Physicists Discovered New State of Matter

I remember the time when states of matter were pretty simple: Solid, liquid and gas. Then came plasma state, supercritical fluid, Bose -Einstein condensate and more. Now this list of states of matter has grown by one more, with the surprising discovery of a new state dubbed “dropletons” that shows some similarity to liquids but occur under very unlike circumstances. The discovery of new state of matter occurred when a team of scientists at the University of Colorado Joint Institute for Lab Astrophysics were concentrating laser light on gallium arsenide (GaAs) to generate excitons.
Photo credit: Image of the probability of location of an electron hole in the newly discovered dropletons. The electron is located at the centre of the peak, and the heights indicate the probability of hole location. Credit: The Cundiff Group and Brad Baxley, JILA

Excitons are made when a photon strikes a material, mostly a semiconductor. If an electron is knocked loose, or excited, it leaves what is labelled as “electron hole” behind. If the forces of other charges at very close distance keep the electron close enough to the hole in order to feel an attraction, a certain state forms called as an Exciton. Excitons are also called quasiparticles because the holes and electrons act together as if they were like a single particle. If this all sounds a little hard to relate to, then just consider that solar cells are semiconductors, so the development of excitons is one likely step to the production of electricity. An improved understanding of how excitons behave and form could yield ways to harvest sunlight more proficiently. Graduate student Andrew Almand-Hunter was creating biexcitons – two excitons that behave just like a molecule, he did this by concentrating the laser to a dot about 100nm across and leaving it on for smaller and shorter portions of a second.

“But the experiment didn’t behave at all in the way we expected,” Almand-Hunter said. When the pulsations were durable for less than 100 millionths of a second exciton density extended to a dangerous 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 guessed that they had produced something other than biexcitons, but they were not sure what. They called theorists at Philipps-University, Marburg who proposed they had made droplets of 4, 5 or 6 electrons and holes, and built a model of these dropletons' behavior.

The dropletons are small enough to act 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 of it they produce a “quantum fog” of holes and electrons that flow about each other and even swell like a liquid, rather than existing as discrete pairs. Unlike liquids which we are familiar with, dropletons forms a finite size, exterior to which the electron/hole association breaks down.

This discovery has been published in Nature. Possibly the most extraordinary thing is that the dropletons are stable, by the standards of quantum physics.

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