Talks in 90 seconds

At the workshop I'd mentioned, because of time constraints only 11 of the participants had the chance to give full talks. To try to get a better sense of what everyone else was working on, the organizers let anyone else who wanted to speak give a 90 second talk after dinner on the first night. This was a fascinating exercise, taken in good fun by all involved. The time limit was enforced by a big analog timer with a loud buzz.... As you might imagine, conveying what your group is doing given just 90 seconds is a challenge. About 19 people tried this, using anywhere from one to ten (!) powerpoint slides. It worked surprisingly well, actually. A neat idea.

The workshop experience

I just finished attending a three-day workshop near Washington, DC sponsored by the National Nanotechnology Initiative (and other "participating agencies", since technically the US Department of Defense is not strictly part of the NNI, for reasons which escape me), focused on "nanotechnology-enabled sensing". Since I've only backed into anything sensing related in the last couple of years, this was very educational for me. The workshop was put together, in part, by Roger van Zee from NIST and Gernot Pomrenke from AFOSR, and was tasked with producing a report on the topic. The report is actually supposed to be useful to diverse a diverse audience, from technical contacts at the various agencies to Congressional staffers and the White House Office of Science and Technology Policy. I learned a lot, and met many interesting people from other universities, national labs, industry, and federal agencies.

As you might imagine, producing the report is quite a task. You have to corral 30-40 PhD researchers (who all have their own areas of expertise and writing styles), and build up a consensus document that is comprehensive, readable, brief but with some technical depth, and covers an extremely broad topic. Sensing is particularly tricky, since there are many many transduction mechanisms, many many things that people want to detect (including small molecule chemicals, biomolecules, cells, physical variables, EM radiation), and lots of ancillary requirements (biocompatibility? specificity? portability? low power?). It's like herding cats, but in the end I think this report should do a good job of highlighting what nano can do for sensing. (Here's the five-word executive summary: Sensors good, nanosensors much better. See? Who says that it's tough to write for non-scientists....)

Small really is different

Shameless self-promotion part II. The actual scientific result that just came out in Nature is rather surprising. There are two "ordinary" settings for Kondo physics: a magnetic impurity in an otherwise nonmagnetic host (e.g., dilute Mn atoms in Cu), or a quantum dot containing an unpaired electron. In the former case, the conduction electrons of the host metal can lower their kinetic energy by trying to occupy a singly occupied d orbital of the magnetic impurity. However, because of the Coulomb repulsion of the other electrons on the impurity atom, really doing this is classically forbidden by energy conservation. Still, quantum mechanics lets that forbidden state exist as a virtual intermediate state in a scattering process that takes a conduction electron from the host, flips the spin of the impurity atom, and spits out an electron into a different conduction band state. In the quantum dot case, an analogous magnetic dance takes place, in which the spin of the unpaired electron on the dot is flipped, and an electron is transferred across the dot. This Kondo scattering process affects the electronic conduction through the dot in a particular, identifiable way.

The surprising result in our case is that we see indications of this Kondo process in atomic-scale junctions between chemically homogeneous (e.g., all the atoms are Ni) ferromagnetic metals. The data are pretty clear, and indicate that this spin-related process competes with ordinary ferromagnetic exchange in these nanostructures. It would appear, from accompanying theory calculations by our coauthors, that the very act of whittling the ferromagnetic metal down to the atomic-scale junction is enough to mess with the electronic properties of the metal that we'd ordinarily consider to be intrinsic. The bottom line is, when worrying about the magnetic properties of truly nanoscale structures (with many surface atoms), one may need to keep track of relatively exotic ("strong correlation") physics like the Kondo effect.

An ideal collaboration

I just had about the best possible experience with a collaboration that one can expect to have. Indeed, I worry that I've now used up my "collaboration karma". Here's how these things are supposed to work....

Back at the APS March Meeting in 2006, my student Zach was presenting his masters work on electronic conduction through atomic-scale Ni junctions. Specifically, he had been doing some experiments to try and examine whether atomic-scale contacts between ferromagnetic metals had unusually large changes in electrical resistance when placed in a changing magnetic field, as had been reported in the literature. (We found that the answer is "No", but the magnetoresistance does depend in detail on the precise atomic configuration of the device. This work was independently confirmed simultaneously by Dan Ralph's group at Cornell.) Anyway, at the end of the session, I met Carlos Untiedt, who was just getting going as a faculty member at the University of Alicante in Spain. I'd read some of Carlos' earlier work on metal junctions made using mechanical means, and he'd read our work, too. He mentioned to me that the Spanish government has a program that allows Spanish graduate students to spend time abroad working in other labs, and suggested that we try this at some point. I said that this sounded like a good idea, and we should do it.

