Physics Web has a story about new discoveries in excitonic systems with the eye-catching headline BEC's confound at higher temepratures. The main idea is that two exotic systems have been found in which quasi-particles undergo Bose-Einstein Condensation at realtively high temperatures-- 19 Kelvin for a system of "polaritons," and room temperature for a system of "magnons." If you're wondering why those sound like Star Trek particles-of-the-week rather than real particles that you might've heard of before, it's because they're quasi-particles involving several real particles coupled together in some way, and treated as a single entity.
I'm not commenting on this to debate the reality of the particles being condensed, though. I'm commenting on it because the opening paragraphs remind me of something that isn't quite a True Lab Story-- a Real LAb Anecdote, maybe. The relevant bit:
Two separate research groups claim to have witnessed a widespread collapse to the ground state in different quasiparticle systems, raising questions over what can really constitute a Bose--Einstein condensate.
On the surface, the definition of a Bose-Einstein condensate (BEC) appears resolutely clear: when a mass of bosons is cooled below a critical temperature, there is a phase transition wherein a significant portion of the bosons collapse into the first quantum state. Therefore it might come as a little surprise to find doubt surrounding the latest two papers to demonstrate it.
The story is below the fold, and has to do with the clarity of the definition.
Back in 1995, BEC in dilute atomic vapors was demonstrated for the first time. This was a Very Big Deal, and earned a well-deserved Nobel Prize in 2001, but it was not the first demonstration of BEC phenomena, as many sloppily-written stories about this seem to imply.
BEC-related phenomena had been known for years in condensed matter systems-- superfluid helium and various superconductors-- and there was already a huge body of literature dealing with these systems. The condensed matter systems all involve particles with very strong interactions between them (unlike the weakly interacting alkali metal vapors first condensed in 1995), which makes the theoretical treatment of these systems a very difficult problem indeed. Still, there was existing theoretical apparatus, and it was very successful at treating these systems.
I was at NIST at the time, and since we were starting to move into the BEC business, the higher-ups in the lab arranged a talk by a very eminent physicist from the condesed matter/ statistical mechanics community (I won't name him), to explain the known BEC phenomena to us. This particular fellow could hold his own in an arrogance duel with a string theorist, but there was no question that he knew the subject inside and out.
Throughout his presentation, he kept referring to the criterion for BEC as "off diagonal long-range order," which is a very precise mathematical description of what happens at the transition point using the complicated mathematical apparatus developed to deal with condensed matter systems. My advisor kept referring to the criterion as "macroscopic occupation of the ground state," more or less in line with the definition above. The speaker didn't like this much, and objected every time it came up.
After half a dozen questions in which "macroscopic occupation of the ground state" came up, he turned to my advisor and said, "Look, you need to stop thinking in terms of high school physics."
A few months later, my advisor won the Nobel Prize in Physics.
(The distinction between "macroscopic occupation of the ground state" and "off-diagonal long-range order" is important for condensed matter theory, as I understand it, because in systems with reduced dimensionality (1-D or 2-D systems), you don't see the collapse of particles into a single state, the way you do in alkali vapors. There's still a transition of some sort, though, so you need a more precise criterion for what marks the transition point.
(For those of us who work in three dimensions, "macroscopic occupation of the ground state" is a perfectly good operational definition of the transition point, and it's the experimental signature that people working with dilute vapors see. So we more or less stick with that, but whenever I hear that put forth as a definition of BEC, I think "high-school physics!")
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but...can you ALSO talk about those experiments. i just read the article today on physicsweb. they look very interesting.
the "long range off-diagonal order" can be measured by looking at correlation functions, which has been done at least indirectly by letting two parts of the condensate interfere with itself, or in the "four-wave" mixing experiments where 3 pielces of condensate interact and generate a fourth. And knowing your advisor a little, well he's an awful smart fella! I remember working with Luis once at a blackboard, and Bill walked in and saw the scribblings and summarized what we were doing in one sentence. He's kind of scary sometimes!