A primer on faculty searches

It's been suggested that it would be valuable for me to post a brief description of the faculty search process. An obvious disclaimer: this is based on my experience, and may not generalize well to other departments with vastly differing cultures or circumstances. Anyway, here are the main steps in a search:

1. 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).
2. 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.
3. 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). 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.
   
4. 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. A couple of people on the search committee will be particularly in charge of oversight on affirmative action/equal opportunity issues.
   
5. 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.).
   
6. Applications come in and are sorted; rec letters are collated. Each candidate has a folder.
   
7. 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%.
   
8. 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.

Ahhh, Texas.

While I generally enjoy living here, there are times when I really, really don't like living in Texas. For example, earlier today our governor (who is almost certainly going to be re-elected tomorrow, since the opposition to him will be split three ways, or four if you count the libertarian candidate) made me feel extra welcome. From the Dallas Morning News:

Gov. Rick Perry, after a God and country sermon attended by dozens of political candidates Sunday, said that he agreed with the minister that non-Christians will be condemned to hell.

Great. Why did the governor feel the need to talk about this at all? Apparently he feels that he needs to say things like that to get re-elected by my fellow Texas residents. Unsurprisingly, one of his opponents had a bon mot about this that I think says it all:

"He doesn't think very differently from the Taliban, does he?" independent Kinky Friedman said.

The h parameter....

Well, since several folks are commenting on the h parameter, I might as well put in my two cents. The h number is defined here. In brief, if you've published h papers (and no more) that each have h or more citations, then your h number is, well, h. In principle, your h number is not supposed to count self-citations (though once h is above 10 or so, that becomes pretty irrelevant anyway). In some fields (e.g. CS) where people tend to publish on public electronic archives rather than in journals, citations of those preprints are counted. The idea is that the H number is a metric of scientific performance and impact, and is more robust than mere citation counting. Steady output that people actually cite is rewarded more than being one co-author on a Science paper that happened to get 750 citations. There are variations, too. You can calculate the h number divided by a person's years of "professional experience", or actually figure out dh/dt. For a fair comparison between people, one should normalize h numbers by subfield. In condensed matter physics, a typical person near tenure time has an h of around 10. In mid-career, an h of around 20-30 is about the average, and exceptional people like National Academy members tend to have h values higher than 50. The h number can be skewed in certain cases. Some people publish little, but their work can have enormous impact. Others, such as materials growers, can have enormous h numbers because they supply materials used by dozens of experimental groups.

Obviously trying to quantify a person's scientific impact and productivity in one number is a crude and rough thing to do, just as the subject GREs and qualifying exams are often crude indicators of actual aptitude. Just as I think the physics GRE is only really good at identifying outliers (the best 2.5% do very well on it; the worst 2.5% do very poorly; the middle 95% get scores that don't seem to correlate with their actual talent or ability), the h number is similar. I would never dream of assigning too much importance to it in tenure cases. As in grad school or postdoc or faculty applications, detailed letters of recommendation are far more useful, and in my experience correlate much better with actual performance. However, if someone has an h number far outside the expected norm in either direction, I'd like to know that. For example, I heard recently of an externally appointed dean at a research university where the faculty were rather shocked to find that the dean's h number was about 4. Unsurprisingly, people who have had vastly larger scientific impacts don't really like being told what to do or have their decisions scrutinized by someone who has essentially been a professional administrator.

Anyway, I wouldn't lose too much sleep over h numbers. They just get a lot of attention because they're a relatively new idea, and they do seem superior to the previous crude metric, citation counting.

This week in cond-mat

Four papers this time, though brief descriptions. Eventually the semester will ease up a bit and I'll have more time to write.

cond-mat/0610572 - Gabelli et al., Violation of Kirchoff's Laws for a coherent RC circuit
Kirchoff's laws are the basic rules you learn in introductory circuits, and may be suitably generalized to think about high frequency systems. One of the basics is that impedances in series add. In this paper (also published in Science), the authors do some very nice work using gated two-dimensional electron gas to make an effective RC circuit, where part of the R is a quantum point contact. They find that when the whole system is quantum coherent, the basic idea of adding impedances goes out the window. This is neat, and it is a beautifully done experiment, but I don't find the conceptual point to be very surprising at all. Think about this simple case just in the dc limit: a single tunneling barrier has some effective tunneling resistance. A second, identical tunneling barrier has the same resistance. What is the resistance of the series combination of the tunneling barriers? Well, in the incoherent limit, the resistances just add. In the fully coherent limit, you have to worry about interference effects between the barriers, and can even arrive at perfect transmission for the series combination, even though each barrier individually is not very transmissive. This paper's analysis is more general than this, but I can't help but think that it's really the same basic physics at work.

cond-mat/0610634 - Neder et al., Controlled dephasing of electrons by non-Gaussian shot noise
This is another great experiment by the folks at the Weizmann Institute, studying the basic physics of quantum decoherence using an interferometer and a tunable detector at one arm of the interferometer, all made from GaAs 2d electron gas. In earlier work, they've shown that the interference of the electrons in the which-path interferometer can be suppressed in a controlled and continuous way, depending on how "on" the detector is, and how strongly the detector is coupled to the interferometer arm. Here, they work in the quantum Hall limit, and study directly the relationship between the back-action of the detector (via its noise) and the effect on the interference.

cond-mat/0610710 - Scalapino, Numerical studies of the 2d Hubbard model
The 2d Hubbard model is one of the favorite toy models suggested for high Tc. It's a square lattice, with some nearest neighbor hopping amplitude t and an on-site repulsion U so strong that each site can only hold 1 electron. Scalapino has written a review chapter summarizing numerical treatments of this model, and arguing that it has all the essential features of high-Tc. Numerical work in models like this is notoriously difficult computationally, in part because of the requirements that the whole many-body state be antisymmetric under exchange of any two electrons.

cond-mat/0610721- Potok et al., Observation of the two-channel Kondo effect
I want to write more about this later. In brief, David Goldhaber-Gordon and Yuval Oreg had proposed an experimental set-up to implement a tunable version of the long-sought two-channel Kondo model, in which a single localized spin is coupled via tunneling to two independent electronic baths. The 2CK model is of interest because its ground state is not a Fermi liquid (as opposed to the conventional Kondo model and ordinary metals). David's students Ron Potok and Illeana Rau have done the experiment, and the results look very interesting. Using the scaling of the conductance, it looks very much like they have succeeded in getting (at least) very close to the two-channel Kondo state. A cool experiment, and very technically demanding, in part because the temperature scales needed to see the physics are so low.

