This week in cond-mat

Four papers this time, though brief descriptions. Eventually the semester will ease up a bit and I'll have more time to write.

cond-mat/0610572 - Gabelli et al., Violation of Kirchoff's Laws for a coherent RC circuit
Kirchoff's laws are the basic rules you learn in introductory circuits, and may be suitably generalized to think about high frequency systems. One of the basics is that impedances in series add. In this paper (also published in Science), the authors do some very nice work using gated two-dimensional electron gas to make an effective RC circuit, where part of the R is a quantum point contact. They find that when the whole system is quantum coherent, the basic idea of adding impedances goes out the window. This is neat, and it is a beautifully done experiment, but I don't find the conceptual point to be very surprising at all. Think about this simple case just in the dc limit: a single tunneling barrier has some effective tunneling resistance. A second, identical tunneling barrier has the same resistance. What is the resistance of the series combination of the tunneling barriers? Well, in the incoherent limit, the resistances just add. In the fully coherent limit, you have to worry about interference effects between the barriers, and can even arrive at perfect transmission for the series combination, even though each barrier individually is not very transmissive. This paper's analysis is more general than this, but I can't help but think that it's really the same basic physics at work.

cond-mat/0610634 - Neder et al., Controlled dephasing of electrons by non-Gaussian shot noise
This is another great experiment by the folks at the Weizmann Institute, studying the basic physics of quantum decoherence using an interferometer and a tunable detector at one arm of the interferometer, all made from GaAs 2d electron gas. In earlier work, they've shown that the interference of the electrons in the which-path interferometer can be suppressed in a controlled and continuous way, depending on how "on" the detector is, and how strongly the detector is coupled to the interferometer arm. Here, they work in the quantum Hall limit, and study directly the relationship between the back-action of the detector (via its noise) and the effect on the interference.

cond-mat/0610710 - Scalapino, Numerical studies of the 2d Hubbard model
The 2d Hubbard model is one of the favorite toy models suggested for high Tc. It's a square lattice, with some nearest neighbor hopping amplitude t and an on-site repulsion U so strong that each site can only hold 1 electron. Scalapino has written a review chapter summarizing numerical treatments of this model, and arguing that it has all the essential features of high-Tc. Numerical work in models like this is notoriously difficult computationally, in part because of the requirements that the whole many-body state be antisymmetric under exchange of any two electrons.

cond-mat/0610721- Potok et al., Observation of the two-channel Kondo effect
I want to write more about this later. In brief, David Goldhaber-Gordon and Yuval Oreg had proposed an experimental set-up to implement a tunable version of the long-sought two-channel Kondo model, in which a single localized spin is coupled via tunneling to two independent electronic baths. The 2CK model is of interest because its ground state is not a Fermi liquid (as opposed to the conventional Kondo model and ordinary metals). David's students Ron Potok and Illeana Rau have done the experiment, and the results look very interesting. Using the scaling of the conductance, it looks very much like they have succeeded in getting (at least) very close to the two-channel Kondo state. A cool experiment, and very technically demanding, in part because the temperature scales needed to see the physics are so low.