Atomic-resolution images provide fresh insights into a mysterious state found in superconducting materials

Nov 09, 2012
This scanning-tunneling-spectroscopy image of Ca2-xNaxCuO2Cl2 shows the merging of clusters (in green and yellow) where the so-called pseudogap state has developed. Credit: 2012 Yuhki Kohsaka, RIKEN Advanced Science Institute

Superconductivity describes the state of certain materials when they conduct electric currents without any resistance. For superconductivity to develop, these materials generally have to be cooled to temperatures below roughly –140 °C, depending on the material. The family of materials that requires the least amount of cooling is known as cuprate superconductors. These compounds are therefore technologically interesting, but scientists are still working to understand the fundamental mechanism underlying superconductivity in these materials. In fact, determining what makes cuprate superconductors tick is one of the grand challenges in condensed-matter physics.

Now, an international research team, led by Yuhki Kohsaka and Hidenori Takagi from the RIKEN Advanced Science Institute, Wako, has provided fresh perspectives on the behavior of these systems. The researchers, from Japan, the US and the UK, took atomic-resolution images of a cuprate material as it undergoes the transition from a 'normal' solid to a superconductor. A broad pool of data has been accumulated since the discovery of cuprate superconductors in 1986, but only a few studies provided microscopic details of how the superconducting state in cuprate materials emerges.

Using a unique setup, combining an exquisitely stable with high-quality samples, the researchers studied an enigmatic state known as the 'pseudogap state'. This state appears when the parent compound of their material, Ca2CuO2Cl2—which is not a superconductor—is gradually doped with . Once the degree of doping is above a critical level, the material becomes superconducting. At intermediate levels of doping, however, the compound goes through the pseudogap state, whose role is the topic of intense debate among physicists.

Kohsaka and colleagues found evidence that the pseudogap state may be helpful for emergence of the . At very low doping levels, they saw the formation of distinct nanometer-scale clusters that are in the pseudogap state. As they added more dopant atoms, they observed that these clusters start to connect. Intriguingly, full connection happens just as the material becomes a superconductor.

These are important new insights into the microscopic behavior of . But Kohsaka remains cautious: "We do not claim yet a local correlation between the pseudogap and superconductivity. We don't have experimental evidence strong enough to prove such a correlation. But establishing this connection will be an important direction of future study."

Explore further: Physicists unlock nature of high-temperature superconductivity

More information: Kohsaka, Y., Hanaguri, T., Azuma, M., Takano, M., Davis, J. C. & Takagi, H. Visualization of the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator. Nature Physics 8, 534–538 (2012). www.nature.com/nphys/journal/v… 7/abs/nphys2321.html

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Macksb
3 / 5 (2) Nov 10, 2012
"Intriguingly, full connection [of the nanometer-scale clusters in the pseudogap state] happens just as the material becomes a superconductor." Let's focus on that observation. If superconductivity depends on full connection, then there is some form of mass synchrony at work. A pervasive emergent system, with some principle of self-organization, likely produces this superconductivity. As I have proposed and described in other posts, Art Winfree's law of coupled oscillators, circa 1967, is a possible and perhaps likely theoretical explanation. Winfree's law also serves to describe BCS theory: phonon oscillations interact with and organize the oscillations of electrons--combining spin and orbit in exact antisynchrony. High temp superconductivity may involve more complex coupling patterns, perhaps with a different cadence.
johanfprins
1 / 5 (1) Nov 10, 2012
I have tried for years to publish a model that describes the pseudogap-phase, and also the fact that this phase MUST form as a precursor for superconduction to occur.

In some materials, one cannot observe the formation of the pseu-phase, owing to the structure of the electron-energy levels. Even so, also in these materials the same phase must first form before SC can occur.

The reason for this is that this phase is a MOTT-insulator, consisting of localised electron-orbitals. When the distances between these orbitals are too large, SC cannot occur. As soon as the distances become less than a critical value, the localised orbitals convey a S-current by means of quantum fluctuations. The latter can easily be modelled in terms of the (delta)E *delta(t) larger than (hbar) relationship.

