Many roads lead to superconductivity

Sep 10, 2010

Since their discovery in 2008, a new class of superconductors has precipitated a flood of research the world over. Unlike the previously familiar copper ceramics (cuprates), the basic structure of this new class consists of iron compounds. Because the structure of these compounds differs from the cuprates in many fundamental ways, there is hope of gaining new insights into how the phenomenon of superconductivity arises.

In cooperation with an international research group, German researchers from Helmholtz-Zentrum Berlin (HZB) have now discovered a signature that occurs universally among all iron-based superconductors, even if the parent compounds from which the superconductors are made possess different chemical properties. Their findings are published in .

Superconductors are generally produced by "doping" so-called parent compounds, which means introducing foreign atoms into them. There is a strong correlation between magnetism and superconductivity here - both being properties of solids.

Conventional superconductors, such as those used in MRI machines in hospitals, do not like magnetism because it disturbs the interactions that lead to superconductivity within the crystal. It is quite a different story for the celebrated high-temperature superconductors, such as cuprates and iron-arsenic compounds. In these cases, the magnetic forces actually help, even promote the onset of superconductivity. These compounds feature magnetic orders which, if they occur in a , are a telltale sign that the material is suitable to be a high-temperature superconductor.

With the new iron-based superconductors, it turns out that the symmetry of a magnetic order corresponds exactly to the symmetry in the superconductivity signal.

Dimitri Argyriou (HZB) and his colleagues have produced iron-tellurium-selenium crystals and determined their using X-ray and neutron diffraction. They measured the magnetic signals in the crystals by performing neutron scattering experiments on the research reactor BER II of HZB and on the research reactor of the Institute Laue-Langevin in Grenoble.

They discovered that the symmetry of the magnetic order is significantly different from that of other iron-based parent compounds, such as iron-arsenic compounds. Yet, surprisingly, this difference has no impact on the development of superconductivity as a property. It has been detected that the magnetic signal caused by superconductivity - often referred to as the magnetic resonance - has the same symmetry as that of the magnetic order. And this is the same in all iron compounds, and apparently follows a universal mechanism that causes superconductivity for all of these materials.

Dimitri Argyriou describes this property as follows: "Going by what we know about the magnetic order of iron compounds, the iron-tellurium-selenium materials ought not to exhibit any superconductivity. But the opposite is the case: Despite the differences in magnetism, the signature of their superconductivity is the same. If we were now to understand how arises in light of different starting conditions, then we could perhaps develop materials that are superconductive at even higher temperatures."

Explore further: Scientists uncover clues to role of magnetism in iron-based superconductors

More information: Paper online: DOI: 10.1038/NMAT280

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Macksb
3 / 5 (6) Sep 10, 2010
This finding is consistent with a theory that I have posted several times on Phys Org. Click on my macksb name and see my comments.

Briefly, my theory (borrowed from Art Winfree)is that synchronized oscillations cause superconductivity under various conditions. The oscillations that synchronize vary (electromagnetic waves in pnictides, lattice phonons in BCS, etc.) Thus, in the case of the pnictides, the symmetry of a (synchronized) magnetic order will necessarily correspond exactly to the symmetry of a superconductivity signal. Synchronized oscillations produce symmetry which produces superconductivity. It's a general theory for all superconducting materials. The oscillations in question are different for the different superconducting materials, but the outcome--superconductivity--is the same. Ockham's Razor.
Sirinx
2.3 / 5 (3) Sep 10, 2010
my theory (borrowed from Art Winfree)is that synchronized oscillations cause superconductivity under various conditions.
Which conditions? These conditions are primary cause of superconductivity, not the "synchronized oscillations", after then.
Macksb
1 / 5 (2) Sep 10, 2010
Sirinx:

Thanks. I've detailed my specific thoughts in other Phys Org posts, so you may refer to them if you have time. Phys Org has a handy feature that lets you find all posts.

But let me take on the "effect, not cause" assertion that is at the heart of your comment. Yes, you might be right...synchronized oscillations might be an effect, not a cause. But I don't think so. Ockham's Razor says that a general condition found in seemingly disparate cases is likely to be a cause, since it is capable of explaining the entire category. E.g., plate tectonics. That of course is a philosophy, not a proof; but it is worthy of consideration. Particularly so in regard to superconductivity, which now has many forms--all unpredicted. Yes, almost all physicists say the various forms of superconductivity are different and thus require different fundamental explanations. My response is when does that end? When do we change course and say there might be some common denominator?
Macksb
1 / 5 (2) Sep 10, 2010
Citinex:

Glad you asked. Art Winfree described the general conditions for synchronized oscillations. His work, in the 1960's, was extended by Kuramoto in the 70's and more recently by Steve Strogatz (Cornell) (author of Sync). The December '93 Scientific American has a good popular article by Strogatz and Ian Stewart. Or if you want the math, there are some PRL articles around 2007, co-authorized by Strogatz (an applied mathematician).

Two key elements cited by Winfree are the frequency of oscillation and the proximity of the oscillators. Those two correspond nicely with temperature and pressure--the main ingredients in phase transitions. Winfree's theory, by the way, harks back to Huygens in the 1660s, and his two pendulum clocks. Below critical temps (which vary depending on the oscillations) the oscillations may extend their range, for example, and may also reach lower frequencies at which coupling is more likely. At some point, synchrony is forced.
Macksb
1 / 5 (2) Sep 10, 2010
Citinex:

Thanks for engaging in this discussion. My responses:

1. "Synchrony is orthogonal to superconductivity" (n your penultimate post). I disagree, respectfully. Take BCS. The lattice vibrations (oscillations)(phonons)synchronize to the electrons. Then Electron A and Electron B synchronize their two principal oscillations exactly antisynchronously--orbits and spins. Result: Cooper pairs, formed by three synced oscillations. That's BCS theory.

2. "The reason of superconductivity is in the compression of electrons between atoms." I disagree, respectfully. Take BCS again. The electrons that form Cooper pairs are at a considerable distance from each other.

3. "OK, how do high Tc superconductors differ from older low temp ones?" Actually, my theory is based on what I believe to be commonalities among all types of superconductivity. There are difference, of course. For the differences, please reread the 120,000 superconductivity papers in ArXiv.

Macksb
1 / 5 (1) Sep 11, 2010
Thanks for the last three posts. I appreciate the extra effort. You say "effect" and I say "cause." I dealt with that issue briefly in my response to Sirinx above. Now let me add another reason. Synchrony creates perfect organization in the group of oscillators that emit or describe the synchronized oscillations--at least in respect to the synchronized oscillations; there may be other oscillations inherent in the oscillator, of a different nature, that need to be synchronized as well to create an even higher level of order--or symmetry, to use the term associated with superconductivity. I believe that superconductivity requires perfect order, so I believe that synchronized oscillations are a cause (a necessary condition, but not sufficient) of superconductivity, not an effect. But I certainly could be wrong.

Here's another piece of evidence. BCS: s wave symmetry. Pnictide: extended s wave symmetry. Cuprates: d wave symmetry. All are associated with oscillations. Maybe cause.