Ahh, Texas II

One of the things I miss about California is the quaint state politics, like the Peace and Freedom Party, which doesn't sound too bad ("Hey - I like peace and freedom!") until you realize that they're radical Marxists. Texas provides its own amusement, though. From this morning's Houston Chronicle:

AUSTIN — A Texas official who receives any sum of cash as a gift can satisfy state disclosure laws by reporting the money simply as "currency," without specifying the amount, the Texas Ethics Commission reiterated Monday.

The 5-3 decision outraged watchdog groups and some officials who unabashedly accused the commission of failing to enforce state campaign finance laws.

"What the Ethics Commission has done is legalize bribery in the state of Texas. We call on the commission to resign en masse," said Tom "Smitty" Smith, who heads Texas Citizen, an Austin-based group that advocates for campaign finance reform.

Thank goodness I live in a state where officials are allowed to take suitcases full of cash, and it's ok as long as they write down "currency" on their ethics disclosure forms. Wow.

A good scientific interaction

The science: one of my colleagues, Prof. Vicki Colvin, can make magnetite nanoparticles via chemical techniques. Magnetite is an interesting material for a number of reasons, some of which I'll probably write about later. It's a ferrimagnet with a Curie temperature of over 800 K. Nanoparticles smaller than about 40 nm in diameter are single-domain, and those smaller than about 16 nm in diameter are superparamagnetic at room temperature. That means that while the spins are ferrimagnetically ordered, the energy required to reorient the magnetization direction of a given particle is less than the thermal energy scale, kT. Anyway, magnetic particles have been used in the chemical engineering world for a long time to do separations. Make magnetic particles that adsorb your favorite nasty contaminant of water; mix the particles in with the water, and then use large magnetic field gradients to pull the now-dirty particles out, leaving behind cleaned water. Just looking at the magnetic forces on individual magnetic particles tells you that nanoparticles shouldn't work well: the magnetic forces scale like diameter cubed, while viscous forces scale linearly with diameter, and Brownian forces scale like one over the diameter. However, she tried the separation process anyway using a little bench-top separator, and it worked exceedingly well! She asked me why, and after scratching my head for a little while, I realized that the key is actually interactions between the particles. The field gradients near single-domain nanoparticles can readily be 10000 times higher than externally applied gradients, leading to much larger forces than those due to the external field directly. To avoid permanent agglomeration of the particles, yet maximize separability at modest fields, one wants to use the biggest particles that are still single-domain superparamagnets. The real upside is that the nanoparticles have enormous specific surface area. So, a cleanup task that used to take a kilogram of big particles only needs a few grams of nanoparticles.

The sociology: Prof. Colvin could easily have written this up and just thanked me, rather than really inviting my participation and making me a co-author. Instead, she very much wanted my input and gave me ample opportunities to help in the writing of the manuscript. The result was a Science paper, and there is real promise (at least according to our environmental engineering coauthor, who is the expert on cost estimates and water purification) that variations of this work could greatly help in cleaning up arsenic-contaminated drinking water in the developing world. Very cool.

A bad scientific interaction.

The science: One of my collaborators supplies interesting molecules to lots of people. Six months ago, my student and I wrote up a paper on our measurements of some of these. We were pretty pleased with ourselves, because our data lets us make a pretty strong statement about the underlying conduction process in this system.

The sociology: While we wrote this up, a competing big group had been doing measurements on the same molecules with a very different technique. They reached the opposite conclusion as us in their case. At the suggestion of my chemistry colleague, we had a discussion about this with them once we both submitted our papers. There is some chance that we're both right, since the measurement systems are so different, so when we revised our paper, we allowed for that possibility. Our paper came out very quickly - five months ago. In the meantime, our competitors had a much longer review process (this doesn't necessarily say anything about their paper; review can be extremely variable.). Their paper just came out in a different journal. Not only is their wording much stronger than ours (basically stating that their suggested explanation is the only possible conclusion, period). They don't even reference our work, despite having known about it for several months. Not cool.

