Monday, March 31, 2008

Yet more superconductivity fun.

Even more excitement on the novel FeAs-based superconductors. By my count there are seven more papers (0, 1, 2, 3, 4, 5, 6) on tonight's arxiv update, including two different groups demonstrating transition temperatures exceeding 50 K in the Nd version of the compound, and one showing 52 K in the Pr version. Gee, this makes my comments here look prescient. For my next trick, again guided by the periodic table, I suggest that we'll see more rare earth variations. For example, there's no reason not to try the comparatively stable actinides (thorium, protactinium, uranium), or the mostly-filled-f-shell lanthanides (thulium, ytterbium, lutetium) as opposed to the mostly-empty-f-shell ones (La, Ce, Pr). Given that pressure boosts Tc (see paper 1 above), one could try duplicating the effect of pressure by creating more internal stress within the lattice via substitutions of larger atoms between the FeAs layers. Of course, it's easy for me to say this stuff, since I don't actually have to make the compounds....

It's also worth noting that early photoemission data and heat capacity measurements on a couple of the compounds strongly suggest that the superconducting gap is zero (or darned close to it) on at least part of the Fermi surface. This is the case for the cuprates, and exactly not the case in conventional low temperature superconductors.

Sunday, March 30, 2008

Outreach can be fun.

Yesterday I did an "Ask a scientist" event at the Children's Museum of Houston, as part of their Nano Days events. It was fun. I started with the obligatory "sizes of things" slide, and then talked a little about scientists (who want to figure out how things work) and engineers (who want to take what we've learned and make new, useful things). To emphasize that nanoscale tech was all around them, I showed the guts of a Nintendo Wii, including the cell processor and the little accelerometer chip that the Wii uses to figure out what you're doing with the controller. I had made a demo accelerometer prop out of PVC pipe and springs that was a big hit. The most fun part was when I invited the kids up to help me take apart a Wii-mote, while I explained that sometimes breaking things down is the best way to figure out how things work. The high point: an eight-year-old whispering "Awwwwsome!" to his friend after playing with the guts of the Wii-mote. (Note: opening up any Nintendo gear requires this kind of screwdriver....)

A nice piece of physics with an elegant consequence

Kuzmenko et al. from Geneva have a PRL out this week titled "Universal Optical Conductance of Graphite". This is a pretty physics result, where the authors find theoretically and demonstrate experimentally that the optical conductivity of graphite is quantized - each graphene sheet has, at optical frequencies, a sheet conductance that is (\pi/2) e2/h. This is a consequence of the particular band structure of graphene, and I think it's rather impressive that this is robust even when there are multiple graphene layers. You might imagine, a priori, that the interlayer coupling would kill this kind of universality.

Nair et al., in a preprint, arrive at essentially the same result, but present the data in a much more dramatic way that does a great job of emphasizing the consequences of the physics. Many people don't have a good intuition for what optical conductivity means. Nearly everyone, though, has a decent sense of what optical absorption means. These folks demonstrate that the quantized optical conductivity implies that the white light absorption of graphite is quantized (!) in units of the fine structure constant (!!), so that each additional graphene layer absorbs 2.3% of the light incident on it, even though each layer is just one atom thick. The figures in the paper, particularly the last one, do a great job of making this point.

The take-home message about presentation: having a compelling physics story to tell is good, and casting it in terms that a general audience can appreciate with some intuition is even better.

Tuesday, March 25, 2008

The superconductivity fun continues

Another bunch of papers on the arxiv (1, 2, 3, 4, 5). The last one is particularly interesting - the group reports that replacing lanthanum with samarium boosts Tc up to 43 K. This is the first non-cuprate with a transition temperature that high. Again, it's a long way from room temperature, but the fact that there's a system besides the cuprates that shows transition temperatures this high is exciting. This may give us more clues to the mechanism at work - what is the normal state like? Are these doped Mott insulators? Do they have a pseudogap? What is the pairing symmetry?

