This week in cond-mat

A couple of new papers on the arxiv that I find particularly interesting....
cond-mat/0607756 - Zarchin et al., Bunching of electrons in transport through quantum dots
The Weizman Institute's work on transport in quantum dots is generally as good as it gets. I've already written about their experimental prowess in measuring shot noise, and this is another example. Shot noise results from the discrete nature of electronic charge. While the current tells you about the average rate at which electrons are flowing through a circuit, there are fundamental fluctuations in that current that describe the temporal correlations between the electrons. For example, if electrons only flowed through the circuit one at a time in perfectly spaced intervals, there would be no noise. On the other hand, if the electrons were Poisson distributed, there would be a classical current noise of 2eI (in units of amps^2/Hz). The authors here looked at shot noise in gate-defined quantum dots on GaAs/AlGaAs 2d electron gas. The authors found a surprising result. In the finite-bias conductance resonances that happen in these kinds of dots (as the source-drain bias is increased to allow access to another charge state for transport), the shot noise was enhanced over this classical result by as much as a factor of 10. This implies that the electrons are bunching up somehow, traversing the dot in bursts. This is quite odd and unexpected.

cond-mat/0607765 - Kitchen et al., Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions (also out in Nature)
This is a very nice STM paper by Ali Yazdani's group from Princeton. These folks are able to insert single Mn atoms into the surface of a p-doped GaAs wafer, and watch what happens. This is important because ferromagnetic semiconductors like GaMnAs are a key class of materials for those interested in capitalizing on the spin as well as charge of free carriers. What I really find interesting about these measurements is how very different a dopant atom in this semiconductor system looks from the puffy, hydrogenic picture painted in solid state physics textbooks. These kinds of results always re-emphasize to me that serious STM can't be your hobby - it has to be the main focus of your research effort, or you can't be competitive.