Sunday, December 31, 2006

This week in cond-mat

Two papers for this end-of-year post.

cond-mat/0612556 - Vartiainen et al., Nanoampere pumping of Cooper pairs
The single-electron transistor was developed almost twenty years ago, based on the observation that one could now fabricate a metal island (weakly coupled to leads via tunnel junctions) so small that it's capacitive charging energy could significantly exceed kT (this gets easier as T is lowered to within a fraction of a Kelvin of absolute zero, which is now readily achievable). In such a device, the charge on the island is generally well-defined and quantized to an integer number of electrons. By cleverly hooking islands together and cycling gate voltages appropriately, it's possible to make an electron "turnstile", such that one electron at a time may be pumped through the circuit. Doing this at high frequencies, f, would enable (ideally) a noiseless current source (with current ef). That's easier said than done, however, because the intrinsic RC charging timescales of such turnstiles tend to limit the frequency of operation. The Finnish group here has implemented an alternative scheme, using superconducting quantum interference devices (SQUIDs) rather than simple tunnel barriers, and can pump individual Cooper pairs of superconducting electrons through their circuit at a high enough rate to generate nanoamperes of current. This is very impressive, and could lead to real advances in metrology.

cond-mat/0612635 - Pereira et al., Kondo screening cloud and charge quantization in mesoscopic devices
In the Kondo effect, a localized spin coupled to mobile electrons undergoes a spin-flip scattering process that leads to spin correlations in the mobile electrons. At temperatures small compared to the characteristic energy of this process, the local spin is "screened" - that is, it is entangled with a cloud of the mobile electrons, forming a singlet state with no net spin. A question that has been around a long time in the solid state community is, how big is that screening cloud? The only successful attempts to measure the size have been in STM measurements of magnetic impurities on surfaces, as far as I know. In this paper, the authors propose a clever scheme to try this in a model system. One can have the local spin be living in a quantum dot, and use a electrons in a large 1d electronic box instead of truly free electrons to form the Kondo state. The idea is that the size of the Kondo cloud will be detected by looking at the single-particle levels of the 1d system (and varying system effective length). Neat, though tough to do!

Friday, December 29, 2006

Great rhetorical device

From an exchange between CNN's Whitehouse correspondent and a Frances Townsend, the Assistant to the President for Homeland Security and Counterterrorism:
HENRY: You know, going back to September 2001, the president said, dead or alive, we're going to get [Osama bin Laden]. Still don't have him. I know you are saying there's successes on the war on terror, and there have been. That's a failure.

TOWNSEND: Well, I'm not sure -- it's a success that hasn't occurred yet. I don't know that I view that as a failure.
Wow. Cool! I need to start using language that way. My Nobel Prize in Physics is a success that hasn't occurred yet.

Friday, December 22, 2006


This is my first introduction to silly blog games, which I suppose shows that I don't blog too much, since it's taken this long. Anyway, I've been tagged. In this game I'm supposed to grab the nearest book, go to page 123, go to the fifth sentence, and write down the next three sentences. Then I'm to tag three more people, presumably ones that I think will play the game. Hmm. Well, on holiday break the nearest book to me right now is The English Assassin by Daniel Silva. Here are the sentences:
He screamed at the room service boys when they didn't bring his coffee quickly enough. Soon the entire staff and most of the guests at the Hotel Laurens knew about the crazy Boche writer in the attic. On the way to Paris, he had stopped at the airport in Nice, dropped off the rented Mercedes, and collected a Renault.
As for tagging, I suppose I'll go with Rob, the Incoherent Ponderer, and the Female Science Professor. (Wolff, Bernie, I'll get you some other time....)

Saturday, December 16, 2006

This week in cond-mat

Just one paper this week. End-of-semester crunch + trying to write up some new stuff in my group is cutting into my blogging....

cond-mat/0612278 - Jeltes et al., Hanbury Brown Twiss effect for bosons versus fermions.
Hanbury Brown and Twiss did a beautiful experiment using light that has since been extended to examine the quantum statistics of other kinds of particles. Consider a source of particles and a couple of detectors. For Bose particles, the symmetry of the wave function under exchange of the particles implies that particles will tend to bunch. In handwavy language, the Bose distribution favors particles to be in the same state rather than different states, all other things being equal. HB and T showed this bunching in space for photons. Conversely, because of Fermi Dirac statistics (the Pauli principle), fermions tend to anti-bunch. All other things being equal, fermions tend to avoid each other. This antibunching has been seen in electrons in solids as well as in free electrons. The authors of this paper have done a beautiful version of this experiment with cold atoms, using the same trapping setup to look at either 3He or 4He, which are chemically identical but possess Fermi and Bose statistics, respectively. They use a multichannel plate detector to look at the positional correlations between pairs of atoms when they hit the detector, and see the expected HB-T correlations. Extremely clean, like all good atomic physics experiments.

Saturday, December 09, 2006

Rumor mills: don't trust 'em.

