Wednesday, April 25, 2012

Heat flow at the mesoscale

When we teach about thermal physics at the macroscopic scale, we talk in terms of the thermal conductivity, k.  For the 1d problem of a homogeneous rod of cross sectional area A and length L, the rate that energy flows from one end of the rod to the other is given by (kA/L)(Th-Tc), where Th and Tc are the temperatures of the hot and cold ends of the rod, respectively.  Built into this approach is the tacit assumption that the phonons, the quantized vibrational modes of the lattice that carry what we consider to be the thermal energy of the atoms in the solid, move in a diffusive way.  That is, if a phonon is launched, it bounces many times in a random walk sort of motion before it traverses across our region of interest.  Phonons can scatter off disorder in the lattice, or mobile charge carriers (or even each other, if the vibrations aren't perfectly harmonic).  

However, phonon motion doesn't have to be diffusive!   If phonons don't scatter while propagating a certain length scale, their motion is said to be "ballistic".  In this paper, the authors have done a very clever experiment to look at whether there is a significant contribution of ballistic phonons to heat transport in silicon at room temperature on scales considerably longer than the "textbook" mean free path for phonon scattering under those conditions, about 40 nm.  The authors use the interference pattern between two "pump" lasers to produce a (sin^2) intensity pattern (and thus, because of absorption and the electron-lattice coupling, a (sin^2) pattern of elevated temperature) in a suspended Si membrane.  The change in local temperature leads to a small change in local index of refraction.  A low intensity "probe" laser can diffract off the grating pattern set up by the temperature variation.  Depending on how long one waits between pump and probe, the temperature pattern can wash itself out due to phonon transport.  So, by varying the delay between pump and probe and looking at the strength of the diffracted probe signal, they can monitor the time evolution of the temperature profile.  By changing the pitch of the initial interferogram, they can look at thermal transport over different length scales.   They find that there are significant deviations from the expectations of diffusive phonon transport (originally worked out by Klaus Fuchs, among others) up to micron scales, which is pretty darn cool, and important for understanding heat flow in, e.g., computer chips.   Very elegantly done.

Thursday, April 19, 2012

Persistent currents and an impressive experiment

A long while ago, I brought up the topic of persistent currents in normal metal rings.  Please click the link to get the context.  The point is, even in a normal metal (as opposed to a superconductor), if you consider a metal ring small enough that the electrons remain quantum mechanically coherent in going about the ring, the electronic wavefunction must remain single-valued.  That means that the quantum mechanical phase accumulated by an electron diffusing around the ring back to its starting point (to speak in a semiclassical way) has to add up to an integer multiple of 2 pi. Since magnetic flux through the ring tweaks the accumulated phase (via the Aharonov-Bohm effect), a persistent current develops in the ring to make sure that the total phase (that from the electron motion and that from the resulting Aharonov-Bohm contribution) add up to a multiple of 2 pi.  As I'd discussed before, these currents and the magnetic fields they produce tend to be quite small and difficult to detect.

To make matters worse, when an electron scatters off static disorder in a solid, it acquires a phase shift that depends on that particular scattering site.  What this really means is, if you consider an ensemble of nominally identical metal rings, you'll actually get some distribution of persistent currents, because each ring has its own particular configuration of disorder.  Now Jack Harris' group at Yale has done a beautiful measurement, looking at many individual rings and examining the statistics of these persistent currents in the ensemble.  They place each ring at the end of a floppy cantilever.  In the presence of a magnetic field, the magnetic dipole moment from the persistent current exerts a torque on the cantilever, and the results can be detected optically via interferometry.  The experiment requires low temperatures, precision fabrication, and very clean technique.  Very nice.

Tuesday, April 17, 2012

Academic science researchers and economics

This article in the NY Times is rather provocative in several ways. First, it raises the question of whether there is a dramatic rise taking place in the number of journal article retractions (spread across all disciplines). The answer is, it's really not clear, given the enormous increase in the number of published articles. Moreover, it's certainly much easier for people to find, read, and compare articles than ever before. Google Scholar, for example, can see through most pay-walls enough to search for words and phrases, making it far easier than ever before to test for plagiarism. Moving on, the article then looks at whether the culture of academic science research is, for lack of a better word, ailing. There are some choice quotes:
[L]abs continue to have an incentive to take on lots of graduate students to produce more research. “I refer to it as a pyramid scheme,” said Paula Stephan, a Georgia State University economist and author of “How Economics Shapes Science,” published in January by Harvard University Press.

