Sunday, June 25, 2006

This week in cond-mat

Two interesting papers relating to mesoscopic physics on the arxiv this past week:
cond-mat/0606486 - Jakobs et al., Temperature-induced phase averaging vs. addition of resistances in mesoscopic systems
Classically, electrical conduction is well described by Ohm's Law. Take two resistors and put them in series, and the total resistance is just the sum of the two individual resistances. In the quantum world things are more complicated. Imagine an electron incident on a tunneling barrier, such that there is some tunneling amplitude t for transmission, leading to a transmission probability of |t|^2. Now consider two such barriers in series. Classical expectations would lead you to expect a transmission probability for the two-barrier system to be (|t|^2)^2. In fact, depending on the details of the system (the incident energy of the particle, the barrier heights and widths, the separation between the barriers), the full quantum treatment can give transmission probabilities ranging from zero to one (!), because of interference effects. These can be constructive or destructive, depending on just how the multiply reflecting waves bouncing back and forth between the two barriers sort themselves out, in terms of phase differences racked up. On the macroscale, inelastic interactions with the environment act to randomize the relative phases of those waves, washing out interference effects and restoring the classical Ohm's Law result. This is treated really well by Datta in one of his books. Anyway, this paper considers just what happens at finite temperature, even in the absence of true decoherence. Because electrons that dominate conduction have a spread in energy of around kT, they have a spread in wavelengths, and effectively a spread in their phase accumulation as they bounce around between scatterers. This paper looks at the effect of that averaging on the addition of resistances.

cond-mat/0606473 - Gao et al., Cotunneling and one-dimensional localization in individual single-walled carbon nanotubes
This paper is related, in the sense that it actually looks at the temperature dependence of conduction through a one-dimensional system containing randomly distributed scatterers. In this case the system is a single-walled nanotube, which really has 1d band structure because of its geometry. The scatterers are defects or disorder, and the tubes in question are around a micron in length. Gao et al. find that the tubes exhibit activated transport (becoming exponentially more resistive as T approaches 0), though the activation energies can change as temperature is reduced. At the low temperature end they find that the tubes effectively have broken up into a 1d array of quantum dots. They argue that the varying activation energies happen as the effective dot size changes with T. As temperature is decreased, coherence is increased, and higher order tunneling processes ("cotunneling") can enhance interdot conduction. A neat result and a nice idea, though their Fig. 1 raises a common issue that comes up in many such measurements. They take a log-linear plot of resistance vs. 1/T, and have "guide to the eye" lines indicating regimes of different activation energy. Are there really clear multiple regimes, or is the effective activation energy smoothly varying over the whole range? Lines to "guide the eye" should be used with caution....

Friday, June 23, 2006

Voting in this country

I try to keep political commentary to a minimum on this blog, because there are plenty of blogs out there dedicated to that kind of discourse. I do have one observation to make, though. When considering modern politics in the US, what does it say about a political party that an apparently legitimate (that is, recognized, orchestrated, and encouraged at the national level by party leaders) part of their strategy is to suppress voter turnout? It's one thing to try to pander to -- err, energize your base to make sure that they come to the polls in droves. It's quite different to deliberately try to keep people that you think might be voting for the other side away from the polls. You know, by tactics like phone bank jamming sanctioned by the White House, blanket scrubbing of voter rolls in ways virtually guaranteed to bar legitimate voters, shredding voter registration cards of people from one party, challenging the legitimacy of every ballot cast in certain precincts to deliberately slow down the vote in areas dominated by the other party, etc.

Thursday, June 22, 2006

NASA and statistics

On NPR this morning I heard NASA administrator Michael Griffin explaining why he thought it was ok to dismiss safety concerns raised by two of his managers regarding the upcoming shuttle launch. He said that since they'd had 114 flights and never lost a vehicle due to the particular problem area identified by the managers, he found it "unreasonable to think it was likely" that they would lose one in the future. There are so many things wrong with that reasoning it's hard to know where to begin. Couldn't his (2x) predecessor have said almost exactly the same thing about any foam falling off the external tank prior to the final flight of Columbia? Has he ever heard of Poisson statistics? As my thesis advisor said while on the Columbia Accident Investigation Board (Times of London, April 30, 2005): "[T]he risk of a serious failure is between 1 and 2 per cent a launch, or between 24 and 43 per cent over the 28 missions still planned"
Griffin's no dummy - what he really wants to say (but can't do so explicitly because it would be so impolitic) is that he considers the risks acceptable, given that the alternative is to declare the shuttle program done because they can't retroactively fix this design flaw.
What a mess.

