Ask me something.

I haven't done this in a while.  Are there any particular subjects that you would like me to address, or concepts to explain?  It's a busy semester, but I can try....

Some interesting links: Books and news

Here are some things that I wanted to share with my readership:

• My friend Arjendu Pattanayak (founder of a very good blog) pointed out to me this book by Kittel.  It's really nice - it is very concise and tightly written without being incomplete, and it's cheap.
• On a lighter note, Science...For Her! is a book by a friend of a friend.  The introductory video is here.  Attention Physics Today:  I volunteer to review this book when it comes out.  Seriously, I'd be happy to do it, and I think it would be great for some amount of wry humor to make its way into the pages of PT.  
• Similarly, Randall Munroe's book What If? is magnificent.   Attention Physics Today:  I volunteer to review this one, also.  If you don't review this book, you are entirely humorless.
• The MIT Technology Review has a fun article in it about topological quantum computing with non-Abelian anyons.  The reason it's fun is that it talks about the people involved (including my postdoctoral mentor) and manages to avoid becoming overly technical. 
• A few people have pointed out to me that Lockheed Martin has made a rather strong press statement regarding a fusion reactor scheme being developed by Skunk Works (the folks who brought us the SR-71 and the F-117, among other things).   This is potentially interesting, but it's really hard to tell whether this is all vaporware so far.  It looks like a magnetic mirror configuration, something that has been explored extensively over the last few decades, and they don't provide enough technical discussion to figure out what they're doing that's different.  Still, there seem to be many takers trying alternatives to tokomaks (Washington, and what Nature termed "fusion upstarts"), and it's surely worth a shot.  
• I listened in on a conference call today from Benefunder.  These people are trying to come up with an alternative philanthropic approach to research funding that isn't crowdsourcing.  Any commenters already sign up with them?

Quantitatively, how amazing are modern electronics technologies and materials?

I've talked before about how condensed matter/materials physics/engineering is so ubiquitous that it somehow fades into the background, and people don't appreciate how truly wondrous it is.  I thought I'd compile a few stats to try and drive this home.

• A typical car contains something like 30,000 discrete parts, if you count down to the smallest individual screw.  By comparison, a typical microprocessor has around (to make the numbers work out conveniently) 3 billion transistors.  That's a factor of a million more constituents.  Bear in mind that essentially all of those transistors work, without fail, for a decade or more.  (When was the last time you actually had a processor failure, rather than a power supply or hard drive issue?).   Imagine taking a million cars, and claiming that they will all run, flawlessly, with no broken parts, for a decade.  
• Parallel manufacturing is a wonderful thing.  If you built the 3 billion transistors serially at a rate of one per second, it would take around 95 years to put together a processor.  
• There is a famous study that proved that Kansas is actually flatter than a pancake.  Perfect flatness would correspond with their flatness metric equalling 1, and they found that Kansas has a flatness of 0.9997.  By that measure, a 300 mm silicon wafer used to fabricate chips would have a flatness on the order of 1 - (30 nm/300mm) = 1 - 10^-7.  If your dining room table was that flat, the typical height of a surface defect would be well under the wavelength of visible light.  If Kansas was that flat, the tallest feature in the state would be a few cm high.
• The worst silicon purity acceptable for Si electronics processing is around 0.1 parts per billion.  That means that a single impurity atom in such silicon is more rare than, well, you as a member of the population of the earth.  
• We have the ability to position particular devices with (roughly) few nm precision and accuracy on a processor of cm scale.  That's equivalent to being able to place an item on your desk in a particular place to within about 1/50th the diameter of a human hair.

If none of this impresses you, you're pretty jaded.

Chapman Lecture - Paul McEuen

We were very fortunate last week to host Paul McEuen for our Chapman Lecture series (previous speakers here).  NAS member, successful novelist, director of LASSP at Cornell - typical underachiever.  The talk was tremendous fun, a look at several cool experiments going on in his lab examining the mechanical properties of graphene (which acts surprisingly like paper, and taught me about the Foppl/von Karman number, \(YL^2/\kappa\), where \(Y\) is the Young's modulus, \(L\) is a relevant length scale, and \(\kappa\) is the bending stiffness) and nanotubes.  The best part of the talk (apart from the rendition of the Cornell alma mater as played by electrically plucked carbon nanotubes) was the palpable sense of joy that he conveyed to the students in the audience.  He clearly really enjoys the playing-with-toys aspect of research!

