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

Seoung‐Hun Kang, Haoxiang Li+11 more|arXiv (Cornell University)|Feb 27, 2023

Topological Materials and Phenomena20 citations

TL;DR

The paper uses ARPES, STM, and DFT to study ScV6Sn6 and reports a temperature-dependent new surface band and a Lifshitz transition associated with CDW formation, without a Fermi-level gap.

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.

Motivation & Objective

• Investigate the electronic structure of ScV6Sn6 across the CDW transition.
• Identify how CDW affects band structure, Fermi surface, and possible nesting scenarios.
• Determine the origin of any new bands appearing in the CDW state.
• Assess whether CDW features gap the Fermi surface or arise from surface states.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) to map band structure and Fermi surfaces.
• Scanning tunneling microscopy/spectroscopy (STM/S) to probe local density of states and surface termination effects.
• First-principles density functional theory (DFT) calculations with spin-orbit coupling to model bulk and surface electronic structures.
• Unfolding of band structures for comparison with ARPES along high-symmetry directions.
• Orbital-resolved analysis focusing on V d orbitals, especially dz^2 character.

Experimental results

Research questions

• 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?

Key findings

• 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.