Iron-based high-temp superconductors show unexpected electronic asymmetry

Jun 20, 2012
This image shows a microscopic sample of a high-temperature superconductor glued to the tip of a cantilever. To study the magnetic properties of the sample, scientists applied a magnetic field and measured the torque that was transferred from the sample to the cantilever. Credit: Shigeru Kasahara/Kyoto University

Japanese and U.S. physicists are offering new details this week in the journal Nature regarding intriguing similarities between the quirky electronic properties of a new iron-based high-temperature superconductor (HTS) and its copper-based cousins.

While investigating a recently discovered iron-based HTS, the researchers found that its were different in the horizontal and vertical directions. This electronic asymmetry was measured across a wide range of temperatures, including those where the material is a superconductor. The asymmetry was also found in materials that were "doped" differently. Doping is a process of chemical substitution that allows both copper- and iron-based HTS materials to become superconductors.

"The robustness of the reported asymmetric order across a wide range of chemical substitutions and temperatures is an indication that this asymmetry is an example of collective caused by quantum correlation between electrons," said study co-author Andriy Nevidomskyy, assistant professor of physics at Rice University in Houston.

The study by Nevidomskyy and colleagues from Kyoto University in Kyoto, Japan, and the Japan Synchrotron Radiation Research Institute (JASRI) in Hyogo offers new clues to scientists studying the mystery of high-temperature superconductivity, one of physics' greatest unsolved mysteries.

Superconductivity occurs when electrons form a that allows them to flow freely through a material without . The phenomenon only occurs at extremely , but two families of layered -- one based on copper and the other on iron -- perform this mind-bending feat just short of or above the temperature of liquid nitrogen -- negative 321 degrees Fahrenheit -- an important threshold for industrial applications. Despite more than 25 years of research, scientists are still debating what causes high-temperature superconductivity.

Copper-based HTSs were discovered more than 20 years before their iron-based cousins. Both materials are layered, but they are strikingly different in other ways. For example, the undoped parent compounds of copper HTSs are nonmetallic, while their iron-based counterparts are metals. Due to these and other differences, the behavior of the two classes of HTSs are as dissimilar as they are similar -- a fact that has complicated the search for answers about how high-temperature superconductivity arises.

One feature that has been found in both compounds is electronic asymmetry -- properties like resistance and conductivity are different when measured up and down rather than side to side. This asymmetry, which physicists also call "nematicity," has previously been found in both copper-based and iron-based high-temperature superconductors, and the new study provides the strongest evidence yet of electronic nematicity in HTSs.

This is Shigeru Kasahara, the study's first author, with the cryogenic apparatus used in the experiments. Credit: Kyoto University

In the study, the researchers used the parent compound barium iron arsenide, which can become a superconductor when doped with phosphorus. The temperature at which the material becomes superconducting depends upon how much phosphorus is used. By varying the amount of phosphorus and measuring electronic behavior across a range of temperatures, physicists can probe the causes of high-temperature superconductivity.

Prior studies have shown that as HTS materials are cooled, they pass through a series of intermediate electronic phases before they reach the superconducting phase. To help see these "phase changes" at a glance, physicists like Nevidomskyy often use graphs called "phase diagrams" that show the particular phase an HTS will occupy based on its temperature and chemical doping.

"With this new evidence, it is clear that the nematicity exists all the way into the superconducting region and not just in the vicinity of the magnetic phase, as it had been previously understood," said Nevidomskyy, in reference to the line representing the boundary of the nematic order. "Perhaps the biggest discovery of this study is that this line extends all the way to the superconducting phase."

He said another intriguing result is that the phase diagram for the barium iron arsenide bears a striking resemblance to the phase diagram for copper-based high-temperature . In particular, the newly mapped region for nematic order in the iron-based material is a close match for a region dubbed the "pseudogap" in copper-based HTSs.

"Physicists have long debated the origins and importance of the pseudogap as a possible precursor of high-temperature superconductivity," Nevidomskyy said. "The new results offer the first hint of a potential analog for the pseudogap in an iron-based ."

