Scientists have observed a new quasi-particle called a π-ton in computer simulations.
We find new particles and quasi-particles all the time. But observing and measuring them can be extremely difficult.
The π-ton is made of two electrons, two electron holes, and an activating photon.
Scientists at the Vienna University of Technology have discovered a new quasi-particle they’re calling a π-ton, or pi-ton, which they identified using computer simulations and look forward to identifying in materials going forward.
In a new paper published in Physical Review Letters, scientists explain how they began to notice a pattern in the holes they observed within samples. A hole, says lead author Karsten Held, of the Institute of Solid State Physics, is the simplest quasi-particle. “Let us imagine, for example, that many atoms are arranged in a regular pattern in a crystal and that there is a moving electron at each atom. Only at one particular atom the electron is missing—this is called a hole,” Held said in a press statement.
There are dozens of identified particles, and maybe 20 identified quasi-particles and collective excitations. Quasi-particles are arrangements of behaviors and features that emerge only inside a solid substance. Electron holes in particular could be the most famous quasi-particle, allowing electrons to pass through semiconductors, for example. A hole is both a quasi-particle unto itself and, in some combinations, part of a more complicated quasi-particle.
Held’s team has observed a new quasi-particle dubbed the π-ton, which includes two electron hole and two electrons, making it unusual and quite a big quasi-particle. And it is activated by absorbing a photon, which the team says the π-ton releases again when it disappears. π-tons in a line alternate in spin so they’re always 180 degrees opposite from their neighbors, which the team said led them to name it π-ton: 180 degrees translates to “an angle of pi, measured in radians,” said researcher Anna Kauch.
A π-ton must, therefore, have all five parts: two electrons, two electron holes, and an absorbed photon. The scientists noticed the quasi-particle while they were hunting for something else: the exciton, a known quasi-particle made of an electron and a hole together. In their simulations, they kept noticing this other arrangement. They believe that in semiconductors interacting with light, the π-ton is a whole new ballgame.
The next step is to identify the π-ton in situ instead of a computer simulation. This can be really hard, because observing and measuring any subatomic particles is fussy and specific to begin with, and quasi-particles are a product of context as much as of their specific makeup. Rudolf Grimm, a fellow Austrian doing research at the University of Innsbruck, explained it this way in 2016:
“You could picture it as a skier on a powder day. The skier is surrounded by a cloud of snow crystals. Together they form a system that has different properties than the skier without the cloud."
Quasi-particles flicker in and out of existence in a nano-blink of an eye. Scientists can observe what happens around them to some extent, or predict effects they can have, but to really watch or even measure quasi-particles in any way, researchers have to prepare special environments. Grimm’s team did this using extremely cold quantum gas in 2016, and that could be possible now for the π-ton. To go from a slowed-down computer simulation to an observable version in real life will take some clever invention.
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