Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

BCS theory

  1. Dec 29, 2003 #1

    Mk

    User Avatar

    What is the BCS theory?
     
  2. jcsd
  3. Dec 29, 2003 #2

    Njorl

    User Avatar
    Science Advisor

    Isn't it the formula the NCAA uses to determine who will be in the high-profile college football bowl games. Evidently, the uncertainty principle applies to it in a macroscopically noticeable way.

    Njorl
     
  4. Dec 29, 2003 #3

    Mk

    User Avatar

    It is the theory on how superconductors work but I don't know much else. Cooper Pairs, crystal latice?
     
  5. Dec 29, 2003 #4
    While it isn't a full-fledged answer to elucidate the nature of the theory, this article does give a pretty succint description of the affairs:

    http://www.nobel.se/physics/laureates/2003/public.html
    http://www.nobel.se/physics/laureates/2003/phyadv03.pdf (a more advanced version of the article, comprised of various equations).
    http://www.physics.carleton.ca/courses/75.364/mp-2html/node16.html#SECTION00017000000000000000

     
    Last edited: Dec 29, 2003
  6. Dec 29, 2003 #5
    So what's actually the most accepted theory to explain superconductivity? RVB theory?
     
  7. Dec 30, 2003 #6
    Yep, it seems the resonating valence bond (RVB) theory is indeed the most acceptable theory we have at our disposal.

    As the article itself explicitly apprise:
     
    Last edited: Dec 30, 2003
  8. Dec 31, 2003 #7
    I seem to remember Brian Green also talking about a BCS theory in his Elegant Universe. It was a theory allowing predictions on the shape of the Calabi-Yau manifolds and I think it's related to this one in superconductivity and he was also calling it Beyond C(I don't remember) States.
     
  9. Dec 31, 2003 #8
    Is possible that Green was talking about BPS states? I know that BPS states appear in string theory, but I'm not aware of any connection between BCS theory and string theory
     
  10. Dec 31, 2003 #9
    sorry, they sound the same, i have no idea about any of them so... I should have remembered that second word. what does BPS come from?
     
  11. Dec 31, 2003 #10
    BPS comes from Bogomolny-Prasad-Sommerfield, three physicists. In superstring theory, BPS states are solitons. D-branes, p-branes and extremal black holes are examples of BPS states in superstring theory
     
  12. Jan 1, 2004 #11
    So soliton theory plays an important role in String/M-theory? That's nice to know, because at the theoretical physics department at my university, they mainly focus on String and soliton theory...
     
  13. Jan 1, 2004 #12
    Soliton theory is a major driving force behind the latest advances in Superstring/M-Theory, especially nowdays. It's the prima field of research, so yeah, i guess it's pretty important :)
     
  14. Jan 20, 2004 #13

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    re: RVB theory. I do not believe it is considered as the most widely accepted theory, at least not for the high-Tc superconductors. In fact, there are a number of experimental evidence that appears to contradict RVB predictions. The current champion of RVB theory is Phil Anderson of Princeton. A preprint by him appeared recently in the e-print archive at

    http://arxiv.org/abs/cond-mat/0311467

    However, it didn't take long for dissenting views of this to appear. See

    http://arxiv.org/abs/cond-mat/0312385

    I have attended many conferences in which RVB scenario isn't "widely accepted". There are still major problems with it.

    re: BCS Theory. It is incorrect to say that BCS theory doesn't work for Type II conventional superconductors. The work of Abrikosov was on just that, the extension of BCS theory to account for these material. In fact, the BCS theory is so powerful that Tony Leggett was able to show that it also can be extended to not only superfluid He4, but also to He3. Both Abrikosov and Leggett, along with Vitaly Ginzburg won the 2003 Physics Nobel prize.

    Zz.
     
  15. Jan 20, 2004 #14

    BCS theory states that Superconductivity occurs through the formation of "Cooper" pairs: At low temperature, electrons starts to attract each other via lattice vibration (phonons) and form pairs, i.e. electron orbits around each other. Now, the "speed" of rotation of one pair is characterized by a number: L=0 (s-wave superconductors), L=1 (p-wave superconductors), L=2 (d-wave superconductor), etc...