Friday, March 21, 2008

A new (apparently unconventional) family of superconductors

I heard about this at the March Meeting, and now it looks like things are picking up steam. There are a number of papers that have started appearing on the arxiv (1, 2, 3, 4, 5, 6, 7 update 8, 9, 10) about a new high temperature superconductor based on the parent compound LaOFeAs. This material has FeAs planes rather reminiscent of the CuO planes in the copper oxide superconductors. It's quite unusual to have an iron-based superconductor, since ferromagnetic correlations are usually associated with killing ordinary superconductivity. More exciting is the fact that this is not directly related to the cuprates and when doped with electrons (by replacing some of the oxygen with fluorine) it has a clear superconducting transition at 28 K. There are indications already that this is an unconventional superconductor, and third-hand rumors suggest that higher Tc values are on the way. It'll be interesting to see where this leads!

Wednesday, March 19, 2008


This is very amusing.

Endowed lectureships

One nice thing about being in a good department that rarely gets discussed is the quality of visitors. We're able to get a good stream of top-notch speakers for colloquia and seminars, and that is very important for maintaining an intellectually rich atmosphere for both the faculty and the students. On top of the usual calendar, we also have a couple of named, endowed lectureships. For example, every year we have a public lecture (followed the next day by a physics colloquium) in honor of William V. Houston (pronounced "how-ston"). The Houston lecturers are usually Nobel Laureates and their visits are very fun. This week we had George Smoot of cosmic microwave background fame, and once again I was reminded how much more we know about cosmology now than when I entered college.

Friday, March 14, 2008

March APS Meeting wrapup

I returned yesterday evening from the March Meeting, and spent much of today helping out with our graduate recruiting weekend for both my department and the applied physics graduate program. Hence the delayed blogging.

My last day at the March Meeting was spent largely flitting from session to session. I saw a very nice pair of talks by David Cobden and one of his students from Washington, showing measurements of the metal-insulator transition in VO2 nano-beams. Vanadium dioxide is allegedly a Mott insulator in its low temperature state, meaning that the on-site repulsion of the d orbitals of the vanadium is so strong and the electronic population is just right so that the whole correlated system is frozen. A bit above room temperature (around 65 C) VO2 becomes metallic, and there's been a lot of interest in understanding the transition, which is accompanied by a lattice distortion. In the new work, suspended beams of the oxide are observed in an optical microscope while the transition is examined. There is optical contrast between the two phases, so one can determine how much of the beam is in each phase in the coexistence region. Moreover, the elastic properties of the beam allow them to infer much information about the phase diagram for the transition, and offer some hints in conjunction with conductance measurements that the metal/insulator transition may be separate from the structural transition.

After this, I went off to the session on charge and orbital ordering to give my own talk about our magnetite results. Then I headed over to a session on molecular electronics. Finally, I ended up over near a focus session on nanotechnology, where there were a couple of nice talks on fabrication methods.

Overall, it was a good meeting - as good as these things usually are. Most of the talks that I saw were pretty decent, and I had some useful conversations with lots of colleagues. Only once or twice did it occur to me that sessions could be more pleasant if someone replaced the usual oven timer for pacing talks with either a giant gong or perhaps one of those big hooks used to pull people off stage in bad vaudeville skits.

Thursday, March 13, 2008

March APS Meeting III

Day 3 in New Orleans continued to be interesting, though I missed some talks so that I could have conversations with a few people, including my program officers from a couple of funding agencies. It's never a bad idea to make sure that the program officers know what you've been doing with their resources.

I started out the day by catching an invited talk by Doug Scalapino talking about his take on the binding "glue" in the high-Tc superconductors. Scalapino uses "glue" to refer to the retarded (time-delayed) interaction that leads to pairing of the electrons. In the low temperature superconductors, the glue in this sense is the retarded phonon interaction - in a sense, one electron leaves behind a lattice vibration that slightly deforms the ion charge distribution, leading to a second electron of opposite momentum to feel a slight residual attraction to the first electron. The screened Coulomb interaction between the electrons is effectively instantaneous (and repulsive). In the high-Tc case, it's not clear what the glue is. Scalapino would argue that it's a spin fluctuation interaction; Phil Anderson would probably argue that there is no important glue in this sense of the term.

