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Quantum droplet discovered

Electrons and holes gather to form tiny, liquidlike particle

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A never-before-seen microscopic droplet has emerged amid a flood of laser light. This quantum droplet is the latest addition to a growing population of quasiparticles: collections of subatomic particles that behave like single entities.

One way to create a quasiparticle is to fire a laser at gallium arsenide, a semiconductor used in solar panels, DVD players and light-emitting diodes (SN: 6/29/13, p. 16). The laser injects energy, causing the material’s resident electrons to jump to a higher energy state. The negatively charged electrons leave behind positively charged holes. Individual pairs of electrons and holes attract one another, creating quasiparticles called excitons.

A team including physicist Steven Cundiff of JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder, used an ultrafast laser to create excitons and push them past their breaking point. After bombarding a gallium arsenide sample with an intense laser pulse, the researchers followed up with a second, weaker pulse. By analyzing the spectrum of the latter pulse once it passed through the sample, they could tell what kind of particles had absorbed energy.

The results indicated that something other than excitons had formed within the sample. Cundiff’s colleagues at Philipps-University of Marburg in Germany looked at the data and theorized that clusters of about 10 electrons and holes had bound themselves into temporarily stable structures. Subsequent experiments at JILA confirmed the existence of this new quasiparticle, the researchers report in the Feb. 27 Nature.

The arrangement of each quasiparticle’s constituent parts is neither rigid, as in a crystalline solid, nor totally random, as in a gas. The quasiparticle falls somewhere in between, making it behave like a liquid. So Cundiff and his team named their discovery a dropleton, short for quantum droplet.

The clumping of electrons and holes interests physicists who want to understand the ways light and matter interact, just as birds flocking and fish schooling are fascinating to biologists, says New York University condensed matter physicist Daniel Turner. These complex interactions of light and matter lead to phenomena with real-world applications, he adds, including solar panels and superconductors.

Cundiff admits that he doesn’t foresee a real-world application for dropletons just yet. But he says that they are worth exploring, particularly because they appear in the commercially important gallium arsenide. Physicists could eventually exploit particle interactions to improve laser diodes that send data encoded in light through Internet and telephone cables.

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