One particular paper caught my eye this week:
cond-mat/0608243 - Nakamura et al., Low-temperature metallic state induced by electrostatic carrier doping in SrTiO3.
The authors of this paper have managed to solve, at least well enough to do the experiment, the surface processing and ohmic contact challenges to make a field-effect transistor on the surface of a n undoped strontium titanate single crystal. At high enough gate voltages, they can accumulate enough carriers in the channel to drop the sheet resistance of the 2d charge layer well below the resistance quantum (~ h/2e^2 ~ 13 kOhms), and see metallic temperature dependence of the channel conductance (that is, the conductance improves with decreasing temperature). Anytime someone does this sort of thing with a new material system it's interesting, and SrTiO3 is particularly noteworthy because it's a perovskite (crystal structure not that different from high Tc materials), it's an incipient ferroelectric (very large dielectric constant as T decreases), and when doped at moderate levels, it's been known to superconduct. Field-effect "doping" is a very nice tool for studying this sort of physics, because the carrier density can be changed without introducing the disorder that comes with chemical doping. I'm actually a co-author on a forthcoming Reviews of Modern Physics paper about this general topic.
Now that you've glanced at that preprint, take a look at this PRL. Those folks have been looking at conduction in a semiconducting polymer, poly(3-hexylthiophene), and claim to observe a metal-insulator transition. The data are very pretty, but I just don't see how the interpretation matches the data well. These folks argue that, because the temperature dependence of the (highly nonlinear) conduction that they measure (at large source-drain voltage) gets weaker with increasing gated charge, and approaches temperature-independence, they are seeing a metal-insulator transition. It seems that the picture is: for high quality polymer films, the potential minima from disorder are relatively shallow, and when the potential is sufficiently tilted (by source-drain), and the deeper minima are filled (by large gated charge), then one can get tunneling (rather than thermal activation) out of the minima, and temperature-indep. conduction. This may well be right, but I really object to calling this a metal-insulator transition. There is no true transition here, and never does conduction improve with decreasing T, as in a metal. Again, the data are good, but the title and language are, to me, an example of wordsmithing. (Full disclosure: one reason this rubs me the wrong way is that in our own work we saw similar weakening of T-dep. several years ago. I would never have thought of calling this a transition to a metallic phase.)