Particle colliders have gotten very, very big, but some collider projects around the world are growing more ingenious just as quickly as the Large Hadron Collider is becoming more powerful. There are colliders that smash together special types of particles, or which smash together particles using only a specific type of controlling energy. But now, a team of German scientists has published a study in Nature describing their new form of collider, which can smash so-called “quasiparticles” together. With the new tool, scientists could study the interactions of things like excitons and “electron holes.”
Quasiparticles are the name we give to certain patterns of behavior in regular particles — in essence, they don’t exist. But engineering is a lot easier if we act as though they do. An electron hole, for instance, is a stable, moving area without electrons, surrounded by electrons. In reality, the hole is a lack of something, but by treating the lack of a negatively charged particle as the presence of a positively charged particle with certain special properties, we can vastly simplify certain challenges in understanding the behavior of matter on this level.
For instance, the exchange of electrons and electron holes is crucial to harvesting energy in solar photovoltaic panels — electrons are excited and move around within the cell, in turn pushing around the no-electron area and, in a certain way of looking at it, swapping places with a “hole.” Trying to work with the dynamics of such a process is almost impossible looking only at electrons. But as a mixture of electron particles and electron hole quasiparticles, each with their own sets of behavior, it makes much more sense.
There are more quasiparticles than just electron holes, however. “Excitons” arise from stable association between electrons and holes. The researchers used their new collider to test the binding energy of excitons — how much energy it takes to pull an exciton apart. Excitons are interesting in part because they can transfer energy via their electron without transferring a net electric charge, which is washed out by the associated hole.
Carbon nanotube “spasers” work by using surface plasmons.
There are quasi-particles called surface plasmons that confine photon-like particles to the surface of a material, which could revolutionize computing by allowing processing at the speed of light. Understanding the dynamics of surface plasmons will be necessary to work past the current problems with cooling and power consumption in optical computing prototypes. They live on a list with many other quasiparticles, with names like magnons and dropletons — none of which are understood as well as we’d like.
So, how do you slam an electron into the lack of an electron? The collider works by using femtosecond pulses of infrared light to create pairs of electrons and excitons in a small sample of test material (in this case tungsten diselenide), and a terahertz electric field accelerates them together at thousands of kilometers per second. In just a few billionths of a second, the quasiparticle collider tears our sample apart and smashes it back together with enough force to create measurable amount of nuclear energy.
Solar power could see rapid advancement with a better understanding of quasiparticle behavior.
The very concept of working with something that is, in reality, just an emergent property of the interactions of many other things, gets at the weirdness of modern quantum science. It shows how physicists have been at least somewhat reasonable to treat things like holes as real entities in the physical world — because that’s exactly how they behave.
It’s an exciting thing to have confirmed, as the researcher talk about wanting to move on from tungsten diselenide to graphene, a super-material that has already been used to transport surface plasmons in the past. The world’s most powerful particle colliders may be the machines that give us fundamental insight into the structure of the universe, but it could well be these more specialized laboratories that give rise to the inventions that shape the future.
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