Tuesday, March 31, 2009

This week in cond-mat

Here are a couple of recent papers that seem likely to generate quite a bit of interest.

arxiv:0903.5359 - Voggu et al., A New Method of Obtaining High Enrichment of Metallic Single-Walled Carbon Nanotubes
One of the major challenges in using nanotubes for various electronics purposes is the large number of tube types. Carbon nanotubes can be metallic or semiconducting depending on just how their graphene-like mesh is wrapped into a cylinder, and most common nanotube growth methods produce a whole mixture of different tube types. Sometimes it would be very nice to produce only a single tube type. Well, the authors here seem to have found an approach that gives them a high yield (90%) of metallic nanotubes, using a particular catalyst chemistry in a carbon arc furnace. Sounds promising, though scale-up to industrial levels (that is, kg quantities of tubes) is likely to be pretty challenging. These approaches also can be devilishly tricky to reproduce - three nominally identical setups can grow different compositions of material because of sensitivity to tiny variations in conditions.

- Hicks et al., Evidence for Nodal Superconductivity in LaFePO from Scanning SQUID Susceptometry
A big outstanding question in the new iron-based superconductors is, what is the pairing symmetry of the superconducting wavefunction? In ordinary low-Tc superconductors, the electrons pair up in "s-wave" pairing. That is, each Cooper pair consists of a spin singlet with zero orbital angular momentum. In contrast, the high-Tc cuprates have d-wave pairing - again a spin singlet, but each Cooper pair has two units of orbital angular momentum. The big significance of this is that the pair wavefunction then must have nodes where it changes sign (just like d orbitals in atoms have nodes as a function of "longitude" when going around the atomic center). The presence of nodes means that there are certain directions in the material where the superconducting gap is zero, and therefore it costs very little energy to make electron- (or hole-)like excitations. These manifest themselves in the temperature dependence of various quantities like magnetic penetration depth. Well, the authors here have used a very powerful (but challenging) technique to measure the penetration depth locally as a function of temperature in LaFePO, and they argue that they see evidence of nodes. This is exciting, because some people argue that the iron pnictides should have s-wave pairing (though a funny kind, where electron and hole pockets in the band structure each have superconducting order with opposite signs of order parameter). It'll be neat to see how this all shakes out....

Thursday, March 26, 2009

One upside of private universities

Private universities are certainly not immune from economic hard times. Heck, Harvard has more money than any of its competition, and they're having to cut back. Stanford is putting $1.2B of construction on hold. Still, private universities usually have enough flexibility to avoid the draconian cuts that can hit public universities when state budgets contract massively. The University of Florida's College of Liberal Arts and Sciences is planning to meet its required budget cut by evisceration of the Department of Geology. Obviously finances are a mess right now, but is it really smarter to pick one science department to cut off at the knees, rather than spreading the pain more evenly? Hat tip to PZ Myers.


Thankfully, the Texas State Board of Education has managed to do the right thing. Barely.

I will make a real CM physics blog posting soon - I'm just working on a couple of papers at the moment.

Tuesday, March 24, 2009

a helpful suggestion

If you're a new equipment vendor at a major conference (say, the March Meeting), don't hand out ballpoint pens so cheap that they run out of ink after less than a week of minimal use. That quality control doesn't exactly inspire confidence. (I know the pens are made by some cheap third-party vendor, but come on....)

Saturday, March 21, 2009

March Meeting wrap-up

Well, that's that for another APS meeting. I actually left late in the day Thursday, so I don't have too much more to report. I know that there was going to be a big session about topological effects in the nu=5/2 fractional quantum Hall state on Friday, but I couldn't be there. I did see the end of a cool session about physics and art, where there were invited talks about the fractal (or, in this case, not) nature of Jackson Pollock paintings, the design of minimal-surface structures like the Water Cube in Beijing, and the use of reflection and projection techniques by some artists. My students also told me that I should've gone to the session that included the talk on how to be a science advisor for Hollywood productions. Anyone else see anything exceptionally cool?