Thus, what these experimenters observed, confirms that my model is correct: The orbitals form, and form SC islands: A soon as the islands form a connective pathway, SC initiates through the material.

ValeriaT
1 / 5 (2) Nov 10, 2012
what these experimenters observed, confirms that my model is correct: The orbitals form.. form SC islands: A soon as the islands form a connective pathway, SC initiates through the material
In this work from 2006 you didn't still mention any "pseudogap" word at all - so I presume, you borrowed this idea from my article about it in 2008 and/or subsequent discussions about it right here at PhysOrg. Please accept my apology, if you could prove the origin of this pseudogap explanation before the 2008. BTW Only working dated link containing "pseudogap" word in the above context will be accepted as an apology.
ValeriaT
1 / 5 (1) Nov 10, 2012
BTW You're writing there
There is of course no reason, why these orbitals couldn't be bi-electron orbitals. In fact, they most probably will be... It will be now postulated, that oxygen atoms..supply electrons, which do form arrays of bi-electron orbitals..
Are you implying, they're forming arrays of Cooper pairs? If yes, why are you fighting against Cooper pair theory so obstinately? If not, in which these bi-electron orbitals differ from Cooper pairs? BTW do you realize, these bi-electron orbitals are way too small for being able to explain the supercritical islands observed in pseudogap state? I know, you're fighting with BCS theory all the time, but you're still maintaining a back-door for it.

BTW The above study is so original neither, if we realize a very similar pictures are encircling the web many years.
johanfprins
3 / 5 (2) Nov 10, 2012
In http://arxiv.org/.../0607227 about it in 2008 and/or subsequent discussions about it right here at PhysOrg.
The fact that you need a Mott-insulator to form before SC can occur follows logically from this paper. Not in one of your papers have you agreed that the charge-carriers must be stationary localised states: So how the hell could I have gotten the idea from your badly-written and incomprehensible paper?
Please accept my apology, if you could prove the origin of this pseudogap explanation before the 2008. BTW Only working dated link containing "pseudogap" word in the above context will be accepted as an apology.
I do not need to apoligise to you since you are claiming nonsense. All my articles on SC in materials have from the word go claimed that one must have localised states forming and that SC initiates when the density of these states reaches a critical density. So what the hell else can this phase be before it reaches the critical density?
johanfprins
3 / 5 (2) Nov 10, 2012
Are you implying, they're forming arrays of Cooper pairs?
Most certainly not. Only a crackpot will believe that electron-pairs can form by exchanging a phonon. I am claiming that localised states, consisting of two electrons can form, as in the case when inserting donor atoms of double valency into a material.
If not, in which these bi-electron orbitals differ from Cooper pairs?
I have explained why!
BTW do you realize, these bi-electron orbitals are way too small for being able to explain the supercritical islands observed in pseudogap state?
The localised orbitals of a Mott-array can form ANY size island
I know, you're fighting with BCS theory all the time, but you're still maintaining a back-door for it.
BCS is BS!! I have stated it OVER and OVER, and do not need a "back-door". In future, young physicists will shake their heads in disbelief that a Nobel Prize could have been awarded for this crackpot model!!
ValeriaT
1 / 5 (2) Nov 10, 2012
So what the hell else can this phase be before it reaches the critical density?
I've no idea..;-) Well - if you think, it could be a pseudogap phase, you should write it explicitly and you shouldn't pretend, you have said it already before years here. If you're saying, you're explaining the pseudogap state within superconductors in this way, you should at least use the word "pseudogap" in such explanations a single times, don't you think?
Only a crackpot will believe that electron-pairs can form by exchanging a phonon
Of course the can be formed, why not? You're himself assuming the existence of electron pairs in your article above cited - in which mechanism these pairs are formed, after then? And please, don't say BCS theory is BS, until you're pushing the very same ideas here.
johanfprins
3 / 5 (2) Nov 10, 2012
So what the hell else can this phase be before it reaches the critical density?
I've no idea..;-) Well - if you think, it could be a pseudogap phase, you should write it explicitly and you shouldn't pretend,
I am not pretending anything.
If you're saying, you're explaining the pseudogap state within superconductors in this way, you should at least use the word "pseudogap" in such explanations a single times,
WHY? To please you? In that article the reason for the formation of a pseudogap was notof major importance at that time. You cannot always address ALL the issues you want in a single manuscript
Only a crackpot will believe that electron-pairs can form by exchanging a phonon
Of course they can be formed, why not? Not by exchanging phonons: By overlapping YES! They do this when they form a covalent bond. A phonon is not needed to form a covalent bond.
johanfprins
3 / 5 (2) Nov 10, 2012
You're himself assuming the existence of electron pairs in your article above cited - in which mechanism these pairs are formed, after then?
Are you really so slow? I have repeated over and over that we know that electrons can pair: If they could not, they would not have had covalent bonds. But in the first place pairs are not needed to have superconduction, and in the second place the Cooper mechanismn is crackpot physics.
And please, don't say BCS theory is BS, until you're pushing the very same ideas here.
I am NOT pushing the very same ideas: Jeesh!!

