'Long-awaited explanation' for mysterious effects in high-temperature superconductors

Jun 07, 2013
Quadrupole order: At each copper atom (grey balls) there is a quadrupole moment. All together, these form a kind of chessboard pattern, whereby the individual squares of the chessboard differ in the orientation of the positively and negatively charged areas (green: positive areas left and right; grey: positive areas top and bottom). At the boundaries between green and grey surfaces, the signs change. Copper atoms close to the boundary have a smaller quadrupole moment than copper atoms in the middle of the areas. Credit: Konstantin Efetov and Hendrik Meier (Institut für Theoretische Physik III)

A German-French research team has constructed a new model that explains how the so-called pseudogap state forms in high-temperature superconductors. The calculations predict two coexisting electron orders. Below a certain temperature, superconductors lose their electrical resistance and can conduct electricity without loss.

"It is not to be excluded that the new pseudogap theory also provides the long-awaited explanation for why, in contrast to conventional metallic superconductors, certain ceramic copper oxide bonds lose their at such unusually high temperatures", say Prof. Dr. Konstantin Efetov and Dr. Hendrik Meier of the Chair of Theoretical at the Ruhr-Universität Bochum. They obtained the findings in close cooperation with Dr. Catherine Pépin from the Institute for Theoretical Physics in Saclay near Paris.

The team reports in the journal Nature Physics.

Transition temperature much higher in ceramic than in metallic superconductors

Superconductivity only occurs at very low temperatures below the so-called . In metallic superconductors, this is close to the absolute zero point of 0 Kelvin, which corresponds to about -273 degrees Celsius. However, crystalline ceramic materials can be superconductive at temperatures up to 138 Kelvin. For 25 years, researchers puzzled over the physical bases of this high-temperature superconductivity.

Pseudogap: energy gap above the transition temperature

In the superconducting state, electrons travel in so-called Cooper pairs through the of a material. In order to break up a Cooper pair so that two free electrons are created, a certain amount of energy is needed. This difference in the energy of the Cooper electrons and the so-called is called an energy gap. In , compounds based on copper oxide bonds, a similar also occurs under certain circumstances above the transition temperature – the pseudogap. Characteristically the pseudogap is only perceived by electrons with certain velocity directions. The model constructed by the German-French team now allows new insights into the physical inside of the pseudogap state.

Two competing electron orders in the pseudogap state

According to the model, the pseudogap state simultaneously contains two electron orders: d-wave superconductivity, in which the electrons of a Cooper pair revolve around each other in a cloverleaf shape, and a quadrupole density wave. The latter is a special electrostatic structure in which every copper atom in the two-dimensional crystal lattice has a quadrupole moment, i.e. two opposite regions of negative charge, and two opposite regions of positive charge. d-wave superconductivity and quadrupole density wave compete with each other in the pseudogap state. Due to thermal fluctuations, neither of the two systems can assert itself. However, if the system is cooled down, the thermal fluctuations become weaker and one of the two systems prevails: superconductivity. The critical temperature at which this occurs can, in the model, be considerably higher than the transition temperature of conventional metallic superconductors. The model could thus explain why the transition temperature in the ceramic superconductors is so much higher.

Cuprates

High-temperature superconductors are also called cuprates. In addition to copper and oxygen, they can, for example, contain the elements yttrium and barium (YBa2Cu3O7). To make the material superconducting, researchers introduce "positive holes", i.e. electron holes into the crystal lattice. Through these, the electrons can "flow" in . This is known as hole doping. The pseudogap state only sets in when the hole doping of the cuprate is neither too low nor too high.

Explore further: Physicists unlock nature of high-temperature superconductivity

More information: Efetov, K. et al. (2013): Pseudogap state near a quantum critical point, Nature Physics. DOI: 10.1038/NPHYS2641

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ValeriaT
1 / 5 (3) Jun 07, 2013
IMO the explanation of pseudogap is way simpler: it's simply the state, where the electrons are forming tiny islands of superconductive phase already, but these islands aren't still mutually connected each other - so that the material as a whole remains nonsuperconductive (but it still exhibits many bulk properties typical for superconductors).
Minich
1 / 5 (2) Jun 07, 2013
Absolute nonsense theory.
New band theory for pseudogap and superconducting gap by Minich will be sent to nature.com soon.
ValeriaT
1 / 5 (4) Jun 07, 2013
In AWT the superconductivity is mediated with chaotically moving electrons, which are fighting for their place around positively charged places inside of atom lattice (so called "holes") like the hungry hens around feeders. Now you can imagine easily, what will happen, if you will increase the number of feeders gradually: if their number will be low, then the hens will fight violently, but they will form mutually insulating islands around sparsely distributed feeders - this situation corresponds the pseudogap phase, which cannot propagate current at distance .

