Chemists propose explanation for superconductivity at high temperatures

Dec 15, 2011 by Kimm Fesenmaier
Path of electrons in the Tahir-Kheli-Goddard model of high-temperature superconductors. The red arrow indicates the percolating pathway through the plaquettes. [Credit: Caltech/Tahir-Kheli/Goddard]

(PhysOrg.com) -- It has been 25 years since scientists discovered the first high-temperature superconductors—copper oxides, or cuprates, that conduct electricity without a shred of resistance at temperatures much higher than other superconducting metals. Yet no one has managed to explain why these cuprates are able to superconduct at all. Now, two Caltech chemists have developed a hypothesis to explain the strange behavior of these materials, while also pointing the way to a method for making even higher-temperature superconductors.

Superconductors are invaluable for applications such as MRI machines because they conduct electricity perfectly, without losing any energy to heat—a necessary capability for creating large magnetic fields. The problem is that most superconductors can only function at extremely low temperatures, making them impractical for most applications because of the expense involved in cooling them.

A value known as the maximum Tc indicates the highest temperature at which a material can superconduct. The superconductor used in MRI—the metal alloy niobium tin—has a maximum Tc of -248˚C. Cooling this material to such a frigid temperature requires liquid helium, a scarce and extremely expensive commodity.

But the cuprates are different. They still operate well below freezing (the highest of the , a cuprate created in 1993, has a maximum Tc of about -135˚C), but some can be cooled using liquid nitrogen. This makes them much more practical, since liquid nitrogen is plentiful and its cost is about a hundredth that of liquid helium.

The ultimate goal, however, is the creation of superconductors that could operate near room temperature. These could improve cell-phone tower signaling and the robustness of the electrical grid, and could one day enable the operation of levitating trains at dramatically reduced fuel costs.

"But to take superconductors to the next level, we need to understand how the known high-temperature superconductors work," says William Goddard III, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech. "After the publication of more than 100,000 refereed papers on the topic, there is still no acceptable explanation, and indeed, there has been no increase in Tc for the last 18 years."

All superconducting cuprates start as magnetic insulators and are transformed into superconductors through "doping," a process that involves removing electrons from the parent compound, either by substituting certain atoms for others or by adding or removing oxygen atoms. Still, no one knows what it is about doping that makes these cuprates superconduct.

Over the last four years, Goddard has published three papers with Jamil Tahir-Kheli, a senior staff scientist at Caltech, building a hypothesis that explains what makes cuprates superconduct. They have been working with a cuprate in which strontium (Sr) atoms are the "dopant atoms," replacing lanthanum (La) atoms. Based on modern quantum-mechanical calculations, Goddard and Tahir-Kheli found that each dopant atom creates a four-center hole on the copper atoms surrounding the strontium, a unit they refer to as a "plaquette." Electrons within the plaquettes form tiny pieces of metal, while those outside the plaquettes are magnetic. This result was completely contrary to the assumptions made by most other scientists about what happens when dopant atoms are added. The problem was, the researchers still did not know how the holes in the plaquettes led to superconductivity.

It took Goddard and Tahir-Kheli five years to figure that out. Their hypothesis is that when enough dopant atoms are added, the plaquettes are able to create a percolating pathway that allows electrons to flow all the way through the material. The magnetic electrons outside the plaquettes can interact with the electrons traveling through the plaquette pathway, and "it is this interaction that leads to the electron pairing—the slight attraction between electrons—that in turn results in superconductivity," Tahir-Kheli says.

The researchers' latest paper, published earlier this year in the Journal of Physical Chemistry Letters, takes the hypothesis a step further by accounting for a mysterious phase seen in cuprate superconductors called the pseudogap. In all superconductors, there is a superconducting , which is the amount of energy required to excite an electron from the superconducting state into a higher energy level not associated with superconductivity. This energy gap vanishes at temperatures above which a material no longer superconducts—in other words, above Tc. But in cuprate superconductors, there is a huge energy gap that persists at temperatures far higher than Tc. This is the pseudogap.

Among scientists, there are two camps on the pseudogap issue. One says that the pseudogap is connected somehow to superconductivity. The other insists that it is not connected, and in fact may be a phase that is competing with superconductivity. Goddard and Tahir-Kheli's theory lands them in this latter camp. "We believe that the pseudogap is decreasing the material's superconductivity," Tahir-Kheli says. "And, once again, its origin is related to the location of the plaquettes."

Goddard and Tahir-Kheli explain the pseudogap by pointing to plaquettes that do not contribute to superconductivity. These plaquettes are isolated; they do not have any other plaquettes directly next to them. The researchers found that there are two distinct quantum states with equal energy within these isolated plaquettes. These two states can interact with nearby isolated plaquettes, at which point the two distinct quantum states become unequal in energy. That difference in energy is the pseudogap. Therefore, to determine the size of the pseudogap, all you need to do is count the number of isolated plaquettes and determine how far away they are from one another.

