Physicists smash particle imitators

Physicists of all stripes seem to have one thing in common: They love smashing things together. This time-honored tradition has now been expanded from familiar particles like electrons, protons, and atomic nuclei to quasiparticles, which act like particles, but aren’t.

Quasiparticles are formed from groups of particles in a solid material that collectively behave like a unified particle (SN: 10/18/14, p. 22). The first quasiparticle collider, described May 11 in Nature, allows scientists to probe the faux-particles’ behavior. It’s a tool that could potentially lead researchers to improved materials for solar cells and electronics applications.
“Colliding particles is really something that has taught us so much,” says physicist Peter Hommelhoff of the University of Erlangen-Nuremberg in Germany, who was not involved with the research. Colliding quasiparticles “is really interesting and it’s really new and pretty fantastic.”

It’s a challenge to control these fleeting faux-particles. “They are very short-lived and you cannot take them out of their natural habitat,” says physicist Rupert Huber of University of Regensburg in Germany, a coauthor of the study. But quasiparticles are a useful way for physicists to understand how large numbers of particles interact in a solid.

One quasiparticle, known as a hole, results from a missing electron that produces a void in a sea of electrons. The hole moves around the material, behaving like a positively charged particle. Its apparent movement is the result of many jostling electrons.

The new quasiparticle collider works by slamming holes into electrons. Using a short pulse of light, the researchers created pairs of electrons and holes in a material called tungsten diselenide. Then, using an infrared pulse of light to produce an oscillating electric field, the researchers ripped the electrons and holes apart and slammed them back together again at speeds of thousands of kilometers per second — all within about 10 millionths of a billionth of a second.

The smashup left its imprint in light emitted in the aftermath, which researchers analyzed to study the properties of the collision. For example, when holes get together with electrons, they can bind into an atomlike state known as an exciton. The researchers used their collider to estimate the excitons’ binding energy — a measure of the effort required to separate the pair.
The collider could be useful for understanding how quasiparticles behave in materials — how they move, interact and collide. Such quasiparticle properties are particularly pertinent for materials used in solar cells, Huber says. When sunlight is absorbed in solar cells, it produces pairs of electrons and holes that must be separated and harvested to produce electricity.

The researchers also hope to study quasiparticles in other materials, like graphene, a sheet of carbon one atom thick (SN: 08/13/11, p. 26). Scientists hope to use graphene to create superthin, flexible electronics, among other applications. Graphene has a wealth of unusual properties, not least of which is that its electrons can be thought of as quasiparticles; unlike typical electrons, they behave like they are massless.