This week in cond-mat

Three papers this time out. The semester is very busy, so not much commentary for a while.

cond-mat/0610352 - Wu et al., Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography
There's been a lot of hubbub about making meso- and nanostructured materials that have negative permeability and permittivity over some limited frequency range. These materials can have very weird optical properties (obey a left-hand rule; refract in the opposite direction than conventional materials; can be used to try and beat the diffraction limit for imaging; can be hyped into Harry Potter-style invisibility cloaks). Here is the first example I've seen of someone making large-area 2d structures with these properties in an interesting frequency range (near-IR, close to the 1.5 micron telecommunications band).

cond-mat/0610413 - Evers and Burke, Pride, prejudice, and penury of ab initio transport calculations for single molecules
I really like this paper, both for what it says and how it says it. The authors go into detail about different calculational approaches used to predict or retrodict electronic transport properties of single molecules. Very often people in this field crank out results using quantum chemistry techniques (density functional theory) and approximate methods without ever pointing out what those methods generally can't handle (strong correlation effects like Kondo; significant interaction corrections; Coulomb blockade). This paper really gets at what works, what doesn't work, why, and what can be done. Similar in topic is a recent preprint from Datta's group, where they look at Coulomb blockade in small molecules.

quant-ph/0610117 - Dyakonov, Is fault-tolerant quantum computation really possible?
I haven't read this one yet, but the abstract is attention-getting. It argues that the math upon which error correction schemes for quantum computers are based is unrealistic in terms of its relationship with real world systems. Therefore, it may be impossible in principle to scale up to large quantum computing systems. Anyone take a look at this and have an opinion?
Update: After reading Dave Bacon's comment, I actually looked at this preprint. Wow. The tone is very colloquial (it's based on a talk), and is hardly subtle, nor is it very convincing as reasoned technical argument. Is this the same Dyakonov as in the Dyakonov-Perel mechanism of spin relaxation? The initials are the same. Not that having something named after you necessarily means that you're right about everything; Brian Josephson's rather unorthodox views on telekinetics and levitation are the classic case in point.

CM Experimental position at Rice

Presumably any serious job-seekers out there would read about this on the AIP website or in Physics Today, but what the heck - it can't hurt to reproduce the ad here:

Faculty Position in Experimental Condensed Matter Physics
Rice University
The Department of Physics and Astronomy at Rice University invites applications for a tenure-track Assistant Professor position in experimental condensed matter physics, in the general area of quantum materials, including strongly correlated electronic systems and quantum nanostructures. This position will complement our existing strengths in condensed matter and materials physics and quantum degenerate gases. Applicants should send a dossier that includes a curriculum vitae, a statement of research and teaching interests, a list of publications, and two or three selected reprints, and arrange for at least three letters of recommendation to be sent to the Chair of the Condensed Matter Search Committee, Dept. of Physics and Astronomy, MS 61, Rice University, 6100 Main Street, Houston, TX 77005. Review of applications will begin in December, and the appointment is expected to be available July 2007. Rice University is an affirmative action/equal opportunity employer; women and underrepresented minorities are strongly encouraged to apply.

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I hope we get some good candidates! I agree firmly with what a competitor of mine from Cornell once said to me about faculty searches: "I aspire someday to be the dumbest person in my department."

Two fun science links

I haven't had a chance to watch these yet, but the Vega Trust in the UK has, on line, four full length lectures on quantum electrodynamics by Richard Feynman from 1979. Someday I'll have the four or five hours available to watch these.

Much shorter, and much more viscerally fun, check out this video to see that alkali metal chemistry really can be fun. (Thanks for the link, Pat!)

This week in cond-mat

Two papers this time around....
cond-mat/0610107 - Butenko et al., Electric field effect analysis of thin PbTe films on high-\epsilon SrTiO3 substrate
This paper is a nice example of using the three-terminal field-effect geometry as a way to probe the states of a material while keeping the disorder fixed. The authors use strontium titanate as the dielectric layer. Since SrTiO3 is almost a ferroelectric, it has an extremely high gateable polarization (gated charge density) at breakdown field. This means that the authors are able to shift the Fermi level over a very broad range, spanning the entire (relatively narrow compared to things like Si or GaAs) energy gap of the PbTe disordered film, and gate in either electrons or holes. They can see the effects of interface states, and the broadening of the conduction and valence bands due to disorder. Their main observation is that the mobility gap in the disordered case is actually larger than the standard band gap in PbTe. Pretty interesting, and written in a reasonably pedagogical style.

cond-mat/0610150 - Liu et al., Experimental observation of the inverse spin Hall effect at room temperature
The spin Hall effect is a neat concept that my friend Jairo Sinova at Texas A&M has been involved with heavily, as has Soucheng Zhang, who taught me many-body physics back in grad school. The basic idea is that, under the right conditions, it is possible for a dc longitudinal current to establish an unequal spin population on the transverse edges of a material (e.g. a GaAs heterostructure). That is, along the two edges of the sample that parallel the current flow, there will be an excess spin population (with no excess electronic population!), with one edge having an excess of spin-up, and the other edge having an excess of spin-down. Here, up and down are relative to the direction normal to the plane of the current flow. This spin population difference is analogous to the voltage difference that develops transverse to the current in the presence of a perpendicular magnetic field in the ordinary Hall effect. Anyway, the bottom line is that one can produce separated spin populations without actually injecting spins from a ferromagnet or something similarly difficult. The spin Hall effect can be intrinsic (due to spin-orbit coupling and a built-in electric field or lack of inversion symmetry in the material) or extrinsic (due to spin-dependent scattering off of disorder in the material). One of the first (the first?) observation of spin Hall was made by Awschalom's group at UCSB, using spatially resolved magneto-optic Kerr to map the spin density.

