Electronic Nature of Charge Density Wave and Electron-Phonon Coupling in Kagome Superconductor KV$_3$Sb$_5$ (2107.02688v2)

Abstract: The Kagome superconductors AV3Sb5 (A=K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV3Sb5. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV3Sb5. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV3Sb5. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV3Sb5 superconductors.

Insight into Charge Density Wave and Electron-Phonon Interaction in KV$_3$Sb$_5$

The paper "Electronic Nature of Charge Density Wave and Electron-Phonon Coupling in Kagome Superconductor KV$_3$Sb$_5$" provides a detailed paper of the electronic structure of the recently discovered Kagome superconductors, particularly focusing on KV$_3$Sb$_5$. This paper employs high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to elucidate the electronic intricacies associated with the charge density wave (CDW) state and electron-phonon coupling in these materials.

Key Findings and Observations

1. Fermi Surface and Band Structure Reconstruction: The paper highlights the reconstruction of electronic structures due to the 2$\times$2 CDW transition in KV$_3$Sb$_5$. Notably, the paper identifies additional features within the Fermi surface and band structure which correspond to CDW-induced band foldings. This signifies the influence of the CDW transition on the electronic landscape, which was previously undetected in similar ARPES measurements.
2. CDW-induced Gap Openings: The authors report distinct CDW-induced band splittings and associated gap openings not only at the Fermi level but also significantly below it. Multiple gaps were identified, including those opening at the pristine and reconstructed Brillouin zone boundaries, with gap sizes reaching up to $\sim$150\,meV. These gap openings were consistent with their DFT calculations, which showed similar features.
3. Momentum-dependent CDW Gap: The paper measures the CDW gap across various Fermi surfaces and finds strong anisotropy for all the V-derived Fermi surface sheets. This reveals a nuanced understanding of how CDW transitions affect momentum in Kagome superconductors, providing insights into their topological properties.
4. Electron-Phonon Coupling: Evidence for electron-phonon coupling is revealed through the observation of peak-dip-hump structures in the EDCs and kinks in the energy dispersions. These are attributed to the interactions between electronic states and phonons, particularly from vanadium vibrations, which are inferred to play a significant role in the CDW phases.

Implications and Future Directions

The paper's findings provide substantial evidence that the CDW phase in KV$_3$Sb$_5$ is largely driven by electron-phonon interactions rather than electron-electron correlations, which were previously hypothesized. This insight suggests the dominance of lattice dynamics in shaping the electronic properties, thus influencing superconductivity mechanisms within these superconductors.

The implications of these results extend to a deeper understanding of CDW states, especially in relation to topological features and superconductivity. This knowledge potentially opens avenues for exploring new types of CDW-related phenomena in other Kagome lattice systems and could inform the development of materials with tailored electronic and superconducting properties.

Speculations on Future Developments

Future research could focus on exploring the nature of superconductivity in KV$_3$Sb$_5$, specifically whether unconventional superconductivity can be conclusively identified alongside the CDW state. Additionally, investigations into pressure-induced or temperature-dependent modifications of the electronic structure may reveal other exotic phases or transitions. As the field progresses, refined experimental techniques and theoretical models might further illuminate the complex interplay between topology, electron correlations, and lattice vibrations in these intriguing materials.