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posted by LaminatorX on Monday March 03 2014, @10:00AM   Printer-friendly
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."

 
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  • (Score: 5, Informative) by cosurgi on Monday March 03 2014, @03:45PM

    by cosurgi (272) on Monday March 03 2014, @03:45PM (#10083) Journal

    I have a pretty fresh view of that, since I just passed an exam on condensed matter physics (with best grade :). So let me tell you that exciton is a pretty amazing thingy, it is an electron-hole pair that is pretty stable. An entirely different thing than you asked. It all happens inside a crystal of GaAs. Just like diamond or silicon (which is used to make processors) is a crystal. Inside this crystal, you have about 10^23 electrons on varoius energy levels. They occupy "layers" of possible energies, just like there are orbitals in hydrogen, recall 1s, 2s, 2p thing? The same is inside a crystal, but with 10^23 electrons, not just five electrons.

    And so, sometimes an electron can get kicked off from a "layer" full of 10^23 electrons. So now you have there 10^23 - 1 electrons. Pretty complex if you try to calculate it that way, with so many electrons. Much simpler to assume that you have a full layer and 1 hole in it. This hole has therefore positive charge.

    The electron cannot go down in energy levels to fill the hole, because there is no available transtion in terms of wavefunctions (Shrodinger equation is involved here). So in positional coordinates they both overlap each other and coexist together. And they are attracted to each other because hole has positive charge, and electron has negative charge. The system has exactly the same hamiltonian and solution to Shrodinger's equation as if you were solving the hydrogen atom (a two particle system: in hydrogen it is proton and electron). Only the masses to plug into equation are different (the hole has an effective mass, not a real mass). But in fact it is more close to the positronium, because the effective masses of both compounds are roughly of the same order of magnitude (more interestingly you get a similar energy structure when solving a two-quark system).

    So this dropleton are two negatively (electrons) and two positively (holes) charged particles, that cannot annihilate each other. Indeed that could be a molecule. I can see that indeed such a configuration would have lower energy and therefore might be stable. I am not sure what they can make of that. maybe exploit the fact that it is a boson, and infinitely many such dropletons can coexist at the same energy level, and they don't collide with each other. This could lead to some superconducting pssibilities. But it depends how many such dropletons you could make, and if you could force them to exchange (pairs of holes)/(pairs of electrons) between each dropleton, becuase only then the current could flow, I guess.

    So you get a picture. This dropleton is something that happened when two electrons got kicked up into higher energy layer, from a layer full of 10^23 electrons, and those two electrons are attracted to the holes that they created, but cannot go back (for long time), because the transition has near zero probability.

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  • (Score: 2) by NovelUserName on Monday March 03 2014, @09:10PM

    by NovelUserName (768) on Monday March 03 2014, @09:10PM (#10233)

    I think what the parent was asking is : "why is this a new class of matter, rather than an interesting subclass of existing matter" The reason for this question is that, to the layman, the exiton sounds like its a special subclass of matter with excited electrons- which aren't new forms of matter. Rather these are energized forms of existing matter. So the question is: why is an exiton special enough that we stop calling it an excitation state, and start calling it a new type of matter.

    I at least have trouble extracting the answer from your post. Is it simply that the exiton is stable compared to the normal means by which electrons change orbitals?

    • (Score: 1) by cosurgi on Tuesday March 04 2014, @12:34PM

      by cosurgi (272) on Tuesday March 04 2014, @12:34PM (#10573) Journal

      Honestly I am not sure if it is rightful to call this a "new state of matter". Maybe, and the reasoning could be following: the equations that can be used to describe exciton are exactly the same that you use to describe a single hydrogen atom in vacuum. The physical difference is that it all happens not in vacuum but in a sea of 10^23 electrons, and the hole's mass (and to be precise, electron's mass also) is a so called "effective mass".

      So this is what makes it special: hydrogen atom is the simplest possible system, which has well known solutions. And you can do similar experiments, measurements & predictions on exciton to what you can do with hydrogen atom.

      If they somehow manage to create 10^22 dropletons inside a crystal, then I would agree that this is a new state of matter. Because their properties would be very interesting, and maybe bizarre. Especially because they would be allowed to occupy the same energy levels (bosons). It could be similar to something what a high-temperature Bose-Einstein condensate might be if it were allowed to exists in high temperatures. But would be different enough from Bose-Einstein condensate to call it a new state of matter because: 1) high temperature, 2) inside a crystal.

      A word about "high temperature". For metals, and electrons flowing there a high temperature is 50000K, but for superconductors high temperature is 50K. For this dropleton state of matter I would expect high temperature to be rather on order of 10K, but that's just a guess.

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      • (Score: 1) by cosurgi on Tuesday March 04 2014, @07:20PM

        by cosurgi (272) on Tuesday March 04 2014, @07:20PM (#10885) Journal

        actually the more I think about that the more I am inclined to suspect that "high temperature" for dropletons might be around 1000K. But this is the first time I heard about them, so you can see how wild is are my guesses. If it's indeed 1000K, then it could be really interesting to investigate and may yield some useful applications.

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  • (Score: 1) by gottabeme on Tuesday March 04 2014, @07:02AM

    by gottabeme (1531) on Tuesday March 04 2014, @07:02AM (#10479)

    I'm trying to picture this. I'm guessing the "hole" is not literally a particle. So I'm imagining an electron orbiting around a point in space, but not orbiting the point directly, rather a point offset by an imaginary pair particle, and the point of the "binary orbit" itself orbiting around the nucleus.

    Is that a reasonable way to visualize it?

    • (Score: 1) by cosurgi on Tuesday March 04 2014, @12:19PM

      by cosurgi (272) on Tuesday March 04 2014, @12:19PM (#10568) Journal

      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

        by gottabeme (1531) on Wednesday March 05 2014, @12:17AM (#11075)

        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

          by cosurgi (272) on Wednesday March 05 2014, @01:02AM (#11088) Journal

          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

            by cosurgi (272) on Wednesday March 05 2014, @08:53AM (#11220) Journal

            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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