MadSci Network: Physics

Re: Why does BEC work only for certain elements?

Date: Sun Mar 1 16:18:45 1998
Posted By: Samuel Silverstein, faculty, physics, Stockholm University
Area of science: Physics
ID: 886730134.Ph

This is an extremely good question, and the answer is not obvious. I'm afraid that my answer will take a little time, and at first will not seem to be directly related to your question, but please bear with me, as it will all come together at the end (I hope!)

To begin with, I gather you have read some of the excellent resources on the web on Bose-Einstein condensates. If you haven't gone to, this is one of the best introductions I have ever seen on the subject. Another good resource is .

Bose-Einstein condensates are a phenomenon unique to groups of particles with integer spins, known as bosons . According to the laws of quantum mechanics, many bosons can be in the same place at once without interfering with each other, while particles with half-integer spins (called fermions) cannot occupy the same spot, and therefore repel each other.

One example of a boson is the photon, which is the smallest unit of light, and also carries the electromagnetic force. In particle physics, we say that its "spin" equals 1. You can fit as many photons in once place as you want, which is the principle behind the laser, which concentrates a lot of photons with the same wavelength in a narrow beam.

The ordinary units of matter, including protons, neutrons, and electrons, are said to have "spin = 1/2". So, individually, they don't want to get too close to each other. However, it IS possible to add half-integer spins to turn them into integer spins. A classical example of this is Helium 4, which has 2 protons, 2 neutrons, and 2 electrons. So, if you add all of these together, you get an integer (1/2 + 1/2 = 1, etc). So although Helium 4 is made up of a bunch of spin-1/2 fermions, it is a boson! That means that if you get helium 4 cold enough, it starts to show strange properties. Very cold liquid helium is a superfluid, which means that it has exactly zero viscosity, and sound waves travel through it without any resistance, since the moving atoms in the sound wave do not interfere with the ones in their path.

Early efforts to produce a Bose-Einstein condensate (BEC) involved trying to cool down Helium-4 to extremely small temperatures. This is technically difficult, and it took a long time to get even close to the tempurature you needed to produce a BEC. Some BEC behavior was seen, but not nearly as spectacular as in the experiments that you have read about recently.

Another, phenomenon similar to superfluidity is superconductivity, in which electricity flows through certain cold materials with absolutely zero resistance. Since electrons (which have spin-1/2) are the carriers of electricity, this would seem to have little to do with the behavior of bosons! But...the way it works is this: when the superconducting material gets cold enough, the electrons inside get together in pairs. The pairs are, therefore, bosons, and can go through the superconductor without interference.

By now, you are probably wondering what all this has to do with your question. Well, we are nearly there! :-)

As I indicated before, the real problem in producing BECs is getting a bunch of atoms cold enough. In the last decade, new techniques in cooling and trapping large groups of atoms using crossed lasers allowed scientists to cool bunches of atoms to temperatures far lower than ever before. Many of the experiments so far have used elements from the first column of the periodic table (hydrogen, lithium, rubidium, sodium, etc), which are known as "alkali-metal gases". These elements are useful because their atomic spectra include visible wavelengths, which make them easy to trap and cool using visible-light lasers.

One small problem though...since they have a single electron in their outer orbital, these atoms are fermions, and not bosons! BUT...if one puts the bunch of atoms in a magnetic field and lines up the spins of all of the atoms in the bunch, the atoms will pair up with each other like the electrons in a superconductor, and form into bosons! Cool these pairs down enough in the laser trap, and you have a Bose-Einstein Condensate.

So, after all this introduction, the answer is this: to form a BEC, you have to either have a bunch of identical atoms or particles with integer spin (like Helium 4), or you have to make identical spin-1/2 atoms or particles join up into integer-spin pairs. Results have been published (to my knowledge) on Bose-Einstein condensates using rubidium, sodium, and lithium. There have been efforts to do this with hydrogen, although I an not sure whether these efforts have succeeded yet. I see no compelling reason why we will not see even more kinds of BECs in the future, as long as there are clever scientists out there to think up new ways to do things!

I hope this helps!

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