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What happens when really powerful magnets—capable of producing magnetic fields nearly two million times stronger than Earth’s—are applied to materials that have a “super” ability to conduct electricity when chilled by liquid nitrogen? A team of scientists set out to answer this question in one such superconductor made of the elements lanthanum, strontium, copper, and oxygen (LSCO). They discovered that the electrical resistance of this copper-oxide compound, or cuprate, changes in an unusual way when very high magnetic fields suppress its superconductivity at low temperatures.
“The most pressing problem in condensed matter physics is understanding the mechanism of superconductivity in cuprates because at ambient pressure they become superconducting at the highest temperature of any currently known material,” said physicist Ivan Bozovic, who leads the Oxide Molecular Beam Epitaxy Group at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and who is a coauthor of the Aug. 3 Science paper reporting the discovery. “This new result—that the electrical resistivity of LSCO scales linearly with magnetic field strength at low temperatures—provides further evidence that high-temperature superconductors do not behave like ordinary metals or superconductors. Once we can come up with a theory to explain their unusual behavior, we will know whether and where to search for superconductors that can carry large amounts of electrical current at higher temperatures, and perhaps even at room temperature.”
(Clockwise from back left) Brookhaven Lab physicists Ivan Bozovic, Anthony Bollinger, and Jie Wu, and postdoctoral researcher Xi He used the molecular beam epitaxy system seen above to synthesize perfect single-crystal thin films made of lanthanum, strontium, oxygen, and copper (LSCO). They brought these superconducting films to the National High Magnetic Field Laboratory to see how the electrical resistance of LSCO in its "strange" metallic state changes under extremely strong magnetic fields.
Cuprates such as LSCO are normally insulators. Only when they are cooled to some hundred degrees below zero and the concentrations of their chemical composition are modified (a process called doping) to a make them metallic can their mobile electrons pair up to form a “superfluid” that flows without resistance. Scientists hope that understanding how cuprates achieve this amazing feat will enable them to develop room-temperature superconductors, which would make energy generation and delivery significantly more efficient and less expensive.
In 2016, Bozovic’s group reported that LSCO’s superconducting state is nothing like the one explained by the generally accepted theory of classical superconductivity; it depends on the number of electron pairs in a given volume rather than the strength of the electron pairing interaction. In a follow-up experiment published the following year, they obtained another puzzling result: when LSCO is in its non-superconducting (normal, or “metallic”) state, its electrons do not behave as a liquid, as would be expected from the standard understanding of metals.
“The condensed matter physics community has been divided about this most basic question: do the behaviors of cuprates fall within existing theories for superconductors and metals, or are there profoundly different physical principles involved?” said Bozovic.
Continuing this comprehensive multipart study that began in 2005, Bozovic’s group and collaborators have now found additional evidence to support the latter idea that the existing theories are incomplete. In other words, it is possible that these theories do not encompass every known material. Maybe there are two different types of metals and superconductors, for example.
“This study points to another property of the strange metallic state in the cuprates that is not typical of metals: linear magnetoresistance at very high magnetic fields,” said Bozovic. “At low temperatures where the superconducting state is suppressed, the electrical resistivity of LSCO scales linearly (in a straight line) with the magnetic field; in metals, this relationship is quadratic (forms a parabola).”