Sunday, September 30, 2007

This week in cond-mat

Two recent papers in cond-mat this time, both rather thermodynamics-related. That's appropriate, since I'm teaching undergrad stat mech these days.

arxiv:0709.4181 - Kubala et al., Violation of Wiedemann-Franz law in a single-electron transistor
The Wiedemann-Franz law is one of those things taught in nearly every undergraduate solid-state physics class. It also happens to be extremely useful for doing cryogenic engineering, as I learned during my grad school days. The idea is simple: simple kinetic theory arguments (and dimensional analysis) imply that the conductivity of some parameter via some excitations is given by the product (carrying capacity of that parameter per excitation)*(speed of excitation carrying that parameter)*(mean free path of that excitation), with some geometric factor out in front (e.g., 1/3 for three dimensional diffusive motion of the excitation). For example, the electrical conductivity in a 3d, diffusive, ordinary metal is (1/3)(e)(v_F)(\ell), where e is the electronic charge, v_F is the Fermi velocity for conduction electrons, and \ell is the mean free path for those electrons (at low T, \ell is set by impurity scattering or boundary scattering). However, in a normal metal electrons can also carry thermal energy with some heat capacity c_v per electron that scales like T, while the speed and mean free path of the electrons are as above. This implies the Wiedemann-Franz law, that the ratio of the thermal conductivity to the (electrical conductivity*T) in an ordinary metal should be a constant (the Lorenz number, ~25 nanoOhms W/K^2). Deviations from the W-F law are indicators of interesting physics - basically that simple metal electrons either aren't the dominant carriers of the electrical current, or that the charge carriers don't carry thermal energy as normal. This paper is a theory piece by the Helsinki group showing that the W-F law fails badly for single-electron transistors. In particular, in the co-tunneling regime, when current is carried via quantum coherent processes, the Lorenz number is predicted to be renormalized upward by a factor of 9/5. This will be challenging to measure in experiments, but exquisite thermal conductivity measurements have been performed in similar systems in the past.

arxiv:0709.4125 - Allahverdyan et al., Work extremum principle: structure and function of quantum heat engines
Marlan Scully (also here) caused a bit of a flurry of excitement a few years ago by proposing a form of heat engine that uses quantum coherence and its destruction to do work, in addition to the conventional approach of using two thermal baths at different temperatures. This paper is a theoretical analysis of some such quantum heat engines. Carnot can sleep easy - in the end you can't violate the Carnot efficiency even with quantum heat engines, if you dot all the "i"s and cross all the "t"s. Neat to think about, though, and of some experimental relevance to the cold atom community, who can prepare highly coherent atomic gases at very low temperatures. This paper is long and detailed and I don't claim to have read it in depth, but it looks interesting.

Tuesday, September 25, 2007

Revised: Primer on faculty searches, part I

It's that time of year again, with Chad Orzel and the Incoherent Ponderer both posting about the faculty job market and job hunting. So, I'm recycling a post of mine from last year describing the search process, at least the way it's done at Rice. I'm going to insert some revisions that are essentially tips to would-be candidates, though I think the IP has already done a good job on this, and some are basically common sense. An obvious disclaimer: this is based on my experience, and may not generalize well to other departments with vastly differing cultures or circumstances.

