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High Temperature Bose-Einstein Condensation Observed
ultracool writes "Two separate research groups claim to have observed Bose-Einstein condensation (BEC) in quasiparticles at much higher temperatures than atomic BEC — one at 19 Kelvin and the other at room temperature. The 19 K BEC was composed of half-matter, half-light quasi-particles called polaritons, and the room temperature condensate was composed of 'magnons' (packets of magnetic energy). There is some skepticism among physicists as to whether these really are BECs. If they are true BECs, these experiments are the first evidence of them in the solid state." Just in case you need a brush up on BEC, like I did, check out the Wikipedia article on Bose-Einstein condensation.
I thought Bose-Einstein condensate was a completely different state of matter. How then, could it appear in a "solid state"?
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Good question. And damn hard to explain in terms that don't sound insane to the layman
Thing is, the condensed particles here aren't the particles that make up the solid. They're not quite real particles, even. They're so-called quasiparticles, which are a fancy way condensed-matter physicists have of describing what the rest of us call "interactions". Each interaction has its own kind of quasiparticle, (and some silly name ending with -on) and they're basically described just like real particles are. The trick is you can describe the system in terms of these virtual particles instead of the real ones and simplify the problem.
To give an analogy, you could think about a bubble moving through some liquid. The bubble isn't actually a real particle - it's just the overall effect of a bunch of gas molecules pressing and bouncing against the liquid molecules. But thinking of it as just a "bubble particle" is a lot simpler.
Anyway. So the condensate here isn't made up of the solid's atoms. It's made up of quasiparticles. And this is why there's some debate on whether this should be called a BEC or not. On one hand, they can, and do have coherence here. On the other hand, they're just not really real!
But it's also pointed out they're extremely short-lived. It's indeed questionable if you can call something a BEC if it's short-lived, because a BEC is supposedly a low and stable state. (So the question becomes "How stable should it be to be a BEC?") But regardless of that, it's no less interesting.
My guess is, people will probably continue to call every BEC-like kind of condensate a BEC. When the need arises to distinguish the two, they'll have to invent a new term for that context, like "quasiparticle condensate" or something.
Let me attempt a hopefully-understandable explanation. I'm a graduate student in experimental condensed-matter physics.
You can think about it in a coneptually-easier way by thinking about vibrations, which is more intuitive. The simplest model in which to think about vibrations would be in one dimension. Imagine you have a collection of some equal masses, equally spaced, with equal springs between each of those masses. If you excite the system anwhere (ie, push some of the masses), it will vibrate throughout the whole system because each 'atom' is coupled through the springs. The individual excitations of such a system would be the collective 'modes' of oscillation of the system. A mode is a specific oscillation that once set up will continue uninterrupted (without friction). For a simple one-dimensional system like the modes would be a sinusoidal oscillations of the system, where the wavelength of each mode would be the twice the length of the 'crystal' divided by an integer. See the wiki page on the Normal Mode with a cute animation.
You can extend this to three-dimensions by considering a three-dimensional grid of massive atoms, connected by springs. Real crystals don't have to be cubic, they can have a number of various arrangements (hexagonal, trigonal, diamond structure), and the effective spring constants can be different in different directions. But N masses, in 3 dimensions, will have 3N distinct modes. What's important to see is that each mode would have its own frequency, and wavelength, and typically the speed of propagation of each mode doesn't have to be the same. Also of note is that each mode has its own energy.
If you now consider a real crystal, and apply these same concepts but within the realm of quantum mechanics, you get a similar result, but each 'mode' now becomes a 'quanta' of lattice vibration. These vibration quanta are called phonons, which are bosons (they have spin 0, and bosons have integer spin). Even a small chunk of crystal will have on the order 10E23 atoms, so this is a huge number of allowed quantizations, and they can be thought of as a continuum. Each allowed 'mode' will again have its own frequency, wavelength, and energy. If you have a chunk of crystal at any non-zero temperature, any of the modes above the ground state (the ground state is the mode with the lowest allowed energy) can be 'occupied' with a finite probability. As you approach zero temperature, the probability of any mode above the ground state being occupied approaches zero.
A Bose-Einstein Condensate refers to an effective phase transition that happens as you cool the system and it becomes harder to excite the higher energy states as system becomes highly occupied in the ground state. There is a phase transition, the presence of which can be manifested by different qualities in things like specific heat, magnetization, magnetic susceptibility, etc. The crystal is still a solid crystal per-se (meaning it has a well-defined atomic ordering) but the occupations of the various modes of the system will drastically change, building into a near divergence at the ground state.
In the 'magnon' case as mentioned in TFA, you can think of it like phonons described above, but instead of two atoms exchanging vibrational energy, they are exchanging magnetic energy. Each electron is a spin-1/2 dipole (a fermion, not a boson), and there are interactions between two neighboring spins. Spin interactions are highly model dependent, meaning the types of atoms and shape of the crystal has huge impact on the interactions, which is why some materials are magnetic and some are non-magnetic. If you quantize the magnetic interactions you get spin-waves or magnons, similar to the sine-wave vibrational modes of the lattice above except the direction of the spin-moment changes instead of the atom displacement in the lattice.
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Sorry that I'm unable to boil all of quantum mechanics and solid-state physics into a single easily-comprehensible slashdot posting, while spending a maximum of 15 minutes writing it. I included a few mentions to wikipedia (eg on modes) to aid you, and also quoted certain terms for you to look up on your own. Any quoted terms below, please look up yourself if you don't understand. This post can hopefully get you started. But I can't believe I'm being criticized for spending my own time trying to help someone entirely unfamiliar with the field understand something.
A mode is the collective motion of the atoms in the crystal, not a single frequency. A mode will oscillate at a specific frequency, however. If you write the 'equations of motion' for all atoms in the crystal in 'matrix' form, the modes would correspond to the 'eigenvalues' of that matrix. I'm sure these sentence will confuse you, but again, I can't boil linear algebra anad its application to mechanics down into a few understandable sentences to be comprehended in only a few minutes. f I tried to go too basic into all the details that post would evolveee into a textbook sized tome.
So a crystal will have several different modes. This is very much like quantum mechanics, where energy states are quantized, and each so-called 'eigenstate' has a specific 'wavefunction' associated with it. These oscillatory modes are called 'phonons', which are 'bosons'. The 'magnons' referred to in the articles are different modes. In those cases it's not vibrations they're 'quantizing' but magnetic interactions. The electrons on the atoms in the lattice are tiny 'magnetic dipoles', which can rotate, interact with magnetic fields, interact with other nearby electrons, etc. Again, if this paragraph confuses you then look up the terms in quotes.
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