Origin of charge density wave in topological semimetals SrAl4 and EuAl4

Topological semimetals in BaAl 4 -type structure show many interesting behaviors, such as charge density wave (CDW) in SrAl 4 and EuAl 4 , but not the isostructural and isovalent BaAl 4 , SrGa 4 , and BaGa 4 . Here using Wannier functions based on density functional theory, we calculate the susceptibility functions with millions of k -points to reach the small q -vector and study the origin and driving force behind the CDW. Our comparative study reveals that the origin of the CDW in SrAl 4 and EuAl 4 is the strong electron-phonon coupling interaction for the transverse acoustic mode at small q -vector along the Γ-Z direction besides the maximum of the real part of the susceptibility function from the nested Fermi surfaces of the Dirac-like bands, which explains well the absence of CDW in the other closely related compounds in a good agreement with experiment. We also connect the different CDW behaviors in the Al compounds to the macroscopic elastic properties. Charge density waves (CDW) are long periodic rearrangement of electronic structure, that are typically coupled with distortions of the underlying crystal lattices, and the driving mechanism of which can vary from one material to another. Here, the authors use density functional theory to investigate the origin of CDW in SrAl4 and EuAl4, finding that strong electron-coupling plays an important role, which also explains the absence of CDW in BaAl4.