ValeriaT
1 / 5 (1) Nov 10, 2012
From the above article of yours
.."There is of course no reason, why these orbitals couldn't be bi-electron orbitals. In fact, they most probably will be... It will be now postulated, that oxygen atoms..supply electrons, which do form arrays of bi-electron orbitals"
In which way these "bi-electron orbitals" differ from Cooper pairs? How their electrons are kept together if not phonon exchange? It's your theory and you're saying, it differs from BCS theory - so it's your problem to prove it. It seems for me, if you replace the "bi-electron orbital" with "Cooper pair", then we get the very same theory, like the BCS. The fact, you're "postulating something" doesn't mean, you're providing a better explanation for it, than the BCS theory does.
johanfprins
1 / 5 (1) Nov 10, 2012
In which way these "bi-electron orbitals" differ from Cooper pairs? How their electrons are kept together if not phonon exchange?
Just like electrons usually pair when their wave-functions overlap. If the latter was not possible we would not have had chemistry
It's your theory and you're saying, it differs from BCS theory - so it's your problem to prove it.
It has been proved for more than 100 years in chemistry laboratories.
It seems for me, if you replace the "bi-electron orbital" with "Cooper pair", then we get the very same theory, like the BCS.
No you do not, since these bi-electron orbitals are not Cooper pairs and secondly they are subject to Boltzmann statistics NOT Bose-Einsteun statistics.
The fact, you're "postulating something" doesn't mean, you're providing a better explanation for it, than the BCS theory does.
If you could have comprehended physics you would have seen that pair-formation is not required in my model for SC to occur!
ValeriaT
1 / 5 (1) Nov 10, 2012
Just like electrons usually pair when their wave-functions overlap. If the latter was not possible we would not have had chemistry
But the electrons in Cooper pairs have their wave function overlapped too. This is why we are calling these pairs entangled: it's just another name for the "overlapping of quantum function".
since these bi-electron orbitals are not Cooper pairs and secondly they are subject to Boltzmann statistics NOT Bose-Einsteun statistics
If their wave functions really do overlap as you just said, then they cannot be a subject of Boltzmann statistics anymore. IMO you're just confused in this matter and you're developing a new description for an old concept. After all, in the same way, like you describing the particle-wave duality with gravitational lens-wave duality: it's the reinvention of wheel of the same category.
Torbjorn_Larsson_OM
5 / 5 (1) Nov 10, 2012
Ah, interesting naturally.

@ Macksb: The connection-to-emergence is possible but only one pathway among many. For example, a connection of riverlets may still be a delta, not a convergence of rivers.

