Monday, August 4, 2014

Solid, liquid, gas, plasma: Now a new state of matter is discovered!!

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 to create excitons.

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

This is a big deal, because solar panels are semiconductors, and 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. A laser was used to create the hole, and the laser was left on for shorter and shorter fractions of a second.

When the pulses were lasting less than 100 millionths of a second, biexciton density reached a critical threshold. Almand-Hunter expected to see the energy of the biexcitons increase as the laser generated more holes. But the energy actually decreased!

The team realized they had created something other than biexcitons.

Physics theorists at Philipps University in Marburg, Germany, suggested that the Colorado team had created droplets of four, five, or six biexcitons. They called them "dropletons".

When the density of biexcitons reached a certain threshold, the biexcitons dissolved and the electrons and holes arranged themselves in a new formation. It looks and acts like a particle (e.g., it obeys the laws of quantum mechanics). But it isn't a particle: Inside the well-defined droplet-shaped space, the electrons and holes flow around one another like particles in a droplet of liquid.

Dropletons are big, too: They are 10 times larger than a biexciton and about as big as some of the smallest bacteria.

Interestingly, dropletons exist only inside matter; they don't exist outside it and can't be separated from the matter from which they were created.

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