A primer on faculty searches, part II

Continuing my description of the faculty search process.... Candidates come in for interviews. Each interview is a two-day affair. The candidate is scheduled to meet pretty much everyone on the search committee, and maybe a couple of other people in addition if there's time. Usually at 4:00pm on the first day, we have the candidate give a departmental colloquium. We point out to them when we invite them that the audience for our colloquia is very general - it can include undergrads, and the areas of research of the faculty members in the crowd can range from astrophysics to biophysics to high energy to AMO and CM. The point is we want to see how well the candidate communicates to a general audience about their work (usually their postdoc results, with a slide or a few at the end on future directions). After the talk is dinner with a couple of members of the committee. There are more visits on the second day. In the late morning (usually), the candidate sits down with the committee and gives a shorter (say 20 minutes) research plan talk. This is where the candidate tells us what they want to do in the near and longer term, and what kind of resources they think they'll need to do it (e.g. a big laser system, or a dilution refrigerator, or access to fab and microscopy tools, etc.). This is generally less formal than the colloquium, but it needs to be taken seriously, since this is really where the committee gets a sense of how the candidate approaches planning research. This is also where discussions about teaching happen. Depending on travel plans, there either is a second dinner, or the candidate heads out at the end of day 2.

Once the candidates have all visited, the committee sits down, compares notes, and comes up with a recommendation for the department to vote on. Once the department has made a decision, the department chair is the one who talks with the candidate about offer details. An unofficial offer letter is then prepared and sent out by the dean. Those in the game know what I mean by "unofficial": full-on offer letters come from the office of the president or the board of trustees, depending on the institution, and are essentially only prepared at the very last minute. The candidate is invited to come for a second visit - to look at lab and office space, meet the dean, bring the spouse or significant other if that's relevant, get a look at real estate, etc.

I'll write a third post about faculty searches with a few generic tips for candidates sometime soon.

Weird Al is the man.

Watch this. It's Weird Al's new video. Bonus points for having the Schroedinger equation show up. This may be my favorite Weird Al song since The Saga Begins.

CIAR Nanoelectronics workshop

Blogging from the scenic Calgary airport, as I wait for my flight back to Houston. I've been attending a very fun nanoelectronics workshop in Banff run by the Canadian Institute for Advanced Research. The CIAR is kind of an institute-without-walls that spans Canada. They have seven (I think) sections, each of which funds some research. One of these sections is Nanoelectronics, and the workshop this weekend was very good. Some highlights of the talks:

• Ted Sargent at Toronto is making optoelectronic devices using semiconductor nanocrystals. His group has succeeded in getting nice surface passivation of PbS nanocrystals, such that they get good photoconductive response in a solution-deposited film of these things. Because the bandgap of the nanocrystals is so small (about 400 meV), they can use these in the mid-IR. In an impressive demo, they took a readout chip for a conventional silicon CCD camera, coated it with their PbS nanocrystals, and voila: instant visible-to-midIR video camera. Neat!
• Supriyo Datta gave a nice talk about the general problem of modeling transport through a system that couples not just to its contacts, but also to the environment. As a story-telling device, he framed the discussion in terms of Maxwell's Demon: can one use the spin-selective transmission of a certain type of barrier (containing paramagnetic impurities) as a way of extracting work from the contacts? This is a solid-state gedanken version of Feyman's ratchet-and-pawl. Unsurprisingly, one can't beat the second law of thermodynamics. You can extract some work from the contacts, but at the cost of increasing the entropy of the barrier. If the barrier is cooled to allow work to be continuously extracted, what you've really done is set up a heat engine running on the temperature difference between the contacts and the barrier. I know this isn't a very coherent summary; the talk was infinitely more lucid.
• Several nice talks about charge transport through molecules. Besides me, there were: Heiko Weber talking about his break junction systems; Latha Venkataraman talking about her break junction systems; Mark Ratner talking about charge transport in DNA; Nicolas Agrait talking about transport through 1d chains of Au atoms; and Philip Kim talking about graphene and nanotubes.
• Mark Reed showed some interesting results on top-down fabrication and surface functionalization of Si nanowires for integrated sensors.
  
• Eli Yablonovitch had some thought-provoking points about nanoelectronics and what we should all really be working on. I told him I wouldn't blog about this until he got it written up, so you'll hear more about this from me once it shows up on the arxiv.

Overall a very good meeting, though I regret not bringing hiking boots or having enough time to go skiing in the gorgeous Canadian Rockies.

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.