Annual Nobel speculation

It's that time of year again - go ahead and speculate away in the comments about possible Nobel laureates in physics or chemistry.  Natural suggestions in physics include Aharonov and Berry for geometric phases, Vera Rubin for dark matter/galaxy rotation curves, Charlie Kane and Shoucheng Zhang (and possibly Molenkamp) for topological insulators, Pendry, Smith, and Yablonovitch and John (oh dear that's four) for metamaterials and/or photonic bandgaps.

Update:  check out Slate's article on deserving women candidates.  Dresselhaus would be a good choice.  (I'm not as big a fan of, e,g., Lisa Randall, who is extremely smart but is in the space of high energy theorists who have not yet had predictions of exotic physics actually verified by experiment.)

AAAS, Science magazine, and figure permissions

Hello readers - As I'd mentioned previously, I've written a nano textbook that's going to come out next year.  I'd like to ask my readership, on the off-chance that someone has a suggested contact:  Please email me if you can suggest a good contact at AAAS/Science, with whom I could have a discussion regarding figure permission fees.  (I'd like to try talking to someone first before turning this into a major blogging topic.)  Thanks.
Update:  I've made contact with an actual person.  We will see what happens....

Penny-wise, pound-foolish: Krulwich blog

I just read that NPR is getting rid of Robert Krulwich's excellent science blog, allegedly as part of cost-cutting.  Cost-cutting?  Really?  Does anyone actually think that it costs a huge sum of money to run that blog?  Surely the most expensive part of the blog is Robert Krulwich's time, which he seems more than willing to give.  Seriously, we should find out what the costs are, and have a kick-starter project to finance it.  Come on, NPR.

The persistent regional nature of physics

In the 21st century, with the prevalence of air travel, global near-instantaneous communications, and active cultures of well-financed scientific research on several continents, you would think that the physics enterprise would be thoroughly homogenized, at least across places with similar levels of resources.  Sure, really expensive endeavors would be localized to a few places (e.g., CERN), but the comparatively cheap subfields like condensed matter physics would be rather uniformly spread out.

Strangely, in my (anecdotal, by necessity) experience, that doesn't seem to be the case.  One area of my research, looking at electronic/optical/thermal properties of atomic and molecular-scale junctions, has a very small number of experimental practitioners in the US (I can think of a handful), though there are several more groups in Europe and Asia.  Similarly, the relevant theory community for this work, with a few notable exceptions, is largely in Europe.   This imbalance has become clear in terms of both who I talk with about this work, and where I'm asked to speak.  Interestingly, there are also strong regional tendencies in some of the citation patterns (e.g., European theorists tend to cite European experimentalists), and I'm told this is true in other areas of physics (and presumably chemistry and biology).  I'm sure this has a lot to do with proximity and familiarity - it's much more likely for me to see talks by geographically proximal people, even if it's equally easy for me to read papers from people all over the world.

Basically, physics areas of pursuit have a (surprising to me) large amount of regional specialization.  There's been a major emphasis historically on new materials growth and discovery in, e.g., Germany, China, and Japan compared to the US (though this is being rectified, in part thanks to reports like this one).  Atomic physics w/ cold atoms has historically been dominated by the US and Europe.   I'm sure some of these trends are the result of funding decisions by governments.   Others are due to the effect of particularly influential, talented individuals that end up having long-lasting effects because the natural timescale for change at universities is measured in decades.  It will be interesting to see whether these inhomogeneities smooth out or persist over the long term.

Hype, BICEP2, and all that.

It's been a few years since I've written a post slamming some piece of hype about nanoscience.  In part, I decided that all this-is-hype posts start to sound the same and therefore weren't worth making unless the situation was truly egregious or somehow otherwise special.  In part, I also felt like I was preaching to the choir, so to speak.  That being said, I think the recent dustup over the BICEP2 experiment is worth mentioning, as an object lesson.   

