  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.
