## Wednesday, February 25, 2009

### I'll bet that this looks good to the NSF as outreach....

Here is a neat little video of Prof. Jim Kakalios, who acted as a science advisor on the forthcoming Watchmen movie. Watchmen is a graphic novel by Allen Moore, set in a dystopian alternate 1985, in a world where there really are "costumed vigilantes". The story looks at what kind of people would dress up in costumes and fight crime (answer: damaged people), and what it would do to society if there really was a single being (who happens to be American) with godlike superpowers.

Anyway, it sounds like Prof. Kakalios had a great time, and helped the movie producers get certain things to look right (e.g., a 1959 physics lab; a 1985 physics lab; equations on chalkboards in the background). This sort of thing seems like it would be great fun. So, to all you Hollywood producers out there, call me :-)

## Saturday, February 21, 2009

### This week in cond-mat

I know that it's been a while since I've done one of these. It's not because of a lack of interesting papers on the arxiv; rather, it's been entirely due to my own lack of time. On to a couple of interesting papers from this week....

arxiv:0902.3014 - Miroschnichenko et al., Fano resonance in nanoscale structures
This is an article intended for Reviews of Modern Physics that takes pedagogical, unifying look at Fano resonances, particularly in nanostructures. (I've linked to the version with high-res figures.) A Fano resonance is a particular kind of (in general) asymmetric resonance lineshape that results from interference between, for example, [direct transmission into a continuum of states] and [transmission involving scattering from a resonant level]. The end result is a resonance lineshape that can look like resonant transmission, resonant absorption, and a variety of asymmetric shapes in between. Originally proposed by Ugo Fano to explain phenomena in atomic physics, Fano resonances are all over the place in nanoscale systems. This paper actually gives about as nice a pedagogical description of this physics as you're going to find.

arxiv:0902.3305 - Deshpande et al., Spatially-resolved temperature measurements of electrically heated carbon nanotubes
A major issue in nanoscale electronic transport experiments is the question of dissipation and energy relaxation. By applying a voltage across a nanostructure in a measurement of conduction, one is driving the electronic distribution (which electronic states are occupied as a function of energy or momentum) out of thermal equilibrium. Eventually, the electrons rethermalize, transferring their energy to one another and to the vibrational modes of the material in question. How this happens in detail can be quite complicated, and nonthermal distributions of electrons and vibrations can exist over relatively long distance scales (say hundreds of nanometers at low temperatures). These folks have been able to use scanning Raman microscopy to map out the local lattice temperature of carbon nanotubes as current is passed through them. In this case they are looking at the shift of a particular nanotube vibrational mode, and using that as an effective thermometer. It's a pretty experiment that demonstrates how much we can learn by combining electronic measurements with complementary techniques.

## Friday, February 20, 2009

### Nice animation re: the credit crisis

I just saw this (link to video), and I was quite impressed. Mr. Jarvis (the animator) has a real pedagogical gift. I wonder if one could make videos this cleanly done about CM physics....

## Wednesday, February 18, 2009

### What is a phonon?

In hindsight, I suppose that I should have addressed phonons earlier. A phonon is a quantized sound wave - a collective vibrational mode of a solid (or liquid). In a crystalline solid, the idea is that the atoms in the solid are displaced, at any given instant, from their equilibrium positions. For a single phonon, the instantaneous displacements are periodic in space (that is, there is some wavelength, where atoms separated by an integer number of wavelengths are displaced the same amount). The displaced atoms feel restoring forces due to their interactions with neighbors, and will tend to oscillate in time around their equilibrium positions. When the wavelength is much longer than the interparticle separation, the frequency of those oscillations times the wavelength gives the speed of sound for the material - phonons propagate along at the speed of sound. In general, the speed of sound can depend on the direction of propagation as well as the direction of the direction of the displacement. If the displacement is along the direction of propagation, the sound is longitudinal; if the displacement is normal to the direction of propagation, the sound is transverse.

The quantum nature of phonons comes in when one discusses their energy content. In a classical mechanical oscillator, you can dump in as much energy as you want; the energy content is proportional to the square of the amplitude of the oscillation, and that can be varied continuously. In a quantum mechanical oscillator of frequency f, the energy content of that oscillator can only take on discrete values, (n + 1/2)hf, where n is a nonnegative integer. This is a subtle yet hugely important distinction. Mathematically it explains a major contribution to the heat capacity of crystalline solids at low temperatures (and it's very strongly related to the form of blackbody radiation when one is worrying about photons rather than phonons).

