Monday, August 31, 2009

Two nanoscience tidbits

Since nearly everyone else in the science blogging world has touched on this (see here, here, here, here, and here, to name a few), I might as well do so, also.  Leo Gross and coworkers at IBM Zurich have used an atomic force microscope to do something incredibly impressive:  They have been able to image the bonding orbitals in individual pentacene molecules with better than atomic resolution, using the very short-range forces that contribute to the "hard core repulsion" between atoms.  Atoms tend to be attracted to each other on nanometer scales, even in the absence of strong chemical interactions, due to the van der Waals interaction, which comes from the fluctuating motion of their electron clouds.  At very short distances, though, atoms effectively repel each other extremely strongly, both from the Coulomb interaction (electrons don't like each other overly much) and the effects of the Pauli Exclusion Principle.  Gross and colleagues accomplished this feat by working in ultrahigh vacuum (around 10-15 of atmospheric pressure) and at 5 K, and by deliberately attaching a single CO molecule to their conducting atomic force microscope (AFM) tip.  It's a heck of a technical achievement for AFM.  Atomic resolution has been demonstrated before, but this kind of sensitivity is remarkable.  (FWIW, I once heard one of the major coauthors, Gerhard Meyer, speak at a meeting about the same group's ultrahigh resolution STM work.  He seemed very low key about their obviously impressive achievements - amazingly so.  I hope he got excited about this!)

Also, a group at Berkeley has made a laser based on a CdS nanowire, and like the result mentioned last week, this gadget uses plasmons (this time in a Ag film) to act as an effective cavity.  Clearly using the extreme confinement of some plasmon modes to do photonics is going to be a growth industry.

Sunday, August 30, 2009

Industrial R&D

I've felt for a long time that the current business climate, which punishes rather than rewards long-term research investments by companies, is misguided. When most stock is owned and traded by institutional investors and large funds who don't have any interest in holding particular companies for the long term, and when executive compensation massively overvalues year-over-year growth (because we all know that 40% annual growth in cell phone sales is sustainable forever, right? There's no such thing as market saturation, is there?), you end up where long-term investment is viewed by company boards as a misuse of resources. This article in Business Week makes some interesting arguments on ways to try and fix this. Unfortunately I think most of these ideas are not very compelling or likely. Norm Augustine had an interesting suggestion: scale the capital gains tax rate inversely with the amount of time one owns a stock. If someone holds a stock less than a year, tax the capital gains at 90%. If they own the stock 10 years or more, tax the capital gains at nearly 0%. Interpolate appropriately. The idea here is to set up a system that incentivizes long-term investment, which in turn is more likely to support industrial research. Hard to see how such an overhaul would ever get passed in Congress, though. I imagine the financial industry would crush it like a bug, since anything that slows down trading is viewed as interference in the free market, or, more cynically, interference in their enormous transaction fee profits.

Wednesday, August 26, 2009

How we fund grad students

As new grad students flood onto campuses across the US, I just got around to reading this piece in Science from a few of weeks ago about Roald Hoffman's idea for changing the way we support grad students in the sciences and engineering. Most S&E grad students in the US are supported by a mix of teaching assistantships (TAs), research assistanceships (RAs), and fellowships. A typical S&E grad student at an American university shows up and is supported during their their first year by a mix of university funds and pay for teaching. They then often make the transition to being supported as an RA by research funds obtained by their advisor through research grants. (Some remain as TAs - this is more common at large, public institutions with large undergraduate teaching needs.) Some relatively small fraction of S&E grad students are supported instead by fellowships, awarded competitively by agencies like NSF, DOE, DOD, NIH, etc. or by private foundations such as the Hertz Foundation.

Prof. Hoffman suggests that we should move to a system where all grad student support is fellowship-based. The idea is that this will (a) fund only the best students; (b) allow students much greater independence since an advisor will no longer be able to say "You have to do boring experiment #23 because that's what the grant that's paying your salary says we're going to do"; (c) result in better mentoring b/c faculty will no longer view students as "hands". Now, there's basically no way to see how such a drastic change in the system would ever happen, but it's worth looking at the idea.

As someone lucky enough to have a fellowship in grad school, I understand the appeal from the student side. Independence is great - it means that you and your advisor are freed from the stress of worrying that your grant won't get renewed when you're in year 3 of your program. It means that you are a free agent.

However, I think Hoffman's idea would be a disaster, for two main research-related reasons (not to mention the challenge of how you'd handle TA duties at large places that suddenly had many fewer grad students). First, there is little doubt that this would skew an already tilted system even further in favor of the top, say, 20 institutions in the country. Right now it's possible for good researchers at second tier universities to write grants, hire students, and do research. Imagine instead if the only source of student support were competitive external fellowships. It's all well and good to talk about overproduction of PhDs, and say that drastically reducing the number of grad students would be good for employment and salaries. There is a point to that. However, you would effectively end research as an enterprise at many second and third-tier schools, and there are a fair number of really good programs that would go away. Second, since federally funded fellowships would presumably only go to US citizens, this idea would drastically reduce international PhD students in S&E. That, too, would be a mess. Some of our best students are international students, and whether or not they stay in the US after their degrees, training these people is a valuable service that the current US system provides.

It is worth considering other funding schemes, though. I know that in the UK students are supported through their PhD, rather than on a schedule set by external grant deadlines. Perhaps some of my UK readers could comment on the pluses and minuses of this approach.

