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!

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.

Tagged.

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....)

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.

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.

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.

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.