Communications of the ACM
Team Finds New Order in Exotic Superconductors
Credit: University of Maryland
All known superconductors — materials that conduct electricity with no energy loss — require cooling to very low temperatures. Even the best "high temperature" superconductors, those made of copper-oxide ceramics called cuprates, operate at sub-freezing temperatures colder than the coldest places on Earth. These cuprates require cooling with liquid nitrogen, which is not practical for many potential superconducting applications, such as smart power grids, advanced wireless communications, power-storage technologies, and new imaging systems.
However physicists from the University of British Columbia and the University of Maryland have made a new investigation into a phenomenon called "charge ordering." Charge ordering appears to compete with superconductivity in cuprates and may be a key to understanding this class of superconductors.
Charge ordering behavior, previously observed only in a class of cuprates known as hole-doped cuprates, has now been detected by the UMD/UBC team in electron-doped cuprate superconductors for the first time. Doping involves adding impurities to the cuprate materials to produce either electrons or holes, which spur cuprate materials to exhibit unusual behaviors, such as superconductivity.
Their findings are described in "Charge Ordering in the Electron-doped Superconductor Nd2–xCexCuO4," published online in the journal Science, and suggest that charge order may be a universal feature of high-temperature superconductors. Because charge order appears to compete with superconductivity in cuprates — and thus lower the temperature at which superconductivity takes place — figuring out how to control or neutralize this competing phenomena might allow cuprates materials to superconduct at higher temperatures. In addition, gaining a better understanding of charge ordering may help scientists determine the specific mechanism for superconductivity in cuprates and eventually lead to the synthesis of materials that can superconduct at room temperature.
"This study's surprising results indicate that charge order must play a very important, as yet unknown, role in high-temperature superconductivity," says Richard Greene, UMD professor of physics. "The cause of high-temperature superconductivity continues to be a major unsolved question in condensed matter physics 28 years after its discovery."
In superconductors, electrons overcome their repulsion and form pairs that move in unison and conduct electricity without resistance. In a charge-ordered state, interaction between electrons keeps them locked into a rigid pattern, which limits their ability to make the freely moving pairs required for superconductivity.
"The universality of charge ordering across very different materials shifts our perspective, and could propel future breakthroughs. We need to understand how charge ordering is formed in materials and ideally tune it, allowing superconductivity to occur at temperatures closer to room temperature," says Eduardo H. da Silva Neto, a postdoctoral fellow with UBC's Quantum Matter Institute and the Max-Planck-UBC Centre for Quantum Materials, who led the study's experimental work with former UBC Ph.D. student Riccardo Comin.
Superconductors have found use most prominently in medical imaging (MRI machines) and Japanese Maglev 'Bullet' trains, but power utilities, electronics companies, the military, and theoretical physics have all benefited strongly from the discovery of these materials. Accomplishing the decades-old goal of finding a room temperature superconductor could transform energy use and spur revolutionary new technologies and developments in many fields.
For the current study, UBC researchers led by Andrea Damascelli conducted resonant X-ray scattering studies on electron-doped cuprate superconductor samples prepared by UMD physics postdoctoral fellow Yeping Jiang to confirm the presence of charge ordering.
Following the confirmation, the researchers investigated a possible prerequisite for charge ordering, and consequently the suppression of superconducting properties — the "pseudogap." This gap in the energy level of a material's electronic spectrum has been closely associated with superconductivity and has been documented to exist at temperatures just above those that give rise to superconductivity.
In the current study, the researchers found that charge ordering gradually developed in the electron-doped cuprate samples at a temperature much higher than the pseudogap, contrasting previous observations in hole-doped cuprates. The findings indicate the pseudogap is not a prerequisite for charge ordering or superconductivity in electron-doped materials. This knowledge could be an important clue to solving the 28-year-old mystery of the cause of high-temperature superconductivity, according to Greene.
"Our next experiments will be to study the doping dependence and temperature dependence of the charge order in electron-doped cuprates in the hopes of better understanding its role in high-temperature superconductivity," Greene says.
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