"The electrons involved in the pseudogap are wasted electrons because they do not contribute to superconductivity," Goddard says. "What is important about them is knowing that, since the pseudogap comes from isolated plaquettes, if we were to control dopant locations to eliminate isolated plaquettes, we should be able to increase the superconducting temperature."

Goddard and Tahir-Kheli predict that by carefully managing the placement of dopant atoms, it might be possible to make materials that superconduct at temperatures as high as -73˚C. They note that such an improvement after 18 years of stagnation would mark a significant step toward the creation of truly high-temperature superconductors with practical implications for the energy and health sectors.

The pseudogap paper, "Origin of the Pseudogap in High-Temperature Cuprate " was published by The Journal of Physical Chemistry Letters.

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epsi00
3.3 / 5 (10) Dec 15, 2011
Oops, chemists explaining high temperature superconductivity. Wait until the physicists hear about it.
rawa1
1.1 / 5 (13) Dec 15, 2011
here are two camps on the pseudogap issue. One says that the pseudogap is connected somehow to superconductivity. The other insists that it is not connected, and in fact may be a phase that is competing with superconductivity.
Pseudogap state is apparently related to superconductivity. In AWT model the superconductivity arises, when the electrons are attracted mutually to the positively charged places of lattice like the hungry hens around feeders arranged in lines (hole stripes). The mutual competition of repulsive electron forces enables the electrons to move freely through atom lattice. But the superconductivity arises only when the distance between holes isn't too large or too small. If it's too large, the electrons form an isolated islands of superconductive phase around holes, which exhibit most of bulk properties of superconductors - but they're not still electrically conductive. as a whole This is just what the pseudogap phase/state means.
rawa1
1.1 / 5 (13) Dec 15, 2011
The introduction a too many holes into material (so-called the overdoping) kills the superconductivity too. We should realize, the electrons condensing around holes exert strongly repulsive forces to the surrounding atoms and they're trying to expand their lattice, which decreases the level of compression of superconductive electron fluid. The attractive and repulsive forces must be in balance.

This simple model explains, why many elements (like the alkali metals) never become superconductive even under pressure, although they've pile of free electrons around atoms available. These elements lack the elongated binding d- f- orbitals, which would keep the whole atoms together in compact lattice. As the result, alkali metals are soft and no sufficiently compressed electrons can exist between them. The good superconductors are forming rigid cages enabling the internal repulsive forces between electrons, which explains, why they're so brittle.
rawa1
1 / 5 (12) Dec 15, 2011
Before ten years the physicist J.F.Prins revealed surprising effect. He injected the positively charged oxygen holes beneath the surface of thin artificial diamond layers. The electrons, who are strongly attracted to these positive charges cannot reach the holes, so they're condensing above the surface in invisible, still superconductive layer. In this case the superconductor is formed with electrons floating at the vacuum above superconductor, which enables to design artificial superconductor.
http://www.aether...ctor.gif
In this proposal a thin wire covered with insulating layer is subjected a strong positive potential, which will attract the free electrons to the surface of insulating layer and formation of dense superconductive phase, which will conduct the current through vacuum above surface of wire. With switching of voltage we can shut down superconductivity easily, so we can use this device as an ideal switch or transistor.
bewertow
Dec 15, 2011
This comment has been removed by a moderator.
hjbasutu
1 / 5 (1) Dec 15, 2011
this is a job for Physicists ok?
Eikka
5 / 5 (3) Dec 15, 2011
this is a job for Physicists ok?


Chemistry, at its fundamental level, is physics. When you go into more detail, you get particle physics.
FrankHerbert
1 / 5 (52) Dec 15, 2011
I cannot understand the manufactured rivalries between branches of science here when I'm pretty sure the people perpetuating them have NOTHING to do with the fields they are cheer leading/trashing.

Don't we have sports teams for this shit?
Callippo
1 / 5 (6) Dec 15, 2011
@rawa1 you're babbling nonsense
Why you're so sure with it? Or better to ask, what I said wrong, exactly?
Osiris1
4 / 5 (4) Dec 15, 2011
Sounds like a job for creative metamaterials like those use in cloaking research...to get that magic plaquette placement. C'mon guys and gals, stop arguing and get creative for the good of all!
that_guy
5 / 5 (2) Dec 15, 2011
Propose anything you want, but until you can propose a way to prove/disprove either hypothesis, or another experiment that can show what's going on, I couldn't care less.

The problem is that most superconductors can only function at extremely low temperatures,

Why bother to note that it's 'most'? You mean as opposed to one or two superconducting states that are under even more extreme circumstances, such as 100,000 atmospheres?