Anyway, in this paper the authors claim to observe the inverse spin Hall effect. That is, they establish an unequal spin population between edges of a sample using a spatially varying intensity of circularly polarized light to generate polarized carriers. Then, they observe a dc current transverse to the spin density gradient. The data look pretty convincing, though I'm no expert in photophysics of III-V materials.

One last post about hype

...and then I'll get back to science. The pending announcement of the Nobel Prizes in physics and chemistry next week should provide some good fodder for discussion, as well as more arxiv stuff.

Anyway, there was an interesting article in yesterday's Wall Street Journal about the meaning of the word "breakthrough" and its overuse in technology company press releases. Take a look - it's interesting, and confirms what many of us already knew: far more incremental work is being sold as "breakthroughs" now than in the past. The same is true in science as well, though we don't do it to bump up the share price; we end up doing it because the cultural pressures to put out a press release with each publication are seemingly always increasing.

Great subject line

You know you're dealing with a serious, high quality scientific journal when the publisher spams basically anyone who's ever reviewed an article for any of their publications to advertise the journal. For example, I laughed out loud this afternoon when I got an email from Wiley Interscience with the subject line: Reasons You Should Be Publishing In Aggressive Behavior . Hee hee. Maybe the email was misaddressed and was supposed to go to Angry Physics.

Statement on scientific integrity in policy-making

This is one of my (thankfully, for some) rare forays into political issues on this blog. If you're a practicing scientist, engineer, or just a concerned citizen, please go here, read the statement, and sign the petition if you think it's an important issue. I know not everyone agrees with the Union of Concerned Scientists on every issue, but I think they're right about this one: science should speak for itself in policy-making, not be censored, manipulated, or heavily edited for political ends, by either party.

New Scientist: WTF?

Sci-Fi author Greg Egan, via John Baez: Save the New Scientist. Basically the British magazine New Scientist used to be very good - like Scientific American before they started cutting content for the sake of flashier visuals (basically trying to look more like Discover, though to be fair, Scientific American still has scientists do the actual writing, which is very nice). Over time, it's devolved to the point of being a conduit for press releases from the worst of the hype-spewers. The coup de grace that pushed Egan to write his plea was this COVER article, which conveniently neglects to point out that the gizmo in question would have to violate conservation of momentum if it works as described. Sad. A "science" magazine publishing, as a cover story, non-peer-reviewed junk that doesn't pass the laugh test.
(Not too much on cond-mat this week that seems good for the general blog reader - I'm sure more will come up soon.)

This week in cond-mat

Two papers from the past week, the first of which gives us a chance to discuss one of the on-going controversies in condensed matter physics.
cond-mat/0609301 - Lai et al., Linear temperature dependence of conductivity in Si two-dimensional electrons near the apparent metal-to-insulator transition
For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder. So, the question is whether the real (interacting) case, with an apparent transition between metallic and insulating states, is profound (that is, a real quantum phase transition) or not (e.g., a percolative transition caused by the system breaking up into disconnected puddles of carriers as the concentration is lowered). There are some interesting pieces of evidence pointing in each direction. This paper weighs in using very nice Si quantum wells in SiGe, showing evidence consistent with a percolative crossover in the conductivity. Anyone out there care to comment on the state of this debate in general? Has there been a really slam dunk experiment out there that I've missed by avoiding this problem?

cond-mat/0609297 - Naik et al., Cooling a nanomechanical resonator with quantum back-action (also available in Nature)
This paper is one I need to read more carefully. These folks have constructed a nanomechanical resonator (operates at about 20 MHz), and are using a superconducting single-electron transistor (SSET) measured at high frequency to detect the resonator's motion. This is a great system for testing ideas about quantum measurement and back-action of the detector on the system being measured. In this case, they find that for the right settings of the SSET detector, they can actually cool the resonator (as determined by the noise temperature of the resonator, inferred from the readout of the detector) using the detector. The claim is that this is analogous to laser cooling in some sense, bit without a closer reading, I don't see how this really works. This shows that I need to think more and read more about this detector back-action business.

Packard Fellows meeting

Sorry for the downtime; the semester has begun, and this past week I also went to the annual meeting of Packard Fellows, which is a wonderful chance to hear technical talks from people working in all sorts of fields of the natural sciences, engineering, and mathematics. I will return with more cond-mat article discussions soon. In the mean time, I wanted to highlight two particular condensed matter talks that I heard at the meeting:

First, Hongkun Park spoke about his recent very interesting work on electronic properties of VO2 nanowires (actually bars - they grow from the vapor phase into long wires with square cross-sections). Some of this has been published. Vanadium dioxide is a weird material. It's supposed to be a Mott insulator at low temperatures, meaning that electronic interactions are so strong that the charges lock into place rather than being free to move around. At higher (not much higher than room) temperature, the material undergoes a first-order structural and electronic phase transition to a metallic state. Prof. Park's group has been playing with these nanowires, and found some amazing phenomena. For example, when the wires are sitting on a surface, the constraint of the surface strain plus the structural phase transition lead to the wires breaking up spontaneously into domains of metallic and insulating regions, and those domains can be (a) imaged with an optical microscope, (b) pushed around by flowing a current, and (c) made to oscillate back and forth because of resistive heating effects. Also, in suspended wires, the metal/insulator phase transition can be incredibly sharp, leading to the possibility of novel temperature sensors. Very neat.
Update: (9/27/06) This has just appeared in Nano Letters.

