Enhanced transport length of spin-helical Dirac fermions in disordered 3D topological insulators

The transport length $l_{\text{tr}}$ and the mean free path $l_{\text{e}}$ are experimentally determined for bulk and surface states in a Bi${}_2$Se${}_3$ nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of $l_{\text{tr}}$, which confirms theoretical predictions \cite{Culcer2010}. Despite strong disorder ($l_{\text{e}}\approx 30$~nm), our result further points to the long-range nature of the scattering potential, giving a large ratio $l_{\text{tr}}\mathrm{/}{l}_{\text{e}}\approx 8$ that is likely limited by a finite bulk/surface coupling. This suggests that the spin-flip length could reach the micron size in disordered 3D topological insulator nanostructures with a reduced bulk doping, even if due to charge compensation.