Tiny wires trigger electric reversal

From Baltimore, at a meeting of the American Physical Society

Physicists have observed an unexpected reversal of conductive behavior in ultracold, ultrathin zinc wires.

Typically, a metal wire more readily superconducts, or transports electricity without resistance, when it spans superconductive electrodes. However, that wire loses its superconductivity if strung between electrodes of normal metals.

Yet in recent experiments, ultrathin zinc wires did just the opposite: They conducted normally when between superconductive electrodes but became superconductive when between normal electrodes.

The reversal is “very stunning, very surprising,” says theoretical physicist Dung-Hai Lee of the University of California, Berkeley.

Led by Moses H.W. Chan, researchers at Pennsylvania State University in State College observed the contrary conductivity. They created nanoscale-diameter wires within pores in thin membranes of polycarbonate or aluminum oxide and then placed the membranes between pairs of metal electrodes. The electrodes’ shapes made it possible to measure the electrical properties of nanowires one at a time.

Penn State team member Ming-Liang Tian reports that the wires’ conductivities depended on their thicknesses and lengths, as well as on the types of metals making up both the wires and electrodes.

For instance, when connected between superconductive electrodes made of tin or indium, zinc nanowires 40 nanometers in diameter anomalously exhibited normal conductivity. But when a magnetic field suppressed the superconductivity of the electrodes, the zinc nanowires unexpectedly turned superconductive.

In contrast, tests of 40-nm wires made of tin, of 70-nm zinc wires, and 40-nm zinc wires sandwiched between lead electrodes, found that the usual conductivity rules prevailed.

To explain the extraordinary reverse behavior, Lee and his colleagues theorize that a known inability of superconductive electrodes to absorb small amounts of energy results in a buildup of quantum disturbances in some wires. Those disturbances, in turn, destroy the wires’ superconductivity. Conversely, normal-metal electrodes shunt away some of that disruptive energy, allowing superconductivity to reestablish in the wires.

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