Finding the 'heart' of an obstacle to superconductivity

Finding the 'heart' of an obstacle to superconductivity
Schematic of a copper oxide crystal lattice shows the bonds with oxygen atoms "east and west" and "north and south" of copper atoms (actually "northeast" and "northwest" in this image. Copper atoms are represented by dots. An asymmetry in electron density between the two bonds creates the appearance of "density waves" across the sample. At right, how it looks to the scanning tunneling microscope. Credit: Davis Lab

A team at Cornell and Brookhaven National Laboratory has discovered that previously observed density waves that seem to suppress superconductivity are linked to an electronic "broken symmetry," offering an important clue to why superconductivity doesn't happen at higher temperatures.

"This exotic state has been predicted for decades," said J.C. Séamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences at Cornell and director of the Center for Emergent Superconductivity at Brookhaven National Laboratory. "It is a pattern of in a crystal that has never been seen experimentally before."

The results were reported July 2 online in the Proceedings of the National Academy of Sciences.

Superconductivity, where an electric current moves with zero resistance, is seen in metals cooled almost to absolute zero (−273.15 degrees Celsius or −459.67 Fahrenheit). Newly discovered materials superconduct at temperatures up to around 150 degrees C above absolute zero. Moving that temperature higher – even up to room temperature – could lead to a revolution in motors, generators and power transmission.

Among the more interesting and important new superconductors are cuprates, which are made up of layers of copper and oxygen atoms interweaved with layers of other elements. Electric charges around atoms in the "doping" layers alter the arrangement of electrons in the copper oxide layers, allowing some electrons to form tightly linked "Cooper pairs" that can move freely to conduct electricity without resistance.

With little or no doping, cuprates are, paradoxically, insulators. With enough doping they achieve superconductivity. In between is a phase called the "pseudogap," where many electrons that ought to be available to form Cooper pairs appear to be absorbed by another electronic state. The pseudogap state often appears at much higher temperatures, so finding how it holds off the superconductive state is a major scientific and practical goal.

Davis studies these materials with a scanning tunneling microscope (STM), that can scan a surface in steps smaller than the width of an atom, reading the energy levels of electron. Observations have shown that in the pseudogap state, electrons in cuprates form "density waves" – alternating rows of many or fewer electrons, like the alternating compression of air molecules in sound waves. Theorists suggest the presence of these waves might block superconductivity.

The copper oxide layers in cuprates are made up of L-shaped cells consisting of a joined to two oxygen atoms, linked together in orderly rows. Another pseudogap phenomenon previously observed at Cornell is that the energy levels of the electrons are mismatched between "east-west" versus "north-south" bonds.

In a radical new approach, the researchers created separate images that showed the electronic structure of a cuprate just around the copper atoms, just around the oxygen atoms "east and west" of the copper and just around the "north and south." They found that the asymmetry varies gradually from one unit cell of the crystal to the next, and a graph of the changes looks like the familiar sine wave, paralleling the observed density waves.

"You have to know your enemy to vanquish him," Davis said. "Here at last we reveal the identity of that enemy. That density wave is modulating the asymmetry. There are more electrons 'north' than 'west' of a copper atom at one location. As you pull back you see that this property is varying in space so that, after half a wavelength, there are more electrons 'west' than 'north' of their copper atom; after a full wavelength, there are more electrons to 'north' than 'west' once again. Then the cycle repeats."

Eun-Ah Kim, associate professor of physics, and Michael Lawler, adjunct professor of physics, first suggested that Davis look for the broken symmetry. The idea of a relationship between that and density waves was proposed by Subir Sachdev of Harvard University. "It would be a terrible coincidence if you discovered this broken symmetry, and you discovered the modulating it, and yet they were not related," Davis concluded.

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Jul 23, 2014
This is a periodic oscillation. More electrons north, then more west, then more north, then more west. The period is one wave length. The north cycle is one half out of phase with the west cycle. This is the exact result predicted by Art Winfree's law of coupled periodic oscillators for a two oscillator system. In more than 100 physorg posts over four years I have predicted that Art's law drives superconductivity of all types--new and old.

