This week in the arxiv

Two papers from the past week that caught my eye....

arxiv:0805.3309 - Bunch et al., Impermeable atomic membranes from graphene sheets
This is a nice piece of work from Cornell combining the techniques from three research groups to look at the permeability of single-layer graphene sheets. The authors prepare freely suspended graphene trampolines and apply controlled pressure differences across them. They use scanned probe methods to measure the membrane shape, which ends up being well described by elasticity theory assuming that the elastic modulus for the graphene sheet is about 10¹² Pa (that's big but not unexpected). By watching that shape as a function of time, they can tell how long it takes the pressure inside the chamber (sealed off by the graphene) to equilibrate with the outside environment. Elegant.

arxiv:0805.2414 - Finck et al., Area dependence of interlayer tunneling in strongly correlated bilayer 2d systems at nu(total)=1.
I've written before about two-dimensional electronic systems (2des), and how they are very useful for looking at all sorts of rich physics such as the fractional quantum Hall effect. This experiment looks at a variation on this theme. For a while now it's been possible to make two high quality 2des separated by a thin barrier - thin enough that the charges in one layer can feel the charges in the other layer via the Coulomb interaction. Since like charges repel, if the two layers have the same density of electrons, a favored low energy state would have every electron in the upper layer accompanied by a hole (the absence of an electron) in the lower layer. If the barrier is sufficiently thin, tunneling can take place between the two layers. One fascinating observation has been that this interlayer tunneling, under certain circumstances, can look very much like the kind of Josephson tunneling that one gets between superconductors. One nagging question out there has been whether the very sharp tunneling seen is a bulk effect (and taking place over the whole area where the two layers are tuned to each other) or something else (e.g., an edge effect, like many quantum Hall phenomena). This experiment shows that the tunneling really is proportional to the area, and thus is a bulk effect. This is a tough experiment, requiring great samples, demanding fabrication, and very sensitive measurements at low temperatures.