Energy.

As US readers have probably heard by now, Steve Chu has been selected by President-Elect Obama to be the new Secretary of Energy. I think that this is very good news. He's extremely smart, and he's been involved in pretty much the entire research enterprise, from his days at Bell Labs to running a research group at Stanford to serving as department chair to acting as director at LBL. He's been actively worrying about both basic and applied research and understands the actual energy demands of the country and the world. As the finance folks say, past performance is not necessarily indicative of future results, but this appointment gives me real cause for optimism. Steve Chu is extremely qualified for this.

What to do about public perception of science

From the comments on my last post, it's clear that there are some number of science types out there who view the situation as hopeless: the public is poorly informed and his bigger things to worry about; despite having direct evidence every day of the importance of science (ubiquitous computers, lasers, GPS, MRI, DNA testing) the public feels that science is somehow not relevant to their lives and finds even the basic concepts inaccessible; because there is no financial incentive for people to learn about science they won't; etc. While there is a grain of truth to these comments, there is plenty of evidence that is more hopeful. There is no question that certain science topics capture the public imagination: Mars rovers, using genetic technology to cure diseases or identify relationships between individuals or species, the LHC (talk about an effective marketing job, at least to some segment of the population).

Chad Orzel has many good things to say about what scientists can do to help improve the situation, and I won't repeat them here. If you are personally trying to do outreach, I do have one suggestion. Remember that people like a compelling story and interesting characters. The story can be a scientific one (Longitude), a personal one (Genius), or a large-scale drama (The Making of the Atomic Bomb), but it is possible to capture and hold people's attention on scientific subjects. I'm not suggesting that everyone should go out and try to write a popular book on science, but try to remember what makes the best of those books successful.

Do people just not care about science and technology?

CNN seems to think that's the case. As others have pointed out (1, 2), they've decided to close down their science and technology division and fold that reporting back into their general news category. So, what is the message here? That the ad revenue CNN can generate from having good science journalism doesn't justify the expense? (I'm sure that they'll claim quality of reporting won't change, but realistically you're not going to get good science journalism if it's a part-time gig for people who are spending more of their time reporting on Jennifer Aniston's latest romantic trevails.) What does it say about our society that we're completely dependent on technology (and therefore science), and yet pursuing science or engineering is viewed as "nerdy" and even accurately reporting on it is judged not worth the expense? Sorry for the buzz-kill of a Friday post, but this really depresses me.

This week in cond-mat

Two brief mentions this week.

arxiv:0811.4491 - M. Häfner et al., Anisotropic magnetoresistance in ferromagnetic atomic-sized metal contacts
Over the last few years there's been quite an interest in the effect of magnetic fields on the electrical resistance of atomic-scale contacts between ferromagnetic metals (e.g., Ni). As pointed out by last year's Nobel in physics, if one can create nanostructures with very large magnetoresistive effects, there can be immediate applications in magnetic data storage. Recent investigations (for example, here and here) have shown dramatic variations in the magnetoresistance of such contacts from device to device. It's pretty clear that the atomic-scale details of the structures end up mattering (which is pretty cool, but rather discouraging from the technological side). This paper is a theoretical look at this issue, emphasizing that this sensitivity to details likely results from the fact that those last few atoms at the contact are undercoordinated - that is, they have fewer neighbors than atoms in the bulk of the magnetic metal.

arxiv:0812.0048 - Masubuchi et al., Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope
I've been waiting for a paper like this to show up. It's been known for about a decade now that one can use the tip of an atomic force microscope to do very local electrochemistry. This has been exploited to make interesting metal/metal oxide structures, designer surface details on Si, and impressive quantum dot systems in GaAs. These folks have done the same on graphene. Nice.
UPDATE: As was pointed out in the comments, I had overlooked two earlier reports of AFM oxidation of graphene, here and here.

You've got to be kidding.

Don't take this as a comment one way or the other on D-wave's actual product claims about building adiabatic quantum computers, but check out the image at right from their presentation at a conference on supercomputing about "disruptive technologies". This may be great for raising [edited in response to comment] enthusiasm, but it's almost self-parody. Remember, true disruptive technologies reshape the world. Examples include fire, agriculture, the wheel, the transistor, the digital computer, the laser, and arguably the internet (if you consider it separately from the computer). Must be nice to claim, on the basis of one real data point and one projected data point, that a product is on a disruptive trajectory (in productivity (arbitrary units) vs. time (arbitrary units)). Wow.

