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from the (sigh)-still-no-Puerto-Ricoton dept.
amblivious writes:
"Researchers investigating the creation of biexcitons noticed an unexpected drop in energy when creating multiple biexcitons in gallium arsenide, leading to the discovery of a new state of matter; the dropleton. Excitons are quasi-particles created when a photon knocks an electron loose from a material, causing an electron hole. If the forces of other charges nearby keep the electron close enough to the hole a state known as an exciton forms where the combined electron and hole act together as though they are a single particle. Biexcitons consist of two of these quasi-particles and collectively behave like a molecule. In this discovery several excitons are behaving together in a 'quantum fog' and behave like a droplet, hence the name.
See the article in Nature for more information."
(Score: 1) by cosurgi on Tuesday March 04 2014, @12:19PM
In fact there is no nucleus in this picture. When you have 10^23 electrons and 10^23 nuclei our math & computers are too weak to cope with that. Therefore we assume that you have a sea of free (free from interaction with nucleus) electrons that occupy different energy layers. And this is true enough for this model to work, because when you take 10^23 atoms, and put them close enough (like they are in a crystal) then their outer electrons are free to move around between all the atoms that create the crystal (that is why you have conduction of electricity, and this particular crystal is called "metal" then). Then, similarly to the hydrogen atom, you have energy layers (aka. orbitals - but we call them layers, because each "orbital"/layer contains like 10^22 electrons (if we assume that there are 10 equally occupied layers)).
And then 1 electron goes from lower layer (with 10^22 occupied places) into higher layer (aka. an orbital with 10^22 vacant places), and wishes to go back to the hole that he created, but can't do that, because the probability of this happening is too low.
So there is no nucleus, just an electron in a sea of (10^23)-1 electrons, orbiting a hole. And this binary system has the same solutions as a hydrogen atom. The electron can occupy 1s, 2s, 2p, 3s, 3p, 3d orbits around this hole. And this electron-hole pair (called an exciton) is moving freely in the sea of electrons, and the formulas that describe their behavior are exactly similar as if it all was happening in a vacuum, not inside 10^23 electrons (the only physical difference is that a hole is impossible to create in vacuum).
Yeah, hard to explain :)
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(Score: 1) by gottabeme on Wednesday March 05 2014, @12:17AM
Thanks, that was great.
One question though--and if this is too deep for here, that's ok--but how do we "know" that, for example, the formulas are the same as for a vacuum? We can't observe individual electrons, right? So isn't this essentially an educated guess about what the electrons are doing, and a set of conclusions based upon logically extending other guesses/conclusions?
By the way, someone mod this guy up! (Probably too late now. :( )
(Score: 1) by cosurgi on Wednesday March 05 2014, @01:02AM
exactly. It is an educated guess. The only reason to think that it is exactly the same formulas is that the results agree with experimental measurements with great accuracy. If they stop agreeing, then it means that we need a new theory :)
We are not observing individual electrons here. But we can measure the extra energy level (actually all of them: 1s, 2s, 2p, ...) created by this exciton pair. We have spectrometers that have remarkable resolution, and they allow us to see those levels, just like we are observing those energy levels (using spectrometers also) in a hydrogen atom.
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(Score: 1) by cosurgi on Wednesday March 05 2014, @08:53AM
Oh, one more thing - in fact we know that this model is sometimes overly simplified. And when experimental results stop agreeing with this theory we know in fact that this is due to this simplification. There are more complex models too, which work when the simplest one stops working. Condensed matter physics is very difficult, because if you try to calculate explicitly 10^23 atoms - you are dead in the water - there is no enough computer memory. So then we are using periodic boundary conditions and many other tricks to reduce memory footprint.
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