• If the BICEP2 collaboration had only posted their paper on the arxiv and said that the validity of their interpretation depended on further checks of the background by, e.g., the PLANCK collaboration, no one would have batted an eye.  They could have said that they were excited but cautious, and that, too, would have been fine.  
• Where they (in my view) crossed the line is when they orchestrated a major media extravaganza around their results, including showing up at Andre Linde's house and filming his reaction on being told about the data.  Sure, they were excited, but it seems pretty clear that they went well beyond the norm in terms of trying to whip up attention and recognition.
• While not catastrophic for science or anything hyperbolic like that by itself, this is just another of the death-by-1000-cuts events that erodes public confidence in science.  "Why believe what scientists say?  They drum up attention all the time, and then turn out to be wrong!  That's why low fat diets were good for me before they were bad for me!"
• Bottom line:  If you are thinking of staging a press conference and a big announcement before your paper has even been sent out to referees, please do us all a favor and think again. 

When freshman physics models fail

When we teach basic ac circuits in second semester freshman physics, or for that matter in intro to electrical engineering, we introduce the idea of an impedance, \(Z\), so that we can make ac circuit problems look like a generalization of Ohm's law.  For dc currents, we say that \(V = I R\), the voltage dropped across a resistor is linearly proportional to the current.  For reactive circuit elements and ac currents, we use complex numbers to keep track of phase shifts between the current and voltage.  Calling \(j \equiv \sqrt{-1}\), we assume that the ac current has a time dependence \(\exp(-j \omega t\).  Then we can say that the impedance \(Z\) of an inductor is \(j \omega L\), and write \(V = Z I\) for the case of an ac voltage across the inductor.

Where does that come from, though?  Well, it's really Faraday's law.  The magnetic flux through an inductor is given by \(\Phi = LI\).  We know that the voltage induced between the ends of such a coil is given by \(-d\Phi/dt = L (dI/dt) + (dL/dt) I\), and in an ordinary inductor, \(dL/dt\) is simply zero.  But not always!

Last fall and into the spring, two undergrads in my lab (aided by two grad students) were doing some measurements of inductors filled with vanadium dioxide powder, a material that goes through a sharp first-order phase transition at about 65 \(^{\circ}\)C from a low temperature insulator to a high temperature poor metal.  At the transition, there is also a change in the magnetic susceptibility of the material.  What I rather expected to see was a step-like change in the inductive response going across the transition, and an accompanying step-like change in the loss (due to resistive heating in the metal).  Both of these effects should be small (just at the edge of detection in our scheme).  Instead, the students found something very different - a big peak in the lossy response on warming, and an accompanying dip in the lossy response on cooling.  We stared at this data for weeks, and I asked them to run a whole variety of checks and control experiments to make sure we didn't have something wrong with the setup.  We also found that if we held the temperature fixed in the middle of the peak/dip, the response would drop off to what you'd expect in the absence of any peak/dip.   No, this was clearly a real effect, requiring a time-varying temperature to be apparent, and eventually it dawned on me what was going on:  we were seeing the other contribution to \(d\Phi/dt\)!  As each grain flicks into the new phase, it makes a nearly singular contribution to \(dL/dt\) because the transition for each grain is so rapid.

This is analogous to the Barkhausen effect, where a pickup coil wrapped around a piece of, e.g., iron and wired into speakers produces pops and crackling sounds as an external magnetic field is swept.  In the Barkhausen case, individual magnetic domains reorient or domain walls propagate suddenly, also giving a big \(d\Phi/dt\).  In our version, temperature is causing sudden changes in susceptibility, but it's the same basic idea.

This was great fun to figure out, and I really enjoy that it shows how the simple model of the impedance of an inductor can fail dramatically if the material in the coil does interesting things.  The paper is available here.

What is a "bad metal"? What is a "strange metal"?

Way back in the mists of time, I wrote about what what physicists mean when they say that some material is a metal.  In brief, a metal is a material that has an electrical resistivity that decreases with decreasing temperature, and in bulk has low energy excitations of the electron system down to arbitrarily low energies (no energy gap in the spectrum).  In a conventional or good metal, it makes sense to think about the electrons in terms of a classical picture often called the Drude model or a semiclassical (more quantum mechanical) picture called the Sommerfeld model.  In the former, you can think of the electrons as a gas, with the idea that the electrons travel some typical distance scale, \(\ell\), the mean free path, between scattering events that randomize the direction of the electron motion.  In the latter, you can think of a typical electronic state as a plane-wave-like object with some characteristic wavelength (of the highest occupied state) \(\lambda_{\mathrm{F}}\) that propagates effortlessly through the lattice, until it comes to a defect (break in the lattice symmetry) causing it to scatter.  In a good metal, \(\ell >> \lambda_{\mathrm{F}}\), or equivalently \( (2\pi/\lambda_{\mathrm{F}})\ell >> 1\).  Electrons propagate many wavelengths between scattering events.  Moreover, it also follows (given how many valence electrons come from each atom in the lattice) that \(\ell >> a\), where \(a\) is the lattice constant, the atomic-scale distance between adjacent atoms.