Here are the main steps in a search:
  • The search gets authorized. This is a big step - it determines what the position is, exactly: junior vs. junior or senior; a new faculty line vs. a replacement vs. a bridging position (i.e. we'll hire now, and when X retires in three years, we won't look for a replacement then).
  • The search committee gets put together. In my dept., the chair asks people to serve. If the search is in condensed matter, for example, there will be several condensed matter people on the committee, as well as representation from the other major groups in the department, and one knowledgeable person from outside the department (in chemistry or ECE, for example). The chairperson or chairpeople of the committee meet with the committee or at least those in the focus area, and come up with draft text for the ad.
  • The ad gets placed, and canvassing begins of lots of people who might know promising candidates. A special effort is made to make sure that all qualified women and underrepresented minority candidates know about the position and are asked to apply (the APS has mailing lists to help with this, and direct recommendations are always appreciated - this is in the search plan). Generally, the ad really does list what the department is interested in. It's a huge waste of everyone's time to have an ad that draws a large number of inappropriate (i.e. don't fit the dept.'s needs) applicants. The exception to this is the generic ad typically placed by MIT and Berkeley: "We are looking for smart folks. Doing good stuff. In some area." They run the same ad every year, trolling for talent. They seem to do ok. The other exception is when a university already knows who they want to get for a senior position, and writes an ad so narrow that only one person is really qualified. I've never seen this personally, but I've heard anecdotes.
  • In the meantime, a search plan is formulated and approved by the dean. The plan details how the search will work, what the timeline is, etc. This plan is largely a checklist to make sure that we follow all the right procedures and don't screw anything up. It also brings to the fore the importance of "beating the bushes" - see above. A couple of people on the search committee will be particularly in charge of oversight on affirmative action/equal opportunity issues.
  • The dean meets with the committee and we go over the plan, including a refresher for everyone on what is or is not appropriate for discussion in an interview (for an obvious example, you can't ask about someone's religion.).
  • Applications come in and are sorted; rec letters are collated. Each candidate has a folder.
  • The committee begins to review the applications. Generally the members of the committee who are from the target discipline do a first pass, to at least wean out the inevitable applications from people who are not qualified according to the ad (i.e. no PhD; senior people wanting a senior position even though the ad is explicitly for a junior slot; people with research interests or expertise in the wrong area). Applications are roughly rated by everyone into a top, middle, and bottom category. Each committee member comes up with their own ratings, so there is naturally some variability from person to person. Some people are "harsh graders". Some value high impact publications more than numbers of papers. Others place more of an emphasis on the research plan, the teaching statement, or the rec letters. Yes, people do value the teaching statement - we wouldn't waste everyone's time with it if we didn't care. Interestingly, often (not always) the people who are the strongest researchers also have very good ideas and actually care about teaching. This shouldn't be that surprising. As a friend of mine at a large state school once half-joked to me: 15% of the faculty in any department do the best research; 15% do the best teaching; 15% do the most service and committee work; and it's often the same 15%.
  • Once all the folders have been reviewed and rated, a relatively short list (say 20-25 or so out of 120 applications) is arrived at, and the committee meets to hash that down to, in the end, five or so to invite for interviews. In my experience, this happens by consensus, with the target discipline members having a bit more sway in practice since they know the area and can appreciate subtleties - the feasibility and originality of the proposed research, the calibration of the letter writers (are they first-rate folks? Do they always claim every candidate is the best postdoc they've ever seen?). I'm not kidding about consensus; I can't recall a case where there really was a big, hard argument within the committee. I know I've been lucky in this respect, and that other institutions can be much more fiesty. The best, meaning most useful, letters, by the way, are the ones who say things like "This candidate is very much like CCC and DDD were at this stage in their careers." Real comparisons like that are much more helpful than "The candidate is bright, creative, and a good communicator." Regarding research plans, the best ones (for me, anyway) give a good sense of near-term plans, medium-term ideas, and the long-term big picture, all while being relatively brief and written so that a general committee member can understand much of it (why the work is important, what is new) without being an expert in the target field. It's also good to know that, at least at my university, if we come across an applicant that doesn't really fit our needs, but meshes well with an open search in another department, we send over the file. This, like the consensus stuff above, is a benefit of good, nonpathological communication within the department and between departments.
That's pretty much it up to the interview stage. No big secrets. No automated ranking schemes based exclusively on h numbers or citation counts.

