Zeroing in on quantum effects: New materials yield clues about high-temperature superconductors

May 28, 2010

(PhysOrg.com) -- A team of U.S. and Chinese physicists are zeroing in on critical effects at the heart of the latest high-temperature superconductors -- but they're using other materials to do it.

In new research appearing online today in the journal , the Rice University-led team offers new evidence about the quantum features of the latest class of high-temperature superconductors, a family of iron-based compounds called "pnictides" (pronounced: NICK-tides).

"In correlated electron systems like the pnictides and their parent compounds, the electrons are caught in a competition between forces," said Rice physicist Qimiao Si, a co-author of the study. "On the one hand, they are compelled to move around, and on the other, they are forced to arrange themselves in a particular way because of their desire to repel one another. In this study, we varied the ratio between these competing forces in an effort to find the tipping point where one takes over from the other."

The aim of the research is to better understand the processes that lead to high-temperature superconductivity. If better understood and developed, high-temperature superconductors could revolutionize electric generators, MRI scanners, high-speed trains and other devices. In today's wiring, electricity is lost due to resistance and heating. This happens because electrons bump and ricochet from atom to atom as they pass down wires, and they lose a bit of energy in the form of heat each time they bounce around.

Almost a century ago, physicists discovered materials that could conduct electrons without losing energy to resistance. These "superconductors" had to be very cold, and it took physicists nearly 50 years to come up with an explanation for them: The electron-electron repulsion in these low-temperature superconductors was so weak that with the mediation of lattice vibrations, electrons overcame it, paired up and glided freely without the bumping and heating.

That explanation sufficed until 1986, when physicists discovered new materials that became superconductors at temperatures above 100 kelvins. These "high-temperature superconductors" were made of layers of copper alloys sandwiched between layers of nonconducting material that were laced, or "doped," with trace amounts of material that could contribute a few extra electrons to the mix.

Physicists quickly realized their existing theories of superconductivity could not explain what was happening in the new materials. For one thing, the undoped versions of the compounds didn't conduct electricity at all. Their electrons -- due to their desire to repel one another -- tended to lock themselves a comfortable distance away from their neighbors. This locked pattern was dubbed "Mott localization," which gives rise to an insulating state.

In 2008, the search for answers took another turn when a second class of high-temperature superconductors was discovered. Dubbed the pnictides, these new iron-based superconductors were also layered and also needed to be doped. But unlike their copper cousins, undoped pnictides were not Mott insulators.

"Mott localization doesn't occur in the undoped pnictides, but there is considerable evidence that the electrons in these materials are near the point where Mott localization occurs," Si said. "This proximity to Mott localization endows the system with strong quantum magnetic fluctuations, which we believe underlie the in the pnictides."

In all high-temperature , the iron or copper atoms in the conducting layers form a grid-like, checkerboard pattern.

In work published earlier this year, Si and colleagues replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This subtle change brought the iron atoms in the checkerboard a tad closer together, and that changed the amount of energy that was compelling to move between the iron atoms. The experiments confirmed a 2008 prediction of Si and, University of California, Los Angeles (UCLA) theorist Elihu Abrahams, who had predicted that boosting the electrons' kinetic energy would drive the pnictides further away from the Mott tipping point.

In the latest tests, Si and colleagues at Rice, China's Zhejiang University, UCLA, Los Alamos National Laboratory and the State University of New York at Buffalo (SUNY-Buffalo) sought to move the system in the other direction, toward Mott localization.

"We wanted to decrease the kinetic energy by expanding the distance between iron atoms in the lattice," said study co-author Jian-Xin Zhu, a theorist from Los Alamos. "Unfortunately, there is no pnictide material with those properties."

So the team's experimentalists, Rice's Emilia Morosan and Zhejiang's Minghu Fang, hit upon the idea of substituting a similarly patterned material called an iron oxychalcogenide (pronounced: OXY-cal-cah-ge-nyde). Like the iron pnictides, iron oxychalcogenides are layered materials. But compared with the pnictides, the distance between iron atoms is expanded in the oxychalcogenides.

Tests on the new materials confirmed the theoretical predictions of the team; a slight expansion of the iron lattice pushed the system into a Mott insulating state.

"Our results provide further evidence that the undoped iron pnictide parent compounds are on the verge of Mott localization," Abrahams said.

