By definition metals and insulators are very different – but now Princeton physicists have accidentally discovered an unexpected quantum behavior in an insulator that was thought to be unique to metals. The find suggests a brand new type of quantum particle, which the team calls a neutral fermion.

Basically speaking, metals conduct electricity and insulators don’t. On the molecular level, that comes down to how freely electrons can move through the materials – in metals, electrons are very mobile, while insulators obviously have high resistance that prevents them moving much.

As a side effect of this, metals can exhibit a phenomenon known as quantum oscillations. When exposed to a magnetic field at very low temperatures, electrons can shift into a quantum state that causes the material’s resistivity to oscillate. This doesn’t happen in insulators, however, since their electrons don’t move very well.

Or at least that was the conventional thinking for the better part of a century. In the new study, the Princeton researchers accidentally discovered quantum oscillations in an insulator for the first time.

The team was working with tungsten ditelluride, which behaves like a metal in bulk but becomes an insulator when it’s shaved down into a two-dimensional form like graphene. While measuring the resistivity of the monolayer material under a magnetic field, they found that it began to oscillate.

“This came as a complete surprise,” says Sanfeng Wu, senior author of the study. “We asked ourselves, ‘What’s going on here?’ We don’t fully understand it yet.”

The phenomenon can’t be explained by current theories, but the researchers have put forward their own hypothesis. They say it may not be the electrons themselves that are oscillating. Rather, the strong interactions might be creating create new quantum particles that exhibit the observed effect.

Since insulators block charged particles, like electrons, from moving freely, these new particles would have to have a neutral charge. These hypothetical neutral fermions could then exhibit the observed quantum oscillations.

“If our interpretations are correct, we are seeing a fundamentally new form of quantum matter,” says Wu. “We are now imagining a wholly new quantum world hidden in insulators. It’s possible that we simply missed identifying them over the last several decades.”

The team says that more work will need to be done to verify if neutral fermions do exist, or if there’s some other explanation for the observed oddities.

The research was published in the journal Nature.