Wednesday, March 18, 2009

General conference observations

  • For the love of heaven, turn off your cell phones. Yes, that includes you, conference speaker who has a lavalier microphone attached right to your breast pocket containing your vibrate-mode phone.
  • Please check to make sure that your laptop actually talks to the projector before the actual moment that you're supposed to start speaking. This goes double for you if you're running linux, windows-on-a-mac, or any other unusual combination.
  • It's unfortunate when you're a focus topic organizer who is supposed to chair a session, and you don't or can't make it to the meeting. It's worse when you're not there to chair the session, and one of your invited speakers doesn't show up either.
  • I can't believe that it took me three days to realize that soda in the vending machine by the registration desk costs half as much as soda at the food concession.

March Meeting, day 3

More interesting talks on the third day of the meeting. Two invited talks stand out in particular for me. The first was by Prof. Peumans at Stanford, ostensibly on using surface plasmons to try to enhance photovoltaics, though it was actually more broad than that. In organic solar cells, there's a competition between trying to get good absorption of incident light (thus favoring thick polymer films) and trying to extract the charge efficiently (leaning toward thin polymer films). It would be great if there was a way to enhance absorption of light while preserving the thin film layers that improve charge transport. With conventional ray optics (the far field limit) in Si solar cells, this can be done by roughening the material surfaces. That way, you enhance the likelihood of total internal reflection, and you can capture a lot more light (proportional to the square of the index of refraction of the semiconductor). The question is, can one gain something by working in the near field and using interference, as opposed to working in the far field limit. The natural approaches that come to mind are: (1) use plasmonic structures as optical antennas, trying to focus and concentrate the light into the PV material; (2) make some kind of broad-band cavity to hold the light in; and (3) make some sort of waveguiding structure to increase the interaction of the light and the PV material. In the end, Peumans made a convincing argument that the near field doesn't have any fundamental advantage over ray optics approaches, but it can actually deliver better results in practice.

The other cool talk was by Dan Rugar at IBM, speaking about the latest results on using a form of cantilever-based force microscopy to do nuclear magnetic resonance imaging at the nanoscale. They now have the sensitivity to see ~ 1000 nuclear spins, with a spatial resolution of around 4 nm. It's a real tour de force experiment. The part that really impressed me was not actually the imaging that they did of the hydrogen (protons) in a tobacco mosaic virus. Rather, I was blown away that they could see, very clearly, the signal from the one or two monolayers of physisorbed hydrocarbon contamination (or, as we technical types call it, "goo") on the surface of their cantilever. Rugar has been working on this idea for years, and the progress has been very impressive.

Tuesday, March 17, 2009

March Meeting, days 1 and 2

Two days of the March Meeting are over, and it's been fun so far. We've really been lucky with the weather, and it looks like tomorrow will also be nice. I'm from Pittsburgh originally, and it's rather surreal to be going to a conference in my old home town. At least I still remember where a bunch of restaurants are, though there's been a lot of turnover. (For those at the meeting, if you don't mind a little bit pricey, I recommend the Fish Market attached to the Westin, as well as Eleven. I also had some great inexpensive Indian food at Spices of India.)

Talks have been fun, though this year I don't seem to have had the time to go to as many sessions outside my immediate interests as in years past. I did catch an all-invited session about the recent exciting work on LaAlO3/SrTiO3 interfaces. It's been known for a few years now that the interface between these two insulating oxides can play host to a 2d electron gas, for reasons that are still under fierce debate. My friend Yuri Suzuki gave a very nice talk explaining their tests trying to get to the bottom of this, and my former Bell Labs colleague Harold Hwang described his group's work on trying to tune the density and type of charge carriers at the interface using techniques similar to modulation doping. There were also talks by the team working on using a conducting AFM to pattern conducting regions of this interface, with an eye toward devices. Neat stuff.

