Stretching-enhanced ballistic thermal conductance in graphene nanoribbons

We combine the nonequilibrium Green's function method with the elastic theory to investigate the ballistic thermal transport in graphene nanoribbons under homogeneous uniaxial stretching applied in the longitudinal direction. Remarkable enhanced thermal conductance is found for the samples with width in the appropriate range. The enhancement ratios could be up to 17% and 36% for the 5 nm width zigzag and armchair graphene nanoribbons, respectively. This enhancement effect results from a lot of dispersive phonon modes which are converged to the low-frequency region. In addition, the transverse shrinkage induced by the Poisson effect is beneficial for enhancing thermal conductance, while the transverse stretching has only a modest modification on thermal conductance. It is also observed that for 2.6 nm width nanoribbons the power-law temperature dependence, T^β, of thermal conductance at low temperatures is independent of strain, with β=1 below a critical temperature about 10 K and βsime2 at 20 K <=T<= 50 K, which differ markedly from β=1.5 in the two-dimensional case. Moreover, above the critical width 11 nm, the armchair nanoribbon displays higher phonon thermal conductance than the same-width zigzag nanoribbon.