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.
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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
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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.