[论文解读] Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor

Zehao Wang, Yucheng Guo+34 more|arXiv (Cornell University)|Jun 7, 2024

Quantum, superfluid, helium dynamics被引用 6

一句话总结

该论文将 ARPES 与 DFT 结合，揭示 CsCr3Sb5 的费米能级附近存在广泛的卡格梅平带，分析轨道选择性重整化，并证明在密度波序跨越中平带发生位移及其在新兴序态与超导性中的潜在作用。

ABSTRACT

In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.

研究动机与目标

• Identify the low-energy electronic structure of CsCr3Sb5 and determine the position and orbital character of kagome flat bands near the Fermi level.
• Determine how electron correlations renormalize DFT-predicted flat bands and compare with polarization-resolved ARPES observations.
• Investigate how the flat band interacts with density-wave order and its evolution with temperature.

提出的方法

• Combine angle-resolved photoemission spectroscopy (ARPES) with polarization dependence to resolve orbital content and band dispersions.
• Use DFT calculations with orbital projections to interpret ARPES data and assign orbital characters to observed features.
• Apply polarization-dependent selection rules to distinguish contributions from Cr d-orbitals (d_xz, d_yz, etc.) and Sb p-orbitals in the flat band.
• Compare measured Fermi surface and van Hove singularities to DFT predictions to assess correlation effects.
• Track temperature dependence of the flat band across the density wave transition to assess its role in emergent order.

实验结果

研究问题

• RQ1 Is there a kagome lattice flat band near the Fermi level in CsCr3Sb5, and what is its orbital composition?
• RQ2 Do electron correlation effects renormalize the flat band position relative to DFT predictions, and how does this compare across different orbital characters?
• RQ3 How does the flat band interact with low-temperature density wave order, and does it shift with temperature?
• RQ4 What is the relationship between the observed flat band and emergent electronic orders, including superconductivity under pressure?

主要发现

• A pervasive kagome flat band near the Fermi level is observed in CsCr3Sb5, extending across a large portion of the Brillouin zone.
• Orbital-resolved ARPES and orbital-projected DFT indicate the flat band has mixed Cr d_{xz}/d_{yz} and possibly d_{xy} character, with correlation effects renormalizing its energy relative to DFT where it sits ~200 meV above E_F.
• The flat band is observed to hybridize with an electron-like Sb p_z band, evidenced by a kink near -70 meV and a corresponding EDC peak, confirming the flat band is intrinsic to the electronic structure.
• Across the density wave transition at T', the flat band shifts toward E_F by about 20 meV, indicating its participation in the emergent electronic order.
• CsCr3Sb5 shows similarities to other kagome systems but uniquely places flat bands near E_F with orbital-selective renormalization, offering a platform to study flat-band-induced electronic orders and superconductivity under pressure.
• The material sits in a regime between strongly ordered kagome magnets and correlation-dominated systems like Ni3In, with pressure-induced superconductivity.