  We discovered a new class of topological crystals, namely linked rings of
crystals. Two rings of tantalum triselenide (TaSe3) single crystals were linked
to each other while crystal growing. The topology of the crystal form is called
a "Hopf link", which is the simplest link involving just two component unknots
linked together exactly once. The feature of the crystals is not covered by the
conventional crystallography.
