  Topological semimetals are characterized by protected crossings between
conduction and valence bands. These materials have recently attracted
significant interest because of the deep connections to high-energy physics,
the novel topological surface states, and the unusual transport phenomena.
While Dirac and Weyl semimetals have been extensively studied, the nodal-line
semimetal remains largely unexplored due to the lack of an ideal material
platform. In this paper, we report the magneto-transport properties of two
nodal-line semimetal candidates CaAgAs and CaCdGe. First, our single
crystalline CaAgAs supports the first "hydrogen atom" nodal-line semimetal,
where only the topological nodal-line is present at the Fermi level. Second,
our CaCdGe sample provides an ideal platform to perform comparative studies
because it features the same topological nodal line but has a more complicated
Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of
our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through
our systematic magneto-transport and first-principles band structure
calculations, we show that our CaTX compounds can be used to study, isolate,
and control the novel topological nodal-line physics in real materials.
