Nature Journal Club

My media onslaught continues. This past week I had a Journal Club contribution in Nature, which was fun and a nice opportunity for a wider audience. Here's a version of it before it was (by necessity) trimmed and tweaked, with added hyperlinks....

Tunable charge densities become very large, with super consequences

The electronic properties of materials depend dramatically on the density of mobile charge carriers. One way to tune that density is through doping, the controlled addition of impurity atoms or molecules that either donate or take up an electron from the rest of the material. Unfortunately, doping also leads to charged dopants that can act as scattering sites.

Fortunately, there is a way to change the carrier concentration without doping. In 1925 J. E. Lilienfeld first proposed what is now called the “field effect”, in which the sample material of interest is used as one electrode of a capacitor. When a voltage is applied to the other (“gate”) electrode, equal and opposite charge densities accumulate on the gate and sample surfaces, provided charge can move in the sample without getting trapped. While the density of charge that can be accumulated this way is rather limited by the properties of the insulating spacer between the gate and the sample, the field effect has been incredibly useful in transistors, serving as the basis for modern consumer electronics.

Recently it has become clear that another of Lilienfeld’s inventions, the electrolytic capacitor, holds the key to achieving much higher field effect charge densities. The dramatic consequences of this were made clear by researchers at Tohoku University in Sendai, Japan (K. Ueno et al., Nature Mater. 7, 856-858 (2008)), who used a polymer electrolyte to achieve gated charge densities at a SrTiO₃ surface sufficiently large to produce superconductivity. While superconductivity had been observed previously in highly doped SrTiO₃, this new approach allows the exploration of the 2d superconducting transition without the disorder inherent in doping.

The most exciting aspect of this work is that this approach, using mobile ions in an electrolyte for gating, can reach charge densities approaching those in chemically doped, strongly correlated materials such as the high temperature superconductors. As an added bonus, this approach should also be very flexible, not needing special substrates. Tuning the electronic density in strongly correlated materials without the associated pain of chemical doping would, indeed, be super.