Wednesday, May 03, 2006

This week in cond-mat

There are three papers I'd like to bring up from the past week or so that I think are pretty neat pieces of physics:

cond-mat/0605061 - Boulant et al., Bloch oscillations in a Josephson circuit.
This is the most recent paper from the Quantronics (quantum electronics) group at Saclay, a collaborative effort that routinely cranks out some of the most elegant and pretty physics experiments using nanodevices. Consider a tunnel junction with some capacitance C. To move a single electron across the junction would generically require an amount of energy (in the form of eV, where e is the electronic charge and V is the dc bias voltage across the junction) that exceeds the capacitive charging energy of the junction, ~ e^2/2C. If such a junction is hooked up to a constant current source, the voltage across the junction is expected to vary like a sawtooth pattern: rising linearly with time until it hits that threshold, and then dropping quickly as the electron tunnels. If one does this with a superconductor, the relevant particles are Cooper pairs with charge 2e, but the effect is the same: a constant current bias should lead to an ac voltage across the junction, with a dominant frequency proportional to the current. These ac voltage wiggles are called Bloch oscillations, and have not been measured directly yet. There's all kinds of reasons why doing so is hard, most related to the fact that it's hard to really make a true constant current source at the relevant frequency scale. Remember, one microamp of current would lead to THz oscillations. Anyway, these folks made a more complicated structure with two junctions, and use that structure to terminate an rf line. When they send rf power into the line and look at the reflected rf coming back, they can see sidebands in the reflected signal offset from the input frequency by the Bloch frequency. It's a very pretty experiment.

cond-mat/0604654 - van der Wolen et al., The Magneto-Coulomb effect in spin valve devices.
This paper is an interesting theory paper by the group of van Wees, who has helped to define the field of mesoscopic physics. It's an examination of the interplay of magnetic effects and Coulomb charging effects in single-electron tunneling structures incorporating ferromagnetic metals. In the absence of the charging effects, the connection between magnetization and electronic transport is responsible for many useful effects like giant magnetoresistance, the basis for the read-head in your hard drive.

cond-mat/0604608 - Onac et al., Using a quantum dot as a high frequency shot noise detector.
Another beautiful and clever experiment from the mesoscopics group at Delft. It is often very challenging to measure high frequency dynamics in nanostructures, since the relatively high impedances of the devices are typically a poor match for most commercial rf electronics and coaxial cables. Life is even more difficult at very low temperatures, where most of the interesting physics often happens, because there are painful experimental constraints that must be obeyed. One method of studying rapid charge variations in quantum dots has been to use a quantum point contact as a charge detector. A QPC is a region of 2d electron gas that has been constricted using gates down to the point where only one or a couple of channels of transmission are left. Sitting on the edge of depletion of a channel, the presence or absence of charge on a nearby quantum dot can strongly change the conductance (and therefore rf impedance) of the QPC. Using rf reflectance methods like those above, this can be monitored at high frequencies. This experiment is the complement of that - by gating and biasing the quantum dot appropriately, dc transport through the dot can be strongly modified by the high frequency fluctuations of the current (shot noise) in the nearby QPC. This is a great approach for studying back-action and measurement: is the dot the detector and the QPC the system, or vice versa?



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