Words of advice about giving talks

I know that there are many many resources out there on the web about how to give scientific talks (see here (pdf), here, here, and here, for example). Still, I have a few pointers to suggest, based on some recent talks that I've seen.

• Know your audience. If you're giving a seminar, remember that you need to give an introduction that is appropriate for first-year graduate students. If you're giving a colloquium, remember that you're facing a diverse crowd that could include (in a physics department) astrophysicists, biophysicists, high energy physicists, etc., as well as their graduate students. Pitch your talk appropriately. This is (at least) doubly important if you're giving a job talk, and as my postdoctoral mentor used to point out, every talk you give is potentially a job talk.
  
• Know your time constraints. Don't bring 140 slides for a 50 minute talk, and don't go way over the allotted time. In fact, for an hour talk slot I'd say aim for 50 minutes.
  
• Avoid jargon; if acronyms are necessary, define them. Just because an acronym or term may be common in your sub-field, don't assume that everyone knows it. Just like most condensed matter people don't know what pseudorapidity means to a high energy physicist, most high energy physicists don't know what ARPES or XAFS are.
  
• Minimize equations, even if (especially if) you're a theorist. You can always have a backup slide with enough math on it to make people's eyes bleed, if you want. For a main slide in a talk, no one (not even the experts) are going to get much out of a ton of equations. If you have to have equations, have a physical interpretation for them.
  
• Don't show big scanned text passages from papers. No one is going to read them.
  
• Explain the big picture. Why is this work interesting? You'd better have an answer that will be intelligible to a non-specialist. Even better, think about how you would explain your work and the point behind it to a sophomore.
  
• If you're giving a ten-minute talk, don't spend two minutes showing and explaining an outline.
  
• Avoid technology party fouls. Make sure that your technology works. Make sure that your fonts are readable and correct. Too many colors, too much animation, too many cutesy transitions - all of these things are distracting.
• Make sure to repeat a question back to the questioner. This helps everyone - the semisleeping audience gets to hear what was asked, and you get to make sure that you're actually understanding the question correctly. No one wins when the speaker and questioner are talking at cross-purposes.

Feel free to add your own recommendations or warnings in the comments.

Hard times.

Wow. I guess those rumors about Harvard getting burned playing hedge fund games are true. They're putting in place a staff hiring freeze and getting ready to cancel faculty searches. Steps like that aren't surprising at, e.g., public universities in states hit hard by the housing crunch or ensuing economic crisis, but for a university with an endowment bigger than the GDP of some countries, this seems rather drastic. Still, it's hard to have too much sympathy for them, since their endowment is coming off a high of nearly $37B.

A (serious) modest proposal

Hopefully someone in the vast (ahem.) readership of this blog will pass this along to someone with connections in the Obama transition team. I've already submitted this idea to change.gov, but who knows the rate at which that gets read.

As part of the forthcoming major economic stimulus package, I propose that the Obama administration fully fund the America Competes initiative immediately. If the goal of the package is to stimulate the economy while doing something for the long-term health of the country (e.g., creating jobs while fixing roads, bridges, etc.), then funding basic research via the various agencies is a great thing to do. Think about it: the US spends less as a percentage of GDP than most of the rest of the developed world on science research. Rectifying that to some degree would (a) help the long-term prospects for technological innovation in the US; (b) create jobs; (c) support the goal of developing energy-related technologies; (d) support our universities, many of which are getting hammered by falling state revenues and/or poor endowment returns. Best of all, you could do all of this and it would be a freakin' bargain! You could double the research funding in NSF, NIH, DOE, NASA, and NIST, and not even come close to the amount of money we've already given to AIG. I'm suggesting something far more modest and much less disruptive. Seriously, ask yourself what's better for the long-term health of the country. Cutting basic science to pay for propping up Goldman Sachs is perverse.

Update: If you think that this is a good idea, I encourage you to submit your suggestion here, here, and/or here.

