Phonon interference in single-molecule junctions

Wave interference allows unprecedented coherent control of various physical properties and has been widely studied in electronic and photonic materials. However, the interference of phonons, or thermal vibrations, central to understanding coherent thermal transport in all electrically insulating materials, has been poorly characterized due to experimental challenges. Here we report the observation of phonon interference at room temperature in molecular-scale junctions. This is enabled by custom-developed scanning thermal probes with combined high stability and sensitivity, allowing quantification of heat flow through molecular junctions one molecule at a time. Using isomers of oligo(phenylene ethynylene)3 with either para- or meta-connected centre rings, our experiments revealed a remarkable reduction in thermal conductance in meta-conformations. Quantum-mechanically accurate molecular dynamics simulations show that this difference arises from the destructive interference of phonons through the molecular backbone. This work opens opportunities for studying numerous wave-driven material properties of phonons down to the single-molecule level that have remained experimentally inaccessible.