2D crystalized bosons

[diogenes]

Can someone explain this for the dimensionally impared?<br /><br />http://www.gatech.edu/news-room/release.php?id=493

 

[rogers_buck]

Here's what I get out of it...<br /><br />Bosons have integral units of angular momentum and are exempted from the pauli-exclusion principal that excludes fermions form occupying the same space. In a B-E condensate, all particles behave as if they have the same wave function, so I suppose that is what as meant as the particles are sitting on top of one another occupying the same space. They don't seem to mention what they mean by boson in this article, but I think we can assume it to mean something other than a photon or a gluon (given the level of technology) and so it is probably refering to a He-4 or larger atom with integral spin.<br /><br />The B-E trap uses resonant lasers to cool the contents of the trap and to hold them in place. <br /><br />"When the repulsive interaction between the bosons is increased, they separate..."<br /><br />Now how do you change the repulsive interaction between these guys? Maybe they do a trick with the cooling lasers or apply some external field, no idea...<br /><br />Crystals are formed by fermions held in place by the energy of the covalent bonds that form the lattice. Apparently, when you increase the repulsive force between the bosons in the trap they form an orderly "free electron fermi-space (bosons style...)" with the covalent bonding energy being replaced by the energy from the trap.<br /><br />If you picture an extremely large and shallow plastic bowl in which you place a hand-full of highly static charged ping-pong balls, you get the idea.<br />

 

[diogenes]

Now, how does one get a 1D or 2 D crystal? Crystal is generly defined as <b>A homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts.</b>

 

[rogers_buck]

Post deleted by rogers_buck<br /><br />Oops, thoiught I had a double post and deleted this one in error...

 

[Maddad]

You are missing the point of Diogenes' question. If you define something as being three dimensional, then what does it mean to talk about it in one or two dimensions?

 

[rogers_buck]

I would suppose that the unit cell for a 2D crystal is constrained to the same plane and constrained to 1D for a 1D crystal. Isn't that just a special case for the simple cubic lattice?<br /><br />I don't know how far to take their crystal analogy. It might be really significant or simply a casual analogy well suited for describing what happened when they turned up the repulsion. Can't really tell from the article. When you tap on a real crystal a phonon is generated that propogates through the lattice - in their crystal a tap would be a quantum event since all of the bosons are part of the BEC? I wonder what the optical properties are?

 

[Maddad]

What makes a crystal a crystal is the arrangement of particles in three dimensions. When you have something that is only arranged in two dimensions, then you no longer have crystaline characteristics. Crystal means three dimensions. It's meaningless to speak of it in two dimensions or one.

 

[rogers_buck]

I'm not sure why you insist upon that definition. A crystal is a symetric grouping of periodic elements. You can have symetry and periodicity in 2D and 1D.

 

[thechemist]

Liquid crystals such as those used in LCD screens have periodicity in only two space dimensions. A general definition of crystallinity would allow it to be just periodicity in molecular arrangement in 1, 2 or 3D. <br />Most crystalline materials we know are 3D ionic crystals, thus crystallinity is usually percieved to imply 3D. <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>