Physics virtual swap meet

Now this is a modest proposal whose time has come. It's common that experimental physicists gradually (or not so gradually) accumulate some pieces of equipment over the years that are laboratory "white elephants". These items are typically acquired for some specific research project or direction, and then over the years as research goals and priorities change, they can end up sitting around gathering dust. Surely someone else somewhere could make productive use of these items. Maybe someone should set up a trading post, where we could list these things and arrange reasonable trades or purchases.

Yes, used equipment vendors exist to address these needs, but they're not not always easy to do deal with, and frequently they offer pennies on the dollar. (For example, at Rice we have an 11-year old electron microscope. It's got a problem that is likely to cost about $10K to fix, though annoyingly the microscope vendor refuses to have a reasonable return policy - if we bought the relevant part and that didn't fix the problem, they'd refuse to take it back even for a restocking fee. Two different used equipment vendors have offered around $8K for the whole SEM (!), while they both offer the same item for sale on their sites for more than $100K.) I also know that people buy and sell scientific equipment on E-bay, but that suffers from some of the same problems. The point here isn't for scientists or universities to make money on this - it's to match up scientists/engineers with equipment that they could use and that currently has the wrong home.

Two papers in Nano Letters

Two new papers in Nano Letters caught my eye.

Danilov et al., "Nanoelectromechanical switch operating by tunneling of an entire C60 molecule"
This is a single-molecule electronic experiment, and it's a pretty neat example of using careful measurements to deduce a fair bit of information about a nanoscale system without a direct microscopic imaging probe. In their experiment the authors find bistable switching between two different conducting configurations of a junction thought to contain a single C₆₀ molecule (as inferred by a signature in the electronic conduction of a well-known vibrational mode of the fullerene). Now, telegraph-like switching isn't new by any stretch, and many nanoscale systems exhibit discrete changes in their properties due to motion of one or a few atoms or molecules. Here, by carefully analyzing the voltage and temperature dependence of the switching, they are able to deduce that the most likely mechanism for the change in configuration is motion of the fullerene between two different sites. I'm not sure that I'm 100% convinced by their interpretation, but the data are quite pretty and the analysis is clever.

Stampfer et al., "Tunable graphene single-electron transistor"
In this work, Klaus Ensslin's group starts from a graphene flake (the now-usual scotch-tape approach) and uses standard patterning methods to make a little graphene region connected by narrow graphene constrictions to source and drain electrodes. Using gate electrodes near the constrictions, they could electrostatically shift the local chemical potential there, shifting the graphene from having electrons charge carriers to holes as charge carriers. By further tuning with an additional gate, they could make a complete single-electron transistor - essentially a puddle of confined charges weakly connected by tunnel barriers to larger reservoirs, with the confinement sufficiently strong that the electrostatic energy cost of changing the puddle population by one charge exceeded the available thermal energy. (This was all done at cryogenic temperatures, since the charging energy of the dot was around 3 meV, or about 35 K.). This is a particularly nice, clean example of using graphene and its relatively unique properties as a platform for nanoscale device fabrication. Several groups are getting very good at this, and if efforts to grow large area, high quality, single-layer graphene succeed, there could be some genuine technological implications.

LHC publicity machine

I understand that the folks at CERN feel like it's important for people to be aware of the LHC and get excited about it - at this point, it looks like it's going to be the only game in town in a few years for the frontier of high energy physics. Still, the steady stream of publicity (much of it arguing that they're going to unlock the secrets of the universe, prove string theory, find evidence of extra dimensions, etc.) is getting to be a bit much. Today comes this article discussing the cooldown of the magnets for the collider and the detector. Technologically impressive to be sure, but the whole "colder than deep space" angle is pretty lame - people have been able to reach these temperatures for nearly 100 years, and superconducting magnets are used in thousands of MRI machines the world over. We get it - it's a big machine. If this is the level of publicity hounding that's going on before they even have a single piece of data, the coverage of the actual physics runs is going to be really oppressive.

