  NaAlSi and NaAlGe are isostructural and isoelectronic semimetals with
topological nodal lines close to the Fermi level. Despite having virtually
identical electronic structures, NaAlSi exhibits superconductivity below Tc =
6.8 K, whereas NaAlGe does not. We investigate NaAlGe by measuring its
electrical resistivity, Hall effect, magnetic susceptibility, and heat capacity
using single crystals. It is revealed that NaAlGe is not a simple semimetal but
rather has an unusual ground state with a small pseudogap of approximately 100
K close to the Fermi level. We argue that the formation of the pseudogap in
NaAlGe is due to an unexpected Fermi surface instability, such as an excitonic
instability, as opposed to the electron-phonon instability that leads to the
formation of the superconducting gap in NaAlSi.