Monday, August 24, 2009

plasmons instead of cavities

Sorry for the delay in posts. The beginning of the new academic year is a hectic time.

This paper
is a very exciting new result. Unfortunately there does not appear to be a publicly accessible version available. Ordinarily, lasing (that is, light amplification by the stimulated emission of radiation) requires a few things. One needs a "gain medium", some kind of optically active system that has (at least one) radiative transition. In this paper, the medium is a dielectric oxide containing dye molecules known to fluoresce at a wavelength of 520 nm. This medium needs to be pumped somehow, so that there are more optically active systems in the excited state than in the ground state. This is called "population inversion". (It is possible to get lasing without inversion, but that's a very special case....) Finally, one generally needs a cavity - an optical resonator of high enough quality that an emitted photon stays around long enough to stimulate the emission of many more photons. The cavity has to be somewhat leaky, so that the laser light can get out. However, if the cavity is too leaky, the optical gain from stimulated emission in the pumped medium can't outpace the cavity losses. The usual approach is to have a rather high quality cavity, made using either dielectric mirrors, total internal reflection, or some other conventional reflectors.

In this paper, however, the authors take a different tactic. They use the near-field from the plasmon resonance of the gold core (not coincidentally, at around 520 nm wavelength) of Au-core-dielectric-shell nanoparticles. Plasmon resonances are often quite lossy, and this is no exception - the Q of the plasmon resonance is around 14. However, the enhanced near field is so large, and the effective mode volume (confinement) is so small, that gain still outpaces loss. When the dye is optically pumped, it is possible to make these nanoparticles lase. This paper is likely to spawn a great deal of further work! It's cool, and there are many clear directions to pursue now that this has been demonstrated.

Thursday, August 13, 2009

This week in cond-mat

Where did summer go? Several interesting things on the arxiv recently. Here are two from this past week that caught my eye.

arxiv:0806.3547 - Katz et al., Uncollapsing of a quantum state in a superconducting phase qubit
This paper first appeared on the arxiv last year, and it made it onto this week's mailings because the authors uploaded the final, published version (PRL 101, 200401 (2008)). This experiment is important as a technical development in the quantum computing community, since the ability to restore some measure of purity to a quantum state after that state gets entangled with some environmental degrees of freedom could be very useful. It is also a great example of why simplistic thought experiments about wavefunction collapse are misleading. A better way to think about this experiment is in (an imperfect) analogy to spin echo in nuclear magnetic or electron spin resonance. In a spin echo experiment, an ensemble of spins is set precessing, and the evidence of their coherent precession gets smeared out as a function of time as the spins "dephase" (get out of sync because of perturbing interactions with other degrees of freedom). However, in these echo experiments, a properly defined external perturbation (a pulse of microwaves) can flip all of these spins around, so that the ones originally going ahead of the pack are put in the back, and the slow ones are put in the front. The spins rephase, or become coherent in their motion again. The authors do something rather analogous here using superconducting devices. Nice!

arxiv:0908.1126 - N. P. Armitage, Electrodynamics of correlated electron systems
I'm not promoting this just because Peter sometimes comments on this blog. This is a great set of lecture notes from a 2008 summer school at Boulder. These notes provide a very good, pedagogical overview of how electromagnetic radiation interacts with the electronic systems of real materials, and how one can use measurements ranging from the THz (mm-wave) to the ultraviolet to infer details of the electronic properties. These sorts of reviews are a wonderful feature of the arxiv.

Wednesday, August 05, 2009

LHC and the hazards of Big Science

This article in the NY Times about the LHC's current problems was interesting. To be fair, the LHC is an incredibly complex undertaking. Making high quality superconducting joints between magnets is a complex business, involving spot-welding annoying materials like niobium-titanium alloys. Testing is a real pain, since room temperature measurements can't always identify bad joints. Still, they clearly didn't design an optimal testing and commissioning regimen. I'm sure they'll get these problems licked, and great science will eventually come out of the machine - it's just a question of how long that'll take. I do wonder, though, if stories like this are, in part, a consequence of their own publicity machine, which has been hammering the general public relentlessly for years about how the LHC is going to unlock the secrets of the universe.

This situation is a prime hazard of Big Science. One thing I definitely like about condensed matter and AMO physics, for example, is that you are often (though not always) in control of your own destiny. Progress is generally not dependent on 1000 other people and 500 vendors and suppliers, nor do you have to hope that some launch schedule isn't screwed up by a hailstorm. The general public needs to know that really good science can be done on a much smaller scale. While the LHC outreach effort is meant to inspire young people into pursuing physics, situations like these delays and the accompanying reporting probably frighten away more people from the field than they attract. If a layperson ends up with the impression that all physics is hugely expensive, and even then doesn't work right, that's not a good thing.

Saturday, August 01, 2009

Chemistry vs. Physics blogging

Interesting. The pseudonymous Kyle Finchsigmate at The Chem Blog just gave some stats about his blogging. He gets something like 6000 unique visitors a day, while I get about 150. Admittedly, we have rather different styles (pseudonymity makes it easy to write with more, umm, gusto, and to slam lousy papers openly, both of which probably make his blog have more broad appeal), and there are a lot more chemists out there than condensed matter physicists. Still, the factor of 40 is a bit intimidating.