Metals and insulators are the yin and yang of physics, their respective material properties strictly dictated by the mobility of their electrons – metals must conduct electrons freely while insulators hold them in place.
So when physicists from Princeton University in the United States found a quantum talk of metals bouncing around inside an insulating compound, they were lost for an explanation.
We will have to wait for further investigations to find out exactly what is going on. But an exciting possibility is that a previously unseen particle is at work, one that represents neutral ground in electron behavior. They call it a ‘neutral fermion’.
“This came as a complete surprise,”
“We asked ourselves, ‘What’s going on here?’ We do not fully understand it yet. “
The phenomenon at the center of the discovery is quantum oscillation. As the term suggests, it involves oscillating back and forth of freely moving particles under certain experimental conditions.
To become a little more technical, the oscillations occur when a material cools to levels where quantum behavior more easily dominates and a magnetic field is applied and varied.
Curvature of 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 commonly used to study the atomic landscape occupied by electrons across a material, specifically in those with metallic properties.
Insulators are thought to be a completely different boiler fish. With their electrons following strict home orders, quantum fluctuations are nothing. At least they should not be.
The team looked at tungsten ditelluride, a strange semimetal that acquires the properties of an insulator when bathed in a magnetic field – and was surprised to see quantum fluctuations.
Despite the shock, they have some thoughts about what could happen. While a liquid charge would make this insulator a conductor (which is a paradox), having neutral particles ‘flow’ would fit the insulator bill 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 charge-neutral fermions,” 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 basic particles are bosons – charge-bearing particles.
A truly neutral particle is also its own antiparticle – and this is something we have seen in bosons, but never fermions.
So finding a true neutral fermion would probably rewrite our understanding of physics, but that’s not what scientists think is happening here – instead, what they have discovered seems more of a neutral quasar particle, which is a quantum type hybrid particle.
To understand what a quasi-particle is, imagine particle physics as a study of music.
Basic particles such as quarks and electrons are individual instruments. They form the basis of a number of larger particles, from three-part rock bands as protons or symphonies as whole atoms.
Bands that are synchronized on opposite scenes can even be seen as a single event – a quasi-particle that plays as one for all purposes.
Quantum weirdness can lubricate the properties of electrons in ways that make up fractions of their charge across space. In other words, some electron quasi-particles will carry some bits of the electron, like its spin, but not its charge, effectively creating a neutral version of itself.
Exactly what flavor of quasiparticle works here (if any) has not yet been worked out, but researchers describe it as a whole new area not only in experiments but in theory.
“If our interpretations are correct, we see a fundamentally new kind of quantum material,” Wu says.
“We now imagine a whole new quantum world hidden in insulators. It is possible that we simply missed identifying them over the last many decades.”
Neutral fermions have 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 days. But so many discoveries in science have come from these timeless words: ‘What’s going on here?’
This research was published in Nature.