Second, Kathryn Moler showed her latest work on scanning SQUID microscopy. Basically it's possible to put an incredibly sensitive magnetometer at the very tip of an AFM-like probe, and image magnetic flux with incredible sensitivity. Most recently her group has been looking at superconducting fluctuations in little superconducting ring structures. Imagine putting a small magnetic flux on a superconducting ring. The fact that the superconducting wavefunction has to be single-valued going around the loop implies that magnetic flux through the loop is quantized. That quantization condition is enforced by spontaneous supercurrents in the loop. Well, for narrow loops its possible to be in a regime where rather than set up those currents, it's more energetically favored for the loop to go "normal". This is the Little-Parks effect. Now, if you imagine a split ring that looks just like the loop but isn't a complete circle, that would be superconducting. Can the topology of the ring really deeply affect the microscopic physics in the superconductor? Superconducting fluctuations in the "normal" ring are part of the answer. Again, a neat technique and a very nice piece of physics.

...and still MORE hype....

I have avoided talking too much about my own research here, with the intent of maintaining a broader perspective on CM and nanoscale physics. However, an example of nano-hype directly related to my own research has come up that I can't just let go (thanks to one of my regular anonymous contributors for pointing out the media aspects of this). Here is a perfectly reasonable theory paper about trying to make single-molecule transistors that operate in a new way. Basically the idea is to somehow (this trivial detail is left as an exercise for the experimentalists, which is actually fine for a theory paper like this) wire up three leads directly to a single small molecule. By varying the voltage on the "gate" lead, the quantum mechanical amplitude for tunneling from the source lead to the drain lead is modulated due to quantum interference. One could imagine (though this isn't discussed in the paper) implementing something like this in a GaAs quantum dot. For example, one could have a little "stub" dot off to the side of a channel connecting the source and drain. If the stub dot was tuned into the Kondo regime via coupling to the channel, then there would be a Fano antiresonance that would suppress source-drain conduction. Same basic idea. Anyway, the concept is sound, and the calculations (though done in some limited approximation on an idealized molecular/lead geometry) show that it's not crazy. Fine.

I have no problem with the science (though experimentally implementing it as conceived will be incredibly difficult). What I do have a problem with is the ensuing media onslought. Read this press release, which got picked up by CNN (broadcast, not the web). Read it all the way through, to the point where the scientist starts talking (I'm not making this up) about little nanobots controlled by computers that use this transistor concept swimming through your bloodstream. AAAAAGGGH! WHY DO PEOPLE DO THIS? Does the Arizona group really think that their paper will have more impact and enable more and better science and technology because of this? Do they think their pending patent on this idea will be more likely to be licensed? Don't they think that this kind of overreaching hype actually hurts the field in the long run?

This week in cond-mat

Two papers that are fun to talk about because of more than just the science:

cond-mat/0608576 - Klimczuk et al., Superconductivity in Mg_10 Ir_19 B_16
This is just a typical example of the kind of neat stuff that can come out of a really outstanding solid-state chemistry group. Bob Cava, formerly of Bell Labs and these days at Princeton, is an impressive materials chemist who has been involved in the discovery and synthesis of an ungodly large number of new materials. The one in the title of this paper is apparently one of a whole new family of superconductors. If someone told me that room temperature superconductivity was discovered, my first guess at the grower would probably be Cava. Just having someone like this on your campus can really make things happen, just like having a fantastic MBE grower. Of course, the total number of people like this who are this successful is very small. You can't just be edisonian - you have to have impressive insight into the chemistry and materials science issues, and you have to have access to the appropriate characterization tools.

cond-mat/0608492 - Hirsch, Do superconductors violate Lenz's Law?
Jorge Hirsch is a very interesting guy. He's very much a political activist, a person interested in developing useful metrics for measuring academic performance, and a condensed matter theorist with his own ideas about superconductivity. When a (type I) superconductor is brought into a region of magnetic field, the superconductor develops screening currents to exclude the magnetic flux. Those currents flow within a penetration depth of the surface of the material, and the result is essentially perfect diamagnetism - this is called the Meissner effect. When those currents get set up, a torque is exerted on the lattice of the superconductor. Basically the paired electrons making up the supercurrent have some orbital angular momentum about the axis of the magnetic field. Since total angular momentum is conserved, the ions of the lattice have to pick up angular momentum going the other way, so that the total remains zero. Hirsch claims (and for fun, is trying to take bets on this to finance an experiment) that there is a big difference between the bring-a-superconductor-into-a-field case, and the cool-through-the-superconducting-transition-in-a-field case. He argues that the torques on the lattice in those two cases should be in opposite directions. I think he's wrong - at the very least, his treatment of this problem is waaaaay to simple. Anyone?

...and speaking of hype....

You know, I like Neil deGrasse Tyson - he does a huge amount for public outreach about science, and has even appeared on the Colbert Report (which I would love to do, since I think Colbert's observations about truthiness and wikiality have a lot to do with science today). However, this article is exactly the sort of hype-ridden malarky that really hurts science in the long run. Our risk of being swallowed by a black hole is negligibly small, and the science in the article is appallingly dumbed down (black holes were long thought to be roughly stationary? With respect to what, exactly?). While it may get people's attention, implying that it's likely that roving black holes in our neighborhood are going to kill us all is not the best way to get science in the public eye.

Is it "vision", or is it BS?

