  The possibility of the $^{14}$C cluster being a basic building block of
medium mass nuclei is discussed. Although $\alpha$ cluster structures have been
widely discussed in the light $N\approx Z$ mass region, the neutron to proton
ratio deviates from unity in the nuclei near $\beta$-stability line and in
neutron-rich nuclei. Thus, more neutron-rich objects with $N>Z$ could become
the building blocks of cluster structures in such nuclei. The $^{14}$C nucleus
is strongly bound and can be regarded as such a candidate. In addition, the
path to the lowest shell-model configuration at short relative distances is
closed for the $^{14}$C+$^{14}$C structure contrary to the case of the
$^{12}$C+$^{12}$C structure; this allows to keep appreciable separation
distance between the $^{14}$C clusters. The recent development of
antisymmetrized quasi-cluster model (AQCM) allows us to utilize $jj$-coupling
shell model wave function for each cluster in a simplified way. The AQCM
results for the $^{14}$C+$^{14}$C structure in $^{28}$Mg are compared with the
ones of cranked relativistic mean field (CRMF) calculations. Although
theoretical frameworks of these two models are quite different, they give
similar results for the nucleonic densities and rotational properties of the
structure under investigation. The existence of linear chain three $^{14}$C
cluster structure in $^{42}$Ar has also been predicted in AQCM. These results
confirm the role of the $^{14}$C cluster as a possible building block of
cluster structures in medium mass nuclei.
