## Monday, April 29, 2013

I just read this article at Science's blog, describing how Lamar Smith, chair of the House science committee, basically wants to gut peer review at the NSF and replace it with something more to the liking of the House Republicans.  This would be catastrophically bad for a large number of reasons.  If they do this, you know it's only a matter of time before they decide to undermine peer review at NIH and DOE Office of Science also.  Gahh.  Can't blog - too incoherently angry.

update:  For what it's worth, this would presumably have a hard time passing the Senate and getting signed into law by the President, though given Congress' tendency to lump zillions of unrelated bills together into giant omnibus legislation, you never know for sure.   NSF is probably not the real long-term target of these types.  Picture what would happen to research if big pharma lobbyists get to have Congress decide what NIH grants should be funded, or if big energy lobbyists do the same for DOE grants (to say nothing of de-funding anything they find politically unacceptable).

## Sunday, April 28, 2013

### Cryogenic dark matter detection, redux

About 3.5 years ago, I posted about the technology used by the CDMS collaboration to look for dark matter using clever solid-state detectors.  These folks have some news that is, as is often the case in any novel particle detection experiment, intriguing but not yet definitive.  This paper is probably the best place to see a summary of the results.  In their CDMS II run, the team had 19 germanium-based and 11 silicon-based detectors (cooled to 0.04 K!) running for five years (2003-2008) in an old mine in Minnesota (to cut down on cosmic ray background).  This paper reports results from the Si detectors, where after a lengthy blind analysis they see three events that look interesting.  Since germanium has a higher atomic number than Si, the idea of running the two materials in parallel was to have a cross-check and provide some information about how the searched-for weakly interacting massive particles (WIMPs) might interact with ordinary matter as a function of energy.  I should also note that the collaboration is now running "SuperCDMS" with a larger mass of germanium (9 kg) since 2011, and will eventually expand up to 200 kg of Ge running in Soudan in Ontario.  It's interesting that their earlier analysis from their Ge detectors reported no candidate events (as far as I can tell), while the analysis of the Si detectors shows three candidate events.  My understanding is that this could have to do with the mass range of the WIMPs, but I would be happy if someone would provide more context in the comments below.  Either way, I think it's great to see how condensed matter physics (and in particular cool device fabrication, as in the superconducting transition-edge sensors used here) can have an impact on Big Questions like dark matter.

## Tuesday, April 23, 2013

### MOOCs and online education

Massive open online courses (MOOCs) and concerns about online education are all the rage these days at universities.  There is a growing recognition of a few key points:  The cost of undergraduate education (in the US at least) continues to increase much more rapidly than inflation; online capabilities are sufficiently advanced now that it is possible, for comparatively little investment, to distribute educational content to many thousands of people at very low cost, in principle having a major pedagogical impact (see, e.g., the Khan Academy, to say nothing of MIT's opencourseware); more than one major concern is springing up trying to guide online education at the university level (see, e.g., coursera and edX).  [Note to self:  find some demo as cool as the thermite reaction to hook people into any online course I ever teach.]  There is clearly a major sense of urgency on the part of university administrators.  To belabor an overused analogy, they are worried that the online education train is leaving the station, and they fear the consequences of getting left behind.

All of these things are true, and I understand the concern.  However, a few points have occurred to me about this, and I'd be happy for some discussion in the comments if people are interested.
1. Many people do not really have the self-discipline to learn in an online-only environment.  I like to think I was a pretty dedicated student (no smart comments from my former classmates, please), and I'm not sure I would have the self-discipline to watch online-only lecture material and do online-only assignments for an entire semester.  Some people do have the personality for this, but I have a hunch that many of them are the same folks who really can check a book out of the library and teach themselves a new subject ab initio.  Most 18 year olds are not like that, and the peer pressure/social environment of having friends physically going to scheduled classes is a major motivator.  Bill Press, when he visited Rice and we chatted about this, pointed out that many people pay real money to take Microsoft online certification courses, and complete them at a high rate.  That's true, but it's also a particular case where the financial benefits of completing that particular course are often very clear to the student, and it's also true that there's a difference between university study and vocational training.
2. It only makes sense to develop online courses where your institution really adds value.  Does anyone think it would be a good idea for every major university to develop their own MOOC for Introductory Calculus?  We could do that, but in the end there will likely be a small handful of truly innovative, extremely well done calc courses.  The market will drive toward some kind of mix-and-match mode of operation (unless the content providers constrain things greatly).
3. The sense of urgency is not unreasonable, but early innovators don't necessarily win the day.  For example, Lycos and Alta Vista were early to the scene in "search", yet comparative latecomer google crushed them.
So, are MOOCs really going to sweep through and destroy the modern university system?  Are physical universities going to become like specialty bookshops and online providers like Amazon?  Let me know what you think.

## Monday, April 22, 2013

### Book review: Alsos

I just found and read a great book, Alsos, by Samuel Goudsmit. The Alsos mission was the Allied dual scientific/military intelligence gathering expedition following the Normandy Invasion, tasked with learning the status of the German atomic program and rounding up German nuclear scientists. Goudsmit, who with Uhlenbeck helped convince people like Pauli of the usefulness of the concept of spin (the intrinsic angular momentum of particles like the electron), was a Dutch Jew, and while he was in the States working on radar, his parents were sent to a concentration camp and killed. The book is fascinating. It's split between the story of the actual mission (which discovered relatively quickly and much to the relief of all involved that the Germans never even got a nuclear pile to go critical) and an indictment of science and industry in a totalitarian regime. It is quite the cautionary tale of the politicization of scientific research and the arrogance of some physicists (Heisenberg fares particularly poorly, to the surprise of no one), told with a wry sense of humor. Highly recommended.

