Monday, May 26, 2008

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.

Sunday, May 25, 2008

This week in the arxiv

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

- 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.

- 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.

Monday, May 19, 2008

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, 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....

Thursday, May 15, 2008

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.

Tuesday, May 13, 2008

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.

- 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.

Saturday, May 10, 2008

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.