Scientists, the media, and desperation

I could've predicted this. Given current energy concerns, it's not at all surprising that the various media are ready to give airtime and column space to wacky stories like this one. The temptation must be irresistible: the public is desperate; the story itself is great TV - the lone inventor, persevering in the face of opposition from those stodgy old scientists; they can even put in quotes from the would-be inventor and the scientists and claim to be covering "both sides". You know the drill: "This conventional scientist says that if he drops this pencil it will fall to the ground. Others disagree! The controversy, up next after this commercial message." News flash: sometimes it doesn't make any sense to cover "both sides".

I think the part that frustrates me the most is the misperception by part of the public and some of the media that scientists want these alleged breakthroughs to fail. Nothing could be further from the truth! If someone discovered cheap, inexhaustible energy because of a remarkable revolutionary breakthrough, we'd love it - it'd be the most exciting time in science since the quantum revolution. The problem is, though, that keeping an open mind doesn't mean lowering your scientific standards because you'd like to believe the result. I, for one, am not holding my breath about hydrino power.

slow blogging + pnictide fun

I'll be traveling (work + vacation), so blogging will be slow until July 15 or so.

Before I go, I wanted to point out that the plot continues to thicken regarding the pairing symmetry of the new iron pnictide superconductors. For example, this paper reports scanning SQUID microscopy on a sample of one of the compounds, with no apparent evidence for sign flips in the order parameter that you might expect if the material was, e.g., d-wave like the cuprates. In contrast, this paper argues that scanning tunneling spectroscopy data resemble d-wave expectations. This paper reports photoemission studies showing that the compounds have quite a complicated band structure and suggests that different parts of the Fermi surface may have different phenomenology. That sounds reminiscent of this theory paper, but I haven't read them in detail.

What makes an experiment "good"

Recently I've had some conversations with a couple of people, including someone involved in journalism, about what makes a physics experiment good. I've been trying to think of a good way to explain my views on this; I think it's important, particularly since the lay public (and many journalists) don't have the background to judge realistically for themselves the difference between good and bad scientific results.

There are different kinds of experiments, of course, each with its own special requirements. I'll limit myself to condensed matter/AMO sorts of work, rather than high energy or nuclear. Astro is a whole separate issue, where one is often an observer rather than an experimenter, per se. In the world of precision measurement, it's absolutely critical to understand all sources of error, since the whole point of such experiments is to establish new limits of precision (like the g factor of the electron, which serves as an exquisite test of quantum electrodynamics) or bounds on quantities (like the electric dipole moment of the electron, which is darned close to zero as far as anyone can tell, and if it was nonzero there would be some major implications). Less stringent but still important is the broad class of experiments where some property is measured and compared quantitatively with theoretical expectations, either to demonstrate a realization of a prediction or, conversely, to show that a theoretical explanation now exists that is consistent with some phenomenon. A third kind of experiment is more phenomenological - demonstrating some new effect and placing bounds on it, showing the trends (how the phenomenon depends on controllable parameters), and advancing a hypothesis of explanation. This last type of situation is moderately common in nanoscale science.

One major hallmark of a good experiment is reproducibility. In the nano world this can be challenging, since there are times when measured properties can depend critically on parameters over which we have no direct control (e.g., the precise configuration of atoms at some surface). Still, in macroscopic systems at the least, one should reasonably expect that the same experiment with identical sample preparation run multiple times should give the same quantitative results. If it doesn't, that means (a) you don't actually have control of all the parameters that are important, and (b) it will be very difficult to figure out what's going on. If someone is reporting a surprising finding, how often is it seen? How readily is it reproduced, especially by independent researchers? This is an essential component of good work.

Likewise, clarity of design is nice. How are different parameters in the experiment inferred? Is the procedure to find those values robust? Are there built-in crosschecks that one can do to ensure that the measurements and related calculations make sense? Can the important independent variables be tweaked without affecting each other? Are the measurements really providing information that is useful?

Good analysis is also critical. Are there hidden assumptions? Are quantities normalized in sensible ways? Do trends make sense? Are the data plotted in ways that are fair? In essence, are apples being compared to apples? Are the conclusions consistent with the data, or truly implied by the data?

