Co-existing heat currents in opposite directions in graphene nanoribbons

Using molecular dynamics simulations, we create an unprecedented scenario in graphene nanoribbons: co-existence of two heat currents in opposite directions at the same location. One heat current is carried by flexural mode phonons, and the other one by transverse/longitudinal modes phonons in the opposite direction. The local apparent thermal conductivity (κapp) varies in a very large range: −468 to 1434W/mK. The negative κapp does not violate the second law of thermodynamics. It is a combined effect of the much higher thermal conductivity of flexural mode phonons and the weak coupling between them and transverse/longitudinal modes phonons.