Decoupled thermal and electric response to external excitations in graphene

Thermal and electrical conductivity are the two most critical material properties in the design of miniaturized modern devices and the cooling of integrated circuits. In principle, thermal and electrical energy transport are two independent physical processes because they are associated with different energy carriers, i.e., phonons that carry heat and electric charges that carry current. However, it is still unknown how the two kinds of energy would spread in a material, such as graphene, with both high thermal conductivity and high electric conductivity at the same time in response to an external excitation. In this paper, we show that the thermal energy and electric potential energy of a graphene nanosheet exhibit quite different decoupled transport behaviors subject to an external flexural or planar excitation by using large-scale ab initio nonequilibrium molecular dynamics simulations up to thousands of atoms. It is found that the thermal energy has a higher transport velocity than the electric potential energy in response to a flexural excitation, while the electric potential energy has a higher transport velocity in response to a planar excitation. The dependence of transport behavior on the excitation strength is investigated. We find that, induced by the different responses to the excitation strength, the thermal energy and electric potential energy possess similar variations subject to the flexural excitations and opposite variations subject to the planar excitations. Anomalous diffusion of the thermal and electric potential energy in this nonequilibrium excitation process is also studied to understand the decoupled carrier mobilities. Furthermore, the cross-correlation function between the thermal energy and the electric potential energy is calculated to numerically demonstrate the decoupled variation. After an initial sharp drop, the cross-correlation function exhibits an exponential decay subject to both flexural and planar excitations. Our findings provide insight into the complex transport behavior of thermal and electric potential energy in crystalline solids and a promising method for designing adjustable thermal and thermoelectric devices.