  Dirac semimetals (DSMs) are classified into different phases based on the
types of the Dirac fermions. Tuning the transition among different types of the
Dirac fermions in one system remains challenging. Recently, KMgBi was predicted
to be located at a critical state that various types of Dirac fermions can be
induced owing to the existence of a flat band. Here, we carried out systematic
studies on the electronic structure of KMgBi single crystal by combining
angle-resolve photoemission spectroscopy (ARPES) and scanning tunneling
microscopy/spectroscopy (STM/STS). The flat band was clearly observed near the
Fermi level. We also revealed a small bandgap of ~ 20 meV between the flat band
and the conduction band. These results demonstrate the critical state of KMgBi
that transitions among various types of Dirac fermions can be tuned in one
system.

Materials Science