What is the role of Pauli's exclusion principle in superfluidity?

[Maxwells Demon]

Super fluids are fascinating. Got this from Wikipedia, but I have also read it at other sites/books..

"Superfluidity is a phase of matter or description of heat capacity, in which, supercondictivity and "unusual" effects are observed when liquids of typically helium-III or hydrogen overcome the force of gravity and friction by surface interaction"

http://en.wikipedia.org/wiki/Superfluid


What are the possibilities with this?
Comments?

 

[chuckschuldiner]

Graduate student E Kim and Prof M Chan from Penn State have published results in Nature, Science and PRL recently on the probable observation of supersolid Helium. Though these results have to be confirmed, this means that all states of matter can Bose-Einstein condense. So weird!

 

[Maxwells Demon]

please please explain more.. superSOLID? Bose-Einstein condense??

 

[chuckschuldiner]

I am not an expert but i will say whatever i know. In the case of Bose-Einstein statistics one can have as many particles (bosons) as one likes occupying a particular energy eigenstate. Contrast this with Fermi-Dirac statistics where you can have a maximum of one fermion occupying a single-particle state. Phonons are examples of Bosons while electrons are Fermions. In a degenerate bose system, the normal situation is that you have a small fraction of the total number of bosons occupying a particular mode i.e bosons are distributed evenly among the available modes. This kind of system is a normal bose fluid. However, when bosons (like say He-4 atoms) are cooled to extremely low temperatures, it is possible to have a large number of atoms in the lowest energy eigen state (as there is no restriction on the number of Bosons) and a much lesser number of atoms in the next higher energy state. Because of a large number of particles present in one single macroscopically occupied mode, that modes's quantum nature will start becoming apparent. A He-4 superfluid is a Bose-einstein condensate. Since all the particles are already at the lowest energy level, they can't decrease their energy further so they cannot dissipate any energy because of friction. So the fluid will flow without viscosity effects. If you have He-4 at very low temperature and high pressure then there is a possibility of having crystalline long-range order so that the superfluid becomes a supersolid. Hope this helps!

 

[ZapperZ]

We need to temper this with a HUGE precautionary warning here. The supersolid phenomena in He4 is still unconfirmed and debated. I highlighted a https://www.physicsforums.com/showpost.php?p=1069165&postcount=16" in Science that suggested the possibility of superfluid flow along the grain boundary of the solid He4 that would account for many of the observations.

So at this stage, we need to be careful here about this issue. I also think, in light of what the OP knows and don't know, "supersolid" should never have been brought up in here, especially when more well-verified phenomena such as superfludity and supercurrent can easily be used as examples.

Zz.

 

[Maxwells Demon]

this was the part I understood:

"A He-4 superfluid is a Bose-einstein condensate. Since all the particles are already at the lowest energy level, they can't decrease their energy further so they cannot dissipate any energy because of friction. So the fluid will flow without viscosity effects."this is the terms I'm not familiar with:

bosons
eigenstate
Fermi-Dirac
single-particle state
PhononsYou don't have to explain it all if you don't want to.. I'm only in high school and I'm from a non-English country.. It all makes it harder to understand.. :P

 

[Maxwells Demon]

ZapperZ I'm just curious.. If you can tell me some exiting and interesting stuff about superconductors or superfluidity I'm very willing to listen..

 

[chuckschuldiner]

hi ZapperZ...in hindsight, u are probably right. for some reason, i assumed OP was a grad student like me!

 

[chuckschuldiner]

Hi MD...sorry i thought you were a grad student!

You may have heard of Pauli's exclusion principle in the context of electrons in atoms which states that no two identical electrons can have the same quantum numbers. i.e n,l,m and the spin quantum number say 's' cannot all be the same for two electrons.

Fermions (which include electrons and protons) obey the Pauli's exclusion principle which states that no two fermions can occupy the same quantum mechanical state at the same time. You can think of a state of an electron in an atom as being described by the quantum numbers n,l,m and s.

On the other hand, many bosons can have the same quantum state. The Bose-Einstein condensation and the superfluidity of He-4 is a consequence of this property.