Tips for candidates:
  • Don't wrap your self-worth up in this any more than is unavoidable. It's a game of small numbers, and who gets interviewed where can easily be dominated by factors extrinsic to the candidates - what a department's pressing needs are, what the demographics of a subdiscipline are like, etc. Every candidate takes job searches personally to some degree because of our culture, but don't feel like this is some evaluation of you as a human being.
  • Don't automatically limit your job search because of geography unless you have some overwhelming personal reasons. I almost didn't apply to Rice because neither my wife nor I were particularly thrilled about Texas, despite the fact that neither of us had ever actually visited the place. Limiting my search that way would've been a really poor decision.
  • Really read the ads carefully and make sure that you don't leave anything out. If a place asks for a teaching statement, put some real thought into what you say - they want to see that you have actually given this some thought, or they wouldn't have asked for it.
  • Research statements are challenging because you need to appeal to both the specialists on the committee and the people who are way outside your area. My own research statement back in the day was around three pages. If you want to write a lot more, I recommend having a brief (2-3 page) summary at the beginning followed by more details for the specialists. It's good to identify near-term, mid-range, and long-term goals - you need to think about those timescales anyway. Don't get bogged down in specific technique details unless they're essential. You need committee members to come away from the proposal knowing "These are the Scientific Questions I'm trying to answer", not just "These are the kinds of techniques I know".
  • Be realistic about what undergrads, grad students, and postdocs are each capable of doing. If you're applying for a job at a four-year college, don't propose to do work that would require an experienced grad student putting in 60 hours a week.
  • Even if they don't ask for it, you need to think about what resources you'll need to accomplish your research goals. This includes equipment for your lab as well as space and shared facilities. Talk to colleagues and get a sense of what the going rate is for start-up in your area. Remember that four-year colleges do not have the resources of major research universities. Start-up packages at a four-year college are likely to be 1/4 of what they would be at a big research school (though there are occasional exceptions). Don't shave pennies - this is the one prime chance you get to ask for stuff! On the other hand, don't make unreasonable requests. No one is going to give a junior person a start-up package comparable to a mid-career scientist.
  • Pick letter-writers intelligently. Actually check with them that they're willing to write you a nice letter - it's polite and it's common sense. Beyond the obvious two (thesis advisor, postdoctoral mentor), it can sometimes be tough finding an additional person who can really say something about your research or teaching abilities. Sometimes you can ask those two for advice about this. Make sure your letter-writers know the deadlines and the addresses.
I'll revise more later if I have the time.

Monday, September 24, 2007

2007 Nobel Prize in Physics

Time for pointless speculation. I suggest Michael Berry and Yakir Aharonov for the 2007 physics Nobel, because of their seminal work on nonclassical phase factors in quantum mechanics. Thoughts?

Saturday, September 22, 2007

Two seminars this past week

I've been remiss by not posting more interesting physics, either arxiv or published. I'll try to be better about that, though usually those aren't the posts that actually seem to generate comments. For starters, I'll write a little about two interesting condensed matter seminars that we had this week. (We actually ended up with three in one week, which is highly unusual, but I was only able to go to two.)

First, my old friend Mike Manfra from Bell Labs came and gave a talk about the interesting things that one sees in two-dimensional hole systems (2dhs) on GaAs (100). Over the last 25 years, practically a whole subdiscipline (including two Nobel prizes) has sprung up out of our ability to make high quality two-dimensional electron systems (2des). If you have a single interface between GaAs below and AlxGa(1-x)As above, and you put silicon dopants in the AlGaAs close to the interface, charge transfer plus band alignment plus band bending combine to give you a layer of mobile electrons confined in a roughly triangular potential well at the interface. Those electrons are free to move within the plane of the interface, but they typically have no ability to move out of the plane. (That is, the energy to excite momentum in the z direction is greater than their Fermi energy.) Now it's become possible to grow extremely high quality 2dhs, using carbon as a dopant rather than silicon. The physics of these systems is more complicated than the electron case, because holes live in the valence band and experience strong spin-orbit effects (in contrast to electrons in the conduction band). In the electron system, it's known that at relatively low densities, low temperatures, and moderate magnetic fields, there is a competition between different possible ground states, including ones where the electron density is spatially complicated ("stripes", "bubbles", "nematics"). Manfra presented some nice work on the analogous case with holes, where the spin-orbit complications make things even more rich.

