Wednesday, October 31, 2007

In honor of Halloween....

Three of my favorite science-related quotes from the movies, all from Ghostbusters:

Dean Teager: Your theories are the worst kind of popular tripe; your methods are sloppy, and your conclusions are highly questionable. You are a poor scientist, Dr. Venkman.
Ray Stantz: Personally, I like the University. They gave us money and facilities, we didn't have to produce anything. You've never been out of college. You don't know what it's like out there. I've worked in the private sector. They expect results.
Peter Venkman: Back off, man! I'm a scientist!

Any other good ones to share? (Real science post coming in a day or two....)

Friday, October 26, 2007

Jobs jobs jobs

I figure it's probably a good idea to take advantage of the staggeringly enormous readership of this blog to point out several searches going on at Rice right now.

First, three searches are going on here at Rice in the Physics and Astronomy department at the moment. These are:
There is also an experimental nanophotonics search going on in Electrical and Computer Engineering.

Finally, the Chemistry department is doing a search for inorganic or physical chemists, broadly defined. The ad is on the departmental homepage.

Share and enjoy! If you want to discuss what Rice is like as a faculty member, please feel free to contact me and I'll be happy to talk.

Friday, October 19, 2007

Three papers and a video.

Three interesting papers on ASAP at Nano Letters at the moment: and are both papers where people have taken graphite flakes, oxidized them to make graphite oxide, and then suspended the graphene oxide sheets in solvent. They then deposit the sheets onto substrates and made electronic devices out of them after trying to reduce the graphene oxide back to just graphene. There are a couple of people here at Rice trying similar things from the chemistry side. Interesting that a number of groups are all working on this at about the same time. That's one reason why it can be dangerous to try to jump into a rapidly evolving hot topic - it's easy to get scooped.

This one is a cute paper titled "Carbon nanotube radio". The science is nicely done, though not exactly surprising. AM radio works by taking an rf carrier signal and demodulating it to get back just the envelope of that carrier signal. Back in the early 20th century (or more recently, if you bought an old kit somewhere), people used to do the demodulating using a diode made semi-reliably by jamming a metal needle (a "cat's whisker") into a lead sulfide crystal - hence the term "crystal radio". It's simple trig math to see that a nonlinear IV curve (one with a nonzero d^2I/dV^2) can rectify an ac signal of amplitude V0 to give a dc signal of (1/4)(d^2I/dV^2)V0^2. Well, in this case the nonlinear element is a nanotube device. Cute, though I have to admit that I found the media hype a bit much. Wilson Ho did the same essential thing very nicely with an STM, but didn't talk about atomic-scale radio receivers....

Lastly, via Scott Aaronson, a link to a fantastic math presentation. Watch the whole thing - this really is a model of clarity and public outreach. On a bitter-sweet note, in the credits at the end I realized that one of the people responsible for this was an acquaintance from college who has since passed away. Small world.

Tuesday, October 16, 2007

This week in cond-mat

Real life continues to be very busy this semester. Two interesting papers on the arxiv this week....

- Fratini et al., Current saturation and Coulomb interactions in organic single-crystal transistors
The technology finally exists to do what He Who Must Not Be Named claimed to have done: use a field-effect geometry to gate significant charge densities (that is, a good fraction of a charge carrier per molecule) into the surface of a clean single crystal of an organic semiconductor. The Delft group has used Ta2O5 as a high-k gate dielectric, and are able to get 0.1 holes per rubrene atom in a single-crystal FET geometry. In typical organic FETs, increasing the charge density in the channel improves transport by filling trap states and by moving the chemical potential in the channel toward the mobility edge in the density of states. Surprisingly, Fratini et al. have found that the channel conductance actually saturates at very high charge densities instead of continuing to increase. The reason for this appears to be Coulomb interactions in the channel due to the high carrier density and the polaronic nature of the holes. The strong coupling between the carriers and the dielectric layer leads to a tendency toward self-trapping; add strong repulsion and poor screening into the mix, and you have a more insulating state induced by this combination of effects. Very interesting!

