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Any atomic nucleus with orbiting electrons fulfills that requirement, you could loosely call those time crystals too.

Their half life for most stable atoms is so long that perpetual applies for most practical uses of the word.

Which makes me wonder if these time crystals have a half-life too.

Electrons do not orbit atomic nuclei in the quantum description of atoms. Instead, in the lowest energy state the electron wave function is spread out in a sphere around the nucleus, and this wave function stays constant in time. Almost all quantum systems do something like this in the ground state, which shows why the time crystal is something new and interesting.

It must be able to tunnel into some other structure. The half-life is defined by how much energy it needs to "melt" as the article says.

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