Iron-pnictide electron orbital pairing promises higher-temperature superconductors

May 17, 2011 by Stuart Mason Dambrot feature
Iron-pnictide electron orbital pairing promises higher-temperature superconductors
The three types of glue for superconducting electrons: lattice vibrations (top), electron spin (middle), and electron orbital (bottom). The yellow spheres represent Cooper pairs of electrons. Courtesy: Takahiro Shimojima.

(PhysOrg.com) -- The quest to develop a so-called room-temperature superconductor – one that exhibits lossless electronic transmission – has long fueled both popular and scientific imagination. At the same time, however, ongoing efforts to raise the still-frigid temperatures at which certain materials display superconductivity are making incremental progress. That research – historically based on lattice and/or spin-based interpretations of electron pairing – has now taken a potentially significant step forward thanks to a theoretical view of how electron orbital pairing in a class of materials known as ferropnictides may provide a new road to high transition temperature superconductivity.

The paper, Orbital-Independent Superconducting Gaps in Iron Pnictides, was published in the April 29, 2011 issue of Science. Lead researcher Takahiro Shimojima (affiliated with the University of Tokyo’s Institute for Solid State Physics, and the Japan Science and Technology Agency’s Core Research for Evolutional Science and Technology) notes that while high transition temperature superconductivity of up to 55K (-218C) in ferropnictides was first observed in 2008, this behavior is not predicted from standard based on lattice vibrations. Therefore, says Shimojima, an alternate explanation was needed.

(Ferropnictides – also known as Fe-pnictides and iron pnictides – are compounds classified as members of the so-called nitrogen group, a periodic table group that includes nitrogen, phosphorus, arsenic, antimony, bismuth, and ununpentium – elements with five in their outermost shell. Pnictides are binary compounds of this group. The new class of Shimojima and his team investigated has conducting layers of iron and arsenic.)

Shimojima and his team focused on what is know as the superconducting gap magnitude – the strength of Cooper pair electron pairing – in various electron orbitals. Using laser-ARPES (laser angle-resolved photoemission spectroscopy), the team determined that electron orbitals, not spin, account for ferropnictide superconductivity. They therefore concluded that electron orbitals – specifically, orbital fluctuations, interorbital pairing induced by magnetism, or a combination of the two – are a third way in which electrons form Cooper pairs.

What prompted the team to consider orbital pairing as a novel binding mechanism in HT superconductivity? “An important insight was Fermi surface orbital polarization in the anti-ferromagnetic metal phase of parent compound BaFe2As2 observed by laser-ARPES,” notes Shimojima. “It was surprising for us that its ground state is realized through orbital-dependent electronic reconstruction. We also found that this result indirectly supported the orbital ordering proposed by theoretical research. We then became curious about the effect of the orbital degrees of freedom on the material’s superconductivity.”

Laser-ARPES was the key tool in the team’s discovery. “One very important technique we developed was the variable laser polarization for examining orbital characteristics,” says Shimojima. “We also achieved high-energy resolution and bulk sensitivity in order to capture high quality data about the superconducting gaps. As a result, we were able to separate the two peak structure in the (Ba,K)Fe2As2 spectrum, which previously makes difficult to interpret ARPES data.”

Going forward, Shimojima points out, if other materials having several entangled orbitals near Fermi level are discovered, it may have the potential to show even higher transition temperature due to orbital pairing. “For example,” he adds, “if the approaches room temperature, superconducting wires for lossless electronic transportation and storage will quickly be deployed worldwide.”

And so the dream lives on.

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More information:
* Orbital-Independent Superconducting Gaps in Iron Pnictides, Published Online 7 April 2011, Published in Science 29 April 2011: Vol. 332 no. 6029 pp. 564-567, DOI: 10.1126/science.1202150
* University of Tokyo Institute for Solid State Physics
* Japan Science and Technology Agency Core Research for Evolutional Science and Technology

4.5 /5 (11 votes)

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TheQuietMan
5 / 5 (2) May 17, 2011
Numbers are always nice. They mention higher transition temperatures, but give no real data. This puts the research firmly in the range of speculation in my book. Even a 10 degree increase would be great. The current record as I understand it is 135°K, or -105°C.

Dagnabit, I want my room temperature superconductor in my lifetime!
bugmenot23
4.5 / 5 (2) May 17, 2011
[quote]The current record as I understand it is 135°K, or -105°C.[/quote]
Actually the current record id 18°C. Check out http://www.superconductors.org/20C.htm
sstritt
2 / 5 (3) May 17, 2011
[q}[quote]The current record as I understand it is 135°K, or -105°C.[/quote]
Actually the current record id 18°C. Check out http://www.superc.../20C.htm
Wonder why this wasn't big news?
shavera
not rated yet May 17, 2011
>Wonder why this wasn't big news?

Is there any peer reviewed source for the information? or is it just that one website "reporting their findings"?
TheQuietMan
5 / 5 (1) May 17, 2011
I had heard about near room temperature superconductors, but they require such ridiculous conditions (such as high pressures) they aren't even close to practical. This may be related, and if it can not be made into a wire I wouldn't consider it anything except an oddity. I would not put them into the running in the slightest. If this is them I will go with my original statement.

It is like metallic hydrogen, a worthy goal, but not there yet.
that_guy
5 / 5 (2) May 17, 2011
I checked out the link from bug and stritt. It appears somewhere between physorg and complete hack in appearance. Aside from the fact that they are obviously bots.

However - the research it cites appears to be correct - but the nuance of it is that detecting nanometer scale pockets of transition zones doesn't make it a superconductor. the current record is still -105C.

I would like to point out that I am happy with the researchers using the scientific term "higher temperature superconductors" rather than "high temp..." for a substance that only works when it is colder than walking outside in antarctica in the middle of winter.
kaasinees
1 / 5 (2) May 17, 2011
Correct me if i am wrong but arent temperature and pressure different causes causing the same effect?
that_guy
not rated yet May 18, 2011
Yes and no. Under certain circumstances, under high pressure, you can get higher than room temperature superconductivity. (Although we generally only count superconductivity at ambient pressure) But the effect is not directly relational, ratcheting up the pressure wouldn't necessarily be like lowering the temperature, and generally won't cause superconductivity. There are different things at work.
Macksb
2 / 5 (4) May 20, 2011
This result is consistent with a theory I have described in several posts on PhysOrg. All three mechanisms described above--lattice vibrations, electron spin, and electron orbital--are oscillations. Specifically, limit cycle oscillations. More than 40 years ago, Art Winfree showed that a system of limit cycle oscillations, of any type, has a tendency to synchronize. See Sync, a book by Steve Strogatz of Cornell.

Winfree's theory has never been applied to physics...but it does apply. BCS theory can be restated in Winfree's terms. Works for HTS as well, and both types of superfluids.

In fact, the quantum world is itself a world of limit cycle oscillations, and nothing else. A quantum is a limit cycle of energy. Planck's constant describes how that limit cycle is expressed. My prior posts have explained how lattice vibrations (aka phonons), electron orbits and electron spin can synchronize, individually or in combination. Cooper pairs are a double synchrony (spin and orbit).