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JoeDuff
1 / 5 (2) May 29, 2010
The conceptual explanation of high Tc superconductivity is surprisingly easy. In normal conductor the repeated acceleration and deceleration of electrons is the main source of ohmic loses, as electrons are squeezing through holes in crystal lattice.

These troubles can be avoided, if we compress electrons between many other electrons. These thingies are repelling mutually at distance and when their density increases sufficiently, then the motion of individual electrons isn't driven by location of atoms in lattice anymore, but by motion of other electrons.

Such system will change into chaotic electron superfluid, where particles are interacting via transverse waves without friction, rather then via longitudinal waves.

In high Tc superconductor such state can be achieved by doping of material with positively charged atoms, so called the holes. These holes are attracting electrons in their neighborhood, thus forming less or more continuous phase of electron superfluid.
JoeDuff
1 / 5 (2) May 29, 2010
IMO the same result could be achieved in simple device, where we would attract electrons toward nonconducting surface by strong external field.

We can imagine such device as a thin insulated wire surrounded in vacuum by heated counter electrode, which is serving there as a source of free electrons. If the mechanic and dielectric strength of insulation would be sufficient, then the electrons could form a compact super-conductive layer around insulator covering the central wire.

Such artificial superconductor could be even switched on and off by switching external voltage source, i.e. it could serve as an transistor.

It's a pity, contemporary theories are serving mostly as a salary generators for physicists, who aren't motivated into real applications. In addition, the lack of intuitive understanding of this phenomena slows down the technical progress in this area a lot.
MaxwellsDemon
5 / 5 (1) May 29, 2010
Then do something about it. Go ahead and invent the artificially-superconductive transistor, thereby proving that your new theoretical model of superconductivity is superior to the latest theories.

Simple.
Parsec
5 / 5 (3) May 29, 2010
JoeDuff - ditto on what MaxwellsDemon said. Its always easy to pontificate silliness from the sideline, but unless you can demonstrate the math and the result, you just blowing smoke.

Of course, you would probably require a salary to support yourself during development, which would then attach you to the salary generator feeding trough. Life is tough when asked for a little consistency isn't it?
akotlar
5 / 5 (4) May 29, 2010
Joe, it's always possible that you know something that they don't, but I recommend you don't start your Nobel-winning dissertation with, "The conceptual explanation for high Tc superconductors is surprisingly easy," before going on to say that contemporary physicists are missing the truth because they lack an "intuitive understanding".

It seems reasonable to assume that if you were able to intuitively grasp such an easy concept the philandering physicists involved in superconductivity research would had an easier time devising room temperature superconductors.
JoeDuff
1 / 5 (1) May 29, 2010
But this model is really quite simple. Suppose we have long vertical pipe with some electrons attached to inner side of it. When we push some free electron into it, will it fall through the pipe?

Nope, it will hang into some position between fixed electrons, because electrons are repulsing mutually, so that every free electron just tries to find the energetically most stable position. The electron will be in Mott's insulating state.

But what will happen, if we increase the number of electrons attached to the walls of pipe? The repulsive action of fixed electrons will overlap mutually and at the moment, when quantum motion of movable electron will be able to overpass the difference in electrostatic field distribution, the electron will pass freely through the pipe like through vacuum.

Which is basically what we need to do for preparation of real superconductor. I'm just explaining, how this stuff works, because I do believe, it could be interesting for you.
JoeDuff
1 / 5 (1) May 29, 2010
If not, then sorry - but after then I don't understand, why you're visiting articles about it here.

My point is, we can attempt for construction of artificial superconductor by now, we needn't to find any exact theory for it. Believe it or not, but rockets are flying even without any math involved into their development. Old Chinese have produced them from bamboo and used them as a weapons in their wars for many years.

And this model is not rocket science at all - it's completely safe stuff.

..unless you can demonstrate the math and the result, you just blowing smoke...
You don't understand the stuff, until you're not able to explain it to Einstein's grandma.

Look, no math was ever mentioned here. You understand the stuff and after then you can explain it - or not.
JoeDuff
1 / 5 (1) May 29, 2010
you would probably require a salary to support yourself during development
I don't want to develop such idea and my advices are free. I just don't want to spend my tax money in development of some complex theories, when the applications can be quite simple. Would you spend your money for theory describing the air flow in flute, when you would need to earn money by production of flutes?