arxiv:0710.2323 - Degen et al., Controlling spin noise in nanoscale ensembles of nuclear spins
Dan Rugar
at IBM has been working on magnetic resonance force microscopy for a long time, and they've got sensitivity to the point where they can detect hundreds of nuclear spins (!). (That may not seem impressive if you haven't been following this, but it's a tour de force experiment that's come very far from the initial work.) The basic idea of MRFM is to have a high-Q cantilever that is mechanically resonant at the spin resonance frequency and coupled via magnetic interactions to the sample - that way the polarized spins precess, they drive the cantilever resonance mode. When they look at such a small number of spins, the statistical fluctuations in the spin polarization are readily detected. This is a problem for imaging, actually - the timescale for the natural fluctuations is long enough that the signal bops around quite a bit during a line scan. Fortunately, Degen et al. have demonstrated in this paper that one can deliberately randomize the magnetization by bursts of rf pi/2 pulses, and thus suppress the fluctuation impact on imaging by making the effective fluctuations much more rapid. This is a nice mix of pretty physics and very clever experimental technique.

Wednesday, October 10, 2007

Giant magnetoresistance

I think it's great that the physics Nobel this year went for giant magnetoresistance (GMR). GMR is intrinsically a quantum mechanical effect, an example of a nanoscale technology that's made it out of the lab and into products, and one of the big reasons that you can buy a 500GB hard drive for $100. (Good job, Sujit, for the advanced pick!).

The story in brief: Back in the ancient past (that is, the 1980s), the read heads on hard drives operated based on the anisotropic magnetoresistance (AMR). For band structure reasons, the electrical resistivity of ferromagnetic metals depends a bit on the relative orientations of M, the magnetization, and J, the current density. In the common NiFe alloy permalloy, for example, the resistivity is about 2% larger when M is parallel to J than when M is perpendicular to J. To read out the bits on magnetic media, a strip of very coercible magnetic material was used, and the fringing fields from the disk media could alter the direction of that strip's M, leading to changes in the resistance that were translated into voltage changes that correspond to 1s and 0s.

In the late 1980s, Fert and Grunberg demonstrated that stacks of nanoscale layers of alternating magnetic and nonmagnetic metals had remarkable magnetoresistive properties. When the M of the FM layers are aligned, the mobile electrons can move smoothly between the layers, leading to relatively low resistance. However, when the M of the FM layers are anti-aligned, there is a mismatch between the densities of states for spin-up and spin-down electrons between anti-aligned layers. The result is enhanced scattering of spin-polarized electrons at the interfaces between the normal and FM layers. (Crudely, a spin-down electron that comes from being the majority spin in one FM layer goes through the normal metal and runs into the anti-aligned FM layer, where that spin orientation is now the minority spin - there are too few empty states available for that electron in the new FM layer, so it is likely to be reflected from the interface.) More scattering = higher resistance. The resulting GMR effect can be 10x larger than AMR, meaning that read heads based on GMR multilayers could read much smaller bits (with smaller fringing fields) for the same signal-to-noise ratio.

Thursday, October 04, 2007

Challenges in measurement

This post is only going to be relevant directly for those people working on the same kind of stuff that my group does. Still, it gives a flavor of the challenges that can pop up unexpectedly in doing experimental work.

Often we are interested in measuring the electronic conductance of some nanodevice. One approach to doing this is to apply a small AC voltage to one end of the device, and connect the other end to something called a current preamplifier (or a current-to-voltage converter, or a glorified ammeter) to measure the amount of current that flows. It's possible to build your own current preamp, but many nanodevice labs have a couple of general purpose ones lying around. A common one is the SR570, made by Stanford Research. This gadget is pretty nice - it has up to a 1 MHz bandwidth, it has built-in filter stages, it is remotely programmable, and it has various different gain settings depending on whether you want to measure microamps or picoamps of current.

Here's the problem, though. One of my students observed that his devices seemed to fail at a surprisingly high rate when using the SR570, while the failure rate was dramatically lower when using a different (though more expensive) preamp, the Keithley 428. After careful testing he found that when the SR570 changes gain ranges (there is an audible click of an internal relay when this happens, as the input stage of the amplifier is switched), spikes of > 1V (!) lasting tens of microseconds show up on the input of the amplifier (the part directly connected to the device), at least when hooked up to an oscilloscope. Our nanoscale junctions are very fragile, and these spikes irreversibly damage the devices. The Keithley, on the other hand, doesn't do this and is very quiet. Talking to SRS, this appears to be an unavoidable trait of the SR570. We're working to mitigate this problem, but it's probably good for people out there in the community using these things to know about this.