Spin

Many particles possess an internal degree of freedom called "spin" that is an intrinsic amount of angular momentum associated with that particle. The name is meant to evoke a spinning top, which has some rotational angular momentum about its axis when, well, spinning. Electrons have "spin 1/2", meaning that if you pick a convenient axis of reference ("quantization axis") that we'll call z, the z-component of the electron's spin angular momentum is either +1/2 hbar or -1/2 hbar. All too often we treat spin in a rather cavalier way. When people talk about "spintronics", they are interested in using the spin degree of freedom of electrons to store and move information, rather than using the charge as in conventional electronics. One complication is that while charge is strictly conserved, spin is not. If you start off with a population of spin-aligned electrons and inject them into a typical solid, over time the spin orientation of those electrons will become randomized. Now, angular momentum is strictly conserved, so this relaxation of the electron spins must coincide with a transfer of angular momentum to the rest of the solid. Feynman pointed this out (somewhere in vol. III of his lectures on physics) - if you fire a stream of spin-polarized electrons into a brick hanging on the end of a thread, you are really applying a torque to the brick since you are supplying a flow of angular momentum into it, and the thread will twist to apply a balancing torque. Well, Zolfagharkhani et al. have actually gone and done this experiment. They use a ferromagnetic wire to supply a polarized spin current and an extremely sensitive nanomechanical torsional oscillator to measure the resulting torque. Very nice stuff.

Nature Journal Club

My media onslaught continues. This past week I had a Journal Club contribution in Nature, which was fun and a nice opportunity for a wider audience. Here's a version of it before it was (by necessity) trimmed and tweaked, with added hyperlinks....

Tunable charge densities become very large, with super consequences

The electronic properties of materials depend dramatically on the density of mobile charge carriers. One way to tune that density is through doping, the controlled addition of impurity atoms or molecules that either donate or take up an electron from the rest of the material. Unfortunately, doping also leads to charged dopants that can act as scattering sites.

Fortunately, there is a way to change the carrier concentration without doping. In 1925 J. E. Lilienfeld first proposed what is now called the “field effect”, in which the sample material of interest is used as one electrode of a capacitor. When a voltage is applied to the other (“gate”) electrode, equal and opposite charge densities accumulate on the gate and sample surfaces, provided charge can move in the sample without getting trapped. While the density of charge that can be accumulated this way is rather limited by the properties of the insulating spacer between the gate and the sample, the field effect has been incredibly useful in transistors, serving as the basis for modern consumer electronics.

Recently it has become clear that another of Lilienfeld’s inventions, the electrolytic capacitor, holds the key to achieving much higher field effect charge densities. The dramatic consequences of this were made clear by researchers at Tohoku University in Sendai, Japan (K. Ueno et al., Nature Mater. 7, 856-858 (2008)), who used a polymer electrolyte to achieve gated charge densities at a SrTiO₃ surface sufficiently large to produce superconductivity. While superconductivity had been observed previously in highly doped SrTiO₃, this new approach allows the exploration of the 2d superconducting transition without the disorder inherent in doping.

The most exciting aspect of this work is that this approach, using mobile ions in an electrolyte for gating, can reach charge densities approaching those in chemically doped, strongly correlated materials such as the high temperature superconductors. As an added bonus, this approach should also be very flexible, not needing special substrates. Tuning the electronic density in strongly correlated materials without the associated pain of chemical doping would, indeed, be super.

This week in cond-mat

One paper today in the arxiv:
arxiv:0811.2914 - Zwanenburg et al., Spin states of the first four holes in a silicon nanowire quantum dot
This is another typically exquisite paper by the Kouwenhoven group at Delft, in collaboration with Charlie Lieber at Harvard. The Harvard folks have grown a Si wire segment in the middle of a long NiSi wire. The NiSi ends act as source and drain electrodes for conduction measurements, and the Si segment acts as a quantum dot, with the underlying substrate acting as a gate electrode. As usual, the small size of the Si segment leads to a discrete level spectrum, and the weak electronic coupling of the Si segment to the NiSi combined with the small size of the Si segment results in strong charging effects (Coulomb blockade, which I'll explain at length for nonexperts sometime soon). By measuring at low temperatures very carefully, the Delft team can see, in the conductance data as a function of source-drain voltage and gate voltage, the energy level spectrum of the dot. By looking at the spectrum as a function of magnetic field, they can deduce the spin states of the ground and excited levels of the dot for each value of dot charge. That's cute, but the part that I found most interesting was the careful measurement of excited states of the empty dot. The inelastic excitations that they see are not electronic in nature - they're phonons. They have been able to see evidence for the launching (via inelastic tunneling) of quantized acoustic vibrations. Figure 5 is particularly nice.

