Monday, September 11, 2006

Packard Fellows meeting

Sorry for the downtime; the semester has begun, and this past week I also went to the annual meeting of Packard Fellows, which is a wonderful chance to hear technical talks from people working in all sorts of fields of the natural sciences, engineering, and mathematics. I will return with more cond-mat article discussions soon. In the mean time, I wanted to highlight two particular condensed matter talks that I heard at the meeting:

First, Hongkun Park spoke about his recent very interesting work on electronic properties of VO2 nanowires (actually bars - they grow from the vapor phase into long wires with square cross-sections). Some of this has been published. Vanadium dioxide is a weird material. It's supposed to be a Mott insulator at low temperatures, meaning that electronic interactions are so strong that the charges lock into place rather than being free to move around. At higher (not much higher than room) temperature, the material undergoes a first-order structural and electronic phase transition to a metallic state. Prof. Park's group has been playing with these nanowires, and found some amazing phenomena. For example, when the wires are sitting on a surface, the constraint of the surface strain plus the structural phase transition lead to the wires breaking up spontaneously into domains of metallic and insulating regions, and those domains can be (a) imaged with an optical microscope, (b) pushed around by flowing a current, and (c) made to oscillate back and forth because of resistive heating effects. Also, in suspended wires, the metal/insulator phase transition can be incredibly sharp, leading to the possibility of novel temperature sensors. Very neat.
Update: (9/27/06) This has just appeared in Nano Letters.

Second, Kathryn Moler showed her latest work on scanning SQUID microscopy. Basically it's possible to put an incredibly sensitive magnetometer at the very tip of an AFM-like probe, and image magnetic flux with incredible sensitivity. Most recently her group has been looking at superconducting fluctuations in little superconducting ring structures. Imagine putting a small magnetic flux on a superconducting ring. The fact that the superconducting wavefunction has to be single-valued going around the loop implies that magnetic flux through the loop is quantized. That quantization condition is enforced by spontaneous supercurrents in the loop. Well, for narrow loops its possible to be in a regime where rather than set up those currents, it's more energetically favored for the loop to go "normal". This is the Little-Parks effect. Now, if you imagine a split ring that looks just like the loop but isn't a complete circle, that would be superconducting. Can the topology of the ring really deeply affect the microscopic physics in the superconductor? Superconducting fluctuations in the "normal" ring are part of the answer. Again, a neat technique and a very nice piece of physics.

3 comments:

Dave Cobden said...

Doug, Hongkun's VO2 nanowires are indeed remarkable things. I've not seen him talk about them, unfortunately. There is a preprint on his web site about the periodic domains, but it doesn't mention domain motion. Do you know if they have an explanation of current-driven domain motion? In the olden days when the material was studied in the bulk domain motion was probably due to typical nonlinear self-heating stuff, but it is also possible that the domains move at a rate proportional to current for very small currents and that would be a rather new phenomenon, I think.

Doug Natelson said...

Hi Dave - yes, they've seen domain motion, and the direction of the motion does depend on the direction of the current (though I don't know what the sign is, or whether the rate is linear in the current). I think he mentioned self-heating as part of the mechanism, but don't quote me - the talk was only 15 minutes and to a general audience, so the details were necessarily sparse.

Alison Chaiken said...

it is also possible that the domains move at a rate proportional to current for very small currents and that would be a rather new phenomenon, I think.

As an older person I have to ask, doesn't motion of domains in response to currents sound a lot like the sliding of charge-density waves?

Those with an interest in density wave phenomena are urged to correct this:

http://en.wikipedia.org/wiki/Spin_density_wave

when the wires are sitting on a surface, the constraint of the surface strain plus the structural phase transition lead to the wires breaking up spontaneously into domains of metallic and insulating regions,

Sounds a lot like the Lai et al. MOSFET work in your other more recent posting.