whoa... composite particles can have integer spin, such as helium-4, which makes them *bosons*. An aggregate of bosons can share the same quantum state. no exclusion principle! superfluidity and bose-einstein condensates make more sense now.

@jb55 @jb55 huh, neat! I've learned about superfluid helium before in the context of de Broglie wavelengths, but I'm not sure I've seen these effects described as fermionic phenomena specifically. At first glance, this makes sense to me in terms of the exclusion principle (at least, as much as quantum things ever 'make sense'..)

I found the bits about hyperfine interactions to be interesting as well, seems to help illustrate the relative influence of the electrons/nucleus on these phenomena?


@htimsxela after doing some more reading about superfluidity I'm probably overstating the "bosonic" aspect of helium-4. fermions like helium-3 can reach superfluidity at lower temperatures.

my intuition of this is that the wavefunctions of the composite particles can exchange with each other at higher temperatures due to their symmetry... but apparently it's still not a resolved problem?

lots of interesting papers on this. reading this one now by feynman:


@jb55 (ahh, typo on my part-- fermionic/bosonic)

I have more of a background in chemistry than physics, so a lot of my intuitions come from that realm. This has made we wonder if there is a difference in superfluid temperature between He3/4, due to a difference in mass (momentum/wavelength). And sure enough... another rabbit hole!

He4 goes hyperfluid at ~2.7K, while He3 does so at ~0.0025K. Thats a large difference! Several orders of magnitude.

@jb55 the He-3 wiki article has some info on this under the 'cryogenics' section, which does specifically mention the spin effects of bosonic/fermionic systems as a contributing factor. Interestingly, it seems that He3 can go super-fluid due to a fermionic pairing effect (similar to cooper's pairs, in relation to superconductivity).


This seems to be a seminal paper on the topic, might have to sci-hub it later:

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