In such an environment, a high-profile paper can mean the difference between a career in science or leaving the field. “It’s becoming the price of admission,” Dr. Fang said.

The scramble isn’t over once young scientists get a job. “Everyone feels nervous even when they’re successful,” he continued. “They ask, ‘Will this be the beginning of the decline?’ ”


“What people do is they count papers, and they look at the prestige of the journal in which the research is published, and they see how many grant dollars scientists have, and if they don’t have funding, they don’t get promoted,” Dr. Fang said. “It’s not about the quality of the research.”

Dr. Ness likens scientists today to small-business owners, rather than people trying to satisfy their curiosity about how the world works. “You’re marketing and selling to other scientists,” she said. “To the degree you can market and sell your products better, you’re creating the revenue stream to fund your enterprise.”
I don't want to quote any more for fear of running afoul of fair use. Read the article. This does hit some of the insecurities felt by any reasonable US faculty science or engineering researcher. I would dispute the pyramid scheme comment because it's based on a false premise, that every doctoral student is looking to become a professor and is crushed if they don't get a faculty position. The prestige paper comments are more worrisomely accurate.

Sunday, April 15, 2012

Getting the most out of an experimental technique

This post is a mini-summary of a Perspectives piece I wrote for ACS Nano.  One conceptually simple way to measure the electronic properties of materials at the atomic scale is to use a "break junction".  Imagine taking a metal needle touching a metal surface, and slowly lifting up on the needle.  At some point, the needle will come out of contact with the surface.  As it does so, at the last instant, the contact between the two will take place only at the atomic scale.  If you hook up one end of a battery to the needle and the other through an ammeter to the metal surface to measure the flow of current, you can measure the electrical conduction throughout this process.  Thanks to the availability of high speed electronics these days, it is possible to record conductance, G, vs. time data throughout the process.  A standard analytic approach is then to compile a histogram of all the data points, counting how many times each value of G is measured.  As explained here, the most stable junction configurations naturally have more data points, and this will lead to peaks in the conductance histogram at the values of conductance corresponding to those configurations.   Molecules may be incorporated into such junctions (as I've written about here).  Since it's possible to set up a system to make and break junctions repeatedly and rapidly in an automated way, this approach has proven very fruitful and revealing.

Of course, only looking at the histograms is wasteful.  You actually have an enormous amount of additional information contained in the G vs. t traces.  For instance, you can check to see if the occurrence of a "plateau" in G vs. t at one conductance level always (or never!) correlates with a similar plateau at a different conductance value.  These kinds of cross-correlations are best represented in two-dimensional histograms of various types.  Makk et al. have written a very clear and tutorial paper about how this works in practice, and what kinds of things one can learn from such analyses.  It's definitely worth a read if you work on this stuff, and it's also a great lesson in how as much of your data as possible.

Monday, April 09, 2012

DOI numbers, Web of Science, and article numbers

Two recurring complaints about bibliographies and citations for papers and proposals:
  • Most people really like DOI, a system meant to assure that reference materials like journal articles get an effectively permanent web address, something that will "always" point to that article.  It's become very very popular, and every online journal that I know provides a doi reference for each article.  It shows up in every Web of Science reference these days, too, if it exists.  So, why can't Web of Science make those doi numbers a clickable link?  That is, instead of forcing me to copy and paste the doi into a browser URL line with "" stuck in front, why not just make the doi itself a link to that?  I mean, why would anyone just want the doi without the link??  Is this some weird bs rule about Web of Science not wanting to have direct links?
  • How come Physical Review handles bibliographic information so badly when it comes to article numbers?   A number of years ago, Phys Rev switched from old-fashioned page numbers for articles to 6-digit article numbers.  Unfortunately, when you try to export bibliographic information for reference management software, for many Phys Rev articles, the automatic response is to stick the article number (which replaced the page number for all practical purposes) in some completely random field, and instead list the page numbers as either blank or the oh-so-useful "1-4" for a four-page article.  Can someone please fix this?  
Both of these are trivial, silly things, but I'd be willing to be that hundreds of person-hours (at least) are lost per year dealing with the latter one.

