@Motodrachen I can't find it now but I remember seeing a paper about making neutral helium compounds by replacing one of the helium's electrons with a muon, making it act a lot more like a hydrogen atom
@Motodrachen I suspect it would actually not be "copper with muons" in actual composition if it's to act anything remotely like copper with electrons. The nuclei would probably be much lighter than copper, which would offset the density problems somewhat.
That said, maybe the nucleus isn't protons and neutrons at all but some other baryon--of course, that would make it heavier. Maybe a mesonic nucleus?
@Motodrachen it occurs to me that muons can only decay into electrons (plus neutrinos) as there are no other lighter charged leptons. So you could have a stable muon if all of the possible electron states it could decay to are filled. I'm not sure exactly what that would imply and I don't know if you could make it stable but it's food for thought
@Motodrachen why am I putting so much thought into this