Mechanical force to control thermoelectric voltage

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The magnitude and sign of the thermoelectric voltage across atomic-scale gold junctions can be controlled by applying a mechanical strain to deform the contact minutely and accurately while the structure of the surrounding material remains unaffected, say researchers from Tokyo Institute of Technology.

The thermoelectric effect – a voltage difference created across a junction of two wires held at different temperatures – has been used in various applications such as thermoelectric power generators, thermoelectric refrigerators, and temperature measurement.

When the cross section of the junction contact is reduced to a few atoms, quantum interferences among electrons affect the transport of electrons across the junction. These interferences are strongly dependent on the structure, including minute defects, of the atomic-scale contact and surrounding material, which determine electrical properties such as conductance and thermoelectric voltage.

So far, quantum interference effect in atomic-scale metal contacts has not found much application, because of the difficulty in precisely controlling atomic structures.

In this study, minute deformations were performed through bending of the junction's substrate by using a piezoelectric transducer and by maintaining a low temperature environment so that the atoms did not gain sufficient kinetic energy to vibrate strongly and cause random deformations of the structure.

As the contact was elongated, the conductance decreased in a step-wise manner, and the thermoelectric voltage varied sharply with changes in sign. According to the researchers, the electrical properties were restored to their initial values when the contact was compressed back to its initial structure.

A suitable range of elongation that causes a step-like change in conductance with a change in sign of the thermoelectric voltage was used to create a voltage switch.

According to the research team, this is the first report of successful manipulation of quantum interference of electrons in metal nanostructures through external mechanical force. Potential applications include thermopower generation, measurement techniques in materials science, and solid-state electronic devices.