Observation of Flat Band, Dirac Nodal Lines and Topological Surface States in Kagome Superconductor CsTi${}_3$Bi${}_5$

Abstract

A kagome lattice of 3d transition metals hosts flat bands, Dirac fermions and saddle points. It provides a versatile platform for achieving topological superconductivity, anomalous Hall effect, unconventional density wave order and quantum spin liquid when the strong correlation, spin-orbit coupling or magnetic order are involved in such a lattice. Here, using laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations, we investigate the electronic structure of the newly discovered kagome superconductor CsTi${}_3$Bi${}_5$, which is isostructural to the AV${}_3$Sb${}_5$ (A=K, Rb or Cs) kagome superconductors and possesses a perfect two-dimensional kagome network of Titanium. We directly observed a strikingly flat band derived from the local destructive interferences of Bloch wave functions within the kagome lattices. We also identify the type-II Dirac nodal loops around the Brillouin zone center, the type-III Dirac nodal loops around the zone corners and type-III Dirac nodal lines along the k${}_z$ direction. In addition, around the Brillouin zone center, Z2 nontrivial topological surface states are also observed which is formed from the band inversion due to strong spin orbital coupling. The simultaneous existence of such multi-sets of nontrivial band structures in one kagome superconductor not only provides good opportunities to study related physics in the kagome lattice but also makes CsTi${}_3$Bi${}_5$ an ideal system to realize noval quantum phenomena by manipulating its chemical potential with chemical doping or pressure.

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• Superconducting, topological and transport properties of kagome metals CsTi${}_3$Bi${}_5$ and RbTi${}_3$Bi${}_5$
• FeGe as a building block for the kagome 1:1, 1:6:6, and 1:3:5 families: hidden d-orbital decoupling of flat band sectors, effective models and interaction Hamiltonians
• Kagome metals
• Diverse electronic landscape of the kagome metal YbTi3Bi4
• Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film
• Topological electronic structure and electronic nematicity in candidate kagome superconductors, ATi${}_3$Bi${}_5$ (A = Rb, Cs)
• Tuning electronic pairing by uniaxial strain in kagome lattices
• Interplay of $d$- and $p$-states in RbTi${}_3$Bi${}_5$ and CsTi${}_3$Bi${}_5$ flat-band kagome metals
• Electronic structure of $A$V${}_3$Sb${}_5$ kagome metals
• The de Haas-van Alphen quantum oscillations in the kagome metal RbTi3Bi5
• High-order van Hove singularities and nematic instability in the kagome superconductor CsTi${}_3$Bi${}_5$
• Long-range Ising spins models emerging from frustrated Josephson junctions arrays with topological constraints
• Infrared spectroscopy study of kagome material CsTi${}_3$Bi${}_5$
• van Hove Singularity-Driven Emergence of Multiple Flat Bands in Kagome Superconductors
• Observation of multiple flat bands and van Hove singularities in the distorted kagome metal NdTi3Bi4
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