Studying thermal transport at the nanoscale could help researchers better understand quantum transport phenomena that have no classical analogues. In nanoscale devices, for example, metallic atomic-sized contacts and single-molecule junctions, charge and energy transport is governed by quantum mechanics. Indeed, electrical conductance, shot noise, thermopower and Joule heating are completely dominated by quantum effects in these atomic and molecular scale devices – even at room temperature. Such devices therefore behave completely differently to macroscale ones.

Measuring thermal transport in such tiny systems is no easy task though, and most techniques to date only worked at sub-Kelvin temperatures. Now, a team of researchers led by Pramod Reddy and Edgar Meyhofer of the University of Michigan at Ann Arbor has developed a new way to measure the thermal conductance of metallic wires down to the single-atom limit and so observed quantized thermal transport at room temperature in single-atom junctions made of gold (Au) or platinum (Pt).

“Conceptually simple” measurement scheme

The researchers used calorimetric scanning thermal microscopy probes that were made in the Lurie nanofabrication facility at the University of Michigan. The probes have a very large thermal resistance (of 1.3 x 106 K/W) and contain Pt thermometers that can distinguish between temperatures as small as 0.6 mK. They are also stiff and thermally isolated from their environment thanks to T-shaped beams of silicon nitride incorporated into their structure.

“The measurement scheme is conceptually simple,” Longji Cui and Wonho Jeong, lead authors of the study tell “We move the picowatt-resolution scanning tip, which is coated with either Au or Pt, towards an Au- or Pt-coated heated substrate until we have established an electric conductance of around 3000 Ohms,” they explain. “When we then withdraw the probe, the junction thins down, forming an atomic junction. The heat flow from the hot substrate into the probe results in a temperature increase in the probe that can be detected by the Pt thermometer.

“Since the temperature rise is proportional to the thermal conductance, we can easily quantify the thermal conductance of the junction as the probe is withdrawn.”

Observing quantum effects at room temperature

The study shows for the first time how measurements can be performed on single-atom junctions formed on the pristine surfaces of metals to systematically probe thermal transport at the atomic scale (which is the smallest conceivable size for devices), they add. “This means we can observe quantum effects at room temperature, and the insights obtained from this work will prove critical in the development of future nanotechnologies that seek to build ever smaller electronics and memory devices.

“The experimentally observed results of heat transport in Au and Pt atomic junctions are in excellent agreement with the state-of-the-art calculations performed Juan Carlos Cuevas and colleagues at the Universidad Autónoma de Madrid and Fabian Pauly and Peter Nielaba at the University of Konstanz,” they say. This strong agreement between theory and experiment provides greater confidence in our observed quantization results.”

The researchers, reporting their work in Science DOI: 10.1126/science.aam6622, expect that the tools they have developed in their experiments will allow them to measure heat transport in atomic and molecular chains that have been modelled theoretically for several decades but never actually studied in the laboratory.