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FuriousGeorge
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Why doesn't BCS theory work for explaining cooper pairs in cuprate superconductors?
Foyevtsov said:BCS says that if material has magnetic ion impurities (even very small amount) than this would destroy superconductive state (because of the interactions between Cooper pairs and unfinished atoms shell). Since the energy of coupling is much less than 1 eV pairs would be uncoupled by magnetism. YBCO ( Fe-based, Heavy-fermion systems are materials with magnetic ions and some of them even magnetically ordered).
ZapperZ said:Actually, the "coupling" in BCS is quite independent of the nature of the glue. So in principle, one could have magnetic coupling, such as spin fluctuation, as the glue in a BCS-type coupling. This is why this scenario has been one of those proposed for the mechanism in high-Tc superconductors. So while it is true that magnetic impurities can be detrimental to conventional superconductors, it doesn't automatically rule out BCS-type description just because the cuprates have an antiferromagnetic ground state.
Zz.
Foyevtsov said:Agree, but for now it was only one glue was found: phonon-glue. Spin fluctuation is a theory so far (with some possible scenarios). BSC perfectly explains low temperature SC with phonons but it definitely cannot be a phonons at high temperatures (because of interactions will be too strong for coupling) and it cannot be (at least only) phonons for the low-Tc (heavy-fermions, charge transfer salts) because of the magnetic order. So, theory which will explain unconventional SC would be not a BSC (at leas biggest part of it), otherwise unconventional SC would be explained for now with some assumptions to introduce magnetism here, but it doesn't.
ZapperZ said:Actually, for electron-doped cuprates, the phonon picture works pretty well. Certainly many of the ARPES spectra seem to be in agreement with the phonon coupling.
Zz.
Foyevtsov said:Please, do not tell that you really believe that in cuprates phohons can be responsible for the pairing.
Concerning pairing (as far as I remember) Cooper (before BSC) proposed that it can be some glue and after they three together create elegant theory which hold inside this phonon-caused coupling, isn't it?
ZapperZ said:And if you had browsed the arXiv preprint yesterday, you would have noticed http://arxiv.org/abs/0808.0802" of the electron-doped compound that argued for phonons as being responsible for that observation. This is in addition to all the ARPES papers that have already been published out of Stanford and Z.X. Shen's group on the electron-doped cuprate. It is certainly premature to simply dismiss those (and several other scenarios including Anderson's RVB theory) at this point based on what we have.
Zz.
Foyevtsov said:Don't worry about the word "belief".
I've read that paper. Well, they said "most likely", means that they cannot prove (disprove) it. It sounds the same as "magnetic glue" or "spin density wave" stories, isn't it?. I am not very confident with these kind of materials, but thank you for this motivation. Unfortunately, (for this talk but not for me) I am on conference now, but later I'll look on it closer. Hear you soon.
Count Iblis said:I've read some time ago that the Casimir effect between the cuprate planes could explain superconductivity in this case, basically because it lowers the energy of the system once superconductivity sets in.
badphysicist said:What about this paper I read recently exploring the possibility of superconductivity from tunneling between copper oxide layers?
http://arxiv.org/pdf/0807.0889"
His english is a bit unpolished at parts, but it seems like an interesting idea. He claims to have calculated the pseudogap correctly for two materials so far. Any major arguments against it?
May i correct your statement?Foyevtsov said:BSC perfectly explains low temperature SC with phonons but it definitely cannot be a phonons at high temperatures (because of interactions will be too strong for coupling)
Minich said:May i correct your statement?
If we apply BCS theory to HTSC assuming phonon glue, then it is true, that BCS needs to have phonon frequencies which don't exist in cuprates. So the BCS must be incorrect for cuprates or must be modified.
1 What is wrong? Can You disclose your argument straight here. What does "the phonon half-breathing mode in the CuO plane" can make wrong?ZapperZ said:This is wrong. Read about the phonon half-breathing mode in the CuO plane:
S. Ishihara and N. Nagaosa, Phys. Rev. B. 69, 144520 (2004)
Z.X. Shen et al. Philosophical Magazine 82, 1349 (2002)
Zz.