In a comment to my previous post, Alison Chaikin tries to put the Steorn business in perspective, pointing out that many grant proposals contain an awful lot of highly improbable exaggeration of potential, too. This is going to sound self-righteous, but I'll say it anyway: I really wonder sometimes if I hamper my own academic impact (defined, say, by funding levels, citations, publications in glossy journals) because of my low tolerance for bullshit. For example, our single-molecule transistor work is really nice science, with a good mix of physics and chemistry. However, when I give talks, I try to point out that, at least as implemented now, these devices are very unlikely to be good for high speed, high performance computers. There are some reasons to be optimistic, and there remains a large amount of great basic science as well as engineering to do before we can really assess whether these gadgets will, in something like their present form, be technologically useful.

Statements like that, while realistic, are much less likely to inspire DARPA to hand me $250K/yr for three years than if I said "Within three years we [always use the royal "we" :-) ] will roll out commercial devices using single-molecule switches that operate at room temperature and GHz frequencies." The fact that this is an unrealistic goal is often irrelevant - it shows self-confidence, aggressiveness, and a vision to change the world. I'm reminded of footage of GWB debating Anne Richards for TX governor back in '94. When asked about possibly legalizing gambling in Texas in some form, Gov. Richards gave a very carefully worded, two paragraph response, explaining that this was worth considering provided it was handled correctly and that some of the taxes went to fund education and children's health programs, etc. George Bush's response was "I'm against it. I think it's a bad idea." The short, definitive, ambitious statement often beats nuance and realism - even in science.

There are some in academia (I've been told this explicitly) who view this grantsmanship stuff as an interplay between Big Picture Visionaries, and "Detail People". The Visionaries want to change the world, and often feel hectored by the Detail People, who they perceive as narrow and uncreative. Of course, the Visionaries need Detail People, since they're the ones who actually make things work. What do you all think about this? I think Vision in this context can be dangerously close to hucksterism.

A scam, or self-delusion?

By now you've probably heard about Steorn, a company of dubious provenance (used to be an e-business of some kind back during the .com boom) with no clear technical expertise that placed a full-page ad in The Economist last week. They claim to have developed a device that produces more energy than it takes to run - essentially a perpetual motion machine of the first kind. They go further than that, anecdotally claiming that scientists and engineers at reputable places have tested this gadget and agree that it really does produce energy seemingly from nowhere, but none of those folks have been willing to speak on the record. So, Steorn is trying to put together a "jury" of 12 scientists to test their gizmo. This has many many of the hallmarks of a pseudoscientific scam, complete with an utter lack of technical detail, and the company wanting to decide who does the testing. In fact, they actually won't let the scientists do tests - just examine records of the tests and data. Presumably they'll also say something like "Pay no attention to the man behind the curtain." On the other hand, it's hard to see what they gain by spending close to $200K on an ad, if the net result is a huge pile of negative publicity - I suppose they're just hoping some gullible rich person will believe that The Scientific Orthodoxy is suppressing this incredible breakthrough, and that big investment will follow. Place your bets on whether we'll ever even hear from these folks again....

Recently in cond-mat

Two recent papers that I find particularly interesting (both of which have now come out in print as well)....
cond-mat/0603442 - Sela et al., Fractional shot noise in the Kondo regime (also PRL 97, 086601 (2006)).
As I've discussed before, shot noise is noise that results from the fact that charge comes in discrete chunks. For strongly correlated systems, when the low energy excitations of the system can't be nicely described as single quasiparticles that act like "free" electrons, there can be dramatic signatures in the shot noise. These authors argue that such an effect should be present in the shot noise that results when current flows through a quantum dot in the Kondo regime - that is, when an unpaired spin on the dot is strongly entangles with the conduction electrons of the leads via higher order tunneling processes. The claim is that the effective charge of the carriers measured via shot noise is actually 5/3e, rather than simply e. This would be very neat.

cond-mat/0608459 - Koppens et al., Driven coherent oscillations of a single electron spin in a quantum dot (also Nature 442, 766 (2006)).
Once again, the Kouwenhoven group at Delft turns out a gorgeous piece of experimental work. This time, not only do they succeed in electrically measuring single-electron spin resonance. They go further, and demonstrate that they can coherently manipulate the spin, placing it into, e.g., a superposition of "up" and "down", and watching the Rabi oscillations back and forth. Wow. This is a real tour de force experiment, when you consider that the whole system needs to work at mK temperatures.

Aphorisms

Just returned from a conference at which I somehow managed not to hurt anyone with my laser pointer, and I picked up a couple of aphorisms from Tom Jackson, an EE professor at Penn State:

Jackson's 2nd rule of engineering (paraphrased): Don't argue with idiots; bystanders have a hard time telling the difference.

Jackson's 1st rule of engineering: Don't polish turds.

These brought to mind a couple of favorites from grad school:

Rogge's rule: When soldering, there is no such thing as too much flux.
O'Keefe's contradiction: Too much flux makes solder run like piss.
Salvino's rule: Any hose may be connected to any other hose with the appropriate hose clamp.
Gilroy's maxim: Graduate school is the process of continually lowering your expectations.
Natelson's variation: Graduate school is the process of continually increasing your cynicism.

Anyone out there got some other good ones?

This week in cond-mat (mini-version + digression)

One particular paper caught my eye this week:
cond-mat/0608243 - Nakamura et al., Low-temperature metallic state induced by electrostatic carrier doping in SrTiO3.
The authors of this paper have managed to solve, at least well enough to do the experiment, the surface processing and ohmic contact challenges to make a field-effect transistor on the surface of a n undoped strontium titanate single crystal. At high enough gate voltages, they can accumulate enough carriers in the channel to drop the sheet resistance of the 2d charge layer well below the resistance quantum (~ h/2e^2 ~ 13 kOhms), and see metallic temperature dependence of the channel conductance (that is, the conductance improves with decreasing temperature). Anytime someone does this sort of thing with a new material system it's interesting, and SrTiO3 is particularly noteworthy because it's a perovskite (crystal structure not that different from high Tc materials), it's an incipient ferroelectric (very large dielectric constant as T decreases), and when doped at moderate levels, it's been known to superconduct. Field-effect "doping" is a very nice tool for studying this sort of physics, because the carrier density can be changed without introducing the disorder that comes with chemical doping. I'm actually a co-author on a forthcoming Reviews of Modern Physics paper about this general topic.