Another property of a conventional metal:  At low temperatures, the temperature-dependent part of the resistivity is dominated by electron-electron scattering, which in turn is limited by the number of empty electronic states that are accessible (e.g., not already filled and this forbidden as final states due to the Pauli principle).    The number of excited electrons (that in a conventional metal called a Fermi liquid act roughly like ordinary electrons, with charge \(-e\) and spin 1/2) is proportional to \(T\), and therefore the number of empty states available at low energies as "targets" for scattering is also proportional to \(T\), leading to a temperature-varying contribution to the resistivity proportional to \(T^{2}\).

A bad metal is one in which some or all of these assumptions fail, empirically.  That is, a bad metal has gapless excitations, but if you analyze its electrical properties and tried to model them conventionally, you might find that the \(\ell\) that you infer from the data might be small compared to a lattice spacing.   This is called violating the Ioffe-Mott-Regel limit, and can happen in metals like rutile VO₂ or LaSrCuO₄ at high temperatures.

A strange metal is a more specific term.  In a variety of systems, instead of having the resistivity scale like \(T^{2}\) at low temperatures, the resistivity scales like \(T\).  This happens in the copper oxide superconductors near optimal doping.  This happens in the related ruthenium oxides.  This happens in some heavy fermion metals right in the "quantum critical" regime.  This happens in some of the iron pnictide superconductors.  In some of these materials, when some technique like photoemission is applied, instead of finding ordinary electron-like quasiparticles, a big, smeared out "incoherent" signal is detected.  The idea is that in these systems there are not well-defined (in the sense of long-lived) electron-like quasiparticles, and these systems are not Fermi liquids.

There are many open questions remaining - what is the best way to think about such systems?  If an electron is injected from a boring metal into one of these, does it "fractionalize", in the sense of producing a huge number of complicated many-body excitations of the strange metal?  Are all strange metals the same deep down?  Can one really connect these systems with quantum gravity?  Fun stuff.

What is the Casimir effect?

This is another in an occasional series of posts where I try to explain some physical phenomena and concepts in a comparatively accessible way.  I'm going to try hard to lean toward a lay audience here, with the very real possibility that this will fail.

You may have heard of the Casimir effect, or the Casimir force - it's usually presented in language that refers to "quantum fluctuations of the electromagnetic field", and phrases like "zero point energy" waft around.  The traditional idea is that two electrically neutral, perfectly conducting plates, parallel to each other, will experience an attractive force per unit area given by \( \hbar c \pi^{2}/(240 a^{4})\), where \(a \) is the distance between the plates.  For realistic conductors (and even dielectrics) it is possible to derive analogous expressions.  For a recent, serious scientific review, see here (though I think it's behind a paywall).

To get some sense of where these forces come from, we need to think about van der Waals forces.  It turns out that there is an attractive force between neutral atoms, say helium atoms for simplicity.  We are taught to think about the electrons in helium as "looking" like puffy, spherical clouds - that's one way to visualize the electron's quantum wave function, related to the probability of finding the electron in a given spot if you decided to look through some experimental means.  If you imagine using some scattering experiment to "take a snapshot" of the helium atom, you'd find the two electrons located at particular locations, probably away from the nucleus.  In that sense, the helium atom would have an "instantaneous electric dipole moment".  To use an analogy with magnetic dipoles, imagine that there are little bar magnets pointing from the nucleus to each electron.  The influence (electric field in the real atom; magnetic field from the bar magnet analogy) of those dipoles drops off in distance like \(1/r^{3}\).  Now, if there was a second nearby atom, its electrons would experience the fields from the first atom.  This would tend to influence its own dipole (in the magnet analogy, instead of the bar magnets pointing on average in all directions, they would tend to align with the field from the first atom, rather like how a compass needle is influenced by a nearby bar magnet).   The result would be an attractive force, proportional to \(1/r^{6}\).