Sunday, April 08, 2012

Commitment and conflicts

One of the various hats I wear right now is chair of Rice's university committee on research, and one topic that has come up lately (in the context of the US government's new regs about conflict of interest) is the discussion of "commitment". Conflict of interest is comparatively simple to explain to people - everyone grasps the idea that financial or other compensation that may give the appearance of affecting your scholarly objectivity is potentially a conflict of interest. Commitment is a more challenging concept. Most universities expect their science and engineering faculty in particular to spend some of their time doing things that are not immediately, directly connected to their simplest academic duties (teaching courses, supervising research students and postdocs, performing university service). For example, technical consulting isn't that unusual. Likewise, there are other broadly defined academic duties that can come up (serving on advisory or editorial boards; professional society work) that can enhance the academic mission of the university in a higher order way. However, it's clear that there have to be limits of some kind on these auxiliary activities - we would all agree that someone who does so much alternative work that they can't teach their classes or adequately do their normal job is having problems with time allocation. The general question is, how should a university manage these situations - how are they identified, how are they mitigated, and what are the consequences if someone is knowingly going over the line (e.g., spending three working days per week running the day to day operations of a startup company rather than doing their academic job)? Things get particularly complicated when you factor in disciplines that basically demand external work (architecture, business school), and the increasingly common practice of special appointments at foreign universities. If anyone has suggestions of universities with what they think are especially good approaches (or lousy ones, for that matter) to this issue, please post in the comments.

Tuesday, April 03, 2012

An open letter to Neil deGrasse Tyson

Hello, Dr. Tyson. First, let me say that I'm a huge fan. You do the scientific community a tremendous service by being such an approachable, clear spokesman, maintaining scientific accuracy while also entertaining the public. Astronomy is a great side interest of mine (like many scientists and engineers), and I really wanted to be an astronaut for a while (until my eyes were demonstrably lousy); that's why on some gut level I enjoyed your call for a renewed vigor in space exploration.

However, my brain's response to your call is, is this really the best strategy? Much as I'd love to one day walk on the moon or Mars, I can't help but be deeply skeptical of NASA's ability to allocate resources. Right now their annual budget is about $17B, more than twice that of the NSF, and more than three times that of the DOE Office of Science. While the achievements of the robotic spacecraft missions are truly amazing, much of the rest of NASA seems very dysfunctional. I'll admit, my impression colored by my thesis advisor's experience on the Columbia accident investigation board, my knowledge of the ISS (hint: the Soyuz "lifeboats" where the ISS crew shelters in case of debris impact? They're actually the most debris-vulnerable part of the ISS.), and the fact that NASA has employees that do things like this and this at some rate.

If taxpayers are going to be persuaded to invest another $17B/yr in federally funded research, I think a much more compelling case needs to be made that NASA is the place for that investment, given the alternatives. Yes, NASA's history and subject matter are inspiring, but you need to convince me that NASA as an agency will really get value out of that investment, given that their recent leadership has been singularly unimpressive.

PS - If you ever need a sub to go onto Colbert in your stead, please call.

Monday, April 02, 2012

Several items

My apologies to my readers for low blogging rate recently. Multiple papers, proposals, teaching, travel, etc. have all contributed to this slow-down. Here are a few brief items to consider:
  • The (nearly) final details have come out regarding the OPERA experiment.  Goodbye, superluminal neutrinos - we hardly knew ye.    Would've been fun!
  • It would appear that one can correlate political affiliation in the US with the somewhat ill-defined concept of "trust in science".  Much as it's tempting to make a wry comment here, I suspect that some of this is due to the very disparate nature of those self-identifying as "conservative" these days.  Either way, this is a problem, though.  Science (in the sense of careful, rigorous testing of hypotheses that allege predictive power) is an incredibly useful way to look at much of the world, and I would hope that this would be appreciated by the vast majority of people out there.
  • Someone has advanced the idea that Mitt Romney is a quantum object.  Clearly we should put him through some sort of interferometer to test this idea.  Alternately, he should interlace his fingers and make a loop with his arms - we can then thread magnetic flux through him and see if his response about the individual mandate for healthcare oscillates as the magnetic field is swept.
  • Visiting NSF is always enlightening.  I really hadn't appreciated before the quantitative problem that they face in proposal evaluation and administration:  the number of proposals that are submitted has more than doubled in the last few years, while their staffing has remained unchanged.  Even apart from overall resource problems (e.g., the runaway positive feedback cycle, when people realize that the odds of funding are bad, so they submit more proposals, making the odds of funding worse), just the challenge of properly handling all the paperwork is becoming incredibly difficult.
  • April Fools is always fun on the web.  This is one of my favorites.