Now that you've glanced at that preprint, take a look at this PRL. Those folks have been looking at conduction in a semiconducting polymer, poly(3-hexylthiophene), and claim to observe a metal-insulator transition. The data are very pretty, but I just don't see how the interpretation matches the data well. These folks argue that, because the temperature dependence of the (highly nonlinear) conduction that they measure (at large source-drain voltage) gets weaker with increasing gated charge, and approaches temperature-independence, they are seeing a metal-insulator transition. It seems that the picture is: for high quality polymer films, the potential minima from disorder are relatively shallow, and when the potential is sufficiently tilted (by source-drain), and the deeper minima are filled (by large gated charge), then one can get tunneling (rather than thermal activation) out of the minima, and temperature-indep. conduction. This may well be right, but I really object to calling this a metal-insulator transition. There is no true transition here, and never does conduction improve with decreasing T, as in a metal. Again, the data are good, but the title and language are, to me, an example of wordsmithing. (Full disclosure: one reason this rubs me the wrong way is that in our own work we saw similar weakening of T-dep. several years ago. I would never have thought of calling this a transition to a metallic phase.)

This week in cond-mat

Two papers for now....
cond-mat/0608069 - Zhou et al., First direct observations of Dirac fermions in graphite
This paper is also in press at Nature Physics. The authors take angle-resolved photoemission spectroscopy (ARPES), and apply it to high purity graphite. ARPES is a very impressive technique - a really nice (highly collimated, bright, well-controlled energy - like from a synchrotron) x-ray beam is incident in a carefully controlled geometry on a sample, and the photoelectrons kicked out of the material are detected in an angularly resolved way. Applying conservation of momentum and energy lets one use this method to extract (2d) band structure information about the material. In high Tc compounds, for example, ARPES has contributed greatly to the understanding of "Fermi Arcs" and so forth. Anyway, these folks look at graphite, and find that massless Dirac fermions really do describe well the 2d band structure of this material. They also see some "boring" carriers in there, with parabolic dispersion (that is, energy proportional to the square of carrier momentum, indicating that the effective mass is a well-defined concept). Finally, they see signs that impurities and defects lead to electrons sitting in there. So, the electronic transport physics in this stuff is "rich", meaning very complicated. This is a good example of applying a highly refined tool to a new (yet very old) material system.

cond-mat/0608159 - Sellier et al., Transport spectroscopy of a single dopant in a gated silicon nanowire
The authors here have done a very elegant experiment. They've taken doped Si on insulator, and etched it to form an "island" with source, drain, and gate leads. That island contains a single dopant atom, and by performing low temperature conductance measurements, including significant magnetic fields, they've been able to look at two charge states of that single dopant, and compare with long-held models (D0 and D- configurations) of how dopants sit in Si. The single arsenic donor acts like an extremely small quantum dot, having electron addition energies exceeding 15 meV. This is the kind of experiment that is conceptually simple, but actually doing the work has real experimental challenges.

hot topics and controversies

As was suggested in a recent comment, now that a nonzero number of condensed matter and nano people are (apparently) reading this blog (at least occasionally), this could be a fun opportunity to have a series of discussions about the hot topics and controversies out there in the world of condensed matter and nanoscale science. The idea would be to take maybe one topic a week, give a relatively gentle introduction to the subject, and then have some discussion, just for fun. This only works, of course, if enough people contribute to make the discussion interesting, rather than just me pontificating (though I suppose that would be de rigour for a blog). As a preamble, I suggest trying to generate a list of topics. Here are a few off the top of my head:

• 2d metal-insulator transition - What is the mechanism for the apparent metal-insulator transition in 2d electron and hole systems at low densities? Is it profound or not?
• High-Tc - what is the mechanism of high temperature superconductivity? What is the ultimate limit of Tc? What is the "bad metal", and what is the pseudogap, really? How important are stripes and checkerboards? Is the phrase "doped Mott insulator" really a generic description of these systems?
  
• Quantum criticality and heavy fermions - Do we really understand these systems? What are the excitations in the "local moment" phase? What is the connection to high-Tc, if any?
• Manganites - What sets the length scale for inhomogeneities in these materials?
  
• Quantum coherence and mesoscopics - Do we really have a complete understanding of mesoscopic physics and decoherence at this point? What about in correlated materials?
  
• Quantum Hall systems - Are there really non-Abelian states at certain filling factors? In bilayers, is there excitonic condensation?
• 1d systems - Is there conclusive evidence of spin-charge separation and Luttinger liquid behavior in semiconductor nanowires? Nanotubes?
• Mixed valence compounds - Is there or is there not charge ordering at low temperatures in Fe3O4, something that's been argued about for literally 60 years now?
• Two-channel Kondo physics - Is there firm evidence for the two-channel Kondo effect and non-Fermi liquid behavior in some physical system?
• Molecular electronics - Is there really improving agreement between experiment and theory? Can novel correlation physics be studied in molecular systems? Can molecules exhibit intrinsic (to the molecule) electronic functionality?
• Organic semiconductors - What is the ultimate limit of charge mobility in these materials? Are there novel electronic correlation effects to be seen? Can one see a metal-insulator transition in these systems?
• Nanomechanical systems - Can we demonstrate true "quantum mechanics", in the sense of a mechanical system that acts quantum mechanically?
• Micro/nano systems to address "fundamental physics" - Can we measure gravity on the 100 nm length scale? Are there experiments with Josephson junctions that can probe "dark energy"?

What am I leaving out? Any other suggestions?

