  The unique properties of topological semimetals have strongly driven efforts
to seek for new topological phases and related materials. Here, we identify a
critical condition for the existence of linked nodal rings (LNRs) in symmorphic
crystals, and propose that three types of LNRs, named as alpha-, beta- and
gamma-type, can be obtained by stacking semiconducting layers. Several
honeycomb structures are suggested to be topological LNR semimetals, including
layered and "hidden" layered structures. Transitions between the three types of
LNRs can be driven by external strains. Interesting surface states other than
drumhead states are found in these topological materials.
