Meet the Dropleton—a “Quantum Droplet” That Acts Like a Liquid

Physicists have created a new composite "quasiparticle" that could help probe the quantum mechanics of many particles working together

Credit: Baxley/JILA

Part particle, part liquid, a newly discovered "quasiparticle" has been dubbed a quantum droplet, or a dropleton. The dropleton is a collection of electrons and "holes" (places where electrons are missing) inside a semiconductor, and it has handy properties for studying quantum mechanics.
The new entity is termed a quasiparticle because it is not an elementary particle, like the quarks and electrons that make up atoms. Rather, it is a composite. Like other quasiparticles, the dropleton—the first quasiparticle found to behave like a liquid—can exist only inside solid materials. "It's a particle inside matter, and it is an entity whose properties are determined by its environment," says Mackillo Kira of Philipps University Marburg in Germany, one of the co-discoverers. Quasiparticles can form in semiconductors because semiconductors' atoms are organized into a lattice by the bonding of their valence (outer shell) electrons. This arrangement allows a conglomeration of electrons and holes to effectively travel though the material as a coherent entity. Thinking about these conglomerations as quasiparticles is a way to simplify the math describing the complex quantum mechanics of many particles within a solid.
The dropleton was not predicted in advance, so its creation during the experiment came as a surprise, Kira says. It arose when researchers sent energy pulses from a superfast laser at a gallium arsenide
semiconductor. The pulses created excitons—pairs of holes—in the material. When the density of excitons reached a certain threshold, the pairs dissolved and the electrons and holes arranged themselves in new formations. Inside the particle the electrons and holes flow around one another like particles in a liquid confined within a small droplet. "It's like a quantum form of a typical liquid," Kira says. He and his colleagues reported their discovery in the February 27 issue of Nature (Scientific American is part of Nature Publishing Group).
The unexpected quasiparticle got its name when the researchers realized, "It has to be a new particle, it has a small size, it has liquid properties," Kira recalls. "Okay, let's call it a dropleton."
"This is new physics, not just a small detail of well-established physics," says Glenn Solomon of the Joint Quantum Institute in Gaithersburg, Md., who was not involved in the research. "Hopefully, it will spark a variety of experiments." In particular, the discovery could help physicists understand the quantum mechanics of "many-body systems" in which large numbers of particles interact. "The results show that interesting effects can appear in many-body systems," says Manfred Bayer of the Technical University of Dortmund in Germany, who is also unaffiliated with the research team.
In experiments the particles ranged in size but required at least four electron-hole pairs as ingredients to be stable. This characteristic puts them in a new class of quasiparticle. The research "brings quantitative support and also fundamental insight into the nature of this correlated state of few (greater than four) but not many (less than 100) electrons and holes," says Alfred Leitenstorfer of the University of Konstanz in Germany, who was not involved in the study.
Dropletons last for only about 25 picoseconds (trillionths of a second), but that makes them relatively long-lived for complex quasiparticles. They are stable enough, for example, to allow scientists to experiment on them. Such experiments, because of dropletons' size, could provide an intriguing probe into the quantum interactions of light and matter. At around 200 nanometers wide, they are more than 10 times larger than single exciton pairs and about as big as some of the smallest bacteria. The laser light used to excite the material in the experiment had a wavelength of 800 nanometers, which is not too much larger than the quasiparticles themselves. "Classical optics can detect only objects that are larger than their wavelengths, and we are approaching that limit," Kira says. "It would be really neat to not only detect spectroscopic information about the dropleton, but to really see the dropleton."

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