In this description, we ignored that it takes time for the fields from the first atom to propagate to the second atom.  This is called retardation, and it's one key difference between the van der Waals interaction (when retardation is basically assumed to be unimportant) and so-called Casimir-Polder forces.   

Now we can ask, what about having more than two atoms?  What happens to the forces then?  Is it enough just to think of them as a bunch of pairs and add up the contributions?  The short answer is, no, you can't just think about pair-wise interactions (interference effects and retardation make it necessary to treat extended objects carefully).

What about exotic quantum vacuum fluctuations, you might ask.  Well, in some sense, you can think about those fluctuations and interactions with them as helping to set the randomized flipping dipole orientations in the first place, though that's not necessary.  It has been shown that you can do full, relativistic, retarded calculations of these fluctuating dipole effects and you can reproduce the Casimir results (and with greater generality) without saying much of anything about zero point stuff.  That is why while it is fun to speculate about zero point energy and so forth (see here for an entertaining and informative article - again, sorry about the paywall), there really doesn't seem to be any way to get net energy "out of the vacuum".

Two cool papers on the arxiv

The beginning of the semester is a crazy time, so blogging is a little light right now.  Still, here are a couple of recent papers from the arxiv that struck my fancy.

arxiv:1408.4831 - "Self-replicating cracks:  A collaborative fracture mode in thin films," by Marthelot et al. 
This is very cool classical physics.  In thin, brittle films moderately adhering to a substrate, there can be a competition between the stresses involved with crack propagation and the stresses involved with delamination of the film.  The result can be very pretty pattern formation and impressively rich behavior.  A side note:  All cracks are really nanoscale phenomena - the actual breaking of bonds at the tip of the propagating crack is firmly in the nano regime.

arxiv:1408.6496 - "Non-equilibrium probing of two-level charge fluctuators using the step response of a single electron transistor," by Pourkabirian et al. 

I've written previously (wow, I've been blogging a while) about "two-level systems", the local dynamic degrees of freedom that are ubiquitous in disordered materials.  These little fluctuators have a statistically broad distribution of level asymmetries and tunneling times.  As a result, when perturbed, the ensemble of these TLSs responds not with a simple exponential decay (as would a system with a single characteristic time scale).  Instead, the TLS ensemble leads to a decaying response that is logarithmic in time.  For my PhD I studied such (agonizingly) slow relaxations in the dielectric response and acoustic response of glasses (like SiO2) at cryogenic temperatures.   Here, the authors use the incredible charge sensitivity of a single-electron transistor (SET) to look at the relaxation of the local charge environment near such disordered dielectrics.  The TLSs often have electric dipole moments, so their relaxation changes the local electrostatic potential near the SET. Guess what:  logarithmic relaxations.  Cute, and brings back memories of loooooong experiments from grad school.

Science and engineering research infrastructure - quo vadis?

I've returned from the NSF's workshop regarding the successor program to the NNIN.  While there, I learned a few interesting things, and I want to point out a serious issue facing science and engineering education and research (at least in the US).

