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Quantum Fluids & Theory 
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#1
Dec3010, 10:55 PM

PF Gold
P: 795

Is there a complete theory for quantum fluids (superfluidity) now?
In this book (published 1990) it says "Because of the dominant importance of quantum mechanics for He II that fluid is called a quantum fluid. Its theory is still not completed today. Certain features can still not be accounted for quantitatively." 


#2
Dec3110, 05:36 AM

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PF Gold
P: 29,239

While this is still a forum, we try to maintain a higher standard than other forums on the net, and requires proper citation as close to actual scientific publication as possible. To answer your question, it would be rather silly that several Nobel Prizes have been given for theoretical description of superfluidity, including the one given to Tony Leggett for He3, if we don't have a proper theory for it. The term "completed" is vague. Does that fact that we don't have a complete solution to the classical 3body problem implies that classical Newtonian physics is "incomplete"? Zz. 


#3
Jan111, 03:55 PM

P: 113

It's still accurate to say that we don't have a complete theoretical description of superfluidity in He4. Superfluidity in he3 and conventional superconductivity are described well with bcs type theories



#4
Jan111, 08:42 PM

P: 981

Quantum Fluids & Theory
Also, I would strongly disagree with "conventional superconductivity are described well with bcs type theories"  those theories (as everyone is aware, I hope) are neglecting some reasonably important things for practical applications  the role of disorder, magnetism, strong phononcoupling, multiband effects, etc. BCS is useful as an educational tool, but for real things people use a "more complete" description. 


#5
Jan111, 09:24 PM

P: 113

What I mean to say, and perhaps I'm mistaken, is that BCS theory is a theory which can make quantitative predictions of the superconducting gap, for example. Is there a similar predictive quantitative theory for superfluid helium4? The twofluid model is phenomenological, so I was thinking of something more "firstprinciples" than that.



#6
Jan211, 07:36 AM

Emeritus
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PF Gold
P: 29,239

So the reference to "BCS theory" isn't simply the 1957 theory, but rather the "philosophy" (for lack of a better word) of the theory. Zz. 


#7
Jan211, 10:24 AM

P: 981

However, to nnnm4 I would like to point out that it is still be bit far to call BCS actually predicting the superconducting gap  it predicts a certain set of relations between things (e.g. superconducting gap vs. critical temperature) but since the "microscopic" input parameters are not really independently determinable, I would not say that we "understand" superconductors better than liquid He4 (or He3). 


#8
Jan1011, 10:57 AM

P: 8

I think superfluid is clear known, but quantum fluids is a much 'wide' concept.
quantum fluid= interacting atoms or electrons without long range order? 


#9
Jan1111, 04:20 AM

Emeritus
Sci Advisor
PF Gold
P: 29,239

To me, quantum fluid is a quantum manybody interaction of a correlated systems, typically electronic systems. This, therefore, covers a wide range of phenomena, and includes also manybody theories such as BCS, Fermi Liquid, etc.. etc. One can see a summary/list of such things here: http://web.mit.edu/redingtn/www/netadv/Xqufluid.html Unfortunately, in this discussion, the OP seems to have gone AWOL, so it is hard to figure out what exactly is being asked here. Zz. 


#10
Jan1111, 09:05 AM

P: 9

Thanks zz, my definition is wrong. I think what I described is "quantum liquid" rather than fluid
but i still dont know the definition of qfluid What's the difference between fluid and liquid? 


#11
Jan1111, 09:13 AM

P: 8

Sorry, I misuse my friend arielleon's account.
That's me 


#12
Jan512, 06:02 AM

P: 227

BTW, I heard somewhere that you can't predict what materials can become a hightemperature superconductor, you have to cool them down and see. (I think this was in some issue of the new scientist). Is this true, and if so, why not? 


#13
Jan612, 11:40 AM

P: 419

There are really two kinds of fundamental problems: 1) While it is perfectly possible to write down the microscopic equations which govern the electronic structures of such systems (at the atomic level), those equations are too complex and involve too many degrees of freedom to be solved with any brute force method. E.g., superconductors will involve long range order, and you cannot simply calculate the wave function of an 1mm x 1mm x 1mm block of material, because it contains too many atoms. Thus, in order to make predictions, you generally need to assume a simplified model (e.g., some kind of generalized Hubbard or Anderson model) and then determine the parameters of that model. 2) The mechanisms which can lead to superconductivity are neither known comprehensively, nor fully understood[*]. Both of those are prerequisites in order to be able to definitely tell whether and where a material will undergo a phase transition to the superconducting state. If you miss certain possible mechanism of superconductivity, you have no way of ever finding it because you don't know what to look for! [*] In practice that would mean that you need to know *all* possible physical models which can exhibit superconducting behavior, and then somehow try to understand whether a given material fits to that model. 


#14
Jan1112, 02:00 AM

P: 91

Sir Rudolf Peierls Selected Private and Scientific Correspondence Vol.2 Red selected by Minich I know the V. Ginzburg (Nobel Prize winner 2003) saying that for He4 the theory analogous to BCS does not exist. So did Feynman asked everybody, who could get formular for heat capacity of He4 near λ point, publish it (Kikuchi's he considered to be wrong)!!! As i know nobody could get formular till now (2012 january) :) Landau never recognized, that bose einstein condensation means superfluidity :) 


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