The types of superconductivity we know are not emergent, but pseudoparticle (Cooper pair) pathways. Extraordinary claims need extraordinary evidence, but we have mundane predictions instead.

@ johanfprins: Here you go with your trial publications again. Or is it the same one?

No matter, the hypothesis that pseudogap playes a role are not sufficiently constrained to be a fact. "the pseudogap state, whose role is the topic of intense debate " says it all.

Including that you are crackpot trolling as tested well by your many concurrent rambling threads, leading from nowhere to nowhere. ValeriaT (who is more deluded, there isn't even an aether!) and johanfprins is a pair made for the crackpot dungeon.
johanfprins
1 / 5 (2) Nov 10, 2012
But the electrons in Cooper pairs have their wave function overlapped too.
No they are not: Inherent in "particle-exchange" is the assumption of no overlapping.
This is why we are calling these pairs entangled
Cooper pairs cannot form at all: And even if they could they cannot be entangled: This is a QFT fallacy.
since these bi-electron orbitals are not Cooper pairs and secondly they are subject to Boltzmann statistics NOT Bose-Einsteun statistics
If their wave functions really do overlap as you just said, then they cannot be a subject of Boltzmann statistics anymore. If they all overlap, you are correct: But if they they overlap in pairs, these pairs still act as separate entities: They MUST then act as distinguishable entities; and such entities adhere to Boltzmann statistics. If they ALL totally overlap they cannot act as distinguishabkle charge-carriers and the whole argument becomes ridiculous..
ValeriaT
1 / 5 (1) Nov 10, 2012
Cooper pairs cannot form at all: And even if they could they cannot be entangled: This is a QFT fallacy.
Why not? The "QFT fallacy" argument is a tautology, i.e. just another logical fallacy.
They MUST then act as distinguishable entities; and such entities adhere to Boltzmann statistics
But the BCS theory doesn't consider Cooper pairs a distinguishable entities: it just attributes them a property of boson condensate. Which is why they do participate in superconductivity. Anyway, you still didn't explain, in which your "bi-electron orbitals" differ from Cooper pairs with the exception, they "just cannot be held together with phonons because of QFT" - which is rather weak argument even in conditions of PO forum.
johanfprins
1 / 5 (2) Nov 10, 2012
But the BCS theory doesn't consider Cooper pairs a distinguishable entities:
Oh yes it does: The model even calculate their size. If they are indistighuishable you will not be able to do this
it just attributes them a property of boson condensate.
It does NOT
Which is why they do participate in superconductivity.
This not the reason. If they are indistiguisghable they will NOT be able to act as charge-carriers ar all.
Anyway, you still didn't explain, in which your "bi-electron orbitals" differ from Cooper pairs with the exception, {/q] Stop lying and playing games: I have done this clearly and noted that pairs are not mandatory to have SC. Is this the same as BCS?
which is rather weak argument even in conditions of PO forum.

My best argument is that my model has so far exolained all the experimental data that I know off for suoerconduction in all materials: Does BCS do this. Not bloody likely!
ValeriaT
not rated yet Nov 10, 2012
The model even calculate their size. If they are indistinguishable you will not be able to do this

The fact, the twins are indistinguishable each other doesn't mean, you cannot measure their size - on the contrary, it's way simpler...

BTW The recent observation of Cooper pairs in aromatic hydrocarbons
Macksb
1 / 5 (1) Nov 11, 2012
Torbjorn: Thanks for your comment. Yes, my analysis is one possible pathway among many. (Your riverlet analogy is not apt, however. Riverlets in deltas diverge. Deltas fan out.)

I also agree, more or less, that the types of superconductivity we know are not emergent. That is true if by "know" you mean only BCS superconductivity; and perhaps mag diboride justifies the "s" that you put on "type," but barely.

