This week in PRL (last year in cond-mat)

Real life continues to limit my blogging time. I'll hopefully be posting more often again soon. In the mean time, here's a neat paper that just came out in Phys. Rev. Lett. today, and was actually on the arxiv last year:

cond-mat/0603079 - Matthey et al., Electric field modulation of transition temperature, mobile carrier density and in-plane penetration depth in NdBa2Cu3O(7-delta) thin films

In this work the authors grow (by sputtering) underdoped high-Tc superconducting films on top of a SrTiO3 gate dielectric with an underlying gate electrode. A number of people (e.g. Allen Goldman's group at Minnesota) have played with SrTiO3 as a high-k gate dielectric to do experiments involving large gated charge densities. It's almost a ferroelectric, so it is possible to get an extremely large electric polarization in that material. The reason to do this is that in principle it allows you to tune the carrier density in an overlying material via the field effect: in a properly designed field-effect transistor, applying a potential difference between the gate electrode and the source/drain electrodes capacitively accumulates or depletes charge at the interface between the overlying material and the dielectric. Of course, there's no guarantee that the interface is nice, and that all the gated charge is actually mobile, even at "clean" interfaces between simple materials (Si, SiO2). However, if you can get it to work, you can tune charge density (at least in a thin layer of material) without accompanying changes in disorder that result from chemical doping. Anyway, the authors of this paper have managed to get this approach to work surprisingly well in this essentially 2d (the sample is only 3-4 unit cells thick) high-Tc material, and can electrostatically tune the transition temperature by about a factor of two within a given sample. This has allowed them to do detailed studies of the superconductor-insulator transition that happens as a function of carrier density, without having to worry about variable disorder. (This kind of phase transition, driven by a control parameter rather than temperature, can occur at T=0 and is called a quantum phase transition.) Very nice stuff. They've been working on this for several years, and it's nice to see them succeed. I met Jean-Marc Triscone, the PI, when we were working on this.