Anomalous phonon diffusion in isotopically disordered armchair-edge graphene nanoribbons


Nobuya Mori, Osaka University

It is possible to control the thermal conductance of materials independently from the electrical conductance by utilizing the isotopic doping method.  We have studied effects of isotope impurities on the phonon transport in isotopically disordered armchair-edge graphene nanoribbons (AGNRs).

For conventional semiconductors, acoustic phonon has a finite group velocity in the low-frequency limit, which may cause anomalous phonon diffusion like a Lévy flight.  For graphene, in-plane phonon has a linear dispersion while out-of-plane phonon has a quadratic dispersion.  The group velocity of the out-of-plane mode thus vanishes in the low frequency limit, which could lead to different phonon transport characteristics.  It is, therefore, interesting to investigate whether there is a difference between the thermal conductance of the in-plane mode and that of the out-of-plane mode in isotopically disordered AGNRs.

We calculate the channel length, L, dependence of the thermal conductance, K, and find that the in-plane thermal conductance exhibits the super-diffusive nature of K∝L-1⁄2.  On the other hand, the out-of-plane thermal conductance shows a normal diffusion-like behavior (K∝L-1).