Quantum materials with layered kagome structures have attracted significant attention due to their potential to exhibit exotic quantum phenomena. The two-dimensional (2D) network of corner-sharing triangles in the kagome lattice can lead to flat bands, Dirac cones, and saddle points in the electronic structure, which, in the presence of spin-orbit coupling, magnetic ordering, or strong correlations, can give rise to spin liquid phases, topological insulator behavior, unconventional superconductivity, the fractional quantum Hall effect, and unconventional density wave orders. However, definitive identification of these unique electronic structures and their relationship to exotic quantum phenomena remains challenging. The kagome superconductors AV₃Sb₅ (A = K, Rb, or Cs) have been extensively studied, exhibiting anomalous Hall effects, unconventional charge density waves (CDWs), pairing density waves, and possibly unconventional superconductivity and nematic phases. The origin of these properties remains debated, with the role of the flat band requiring further investigation. CsTi₃Bi₅, isostructural to the AV₃Sb₅ family, offers a valuable opportunity to study the intrinsic electronic structure of the kagome lattice in the absence of CDW order, providing a crucial reference point for understanding the origin of various quantum phenomena observed in related kagome materials. This study aims to elucidate the relationship between the unique electronic structure and the emergent properties in CsTi₃Bi₅ through high-resolution laser-based angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory (DFT) calculations.

Extensive research has focused on the AV₃Sb₅ (A=K, Rb, Cs) family of kagome superconductors, revealing diverse phenomena like anomalous Hall effects, unconventional charge density waves, and superconducting states. However, the interplay between the unique electronic structure of the kagome lattice (flat bands, Dirac cones) and these observed phenomena remains unclear. While some ARPES studies have reported the observation of flat bands in various kagome materials (GdVSn₄, YMnSn₆, CoSn, Fe₃Sn₂, FeSn), clear evidence in the AV₃Sb₅ family remains scarce. The absence of a clear understanding of the flat band's role necessitates further investigation into this fundamental aspect of kagome lattice materials.

The electronic structure of CsTi₃Bi₅ was investigated using high-resolution laser-based ARPES, combined with DFT calculations. High-resolution ARPES measurements were performed using a lab-based system equipped with a 6.994 eV vacuum-ultra-violet (VUV) laser and a hemispherical electron energy analyzer. The laser spot was focused to ~10 μm to minimize the effect of sample inhomogeneity, and the light polarization was varied. The energy resolution was 1 meV, and the angular resolution was 0.3 degrees. Samples were cleaved in situ at 20 K and measured under ultrahigh vacuum. DFT calculations using the projector augmented-wave pseudopotential method within the Vienna ab initio simulation package (VASP) were performed. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was used for the exchange-correlation functional. Structural optimization was performed until atomic forces were less than 1 meV/Å, with a plane-wave cutoff energy of 520 eV and a 20 × 20 × 12 Monkhorst-Pack k-points grid. Wannier90 and WannierTools were used for fitting Wannier functions, constructing tight-binding models, and calculating surface spectral functions. The parity of the wavefunctions at time-reversal invariant momenta (TRIMs) was calculated to determine the Z₂ topological invariant.

ARPES measurements revealed five Fermi surface sheets: three electron-like pockets around Γ, an electron-like triangular pocket around K, and a small hole-like pocket around M. DFT calculations showed that low-energy bands originate primarily from Ti 3d orbitals, exhibiting characteristic kagome lattice features: a flat band, two saddle points at M, and a Dirac point at K. The flat band, primarily from Ti 3dxy orbitals, is nearly dispersionless along K-M-K but dispersive along Γ-M and Γ-K. Analysis suggests that the flat band arises from destructive interference of Bloch wave functions. An unexpected spectral weight buildup between -0.25 and -0.50 eV across the entire Brillouin zone was observed. DFT calculations also revealed type-II and type-III Dirac nodal loops and lines in 3D momentum space, consistent with ARPES measurements. Type-II loops formed hexagons around Γ and A, while type-III loops formed triangles around K and H. Finally, Z₂ nontrivial topological surface states were observed near Γ, attributed to band inversion and strong spin-orbit coupling. These states are located between ε and δ bands, consistent with DFT calculations of surface spectral functions.

The observed flat band in CsTi₃Bi₅, located closer to the Fermi level (-0.25 eV) than in AV₃Sb₅, provides clear evidence of this characteristic kagome lattice feature. The identification of type-II and type-III Dirac nodal loops, along with Z₂ topological surface states, further highlights the rich topological properties of CsTi₃Bi₅. The unexpected spectral weight buildup requires further investigation, but its correlation with the flat band suggests a potential intrinsic effect related to electron scattering. The tunability of electronic structures in the 135 system by manipulating chemical composition offers a pathway for realizing exotic quantum phenomena by bringing characteristic electronic structures closer to the Fermi level, potentially influencing superconductivity and other properties.

This study provides compelling spectroscopic evidence for a flat band, Dirac nodal lines, and topological surface states in the kagome superconductor CsTi₃Bi₅. The observed features are consistent with DFT calculations, revealing a rich interplay of electronic structure characteristics. The tunability of these features through chemical composition suggests a promising avenue for engineering novel quantum phenomena in kagome materials. Future studies should focus on the origin of the observed spectral weight buildup and explore the implications of the identified electronic features for the superconductivity and other emergent properties in this material.

The observed spectral weight buildup between -0.25 and -0.50 eV remains unexplained. While the correlation with the flat band is noted, further investigation is needed to confirm its intrinsic nature. Additionally, the full characterization of the Dirac nodal lines would require ARPES measurements at different kz values using varying photon energies.