Jun 17 2019
Researchers aiming to decode the mechanism behind superconductivity in “stripe-ordered” cuprates have identified an extraordinary metallic state when trying to turn superconductivity off.
A phase diagram of LBCO at different temperatures and magnetic field strengths. Colors represent how resistant the material is to the flow of electrical current, with purple being a superconductor with no resistance. When cooled to near absolute zero with no magnetic field, the material acts as a 3D superconductor. As the magnetic field strength goes up, 3D superconductivity disappears, but 2D superconductivity reappears at higher field strength, then disappears again. At the highest fields, resistance grew, but the material retained some unusual metallic conductivity, which the scientists interpreted as an indication that charge-carrier pairs might persist even after superconductivity is destroyed. (Image credit: Brookhaven National Laboratory)
Cuprates are copper-oxide materials that have alternating regions of magnetism and electric charge. The researchers discovered that under their experimental conditions, the material retains some amounts of conductivity even after it loses its potential to carry electrical current without any energy loss, and possibly the electron or hole pairs needed for its superconducting superpower.
This work provides circumstantial evidence that the stripe-ordered arrangement of charges and magnetism is good for forming the charge-carrier pairs required for superconductivity to emerge.
John Tranquada, Physicist, Brookhaven National Laboratory, U.S. Department of Energy
Tranquada, along with his co-authors from the National High Magnetic Field Laboratory at Florida State University, where some of the work was carried out from Brookhaven Lab, has detailed the findings in a paper recently reported in Science Advances. In addition, an associated paper in the Proceedings of the National Academy of Sciences by co-author Alexei Tsvelik, a theorist at Brookhaven Lab, gives a better understanding of the hypothetical underpinnings for the observations.
The researchers were examining a specific formulation of lanthanum barium copper oxide (LBCO) that displays a strange form of superconductivity at temperatures of 40 K (–233 °C)—that is comparatively warm in the world of superconductors. Traditional superconductors have to be cooled with liquid helium to temperatures close to –73 °C (absolute zero or 0 K) in order to carry current with no energy loss. If the mechanism behind this “high-temperature” superconductivity is known, it may lead to the finding or strategic design of superconductors working at higher temperatures.
“In principle, such superconductors could improve the electrical power infrastructure with zero-energy-loss power transmission lines,” stated Tranquada, “or be used in powerful electromagnets for applications like magnetic resonance imaging (MRI) without the need for costly cooling.”
The mystery of high-Tc
About 33 years ago. LBCO was the first high-temperature (high-Tc) superconductor to be discovered. It includes copper-oxide isolated by layers made up of barium and lanthanum. Compared to lanthanum, barium contributes fewer electrons to the copper-oxide layers, and hence, at a specific ratio, the imbalance leads to vacancies of electrons called holes in the cuprate planes.
Such holes can serve as charge carriers and combine together, similar to electrons, and at temperatures less than 30 K, current can travel via the material without any resistance in three dimensions—both between and within the layers.
This material has an odd trait where the holes separate into “stripes” in the copper-oxide layers at the specific barium concentration, and these stripes alternate with regions of the magnetic alignment. Since this discovery in 1995, there has been an intense debate regarding the contribution of these stripes in inhibiting or inducing superconductivity.
In 2007, Tranquada along with his group came across the most extraordinary form of superconductivity in this material at the greater temperature of 40 K. When the team tried to change the barium amount to just under the amount that enabled 3D superconductivity, they noticed 2D superconductivity—implying not between the copper-oxide layers but rather just within them.
“The superconducting layers seem to decouple from one another,” stated the theorist, Tsvelik.
Within the layers, the current can still flow without any loss in all directions; however, resistivity exists in the direction perpendicular to the layers. Such an observation was considered as a sign that charge-carrier pairs were creating “pair density waves” with orientations that are perpendicular to each other in adjacent layers.
That’s why the pairs can’t jump from layer to another. It would be like trying to merge into traffic moving in a perpendicular direction. They can’t merge.
Alexei Tsvelik, Study Co-author and Theorist, Brookhaven National Laboratory
Superconducting stripes are hard to kill
In the latest experiment, the researchers explored deeper into studying the origins of the unique superconductivity in the exclusive formulation of LBCO by attempting to destroy it. “Often times we test things by pushing them to failure,” said Tranquada. The researchers’ destruction technique was subjecting the material to strong magnetic fields created at Florida State.
As the external field gets bigger, the current in the superconductor grows larger and larger to try to cancel out the magnetic field. But there’s a limit to the current that can flow without resistance. Finding that limit should tell us something about how strong the superconductor is.
John Tranquada, Physicist, Brookhaven National Laboratory, U.S. Department of Energy
For instance, if the stripes of magnetism and charge order in LBCO are not good for superconductivity, it should be destroyed by a modest magnetic field. “We thought maybe the charge would get frozen in the stripes so that the material would become an insulator,” added Tranquada.
However, the superconductivity turned out to be relatively stronger.
Brookhaven physicist Genda Gu developed perfect LBCO crystals, which were used by Yangmu Li, a postdoctoral fellow who works in Tranquada’s laboratory, to measure the conductivity and resistance of the material under different conditions at the National High Magnetic Field Laboratory.
At just above the absolute zero temperature and without the presence of the magnetic field, the material displayed full, 3D superconductivity. Maintaining the temperature constant, the researchers had to increase the external magnetic field considerably in order to make the 3D superconductivity disappear. More surprisingly, when the field strength was increased further, the resistance inside the copper-oxide planes again went down to zero.
“We saw the same 2-D superconductivity we’d discovered at 40K,” stated Tranquada.
The 2D superconductivity was destroyed when the field was ramped up further; however, it never fully destroyed the ability of the material to carry standard current.
“The resistance grew but then leveled off,” observed Tranquada.
Signs of persistent pairs?
When more measurements were made under the highest-magnetic-field, the charge-carriers in the material, although no longer superconducting, could still exist as pairs, said Tranquada.
The material becomes a metal that no longer deflects the flow of current. Whenever you have a current in a magnetic field, you would expect some deflection 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 deflection.
Alexei Tsvelik, Study Co-author and Theorist, Brookhaven National Laboratory
To put this in simple terms, even after the superconductivity is damaged, the material still retains one of the main signatures of the “pair density wave” that is typical of the superconducting state.
“My theory relates the presence of the charge-rich stripes with the existence of magnetic moments between them to the formation of the pair density wave state,” stated Tsvelik. “The observation of no charge deflection at high field shows that the magnetic field can destroy the coherence needed for superconductivity without necessarily destroying the pair density wave.”
Together these observations provide additional evidence that the stripes are good for pairing. We see the 2D superconductivity reappear at high field and then, at an even higher field, when we lose the 2D superconductivity, the material doesn’t just become an insulator. There’s still some current flowing. We may have lost coherent motion of pairs between the stripes, but we may still have pairs within the stripes that can move incoherently and give us an unusual metallic behavior.
John Tranquada, Physicist, Brookhaven National Laboratory, U.S. Department of Energy
The DOE Office of Science funded the study. The National Science Foundation supports the National High Magnetic Field Laboratory at Florida State University.