I then chaired my session, which was fun but tiring. One particularly cute experiment was from the Weig/Kotthaus group at Munich. They are trying to use nanomechanical resonators as charge shuttles. The idea is a bit like a bucket brigade. Have a metal island be suspended on a resonant beam between a source and a drain electrode. When set up ideally and driven at resonance, the island will swing back and forth between the source and drain like the clapper between the bells of an old alarm clock. When the island gets close to the source, an electron can tunnel onto the island. Ideally Coulomb blockade would ensure that it's one and only one electron. Then the island can swing over to the drain electrode, and drop off that electron. The experiment was elegant - they make many resonators at once and wire them all up in parallel. The clever bit is that they have each resonator tailored with a different mass, so that they can selectively drive just the one that they want. They drive mechanically, by shaking the whole chip back and forth, to avoid electrical crosstalk trouble. It should be very nice when they can get the structures even smaller and colder, to see strong Coulomb blockade effects.

Tuesday, March 11, 2008

March APS Meeting II

The March Meeting continues. Other topics that seem relatively hot (based on the number of abstracts) compared to previous years include thermoelectrics and ultracold gases and fluids. The latter are really at the border between condensed matter and atomic/molecular/optical physics, and it's interesting to see the merger of the two disciplines. While the ultracold gases provide an exquisitely clean, tunable environment for studying some physics problems, it's increasingly clear to me that they also have some significant restrictions; for example, while optical lattices enable simulations of some model potentials from solid state physics, there doesn't seem to be any nice way to model phonons or the rich variety of real-life crystal structures that can provide so much rich phenomenology.

Anyway, I saw some very pretty talks today. Taking the prize for coolest graphics in a presentation were definitely two talks from an invited session on Kondo physics. The first was by Andreas Heinrich, giving an overview of the IBM Almaden's use of scanning tunneling microscopy to examine magnetic anisotropy and Kondo physics on the single atom level. The second was by Hari Manoharan of Stanford, who showed around three experiments, the most elegant of which involved using STM of magnetic atoms to demonstrate that sometimes it's possible to really extract phase information about superpositions of quantum states. Basically he showed that one could make a designer system (an elliptical corral that confines the Cu(111) surface states) and then use STM spectroscopy based on the Kondo properties of Co atoms on the Cu(111) surface to identify specific superpositions of the eigenstates of that corral.

Another interesting series of talks took place in a session that I organized, where Lindsay Moore of the Goldhaber-Gordon group at Stanford discussed some recent studies of the so-called "0.7 anomaly". In 2d electron gas, it is possible to use gates to create a 1d constriction for a small number of electronic modes. This is called a quantum point contact (QPC). In zero magnetic field, as the point contact is pinched off the conductance of the QPC drops in quantized steps of 2e2/h until it falls to zero. The 0.7 anomaly is the appearance of an extra plateau in the conductance at around 0.7 x 2e2/h. People have been bandying about possible explanations for this feature for a while now, and finding new probes to apply is a popular tactic. The following contributed talk was by Alex Hamilton from UNSW, who had looked at the 0.7 anomaly in 2d hole systems. The holes have strong spin-orbit scattering effects that, through the study of response to applied magnetic fields, allow one to demonstrate convincingly that the 0.7 anomaly clearly has some mechanism related to spin. Nice.

