This week in cond-mat

Two papers for this end-of-year post.

cond-mat/0612556 - Vartiainen et al., Nanoampere pumping of Cooper pairs
The single-electron transistor was developed almost twenty years ago, based on the observation that one could now fabricate a metal island (weakly coupled to leads via tunnel junctions) so small that it's capacitive charging energy could significantly exceed kT (this gets easier as T is lowered to within a fraction of a Kelvin of absolute zero, which is now readily achievable). In such a device, the charge on the island is generally well-defined and quantized to an integer number of electrons. By cleverly hooking islands together and cycling gate voltages appropriately, it's possible to make an electron "turnstile", such that one electron at a time may be pumped through the circuit. Doing this at high frequencies, f, would enable (ideally) a noiseless current source (with current ef). That's easier said than done, however, because the intrinsic RC charging timescales of such turnstiles tend to limit the frequency of operation. The Finnish group here has implemented an alternative scheme, using superconducting quantum interference devices (SQUIDs) rather than simple tunnel barriers, and can pump individual Cooper pairs of superconducting electrons through their circuit at a high enough rate to generate nanoamperes of current. This is very impressive, and could lead to real advances in metrology.

cond-mat/0612635 - Pereira et al., Kondo screening cloud and charge quantization in mesoscopic devices
In the Kondo effect, a localized spin coupled to mobile electrons undergoes a spin-flip scattering process that leads to spin correlations in the mobile electrons. At temperatures small compared to the characteristic energy of this process, the local spin is "screened" - that is, it is entangled with a cloud of the mobile electrons, forming a singlet state with no net spin. A question that has been around a long time in the solid state community is, how big is that screening cloud? The only successful attempts to measure the size have been in STM measurements of magnetic impurities on surfaces, as far as I know. In this paper, the authors propose a clever scheme to try this in a model system. One can have the local spin be living in a quantum dot, and use a electrons in a large 1d electronic box instead of truly free electrons to form the Kondo state. The idea is that the size of the Kondo cloud will be detected by looking at the single-particle levels of the 1d system (and varying system effective length). Neat, though tough to do!