Electron pairs (or "holes") can survive the effort to eliminate superconductivity

Scientists seeking to understand the underlying mechanism of superconductivity in "striped" cuprates – copper oxide materials with alternating areas of electrical charge and magnetism – have discovered an unusual metallic state when trying to deactivate the superconductivity. They discovered that, under the conditions of their experience, even after the material has lost its ability to withstand the electrical current without loss of energy, it retains some conductivity, even the electron pairs (or of holes) necessary for its superconducting superpower.

"These works provide circumstantial evidence that ordered arrangement by charge and magnetism bands is good for forming the charge carrier pairs necessary for the emergence of superconductivity," said physicist John Tranquada. Brookhaven National Laboratory of the US Department of Energy.

Tranquada and his co-authors of Brookhaven Lab and the National High Magnetic Field Laboratory of Florida State University, where some of the work was done, describe their findings in a recently published article in Progress of science. A related document in the Proceedings of the National Academy of Sciences by co-author Alexei Tsvelik, theorist at Brookhaven Lab, gives an overview of the theoretical foundations of the observations.

Scientists were studying a particular formulation of copper, barium and lanthanum oxide (LBCO) with an unusual form of superconductivity at a temperature of 40 Kelvin (-233 degrees Celsius). It is relatively hot in the field of superconductors. Conventional superconductors must be cooled with liquid helium at temperatures close to -273 ° C (0 Kelvin or absolute zero) to support the current without loss of energy. Understanding the mechanism behind this "high temperature" superconductivity could guide the discovery or strategic design of superconductors operating at higher temperatures.

"In principle, such superconductors could improve the power supply infrastructure with energy transmission lines at zero loss of energy," said Tranquada, "or be used in Powerful electromagnets for applications such as magnetic resonance imaging (MRI) without the need for expensive cooling. "

The mystery of high-tc

LBCO was the first high temperature (high temperature) superconductor discovered about 33 years ago. It consists of layers of copper oxide separated by layers of lanthanum and barium. Barium contributes less to electrons than lanthanum to copper oxide layers. Thus, at a given ratio, the imbalance leaves empty spaces of electrons, called holes, in planes in cuprate. These holes can serve as charge carriers and couple, just like electrons, and at temperatures below 30K, the current can pass through the material without resistance in all three dimensions, both in and between layers.

A strange feature of this material is that, in copper oxide layers, at the particular barium concentration, the holes separate into "bands" alternating with areas of magnetic alignment. Since this discovery, in 1995, the role of these bands in the induction or inhibition of superconductivity has been the subject of much discussion.

In 2007, Tranquada and his team discovered the most unusual form of superconductivity in this material at temperatures above 40K. If they modified the amount of barium just below the amount allowing 3-D superconductivity, they observed 2-D superconductivity, that is, inside the layers. Copper oxide but not between them.

"The superconducting layers seem to decouple from each other," said theorist Tsvelik. The current can still flow without loss in all directions within the layers, but there is a resistivity in the direction perpendicular to the layers. This observation has been interpreted as a sign that charge carrier pairs formed "pair density waves" with perpendicular orientations to each other in neighboring layers. "It's for this reason that pairs can not jump from one layer to the other.This would be like trying to blend into a traffic moving in a perpendicular direction.They can not to merge, "said Tsvelik.

Superconducting scratches are hard to kill

In the new experiment, scientists deepened their exploration of the origins of unusual superconductivity in the special formulation of LBCO by attempting to destroy it. "Often we test things by pushing them to failure," said Tranquada. Their method of destruction exposed the material to powerful magnetic fields generated at Florida State.

"As the external field grows, the current in the superconductor gets bigger and bigger to try to cancel the magnetic field," Tranquada explained. "But there is a limit to the current that can flow without resistance, and finding that limit should tell us something about the power of the superconductor."

For example, if the load order bands and magnetism in LBCO are bad for superconductivity, a modest magnetic field should destroy them. "We thought maybe the load would be frozen in the scratches so the material would become an insulator," Tranquada said.

But superconductivity has proven to be much more robust.

Yangmu Li, a postdoctoral researcher working in the Tranquada laboratory, took measurements of the resistance and conductivity of the material in various conditions at the National Laboratory of High Magnetic Fields, using perfect LBC crystals grown by Genda. Gu, Brookhaven physicist. At a temperature just above absolute zero in the absence of a magnetic field, the material exhibits a complete superconductivity in 3D. By keeping the temperature constant, scientists had to greatly increase the external magnetic field to remove 3D superconductivity. Even more surprising, when they further increased the intensity of the field, the resistance in the copper oxide planes went down to zero!

"We saw the same two-dimensional superconductivity that we found at 40K," said Tranquada.

The field acceleration still destroyed 2D superconductivity, but it never completely destroyed the ability of the material to withstand the ordinary current.

"The resistance increased but then stabilized," Tranquada noted.

Signs of persistent pairs?

Additional measurements in the higher magnetic field indicated that charge carriers in the material, although no longer superconducting, may still exist in pairs, Tranquada said.

"The material becomes a metal that no longer deflects the flow of current," Tsvelik said. "Whenever you have a current in a magnetic field, you expect a deviation of the charges – electrons or holes – in the direction perpendicular to the current [what scientists call the Hall effect]. But that's not what happens. There is no deviation. "

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

"My theory links the presence of charge-rich bands to the existence of magnetic moments between them at the formation of pair density wave state," Tsvelik said. "The observation of a high-field-free deflection shows that the magnetic field can destroy the coherence necessary for superconductivity without necessarily destroying the pair density wave."

"Together, these observations provide further evidence that scratches are good to couple," said Tranquada. "We see two-dimensional superconductivity reappearing at a high field, then, at an even higher field, when we lose two-dimensional superconductivity, the material does not just become an insulator, there is still flowing current. We may have lost consistent movement of pairs between bands, but we can still have pairs in bands that can move incoherently and give us unusual metallic behavior. "