A Georgia Tech physics group has discovered how and why the electrical conductance of metal nanowires changes as their length varies. In a collaborative investigation performed by an experimental team and a theoretical physics team, the group discovered that measured fluctuations in the smallest nanowires' conductance are caused by a pair of atoms, known as a dimer, shuttling back and forth between the bulk electrical leads.
The team formed niobium nanowires using the mechanically controlled break junction technique – that is bending a thin nanofabricated strip of niobium until it breaks. In the final stage before the strip breaks completely, all that's left is a nanowire made of a short chain of niobium atoms that bridge the gap between the two sides of the strip.
Conducting the experiment at 4.2 degrees Kelvin (far below niobium's superconductivity transition temperature of 9.2 Kelvin), as well as performing measurements above the transition temperature, Marchenkov's team measured the electrical conductance of the atomic nanowire as it is stretched during the bending of the strip. As this bending occurs, the atoms separate from each other. The researchers were capable of controlling this separation with a precision better than 1 picometer (one thousandth of a nanometer), which is about 100 times smaller than the typical size of atoms.
As the nanowire is slowly pulled, the conductance drops. The drop in conductance was gradual until a rapid decrease in the conductance was observed in a narrow region of just 0.1 angstrom . Upon further pulling of the wire, the conductance resumed its gradual decline.