Minich said:1 What is wrong? Can You disclose your argument straight here. What does "the phonon half-breathing mode in the CuO plane" can make wrong?
2. The main topic question is connected with BCS theory. By the way I don't found any "BCS" word in the second paper (Z. -X. Shen1,...) . Is "RVB picture in terms of the t-J models" the BCS model?
3. When anybody says about "strong" phonon coupling we have the right question him what does he mean? Is it phonon frequencies are very high or phonon amplitudes are very strong or both? BCS theory regarded maximum phonon frequencies as one of major factors for high Tc. And BCS didn't pay attention to possibility of phonon AMPLITUDE to be very strong and to consequences of such approach. But as i wrote the father of phonon picture of superconductivity Froehlich showed the example where phonon AMPLITUDE is very impotant.
4. I don't see anything "wrong" even after reading one of recommended papers and after being thought about what does it mean "wrong".
If we apply BCS theory to HTSC assuming phonon glue, then it is true, that BCS needs to have phonon frequencies which don't exist in cuprates. So the BCS must be incorrect for cuprates or must be modified.
Yes, I see. I think you have severely misunderstood me. I have opposite opinion, that in majority of known HTSC cases phonon mechanizm is the dominant one. Even opposite to BCS conclusion, that Tc can't be more than the highest phonon FREQUENCY in the sample. I pointed out to the work of Froehlich as an example of another approach, where phonon AMPLITUDE is very impotant, and the gap can greately (10,100, 1000 times,...) exceed the MAXIMUM phonon frequency in the sample, and phonon wavelength (connected with phonon frequency) must be one half of electron wavelength at Fermi surface.ZapperZ said:You claim that there are no phonon modes for the cuprates:
Those papers clearly show that there IS at least one phonon mode that can sustain the coupling strength called for with such high Tc. This has been used to justify the use of such phonon modes within the BCS theory
Zz.
Minich said:Yes, I see. I think you have severely misunderstood me. I have opposite opinion, that in majority of known HTSC cases phonon mechanizm is the dominant one. Even opposite to BCS conclusion, that Tc can't be more than the highest phonon FREQUENCY in the sample. I pointed out to the work of Froehlich as an example of another approach, where phonon AMPLITUDE is very impotant, and the gap can greately (10,100, 1000 times,...) exceed the MAXIMUM phonon frequency in the sample, and phonon wavelength (connected with phonon frequency) must be one half of electron wavelength at Fermi surface.
In my deleted message it was clearly stated, what i mean. Please consider as final my previous message. I hope it contains no misunderstanding.ZapperZ said:This is very confusing.
First you claim that there are no phonon modes that BCS can use for the cuprates. In the next breath, you then claim that the phonon mechanism is dominant in these compounds. These are highly inconsistent.
Zz.
The BCS Theory is a theory proposed by John Bardeen, Leon Cooper, and John Schrieffer in 1957 to explain the phenomenon of superconductivity. It states that at low temperatures, electrons can pair up and move through a material without resistance, creating superconductivity.
Cooper pairs are pairs of electrons that are bound together by lattice vibrations in a material. These pairs have opposite spin and a net spin of 0, which allows them to move through the material without resistance and contribute to superconductivity.
Cuprate superconductors are a type of high-temperature superconductor that exhibit superconductivity at temperatures higher than traditional superconductors. They are important in the BCS Theory as they have been shown to exhibit characteristics that align with the predictions of the theory, such as the formation of Cooper pairs.
One of the major challenges in understanding superconductivity in cuprate materials is the complexity of their electronic structures. These materials have multiple layers of atoms and different types of atoms, making it difficult to fully understand the interactions between the electrons that lead to superconductivity.
The BCS Theory explains the critical temperature for superconductivity in cuprate materials by stating that the formation of Cooper pairs is dependent on the strength of the electron-phonon interactions in the material. As the critical temperature is reached, these interactions weaken, causing the Cooper pairs to break apart and the material to lose its superconducting properties.