If you add too many feeders into farm, the the hens will form a continuous phase around feeders, but their density will be low there, as the hens will not have any reason for fight anymore. This analogy explains, why the (existence of) pseudogap is limited to underdoped superconductors only and why the overdoping of superconductor with holes kills the superconductivity as well. You need to maintain the proper density of holes.
ValeriaT
1 / 5 (4) Jun 07, 2013
Absolute nonsense theory.
IMO this model is too centric to Cooper pairs. In high temperature superconductors like the cuprates the spin-spin interactions between electrons play a marginal role, because they're quite weak. The electrons do not form a Cooper pairs, but rather a "Cooper teams", i.e. the whole stripes of electrons, each of which are of opposite spin. 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, and superconductivity occurs because these hole-pairs march collectively along the channels, like trams on pairs of tramlines running between the blocks of houses. We can clearly see these structures with neutron diffraction, for example.
ValeriaT
1 / 5 (4) Jun 07, 2013
One consequence for example is the fact, that Josephson junctions between cuprates tunnel at much higher distances, than the standard Cooper pair theory predicts (it exhibits so-called "Giant Proximity Effect", applying to distances larger than some 20 nm). But not everything is wrong with the above model.We should realize, it's a model of Fermi surface geometry in pseudogap state - not the theory of superconductive state, neither the band gap theory of Minich. It considers checkerboard charge-ordered state, which can be really seen at the underdoped samples. And I do like the concept of mutual duality of transverse d-wave and longitudinal quadrupole density wave, which corresponds the duality of EM and gravitational waves in vacuum.
shavera
4.4 / 5 (7) Jun 07, 2013
unfortunately ValeriaT (zephyr, etc.) Science is more than just cute stories. The work done by these researchers comes from years of experiments and decades of observations before that. It's fine if you want to tell your little fictions, but of course, without any data or developed and confirmed theory, they're little more than that.
ValeriaT
1 / 5 (4) Jun 08, 2013
Science is more than just cute stories.
For scientists indeed yes but I'm talking to laymen here. The laymen deserve intuitive, easy to follow explanation of these phenomena. The nature of contemporary physics is descriptive, it doesn't bother with "WHY" questions, so there is opened field for people like me. Anyway, I'm presenting it here simply because these analogies work - if they wouldn't work or if they would violate the logics of the results, I wouldn't present them here.
without any data or developed and confirmed theory
I'm always linking the data supported with my insights. I'm hated with big science, because I'm representing a break of informational monopoly, from the same reason, why Linux is hated with Microsoft. I'm just enabling to understand some connections better/earlier than they're presented with mainstream, who takes a lotta money from government for it.
johanfprins
2.2 / 5 (5) Jun 09, 2013
unfortunately ValeriaT (zephyr, etc.) Science is more than just cute stories. The work done by these researchers comes from years of experiments and decades of observations before that. It's fine if you want to tell your little fictions, but of course, without any data or developed and confirmed theory, they're little more than that.
Well said and absolutely true: The only problem is that this model is claptrap, but not for the reasons (if you can call his hallucinations "reasons") given by ValeriaT. The pseudogap is the transition to a Mott-insulator, and the lower temperature transition occurs when the density of these Mott-orbitals becomes high enough so that tunnelling by means of quantum-fluctuations can occur: Voila! Superconduction. Electron-pairs are NOT required!
ValeriaT
1 / 5 (3) Jun 09, 2013
the lower temperature transition occurs when the density of these Mott-orbitals becomes high enough
And I'm just explaining, why the "density of Mott-orbitals becomes high enough", being one step ahead of you in this way. In history of science was not quite uncommon, when the explanations of existing theories were called "hallucinations". No theorist is indeed particularly happy, when his "explanations" turn out to be ad-hoced description of reality without deeper reasoning.
johanfprins
1 / 5 (2) Jun 10, 2013
the lower temperature transition occurs when the density of these Mott-orbitals becomes high enough
And I'm just explaining, why the "density of Mott-orbitals becomes high enough", being one step ahead of you in this way.
Where have you "explained" this? Hand-waving and cartoons of ducks swimming within a non-existent aether are not scientific explanations.

If you are ahead of me write down the equation for the critical density of Mott-orbitals, in terms of their binding energy, at which superconduction occurs. I have already done this 8 years ago!
johanfprins
1 / 5 (3) Jun 10, 2013
@Natello: You are really boring and completely dishonest as proved by the corrupt manner in which you post under different names. It is simple to explain in terms of real physics using Mott-orbitals and their binding energies why any material superconducts and why other materials cannot. No intuition required! I never rely on intuition since it is usually misleading.

The Joe Eck data is a blimp within a curve that shows resistivity all the way: This is hardly proof of superconduction.
johanfprins
2 / 5 (4) Jun 11, 2013
I never rely on intuition since it is usually misleading
Yes it indeed is - for idiots. But I'm not talking about intuition, but about logical reasoning.


Then why have not yet reasoned any logic EVER on this forum? You obviously do not understand what the word "logic" means. "Logic" is NOT a duck paddling and farting bubbles within a non-existing aether.

why any material superconducts and why other materials cannot
You cannot explain it logically - this is the whole problem.


I have done so and posted it on my website. It is not just logic but also impeccable mathematics to back it up. But this is of course where YOU get lost: You do not understand ANY mathematics at all. Sue your Kindergarten teacher!
johanfprins
2 / 5 (4) Jun 11, 2013
It is not just logic but also impeccable mathematics to back it up
Nope, logical reasoning doesn't require any math with exception of predicate logics. It's the basis, i.e. skeleton of formal model, which is based upon it. We aren't combining the math equations randomly and freely, we are combining the with using of the formal logics of the physical model.
Anybody with a modicum of grey matter between his/her ears who reads this comment by you will immediately realise that you are an insane moron! Nuff said!