• The NNIN has been (since 2010) essentially level-funded at $16M/yr for the whole program, and there are no indications that this will change in the foreseeable future.  (Inflation erodes the value of that sum as well over time.)  The NNIN serves approximately 6000 users per year (with turnover of about 2200 users/yr).  For perspective, a truly cutting edge transmission electron microscope, one instrument, costs about $8M.  The idea that the NNIN program can directly create bleeding edge shared research hardware across the nation is misguided.
• For comparison, the US DOE has five nano centers.  The typical budget for each one is about $20M/yr.  Each nano center can handle around 450 users/yr.  Note that these nano centers are very different things than NNIN sites - they do not charge user fees, and they are co-located with some truly unique characterization facilities (synchrotrons, neutron sources).  Still, the DOE is spending seventeen times as much per user per year in their program as the NNIN.
• Even the DOE, with their much larger investment, doesn't really know how to handle "recapitalization".  That is, there was money available to buy cutting edge tools to set up their centers initially, but there is no clear, sustainable financial path to be able to replace aging instrumentation.  This is exactly the same problem faced by essentially every research university in the US.  Welcome to the party.  
• Along those lines:  As far as I can tell (and please correct me if I'm wrong about this!), every US federal granting program intended to have a component associated with increasing shared research infrastructure at universities (this includes the NSF MRI program, MRSEC, STC, ERC, CCI; DOE instrumentation grants, DOE centers like EFRCs, DOD equipment programs like DURIPs) is either level-funded or facing declining funding levels.  Programs like these often favor acquisition of new, unusual tools over standard "bread-and-butter" as well.  Universities are going to have to rely increasingly on internal investment to acquire/replace instrumentation.  Given that there is already considerable resentment/concern about perceived stratification of research universities into "haves" and "have-nots", it's hard to see how this is going to get much better any time soon.
• To potential donors who are really interested in the problem of graduate (and advanced undergrad) science and engineering hands-on education:  PLEASE consider this situation.  A consortium of donors who raised, say, $300M in an endowment could support the equivalent of the NNIN on the investment returns for decades to come.  This can have an impact on thousands of students/postdocs per year, for years at a time.  The idea that this is something of a return to the medieval system of rich patrons supporting the sciences is distressing.  However, given the constraints of government finances and the enormous sums of money out there in the hands of some brilliant, tech-savvy people who appreciate the importance of an educated workforce, I hope someone will take this possibility seriously.  To put this in further perspective:  I heard on the radio yesterday that the college athletics complex being built at Texas A&M University costs $400M.  Think about that.  A university athletic booster organization was able to raise that kind of money for something as narrowly focused (sorry, Aggies, but you know it's true). 

Distinguishable from magic?

Arthur C. Clarke's most famous epigram is that "Any sufficiently advanced technology is indistinguishable from magic."  A question that I've heard debated in recent years is, have we gone far enough down that road that it's adversely affecting the science and engineering education pipeline?  There was a time when young people interested in technology could rip things apart and actually get a moderately good sense of how those gadgets worked.  This learning-through-disassembly approach is still encouraged, but the scope is much more limited. 

For example, when I was a kid (back in the dim mists of time known as the 1970s and early 80s), I ripped apart transistor radios and at least one old, busted TV.  Inside the radios, I saw how the AM tuner worked by sliding a metal contact along a wire solenoid - I learned later that this was tuning an inductor-capacitor resonator, and that the then-mysterious diodes in there (the only parts on the circuit board with some kind of polarity stamped on them, aside from the electrolytic capacitors on the power supply side) somehow were important at getting the signal out.  Inside the TV, I saw that there was a whopping big transformer, some electromagnets, and that the screen was actually the front face of a big (13 inch diagonal!) vacuum tube.  My dad explained to me that the electromagnets helped raster an electron beam back and forth in there, which smacked on phosphors on the inside of the screen.  Putting a big permanent magnet up against the front of a screen distorted the picture and warped the colors in a cool way that depended strongly on the distance between the magnet and the screen, and on the magnet's orientation, thanks to the magnet screwing with the electron beam's trajectory. 

Now, a kid opening up an ipod or little portable radio will find undocumented integrated circuits that do the digital tuning.  Flat screen LCD TVs are also much more black-box-like (though the light source is obvious), again containing lots of integrated circuits.  Touch screens, the accelerometers that determine which way to orient the image on a cell phone's screen, the chip that actually takes the pictures in a cell phone camera - all of these seem almost magical, and they are either packaged monolithically (and inscrutably), or all the really cool bits are too small to see without a high-power optical microscope.  Even automobiles are harder to figure out, with lots of sensors, solid-state electronics, and an architecture that often actively hampers investigation. 

I fully realize that I'm verging on sounding like a grumpy old man with an onion on his belt (non-US readers: see transcript here).  Still, the fact that understanding of everyday technology is becoming increasingly inaccessible, disconnected with common sense and daily experience, does seem like a cause for concern.  Chemistry sets, electronics sets, arduinos and raspberry pi-s, these are all ways to fight this trend, and their use should be encouraged!

Some quick cool science links

Here are a few neat things that have cropped up recently:

• The New Horizons spacecraft is finally getting close enough to Pluto to be able to image Pluto and Charon orbiting about their common (approximate, b/c of other moons) center of mass.
• The Moore Foundation announced the awardees in the materials synthesis component of their big program titled Emergent Phenomena in Quantum Systems.  Congratulations all around.  
• Here's a shock:  congressmen in the pockets of the United Launch Alliance don't like SpaceX.
• Cute toy.
• The Fields Medal finally goes to a woman, Maryam Mirzakhani.  Also getting a share, Manjul Bhargava, who gave the single clearest math talk I've ever seen, using only a blank transparency and a felt-tip pen.

