Metals and insulators are the yin and yang of physics, their respective material properties strictly dictated by their electrons' mobility - metals should conduct electrons freely, while insulators keep them in place.

So when physicists from Princeton University in the US found a quantum quirk of metals bouncing around inside an insulating compound, they were lost for an explanation.

We'll need to wait on further studies to find out exactly what's going on. But one tantalising possibility is that a previously unseen particle is at work, one that represents neutral ground in electron behaviour. They're calling it a 'neutral fermion'.

"This came as a complete surprise," says physicist Sanfeng Wu from Princeton University in the US.

"We asked ourselves, 'What's going on here?' We don't fully understand it yet."

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