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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: 5, Informative) by kebes on Monday March 03 2014, @03:33PM
A quasiparticle is a "collective excitation" of some other, more fundamental, particles/forces. I think the easiest one to understand is the phonon [wikipedia.org], which is a collective vibration of many atoms in a solid, which give rise to a 'localized vibration packet' that travels through the material. This becomes intuitive when you think of it in terms of sound: a sound wave travelling through air is really a bunch of gas molecules that are bumping against each other. A vibration source generates a sequence of density changes in the air, which propagate. The collective behaviour leads to a wave travelling through the air, which we call sound. In solids, phonons are the lattice vibrations that transmit sound in the solid (and in fact can be thought of as the way in which mechanical forces more generally are transmitted through the material). Coming back to quasiparticles, the crucial point is that the emergent phenomenon has properties that make it describable without worrying about (or even understanding) some of the underlying physics. So, you can model sound waves without worrying about air molecules, and even calculate their reflection, interference, etc. With phonons, we call them quasiparticles because they really do have many particle-like behaviours. They are 'effective particles', that travel with well-defined velocity, can be reflected or refracted at interfaces; these quasiparticles can even interact with each other: attracting or repelling depending on type, etc. So when physicists label something a quasiparticle, it's because the collective behaviour exhibits particle-like properties: it's a whole bunch of underlying particles that are acting in unison as a single effective entity. This not only makes it simpler to understand complex systems, but it also invariably points to some deep insights about how the system behaves.
A exciton [wikipedia.org] is a quasiparticle you get when you excite an electron in a material. The electron is knocked-out of its usual spot, leaving behind a positively-charged vacancy: an 'electron hole [wikipedia.org]'. This strongly-couple electron-hole pair (which only exists because of the excitation; i.e. energy input) has its own unique particle-like characteristics. In some cases, this exciton can separate into an electron and a hole, which are called polarons [wikipedia.org]. The 'hole' is another neat quasiparticle: it's a positive charge moving through the material, but in reality, no positive charges are moving; instead, all the negative charges (electrons) in the material are hopping around to fill the vacancy, leading to an effective motion of the positive charge. The negatively-charged polaron is also interesting: even though it's a real electron hopping around, the interaction with the medium can be thought of in terms of an 'effective electron' (whose effective mass is different from the mass of the fundamental electron particle).
Biexcitons [wikipedia.org] can arise through the interaction of two excitons. And TFA describes how they excite gallium arsenide to generate a whole bunch of excitations that all interact. Their interactions are reminiscent of a liquid (e.g. the 'pair correlation' between particle positions has the same kind of order one seems in a fluid: local preferential spacing of the particles, but long-range disroder), and so they are drawing an analogy between this collective excitation, and a small droplet of liquid.
(Score: 1) by amblivious on Monday March 03 2014, @11:49PM
Man, I'm so glad someone explained that for me ;-)
Thanks
(Score: 1) by Taibhsear on Tuesday March 04 2014, @07:45PM
This is the one part my brain was having trouble with. Thanks for clarifying. So the original electron that was excited to a higher energy level (creating the electron hole) can't just drop down to the original energy level and emit a photon (like in fluorescence, etc.) because of the sea of other electrons? And so another electron fills the first electron hole, leaving one behind themselves (the second electron hole), which yet another electron fills (creating a third hole)... Yes?
(Score: 2) by kebes on Friday March 07 2014, @01:22AM
There are various ways the excited electron can lose energy and drop back down to the ground-state, thereby eliminating both the negative and positive free charges (fluorescence being one way, thermalization being another,