The nematic order in the barium iron arsenide was revealed during a set of experiments at Kyoto University that measured the rotational torque of HTS samples in a strong magnetic field. These findings were further corroborated by the results of X-ray diffraction performed at JASRI and aided by Nevidomskyy's theoretical analysis. Nevidomskyy and his collaborators believe that their results could help physicists determine whether electronic nematicity is essential for HTS.

Nevidomskyy said he expects similar experiments to be conducted on other varieties of iron-based HTS. He said additional experiments are also needed to determine whether the nematic order arises from correlated electron behavior.

Nevidomskyy, a theoretical physicist, specializes in the study of correlated electron effects, which occur when electrons lose their individuality and behave collectively.

"One way of thinking about this is to envision a crowded stadium of football fans who stand up in unison to create a traveling 'wave,'" he said. "If you observe just one person, you don't see 'the wave.' You only see the wave if you look at the entire stadium, and that is a good analogy for the phenomena we observe in correlated electron systems."

Nevidomskyy joined the research team on the new study after meeting the lead investigator, Yuji Matsuda, at the Center for Physics in Aspen, Colo., in 2011. Nevidomskyy said Matsuda's data offers intriguing hints about a possible connection between nematicity and high-temperature superconductivity.

"It could just be serendipity that nematicity happens in both the superconducting and the nonsuperconducting states of these materials," Nevidomskyy said. "On the other hand, it could be that superconductivity is like a ship riding on a wave, and that wave is created by electrons in the nematic collective state."

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User comments : 22

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3 / 5 (2) Jun 20, 2012
Oooo I've got a brand new Iron HTS, you've got nematicity,
I think that we should get together and make electrons free!
I've been looken around awhile, that Nobel Prize is for me,
Cuz I've got a brand new Iron HTS & you've got nematicity!
1 / 5 (3) Jun 20, 2012
The electrons are known to form superconductive stripes within superconductors. Therefore if we take a sufficiently small sample formed with monocrystal, we can observe some anisotropy. I'd rather say, the experimental arrangement of this study was motivated just with the effort to find such a "surprise". I tried to explain the "pseudogap" phase here.
1 / 5 (2) Jun 20, 2012
So might this produce some sort of thermoelectric material? Wouldnt that be nice - efficient solid state heat to electricity.
1.3 / 5 (3) Jun 21, 2012
This article should be read in conjunction with a PhysOrg article dated June 11 entitled "Theory Unifies Superfluids and Other Weird Materials," describing work by Prof. Murayama and grad student H. Watanabe (Cal). That article discusses collective behavior, such as that of spins or phonons, as agent behind spontaneous symmetry breaking. The above article focuses on "collective electronic behavior," finding significant similarity between the two high temp superconductors--cuprates and iron-based. Same idea.

I have made many posts in which I propose a theory of coupled or synchronized oscillators, based on work done by Art Winfree in the late 1960s. See article by Steve Strogatz and Ian Stewart, Scientific American Dec. 1993, "Coupled Oscillators and Biological Synchronization." You can Google it on the web and get a copy. Oregon State website.

Simply take Art Winfree's theory and apply it to physics, a world replete with quantum (periodic) oscillations. They self-organize.
not rated yet Jun 21, 2012
Let me get this straight, the asymmetry must be due to gravity (aka vertical) right?
1 / 5 (1) Jun 21, 2012
I hope, they rotated the magnet, not the sample. I presume, the superconductor monocrystals exhibit directional Meissner effect, because they contain hole stripes oriented by some crystal axis preferably.
1 / 5 (1) Jun 21, 2012
Note in this connection Colin Humphreys theory:

Humphreys believes that existing theories fail because they do not take into account the distribution of the holes. He argues that each copper-oxide plane consists of square "nanodomains", separated by channels that are one unit-cell wide - rather like a grid of streets surrounding blocks of houses. Holes at the edges of adjacent blocks are magnetically paired, he says, and superconductivity occurs because these hole-pairs march collectively along the channels, like trams on pairs of tramlines running between the blocks of houses. There is one hole on each tramline, according to the model, and the pairs of holes move down the channels, hopping from oxygen to oxygen via adjacent copper sites.
not rated yet Jun 21, 2012

superconductors typically are crystalline in composition, and have alternating layers with different chemicals some the odd layer has one set of molecules and the even layers have a different set of molecules. Vertical means traversing the layers -- while horizontal means running along the plane of the layers.

