researchers at the tokyo institute of technology (japan) have demonstrated precise and fully reversible switching of the polarity of voltage produced by the thermoelectric effect across a gold junction with an atomic-scale contact, according to a report from tokyo tech.

 schematic view of the experimental setup. the inset shows microscopic images
of the contact after it is broken by elongation. (tokyo institute of technology)

the report noted that this is the first successful “manipulation of quantum interference of electrons in metal nanostructures through external mechanical force.” this could have significant impact on thermopower generation and solid-state electronics.

“when the cross-section of the junction contact is reduced to a few atoms, quantum-mechanical effects or, specifically, quantum interferences among electrons affect the transport of electrons across the junction,” the article explained. “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.”

scientists provided a mechanical strain on the contact to deform it at the nanoscale level using a piezoelectric transducer to bend the junction’s substrate and maintaining a low-temperature to avoid interference from the kinetic energy of the atoms.

“as the contact was elongated,” the article continued, “the conductance decreased in a step-wise manner, and the thermoelectric voltage varied sharply with changes in sign. remarkably, these changes were perfectly reversible: the electrical properties were restored to their initial values when the contact was compressed back to its initial structure.”

this discovery led the scientists to create a voltage switch, which can control and reverse the voltage by elongating or compressing the contact. the switch worked for at least 20 cycles of elongation and compression.

“further, the scientists theoretically proved that the switching is caused by the change of quantum-interference states of electrons due to the mechanical modification of the structure of the contact,” the article added. “a theoretical model of the junction that the scientists constructed using density functional theory accurately predicted the changes of electrical properties with varying deformation.”

the research was recently published in scientific reports. the abstract stated:

“the thermoelectric voltage developed across an atomic metal junction (i.e., a nanostructure in which one or a few atoms connect two metal electrodes) in response to a temperature difference between the electrodes, results from the quantum interference of electrons that pass through the junction multiple times after being scattered by the surrounding defects.

“here we report successfully tuning this quantum interference and thus controlling the magnitude and sign of the thermoelectric voltage by applying a mechanical force that deforms the junction. the observed switching of the thermoelectric voltage is reversible and can be cycled many times. our ab initio and semi-empirical calculations elucidate the detailed mechanism by which the quantum interference is tuned.

“we show that the applied strain alters the quantum phases of electrons passing through the narrowest part of the junction and hence modifies the electronic quantum interference in the device. tuning the quantum interference causes the energies of electronic transport resonances to shift, which affects the thermoelectric voltage.

“these experimental and theoretical studies reveal that au atomic junctions can be made to exhibit both positive and negative thermoelectric voltages on demand, and demonstrate the importance and tunability of the quantum interference effect in the atomic-scale metal nanostructures.”