Revealing the tug of war within graphene

Graphene transports heat by atomic vibrations (phonons) that either move in in-plane or out-of-plane.  Our work reveals a counterintuitive effect in graphene: the in-plane and out-of-plane vibrations are at a very different temperatures. This can occur because, in flat graphene sheets, there is very little interaction between these two types of phonons. This effect is shown by periodically heating freely suspended graphene membranes with a laser, and carefully analyzing their mechanical motion. When the temperature in graphene is increased the in-plane phonons try to expand, while the out-of-plane phonons try to shrink the material, like in a tug of war between the two types of phonons.  Whereas conventional techniques could only measure the final result of this tug of war, our new technique is able to show that initially the faster in-plane phonons extend the graphene, and only later the out-of-plane phonons cause an even larger contraction of the wire, winning the tug of war. Since the thermal expansion force is a measure of the temperature, we can determine whether the temperatures of the in-plane and out-of-plane directions are different. We find that they can indeed differ by as much as a factor of 5, showing that heat transport in graphene cannot be described in the same way as everyday materials.