transition is observed near the M point, indicating a Lifshitz transition as a saddle point crosses the Fermi level.
• An additional band emerges in the CDW state near the Fermi level, attributed to the surface kagome layer, with dz^2 character from surface V atoms.
• No energy gap is detected at the Fermi level in the CDW state, suggesting the CDW is not a conventional Fermi surface nesting instability.
• DFT and ARPES show bulk kagome features such as a Dirac point at K and a saddle point at M, with surface termination affecting the observed bands.
• The newly observed surface band aligns with CDW-induced surface reconstruction and is supported by STM/LDOS measurements.
• The Lifshitz transition is linked to the van Hove saddle point moving above E_F as temperature decreases below T_c, correlating with changes in resistivity reported in prior studies.