Metals and insulators are the yin and yang of physics, their respective material properties strictly dictated by the mobility of their electrons – metals should conduct electrons freely, while insulators hold them in place.
So when physicists at Princeton University in the United States found a strange quantum of metals bouncing inside an insulating compound, they were lost for an explanation.
We will have to wait for further studies to find out exactly what is happening. But a tempting possibility is that a previously unseen particle works, one that represents a neutral ground in the behavior of electrons. They call it “neutral fermion.”
“It was a complete surprise,” says physicist Sanfeng Wu of Princeton University in the United States.
“We asked ourselves, ‘What’s going on here?’ “We still don’t fully understand it.”
The phenomenon at the center of the discovery is the quantum oscillation. As the term suggests, it involves the forward and backward oscillation of free-moving particles under certain experimental conditions.
To become a little more technical, oscillations occur when a material is cooled to levels where quantum behaviors dominate more easily and a magnetic field is applied and varied.
Wrapping the magnetic field up and down causes unconnected charged particles, such as electrons, to slide between energy bands called Landau levels.
It is a commonly used technique to study the atomic landscape occupied by electrons on a material, especially in those with metallic properties.
Isolators are considered to be another kettle of fish. With their electrons following strict commands to stay home, quantum oscillations are not a thing. At least, it shouldn’t be.
The team looked at tungsten dithellurid, which is a strange semimetal that takes on the properties of an insulator when bathed in a magnetic field – and was surprised to see quantum oscillations happen.
Despite the shock, they have some thoughts about what might happen. While a flowing load would make this insulator a conductor (which is a paradox), if the “flow” of neutral particles would match the bill of the insulator and the quantum oscillator, which makes more sense.
“Our experimental results conflict with all existing theories based on charged fermions, but could be explained in the presence of neutrons with a neutral charge,” adds colleague Pengjie Wang.
The only problem is that truly neutral fermions should not exist, according to the standard model of particle physics.
Fermions are particles that resemble the “Lego blocks” of matter, while the other type of fundamental particles are bosons – charge-bearing particles.
A truly neutral particle is also its own antiparticle – and that’s what I’ve seen in bosons, but never in fermions.
So finding a truly neutral fermion would probably rewrite our understanding of physics, but that’s not what the researchers here think – instead, they think what they’ve detected is more of a quasi-neutral particle, which is a quantum type of hybrid particles.
To understand what a quasi-particle is, imagine particle physics as a study of music.
Fundamental particles such as quarks and electrons are individual instruments. They form the basis of a variety of larger particles, from three-piece rock bands, such as protons or symphonies, such as whole atoms.
Bands that play synchronized on opposite stages can even be seen as a single event – a quasi-particle that, for all intents and purposes, plays as one.
Quantum oddity can scatter the properties of electrons in ways that make fractions of their charge in space. In other words, some quasi-electron particles will carry some bits of the electron, such as its rotation, but not its charge, effectively creating a neutral version of it.
Exactly what flavor of the quasi-particles works here (if any) is yet to be worked out, but researchers describe it as completely new territory not only in experimentation but in theory.
“If our interpretations are correct, we see a fundamentally new form of quantum matter,” says Wu.
“Now we imagine a completely new quantum world hidden in isolators. We may have simply failed to identify them in the last few decades.”
Neutral fermions have a potential role in improving the stability of quantum devices, so finding evidence here would be more than an academic curiosity, with promising practical applications.
It’s still the first days. But so many discoveries in science have emerged from those timeless words: “What’s going on here?”
This research was published in The nature.