This week in cond-mat

It's been a busy summer, hence the sparseness of my recent postings. Here are a couple of papers that caught my eye this past week.

arxiv:0707.1923 - Hogele et al., Quantum light from a carbon nanotube
Here the authors do careful time-resolved photoluminescence experiments on individual single-walled carbon nanotubes. By studying the time distribution of photon production, they can get insights into the exciton (bound electron-hole) dynamics that lead to light emission. They find evidence that photons are produced one-at-a-time in these structures, and that multiphoton processes are strongly suppressed. Perhaps nanotubes could be useful as sources of single photons, strongly desired for quantum cryptography applications.

arxiv:0707.2091 - Quek et al., Amine-gold linked single-molecule junctions: experiment and theory
This is a nice example of a mixed experiment/calculation paper in molecular electronics that actually has an interesting point. Very pretty experimental work by Venkataraman et al. at Columbia has shown that NH2-terminated molecules form better-defined contacts with Au electrodes than the conventional thiol (sulfur)-based chemistry. For example, looking at huge data sets from thousands of junction configurations, benzene diamine glommed into a Au break junction has a well-defined most likely conductance of around 0.0064 x 2e^2/h. Now theory collaborators have done a detailed examination via density functional theory of more than a dozen likely contact geometries and configurations for comparison. The calculations do show a well-defined junction conductance that's robust - however, the calculations overestimate the conductance by a factor of seven compared to experiment. The authors say that this shows that DFT likely misses important electronic correlation effects. Hmmm. It's a neat result, and now that they mention it, the almost every non-resonant molecular conduction calculation I've ever seen based on DFT overestimates the conduction by nearly an order of magnitude. The only underestimates of molecular conduction that come to mind are in the case of Kondo-based mechanisms, which can strongly boost conductance and are always missed by ordinary DFT.