They employed it to illustrate the quantization of the electrical part of the thermal conductance. The study also reveals that the Wiedemann-Franz law, a relation from classical researchers, remains valid down to the atomic level.
To instigate the test object is a microscopic gold wire. This wire is pulled till its cross section is just one atom wide and a chain of gold atoms forms, before it eventually breaks. The researchers send electrical current through this atomic chain that is through the thinnest wire conceivable. With the aid of distinct theoretical models the scientists can predict the conductance value of the electrical transport, and also confirm it by experiment.
This electric conductance value showcases how much charge current flows when an electronic voltage is applied. The thermal conductance that indicates the volume of heat flow for a difference in temperature could not yet be estimated for such atomic wires.
Now the question was whether the Wiedemann-Franz law that states that the electronic conductance and the thermal conductance are proportional to each other remains valid also at the atomic scale. Usually, electrons as well as atomic oscillations contribute to heat transport. Quantum mechanics has to be employed, at the atomic level to describe both the electron and the phonon transport. The Wiedemann Franz law, but, only describes the relation between macroscopic electronic properties. Therefore, initially the scientists had to identify how high the contribution of the phonons is to the thermal conductance.
The doctoral scientists Jan Klockner and Manuel Matt did complementary theoretical estimations, which revealed that usually the contribution of phonons to the heat transport in atomically thin gold wires is less than 10% and thus is not decisive. At the same time, the simulations confirm the applicability of the Wiedemann-Franz law. Manuel Matt employed an effective, albeit less precise method that provided statistical results for numerous gold wire stretching events to estimate the electronic part of the thermal conductance value, while Jan Klockner applied density functional theory to estimate the phononic and electronic contributions in individual contact geometries.
The quantization of the thermal conductance in gold chains, as proven by experiment, eventually results from the combination of three factors- the quantization of the electronic conductance value in units of the so-called conductance quantum, the negligible role of phonons in heat transport and the validity of the Wiedemann-Franz law.
The results of the study make it possible to analyse heat transport not just in atomic gold contacts but numerous other nanosystems. They offer opportunities to experimentally and theoretically explore numerous fundamental quantum heat transport procedure that may help to employ energy more effectively, for instance by exploiting thermoelectricity
Filed Under: News