Fast forward to the beginning of 2008, when Carlos and I got back in touch. He had a very good student eager and interested to come and visit, and, even better, they had some exciting data that they'd been taking in mechanically-controlled (STM-style, middle of this page) atomic-scale metal junctions. The main advantage of mechanical junctions is that you can break and re-form them many times, giving you serious statistical information about junction properties. Now, in my lab we often use an alternative technique for making atomic-scale junctions that doesn't involve mechanical motion. While our method (electromigration) is more time-consuming and therefore not well suited to really large statistical samples, it has one main advantage: the junctions we make have enough geometric stability that we can look at a single junction over many temperatures. This can't really be done in STM-style junctions. This was a relatively rare situation: there was an ideal point of scientific collaboration, and we had the person and the resources to make things happen.

So, we did it. Carlos' student, M. Reyes Calvo, came and spent a little under four months working in my lab with my group. She was able to make junctions with our approach that were analogous to the ones that she'd been studying in Spain, and measured them as a function of temperature in our system. The results were very nicely consistent with her data from Spain, and the whole scientific story hung together well. After her visit and a number of fun conversations with theorist colleagues at Alicante, a paper was written that came out today in Nature. It just doesn't work any better than that. I'll write about the science in a separate post....

Nice speech.

President Obama addressed the US National Academy of Sciences this morning (video link here). Students, take note. In addition to boosting funding for basic research and making the R&D tax credit permanent, he's talking about tripling (!) the number of NSF graduate fellowships.

Just stop.

Attention TX state and federal officials with R next to their names. Let me clue you in on a couple of points.
1) Secession is not an option. See the US Civil War.
2) TX does not have the authority to break up into smaller states autonomously. That went out the window when TX was re-admitted to the Union after the Civil War.

Bloviating about this pointless drivel makes the entire state look bad. Don't you realize that this garbage makes it difficult to convince smart people to move here, because it looks like the state is governed by idiots?

(This is my last Texas post for a long while - I promise.)

Random favor....

I use the free version of google analytics to do some simple tracking of page views, etc. on both this blog and on my group webpage, just for fun. For some strange reason, the little javascript doodad that allows google to track hits works just fine on all of my group-related pages (like this one and this one), but fails on my publications page. If someone out there with greater expertise or sharper eyes than me could take a look at the html source and explain to me what's wrong with my publications page, I'd greatly appreciate it. Thanks. UPDATE: Thanks - all fixed, I think. Behold the power of teh intarwebs.

Cold fusion, the longer story.

I fully expect angry comments about this....

Here's how a cold fusion experiment is supposed to work, broadly. One takes an electrochemical cell containing either regular water or D₂O, and as one electrode uses palladium (prepared in some meticulous way, to be discussed further below). Then one sets the electrochemical conditions such that hydrogen (or deuterium) ions are electrochemically favored to go into the palladium lattice, up to some very high loading. It's been known for decades that Pd likes to take up hydrogen, so the fact that one can do this is of no surprise. Now, while all this is going on, one carefully monitors the temperatures of the electrodes, the water, etc. The experimental claim, coarsely described, is that after some time, cells containing heavy water under these conditions begin to get hot (but not cells containing ordinary water!). Ideally one does good calorimetry and can measure the amount of energy that comes out of the cell in the form of heat, vs. the amount of energy put in in the form of integrated electrochemical current times voltage. The claim is that in some such experiments, the inferred amount of energy out is much larger than the electrical energy in. This is "excess heat".

