This week in cond-mat

Real life continues to be very busy this semester. Two interesting papers on the arxiv this week....

arxiv:0710.2845 - Fratini et al., Current saturation and Coulomb interactions in organic single-crystal transistors
The technology finally exists to do what He Who Must Not Be Named claimed to have done: use a field-effect geometry to gate significant charge densities (that is, a good fraction of a charge carrier per molecule) into the surface of a clean single crystal of an organic semiconductor. The Delft group has used Ta2O5 as a high-k gate dielectric, and are able to get 0.1 holes per rubrene atom in a single-crystal FET geometry. In typical organic FETs, increasing the charge density in the channel improves transport by filling trap states and by moving the chemical potential in the channel toward the mobility edge in the density of states. Surprisingly, Fratini et al. have found that the channel conductance actually saturates at very high charge densities instead of continuing to increase. The reason for this appears to be Coulomb interactions in the channel due to the high carrier density and the polaronic nature of the holes. The strong coupling between the carriers and the dielectric layer leads to a tendency toward self-trapping; add strong repulsion and poor screening into the mix, and you have a more insulating state induced by this combination of effects. Very interesting!

arxiv:0710.2323 - Degen et al., Controlling spin noise in nanoscale ensembles of nuclear spins
Dan Rugar at IBM has been working on magnetic resonance force microscopy for a long time, and they've got sensitivity to the point where they can detect hundreds of nuclear spins (!). (That may not seem impressive if you haven't been following this, but it's a tour de force experiment that's come very far from the initial work.) The basic idea of MRFM is to have a high-Q cantilever that is mechanically resonant at the spin resonance frequency and coupled via magnetic interactions to the sample - that way the polarized spins precess, they drive the cantilever resonance mode. When they look at such a small number of spins, the statistical fluctuations in the spin polarization are readily detected. This is a problem for imaging, actually - the timescale for the natural fluctuations is long enough that the signal bops around quite a bit during a line scan. Fortunately, Degen et al. have demonstrated in this paper that one can deliberately randomize the magnetization by bursts of rf pi/2 pulses, and thus suppress the fluctuation impact on imaging by making the effective fluctuations much more rapid. This is a nice mix of pretty physics and very clever experimental technique.