Collective vibrations of atoms or "phonons" play a crucial role in determining the properties of crystalline materials. For instance, phonons play a key role in heat transport in materials. Now Michael Roukes and co-workers at the California Institute of Technology have measured the quantum unit of thermal conductance - the maximum amount of heat that a single phonon can transport (Nature 404 974).
Quantized electrical conductance was confirmed in experiments with very narrow conducting wires in the late 1980s. The electrons move through the wire along quantum channels, each of which contributes a quantum unit of 2e2/h to the conductance, where e is the charge on the electron and h is the Planck constant. As the voltage difference across the wire increases, more quantum channels open up and the current passing through the wire increases in a step-like fashion. Similar behaviour has been predicted for phonons, with the quantum of thermal conductance being given by G0 = p2k2T/3h, where k is the Boltzmann constant and T is temperature.
Roukes and colleagues used electron beam lithography to make a narrow bridge from silicon nitride: the bridge was 60 nm thick and 200 nm wide. Niobium wires were used to connect the bridge to two pads that were attached to ultrasensitive thermometers. The group warmed one end of the bridge and measured how the thermal conductance – the rate of heating divided by the temperature difference – varied as a function of temperature. They discovered that the thermal conductance fell with temperature to about 1 Kelvin, below which it remained flat at a value consistent with thermal conductance being quantized in units of G0.