Monday, March 10, 2008

March APS Meeting I

Yes, it's that time of the year again, when I get together with 6500-7000 of my closest colleagues and talk physics until our brains are full and it's time to leave. This year the March APS Meeting, the big US national meeting of (mostly) condensed matter physics folks, is in New Orleans. So far the biggest topic at the meeting by a wide margin seems to be graphene, just like last year. There are various divisions of the APS, including the Division of Condensed Matter Physics (DCMP), the Division of Materials Physics (DMP), the Division of Chemical Physics (DCP), and the Division of Polymer Physics (DPOLY). Each division sponsors Focus Topics designed to appeal to their membership and centered around hot ideas of the moment. One challenge in laying out the meeting is coordinating all of their Focus Topic sessions and invited sessions so that we don't end up with what seems to happen every year: head-to-head competition of researchers in a hot field speaking at the same time on similar subjects in different sessions at the meeting. Like this morning, when DMP had "Graphene Transport" at the same time as DCMP's "Electronic Properties of Graphene and Related Structures", or 3.25 hours later when DMP had "Graphene, Graphite, and Related Structures" at the same time as DCMP's "Graphene Transport II". Ahh, coordination.

I spent most of my time in sessions that I helped to organize, and I saw some interesting talks on STM work and single molecule electronic measurement techniques. One talk by Anping Li of Oak Ridge gave me a classic case of stainless steel envy. He has put together a variable temperature 4-probe (!) ultrahigh vacuum scanning tunneling microscope with built-in UHV scanning electron microscope and electron analyzer for scanning Auger microscopy, as well as an integrated UHV deposition chamber with built-in electron diffraction. Wow. Now that's a cool toy! That was followed by a very interesting set of measurements from a group at the University of Tokyo, looking at the electrical properties of truly 2d atomic layers of indium on Si. It's quite fascinating how the temperature dependence of such a film can be changed from metallic (better conduction at low temperatures) to insulating just by the introduction of a very few defects; this is a great demonstration of localization physics.

Sunday, March 09, 2008

Another physicist in Congress

Thanks to a special election to fill the IL-14 seat of former Speaker of the House Dennis Hastert, there is another physicist in Congress, Bill Foster. Good for him. We need more civic scientists.

Friday, March 07, 2008

This week in cond-mat

Super-brief pre-March Meeting blogging. Earlier this week there were five papers that particularly caught my eye on the arxiv. The first three are closely related....

arxiv:0803.0562 - Kiguchi et al., Highly conductive molecular junctions based on direct binding of benzene to Pt electrodes
This paper is from the always impressive van Ruitenbeek group. Here they demonstrate a chemical method of bridging the nanoscale gap between two movable Pt electrodes (in a geometry called a mechanically controllable break junction) by a benzene ring, with direct C-Pt bonds. The result is a junction that has a conductance approaching the conductance quantum, 2e2/h. This is impressive because achieving such strong electronic coupling in single molecule junctions has been challenging in the past without special transport mechanisms (like the Kondo effect). They prove that they have the desired structure by looking at vibrational signatures in the tunneling conductance at finite bias. By comparing regular 12C benzene devices and those made with 13C, they see a distinct isotopic shift in the vibrational modes - the molecule with heavy carbon has lower vibrational frequencies. They also use sub-gap structure in the tunneling (with the electrodes driven to superconduct by the proximity effect, as in this paper) to show that the conductance comes predominantly from a single, highly transmissive channel. Very nice.

arxiv:0803.0582 - Hybertsen et al., Amine-linked single-molecule circuits: Systematic trends across molecular families
This paper summarizes a large and very pretty body of experimental work done by Latha Venkataraman et al., with complementary theory calculations by Hybertsen and collaborators. Using
the STM equivalent of a mechanical break junction, these folks have made comprehensive studies of single-molecule conductance by compiling histograms of tens of thousands of conductance measurements in various junction configurations. This is a nice review of the work, and is an invited paper that is part of a forthcoming special issue of Journal of Physics: Condensed Matter. Our group also has a contribution to that issue.

arxiv:0803.0710 - Prodan and Car, Tunneling conductance of amine-linked alkyl chains
This is a new theory paper that examines one subset of the devices mentioned in 0803.0582. The neat thing about this is that this work uses a novel approach to density functional theory to do the transport calculations.