And really, they should also ditch the 'high temp superconducter' moniker. It only makes sense when compared to extremely cold superconducters. As soon as they get a superconductor at 0 C, it will be a 'super high temp boiling superconductor'.

Maybe they should use a more scientific notation, like say, 200k class superconducter, and 300k class superconducter. (A reference to their Kelvin Temp range)

/rant.
Silverhill
5 / 5 (1) Dec 15, 2011
this is a job for Physicists ok?
Chemistry, at its fundamental level, is physics. When you go into more detail, you get particle physics.
(see http://xkcd.com/435/ )
Osiris1
2.3 / 5 (3) Dec 16, 2011
Do not forget the mathematics,as anyone who has ever taken physical chemistry, or 'P-chem', knows only toooooo well. Have 20 hours of chem in my engineering major along with 20 of math....two minors. Must say, hardest thing about chem is your eyesight. If you are color blind in any degree, pick another major! Those nickle and amine radicals and their many variations of light green would even muddle a Muslim.
Nanobanano
3 / 5 (2) Dec 17, 2011
After the publication of more than 100,000 refereed papers on the topic, there is still no acceptable explanation


Who reads them all?

Nobody.

If you had someone twice as smart as Einstein and read 50 papers per day it would take 7 years just to read them all, else you'd never know if any of them said anything useful at all, or whether they were just nonsense.

Look at all the wasted time and money!

And if anyone found anything obscure that might be important, odds are nobody will even notice, because you can be sure they didn't all read one another's papers, they couldn't possibly have had the time to do so.
Minich
not rated yet Dec 23, 2011
High temperature superconductors and superfluid helium have the same superfluid/superconducting mechanism (the same order parameter). The same mechanism is for pseudogap (dielectric gap in some directions) and supercondicting gap (in some directions, different from pseudogap antinodal directions). These gaps coexist.

For conventional superconductors with half filled zones superradiance of phonons is added to above mentioned order parameter.
Minich
not rated yet Dec 23, 2011
No pairing of electrons is needed so doesn't Bose-Einstein condensation.
Callippo
1 / 5 (1) Dec 23, 2011
High temperature superconductors and superfluid helium have the same superfluid/superconducting mechanism... No pairing of electrons is needed so doesn't Bose-Einstein condensation.
The boson condensate of neutral atoms (helium) differs significantly from boson condensate of charged particles (electrons within superconductors) just with presence of spin-spin interactions. The electrons in HT superconductors are always coupled, even at the high temperatures - they just don't form pairs, but paired groups of electrons, which differ with their spin.
Minich
1 / 5 (1) Jan 01, 2012
High temperature superconductors and superfluid helium have the same superfluid/superconducting mechanism... No pairing of electrons is needed so doesn't Bose-Einstein condensation.
The boson condensate of neutral atoms (helium) differs significantly from boson condensate of charged particles (electrons within superconductors) just with presence of spin-spin interactions. The electrons in HT superconductors are always coupled, even at the high temperatures - they just don't form pairs, but paired groups of electrons, which differ with their spin.

Cooper pairing is the wrong idea for superconductors. So nobel prize for the BCS theory is wrong Prize :)

It is funny, that even very clever physisists, except Matthias, Hirsch and Frohlich, made everybody to repeat this foolish idea :)

Even 1986 event couldn't open the truth: cooper pairing has nothing to do with superconductivity!!!

New inified idea will be published in 2 months :)
Callippo
1 / 5 (1) Jan 01, 2012
Cooper pairing is the wrong idea for superconductors.
It just isn't dominant for HT superconductors. And it's byproduct of superconductivity and bosone condensation, rather than reason of it. Copper pair-less theories are many already, including mainstream theories (BEC, RVB).
Minich
not rated yet Jan 07, 2012
Cooper pairing is the wrong idea for superconductors.
It just isn't dominant for HT superconductors. And it's byproduct of superconductivity and bosone condensation, rather than reason of it. Copper pair-less theories are many already, including mainstream theories (BEC, RVB).

Why are You so sure? :)
Explain me then, pseudogap in cuprates, please.

Can you give one dimensional exactly solvable model of superconductor with formulas for pseudogap Tp and superconducting Tc depending of doping?
Do in 1d case pseudogap and superconducting gap coexist? :)

You can see my answers in two months.
Callippo
1 / 5 (1) Jan 07, 2012
Explain me then, pseudogap in cuprates, please.
I just explained it above (rawa1 Dec 15, 2011). The pseudogap arises, when the material is filled with isolated islands of superconductive phase, so that the material exhibits many local characteristics typical for superconductors (thermal capacity coeficient, for example), but it still doesn't exhibit bulk characteristics of superconductors (Meissner effect and zero resistance). http://www.aether...evel.gif Such situation happens, when the superconductor contains too low concentration of holes.