Workshop on new iron arsenide superconductors

This weekend is a big workshop at the University of Maryland on the new iron arsenide high temperature superconductors. Since it's not really my area, I didn't go. Anyone want to give a little update? Any cool news?

Poor Doug's Almanack

Welcome, readers of Discover Magazine! Thanks for coming by, and I hope that you find the discussion here interesting. The historical target audience of this blog has been undergrads, grad students, and faculty interested in condensed matter (solid state) physics and nanoscience. The readership also includes some science journalists and other scientific/engineering professionals. I would like very much to reach a more general lay-audience as well, since I think we condensed matter types historically have been pretty lousy at explaining the usefulness and intellectually richness of our discipline. Anyway, thanks again.

(By the way, I don't compare in any serious way with Ben Franklin - that was a bit of hyperbole from Discover that I didn't know was coming. Fun science fact: Franklin's to blame that the electron charge is defined to be negative, leading to the unfortunate annoyance that current flow and electron flow point in opposite directions. He had a 50/50 chance, and in hindsight his choice of definition could've been better.)

This week in cond-mat

This week the subject is boundary conditions. When we teach about statistical physics (as I am this semester), we often need to count allowed states of quantum particles or waves. The standard approach is to show how boundary conditions (for example, the idea that the tangential electric field has to go to zero at the walls of a conducting cavity) lead to restrictions on the wavelengths allowed. Boundary conditions = discrete list of allowed wavelengths. We then count up those allowed modes, converting the sum to an integral if we have to count many. The integrand is the density of states. One remarkable feature crops up when doing this for confined quantum particles: the resulting density of states is insensitive to the exact choice of boundary conditions. Hard wall boundary conditions (all particles bounce off the walls - no probability for finding the particle at or beyond the walls) and periodic boundary conditions (particles that leave one side of the system reappear on the other side, as in Asteroids) give the same density of states. The statistical physics in a big system is then usually relatively insensitive to the boundaries.

There are a couple of physical systems where we can really test the differences between the two types of boundary conditions.
arxiv:0811.1124 - Pfeffer and Zawadzki, "Electrons in superlattices: birth of the crystal momentum"
This paper considers semiconductor superlattices of various sizes. These structures are multilayers of nanoscale thickness semiconductor films that can be engineered with exquisite precision. The authors consider how the finite superlattice result (nonperiodic potential; effective hardwall boundaries) evolves toward the infinite superlattice result (immunity to details of boundary conditions). Very pedagogical.

arxiv:0811.0565, 0811.0676, 0811.0694 all concern themselves with graphene that has been etched laterally into finite strips. Now, we already have a laboratory example of graphene with periodic boundary conditions: the carbon nanotube, which is basically a graphene sheet rolled up into a cylinder. Depending on how the rolling is done, the nanotube can be metallic or semiconducting. In general, the larger the diameter of a semiconducting nanotube, the smaller the bandgap. This makes sense, since the infinite diameter limit would just be infinite 2d graphene again, which has no band gap. So, the question naturally arises, if we could cut graphene into narrow strips (hardwall boundary conditions transverse to the strip direction), would these strips have an electronic structure resembling that of nanotubes (periodic boundary conditions transverse to the tube direction), including a bandgap? The experimental answer is, yes, etched graphene strips to act like they have a bandgap, though it's clear that disorder from the etching process (and from having the strips supported by an underlying substrate) can dominate the electronic properties.

Two new papers in Nano Letters

Two recent papers in Nano Letters caught my eye.

