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At a fixed low temperature, decreasing the doping level or increasing the applied magnetic field both suppressed superconductivity, allowing a competing state of electronic order to take over. Dramatic fluctuations appeared in the Hall resistance below a critical temperature, and these fluctuations increased in frequency and magnitude as all samples were further cooled toward absolute zero, indicating that they are of quantum origin.
"The behavior of these fluctuations is opposite to that seen in fluctuations driven by thermal energy, such as the vapor bubbles that appear when water is boiled," said Wu. "The bubbles fade away as the temperature is lowered."
The Hall resistivity measurements compared over the entire range of magnetic fields tested revealed that the samples have "memory" of their prior electronic states. After a magnetic field was applied, the value and sign of the Hall resistivity changed. When the magnetic field was removed, the samples stayed in the same electronic configuration until the field was reapplied—a very unusual property for conductors.
The scientists' data reveal that, at near-absolute-zero temperatures, the superconducting state competes with another state of electronic order characterized by the random distribution of many small charge "clusters," or localized groups of electrons. Unlike the free-flowing electrons in metals and superconductors, the electrons in these clusters are localized and pinned to particular atoms, rendering them immobile and unable to carry current when an electric field is applied. The clusters can hop around and trade places in the lattice as a result of quantum fluctuations.
"This picture explains the weak conductivity of this strange "insulating" state, revealing that the state originates from localization of charges," said Wu. "Our conclusion builds upon our understanding of the insulator-superconductor transition in an important class of high-temperature superconductors. We are one step closer toward our goal of predicting and designing new superconducting materials with superior properties for energy applications."
This research was funded by the Office of Science within the U.S. Department of Energy. Other authors include Anthony Bollinger of Brookhaven Lab and Yujie Sun of Brookhaven Lab and the Chinese Academy of Sciences.
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