Since I'm heavily involved in our faculty search, I can't really talk too much about it online. However, I do want to point out something about rumor mill sites. These have existed for about a decade in the high energy and astrophysics communities, and for the last three years or so there has been a condensed matter/AMO rumor mill site as well. The idea is that this is a way for the candidates (primarily) to keep track of who is interviewing for the various jobs, and who is getting offers. The problem is, these sites are only as good as their sources of information. Now that just about every department puts their seminar calendars online, it's not hard to go through the schedules and try to figure out who is giving job talks. Of course, you have to use a bit of common sense, too. As a cautionary example: yesterday the CM/AMO rumor mill listed two candidates allegedly on the short list for our position. However, (a) both are theorists, (b) one is an AMO person, and (c) our search committee hasn't even had its first meeting yet. Doh! The lesson: don't take the rumor mills very seriously. They can be very wrong, have no fact-checking, and in principle can be manipulated via disinformation.

Saturday, December 02, 2006

This week in cond-mat

Two papers on the arxiv this week that give me an excuse to talk about the standard picture of metals and how bits of it can fail.

cond-mat/0611714 - Pisana et al., Born-Oppenheimer breakdown in graphene
The Born-Oppenheimer approximation is one of the most commonly made in quantum mechanical treatments of atoms, molecules, and solids. It's a specific example of the adiabatic approximation: if the potential energy term V(t) of the single-particle Schroedinger equation changes slowly enough (basically compared to \hbar divided by the energy difference between the single-particle energy levels of the system at some instant in time), then it's ok to say that the true single-particle solutions are well approximated at time t by the solutions to the static Schroedinger equation with V = V(t). The Born-Oppenheimer approximation applies this to electrons around atoms. It assumes that the atoms move slowly compared to the electronic energy timescales, so that one can do calculations of molecular (for example) states by assuming that the ions are fixed in space. This paper reports Raman scattering measurements of the vibrational modes of graphene as a function of gate voltage (and hence electronic density). What they find is that the electronic population affects the lattices vibrational modes in a way that violates the Born-Oppenheimer approximation. I haven't read this very carefully, but this is interesting and surprising, at least to me. Given how well the basic graphene electronic structure can be approximated by a simple tight-binding calculation, a big violation here seems weird.

cond-mat/0611724 - Qazilbash et al., Correlated metallic state of vanadium dioxide
The mean free path is a simple concept: it's the average distance a particle travels before scattering off of something. For a classical gas of hard spheres, the mean free path would be the inverse of (number density times cross-section). For quantum mechanical electrons in a metal, the electrons scatter off anything that breaks the periodicity of the crystal lattice - grain boundaries, defects, impurities, distortions of the lattice due to phonons. The mean free path in a metal is typically found from the conductivity, via something called the Einstein relation. Tacit here is the assumption that the electrons behave like well-defined particles that can propagate along for a while between scattering events. Indeed, a general requirement for the validity of this quasiparticle picture for electronic states in a metal is that the ratio of the mean free path to the wavelength of the electron is much greater than one. If the electron scatters many times before even traveling one wavelength, obviously the traveling wave picture of the electron is not valid. The point of this is that there is a physical lower limit to the mean free path: in a "good metal", the mean free path should never be shorter than the lattice spacing between atoms. This is called the Ioffe-Regel-Mott limit.
Now look at vanadium dioxide, which has a transition at 340 K between a high temperature metallic phase and a low temperature insulating phase. The phase transition is complicated, and includes a change in the unit cell shape. The authors of this paper have used optical techniques to infer the frequency-dependent conductivity in both phases. They confirm that the Ioffe-Regel-Mott limit is violated in the metallic phase at high temperatures, and they infer that the dominant scattering mechanism is due to electron-electron interactions. Basically this is one more nice piece of evidence that VO2 is a "bad metal", in which the quasiparticle way of thinking about distinct electrons isn't really valid.

Tuesday, November 28, 2006

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.

Friday, November 24, 2006

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.

Tuesday, November 21, 2006

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.

Saturday, November 18, 2006

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.

Thursday, November 16, 2006

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.

Sunday, November 12, 2006

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.

Tuesday, November 07, 2006

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.

  9. 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.

Monday, November 06, 2006

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.

Sunday, October 29, 2006

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.

Saturday, October 28, 2006

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.

Wednesday, October 18, 2006

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).

610413 - 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.

Friday, October 06, 2006

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.


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."

Thursday, October 05, 2006

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.

- 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.

Thursday, September 28, 2006

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.

Tuesday, September 26, 2006

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.

Monday, September 25, 2006

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.

Saturday, September 23, 2006

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.)

Friday, September 15, 2006

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.

Monday, September 11, 2006

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.

Friday, September 01, 2006

...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?

Wednesday, August 30, 2006

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?

Monday, August 28, 2006

...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.

Thursday, August 24, 2006

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.

Wednesday, August 23, 2006

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....

Tuesday, August 22, 2006

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.

Wednesday, August 16, 2006


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?

Sunday, August 13, 2006

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

One particular paper caught my eye this week:
- 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.)

Tuesday, August 08, 2006

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.

Wednesday, August 02, 2006

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?

Sunday, July 30, 2006

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.

Monday, July 24, 2006

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?

Sunday, July 23, 2006

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.