Kuemmeth et al., "Measurement of Discrete Energy-Level Spectra in Individual Chemically Synthesized Gold Nanoparticles"
One of the first things that I try to teach student in my nano courses is the influence of nanoscale confinement on the electronic properties of metals. We learn in high school chemistry about the discrete orbitals in atoms and small molecules, and how we can think about filling up those orbitals. The same basic idea works reasonably well in larger systems, but the energy difference between subsequent levels becomes much smaller as system size increases. In bulk metals the single-particle levels are so close together as to be almost continuous. In nanoparticles at low temperatures, however, the spacing is reasonably large compared to the available thermal energy that one can do experiments which probe this discrete spectrum. Now, in principle the detailed spectrum depends on the exact arrangement of metal atoms, but in practice one can look at the statistical distribution of levels and compare that distribution with a theory (in this case, "random matrix theory") that averages in some way over possible configurations. This paper is a beautiful example of fabrication skill and measurement technique. There are no big physics surprises here, but the data are extremely pretty.

Xiao et al., "Flexible, stretchable, transparent carbon nanotube thin film loudspeakers"
This is just damned cool. The authors take very thin films of carbon nanotubes and are able to use them as speakers even without making the films vibrate directly. The idea is very simple: convert the acoustic signal into current (just as you would to send it through an ordinary speaker) and run that current through the film. Because of the electrical resistance of the film (low, but nonzero), the film gets hot when the current is at a maximum. Because the film is so impressively low-mass, it has a tiny heat capacity, meaning that small energy inputs result in whopping big temperature changes. The film locally heats the air adjacent to the film surface, launching acoustic waves. Voila. A speaker with no moving parts. This is so simple it may well have real practical implementation. Very clever.

To quell speculation....

Yes, if asked, I would serve as President Obama's science advisor. (Come on - you would, too, right? Of course, it's easy for me to joke about this since it's about as probable as me being asked to serve as head of the National Science Board.)

This one's easy.

Has Bush been good for science? I agree with ZapperZ: No. How Marburger can argue that research funding has kept pace with inflation is beyond me, given the last three years of continuing resolutions, unless one (a) fudges the definition of research to include a lot of military development, and (b) fudges the definition of inflation to ignore things like food, fuel, and health care costs.

Could Bush have been even worse? Yes.

Statistical physics

This fall I'm teaching Statistical and Thermal Physics, a senior (in the most common Rice physics curriculum, anyway) undergraduate course, and once again I'm struck by the power and profundity of the material. Rather like quantum, stat mech can be a difficult course to teach and to take; from the student perspective, you're learning a new vocabulary, a new physical intuition, and some new mathematical tools. Some of the concepts are rather slippery and may be difficult to absorb at a first hearing. Still, the subject matter is some of the best intellectual content in physics: you learn about some of the reasons for the "demise" of classical physics (the Ultraviolet Catastrophe; the heat capacity problem), major foundational issues (macroscopic irreversibility and the arrow of time; the precise issue where quantum mechanics and general relativity are at odds (or, as I like to call it, "Ultraviolet Catastrophe II: Electric Boogaloo")), and the meat of some of the hottest topics in current physics (Fermi gases and their properties; Bose Einstein condensation). Beyond all that you also get practical, useful topics like thermodynamic cycles, how engines and refrigerators work, chemical equilibria, and an intro to phase transitions. Someone should write a popular book about some of this, along the lines of Feynman's QED. If only there were enough hours in the day (and my nano book was further along). Anyway, I bring this up because over time I'm thinking about doing a series of blog posts at a popular level about some of these topics. We'll see how it goes.

Ahh, Texas, again.

Stories like this one depress me. Is it really any wonder that our state has a difficult time attracting large high-tech companies from, e.g., California and Illinois, even though corporate taxation policies are very friendly here?

Local news

You can learn all sorts of things reading your local paper. For example, I read yesterday that Rice and Baylor College of Medicine are talking about a possible merger. Unsurprisingly, everyone on campus already knew about that, but it's interesting to see the reporter's take on things, including various quotes from unnamed professors.

Today, I read about this. For those who didn't or can't click the link, it's an article about plagiarism. A professor at Texas Southern University's physics department had apparently asked a University of Houston physics professor for an example of a successful grant a couple of years ago. The UH prof gave him a copy of a grant that had been funded by DARPA in 2002-3. The TSU prof then allegedly sent in the same proposal, word for word (though editing out references to the UH prof's work) to the Army Research Lab, which then funded it to the tune of $800K. Lovely. TSU refused the grant, and the investigation is "ongoing".