As requested by Gokul, here it is:
J. Lee et al., Nature v.442, p.546 (2006)
Interplay of electron-lattice interactions and superconductivity in Bi_2Sr_2CaCu_2O_{8+\delta}
Abstract:Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d^2I/dV^2 corresponds to each phonon mode. For high-transition-temperature (high-Tc) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron–boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d^2I/dV^2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d^2I/dV^2 imaging studies of the high-Tc superconductor Bi_2Sr_2CaCu_2O_{8+\delta}. We find intense disorder of electron–boson interaction energies at the nanometre scale, along with the expected modulations in d^2I/dV^2. Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of ^{18}O for ^{16}O throughout the material reduces the average mode energy by approximately 6 per cent—the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.
A review of this paper can be found in both the same issue of Nature, and in Science of the same week. A short overview of it can also be found here:
http://physicsweb.org/articles/news/10/8/4/1
And now, my take on this. This work encompasses both areas of my expertise - tunneling and angle-resolved photoemission (i.e. check out the reference to the "mode" energy). It also continues the on-going battle between phonons and magnetic fluctuations as the mechanism for superconductivity in these cuprate compounded. The phonon scenario has gotten a lot of bruises lately with a number of rather interesting experimental results. However, with this paper, they seem to make a comeback - but have they?
One of the continuing issues in tunneling spectroscopy in these material is the origin of what is known as the "dip-hum structure" in practically ALL of high-Tc tunneling spectroscopy (see the arrows in Fig. 1b of the paper). This is a structure that is at an energy larger than the superconducting gap. A similar structure is seen quite clearly in ARPES measurement, especially at or near the antinodal direction of the crystal momentum space. In conventional superconductors, these structures have been extracted from the tunneling data (the d^2I/dV^2 spectrum) using the McMillan-Rowell inversion and the resulting "modes" matches exactly with the phonon modes for that material. This was one of the most convincing evidence that phonons were responsible for the superconducting mechanism in these materials.
Doing this for high-Tc superconductors isn't that easy. The phonon modes for these materials are still not that well-known. Furthermore, the material is very complicated. To be able to know of phonons are responsible, you can't just do one measurement - you need to do this for different types of phonon spectra and see if the changes in superconductivity follows that trend. The isotope effect is a good example. This is essentially what is done in this paper. They doped the high Tc superconductor with an isotope of oxygen (doping this family of high-Tc superconductors with oxygen introduces holes, which are the charge carriers in this "hole-doped" superconductors). So doping with O-18 means you are introducing a heavier hole as the charge carrier. This changes the phonons spectrum, and in particular, they found that the "mode" energy reduced by the expected amount.
[This "mode" energy is roughly the strength of the coupling between the charge carrier (in this case, the holes) and the boson that is the "force carrier". If you believe in phonons as the mechanism, then this boson is a phonon. If you believe in magnetic fluctuations, then this boson could be a spinon or a magnon. This is the QFT description of interactions in this scenario.]
So is this a slam-dunk evidence for phonons? Nope. There are two issues that are still left dangling:
1. Even by changing the doping oxygen isotope and changing the mode energy, the value of Tc doesn't change! One would expect, as in conventional superconductor, that as one changes the strength of the mode coupling, that Tc would also change. This didn't occur (which is why the isotope effect is still vague in high-Tc compounds).
2. In performing tunneling experiments in this particular material, the "cleave surface" is not the Cu-O plane (where it is believed all superconductivity is occurring), but rather the insulating Bi-O layer. So the charge carrier has to first pass through these insulating barrier. Now, there are many tantalizing evidence that when one dopes this material, not all of the oxygen actually does get doped into the Cu-O later, but rather some gets into the insulating later. The charge carriers making the tunneling adventure can be affected by such a barrier. So the signature that was seen in this paper cannot rule out the insulating layer as the origin of the effects they witnessed.
While this paper certainly gives a strong "straw" to the phonon camps, I still don't see how it can explain a series of other experimental results that it could not be consistent with. This is an on-going battle that will require other results to settle.
Edit: They have uploaded the paper to the e-print ArXiv. You may find it here:
http://arxiv.org/abs/cond-mat/0608149
Zz.