I know that some of this sounds vague. Anyone more eloquent than me want to try to articulate this more clearly?

New comic

Thanks to Tom for having a link on his page to this. Too true! This is also brilliant. Good to have a laugh, despite the passing of George Carlin. One of my favorite Carlin quotes: [W]e have flamethrowers. And what this indicates to me, it means that at some point, some person said to himself, "Gee, I sure would like to set those people on fire over there. But I'm way too far away to get the job done. If only I had something that would throw flame on them."

No "Singularity" for you.

I wasn't going to even mention the idea of a Singularity, but then the IEEE made a point of dedicating an issue of their magazine to the concept. For those who don't know, the term "Singularity" originates with sci-fi author Vernor Vinge, who has written some compelling novels. Proponents of the concept believe that we live in an era of exponentially accelerating technological change, and that at some point (the Singularity) there will be a complete break in the nature of our species and societies, ushering in what some call a transhumanist future. The technologies typically associated with this idea are (1) Drexlerian molecular nanotechnology, so that we can eliminate scarcity by building anything we want anytime we want via (self-reproducing) nanomachines; (2) immortality via nanotechnological or biochemical control over biological processes that lead to senescence; and (3) strong AI, often including the concept of people uploading their minds to constructed hardware. The thing that continues to surprise me about this idea is that so many people seem to take it so seriously.

Hey, I'm all for optimism, and I'm generally bullish on the future of the species despite current scariness and some scientific arguments, but asserting that we will have a transhumanist utopia in twenty or thirty years is a wee bit of a reach, to put it mildly.

New physics building - suggestions? ideas? horror stories?

My university is in the design phase on a new physics building. This is exciting - first, it's a rare opportunity to design new lab space literally from the ground up. Second, new space will make possible some targeted expansion in the experimental directions in our department as well as in the experimental physicsy part of our electrical and computer engineering department.

Anyone out there have suggestions on building design, particularly with regards to laboratory facilities, utilities, HVAC, electrical service, vibrations, etc.? We're already looking at several recently constructed buildings elsewhere to learn lessons about best practices. If you have thoughts on physics buildings that you think are particularly well done (e.g., the electrical wiring system for the labs at the new nano building at Purdue looks extremely clever and well done), or, conversely, specific examples of design ideas that are lousy in practice or implementation, please post in the comments or email me.

Great scientific workshop

Posting from the scenic Newark International Airport.... I just finished attending the 2008 international workshop, ESPMI-08 - Electronic Structure and Processes at Molecular-based Interfaces, at Princeton University, hosted by Antoine Kahn and David Cahen. For me, this was practically the perfect scientific meeting - about 80 attendees, a mix of theorists and experimentalists, and all the talks were very good and pitched at the right level. I'll write more about this later, but for now, two highlights that show that some things are truly universal.

First, we were having a group discussion about organic photovoltaics and the relevant issues, and it was refreshing to see that everyone, even people who have been thinking about these problems for twenty years, starts out thinking about semiconductor interfaces by drawing the un-coupled materials and then thinking about what happens when they are brought into contact. I know I think this way, but it's reassuring to see that no one can just draw complicated band alignment diagrams freehand.

Second, during this morning's session there was a 1-second brownout/power glitch - the air conditioning shut down and restarted; the computer at the front of the lecture hall rebooted. The part that struck me as amusing was how the Princeton faculty immediately motioned to their grad students/postdocs to run off, or ran off themselves, to check on the lab equipment (particularly the UHV systems). I can totally see myself doing that.

Simple numbers

So, if crude oil futures cost at least $126/42 gallon barrel these days, doesn't that imply that the raw starting material for gasoline already costs (once you factor in the time delay between futures contracts and refining) $3/gallon? This suggests to me that the "correct" price for gasoline in the US should be closer to $5-6/gallon, when the refining catches up with futures. (That doesn't even touch on issues about how much of the crude oil pricing is due to speculation vs. actual supply & demand, or how much of this is due to the effective weak dollar policies of the US central bank.)

