Superconductors, materials that offer no electrical resistance at low temperatures, have long been objects of study and fascination. The properties of superconductivity underpin advanced technological applications such as magnetic resonance imaging machines. So called “high-temperature” cuprate superconductors display their superconducting properties at temperatures as high as 100 degrees Kelvin — much higher than traditional superconductors such as lead or tin — and offer the promise of new applications as their properties are increasingly understood.
Now, as recently described in Science magazine, a research effort led by Lawrence Berkeley National Laboratory and including scientists from Los Alamos National Laboratory has uncovered the quantum phase transitions that may be behind the behavior of cuprate superconductors. In an experiment representing the first quantitative study of such quantum phase transitions, researchers studied cerium-cobalt-indium 5 (CeCoIn5), a material with similar crystal structure, transport properties and, key to the research, superconducting properties to the cuprates.
“We found a material that behaves the same way and has the same structure as the cuprates, but, using our magnets, we can put it deep in the normal state that occurs before superconductivity,” said research team member John Singleton. “This allows us to study the precursors to superconductivity in materials that mimic the cuprates.”
Unlike most transitions between phases of matter, quantum phase transitions are not driven by thermal parameters — heat or cold. Instead, the phase transition seen in CeCoIn5 is associated with delocalization of electrons at a transition between Fermi surfaces of different volume. Complementary experiments suggest that the change in Fermi-surface volume is not accompanied by broken symmetry in the material’s crystal structure. In cuprates, similar quantum phase transitions that appear not to be associated with a broken symmetry are thought to underlie the mechanism of superconductivity itself.
Strong magnetic fields enabled discovery
When subjected to magnetic fields of up to 73 Tesla at the National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos, the quantum phase transition in CeCoIn5 was revealed in a quantity called the Hall resistance. At (and only at) very high magnetic fields, the Hall resistance is determined by the number of mobile electrons, allowing it to be used to detect the change in electron density that occurs at the quantum phase transition.
“The high magnetic fields we’re able to achieve at the Pulsed Field Facility help us to detect the gradual, tiny changes that move the material through the quantum phase transition,” said Singleton. “At such high fields, their signature in the Hall effect becomes completely unambiguous.”
Imagining new era of applications for superconductors
Understanding the properties of cuprates may unlock a new era of applications for superconductors. The high-temperature superconductors can be made into tape to make superconducting magnets, a capability that could allow for superconductor-reliant machines of practical size — compact nuclear-fusion reactors, for instance.
Cuprates may also inform the design of new SQUIDS, or superconducting quantum interfacing devices, which detect weak magnetic fields and are employed in studies in biology, medical applications, mineral exploration, geothermal energy surveying and more.
About the MagLab
The National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos is the only pulsed field user facility in the United States. The Pulsed Field Facility develops and maintains a set of powerful, nondestructive pulsed magnets providing fields from 60 Tesla to 101 Tesla with different pulse widths that are tailored to support a wide variety of users.
Paper: “Evidence for a delocalization quantum phase transition without symmetry breaking in CeCoIn5,” by N. Maksimovic, et al. in Science.
Funding: The research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences; the National Science Foundation Graduate Research Fellowship Grant; the Gordon and Betty Moore Foundations EPiQS Initiative; and the Swedish Research Council and the K. and A. Wallenberg Foundation Award. The National High Magnetic Field Laboratories in Tallahassee, Florida, and Los Alamos, New Mexico, are supported by the National Science Foundation.