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.
Thus, when physicists at Princeton University in the United States found a quantum peculiarity of metals bouncing inside an insulating compound, they got lost to get an explanation.
We will have to wait for other studies to know exactly what is going on. But a tempting possibility is that a previously unseen particle is working, one that represents neutral ground in the behavior of electrons. They call him a “neutral fermion.”
“This came as 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 quite understand it.”
The phenomenon at the center of the discovery is quantum oscillation. As the term indicates, it involves the rocking back and forth of free-moving particles under certain experimental conditions.
To get a little more technical, oscillations occur when a material cools to levels where quantum behaviors are more easily mastered and a magnetic field is applied and varied.
Rotating the magnetic field up and down causes unconditioned charged particles, such as electrons, to slip between the energy bands called Landau levels.
It is a technique commonly used to study the atomic landscape occupied by electrons through a material, specifically in those with metallic properties.
Insulators are believed to be another fish boiler. With their electrons following strict home stay orders, quantum oscillations are no thing. At least, they shouldn’t be.
The team examined tungsten ditelluride, which is a strange semimetal that acquires properties of an insulator when bathed in a magnetic field, and was surprised to see quantum oscillations occur.
Despite the shock, they have some ideas about what could happen. Although a fluid charge would make this insulator a conductor (which is a paradox), having a “flow” of neutral particles would fit the quantum insulator and oscillator bill, which makes more sense. .
“Our experimental results conflict with all existing theories based on charged fermions, but could be explained in the presence of charged neutral fermions,” adds fellow Pengjie Wang.
The only problem is that there should be no truly neutral fermions, according to the standard model of particle physics.
Fermions are particles that are like 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 we have seen this in bosons, but never in fermions.
So finding a really neutral fermion would probably rewrite our understanding of physics, but that’s not what researchers think here, but they think that what they’ve detected is rather a neutral quasiparticle, which is a quantum type of particle. hybrids. .
To understand what a quasiparticle 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 wide variety of larger particles, from three-piece rock bands like protons or symphonies like whole atoms.
Bands that play synchronously in opposite scenarios can even be seen as a single event, a quasi-particle that, for all intents and purposes, is being played as one.
Quantum strangeness can tarnish the properties of electrons so that they produce fractions of their charge across spaces. In other words, some electron quasiparticles will carry some bits of the electron, such as its rotation, but not its charge, effectively creating a neutral version of itself.
Exactly the taste of the quasiparticle that works here (if any) has not yet been resolved, but researchers describe it as a completely new territory not only in experimentation, but in theory.
“If our interpretations are correct, we will see a fundamentally new form of quantum matter,” Wu says.
“We are now imagining a whole new quantum world hidden in insulators. We may have simply failed to identify them over the last few decades.”
Neutral fermions play a potential role in improving the stability of quantum devices, so finding evidence of this would be more than an academic curiosity, with promising practical applications.
These are still early days. But so many scientific discoveries have emerged from these timeless words, “What’s going on here?”
This research was published in Nature.