Ahh, missile defense

At the suggestion of my colleague, I want to draw your attention to a very interesting and fun article in today's New York Times (free reg. required). It's about an antimissile laser system developed jointly by the US and Israel. The system works, basically, but is hugely expensive and so large in physical size that deployment is a nightmare. The article is really worth reading, just for the paragraph that begins: "As often happens in the federal development of death rays, parts failed and costs soared."

This week in cond-mat

A couple of new papers on the arxiv that I find particularly interesting....
cond-mat/0607756 - Zarchin et al., Bunching of electrons in transport through quantum dots
The Weizman Institute's work on transport in quantum dots is generally as good as it gets. I've already written about their experimental prowess in measuring shot noise, and this is another example. Shot noise results from the discrete nature of electronic charge. While the current tells you about the average rate at which electrons are flowing through a circuit, there are fundamental fluctuations in that current that describe the temporal correlations between the electrons. For example, if electrons only flowed through the circuit one at a time in perfectly spaced intervals, there would be no noise. On the other hand, if the electrons were Poisson distributed, there would be a classical current noise of 2eI (in units of amps^2/Hz). The authors here looked at shot noise in gate-defined quantum dots on GaAs/AlGaAs 2d electron gas. The authors found a surprising result. In the finite-bias conductance resonances that happen in these kinds of dots (as the source-drain bias is increased to allow access to another charge state for transport), the shot noise was enhanced over this classical result by as much as a factor of 10. This implies that the electrons are bunching up somehow, traversing the dot in bursts. This is quite odd and unexpected.

cond-mat/0607765 - Kitchen et al., Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions (also out in Nature)
This is a very nice STM paper by Ali Yazdani's group from Princeton. These folks are able to insert single Mn atoms into the surface of a p-doped GaAs wafer, and watch what happens. This is important because ferromagnetic semiconductors like GaMnAs are a key class of materials for those interested in capitalizing on the spin as well as charge of free carriers. What I really find interesting about these measurements is how very different a dopant atom in this semiconductor system looks from the puffy, hydrogenic picture painted in solid state physics textbooks. These kinds of results always re-emphasize to me that serious STM can't be your hobby - it has to be the main focus of your research effort, or you can't be competitive.

What the...?!

I thought I'd seen it all this evening when I opened my email to find an extensive warning email about laser pointer safety (!) from the SPIE (presumably sent to me because I'm speaking at an upcoming meeting, not because they think I'm a danger to myself and others when armed with a laser pointer). Remember, laser pointers are all fun and games until somebody loses an eye. This warning actually did include the sentence "NEVER stare directly into the beam of a laser pointer!". Whew! Good thing they warned me, in case my advanced degree hadn't given me sufficient critical thinking skills to reason that out for myself. The last line of the email made clear the real reason for sending it. They boldly declaim that any person using a laser pointer at an SPIE event but not adhering to the outlined safety protocols is personally liable in the event of injuries, and the SPIE is not liable. I consider this direct observational proof that our society has too many risk management and personal injury lawyers.

That paled compared to my reaction to this story, though. It would appear the Purdue University has done a thorough and careful investigation of claims of research misconduct in the case of Rusi Taleyarkhan, the scientist who claims to have used sonoluminescence of deuterated acetone to produce table-top-scale fusion. In the spirit of scientific openness and transparency, Purdue has decided to not make public the result of its investigation. So, either Taleyarkhan is legit, and Purdue is content to let his reputation suffer, or they think he's a fraud, but are content not to tell the scientific community, or some mysterious third alternative. What on earth is Purdue's administration thinking with this? Did they assume noone would notice?

This week in cond-mat

Just two papers this time. For the first, I must make a disclaimer: this is certainly not my area of expertise, and I can't really judge the validity of the results, but the topic is very interesting. I also haven't read either of these in any detail - they just look intriguing.

cond-mat/0607492 - Joly et al., Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics.
I vividly remember a great APS meeting talk by Seth Putterman (I think 10 years ago at the big centennial meeting in Atlanta) on basic pieces of table-top physics that we still don't really understand. One that he mentioned was triboelectricity - the separation of charge due to some frictional process. Remember junior high when you were told to rub a lucite rod with rabbit fur to build up a static charge? Amazingly, we still don't really understand the microscopics of this (unless the situation has changed recently. Any enterprising readers out there know anything about this?). Anyway, this paper is about the fluid analog of this. When a fluid containing ions is placed in contact with the walls of a container, the ion distribution is altered. Depending on the microscopic details of the fluid and the wall material, a sub-monolayer of charge can become practially immobilized at the wall (the Stern layer), and beyond that there extends into the fluid a net charge density (decaying exponentially into the fluid on a scale called the Debye length) set by competition between charge screening and diffusion due to concentration gradients (the appropriate diff-eq is the Poisson-Boltzmann equation). All this stuff is very important when worrying about colloidal suspensions, net charge on nanoparticles in solution, electrochemical scanned probe, etc. When fluid is flowing, slippage of the fluid layer right next to the wall can strongly modify the ion concentrations, and this can have big consequences for electrokinetic processes like electro-osmosis and electrophoresis. That's what this paper is on, and it's directly relevant to lots of micro- and nanofluidics work going on, particularly in the lab-on-a-chip community.

cond-mat/0607354 - Qi and Flatte, Current-induced spin polarization in nonmagnetic semiconductor junctions
Kato et al. showed recently that it's possible to build up a net spin polarization in the carriers in a strained nonmagnetic semiconductor (e.g. GaAs) by applying an electric field (and hence driving current into one side of the semiconductor through a junction, and out the other side). Lots of questions were inspired by this - is this a spin-orbit effect? Is this a spin-Hall effect? Now this new paper argues that the effect is neither of these things, and happens even in the absence of spin-orbit effects and for purely spin-independent scattering mechanisms. The trick seems to be that the mobility of carriers ends up depending nontrivially on the spin polarization (see here) for reasons that I don't currently understand. Seems profound enough that I should try to learn about it, though.