Because they have a wavelength and therefore a wavevector (and an effective momentum) as well as an energy, one can think about processes that involve the emission, propagation, and scattering of phonons - they have particle-like attributes in that sense.

(For a layperson discussion, I'm avoiding subtle distinctions like acoustic vs. optical phonons. If you really care, in acoustic phonons all the atoms within a unit cell move together, while for optical phonons different atoms within a single unit cell move by different amounts.)

## Tuesday, February 17, 2009

### No, it's not a nanorobot.

Once again, nano-hype. This time the subject is this very nice paper from Seeman's group at NYU, in which they use DNA-based tools to perform programmed self-assembly of some cute nanostructures (also made out of DNA). Seeman has been doing pioneering work for years on leveraging the great specificity of DNA chemistry to make interesting nanostructures. The trick is that each nucleic acid base in DNA likes to hydrogen bond with its own particular complementary base. This specificity of binding plays an essential role in eukaryotic biology, and we now know how to engineer it. In this case, Seeman and coauthors set up a situation where user-defined shapes made from DNA (created using "DNA origami") are bound in specific places and not elsewhere. The major innovation is that they've figured out a way to implement a form of error correction, and in principle they can alter the assembly parameters (that is, which peg goes into which hole) on the fly.

This is nice work, but it's not a nanorobot, not by any reasonable definition of the term. Sorry. By the way, the word "robot" doesn't appear anywhere in the paper (except in the title to one of the references).

## Saturday, February 14, 2009

### What is a plasmon?

Continuing my series of posts trying to describe condensed matter topics in relatively non-technical language....

As I've mentioned before, in condensed matter physics, we tend to give particle-like names (that is, ones that end in "-on") to excitations of systems that have well-defined particle-like attributes, like momentum, energy, and angular momentum (such as spin). Plasmons are another example of this, and lately they've become extremely fashionable because it's increasingly clear that they can be technologically useful.

A plasmon is a collective excitation of the electronic "fluid" in a piece of conducting material, like ripples on the surface of a pond are a collective mode of the water molecules of the liquid. The simile here isn't too far off, because like water, the electronic fluid in a metal is pretty close to incompressible. If you push down on the surface of a pond somewhere with a float, the density of the water doesn't change; instead the water elsewhere is displaced, because the water molecules have finite volume and push each other out of the way. The electronic fluid acts similarly, not because of any finite size or even the Coulomb repulsion of the electrons, but mostly because of the Pauli exclusion principle, which tends to keep the electrons out of each others' way.

These electronic ripples can have a well-defined wavelength (which quantum mechanics tells us is related to their momentum). What makes them have a frequency? That is, what makes the plasmon waves wave? When the electrons are displaced, the positive charge left behind exerts an attractive force on the electrons, trying to pull them back to their original positions. This interaction is what makes the plasmons oscillate once they're excited, and these Coulomb interactions are also why plasmons cost energy to excite. These Coulomb interactions with the positive background charge also force plasmons to obey certain boundary conditions at the edges of the host metal. As a result, nanoparticles can have discrete allowed plasmonic modes strongly influenced by particle shape, while larger structures (e.g., thin metal films) can have propagating plasmon modes over a broad range of wavelengths. Typical plasmon frequencies are comparable to the frequencies of visible light (i.e., ~ 1015 Hz). Plasmons decay (into incoherent electron-hole pair excitations), eventually dissipating their energy as the sloshing electrons scatter instead of oscillating smoothly, and as oscillating electric dipoles (and other multipoles) radiate.

Plasmons have gotten so much attention lately for several reasons. They may offer a way of shuttling information around on computer chips that naturally interfaces with optics. Plasmons are also associated with large local electric fields at metal surfaces, which can be very useful for certain kinds of spectroscopies and things like optical trapping. Finally, in properly designed materials, plasmon properties can be manipulated so that the overall optical response of a conducting system can be tuned, leading to lots of hope and hype about "perfect lenses" and "invisibility cloaks".