Then yesterday we had a talk by Satoru Nakatsuji from the ISSP at the University of Tokyo. He was talking about an extremely cool material, Pr2Ir2O7. This material is a metal, but because of its structure it has very complicated low temperature properties. For example, the Pr ions live on a pyrochlore lattice, which consists of corner-sharing tetrahedra. The ions are ferromagnetically coupled (they want to align their spins), but the lattice structure is a problem because it results in geometric frustration - not all the spins can be satisfied. As a result, the spins never order at nonzero temperature (at least, down to the milliKelvin range) despite having relatively strong couplings. This kind of frustration is important in things like water ice, too. In water ice, the hydrogens can be thought of as being at the corners of such tetrahedra, but the O-H bond lengths can't all be the same. For each tetrahedron, two are short (the covalent O-H bonds) and two are long (hydrogen bonds). The result is a ground state for water ice that is highly degenerate, leading to an unusual "extra" residual entropy at T = 0 of R/2 ln 3/2 per mole (in contrast to the classical third law of thermodynamics that says entropy goes to zero at T = 0. The same kind of thing happens in Pr2I2O7 - the spins on the tetrahedron corners have to be "two-in" and "two-out" (see the link above), leading to the same kind of residual entropy as in water ice. This frustration physics is just the tip of the iceberg (sorry.) of what Nakatsuji discussed. Very neat.

Friday, September 14, 2007

The secret joys of running a lab II: equipment

The good news is that we're getting a cool new piece of equipment to be installed here next week. The bad news (apart from the fact that it uses liquid helium - see previous post) is that I've been spending my morning shifting through US import tariff codes trying to come up with a number that will make the shipping agent happy. You might think that the tariff code supplied by the vendor would be good enough. Apparently you'd be wrong. You might think that this would be the job of a customs broker. Again, apparently you'd be wrong. As the Incoherent Ponderer pointed out, there are many aspects of our jobs for which we never receive formal training. Customs agent is one. By the way: can anyone explain to me why US tariff codes are maintained by the US Census Bureau? Ok, so they're part of the Department of Commerce, but this is just odd.

Thursday, September 13, 2007

The secret joys of running a lab: helium.

In my lab, and in many condensed matter physics labs around the world, we use liquid helium to run many of our experiments. At low temperatures, many complicating effects in condensed matter systems are "frozen out", and it becomes easier to understand the effects that remain. Often we are interested in the ground state of some system and want to reduce thermal excitations. Quantum effects are usually more apparent at low temperatures because the inelastic processes that lead to decoherence are suppressed as T approaches zero. For example, the quantum coherence length (the distance scale over which the phase of an electron's wavefunction is well defined before it gets messed up due to inelastic effects of the environment) of an electron in a metal like silver at room temperature is on the order of 1 nm, while that length can be thousands of times longer at 4.2 K, the boiling point of liquid helium at atmospheric pressure. Those kinds of temperatures are also necessary for running good superconducting magnet systems.

The downside of liquid helium is that it's damned expensive, and getting more so by the minute. Running at full capacity I could blow through several thousand liters in a year, and at several dollars a liter minimum plus overhead, that's real money. As a bonus, lately our supplier of helium has become incredibly unreliable, missing orders and generally flaking out, while simultaneously raising prices because of actual production shortages. I just had to read the sales guy the riot act, and if service doesn't improve darn fast, we'll take our business elsewhere, as will the other users on campus. (Helium comes from the radioactive decay of uranium and other alpha emitters deep in the earth, and comes out of natural gas wells.) The long-term solutions are (a) set up as many cryogen-free systems as possible, and (b) get a helium liquifier to recycle the helium that we do use. Unfortunately, (a) requires an upfront cost comparable to about 8 years of a system's helium consumption per system, and (b) also necessitates big capital expenses as well as an ongoing maintenance issue. Of course none of these kinds of costs are the sort of thing that it's easy to convince a funding agency to support. Too boring and pedestrian.

