Topological Phase with Critical-Type Nodal Line State in Intermetallic CaPd

In recent years, realizing new topological phase of matter has been a hot topic in the fields of physics and materials science. Topological semimetals and metals can conventionally be classified into two types: type-I and type-II according to the tilting degree of the fermion cone. Here, it is the first time to report a new topological metal phase with the critical-type nodal line between type-I and type-II nodal line. The critical-type nodal line shows a unique nontrivial band crossing which is composed of a at band and a dispersive band and leads to a new fermionic state. We propose intermetallic CaPd can be an existing topological metal for the new fermionic state, characterized with critical-type nodal line in the bulk and drumhead band structure on the surface. Our work not only promotes the concept of critical-type nodal line, but also provides the material realization to study its exotic properties in future experiments.