By Yong Hu, Ming Shi, PSI
Based on article published in Nature Communications
Transition-metal based kagome materials, hosting corner-sharing triangles, offer an exciting platform to explore intriguing correlated and topological phenomena, including quantum spin liquid, unconventional superconductivity, Dirac/Weyl semimetals and charge density wave (CDW) order. Their emergence originates from the inherent features of the kagome lattice: substantial geometric spin frustration, flat bands, Dirac cones and van Hove singularities (VHSs) at different electron fillings. Recently, a new family of kagome metal AV3Sb5 (A= K, Rb, Cs) with V kagome nets, was found to feature a ℤ2 topological band structure and superconductivity was realized with a maximum Tc of 2.5 K at ambient pressure. Moreover, they exhibit CDW order below TCDW≈78-103 K. Aside from the translational symmetry breaking in this CDW phase, the breaking of additional symmetries, i.e., rotation and time-reversal, was observed upon cooling down towards Tc. Despite evidences supporting a nodeless gap from magnetic penetration depth measurements, double superconducting domes under pressure, a large residual in the thermal conductivity and an edge supercurrent in Nb/K1-xV3Sb5 suggest electronically driven and unconventional superconductivity. It’s widely believed that these exotic correlated phenomena are intimately connected with the multiple VHSs in the vicinity of the Fermi level.
The characteristics of VHS bands are crucial in determining the Fermi surface instabilities. From the perspective of band dispersion around the saddle point, VHSs can be classified into two types: conventional and higher-order, as shown in Figs. 1a (i) and (ii). The higher-order VHS displays a flat dispersion along one direction with less pronounced Fermi surface nesting, generating a power-law divergent density of states (DOS) in two dimensions (2D) instead of a logarithmic divergent one. Moreover, VHSs in kagome lattices possess distinct sublattice features: sublattice pure (p-type) and sublattice mixing (m-type), as shown in Fig. 1b. They induce an effective reduction of the local Coulomb interaction, thereby enhancing the role of non-local Coulomb terms. Therefore, the nature of VHSs is pivotal to understand correlated phenomena, but still remains elusive in the kagome metals AV3Sb5 so far.
To this end, we have performed a comprehensive study on the electronic structure of CsV3Sb5 by combining polarization-dependent Angle-resolved Photoemission Spectroscopy (ARPES) measurements with Density Functional theory (DFT) . The diverse nature of the four VHSs in the vicinity of the Fermi level (EF) is directly revealed (Fig. 1c). We observe three VHSs around the M point below the EF, formed by Vanadium 3d orbitals. Two of them are of conventional p-type, while the other one is of higher-order p-type. In addition, we find a conventional m-type VHS slightly above EF from our theoretical calculations. Furthermore, we show that the sublattice features are also embedded in the Dirac cone around the K point, exhibiting characteristic intensity modulations under various polarization conditions. The crucial insights into the electronic structure, revealed by our work, provide a solid starting point for the understanding of the intriguing correlation phenomena in the kagome metals AV3Sb5.
Figure 1: (a) Schematics of the conventional VHS (i) and higher-order VHS (ii) in two-dimensional electron systems. (b) Two distinct types of sublattice decorated van Hove singularities (VHSs) in CsV3Sb5, labeled as p-type (sublattice pure, upper panel) and m-type (sublattice mixing, lower panel). (c) Experimental band dispersion (i) and calculated bands (ii) along the Γ – K – M – Γ direction. The red arrows mark the multiple VHSs. The data were probed with circularly polarized light, at 200 K, and the Γ – M direction of the sample was aligned with the analyzer slit.
 Yong Hu, Xianxin Wu, Brenden R. Ortiz, Sailong Ju, Xinloong Han, Junzhang Ma, Nicholas C. Plumb, Milan Radovic, Ronny Thomale, Stephen D. Wilson, Andreas P. Schnyder, and Ming Shi, “Rich Nature of Van Hove singularities in Kagome superconductor CsV3Sb5”, Nature Communications 13, 2220 (2022).  Yong Hu, Samuel M. L. Teicher, Brenden R. Ortiz, Yang Luo, Shuting Peng, Linwei Huai, Junzhang Ma, Nicholas C. Plumb, Stephen D. Wilson, Junfeng He, and Ming Shi, “Topological surface states and flat bands in the kagome superconductor CsV3Sb5”, Science Bulletin 67, 495-500 (2022).