nothing to do with gravity as you could turn it on its side and you would still describe it with vertical and horizontal.

in the words of Ender "which way is down?" and everyone knows to point toward the door.
1 / 5 (1) Jun 21, 2012
TK Click: Interesting to see your support for Colin Humphreys. While I have a different take, please note that my theory--that oscillators synchronize their oscillations in one or more Winfree patterns, which results in self-organization--is something that you might wish to endorse. Humphrey critics say "how do these holes organize just so?" Same questions as to Humphreys' trams, tramlines and nanodomains. Art Winfree showed us how and why self organization occurs in biology. I adapt his law to physics.

Has Humphrey updated his 1997 theory to embrace pnictides as well? Or is his theory still unique to cuprate HTS? Under my theory, I believe that the holes self-organize, along with other ingredients, and do so according to Winfree's law. As far as I know, there is no other "law of self-organization" that is remotely satisfactory, even as to cuprates considered in isolation. And certainly there is no other law of self organization that works for all the "supers." Mine does.
1 / 5 (1) Jun 21, 2012
I explained you many times, that the concept of synchronized oscillations of electrons is covered with Cooper pairs in BCS theory, where they synchronized with lattice vibrations (phonons). You're reinventing wheel in this extent.
I believe that the holes self-organize
The "hole" and "doping" concepts in superconductor science have a slightly different meaning, than in the semiconductor physics. The holes are essentially oxidized atoms (atoms with some electrons removed), so they cannot migrate in lattice. In semiconductors the holes are places without electrons and they can more arbitrarily when they're filled with electrons.
1 / 5 (1) Jun 21, 2012
Nice to hear from you Terriva. This article confirms my theory of synchronized oscillators, which provides a common theory to explain the supers and other phases of matter. Specifically, the article says that the cuprates and this particular pnictide have strikingly similarities. I predicted that. This article says that the key agent is "correlated electrons that lose their individuality and behave collectively." That is precisely my theory. The authors even use the Stadium Wave analogy, which I used last week, as you know. They also find a possible pseudogap phase in this pnictide, which is consistent with my theory, and they say that "superconductivity could be like a ship riding on a wave and that wave is created by electrons in the nematic collective state." That is precisely my theory of synchronized oscillators.

All of this is why you found yourself compelled to dismiss the article and suggest that the authors prearranged their result.
1 / 5 (1) Jun 21, 2012
So far I have addressed two of four "supers." My foregoing post shows how this article and my theory dovetail precisely in suggesting a common explanation for cuprate and pnictide superconductivity.

Last week's article "Theory Unifies Superfluids and..." provided great support for my theory specifically as to superfluids and generally as to other quantum phases. See my comments there.

That leaves BCS theory, which is an easy case. Phonons and electrons interact, synchronizing their oscillations to self-organize, producing Cooper pairs. That is my theory. This fits so perfectly that you say I have "reinvented the BCS wheel." That is a virtue, not a vice.

If my broad theory is correct, it must accommodate BCS theory. My argument is that all of these supers (and more) share a common self organizing mechanism--namely, synchronized or coupled oscillations. So when you attack my theory as "reinventing the BCS wheel," you validate my work. I thank you for that.
3 / 5 (2) Jun 22, 2012
Phonons and electrons interact, synchronizing their oscillations to self-organize, producing Cooper pairs. That is my theory.
You should ask for Nobel price after then, as an anonymous poster you cannot get a recognition here, so you're off topic here with it...;-) It seems for me, you just learned about BCS and because it fits your fuzzy ideas about superconductivity, you're trying adopt it. But the fact, you named some particular aspect of more general theory doesn't mean, you invented all of it. The BCS theory is fifty years old and it's developed with full of math. Nobody will care about you and you have no chance with your silly approach. BTW all interactions in physics manifests with some synchronized vibrations, but this synchronicity is not a reason but a consequence of these interactions, which could have a way richer manifestations.
3 / 5 (2) Jun 22, 2012
The fact, the synchronized vibrations aren't reason, but a consequence of some deeper mechanism is the reason of fact, you cannot predict anything with Winfree's "coupled oscillator" model, only to "explain". The absence of predictions on behalf of postdictions is characteristic feature of ad-hoced models based on homologies instead of analogies. You essentially reversed time arrow in your deductions.
1 / 5 (1) Jun 22, 2012
My predictions have been working pretty well. This article and last week's article are but the most recent examples. Art Winfree's predictions have worked pretty well also--still successful after 50 years of testing.