Cooper pairs are a two electron Winfree system. Two individual electrons. The Winfree system in this article is the same, but it operates on two groups of electrons. The groups are coupled by Winfree's law, each group coupled exactly anti-synchronously to the other group. Half out of phase to each other.

This research confirms my theory.

Jul 23, 2014
Winfree's law of coupled periodic oscillators is the plate tectonics that underlies all superconductivity. And much more.

Jul 24, 2014
The state of superconductivity is on line with the mixed valency theory as proposed by the Robin-Day Classification. I mean a super conductor is a mixture of copper oxides and other metal oxides reduced by hydrogen or oxidized by oxygen at high temperature.A good example of a mixed valence compound is copper sulfide (similar to copper oxide) in which X-Ray photo-electron spectroscopy cannot distinguish between Cu(II) and Cu(I) because the electrons are de-localized ( homeless) . These de-localized electrons can display a discontinuity in certain situation. This is the state of superconductivity.

Jul 24, 2014
How many millions of dollars are still going to be wasted in order to measure the obvious without comprehending it? The pseudogap consists of stationary Mott electronic-states which conduct normally by means of hopping and superconduct by means of "tunnelling" when the average near-neighbour distances become less than a critical value. Stop this nonsense about "broken symmetries". The latter is unadulterated BS!!

Jul 24, 2014
The working of room temperature superconductors is already understood and described well, the physicists just waste the money of tax payers for reinventing of wheel. The independent researchers already publish the hot superconductors routinely without expensive research bases, synchrotron beams and twaddling about charge-wave BS.

Jul 25, 2014
The superconductivity state for cuprates is 2 x 2 Cooper pairs, with the east west pair dancing anti synchronously to the north south pair. Much like a square dance: one couple hands together in the center, other couple hands further apart. Or the trot gait of a horse. Each pair is a Cooper pair: one electron matched anti synchronously with its partner, half out of phase to its partner. And each pair is half out of phase to the other pair.

The "super" perovskites are like a horse and rider system. Four legs (the cuprate plane in which Cooper pairs form) that move in certain gaits, and a rider up top (a different atom). This is a 5 oscillator Winfree system. The 5th oscillator (the rider) synchronizes its beat with the 2 x 2 paired motion of the legs. Lots of motion (higher temp than old BCS) but all in a Winfree synchrony.

Below Tc, precise enough for the super state. Above Tc, pseudogap phase: similar, but two groups dancing, not Astaire and Rogers x 2.

Jul 25, 2014
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Jul 25, 2014
My fourth post. My hunch is that some Astaire and Rogers 2 x 2 pairs exist in pseudogap phase, but not enough. This consistent with data in above article if I read it correctly on this detail. Tc will occur when a high enough percentage of the electrons is so paired, which will then flip the switch over to superconductivity. Necessary percentage probably below 40 percent, perhaps well below.

General idea in Winfree terms: Lower temp = lower frequency of oscillations = easier to Winfree sync lattice more precisely, which then enables more electrons to communicate with their counterparts, which leads to more Winfree sync (Cooper pairs) at level of individual electrons.

Jul 26, 2014
Electron-pairing is not the reason for superconduction. Please stop this absurd myth!

Jul 28, 2014
So pseudo gap phase is best viewed as a cousin of, not an obstacle to, the super phase: a group level Winfree pattern of electrons (pseudogap) as compared with an individual level Winfree pattern of electrons ( super).

Winfree's law fits the data and answers all the questions. Such as: Why pseudogap? Why transition from pseudo to super? What micro details in super? Why perovskites? How can super phase and high temp coexist? Winfree law points way to other Cooper patterns, such as 4 way and 6 way (mag diboride, with six insect legs as Winfree model rather than 4 horse legs).

Yes, I know it is heretical to use ordinary natural forms to guess at forms that might emerge in the quantum world. But periodic oscillations exist in nature and quantum physics, and the math of Winfree's Law applies to both. The Winfree patterns must be the same. Winfree's Law is unknown or ignored by physicists--which might explain why superconductivity has confounded the experts for so long.

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