## Friday, April 12, 2013

### Workshop on "Electronic Properties of Carbon-based Nanostructures"

I'm on my way back to the US from this workshop at the Universität Regensburg.  It was a fun and interesting meeting, and the quality of the invited talks was uniformly high.  The city was also very neat.  I'd had no idea that it had managed to escape (almost completely) Allied bombing during WWII, so as a result it has many buildings dating back to the Middle Ages.

On the science side, it was particularly nice to hear some talks from and meet a number of people that whose work I've seen over the years but I'd never met face-to-face before.  For example, Steven Louie (linking to wikipedia since all of the Berkeley servers are inexplicably slow right now) spoke about ways to accurately calculate the optical properties of graphene (including electron-hole interactions properly). Philip Collins showed how it's possible to look at single-molecule biophysics (like the functioning of individual enzyme molecules) by anchoring the molecules of interest to single-walled carbon nanotubes, where the action is transduced into changes in the conductance.  Adrian Bachtold gave a nice overview of their work on optomechanics of nanotubes, which has enabled them to do mass sensing at the resolution of a single atomic mass unit (10-27 kg) and force sensing with similarly impressive sensitivity.  Richard Berndt from Kiel discussed his group's work where they argue that light emission from STM tips shows the signature of shot noise in the current at optical frequencies.  Wolfgang Wernsdorfer, grand poobah of molecular magnetism, presented new results showing amazing control and detection of individual electronic spin lifetimes (in Tb-containing molecules).  For a spin to flip spontaneously, the molecule has to transfer angular momentum to the rest of the world somehow.  In the new experiment, this happens by dumping angular momentum into a carbon nanotube to which the molecule is anchored.  Since the allowed vibrational states of the nanotube can be controlled, this in turn tunes the spin flip rates.  Finally, Klaus Müllen gave an overview of ways to rationally synthesize, by chemical means, graphene flakes, ribbons, and other shapes.  The chemistry is just unreal.

## Thursday, April 04, 2013

### Spin Hall physics

As I mentioned during the APS meeting, Dan Ralph presented some beautiful work (for example) on spin torque devices (where the flow of spin-polarized electrons is able to rotate the magnetization of some "free" ferromagnetic layer of material).  This spin torque business is a fairly mature idea, and the early demonstrations of this effect made use of layered structures (ferromagnet/normal metal/ferromagnet), with the current flowing perpendicular to the layers.  That is, if electrons flow from FM1, some of them are spin-polarized because of the magnetization of FM1, and those polarized electrons traverse the normal layer into FM2.  That works fine, but the most angular momentum you're ever going to transfer that way is $\hbar/2$ per electron, and that assumes that the electrons from FM1 are perfectly polarized.   Suppose you could do better than this.  Is there some way, for a given amount of charge current that you flow, to get more angular momentum transferred?

The answer is "yes", and the key is to leverage the spin Hall effect.  (For a good summary of spin Hall physics, see this paper by one of the progenitors of the field - I'll briefly summarize.)  In the regular Hall effect, we think about charge current flow in a plane in the presence of a perpendicular magnetic field.  The charge carriers experience a Lorentz force from the magnetic field that pushes them in the plane transverse to the direction of the (longitudinal) charge current.  Net charge of one sign piles up at one transverse edge of the sample, and net charge of the other sign piles up at the opposite edge, until the force from the resulting transverse electric field balances the Lorentz force.  (Glad to see wikipedia has fixed the figure in this article.  A few years ago they had the direction of the Lorentz force backward.)  In the spin Hall effect, we again think about current flow in a plane.  However, there is no external magnetic field.  Instead, we have the current flowing in a material with strong spin-orbit scattering (that is, in the reference frame of the moving electron, the effective charge current due to the nuclei seemingly moving by produces enough of a magnetic field in that frame to couple significantly to the spin of the electron.  Fundamentally this is a relativistic effect!).  Because of the coupling of spin to orbital motion, if the charge carriers scatter, the spins self-polarize; spin-"up" electrons will pile up on one transverse edge of the sample, while spin-"down" electrons will tend to pile up on the opposite edge.  The extent to which this happens is determined mostly by the strength of the spin-orbit coupling, which is larger in heavier atoms.

So, Ralph and coworkers have used this effect to great advantage.  Instead of the FM/N/FM layered structure, they make a structure that looks like SO/FM/N/FM, where SO is a strong spin-orbit material, such as tungsten or platinum.  They can flow a current within the plane of the SO layer.  Through the spin Hall effect, this can pump polarized spins perpendicular to the plane, into the adjacent FM layer.  (The electrical resistance vertically through the FM/N/FM stack is a way of monitoring the relative alignment of the FM layers, thanks to the giant magnetoresistance.)  This is particularly clever, because for strong SO coupling in the SO layer, thanks to the large contact area at the SO/FM interface, they can get more like 10 $\hbar$ of angular momentum per electron flowing within the SO layer.   Fascinating to realize that these effects (because they originate from SO physics) are really dramatic experimental proof of the way electric and magnetic fields obey special relativity!