Dropletons : New State of Matter

There was a time when states of matter were simple: Solid, liquid, gas. Then came plasma, Bose -Einstein condensate, supercritical fluid and more. Now the list has grown by one more, with the unexpected discovery of a new state dubbed “dropletons” that bear some resemblance to liquids but occur under very different circumstances.

The discovery occurred when a team at the University of Colorado Joint Institute for Lab Astrophysics were focusing laser light on gallium arsenide (GaAs) to create excitons.

Excitons are formed when a photon strikes a material, particularly a semiconductor. If an electron is knocked loose, or excited, it leaves what is termed an “electron hole” behind. If the forces of other charges nearby keep the electron close enough to the hole to feel an attraction, a bound state forms known as an exciton. Excitons are called quasiparticles because the electrons and holes behave together as if they were a single particle.

If this all sounds a bit hard to relate to, consider that solar cells are semiconductors, and the formation of excitons is one possible step to the production of electricity. A better understanding of how excitons form and behave could produce ways to harvest sunlight more efficiently.

Graduate student Andrew Almand-Hunter was forming biexcitons – two excitons that behave like a molecule, by focusing the laser to a dot 100nm across and leaving it on for shorter and shorter fractions of a second.

“But the experiment didn’t behave at all in the way we expected,” Almand-Hunter said. When the pulses were lasting less than 100 millionths of a second exciton density reached a critical threshold. “We expected to see the energy of the biexcitons increase as the laser generated more electrons and holes. But, what we saw when we did the experiment was that the energy actually decreased!” 

The team figured that they had created something other than biexcitons, but were not sure what. They contacted theorists at Philipps-University, Marburg who suggested they had made droplets of 4, 5 or 6 electrons and holes, and constructed a model of these dropletons' behavior. 

The dropletons are small enough to behave quantum mechanically, but the electrons and holes are not in pairs, as they would be if the dropleton was just a group of excitons. Instead they form a “quantum fog” of electrons and holes that flow around each other and even ripple like a liquid, rather than existing as discrete pairs. However, unlike liquids we are familiar with, dropletons a finite size, outside which the electron/hole association breaks down.

The discovery has been published in Nature. Perhaps the most remarkable thing is that the dropletons are stable, by the standards of quantum physics. While they can only survive inside solid materials, they last around 25 trillionths of a second, which is actually long enough for scientists to study the way their behavior is shaped by the environment. At 200nm wide the dropletons are as large as very small bacteria – a size that can be seen by conventional microscopes.