I saw some very nice talks about single-molecule electronic devices. It's a shame that the Division of Materials Physics and Division of Chemical Physics were unable to avoid scheduling their focus topics (on largely experimental measurements, and on theory of molecular devices, respectively) in direct conflict. A couple of invited talks that were particularly compelling were the ones by Christian Schoenenberger, N.J. Tao, and Mark Ratner. Schoenenberger gave a great review of his group's work on mechanical break junctions to look at single molecules in a solution environment, as well as their recent experiments using arrays of linked metal nanoparticles as an electronic testbed. Tao also discussed single-molecule break junction measurements, particularly emphasizing measurements of force and breaking processes as a window on effective junction temperatures. He also talked about some new experiments using asymmetric molecules as rectifiers, and single-molecule measurements of inelastic electron tunneling spectroscopy. Ratner gave a terrific overview of different theoretical approaches to the problem of calculating molecular conduction properties. Most importantly, he did a great job emphasizing when certain methods work well, and why - he gave me a new way to think about some of the physics in these systems. I also saw a number of good, short talks about single-molecule experiments, with a particular bounty coming from the Venkataraman group and collaborators. Fun stuff.

Sunday, March 15, 2009

APS March Meeting

Well, it's that time of year again, when I get together with 7000 of my (mostly condensed matter) physicist colleagues for the annual March Meeting of the American Physical Society. This year the meeting is in my old hometown, Pittsburgh, though this is the first time I've been to an event in the new convention center here. For the curious, the meeting program is available online here.

For those who don't know, these conferences are an interesting though exhausting experience. It's a chance for faculty (and national lab scientists) to catch up with friends and colleagues and get a sense of the exciting science that's out there. For grad students it's an important opportunity to get experience giving talks and posters, to get the lay of the land in the field as a whole, and to begin to network and look at career options, particularly if you're close to graduating. For postdocs, it's a chance to show your stuff and look around at the options before you.

I'll probably do a little blogging about the meeting this year, but my perspective will be a bit limited, since I won't be session-hopping as much as might be necessary to give a really balanced overview. It'll be interesting to see what the big science story of the meeting turns out to be. Last year, while there was an enormous wealth of talks about graphene in particular, the exciting undercurrent was the rumor/discovery of what are now the new hot topic, the Fe-As based high temperature superconductors.

Tuesday, March 03, 2009

What are magnons?

Another in my continuing series trying to explain some condensed matter concepts in comparatively jargon-free language. So far I've talked about electron-like quasiparticles, phonons, and plasmons. Now we consider magnons, also known as "spin waves". A magnon is another collective excitation, like a phonon or a plasmon, that may be described by a wavelength (or equivalently a wavevector) and an accompanying frequency. In phonons, we were interested in the pattern of atomic displacements away from their equilibrium positions, and we thought about this in a balls-and-springs picture of solids. Magnons, as the name suggests, are intimately related to magnetism. In many materials there are magnetic moments associated with (some or all of) the atoms in the material, and you can think of these moments as little arrows. In a material with "ferromagnetic interactions", the system can lower its energy by having the moments tend to align with each other. In a true ferromagnetic state all of the moments spontaneously align - all of the arrows point in the same direction. Flipping one arrow 180 degrees around would cost quite a bit of energy, since that arrow would then be antialigned with its neighbors. On the other hand, it costs much less energy to move one arrow just a little bit out of alignment with its neighbors. A magnon is a collective excitation where the relative alignment between neighboring magnetic moments is spatially described by some wavelength (That is, start at some arrow. Translating over by one magnon wavelength takes you back to an arrow tilted the same way as the initial arrow.).

Now, when you tilt a magnetic moment in a magnetic field, that moment will feel a torque that will cause it to precess. This is completely analogous to a tilted gyroscope precessing when it feels a gravitational torque. So, each little moment participating in the magnon is precessing around, giving a time-dependence to the local moment orientation.

This has been a very classical description. Quantum mechanics enters in a couple of ways when talking about real materials. First, there are quantum mechanical restrictions on what we can say about different components of an electron's magnetic moment at any one time. Second, like phonons, one can think of these magnons a bit like harmonic oscillators - a given magnon mode with angular frequency \omega can only exchange energy in chunks of size \hbar \omega.