I think the mechanism for the new supers will prove to be emergent. Oscillators couple in pairs, non-emergently; but they also couple in mass synchrony, emergently. See Winfree, Kuramoto, Strogatz, Mirollo. That bifurcation means that a theory of coupled oscillators could explain all of the supers--old and new. Occam would approve. BCS theory says that phonons (oscillations) cause electrons (oscillators) to unite antisynchronously their two principal forms of oscillation, forming a boson--perfect coupling. Which is the result that Winfree predicted for two oscillator systems.
johanfprins
1 / 5 (2) Nov 11, 2012
The fact, the twins are indistinguishable each other doesn't mean, you cannot measure their size - on the contrary, it's way simpler...
This is one of the bigest misconceptions in physics ever: The assumption that in Statistical Thermodynamics "identical" has the same meaning as "indistinguishable".

Consider a bottle filled with say aragon-gass: When modelling the thermodynamics, one has to use Boltzmann statistics: And this statistics is derived by assuming that the atoms are distinguishable. So, according to your agument, argon-atoms cannot be identical! Can you not see what utter rot you are spouting?

BTW The recent observation of Cooper pairs in aromatic hydrocarbons
There is NO PROOF WHATSOEVER that these electron pairs are caused by phonon-exchange. To call an electron pair a Cooper Pair you must first proof that the electrons are glued by virtual phonon exchange. Only a fool will call a paired electron-state a Cooper Pair without this evidence.

johanfprins
1 / 5 (2) Nov 11, 2012
The photons which form a stationary wave within a laser cavity or blackbody-cavity, are not separate entities: That is why they are indistnguishable even though they are identical entities before they form the "multi-particle" wave: There are no "multi-particle" waves since, the separate photons, which can be distinguished, even though they are identical, lose their seperate identical identities when they quantum-mechanically combine to form the macro-wave.

Thus, if a Bose-Einstein Condensate forms, this condensate can also not consist of separate identical enties: The entities must lose their separate existences to form a macro holistic-wave. This does not happen for Cooper Pairs according to the BCS model, and therefore the condensate it models is not a Bose-Einstein Condensate. Since the identical enities, are still distinguishable, such a condensate must be a classical condensate subject to Boltzmann statistics: The latter does not require electron-pairs in order to form.
johanfprins
1 / 5 (2) Nov 11, 2012
We all agree that for SC to occur. you must have a gap in the electron-energy spectrum; and that the electrons above the gap conduct normally, while the electron-states below the gap form SC charge-carriers. According to BCS, this gap starts to form at the critical temperature and then increases to its maximum size at T=0.

A few years ago, compelling evidence was found from inelastic neutron scattering on lead that the full gap, as it manifests at T=0, is already present for temperatures above the critical temperature.

This implies that as you cool the specimen, the normal charge-carriers above the gap decreases since they de-excite to form SC charge-carriers below the gap. This behaviour across a gap is vintage Boltzmann-statistics driven by an activation-energy!

The activation energy is determined by the Fermi-level. At high temperatures the Fermi-level is amongst the normal charge-carriers above the gap. As you cool, the Fermi-level moves down in energy.
johanfprins
1 / 5 (2) Nov 11, 2012
When the Fermi-level moves into the gap, the activation energy of the Boltzmann factor is the difference in energy between the top of the gap and the energy of the Fermi-level within the gap.

Thus when the Fermi-level just enters the gap, the activation is zero and with further cooling the Fermi-level moves to lower energies, so that the activation energy increases to reach the gap size at T=0. Having been a co-invetor of the transistor, Bardeen should hvae known that this is what happens across an energy-gap within a material when cooling: But maybe he did not: Schockley did comment that Bardeen was a fool!

Nonetheless, this normal increase in activation energy, which can be experimentally measured, has been interpreted by Bardeen, Cooper and Schrieffer as an increase in the actual energy-size of the gap owing to Cooper Pair formation. LOL!

The saddest part of all is that the physicists that after them have believed this fallacy for more than 50 years by now!