    The BCS theory works perfectly well for so-called "conventional" superconductors which are S-wave type (L=0) and which can be Type-I OR type-II superconductors.

    The difference between type-I and type-II supercondcutors is given by their response to an external magnetic field: A type-I superconductor does NOT let any external magnetic field penetrates its body while a type-II superconductor can PARTIALLY let a external magnetic field penetrates its body.

    Now, high-Tc superconductors (which experimentally occur to be of type-II as far as their magnetic properties are concerned) seem not to be "conventional" superconductors. Their pairing seems to be of the p-wave or d-wave type. This is still included in the BCS theory.

    However, there are theoretical difficulties (in High-Tc superconductors) to determine the nature of the interaction which induces the paring of electrons: It may be that the lattice vibrations are not the right candidate for High-Tc superconductors. And the RVB model is an alternative to the BCS model. Further experimental work is required to the conclude that debate...

    NOW, the superfluidity of Helium-4 has nothing to do with the BCS theory. The superfluidity of Helium-4 (at 2 Kelvin) is due to a BOSE-EINSTEIN condensation (BEC),

    AND the BCS theory is not EXTENDED, but strictly APPLIES to superfluid Helium-3 (at 2 millikelvin). Simply the interaction leading to the pairing of helium-3 atoms is magnetic in nature and is not due to lattice vibrations which do NOT exist in liquids. Note that the pairing in superfluid helium-3 is a p-wave type (L=1).
     
  16. Jan 20, 2004 #15

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Correct. That's what I was responding to when I pointed out that it is not correct to say that BCS theory doesn't work for all Type II superconductors. I should have clarified that this does NOT include high-Tc superconductors.

    [/i]
    Now, high-Tc superconductors (which experimentally occur to be of type-II as far as their magnetic properties are concerned) seem not to be "conventional" superconductors. Their pairing seems to be of the p-wave or d-wave type. This is still included in the BCS theory.

    However, there are theoretical difficulties (in High-Tc superconductors) to determine the nature of the interaction which induces the paring of electrons: It may be that the lattice vibrations are not the right candidate for High-Tc superconductors. And the RVB model is an alternative to the BCS model. Further experimental work is required to the conclude that debate...
    [/QUOTE]

    I'm not disputing that RVB theory is an alternative. I am disputing that it is "widely accepted". Nothing that I have seen so far would indicate that the community are moving in this direction.

    [/i]
    NOW, the superfluidity of Helium-4 has nothing to do with the BCS theory. The superfluidity of Helium-4 (at 2 Kelvin) is due to a BOSE-EINSTEIN condensation (BEC),

    AND the BCS theory is not EXTENDED, but strictly APPLIES to superfluid Helium-3 (at 2 millikelvin). Simply the interaction leading to the pairing of helium-3 atoms is magnetic in nature and is not due to lattice vibrations which do NOT exist in liquids. Note that the pairing in superfluid helium-3 is a p-wave type (L=1).
    [/QUOTE]

    You are correct. I was being sloppy at using what I term as "BCS" theory. I included the cooper pairing as part of it. However, BCS theory in itself is independent of the "glue" that causes this pairing. While the BCS paper of 1957 did use phonons as the mechanism, the fundamental theory itself is adaptable to any kind of mechanism. So even if spin fluctuations end up being the paring mechanism, that is perfectly acceptable within the BCS model. That's why I pointed out the He3 case (He4 example was the brain-dead part of my posting just now). Incidentally, I believe the ruthenates also have p-wave symmetry AND where BCS theory have more successes than for the cuprates.

    Zz.
     
    Last edited: Jan 20, 2004
  17. Jan 20, 2004 #16
    I agree on that point, i.e. it is far from being "widely accepted".

    I would like to mention that it is always very useful to give references to publications (as you did in your former post). That should be more of an habit in these forum. That helps to clarify what we are talking about.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: BCS theory
  1. String Theory/M-Theory (Replies: 2)

Loading...