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Electron pairs (or pairs of holes) may survive the effort to kill superconductivity



  Electron pairs (or & # 39; hole & # 39; pairs) may survive an attempt to kill superconductivity.
Copper oxide layers of LBCO (the lanthanum barium layers would be in between). 3-D superconductivity occurs when the current in and between the copper oxide layers can flow freely while 2-D superconductivity exists when the current flows freely only within the layers (not perpendicular). The perpendicular orientations of stripe patterns from one layer to the next may be part of what inhibits the movement of current between layers. Picture credits: Brookhaven National Laboratory

Scientists who wanted to investigate the mechanism of superconductivity in "striped" cuprates-copper oxide materials with alternating regions of electric charge and magnetism-discovered an unusual metallic state in an attempt to eliminate superconductivity. They found that under the conditions of their experiment, even if the material loses its ability to carry electric current without energy loss, it maintains some conductivity – and possibly the electron (or hole) pairs required for its superconducting superpower.

"This work provides evidence that the striped ordering of charges and magnetism is good for the formation of the pairs of charge carriers necessary for the formation of superconductivity," said John Tranquada, a physicist at the Brookhaven National Laboratory of the US Department of Energy ,

Tranquada and his co-authors from the Brookhaven Lab and Florida State University's National High Magnetic Field Laboratory, where part of the work was done, describe their findings in an article just published in Science Advances . , A related article in the Proceedings of the National Academy of Sciences by co-author Alexei Tsvelik, a theorist at the Brookhaven Lab, provides insights into the theoretical foundations of observations.

Scientists investigated a specific formulation of lanthanum barium copper oxide (LBCO), which has an unusual form of superconductivity at a temperature of 40 Kelvin (-233 degrees Celsius). This is relatively warm in the field of superconductors. Conventional superconductors must be cooled with liquid helium to temperatures near -273 ° C (0 Kelvin or absolute zero) to conduct power without energy loss. Understanding the mechanism behind such "high-temperature" superconductivity could guide the discovery or strategic design of superconductors operating at higher temperatures.

"In principle, such superconductors could improve the electrical energy infrastructure with lossless power transmission lines," Tranquada said, "or be used in high-performance electromagnets for applications such as magnetic resonance imaging (MRI) without the need for costly cooling."

The Secret of High-Tc

LBCO was the first high-temperature superconductor (High-Tc) was discovered 33 years ago. It consists of layers of copper oxide separated by layers of lanthanum and barium. Barium contributes less electrons than lanthanum to the copper oxide layers, so that in a certain ratio due to the imbalance electron vacancies, so-called holes, remain in the copper planes. These holes, like electrons, can act as carriers and mate, and at temperatures below 30 K, current can flow through the material without resistance in three dimensions – both within and between layers.

A curious feature of this material is that in the copper oxide layers at the particular barium concentration, the holes disintegrate into "stripes" that alternate with areas of magnetic alignment. Since this discovery in 1995, the role of these strips in the induction or suppression of superconductivity has been controversial.

In 2007, Tranquada and his team discovered the most unusual form of superconductivity in this material at a higher temperature of 40K. If they change the amount of barium so that it is just below the 3-D superconductivity, they observe a 2-D superconductivity – only within the copper oxide layers, but not between them.

"The superconducting layers seem to decouple from each other," said the theorist Tsvelik. The current can still flow losslessly in any direction within the layers, but there is a resistivity in the direction perpendicular to the layers. This observation was interpreted as a sign that pairs of charge carriers in neighboring layers form "pairwise" light waves with mutually perpendicular orientations. "That's why couples can not jump from one shift to the next, it's like trying to integrate into the traffic moving in a vertical direction, they can not be brought together," said Tsvelik Kill . 19659005] In the new experiment, the scientists investigated more closely the origins of the unusual superconductivity in the specific formulation of LBCO, trying to destroy it. "We often test things by destroying them," Tranquada said. Their method of destruction was to expose the material to strong magnetic fields generated in the State of Florida.

  Electron pairs (or pairs of holes) may survive the attempt to kill superconductivity.
A phase diagram of LBCO at different temperatures and magnetic field strengths. Colors indicate how resistant the material is to electrical current, where purple is a superconductor without resistance. When the material is cooled to near absolute zero without a magnetic field, it acts as a 3-D superconductor. With increasing magnetic field strength, the 3-D superconductivity disappears, but the 2-D superconductivity occurs again at higher field strength and then disappears again. Resistance grew in the highest fields, but the material retained an unusual metallic conductivity, which the scientists interpreted as an indication that pairs of charge carriers could persist even after superconductivity was destroyed. Picture credits: Brookhaven National Laboratory

"The larger the outer field becomes, the greater the current in the superconductor to balance the magnetic field," Tranquada explained. "But there is a limit to the current that can flow without resistance, and if we find that limit, we know how strong the superconductor is."

For example, if the charge-order stripes and magnetism in LBCO are bad for superconductivity, a modest magnetic field should destroy it. "We thought the charge would freeze into the strip, making the material an insulator," Tranquada said.

However, superconductivity proved to be much more robust.

Using Perfect LBCO Crystals by Brookhaven physicist Genda Gu, Yangmu Li, a postdoctoral researcher working in Tranquada's lab, measured the resistance and conductivity of the material under various conditions at the National High Magnetic Field Laboratory. At a temperature just above absolute zero with no magnetic field present, the material showed complete 3D superconductivity. At constant temperature, the scientists had to raise the external magnetic field significantly, so that the 3D superconductivity disappeared. All the more surprising, as they further increased the field strength, the resistance in the copper oxide planes dropped back to zero!

"We saw the same 2-D superconductivity that we discovered at 40K," said Tranquada. 19659005] The ramp-up of the field further destroyed the 2-D superconductivity, but never fully the material's ability to carry normal current.

"The resistance increased, but then went back," said Tranquada.

Signs of persistent couples?

Additional measurements under the highest magnetic field showed that the charge carriers in the material, although no longer superconducting, can still exist as pairs, Tranquada said.

"The material becomes a metal that does not distract the flow of electricity," Tsvelik said. "If you have a current in a magnetic field, you expect a certain deflection of the charges – electrons or holes – in the direction perpendicular to the current. [what scientists call the Hall effect] But that is not the case. There is no distraction."

In other words, even after the superconductivity has been destroyed, the material retains one of the key signatures of the "pair-density wave" characteristic of the superconducting state.

"My theory refers to the presence of the high-charge stripes with the existence of magnetic moments between them to form the pair-seal state," Tsvelik said. "The observation that there is no charge deflection in the high field shows that the magnetic field can destroy the coherence required for superconductivity without destroying the pairwise direct wave."

Together, these observations provide additional evidence that the stripes are good for mating. Said Tranquada. "We see 2-D superconductivity reoccur at high fields, so if we lose 2-D superconductivity at higher fields, the material will not just become an insulator, it will still flow, we may have lost . " coherent movement of pairs between the strips, but we can still have pairs within the strips that can move incoherently and give us an unusual metallic behavior. "


Discovery of the field-induced pair-density wave state in high-temperature superconductors


Further information:
"Tuning from failed superconductor to fancy insulator with magnetic field" Science Advances (2019). DOI: 10.1126 / sciadv.aav7686, https://advances.sciencemag.org/content/5/6/eaav7686

A.M. Tsvelik. Superconductor Metal Transition in an Odd Paired Superconductor in a Magnetic Field, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1902928116

Provided by
Brookhaven National Laboratory




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Electron pairs (or pairs of holes) may survive the effort to kill superconductivity (2019, June 14)
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