Plagiarism at the professional level

Remember my discussion of plagiarism? Remember how a couple of readers didn't seem to thing that this was necessarily that big a deal, particularly if it was "just" background stuff and not actual data? Well, I'd be curious to know what they think of this case. I hope that someone follows through and notifies the editors at the respective journals. Makes you curious about their other publications, doesn't it?

This week in the arxiv: superconductivity update

Summer writing and travel are eating my blogging time a bit, and I've also agreed to write the occasional nano-related blurb for the ACS. While my posting rate has taken a hit, science has continued to march forward, with a lot of exciting new preprints concerning (relatively) high temperature superconductivity. Here's a sampling....

arxiv:0805.4463 - Matsumoto et al., Superconductivity in undoped T' cuprates with T_c over 30 K
This paper is a perfect example of why materials growers are (unfortunately often unsung) heroes in this field. The authors have come up with a new method for growing cuprate compounds of the form T'Re₂CuO₄, where T'Re is a rare earth from the series (Pr, Nd, Sm, Eu, Gd). Historically these compounds were found to be antiferromagnetic insulators - no superconductivity. In this new work the authors argue that these old results were due to interstitial oxygen leading to pair-breaking. Instead, with the new growth + annealing technique, these compounds are found to exhibit superconductivity with transition temperatures as high as 30 K. These subtleties are why one should always be very careful when looking at suggested compositions in new compounds....

arxiv:0805.4630 - Rotter et al., Superconductivity at 38 K in the iron arsenide (Ba_1-xK_x)Fe₂As₂
This is the first paper I've seen (though I may have missed one) that reports superconductivity in a compound related to the new iron arsenide systems but with two iron arsenide layers per unit cell rather than one. Back in the heyday of the cuprates, the same sort of thing happened - people went from compounds with single copper oxide planes to those with multiple planes per unit cell, and transition temperatures went up. Once again we see how rich the materials landscape can be. Update: as anon. in the comments pointed out, this isn't actually the 2-layer version of the compound. Rather, it's analogous to the so-called "infinite layer" version. My mistake.

arxiv:0806.0063 - Wang et al., Very high critical field and superior J_c-field performance in NdO_0.82F_0.18FeAs with T_c of 51 K
Other exciting features of the new iron arsenide superconductors are their extremely high critical fields and critical currents. If the transition temperatures could be raised a bit (say past 77 K) and the compounds could be made in wire form (certainly not easy in the cuprates; unlikely to be simple in these either since like the cuprates they are brittle), this could be a huge deal for high field magnets and other applications of superconductivity.

arxiv:0805.4616 - Chen et al., The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
arxiv:0806.0249 - Matano et al., Spin-singlet superconductivity with multiple gaps in PrO_0.89F_0.11FeAs
These two papers examine two related compounds with different techniques, trying to figure out how the charge carriers in these iron arsenides pair up to form the Cooper pairs that make up the superconducting condensate state. In the former, measurements of Andreev reflection (a process where an electron in a normal metal approaches a superconductor, two electrons actually cross into the superconductor, and a hole is "retroreflected" back into the normal metal, leading to a pronounced feature in the conductance of the metal/superconductor interface) strongly suggest that the samarium compound acts like an ordinary BCS superconductor. That is, each Cooper pair has zero angular momentum (s-wave pairing); this implies that the superconducting gap is uniform in momentum space, with no nodes. In contrast, the cuprates exhibit d-wave pairing, with a superconducting gap that has a four-lobe structure in momentum space and that goes to zero along four particular crystallographic directions.

The second paper uses NMR measurements of the Pr compound to argue instead that there are multiple gaps, and further that the pairing symmetry is p-wave (which has been seen in superfluid ³He and in strontium ruthenate). At first glance, these two results seem to disagree, though (a) they are talking about different materials, and (b) the Andreev measurements are particularly sensitive to the surface, while the NMR measurements are nontrivial to interpret, at least for nonexperts. Well, this is the fun part - stay tuned, and we'll see how this shakes out.