And vice-versa, even though you would have such a theory developed, it would not help you in construction of well sounding flute. It's all about effectiveness of research.

The situation of electrons in real superconductor is quite complex, it cannot be modeled without computers at all. And vice-versa, even with such model you would have no clue, how to synthesize well behaving superconductor.

This is a problem of quite different category, despite you're probably believing, it isn't and so called the "scientific approach" is the most reliable one.
MaxwellsDemon
not rated yet May 29, 2010
philandering physicists


I had to grant all five stars for coining my new favorite comic science term...

JoeDuff, 'self-professed science advisors' et al: free advice is usually worth the price, and the difference between 'science' and 'junk science' is whether or not you're willing to do the work that it takes to prove (or disprove) your own ideas.
JoeDuff
2.5 / 5 (2) May 29, 2010
There is nothing to prove/disprove - the electrons are mutually repulsing particles, and the rest is just a trivial geometry. In fact it's too trivial for being ignored for thirty years.

Anyway, this model was confirmed before ten years by experiments of J.F.Prins already.

http://www.iop.or...8/3/319/

Regarding the proof, for example string theory exists forty years without proof and nobody cares about willingness of people to (dis)prove it. Ideas are simply true or false from the moment, they were bespoken and later research only can make it less or more obvious. Earth is spherical - and who cares, whether such fact was "scientific" or not?

Anyway, it's evident, neither modern people can accept any new logics, until it's not presented by some formal authority - no matter how simple such logics could be. People are still adhering to religious thinking, based on belief in authorities.
Bonkers
not rated yet May 29, 2010
Well Joe, its late and I'll rise to your challenge, though i think the other two posts have it - never ruin a good Nobel-prize-winning insight with a random diatribe against salaried scientist scum...
Your superconductive thing, an insulated wire in a vacuum with a heated filament nearby looks much like a thermionic diode with an insulated anode. You are right in imagining the surface of the insulator attracting electrons, though without any specific binding to the surface they would be free to "boil off" back into the vacuum, these are reasonably "hot" electrons - and would form a space charge (look it up, its a standard term) around the insulator surface. Perhaps not quite the dense superfluid you imagine. Even if the electron density is enough to form your superfluid, the superconduction is transverse along the insulator - you need another couple of electrodes to tap in and tap out, you can't just put a lump of metal at each end, as rings round the insulator.
Bonkers
not rated yet May 29, 2010
... because cold metal won't emit electrons, even heated metals have a "work function" associated with them, you don't get across the bridge into the vacuum for free. Even if this voltage drop (~1V in best case) a one end only were to be acceptable, and understand superconducting magnets carry 10kA, its still not exactly "wire" is it?
Keep on with the interest though, but understand that solid-state physics is, like, really hard...
JoeDuff
1 / 5 (1) May 29, 2010
never ruin a good Nobel-prize-winning insight with a random diatribe against salaried scientist scum
Such analysis are part of my general theory, so I've no problem with it. I'm just mapping the psychosocial reasons, which lead scientists to ignorance of some fundamental principles the more, the more trivial are.
..though without any specific binding to the surface they would be free to "boil off" back into the vacuum..
I can see no reason, why such electron should leave the surface of insulator, if they're attracted to it in sufficiently strong gradient of electrostatic intensity.
you need another couple of electrodes to tap in and tap out
Of course - at least you're starting to think about it in the same way, like me...

http://tinyurl.com/y8xtr7n

..understand that solid-state physics is, like, really hard..
Why not, but first FET transistor was patented in 1925, for example. So you're not required to be an expert in solid-state to propose such a prototype.
JoeDuff
1 / 5 (1) May 29, 2010
Prof. Prins has used a somewhat different arrangement: he injected oxygen atoms into diamond layer in plasma discharge. These oxygen atoms formed a holes beneath diamond surface and they attracted electrons onto it, thus forming a superconductive layer of electrons above diamond surface even at the room temperature.

As far I know, these experiments were never attempted to replicate - why? Have we some better solution already? Anyway, these pioneering experiments demonstrated clearly (at least for me), how high Tc superconductivity really works.

