Thursday, March 13, 2008

March APS Meeting III

Day 3 in New Orleans continued to be interesting, though I missed some talks so that I could have conversations with a few people, including my program officers from a couple of funding agencies. It's never a bad idea to make sure that the program officers know what you've been doing with their resources.

I started out the day by catching an invited talk by Doug Scalapino talking about his take on the binding "glue" in the high-Tc superconductors. Scalapino uses "glue" to refer to the retarded (time-delayed) interaction that leads to pairing of the electrons. In the low temperature superconductors, the glue in this sense is the retarded phonon interaction - in a sense, one electron leaves behind a lattice vibration that slightly deforms the ion charge distribution, leading to a second electron of opposite momentum to feel a slight residual attraction to the first electron. The screened Coulomb interaction between the electrons is effectively instantaneous (and repulsive). In the high-Tc case, it's not clear what the glue is. Scalapino would argue that it's a spin fluctuation interaction; Phil Anderson would probably argue that there is no important glue in this sense of the term.

I then chaired my session, which was fun but tiring. One particularly cute experiment was from the Weig/Kotthaus group at Munich. They are trying to use nanomechanical resonators as charge shuttles. The idea is a bit like a bucket brigade. Have a metal island be suspended on a resonant beam between a source and a drain electrode. When set up ideally and driven at resonance, the island will swing back and forth between the source and drain like the clapper between the bells of an old alarm clock. When the island gets close to the source, an electron can tunnel onto the island. Ideally Coulomb blockade would ensure that it's one and only one electron. Then the island can swing over to the drain electrode, and drop off that electron. The experiment was elegant - they make many resonators at once and wire them all up in parallel. The clever bit is that they have each resonator tailored with a different mass, so that they can selectively drive just the one that they want. They drive mechanically, by shaking the whole chip back and forth, to avoid electrical crosstalk trouble. It should be very nice when they can get the structures even smaller and colder, to see strong Coulomb blockade effects.

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