Pairing up in insulators

The same pairs of electrons that give rise to superconductivity in the BCS model of those materials can also form in their opposite number - electrical insulators. The finding is entirely counterintuitive, according to Brown University's James Valles. "Cooper pairing is not only responsible for conducting electricity with zero resistance," he explains, "but it can also be responsible for blocking the flow of electricity altogether." The BCS theory, named for John Bardeen, Leon Cooper and Robert Schrieffer, (Cooper is currently at Brown), says that superconducting electrons form pairs that correlate their motion with other electron pairs to flow smoothly and infinitely. Stewart has now observed the same Cooper pairs in bismuth, which is a superconductor in a thick layer but in very thin layer just tens of nanometers thick it is a strong insulator. The findings could help researchers understand the limits of superconductivity and, perhaps, push them to create insulated wires that conduct electricity without heating up. Cooper said the work sheds important and intriguing new light on quantum effects.