Thermal conductance of atomic-scale glass reaches 40% of quantum limit

Heat dissipation limits the performance of nanoscale electronic and quantum devices, yet the fundamental limits of phonon-mediated heat transport at the atomic scale remains unknown. By contrast, atomic-scale electron transport has been extensively studied over the past decades using electronic break-junction platforms. Here we show a custom-fabricated, picowatt-resolution scanning thermal probe that uses thermal feedback to create phononic break junctions at the ultimate atomic scale. We report the formation of atomic junctions of amorphous silicon dioxide (a-SiO₂)-an archetypal glass-capturing, in real time, the transition from a bulk diffusive heat insulator to a ballistic efficient heat conductor as the tip is withdrawn from a nano-contact to an atomic contact. Remarkably, atomically thin glass junctions support phononic thermal conductance up to 40% of the quantum limit at room temperature, approximately an order of magnitude higher than the phonon thermal conductance of crystalline gold. These results, in conjunction with our atomic simulations, demonstrate that intrinsic structural disorder, which strongly affects macroscopic thermal properties, does not fundamentally constrain heat flow at the atomic limit. Instead, we find that it is the Debye frequency that dictates the phononic thermal conductance. We expect our approach to offer an experimental platform to study heat flow via wave-like coherent phonons and enable the engineering of high-performance thermal phononic devices, such as a thermal diode and transistor.