Topological electron and phonon flat bands in novel kagome superconductor XPd5 (X = Ca, Sr, Ba)

Fermionic and bosonic localized states induced by geometric frustration in the kagome lattice provide a unique platform for exploring exotic quantum phenomena in strongly correlated systems. We report the coexistence of electronic and phononic flat bands induced by geometric frustration in the kagome superconductor XPd5 (X = Ca, Sr, Ba). The electronic flat band is located around the Fermi level and possesses a nontrivial topological invariant with $${{\mathbb{Z}}}_{2}=1$$ . Additionaly, we identify multiple van Hove singularities (vHS) arise from the kagome Pd d orbitals, including conventional, higher-order vHS and p-type, m-type vHS. Specifically, our investigation of the vibrational modes of the phononic flat band reveals that its formation originates from destructive interference between adjacent kagome sites. Furthermore, we theoretically reveal that XPd5 exhibits superconducting ground states, with critical temperatures of 4.25 K, 2.75 K, and 3.35 K for CaPd5, SrPd5, and BaPd5, respectively. This work provides a promising platform to explore the Fermion-boson many-body interplay and superconducting states.