Iron-based superconductors exhibit s-wave symmetry

May 18, 2012 By Lori Ann White
False-color plots of the superconducting gap distribution of BaFe2(As0.7P0.3)2. The image on the left shows the superconducting energy gap approaching zero (indicated by color), but at a circular horizontal line-node. This immediately rules out the d-wave pairing symmetry, which would give four vertical line nodes in the diagonal directions. Credit: D.L. Feng, et al.

(Phys.org) -- Condensed-matter physicists the world over are in hot pursuit of a comprehensive understanding of high-temperature superconductivity, not just for its technological benefits but for the clues it holds to strongly correlated electron systems.

One important avenue of investigation is pairing symmetry. It’s a property of Cooper pairs, the bound electron pairs that are a hallmark of all superconductors, whether high-temperature or conventional. The paired electrons act as if they were a single particle, and the energy required to break Cooper pairs is measured by the superconducting gap. The symmetry of the superconducting gap, known as the pairing symmetry, is an important characteristic of Cooper pairs that is intimately related to the mechanism of superconductivity.

In conventional superconductors, the Cooper pairs have s-wave pairing symmetry, which takes the shape of a sphere. In contrast, in the cuprate family of high-temperature superconductors exhibit d-wave pairing symmetry, which looks a bit like a four-leaf clover. The leaves, or lobes, are areas where the superconducting gap is finite. At the points where two leaves join, known as nodes, the superconducting gap goes to zero.

However, iron-based superconductors do not fall nicely into either of these two categories. Some members of this group exhibit characteristics of superconducting gaps with s-wave pairing symmetry, while others show signatures of nodes where the gap becomes zero, as with d-wave pairing symmetry.

The key to resolving this discrepancy remained unknown until recently, when a team of scientists from Fudan University used an instrument  at the Stanford Synchrotron Radiation Lightsource's Beam Line 5-4 to measure the detailed superconducting gap structure of the ferropnictide superconductor BaFe2(As0.7P0.3)2. They discovered a signature that could not have originated from a d-wave pairing – a striking difference from the cuprate family.

This finding, the first measurement of its kind, provides solid experimental evidence that iron-based superconductors fall into the regime of s-wave pairing seen in conventional , and suggests that both nodal and nodeless gaps could arise from the same mechanism. This could lead to a unified theoretical framework for both phenomena, making the research an important step toward unveiling the mechanism of iron-based superconductivity.

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Macksb
2.5 / 5 (4) May 19, 2012
So...what might be the basis for a unified explanation? Here's a suggestion. In BCS theory ("old," low temp superconductivity) it is agreed that lattice oscillations--phonons--coordinate with the oscillations of electrons. Full stop there.

Suppose the same thing happens in high temp superconductivity. In cuprates, suppose a more complex set of oscillations coordinates electrons into two by two pairs, producing four way pairing, like hooves of a horse. And in the pnictides, yet another set of oscillations produces the pairing set forth above. Any set of oscillators may become synchronized under certain conditions, and such coherent behavior will produce one of several different permissible patterns.

Three particle Efimov states, which act like Borromean rings, are one of the four allowed patterns than can arise in a three oscillator system.

The concept is well developed in math and bio-math: Winfree, Kuramoto, Strogatz, Mirollo. Time to apply it to physics.

Terriva
1 / 5 (2) May 19, 2012
..what might be the basis for a unified explanation?
I wrote about it here (orgon) and here (terriva). The reason of superconductivity is the compression of electrons withing their atoms. Within conventional low-Tc I-type superconductors it's compression of s-orbitals, the electrons within high temperature superconductors of II-type are compressed with d- or even f-orbitals of neigbouring atoms. Iron based superconductors are something inbetween apparently. It means, we shouldn't expect very high Tc for iron superconductors (pnictides). This is first intuitive conclusion, which you can get from the above article.

You're apparently trying to promote the Winfrey's "coupled oscillators theory" here under different names. But this theory cannot explain, why the electrons are coupled - even the "old" BCS theory is better in it.
Terriva
1 / 5 (2) May 19, 2012
If we could compress charged electrons or protons bellow piston, we could probably achieve their superconductivity even without any other geometric constrains. Before ten years prof. J.F.Prins observed such a superconductivity. He has shoot the oxygen ions bellow surface of artificial diamond layer. The oxygen ions remained trapped bellow surface and in vacuum they attracted the free electrons to this surface. These electrons did a compact superconductive layer above surface of the diamond, which remained superconductive even above room temperature for few days. These experiments were never replicated, which is typical for every fundamental experiments (hydrogen-nickel cold fusion, Podkletnov antigravity beam, etc) of modern era. At the moment, when physics tries to ignore some experiments instead of replicate them, you should consider the trolling of physicists. It's characteristic sign of pseudo-skepticism.
Terriva
1 / 5 (2) May 19, 2012
IMO this mechanism is quite feasible and we could use it for construction of artificial superconductors and transistors. If we would apply high voltage to thin insulated wire in the vacuum, then the insulator of the wire should become covered with electrons in similar way, like the diamond layers in J.F.Prins's experiments. The insulating surface should become superconductive and this superconductivity could be even controlled with external voltage easily. The only technological problem is the choice of insulator material here - this material should become resistant to electrical breakdown. The high dielectric and mechanical strength is required here - it's probable, the compact diamond layer would be the most promising material, again.
Minich
not rated yet Jun 04, 2012
:)
Lev Landau goes to his pupils and says: There are two news, bad and good.
Good is that superfluidity and any superconductivity are explained by the SAME feature in spectrum of quasiparticles :) !!!!
Bad news from Feynman: Landau have no enough imagination to get correct spectrum of helium quasiparticles and of quasielectrons :( !!!
All the more "the SAME feature", that i suggested as superfluidity criterium for HeII, is INVALID!!! :(