Monday, June 19, 2006

physics sociology

There's been a brewing discussion going on, largely and appropriately in the high energy physics community, about string theory - does it actually have reasonably specific, testable predictions? If not, is it really science in the classic sense?

People can become incredibly personally vested in their ideas in science. In physics in particular there can be a tendency to assume (a) that you're right (duh!), (b) that your ideas have been arrived at by a careful intellectual process (duh! again), and (c) therefore anyone who disagrees with you is either ignorant, not very smart, or hasn't been thinking about things "the right way" (read: your way). Reminds me of Vizzini in The Princess Bride: "Ever hear of Plato? Aristotle? SOCRATES?! Morons." Prior to today, the best example of this attitude that I'd ever seen was at a talk given by a job candidate, who, when asked a very good question by one of my very respected colleagues (who happened to be on the search committee), began his response with "If you think about this a little, you'll see...." Nothing like implying that your potential future employer hasn't considered his question.

Now, though, I've got a new favorite example. From Lubos Motl's well known blog:

(UPDATE: Lubos has removed the page in question, so the link is now broken.)
(UPDATE II: Lubos has put the page back, re-edited, but the new version still conveys his clear view that only high energy theorists, and specifically string theorists, are actually doing science - the rest of us are just wankers, apparently.)

Sorry to say but this is the last well-known physics blog on this planet; all others blogs that claim to have something to do with science are just politically correct tools for crackpots to make their deep misunderstandings of the basics of modern physics ever more powerful and legitimized, and to destroy physics as such at a finite timescale.

Oooooookay. So, everyone else is a complete idiot. Got it. Might as well pack up my computer and quit now.

Sunday, June 18, 2006

Recently on cond-mat

Here are a couple of recent preprints that caught my eye. I'm going to try to get back to chronicling these weekly, if I can find the self-discipline....

cond-mat/0606430 - Streed et al., Continuous and pulsed Quantum Zeno Effect
This experiment is really an atomic physics experiment, but it is on cond-mat, and the physics is very cool. The Quantum Zeno Effect gets its name from Zeno's Paradox: in order to get from point A to point B, a person would first have to get half-way; however, to get to the midpoint between A & B, a person would first have to get half-way to that spot, and so on. Thus, noone can ever get anywhere. While the solution to this apparent paradox lies in the idea of rates and limits (at a given instant, there is something called the velocity that is the rate of change of distance per unit time), one can set up a quantum case where a system really never does get from state A to state B. This is a result of the basic postulates of quantum mechanics: after a measurement of some observable, the system is left in an eigenstate of that observable. If the same observable is measured again before the system has had a chance to evolve (via the Schroedinger equation and whatever the Hamiltonian is), the system will still be in that same eigenstate that was just found. So, if one keeps measuring the system continuously, the state of the system can't evolve. The act of continuous measurement locks the system in that one eigenstate. Ketterle's group at MIT have managed to implement a version of this using a Bose-Einstein condensate of rubidium atoms. Very neat.

cond-mat/0606375 - Reich et al., Observation of magnetism in thin gold films
This paper is already out as an Applied Physics Letter. The authors report sensitive magnetic susceptibility measurements on thin Au films, and find that, depending greatly on substrate and preparation, it is possible for those films to be significantly paramagnetic. This is a bit weird, since Au in bulk is diamagnetic. Of course, there have been reports of weird magnetism in nanostructured Au before, including ferromagnetism in Au clusters and whopping big magnetic effects in the presence of self-assembled monolayers of molecules. All of these effects have been challenging for folks to reproduce and confirm, in part because it really does seem like every little detail about sample prep and interfaces matters. It's always interesting to see how even things that seem like they should be well understood can be rich and complex. My personal theory on these effects is that they involve orbital moments in the Au caused by interfacial charge transfer and the strong spin-orbit scattering in Au. Some theorists seem to have the same idea.