Cold fusion - same old same old.

Once again (and it seems like this happens every couple of years) someone is claiming "success" in a cold fusion experiment. Basically this fellow has made a cell containing some composite of ZrO2 and nanoscale Pd crystals. The claim is that when this cell is filled to moderate pressures (a few bar) with deuterium gas over a couple of days, the cell gets hot (compared to its surroundings) and stays hot for a while (tens of hours), and that ⁴He is detected afterward. Furthermore, the claim is that control experiments with ordinary hydrogen do not produce the long-term heating or helium, and that control experiments without the Pd/ZrO₂ produce no heating at all. People who know next to nothing about nuclear physics argue that the lack of neutrons (from the D+D goes to ³He + n reaction pathway) or gamma rays is fine, since simple p and n counting lets you have D + D goes to ⁴He, despite the fact that the ³He reaction is vastly more favored in ordinary fusion. There continues to be no credible mechanism for getting the D nuclei close enough to each other to get fusion. Now, it's entirely possible that there is weird chemistry going on here, but how come in twenty years of people trying to do this stuff there has yet to be a clean, well-designed experiment done by physicists that is reproducible and actually shows anything interesting? It's grating on many levels that this, an anecdotal discussion of nonconclusive experiments, gets touted online through slashdot, gizmodo, digg, engadget, etc. Extraordinary claims require extraordinary evidence.

This week in the arxiv

Two papers from the past week that caught my eye....

arxiv:0805.3309 - Bunch et al., Impermeable atomic membranes from graphene sheets
This is a nice piece of work from Cornell combining the techniques from three research groups to look at the permeability of single-layer graphene sheets. The authors prepare freely suspended graphene trampolines and apply controlled pressure differences across them. They use scanned probe methods to measure the membrane shape, which ends up being well described by elasticity theory assuming that the elastic modulus for the graphene sheet is about 10¹² Pa (that's big but not unexpected). By watching that shape as a function of time, they can tell how long it takes the pressure inside the chamber (sealed off by the graphene) to equilibrate with the outside environment. Elegant.

arxiv:0805.2414 - Finck et al., Area dependence of interlayer tunneling in strongly correlated bilayer 2d systems at nu(total)=1.
I've written before about two-dimensional electronic systems (2des), and how they are very useful for looking at all sorts of rich physics such as the fractional quantum Hall effect. This experiment looks at a variation on this theme. For a while now it's been possible to make two high quality 2des separated by a thin barrier - thin enough that the charges in one layer can feel the charges in the other layer via the Coulomb interaction. Since like charges repel, if the two layers have the same density of electrons, a favored low energy state would have every electron in the upper layer accompanied by a hole (the absence of an electron) in the lower layer. If the barrier is sufficiently thin, tunneling can take place between the two layers. One fascinating observation has been that this interlayer tunneling, under certain circumstances, can look very much like the kind of Josephson tunneling that one gets between superconductors. One nagging question out there has been whether the very sharp tunneling seen is a bulk effect (and taking place over the whole area where the two layers are tuned to each other) or something else (e.g., an edge effect, like many quantum Hall phenomena). This experiment shows that the tunneling really is proportional to the area, and thus is a bulk effect. This is a tough experiment, requiring great samples, demanding fabrication, and very sensitive measurements at low temperatures.

Public service announcement re: cheating

I want to alert faculty colleagues to a website of which they need to be aware if they teach, particularly undergraduates. I won't link to them since I don't want to drive up their revenue, but it's called cramster.com, and while they bill themselves as a "24/7 study community", what they do is provide links to scanned solution manuals for many many textbooks. What this means is, if you teach a course from a reasonably popular book, you need to be aware that students can and often do buy the homework solutions online. As far as physics goes, they have a rather eclectic assortment. Lots of intro books, and a few major upper level ones (Griffiths; Goldstein; Jackson). If you make up a final exam using problems from the textbook, you're opening yourself up to this problem. If your problem sets contribute a lot to the final grade in a course and you use verbatim problems from the book, again you are almost certainly going to see this on some level. The more you know....

