Machine Design

Buddy, can you spare a dimer?

Niobium nanowires change electrical conductance as they stretch, and now a team of Georgia Tech physicists knows why.

Edited by Lawrence Kren

A model of the niobium nanowire with the electrical leads on the outside and the two dimers. At left, the dimer sits in the center of the leads, giving the nanowire high electrical conductivity. At right, the off-centered dimer lowers conductivity.

Niobium nanowires change electrical conductance as they stretch, and now a team of Georgia Tech physicists knows why. It turns out measured fluctuations in nanowire conductance are caused by a pair of atoms, known as a dimer, shuttling back and forth between the bulk electrical leads.

The nanowires are formed by repeatedly stretching a thin, nanofabricated strip of the material just to the breaking point. This leaves a short chain of niobium atoms bridging the gap between the two sides of the strip. The stretching process and conductance measurements take place at 4.2°K — far below niobium's superconductivity transition temperature of 9.2°K — as well as above the transition temperature. Researchers can control the stretching process to better than 1 picometer (one-thousandth of a nanometer), or about 1100 the size of the atoms involved.

Slowly pulling the nanowire makes conductance gradually drop until, in a narrow region of about 0.1 A, it rapidly falls. Conductance resumes its gradual decline with further pulling of the wire. In this narrow region, conductance jumps between two values. Close to the onset of the rapid drop, conductance is mostly high, followed by random, short periods of decline to significantly lower values. The pattern reverses itself on the other side of the interval. Theoretical simulations help explain why.

At first, the team thought a single atom must be randomly shuttling back and forth between two positions in the space separating the electrical leads, but the data didn't fit. So, they ran simulations with a connected pair of atoms, or dimer. When the dimer sits closer to one lead, electrons have a longer way to hop from the dimer to the other lead, hindering current flow. Centering the dimer between the leads shortens the hop distance and boosts current flow. As the wire stretches, the dimer spends more time closer to one electrical lead than in the center, accounting for the overall conductance loss observed in the experiments, says the group.

UPM Raflatac,

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