## Thursday, February 12, 2009

### Whew, again.

Thankfully, my bad feeling was off-base. According to Speaker Pelosi's summary of the conference committee version of the stimulus (word document here), the NSF will end up with $3B, DOE Office of Science gets$1.6B, there will be an ARPA-E with $400M, NIST will get$580M, NIH will get $8.5B (more than the entire NSF annual budget, by a good fraction) for research and an additional$1.5B for university facilities; and NASA will get $1B, with$400M of that targeted for climate research.

Now all they have to do is actually pass this thing.

I know that many people out there have philosophical objections to this kind of investment being done in a stimulus bill (as opposed to a regular appropriation). I also know that big one-time spikes in funding can be disruptive and harmful in the long term. Still, this is the first decent investment in the physical sciences in years, and it's hard for me to feel misgivings about it given that we've given more than TEN TIMES the total up there in taxpayer dollars to prop up just AIG.

## Wednesday, February 11, 2009

...but I'm prepared to be surprised. For some reason my gut is telling me that the House/Senate conference is going to eviscerate all the science funding in the stimulus except NIH. I hope I'm wrong. I guess we'll find out in a few hours. I'd feel better about this whole business if Grassley wasn't on the conference.

## Tuesday, February 10, 2009

### Experimental physics rules to live by?

I thought that it might be fun to have a discussion about "rules to live by" in experimental physics. Here are a few that I think may qualify, and of course I'd appreciate your suggestions for others....
• Know your apparatus. Don't blindly use a piece of equipment as a black box. Understand how it works. Just because some hand-me-down voltage supply is supposed to put out a square wave doesn't mean that it actually does. You can't blindly use a 10 MOhm input impedance voltage amplifier to measure the voltage dropped across a 1 GOhm load.
• When trying to understand something new, turn every experimental knob as much as you can. You'll be kicking yourself if you decide not to bother cooling the sample below 10 K, and then someone else finds an exciting effect at 9 K. Clearly one needs to strike a balance between time and likelihood of discovery, but in general, if you can tune a parameter, do so.
• Estimate the expected signal size, in real, useful units. Double-check your calculation. My thesis advisor used to tell a story about some students in an advanced undergrad lab who thought their experiment was working well, but it turns out that a wire was actually disconnected, and they'd screwed up the calculation of expected signal size so that the answer agreed with the output of the broken setup.
• Turn knobs finely enough. There are multiple tales out there in physics of discoveries being missed or almost missed because someone was tuning some parameter in coarse steps and skipped over a big feature in the data. That's how superconductivity in MgB2 was missed back in the 1960's, and how SLAC almost didn't co-discover the J/Psi particle.
• Yes, you really do need to reproduce that result. You can see anything once. If the wild, exciting effect you just observed is real, you should be able to see it again if you're careful and diligent.
• Be your own harshest critic. If you won't, the referees surely will.

## Thursday, February 05, 2009

### AAAAAAAGGGH!

I just don't believe it. The "moderate" senators trying to whittle down the stimulus package to avoid a filibuster in the Senate really are suggesting that NSF funding be cut, presumably b/c of the silly porn-viewing incident.

UPDATE: Whew. Thanks in part to hard lobbying by a large number of scientists and engineers, especially the folks at Sciencedebate 2008, the Senate compromise version of the stimulus package was not eviscerated of support for science. It is interesting to note, though, that the NIH will get a boost that exceeds the NSF's entire budget.

## Wednesday, February 04, 2009

### To tide you over....

Proposal deadlines are almost done with, and then I'll try to post more. In the meantime, here's a question to tide you over. Superconductors are classified as "type I" or "type II". In type I superconductors, superconductivity is completely destroyed above some critical externally applied magnetic field, Hc. To be more jargon-y, in these materials the coherence length (the typical spatial extent of pair-like correlations between electrons in the superconductor) is much larger than the magnetic penetration depth (the distance that a magnetic field penetrates into a superconductor before it is screened away by circulating supercurrents). In type II superconductors, above a critical field Hc,1 magnetic flux starts to penetrate the superconductor in the form of vortices (localized regions with nonsuperconducting cores through which magnetic flux is threaded, and around which are circulating supercurrents). Above a second, higher critical field, Hc,2, superconductivity is eventually destroyed. In type II superconductors, the coherence length is much shorter than the penetration depth.

So here's the question: why are almost all the pure elemental superconductors type I, and why are essentially all alloys type II? Is there a simple argument that explains this? If there is, I haven't heard it....