Changing the topic,

arxiv:0803.0719 - Marini et al., Fluctuation-dissipation: response theory in statistical physics
This is a long, comprehensive review article about the deep connection between equilibrium fluctuations and nonequilibrium dissipation. The classic example of this is Johnson-Nyquist noise, the voltage noise in a resistor that results from thermal fluctuations of the electron distribution, and its relationship to the actual resistance that determines dissipation when current flows. I need to find the time to read through this in detail - it looks like a real resource.

arxiv:0803.0568 - Wenzler and Mohanty, Measurement of Aharonov-Bohm oscillations in mesoscopic metallic rings in the presence of high-frequency electromagnetic fields
This is another experiment in an area that I continue to find interesting, the challenge of inferring information about the quantum coherence of electrons in solids. As the intro to this paper reminds, there are ambiguities in how various quantum corrections to electronic conduction define the coherence length - the characteristic distance scale that an electron can travel before its quantum mechanical phase becomes ill-defined due to "decoherence mechanisms" (inelastic interactions that somehow change the state of the environment). This paper examines one such correction, the Aharonov-Bohm effect, when electromagnetic radiation is introduced at a frequency related to the coherence length.

Wednesday, March 05, 2008

Reviewing- why, how, and how often?

I review lots of papers and proposals for various journals and funding agencies. While time allocation is a continual challenge, and while there is no good framework for rewarding this kind of professional service, I think it's important to do my fair share for several reasons. First, I'd like someone else out there to do me the same courtesy - well written, thorough, timely referee reports almost always improve the quality of scientific papers. Sometimes it's just a matter of the referee having fresh eyes and a different perspective; a referee can point out that something which seems obvious to you may not be clear to others. Second, reviewing is a way to keep abreast of what's going on out there in the community. Third, reading articles and proposals and having to write reviews is intellectually stimulating - it gets me to think about new things and areas that aren't necessarily my primary interest.

When writing a report, I try to produce something that's actually useful to the authors (as well as the editors in the case of journal articles). I briefly summarize the main points of the paper or proposal to indicate that I've actually read it and understand the key ideas. For a paper, I emphasize my overall opinion of the work. Then I point out anything that I found unclear or any parts of the argument that don't seem supported by the data or calculations, with an eye toward what would improve the manuscript. I rarely reject papers out of hand, since I rarely get manuscripts to review that I think are hopeless (though some are submitted to inappropriate journals). On the other hand, it's pretty rare that I think something is absolutely flawless (though if my comments are minor I don't ask to see the paper again). I truly don't understand why some people submit two-sentence referee reports that are dismissive - this doesn't help anyone. I also don't understand why a small number of people can be venomous in reviews. Ok, so you didn't like the work for some reason - why get nasty? Just explain rationally why you don't think the paper is right. The point of refereeing is not to fire off insults under the shelter of anonymity - that's what blogs and internet forums are for.

Proposals can be more work. The big questions are usually (1) Does the PI clearly articulate the science or engineering question that is under investigation? (2) Is the plan well considered and likely to lead to good science? (3) How much of this is new and how much is completely incremental? (4) Did the PI(s) include everything that they were required to (e.g., description of prior work, for NSF discussion of outreach and education)? Grade inflation in proposal refereeing makes this process more painful as well. I am well aware that labeling a proposal as merely "good" is the kiss of death.

I have a tough time saying "no" to refereeing requests, and I need to get better at it. Prompt refereeing is important, and it's better to decline to review than to sit on a manuscript for two months. Still, good refereeing is definitely needed. I'm sure many readers have had the experience of a manuscript sitting with editors for a long time because it's tough to find qualified reviewers with the right expertise who also have enough time to review promptly. It is a shame that there isn't some intelligent mechanism for rewarding refereeing. It shows up as a line or so on your CV, even though it's arguably more valuable than serving on some university committees. Ahh well. Off to write some reviews.