Metals and insulators are the yin and yang of physics, their respective material properties are strictly determined 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 US found a quantum oddity of metals bouncing around in an insulating compound, they were lost to an explanation.
We will have to wait for further studies to find out what exactly is going on. But a tantalizing possibility is that a previously invisible particle is at work, a particle that represents neutral terrain in electron behavior. They call it a ‘neutral fermion’.
“This came as a complete surprise,” said physicist Sanfeng Wu of Princeton University in the US.
“We wondered, ‘What’s going on here?’ We don’t quite understand it yet. “
The phenomenon central to the discovery is quantum oscillation. As the term implies, it involves swinging freely moving particles back and forth under certain experimental conditions.
To get a little more technical, the oscillations occur when a material is cooled to levels where quantum behavior more easily dominates and a magnetic field is applied and varied.
Moving the magnetic field up and down causes unbound charged particles, such as electrons, to slide between energy bands called Landau levels.
It is a technique often used to study the atomic landscape occupied by electrons in a material, especially materials with metallic properties.
It is believed that insulators are a whole different story. With their electrons following strict orders to stay at home, quantum oscillations aren’t a thing. At least they shouldn’t be.
The team looked at tungsten ditelluride, a strange semi-metal that takes on the properties of an insulator when immersed in a magnetic field – and was surprised to see quantum oscillations happen.
Despite the shock, they have some thoughts about what could happen. While a flowing charge would make this insulator a conductor (which is a paradox), having neutral particle ‘currents’ would fit the list of insulator and quantum oscillator, which makes more sense.
“Our experimental results contradict all existing theories based on charged fermions, but could be explained in the presence of charge-neutral fermions,” added colleague Pengjie Wang.
The only problem is that truly neutral fermions shouldn’t exist, according to the Standard Model of Particle Physics.
Fermions are particles that look a bit like the ‘Lego blocks’ of matter, while the other type of fundamental particles are bosons – charge-carrying particles.
A truly neutral particle is also its own anti-particle – and this is something we’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 think is happening here – instead, they think what they detected is more of a neutral quasi-particle, which is a quantum-type hybrid particle. .
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-part rock bands as protons or symphonies as whole atoms.
Bands playing in sync on opposing stages can even be viewed as a single event – a quasi-particle that plays as a whole for all intents and purposes.
Quantum alienation can smear the properties of electrons in a way that makes fractions of their charge across spaces. In other words, some electron quasiparticles will carry some bits of the electron, such as its spin, but not its charge, effectively creating a neutral version of itself.
Exactly what quasi-particle flavor works here (if any) has yet to be worked out, but the researchers describe it as a completely new field, not just in experiments, but also in theory.
“If our interpretations are correct, we will see a fundamentally new form of quantum matter,” Wu said.
“We now envision a whole new quantum world hidden in insulators. It is possible that we have simply missed identifying them over the decades.”
Neutral fermions play a potential role in improving the stability of quantum devices, so finding evidence of one here would be more than an academic curiosity, with promising practical applications.
It’s still early. But so many discoveries in science have sprung from those timeless words, “What’s going on here?”
This research is published in Nature.