A time-saving step

This weekend I'll catch up w/ the cond-mat archive. In the meantime, I wanted to point out one amusing piece of Lubos Motl's latest blog posting:
The previous paragraph also clarifies my style of reading these papers. The abstract has so far been always enough to see that these fundamental gerbes papers make no quantitative comparison with the known physics - i.e. physics of string theory - and for me, it is enough to be 99.99% certain (I apologize for this Bayesian number whose precise value has no physical meaning) that the paper won't contain new interesting physics insights.
This attitude is surprisingly common among physicists. In a graduate seminar course at Stanford, someone else in the class showed our (then pre-)Nobel Laureate theorist professor a paper on high temperature superconductivity. After glancing at the title, author list, and abstract, he tossed the paper face-down on the table, and said, "I don't even have to read this to know that this is crap." Sometimes this approach (or its converse) really does work. I certainly have a list of condensed matter and nano experimentalists whose work I presume to be extremely good, because everything I've ever seen from their research groups has been elegant and solid. However, pre-judging results based on who did the work and what the abstract says is exactly the kind of non-scientific, unobjective attitude that emboldens social science types to argue that science and its findings are largely a social construct, etc., a conclusion that I think is way off base (when I drop my pencil from above my desk, it will fall toward the ground at 9.8 m/s^2, regardless of my sociology, preconceptions, or personal beliefs).

A couple of random things

One of the more popular physics blogs, Cosmic Variance, has an interesting post about rumor mill websites. If you aren't familiar with the concept, rumor mill sites have been around for a number of years associated with physics and astrophysics faculty job searches. The atomic/molecular/optical and condensed matter rumor page is here. Mark over at Cosmic Variance has interesting things to say on the subject.

Also, as a follow up: I did hear back from Phys. Rev. Letters about the possible data falsification that I pointed out to their editors. They heard back from the authors of the paper in question, and say that the authors showed them "raw" data, and that it was some sort of image processing artifact that made all the noise in the relevant images really look identical. Hmm. I'm unconvinced, but the editorial office says they're satisfied. If anyone wants to see the paper in question, contact me.

I'll put up more cond-mat and physics related postings soon; I need to tend to a couple of papers from my students, as well as a not-so-minor crisis involving our cleanroom facility.

Conference proceedings

I'm working on a conference proceedings paper for a meeting at which I'm giving an invited talk next month. So, are conference proceedings papers worth it? Does anyone actually read these things, even the ones published in peer-reviewed form? Or are they part of a borderline sleazy scheme by some professional societies and journal publishers (hint: I'm thinking of one that begins with "Elsev" and ends with "ier") to extort money from cash-strapped libraries for volumes noone ever examines? Also, what is the appropriate ettiquette regarding these? I get the impression that many of my colleagues would have no problem either farming out the writing to a student (even though they wouldn't get first authorship), or just bailing on the whole proceedings altogether (which I confess I've done before, too, when other demands on my writing time get too big). Opinions, anyone?

This week in cond-mat

Just returned from the Electronic Materials Conference. Interesting, and generally much more oriented toward engineering than pure physics, but fun nonetheless. I'll be out of commission for the next week or so, so this blog entry will have to tide over my dedicated readership :-)

cond-mat/0606742 - Camino et al., Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit
In the fractional quantum Hall effect, in very clean two-dimensional electron systems (typically formed at the interface between GaAs and AlGaAs layers) at very low temperatures and particular large magnetic fields, the "normal" metallic state of the electrons is unstable. The particular values of magnetic field are those for which the ratio of magnetic flux through the sample (in units of h/e, the so-called flux quantum) to the density of electrons (number of electrons per cm^2) takes on special values, such as three or five halves (corresponding, respectively, to three flux quanta for each electron, and five flux quanta for each pair of electrons). At these special values of magnetic field, the electrons form a correlated state named after Bob Laughlin, who first wrote down a trial many-body wave function to describe it. In a Laughlin state, the electrons can't be treated as nearly independent, as in a normal metal. Instead, when one tries to probe the electronic system, one finds collective excitations (rather than simple electron-like excitations in a normal metal). These collective excitations have very funky properties: they can have fractional charge (in the three flux quanta per electron case, the excitations have charge 1/3 e) and obey fractional statistics.
Fractional statistics are funky. Swap two electrons, and the total wave function picks up a factor of exp(i pi) = -1. Swap two bosons (like two 4He atoms), and the total wave function of the boson system picks up a factor of exp(i 2pi) = 1. Swap two Laughlin quasiparticles, and the total wave function picks up a factor of exp(i alpha), where alpha depends on precisely which fractional state the system is in. Generically alpha can be anything, earning the nickname anyons for particles that obey such statistics.
This paper looks at conductance oscillations as a function of magnetic field in a patch of Laughlin electron fluid that should exhibit fractional statistics and fractional charge of 1/5 e. The authors claim that these oscillations are surprisingly robust as temperature is increased, and that this is evidence of special stability of that state due to topological considerations. I'm not sure I believe the final conclusions, which seem to depend in great detail on precisely knowing the electron temperature. It's a neat experiment, though, and gives real insight into some exotic quantum effects that people think might be useful for building a quantum computer.

cond-mat/0606802 - Costache et al., Spin accumulation probed in multiterminal lateral all-metallic devices.
The authors in this paper look in detail at the magnetoresistive properties of a little piece of aluminum connected to four separate cobalt electrodes. It turns out fortuitously that each of the four cobalt leads can have its magnetization switched independently of the others, and this lets the authors study effects that arise from pumping certain spin polarizations of electrons into the aluminum island. Since aluminum is a low atomic number material, spin-orbit scattering is pretty weak in there, so electrons can maintain their spin polarization for a while. These experiments require extremely clean interfaces between the Co and the Al to work, and provide concrete numbers for spin lifetimes and diffusion lengths in practical materials.

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....

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.

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.

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.

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.

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.

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.

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?