Anyway, I consider the concept of charged particles attracted to insulating surface a quite interesting by itself. What will happen, for example, if we would use deuterium ions instead of electrons? Couldn't we achieve a cold fusion in such way? I know about all these tokamaks and fusors - but sometimes the brute force may not be the best route to overcome Lawson criterion. Sometimes it may be better to think about problem from different side
JoeDuff
1 / 5 (1) May 30, 2010
The Puzzle of High Temperature Superconductivity in Layered Iron Pnictides and Chalcogenides - a recent comprehensive review for everybody, who wants to becom familliar with the actual state of research in this field. Anyway, it will help you to understand too, how remote this research is from comprehensive understanding and actual applications of high Tc superconductors. It's a research industry preprogrammed to many years in advance and fully absorbed with itself.

http://arxiv.org/...92v1.pdf

Mott insulating state is rather hot topic in this field because of many connection to topological insulators and Bose condensates. Most of experimental work in recent years was dedicated to detection of Mott's localization in various materials.
Bonkers
not rated yet May 31, 2010
never ruin a good Nobel-prize-winning insight with a random diatribe against salaried scientist scum
Such analysis are part of my general theory, so I've no problem with it. I'm just mapping the psychosocial reasons, which lead scientists to ignorance of some fundamental principles the more, the more trivial are.

Wow, that's a monumental position you just breezed over. Scientists eh? - retards I call 'em.. Spectacular arrogance needs spectacular genius, and i don't see it from your few hand-wavy arguments. Far from it.
Bonkers
not rated yet May 31, 2010
..though without any specific binding to the surface they would be free to "boil off" back into the vacuum..
I can see no reason, why such electron should leave the surface of insulator, if they're attracted to it in sufficiently strong gradient of electrostatic intensity.

Erm, as i said, these are "hot" electrons - imagine ping-pong balls that never slow down, forming a gas cloud around the insulator surface - no reason why they should superconduct, there is no " cold crystalline order" here. Moreover you haven't addressed how to get a transverse current through them without injection, a lossy process. Oh, and if you did, there would be no need for the heated filament - electrons from the necessary injector would "coat" the insulator anyhow.

Ok, now imagine buried positive charges doing the job of your high field, and make planar arrays of these and stack them. Let's call this material "Graphite" Erm, but it doesn't superconduct, why not?
JoeDuff
not rated yet May 31, 2010
imagine ping-pong balls that never slow down, forming a gas cloud around the insulator surface
If they collide, they should lose energy gradually via EM waves, or not? Anyway, prof, Prins observed condensation of such electrons already above diamond surface, so we can be sure, such process will occur.

..there would be no need for the heated filament..
I believe so. After all, prof. Prins has revealed his superconductivity accidentally just during attempts to develop effective cold cathode with using of diamond layers.

..how to get a transverse current through them without injection, a lossy process..
Of course, there will be some barrier, but as prof. Prins measured, this voltage drop is very low. I presume it's because electron density required for formation of superconductive fluid is comparable to those within metals, so that charge is spreading via surface plasmons through phase interface.
JoeDuff
not rated yet May 31, 2010
now imagine buried positive charges doing the job of your high field, and make planar arrays of these and stack them. Let's call this material "Graphite" Erm, but it doesn't superconduct, why not
I'm not sure, which configuration do you mean, exactly. In high Tc superconductors just the continuous stripes of positively charged holes is what does the superconductivity. But charge carriers are still electrons, which are attracted to holes like birds to grains.

Low temperature superconductivity in graphite was induced by doping too, but graphite planes doesn't held together. There is no force, which could compress electron fluid too much between graphite layers. The same situation occurs during overdoping of cuprates - the repulsive force of electrons expands and ripes atom lattice apart and overdoped superconductor will change into common metal.
KBK
not rated yet Jun 09, 2010
David Radius Hudson and alchemy in general have all, as a group, known all about these particular considerations involving iron, etc for many years. In the case of alchemists, thousands of years.

In the case of Hudson, since about 1987-88 or so. And then he went far, far beyond that.

'Mainstream' and 'known' isn't mainstream and known until it comes out of the darkness of the fringe.

You wanna be on the cutting edge? Then go step out into the darkness of the fringe and to hell with the attitudes and crank ego-based arguments of those who live in the middle of the herd. You gotta get adventurous and grow a thick skin, otherwise you will always be in the middle of the herd, walking the same..old..tired.. grounds. Over and over. Until you die. Same-same.

Your choice.

If you search out Hudson's purposely ridiculed works, and read/hear (mp3's) all there is on the subject you ~~WILL FIND~~ actual room temperature superconductors. More than you know what to do with.

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