Exceptional ballistic transport in epitaxial graphene nanoribbons

The patterning of graphene into graphene nanoribbons is an essential task for the development of graphene based devices. For such ribbons with a well-ordered edge geometry the presence of one-dimensional edge states has been predicted. We use a selective graphitization process on SiC-mesa structures to produce graphene nanoribbons with a width of 40 nm. The local electronic properties of the ribbons are investigated by means of a 4-tip STM. In combination with a SEM, the precise positioning of all four tips on the nanometer range is possible to perform local transport measurements. Furthermore, local tunneling spectroscopy reveals characteristic features of ferromagnetic zig-zag graphene nanoribbons. Transport experiments carried out on the very same ribbon show a conductance of $e^2/h$ for a wide temperature range from 30 K up to room temperature and probe spacings between 1~$\mu m$ and 10~$ \mu m$. Description within the Landauer formalism is possible assuming ballistic transport dominated by a single channel. Transportin the second zeroth subband is only detectable for probe spacings smaller than 1~$\mu m$ due to the short localization length of carriers in this subband manifesting in the increase of the conductance to $2e^2/h$ at probe spacings below 200 nm.