Fermi surface mapping and the nature of charge density wave order in the kagome superconductor CsV$_3$Sb$_5$

Abstract: The recently discovered family of AV$_3$Sb$_5$ (A: K, Rb Cs) kagome metals possess a unique combination of nontrivial band topology, superconducting ground states, and signatures of electron correlations manifest via competing charge density wave order. Little is understood regarding the nature of the charge density wave (CDW) instability inherent to these compounds and the potential correlation with the accompanying onset of a large anomalous Hall response. To understand the impact of the CDW order on the electronic structure in these systems, we present quantum oscillation measurements on single crystals of CsV$_3$Sb$_5$. Our data provides direct evidence that the CDW invokes a substantial reconstruction of the Fermi surface pockets associated with the vanadium orbitals and the kagome lattice framework. In conjunction with density functional theory modeling, we are able to identify split oscillation frequencies originating from reconstructed pockets built from vanadium orbitals and Dirac-like bands. Complementary diffraction measurements are further able to demonstrate that the CDW instability has a correlated phasing between neighboring V$_3$Sb$_5$ planes. These results provide critical insights into the underlying CDW instability in AV$_3$Sb$_5$ kagome metals and support minimal models of CDW order arising from within the vanadium-based kagome lattice.

Fermi Surface Mapping and Charge Density Wave Order in CsV$_3$Sb$_5$

The manuscript titled "Fermi Surface Mapping and the Nature of Charge Density Wave Order in the Kagome Superconductor CsV$_3$Sb$_5$" presents a detailed investigation into the interplay between superconductivity and charge density wave (CDW) order in the kagome metal CsV$_3$Sb$_5$. The research focuses on understanding the impact of CDW orders on the electronic structure, which may have implications for unconventional superconductivity observed in these materials.

Nature of the Charge Density Wave

CsV$_3$Sb$_5$, part of the AV$_3$Sb$_5$ family (A = K, Rb, Cs), is characterized by its superconducting ground state and presence of CDW order. The authors elucidate that the CDW state causes substantial reconstruction of the Fermi surface primarily involving the vanadium orbitals and the intrinsic kagome lattice framework. This observation is critical as it provides an empirical basis for the theoretical models attributing the CDW instability to native electronic interactions at specific electron fillings inherent to the kagome structure.

Experimental Results and Methods

The study employs quantum oscillation measurements to map the Fermi surface changes due to CDW formation. The authors report multiple frequencies in Shubnikov-De Haas (SdH) quantum oscillation data, which they correlate with reconstructed Fermi pockets. These oscillations imply that the CDW impacts the band structure to a considerable degree. Density functional theory (DFT) calculations reveal band reconstructions near the M-point, which are validated through extremal orbit analysis.

The quantum oscillation measurements, complemented with structurally sensitive synchrotron x-ray diffraction, enable the identification of the nature of the CDW order. Such measurements reveal a (0.5, 0.5, 0.25) superlattice modulation that denotes a three-dimensional order with both in-plane and out-of-plane distortions. The diffraction measurements further show that the structural distortion related to CDW does not remove the inversion symmetry, confirmed through second harmonic generation (SHG) experiments.

Implications of the Findings

The findings provide substantial evidence for CDW-induced Fermi surface reconstruction centered around the vanadium orbitals within the kagome lattice in CsV$_3$Sb$_5$. Such insights can inform theoretical models emphasizing symmetry-breaking phenomena in kagome lattices and unconventional superconductivity. The implications of CDW orders extend to predictions of substantial Berry curvature effects, potentially contributing to large anomalous Hall effects observed experimentally.

Future Research Directions

This paper’s results establish a framework for future research to explore minimal models focusing on kagome planes to capture unconventional electronic properties in AV$_3$Sb$_5$ superconductors. Further computational studies using expanded supercell structures may provide deeper insights into the modulation of distortions and the concomitant effects on electronic structures. Additionally, expanding experimental techniques to resolve the precise nature of the out-of-plane modulated distortions will be essential.

In conclusion, the work presents a thorough exploration of the CDW state in CsV$_3$Sb$_5$, offering important insights into the complex interplay of charge order and superconductivity in layered kagome metals. This contributes to an improved understanding of their potential functionality in advanced technological applications related to electronic transport phenomena.