http://www.tcm.ph...fig1.gif

When the material contains too much holes, a Fermi liquid state appears instead: the electrons are forming continuous phase, but their level of mutual compression is too low, so they're moving in ballistic mechanism like fermions, not in waves like the bosons. http://aetherwave...uctivity
Callippo
1 / 5 (1) Jan 07, 2012
Can you give one dimensional exactly solvable model of superconductor with formulas for pseudogap Tp and superconducting Tc depending of doping?
The theorists are developing low dimensional models and the high dimensional caravans goes on. Look, I'm not saying, my explanations are job and salary generator for theorists, who are developing formal models. I'm proposing easy to imagine physical models of situation to help the laymans in orientation through fast growing forest of experimental knowledge. We can imagine, what happens in superconductors, which enables us to design room temperature superconductors. The formal models will not help us with it, until they're not based on easy to follow logics. After all, even the existence of formal model doesn't imply, we could use it for testable predictions at the case, when it relies on the parameters, which are difficult to measure.
Callippo
1 / 5 (1) Jan 07, 2012
In brief, while I respect the work of theorists, with respect to the findings and practical usage of high temperature superconductors their models are essentially useless. Simply because the HT superconductors are highly dimensional chaotic systems, the formal description of which is quite separate job, whose difficulty has nothing to do with the difficulty of finding room temperature superconductor. It's simply different game, not worse or better, but another one. Even if we would have such a model, we couldn't be able to propose ideal superconductor with it. Even if we would have such a proposal, we couldn't be able to cook it. Each step in this route requires quite different qualification, the success of which is independent to others. At the case of hyperdimensional systems like the HT superconductors it's quite apparent, the experimentalists are more successful than the phenomenologists and the phenomenologists are more successful, than the theorists. AWT explains, why it happens.
Callippo
1 / 5 (1) Jan 07, 2012
After all, the lead of experimenters over theorists regarding the finding of HT superconductivity is nothing in compare to the finding of cold fusion, which is even more hyperdimensional system. The models of theorists must remain low-dimensional to remain solvable and as such they're remain a much more simpler, than the hyperdimensional reality, which they're trying to describe. The intuitive thinking of people is essentially hyperdimensional too, so that the nonformal models of reality could become more effective in solving of practical problems of hyperdimensional systems, than these strictly rigorous but low dimensional ones. Despite the theorists will label these intuitive models as desperately fuzzy, naive and indeterministic. But hyperdimensional reality simply IS fuzzy and indeterministic from low dimensional perspective. Try to imagine, how chaotically the 2D shadow of short rod regularly rotating in 3D will appear to move.
Minich
not rated yet Jan 09, 2012
Landau, Bardeen, Cooper, Schriffer, Leggett had got Nobel prizes for wrong theories (helium superfluidity and superconductivity).

It is OBVIOUS for me NOW.

Nobody couldn't declare BCS theory as wrong, except Hirsch and Matthias. But they also couldn't give true theory.
arxiv.org/abs/0901.4099

True theory must be understandable even to barmaid (Rutheford). I hope i have such UNIFIED theory for helium 3 & 4 and ANY superconductor.

This theory is much more simple, than BCS and uses such simple cases as extended(!) Kronig Penney model (Bloch's 3D waves) and extended(!) nonideal bose gas Bogoliubov model plus one nobel prize german (VERY SIMPLE though) effect.

I and Landau graduated from the same Leningrad State University, but i graduated quantum mechanics department, headed by Vladimir Fock, when Fock was alive and i could see him sometimes, so i consider myself as being of Fock school :)

The theory will be disclosed in two months.
Callippo
not rated yet Jan 09, 2012
The theory will be disclosed in two months.
OK, can you for example explain with your theory, which sodium never gets superconductive, while the niobium does?
Minich
not rated yet Jan 12, 2012
The theory will be disclosed in two months.
OK, can you for example explain with your theory, which sodium never gets superconductive, while the niobium does?


Of course, I can. For HTSC (cuprates for example) we can even with simple calculations to predict superconductivity. For HTSC the theory is more evident,than for about half filled zone. For "conventional" superconductors the accuracy of calculation of conductance band must be more accurate than for HTSC case (doped bands).

This will be EVIDENT for every student who knows only
fundamentals of quantum mechanics and doesn't know such tools as feynman diagrams, green functions approach, quantum field theory in solids, sham-kohn approach, et cetera.

Wait a little bit.
Minich
not rated yet Jan 12, 2012
And it will be evident why wrong pairing theories were so popular.
And why helium 4 physicists discovered wrong "two roton bound state" :)
http://iopscience.../8/6/003
High resolution Raman study of the two-roton bound state
C A Murray, R L Woerner and T J Greytak

In 1975 i spent much time trying to explain the results of two roton raman studies of HE 4 (Japan works and USA).

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