Last summer David Cassidy, a scientist at the University of California, Riverside, was busy using silicon to study positronium formation when his team noticed that the positronium, sitting on the silicon surface, didn't behave as it should have.
Setup for positronium formation with silicon at the University of California, Riverside. Image: David Cassidy
As the silicon was heated, the amount of positronium leaving the surface increased, as expected. What was surprising, however, was the fact that, even as the silicon’s temperature was hiked up, the energy of the positronium atoms being ejected from the surface didn’t increase along with it.
The positronium emission energy had nothing to do with the material’s temperature.
“The positronium energy wasn’t changing, which doesn’t make any sense if you think it’s coming from a thermal distribution, because it should obviously get hotter if you heat the target up,” Cassidy says. “That’s how we found out that something different was going on.”
Positronium is an electron-positron pair. It survives for tens of nanoseconds before the two particles annihilate each other.
In the typical recipe, positrons stuck on the surface of some material grab hold of an electron, forming positronium. If the material is heated, the positronium gets a thermal kick and leaves the surface, with an energy that depends on the temperature. The temperature dependence of thermal positronium can be easily measured.
But the positronium in the UC Riverside silicon setup was falling off the material spontaneously.
“It’s metastable, sitting there on the edge of a cliff,” says Allen Mills, head of Cassidy’s group.
What appeared to be pushing it off the cliff was not the heat itself, but a consequence of the rise in temperature: the movement of the electrons in the surface material. As the temperature goes up, the electrons, with positrons clinging to them, move differently. When the arrangement of the positronium is just right, it flies off.
“Our idea was that it has a transient existence that can be thermally activated by moving some electrons from somewhere to somewhere else,” Mills says.
Or, in other words, posits Cassidy, “It’s really the electronic activation that you’re looking at, not the positronium emission.”
That the positronium flies off with a fixed energy means that it’s in a fixed energy state when it’s sitting on the surface. If you can control the state it’s in, you can control the energy with which it’s emitted.
“It’s only held at the surface because it’s not in the right arrangement to leave,” Mills says. “It has to make arrangements, call a babysitter and stuff, to fall off spontaneously.”
The team decided to heat silicon with a laser, bringing electrons to the surface in a controlled way, readying them for take-off. With a laser, they could control the electron population and manipulate their energy states.
Using a laser also enables researchers to activate the electrons in the material without having to heat it up, meaning that positronium production doesn't have to be restricted to materials at hotter temperatures, which is typically the case. Being able to efficiently generate positronium at low temperatures opens up new experimental arenas. Cryogenic traps, for example, are not well suited to positronium experiments that require a metal at 1000 kelvins.
Mills and Cassidy are no strangers to positronium. They were the first to observe di-positronium, a two-positronium molecule, and had made plenty of it, not in silicon, but in porous films where the positronium molecules would hang out in the little voids dotting the film.
The move from porous films to the silicon made sense. Silicon, a semiconductor, is widely used in the tech industry. But beyond its use in broader applications, it turned out it was useful as part of a kind of positronium factory. For one, it’s resilient and easy to maintain. Metals, by contrast, require constant cleaning. And where the porous films, which are insulators, can not only become damaged at low temperatures but also, by nature, build up charge and discourage electron movement, silicon doesn’t. Its conducting electrons are easy for any off-the-shelf laser to bring to the surface.
“It might be a really handy way around those problems,” Cassidy said. “You don’t need any fancy lasers to create the positronium.”
Both Mills and Cassidy emphasize that their guesses as to exactly what’s happening at the silicon surface are just that – hypotheses. As the discovery happened by chance, they hadn’t planned on following the thread of this strange non-correlation between positronium energy and temperature, but they’d love to hear what others might have to say about it.
“I’d really like some theorists to weigh in on this and tell us precisely what’s going on,” Cassidy says. “There are a lot of details related to how the electrons are getting into these particular phases, what those states are and how they interact with each other – all that stuff that somebody more theoretically minded could help with.”
