Liquid helium's unusual properties

[brum]

Ok I am no expert on this, but apparently liquid Helium, when cooled to near absolute zero temperatures (below a certain value which is about 5 K give or take 1 or 2, don't have exact)

About 1 millionth of the viscosity it once had remains. This means it becomes a superfluid.

It climbs UP the sides of test tubes and displays other amazing properties.


If you have some extra information I didn't state here, I'd love to hear it.

Also, my teacher said that the only thing keeping the liquid from escaping into a gaseous state is that all of the atoms are in the same quantum state. This doesn't quite make too much sense to me, could someone spell this out please? I don't see how being in the same quantum state keeps the atoms (barely) together.

 

[damgo]

? I think your teacher was a little confused... liquids typically freeze as they are cooled, not turn to gas!

Many of the atoms do go into the same quantum state, it's called Bose-Einstein condensation -- when almost all of them are in that state, you've got a Bose-Einstein condensation.

It's hard to explain w/o advanced QM, but basically all the atoms in the same state sort of act like one big super-atom.

 

[Integral]

I read a Sci Am article about this years ago (~1977) You might want to look in a liberay for this issue (Can't rember which month but it should be '77 or if that fails '76)

As I recall you have the properties correct, and the reason is due to its Qunatum state. As I recall this can be seen as similar to "wicking" only now due to the compactness of the H atom in its lowest (or nearly so) quantum state it is able to wick through the atomic lattice of larger atoms.

This may not be the "actual" mecanism, others may have more detailed and techincally correct explanaintions.

 

[heumpje]

Well, first of all: He4 becomes a superfluid at exactly 4.217 K For He3 this is around 3.1 K.

Because one deals with bosonic particles, most people assume that helium superfluidity is due to bose-einstein condensation (BEC). This effect arises only for bosonic particles. Bosons can occupy the same state and bose einstein condensation occurs when nearly all particles are in the absolute groundstate.

This is, however, not the case. First of all, the temperature at which BEC should occur (not sure but i thought it was in the order of microKelvins) for He4 is very much smaller then the actual transition temperature.
Secondly, He3 also becomes a superfluid and since He3 atoms have half-integer spin they are fermionic. BEC cannot occur for fermionic particles.

What happens instead is not very well understood. The same thing seems to happen as in a superconductor: due to effective attractive interactions between the atoms a superfluid is the energetically favorable state.

About what your teacher said: that doesn't make much sense to me either.

 

[damgo]

IIRC, He4 is a superfluid at temperatures above T_crit because it has a significant fraction of atoms in the ground state... below T_crit they are all in the ground state and BEC occurs. Also I thought He3 superfluidity only occurred at microkelvin temps and was due to pairing of He3 atoms into effective bosons...

 

[meteor]

Heumpje, I believe that superfluidity in He4 occurs at 2.2K
The temperature of superfluidity for He3 is even more extreme: only a few thousandths of a degree above absolute zero

 

[heumpje]

Auch... I severely screwed up here ... You're 100% right of course since 4.217K is the temperature at which He4 becomes liquid. THe He3 value of 3.1 K is the same mix up. The correct transition temperatures are:

He4 2.172 K
He3 0.0025 K

(i'm not sure about this last value. The transition temperatre strongly depends on the pressure and perhaps this is the transition temp at 34 atm.)

 

[chroot]

The mechanism responsible for He-3 superfluidity and electronic superconduction is the same: Cooper pairing.

- Warren

 

[Creator]

Originally posted by brum
... my teacher said that the only thing keeping the liquid from escaping into a gaseous state is that all of the atoms are in the same quantum state. This doesn't quite make too much sense to me, could someone spell this out please?
. [/B]

Good question (above), but it doesn't seem to have been answered; Your teacher is correct: but an explanation requires a bit of length if you can bear it.[zz)]

Ist, the difference in the 'normal' He4 liquid and the low temp. BEC is due to a difference in the spin (spin angular momentum) states of the two liquids. In its 'normal' state (above critical temp.) He4 acts similar to a normal liquid with each particle (atom) (of spin 1/2) acting relatively independently where 'Fermi statistics' governs the ensomble of particles. Thus individual atoms on the surface, if given a slight boost thermally, can evaporate.

However below t(c), due to Cooper pairing, the 1/2 spins now become Bosonic with integral multiples of spin (spin 0,1) and Bose (BEC) statistics takes over allowing ALL atoms to acquire the exact same quantum energy state (eigenfunction). This unusual state in effects allows the entire fluid to act energetically 'as if' it is one atom, and no single atom can act independent without all the others being changed equivalently. This however requires a huge amount of energy, which is not available thermally from a small local interactions - thus no evaporations of individual molecules are possible until there is enough energy to push the entire system of particles back above the critical temp.

This Bosonic (BEC) state is also what is responsible for other unusual behavior of He4 like superfluidity, macroscopic quantum rotation, etc.

Creator