So, what's the problem? Well, there are several issues.
1) Calorimetry can be a tricky business. This was the main criticism of the original Pons and Fleischmann work. From what I can tell, people have been much more careful about this than twenty years ago.
2) The experiments just aren't reproducible, in many senses of the term. For example, the temperature-vs-time evolutions of nominally identical cells are completely different, and all over the map. There are big fluctuations on many timescales all over the place. Sometimes the thermal output is big, sometimes it's small. This is generally swept aside by those doing the experiments, who take a wildly fluctuating response, integrate it, and claim reproducibility because the net integral ends up having the desired sign. Not the desired magnitude, just the desired sign. What would I expect to see in a well-controlled experiment? Take one large piece of palladium, cut it into thirds, and set up three identical cells. The temperature-time histories of these things should really reproduce. If you can't do that, then you don't have a controlled experiment. This isn't a small thing.
3) The cells stop working after a while. Unsurprisingly the time period varies from cell to cell. Now, why should this happen unless the underlying process is chemical in some way? By the way, some cells (but not all) "revive" when the electrochemical conditions are changed. Again, all of this is massively variable, even between nominally identical cells in the same labs.
4) The claim of excess heat assumes that there's no chemistry taking place. For example, what if I made that assumption and looked at my car engine? The amount of electrical power input by each spark plug is miniscule compared to the total power out. If I neglected chemical reactions, I'd come to the conclusion that something amazing was going on. Furthermore, if I normalized the output power by, say, the number of platinum atoms at the tips of the spark plugs, I might then conclude that the only way of achieving such power out was something like nuclear. That's the hazard of ignoring possible chemical channels. The issue here is that palladium is known to be highly catalytic, and there are certainly diffusion processes within solids that can be strongly influenced by isotopic differences. Moreover, the claim is also that surface prep of the Pd is of absolutely critical importance. Again, this sounds to me like catalysis, not a bulk effect. Now, you'd think this could all be resolved by analytical chemistry - look at the cell materials before and after running. Look at the water before and after running. However, remember that the different folks doing this disagree on basic analytical chemistry issues like the possible production of helium, tritium, etc. That has to make you wonder about how trustworty their collective analyses are.

Now, I'm not saying that there's nothing worth examining here. The DOD clearly thinks its worth looking into, and it would be nice to get this straightened out once and for all. However, 60 Minutes notwithstanding, the work is just not reproducible in the sense that most experimental physicists would use.

Cold fusion.

Tonight 60 Minutes is airing a report on cold fusion research. I haven't seen the report, but this whole business is what I was referring to obliquely back when I wrote this. I'll write more about this soon, since it provides a nice vehicle for talking about good experiments and what we mean when we say that something is "reproducible" and "controlled". The short version: (1) Don't go out and buy palladium futures just yet. (2) There is something weird going on in the experiments, but it's not at all clear that it has anything to do with nuclear processes, since that would require at least two "miracles" (= totally unexpected deviations from established physics). (3) A careful look at this by careful people is probably worthwhile, but massive hype is a bad idea.

This just in....

The governor of Texas doesn't always think before he talks. sigh.

What is temperature?

Another in my off-and-on series about condensed matter physics concepts.

Everyone has an intuitive grasp of what they mean by "temperature", but for the most part only physicists know the rigorous definition. Temperature, colloquially, is some measure of the energy stored in a system. If two systems having different temperatures are placed "in contact" (so that energy can flow between them via microscopic interactions like atoms vibrating into each other), there is a net flow of energy from the high temperature system to the low temperature system, until the temperatures equilibrate. Fun fact: the nerves in your skin don't actually sense temperature; rather, they sense the flow of thermal energy. Metals below body temperature generally feel cold to the touch because they are very effective at conducting away thermal energy. Plastics at the same temperature feel warmer because they are much worse thermal conductors.

Anyway, temperature is defined more rigorously than this touchy-feely business. Consider a system (e.g., a gas) that has a certain amount of total energy. That system can have many configurations (e.g., positions and momenta of the gas molecules) that all have the same total energy. Often there are so many configurations in play that we keep track of the log of the number of available configurations, which we call (to within a constant that gives us nice units) S. Now, what happens if we give the system a little more total energy? Well, (almost always) this changes the number of available configurations. (In the gas example, now more of the molecules have access to higher momenta, for example.) How many more configurations? A simple assumption here is that the change in E is linearly proportional to the change in S. The proportionality factor is exactly T, the temperature. At low temperatures, a given change in E implies a comparatively large change in the number of available configurations; conversely, at high temperatures, a given change in E really doesn't increase the number of available configurations very much. (I know that someone is going to object to my gas example, since even for a single molecule in a box there are, classically, an infinite number of possible configurations for the molecule even if we only keep track of positions. Don't worry about that too much - we have mathematically solid ways to deal with this.)

Those in the know will already be aware that S is the entropy of the system. The requirement that the total S always increase or stay the same for a closed system ends up implying just the familiar properties of temperature and average energy flow that we know from everyday experience.

Writing and word limits

I'm working on an article for nonspecialists about a nano topic, and all I can say is, writing concisely for nonexperts is much more difficult than writing concisely for experts.

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.

arxiv:0903.5260 - 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-T_c 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-T_c 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....

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.

Whew.

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.

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....)

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?

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.

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.