Fortunately, when you work at really nanometer scales, interesting physics often happens at higher temperatures. I've been lucky that two major things going on in my lab right now don't require helium at all. Still, it's bad enough worrying about paying students without the added fun of helium concerns.

UPDATE: See here.

Sunday, September 09, 2007

Other Packard meeting highlights

I'm back from California, and the remainder of the Packard meeting was just as much intellectual fun as the first day. It's great to see so much good science and engineering outside my own discipline. Some fun things I learned:
  • Plants really can communicate by smell (that is, by giving off and detecting volatile compounds).
  • Many flying insects have evolutionarily found wing flap patterns that optimize for minimum energy consumption when hovering.
  • Most of the huge number of insect species in tropical rainforests (at least in New Guinea) are specialist feeders, preferring to eat only one type of plant.
  • When you split a molecular ion (say I2-) into a neutral atom and an atomic ion, the coherent superposition (in this case, 1/\sqrt(2) [(I + I-) + (I- + I)]) can persist even when the atom and ion are separated by more than 10 atomic diameters.
  • Super fancy mass spec plus amazing statistical capabilities can let you do serious proteomics.
  • There may have been as many as four supercontinent phases and two "snowball earth" phases in the last three billion years.
  • If you come up with a computationally efficient way to model viscoelastic materials (e.g. jello, human skin), you can develop virtual surgery tools for reconstructive surgeons, and win an Oscar for special effects by modeling Davey Jones for POTC II.
  • If you develop a DNA microarray chip that lets you cheaply and reliably identify any known virus or the nearest relative of an unknown virus, and you want to use this clinically, the established medical testing companies will react in a very negative way (because they're afraid that if you're successful, they won't be able to keep chargin insurers $3K per possibly unnecessary blood test). The fact that you can save lives won't be of interest to them.
  • Comparing different measurement techniques can really tell you a lot about how cells sense and respond to touch.
  • You can design a Si photonic crystal to act as a superprism and show negative refraction and negative diffraction, all at the same time, over a useful bandwidth near 1.55 microns wavelength (the standard telecommunications band).
I know I'm leaving some out, too. Very fun stuff.

Friday, September 07, 2007

Packard meeting

I'm currently in Monterey thanking the Packard Foundation for their generous support. They're fantastic, and their fellowship has been a godsend that's really given me the flexibility in my research that I've needed. The best part about their annual meetings is that it's a chance for me to listen to good talks pitched to a general audience on an enormously broad set of science and engineering subjects. Some things that I learned yesterday:
  • It's possible to do successful astronomical planet-hunting surveys using 300mm camera lenses to make a telescope array.
  • There are molecules and molecular ions in astronomical gas clouds that are extremely difficult to make and study on earth (e.g., CH5-; C6H7+).
  • The human brain is 2% of the body's mass but uses 20% of the body's oxygen. It also has roughly 10x the concentration of iron, copper, and zinc as other soft tissues on the body.
  • Chemical "noise" (e.g., concentration fluctuations) is essential for some kinds of cell differentiation.
  • There are other photoactive parts in your eye besides rods and cones, and if those other parts are intact, your body clock can still re-set itself even in the absence of vision.
  • Soft tissue can (pretty convincingly) survive inside fossil bones dating back tens of millions of years.
  • Viral phylogeny shows convincingly that HIV did not start from contaminated polio vaccines grown in monkeys, and that HIV came from Africa first to Haiti, and then from Haiti to the US in the late 1960s.
  • Lots of microbes live as biofilms on the ocean floor via chemical energy gained from the decomposition of basaltic rock.