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Is BCS low-temp superconductivity completely accepted?

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kye
#1
Nov17-13, 04:51 PM
P: 168
is the BCS theory of low temperature superconductivity completely accepted already? any doubters here? I encountered this paper that suggests the BCS is incomplete:

http://physics.ucsd.edu/~jorge/abstracts/bcs.pdf

"The most quoted reason given as convincing proof that BCS-electron-phonon theory describes conventional superconductors is the structure in tunneling characteristics detected in normal-insulator-superconductor tunneling experiments, where small wiggles in the tunneling
conductance as function of voltage match the peaks and valleys of the phonon density of states as function of frequency measured in neutron scattering experiments in several materials, most notably Pb[23–25].

I am not disputing the interpretation that the structure in the tunneling conductance reflects the phonon spectrum. As Bernd Matthias said[22], “you can’t ever stop a crystal lattice from rattling”. Even the gap of ordinary semiconductors is modulated (but not caused!) by the electron-phonon interaction and shows an isotope effect[26]. What I am disputing is the interpretation that the small modulation (few %) of the tunneling conductance spectrum by the phonons is proof that superconductivity is caused by lattice vibrations and would not
exist for infinite ionic mass."

Comments of the paper? What stuff is wrong there and which do you and do not agree?
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DrDu
#2
Nov18-13, 01:39 AM
Sci Advisor
P: 3,593
I think this paper has been discussed already. Maybe you can find the relevant thread.
ZapperZ
#3
Nov18-13, 07:56 AM
Emeritus
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P: 29,238
Quote Quote by kye View Post
is the BCS theory of low temperature superconductivity completely accepted already? any doubters here? I encountered this paper that suggests the BCS is incomplete:

http://physics.ucsd.edu/~jorge/abstracts/bcs.pdf

"The most quoted reason given as convincing proof that BCS-electron-phonon theory describes conventional superconductors is the structure in tunneling characteristics detected in normal-insulator-superconductor tunneling experiments, where small wiggles in the tunneling
conductance as function of voltage match the peaks and valleys of the phonon density of states as function of frequency measured in neutron scattering experiments in several materials, most notably Pb[23–25].

I am not disputing the interpretation that the structure in the tunneling conductance reflects the phonon spectrum. As Bernd Matthias said[22], “you can’t ever stop a crystal lattice from rattling”. Even the gap of ordinary semiconductors is modulated (but not caused!) by the electron-phonon interaction and shows an isotope effect[26]. What I am disputing is the interpretation that the small modulation (few %) of the tunneling conductance spectrum by the phonons is proof that superconductivity is caused by lattice vibrations and would not
exist for infinite ionic mass."

Comments of the paper? What stuff is wrong there and which do you and do not agree?
You have missed major parts of this.

First of all, from the conductance data, you then extract the phonon spectrum via the Eliashberg theory. THEN, you look at the strength of the phonon coupling, and then compare that with the BCS theoretical calculation of energy gap. If they match, you have a very convincing evidence that the phonon has something to do with the formation of the gap.

And not only that, you look at other material with different values of the energy gap, and you do the same thing. BCS predicts a clear relationship between these two, and that can be verified.

BTW, the BCS theory is actually independent of the coupling mechanism. You are confusing the theory with the coupling mechanism via phonons. While the BCS paper did use such phonon mechanism, it actually is generic for ANY type of bosonic coupling. That is why in the early days of high-Tc superconductors, the BCS picture was still used even when the coupling mechanism was thought to be due to spin-fluctuations!

So the title of your thread is completely in error if all you're doing is questioning if phonons are accepted as the coupling mechanism.

Zz.


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