Tuesday, June 13, 2006

Amazingly inappropriate ad from a vendor

Wow. Late this afternoon I got an email advertisement from an equipment vendor that was astonishingly over the line of propriety. The company makes plasma tools for processing semiconductors, and they were advertising their upcoming exhibit at Semicon West, the big semiconductor trade show. One of their pieces of equipment is a tool that uses an oxygen plasma to strip away photoresist residue. The email ad included an image of this tool, and a picture of a (apparently supposedly hot) woman with a come-hither look, and big letters saying "I'll strip for you." I'm hardly a zealot of political correctness, but this was so unprofessional that my jaw dropped. This will not result in increased sales. At most companies something like that would be grounds for a harassment complaint.

UPDATE: here is the ad in question, with the vendor blocked out....

Curse you, rotavirus!

Right now I'm the only member of my family not battling some nasty stomach bug. You know it's bad when your spouse calls you to ask you to pick the recovering younger child up at school, because she and the older child are too ill to get in the car.

Saturday, June 10, 2006

Observations about NSF panels

I just returned from an NSF review panel. For those of you that don't know, the NSF peer-reviews all grant proposals, and many programs have a panel review system: an NSF program officer will email you or call you and ask if you are available on such-and-such a date for a panel. If you're willing to do it, you say "yes", and then you're given electronic access to about 8 proposals to review. You do your reviews at your leisure over the next few weeks and upload them via the impressively good web-based system, Fastlane. Then you go to Washington (really Ballston, VA) to NSF headquarters at the appointed time, and sit down in a room with about 10 other reviewers plus the program officer. Everyone has a laptop in front of them, and now you can see each other's reviews. You go through all the proposals (usually about 30 for the whole panel), discuss and compare notes, and in the end write up panel summaries of the reviews that eventually get sent to the proposal writers (PIs, or principal investigators). Typically the proposals are grouped into three categories: "highly recommended" (will actually get funded), "recommended" (on the edge, and may get lucky if there's enough money available), and "not recommended" (no chance). These days the yield of "highly recommended" is 5-15% at NSF, depending on the program. The government pays your travel, and you get a nominal stipend that covers hotel and meals.

A few observations:
  • The main reason to do this is one of citizenship: you can really see the process work, learn how to improve your own proposals, and reassure yourself that the people reviewing the grants have a clue.
  • Why are there never people from top 15 schools at these panels? Are they really only involved in things like site visits for major center proposals? Seriously, I've never seen someone from any Ivy League school, any of the UC schools, MIT, CalTech, Stanford, Illinois, etc. on one of these things. Are they really all that much busier than me?
  • It's painful when someone is on a panel that is not technologically literate enough to handle the web-based system.
  • It's equally painful when someone bails at the last minute, doesn't review their share, and doesn't show up.
  • This is still the best system around. Scary.

Thursday, June 08, 2006

To write, or not to write

I've been talking with a major publishing house about writing a textbook based on my two-semester course sequence, Nanostructures and Nanotechnology I and II. I've been teaching these classes for the last several years, and they've been very successful. The editor has sent out a detailed outline of my ideas, and the feedback from reviewers has been very positive. That's nice and validating, but I remain pretty conflicted about doing this. I know a few things:
  • Every one of my research-active faculty colleagues here looks at me like I'm absolutely stark raving bonkers for even considering this - I should be spending all my resources on my research.
  • Right now, there is no text for this sort of thing at this level. There is real potential for a transformative effect if the book is good. If I wait 5 years, someone else will write the book instead of me.
  • However long I think this will take, it will take longer.
  • It would be very nice to feel like I'm having an educational impact on more than 20 students a year.
  • I'm unlikely to get any support in this (time off from teaching, etc.) from my institution.
  • I have it on good authority that the editor in question is very good, and that this publisher is generally as pleasurable to deal with as any.
So. What to do. Any comments from out there?