Nanotubes by design

There is a paper in this week's issue of Nature (with an accompanying news commentary by my colleague Jim Tour) in which the authors appear to have solved a major, two decade+ challenge, growing single-walled carbon nanotubes of a specific type.   For a general audience:  You can imagine rolling up a single graphene sheet and joining the edges to make a cylinder.  There are many different ways to do this.  The issue is, different ways of rolling up the sheet lead to different electronic properties, and the energetic differences between these different tube types are very small.  When people have tried to grow nanotubes by any number of methods, they tend to end up with a bunch of tube types of similar diameters, rather than just the one they want.

The authors of this new paper have taken an approach that has great visual appeal.  They have used synthetic chemistry to make a planar hydrocarbon molecule that looks like they've taken the geodesic hemisphere end-cap of their desired tube and cut it to lay it flat - like making a funky projection to create a flat map of a globe.  When placed on a catalytically active Pt surface at elevated temperatures, this molecular seed can fold up into an endcap and start growing as a nanotube.  The authors show Raman spectroscopic evidence that they only produce the desired tube type (in this case, a metallic nanotube).  The picture is nice, and the authors imply that they could do this for other desired tube types.  It's not clear whether this is scalable for large volumes, but it's certainly encouraging.

This is very cute.  People in the nanotube game have been trying to do selective synthesis for twenty years.  Earlier this summer, a competing group showed progress in this direction using nanoparticle seeds, an approach pursued by many over the years with limited success.  It will be fun to see where this goes.  This is a good example of how long it can take to solve some materials problems.

Does being a physicist ruin science fiction for me? Generally, no.

For the past few years, as I've been teaching honors freshman mechanics, I've tried to work in at least one homework problem based on a popular sci-fi movie.  Broadening that definition to include the Marvel Cinematic Universe, I've done Iron Man, Captain America, the Avengers.  Yesterday I saw Guardians of the Galaxy, and I've got a problem in mind already.

I've been asked before, does being a physicist just ruin science fiction books and movies for me?  Does bad physics in movies or sci-fi books annoy me since I can't not see it?  Generally, the answer is "no".  I don't expect Star Trek or Star Wars to be a documentary, and I completely understand why bending physics rules can make a story more fun.  Iron Man would be a lot less entertaining if Tony Stark couldn't build an arc reactor with enough storage capacity and power density to fly long distances.  Trips through outer space that require years of narrative time just to get to Jupiter are less fun than superluminal travel.  If anything, I think well-done science fiction can be creatively inspiring.

One thing that does bug me is internally inconsistent bad physics or bad science.  For example, in the book Prey by Michael Crichton, it's established early on that any tiny breach in a window, etc. is enough for the malevolent nanocritters to get in, yet at the climax of the book the author miraculously forgets this (because if he'd remembered it the protagonist would've died).  Another thing that gets me is trivially avoidable science mistakes.  For example, in a Star Trek:TNG episode (I looked it up - it was this one), they quote a surface temperature less than absolute zero.  I'm happy to serve as a Hollywood science advisor to avoid these problems :-)

A book, + NNIN

Sorry for the posting drought.  There is a good reason:  I'm in the final stages of a textbook based on courses I developed about nanostructures and nanotechnology.  It's been an embarrassingly long time in the making, but I'm finally to the index-plus-final-touches stage.  I'll say more when it's in to the publisher.

One other thing:  I'm going to a 1.5 day workshop at NSF in three weeks about the next steps regarding the NNIN.  I've been given copies of the feedback that NSF received in their request for comment period, but if you have additional opinions or information that you'd like aired there, please let me know, either in the comments or via email.

My Nerd Nite talk - video

I mentioned back in February that I'd had the chance to speak at Nerd Nite Houston (facebook link - it's updated more frequently than the website).  It was a blast, and I encourage people in the area to check it out on the last Thursday of each month, location announced on the page, though so far they've all been at Notsuoh. 

Thanks to the fantastic videographic efforts of Jon Martensen, the video of my talk is now available on youtube here.  The talk is about 20 minutes and the rest is the audience Q&A.   All in all, a very fun experience - thanks again to Amado Guloy and the rest of the Nerd Nite folks for giving me the opportunity.