Now that would speed up sample fabrication.

There's no question that one of these would be useful to have in the lab. Check out the whole catalog of their products - fun for all ages.

This week in the arxiv

A couple of interesting papers, two about graphene and one about a weird fluid mechanics effect.

arxiv:0805.1830 - Bolotin et al., Temperature dependent transport in suspended graphene
It's become clear over the last year that a lot of what was limiting the measured electrical transport properties of graphene sheets had to do with interactions between the graphene and the underlying substrate (usually SiO₂). Now multiple groups have started preparing suspended graphene membranes (supported around the edges by oxide) overhanging underlying gate electrodes. By ramping up the current through the suspended membrane, the graphene sheet can be resistively heated in vacuum up to a temperature sufficient to desorb residual contaminants, and electronic properties can be measured without substrate effects. In this paper the Columbia group demonstrates that extremely high mobilities are then possible (well over 100000 cm²/Vs), and by examining the temperature and gate dependence of the conduction they can understand the scattering mechanisms at work as well as residual disorder in the system. Very clean looking data.

arxiv:0805.1884 - Booth et al., Macroscopic graphene membranes and their extraordinary stiffness
The Manchester group has also been very busy. In this paper they show a cute technique to produce large (say 0.1mm in diameter) graphene sheets in a form that's easy to suspend and handle. Basically instead of abrading or cleaving graphite into graphene on top of oxidized Si, they do so on top of Si coated with a layer of e-beam resist. An additional layer of a different sensitivity resist is put on top and patterned, followed by metal deposition. The metal layer forms a frame that goes around the previously identified graphene sheet, and the metal is then used as a seed layer to deposit a more robust Cu layer via electrochemistry. Finally, the original resist layer is dissolved, freeing the graphene+Cu frame for manipulation. They then further study the mechanical properties of these suspended layers, finding that single sheets of graphene are indeed very stiff - much more so than you might think, since they're 1 atom thick. The technique is elegant, and there is one particularly impressive TEM image. Nice SuperSTEM that they have over there in Cheshire.

arxiv:0805.0490 - Amjadi et al., A liquid film motor
Hat tip to arxivblog for pointing this out to me. These folks at Sharif University in Iran have found that DC electric fields can make soap films flow in very interesting and controllable ways. They suggest a few possible mechanisms for this kind of electrohydrodynamic motion, but conclude that none of them are entirely satisfactory. The paper has a minor rendering problem with Fig. 4, but you should definitely watch the movies on their webpage. Very dramatic! Soft CM physics can be inspiring - here's a visually impressive phenomenon that might actually be useful in fluidic applications, and the whole experiment is simple, elegant, and inexpensive. No exotic apparatus required.

The fun parts

In contrast to the previous post, there have been some fun parts of the job lately. Today was commencement, which is always amusing - I get to play dress-up and look like a real academic. If only point 4 in this list was true, then commencement would be much more exciting.

In the lab we've had some genuinely weird data come along, and that can be fun, too. In one kind of structure we're observing a phenomenon that is completely reproducible but for which we have essentially no sensible explanation. We've been messing around with this for a month, and every time we come up with a plan, thinking we know what's going on, nature turns around and proves us wrong. Whatever is going on, it seems interesting. When we figure it out enough to write it up, I'll discuss it further here.

Lastly, after a trip to the movies last week I had the shocking realization that Rice is now partnering with Stark Industries. Sweet. I need to get one of those flying suits.

Copying text without attribution is plagiarism.

Amazingly, there are graduate-level students out there who do not understand this simple, basic fact. When you're writing a scholastic or scientific document, you never copy other people's words - certainly not complete verbatim sentences - without clear attribution and indication that you're quoting someone else. You just don't. Ever. Doing so is plagiarism, and as any kind of professional you should know that it's wrong. Amazingly, some students don't seem to get this point, even when they've been told about this, explicitly, repeatedly, and actually signed documents attesting that they understand this, and when they know that the professor can use this amazing tool called google to figure this sort of thing out.

Just. Don't. Do. It.