BCS theory is unquestionably correct. But it "predicted" that superc could not exist at higher temps, which is why the cuprate superc was discovered accidentally. When a theory is right, but rules out a possibly related phenomenon, there is a chance that it can (and should) be generalized. Occam.

The above article points to connections between cuprates and pnictides. It focuses on synchronized oscillations. 2 for 2. I have said publicly that all superc (old, new and mag diboride) should share a general explanation.

Last week's article ("Theory Unifies Superfluids...") says that the Nobel Prize Nambu Goldstone theory is correct but a special case, and it can be generalized, based on a meta-synchrony of oscillations. 2 for 2. I say the same as to BCS theory.

1 / 5 (1) Jun 22, 2012
OK, superconductors do share "coupled oscillations" and you predicted it. Anything else?
1 / 5 (1) Jun 22, 2012
Happy to oblige, TkC. First, a prediction: all of these puzzling phases will be explained by synchronized or coupled oscillators, and in every case the pattern will be one of the patterns specified by Art Winfree. No exceptions.

Second, consider the Efimov effect, predicted by Efimov in 1970 and experimentally confirmed within the last year or so. Those are synchronized oscillations of a three particle system, with each particle one third out of phase with respect to the other two. That is a Winfree pattern. (Think Borromean rings.) Art did his work two or three years before Efimov. He made the general prediction. I said Art's law applies to physics. Credit goes to Art, not me.

Finally, I say the fractional quantum Hall effect is an example of the Macksb--Winfree law. Every single fraction. If you've read Robert Laughlin's book, you know he is fascinated by the extreme precision of fqHe. I say the precision and the patterns are due to the Macksb--Winfree law.
1 / 5 (1) Jun 22, 2012
First, a prediction: all of these puzzling phases will be explained by synchronized or coupled oscillators
This is not a prediction, this is a guess or let say the postulate of your hypothesis. The predictions are based on robust sequence of logical steps. The simply saying "Winfree's theory predicts" is not a prediction but a wishful thinking. With compare to mainstream physics I don't think, all predictions must be formalized in rigorous way, fuzzy logics is a reproducible logics too - but this logics must be present, or it's religion and not science based on reproducibility of thoughts.
say the fractional quantum Hall effect is an example of the Macksb-Winfree law
You're not supposed to "say" anything here, you should PROVE it. Can you understand the difference? BTW No "Macksb-Winfree law" exists.
1 / 5 (1) Jun 23, 2012
Good advice, Terriva. Thanks. are your predictions working out?
1 / 5 (1) Jun 23, 2012
Compare my explanations of it here.
1 / 5 (1) Jun 23, 2012
I have now read the four posts to which you directed me. Our theories are different, as you well know. But I will make some comparisons, following the order in which you presented your ideas.

1. Stronger shaking of electrons triumphs over thermal vibrations of lattice. Disagree. I believe that lattice vibrations synchronize, which helps the electrons to self-organize.

2. Hens to their feeder. I agree in general terms.

3. Chaotic electron fluid. My theory says fully interconnected electrons, in a Winfree pattern. Okay to call that a fluid. But the opposite of chaotic.

4. Islands in pseudogap phase. Agree pretty much. My theory says islands for a while until oscillations slow down to a critical point at which the self-organization must be universal, due to increased influential range of oscillations.

5. My view: pseudogap friend, not foe.

6. As you know, my ideas are based on Winfree, not Prins. And Winfree himself is based in part on Huygens clocks. I do not downvote.
1 / 5 (1) Jun 24, 2012
There is a new PhysOrg article today: "Discovery of Material with Amazing Properties," June 24. It describes an unusual and precise spin order which arises suddenly in a multiferroic. "Why does it look like this? asks the article.

The answer lies in Art Winfree's work, so this is another confirmation. I have posted comments there.

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