# Why aren't all quantum systems superfluids

Simply I was just wandering why aren't all quantum systems (F-D and BE condensates) superfluids at low temp like He 3 and 4?

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Out of curiosity, could you expand on why you might think they should be in your question? Also, are you really referring to ALL quantum systems or just FD/BE condensates? Strictly speaking as far as we know everything in the universe can be regarded as a quantum system... – joshphysics Feb 25 '13 at 23:22
well why aren't all Bose-Einstein condensates super-fluids at low types, i.e why is it only He 3 and 4, not say argon for example that become super fluids at low temps? – user21119 Feb 25 '13 at 23:28

Fermi-Dirac and Bose-Einstein condensates do indeed share many of the striking features of superfluids like liquid helium, though as wikipedia will tell you the concepts overlap but are not identical.

My favourite superfluid aspect of atom clouds is the formation of quantized vortices when they are spun: the angular momentum will go into creating many $L=\hbar$ vortices instead of the whole thing rotating together.

Isn't that just fantastic? (note also that it took until 2006 to visualize quantized vortices in helium.)

Other features make less sense. Rollin film creeping, for example, makes little sense for a typical cloud of cold atoms, as it's trapped by light and there is no container to get out of. Filtering through porous materials would destroy a cloud condensate by heating alone.

Seen from the other side, why is only helium superfluid when cooled sufficiently? There I'm less sure, but all (most?) other materials will solidify before that; I understand that helium will not freeze at low pressures as the zero-point energy of the lattice would be enough to melt it. (Wikipedia confirms this.)

Other than that, it depends on exactly what you mean by your question.

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The basic reason is that superfluidity (which also includes superconductivity) is assumed by a system when thermal de Broglie wave length ($\lambda_T$) of its particles become larger than inter-particle separation $d$ and when it happens the system should be in its fluid state. Calculations of $\lambda_T$ for the fluid state of all systems (except for liquids $^4$He and $^3$He) reveal that $\lambda_T$ falls far short of their $d$ and if we cool these systems to have an increase in $\lambda_T$, they become solid before they achieve $\lambda_T > d$. Only two liquids that are found to satisfy the said condition are liquids $^4$He and $^3$He and they become superfluid, respectively, at 2.17 K and .93 mK. Conventional theories of superfluidity are not only highly complex in their mathematical formulation but are based on certain premises that are fundamentally erroneous; this has been unequivocally proved in the following papers.

In what follows, quantum field theories of superfluidity and superconductivity (including BCE theory) or similar other theories developed by using other mathematical techniques failed to account for these phenomena completely , clearly and consistently in agreement with experiments in spite of many efforts made over the last several decades.

On the other hand a simple non-conventional theory of superfluidity explains the behaviour of superfluid $^4$He and similar systems (such as trapped dilute gases) to a very good accuracy at quantitative scale; the theory is reported in

http://article.sapub.org/pdf/10.5923.j.ajcmp.20120202.02.pdf
(Amer. J Conden. Matter Phys. 2, 32-52 (2012)

You may look into these papers for a detailed answer to every question that you have in relation to superfluidity. However, if you have any other question that is not answered here please write to me at my email address given these papers.

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Dear user30281: Are you in any way related to the author of the links? For your information, Physics.SE has a policy that it is OK to cite oneself, but it should be stated clearly and explicitly in the answer itself. – Qmechanic Sep 30 '13 at 15:09