Wednesday, February 19, 2014

Ballistic electrons in graphene nanoribbons at room T: whoa!

The de Heer group at Georgia Tech has a paper in this week's Nature where they present some results on graphene nanoribbons that are quite unexpected and exciting.  Rather than exfoliate graphene from graphite, or grow it via chemical vapor deposition, the Georgia Tech group creates graphene via the controlled transformation of silicon carbide.  In this latest work, they used a vicinal substrate (meaning that it is cut slightly off-axis from a high symmetry direction, so that the surface has regularly spaced atomic terraces).  When this substrate is annealed in a particular way, graphene forms across the surface.  Interestingly, on the plateaus, the resulting material appears to be semiconducting (based on tunneling measurements made by scanning tunneling microscopy (STM)), while the steps reconstruct and form sloping sidewalls that have 40 nm wide ribbons that are metallic graphene (as seen through tunneling and photoemission).

Using in situ multiple tungsten STM tips, they are able to measure the conductance of such ribbons as a function of length.  Remarkably, they find that the two-terminal conductance is approximately independent of length (!) over a broad range of lengths (from 0.5 $\mu$m to about 16 $\mu$m) even at room temperature, and it has the very suggestive value of $e^{2}/h$, which is what you would expect for a single quantum channel, with one species of the electronic spin.  This kind of violation of "Ohm's Law" is expected when the electrons travel essentially without scattering from one end of a device to the other.  Ordinarily we can't see this at room temperature in macroscopic conductors, because there are many ways electrons can scatter, including inelastic processes involving lattice vibrations.  The authors have a number of other measurements that are consistent with the implication that a single channel is somehow able to propagate ballistically over these long distances at room temperature.  Indeed, they can use additional tips as "passive" scattering centers; placing an additional tip on the wire makes the conductance drop, presumably because that tip is able to cause back-scattering.

These observations are very interesting, since they suggest that there is some kind of "protected" channel that allows conduction by basically making back-scattering (which would usually contribute to resistance) very disfavored.  The apparent spin polarization (inferred from the conductance value, not measured directly) is also intriguing.  I wonder if the "kink" at the edges of the ribbons where the sidewall transitions to the flat plateaus on either side of the ribbon acts as some sort of source of strong spin-orbit interactions (despite the low $Z$ of carbon) by distorting the graphene lattice.  In any case, it is nice to see a genuinely surprising graphene result.

1 comment:

Anonymous said...

Thanks for pointing out this result Doug. Finally some exciting and simple science coming out of this material!