Superomniphobicity

Thanks to developments in surface science, surface chemistry, and nanoscience, we now understand far more about the microscopic origins of friction than we ever have before.  (When I teach about this, I point out that our depth of knowledge about the detailed physics of friction really didn't advance much between 1600 and 1950.)  One way that this increased basic knowledge is paying dividends is in the design of surface coatings to control interactions between fluids and solid surfaces.  For example:  When sophomore mechanical engineers learn basic fluid mechanics, they are taught about the "no-slip condition", an assumption that turns out to be pretty good in many many macroscopic situations.  The no-slip condition says that when a fluid flows past a solid boundary, the tangential velocity of the fluid goes to zero at the boundary, and only approaches the "bulk" flow velocity some distance away from the surface.  The region where the local flow velocity is suppressed relative to the bulk far-from-wall speed is the boundary layer.  The underlying physics here is that interactions between the fluid molecules and the wall actually stop the fluid molecules adjacent to the wall, and internal interactions between fluid molecules (the origins of viscosity) tug on neighboring layers of molecules and slow those down.  

In fact, we now know that it's possible to tweak the interactions between that layer of fluid and the solid surface, in ways that make the no-slip condition a poor assumption.  We can do this directly through chemistry.  The example you all know is the use of "hydrophobic" coatings (e.g., wax on a car; fluoropolymers like teflon on a non-stick pan).  With the right kind of chemical bonds at the surface, if the fluid molecules interact attractively much stronger with each other than with the surface, the fluid will "bead up".  Water molecules can hydrogen-bond with each other, while attractive interactions with saturated hydrocarbons are much weaker.  Water beads on wax for the same reason that water and oil do not mix.

We can also leverage the surface tension of the fluid (again related directly to the attractive interactions between fluid molecules, compared with surrounding air).  If the surface morphology of the interface is really bumpy on a length scale sharper than the ability of a liquid interface to curve, it is possible to trap air at the interface and have the liquid be resting mostly on air and just a little on the tips of the surface bumps.  This is what happens when you see water running down a lotus leaf.  (Remember "nano-pants"?)

Now that the science behind these phenomena is better understood, people are trying hard to make designer coatings with remarkably extreme versions of these properties.  Something that really "repels" water or other polar liquids is said to be superhydrophobic.  Something that really "repels" oils and waxy, non-polar liquids is said to be superoleophobic.  The ultimate limit is something that manages to have very low surface affinity for both classes of liquids - a superomniphobic interface, achievable through a combination of surface chemistry and morphology control.  Lately there have been claims of achieving this, with some dramatic videos.  There's this one from Michigan, with this video, for example.  However, that coating apparently requires electrospinning to put down.  This demonstration of a two-component spray-on coating is truly amazing to watch.  The big open question here is how robust is the coating.  If it gets degraded by, e.g., exposure to sunlight, or modest abrasion, that would limit its utility.  (It may be chemically nasty as well, given the protective equipment worn by the person applying it, but that may just be showing good sense.)

RIP, Bob Richardson, and the human nature of science

I was saddened to read of the passing of Bob Richardson, who shared the Nobel Prize for Physics in 1996 with his colleague Dave Lee and their former grad student (and my thesis advisor) Doug Osheroff.   I only had a handful of chances to meet with Prof. Richardson, and he was a friendly, classy person every time.  In some ways it's a shame that more people don't have the opportunity to interact with really accomplished scientists (and engineers); the chances I've been exceedingly fortunate to have over the years to meet and talk with prize winners and national academy members have been fun professionally and revelatory in terms of showing the human side of these endeavors.  These people aren't infallible or unapproachable, and in my limited experience the vast majority are neither arrogant nor lacking in social skills (I'm looking at you, Big Bang Theory).  It would be nice if TLC or Discovery or someone would tell the stories of these people in an accessible, fun way, instead of wasting precious bandwidth on ghost-hunting moonshiners who horde ice fishing hauling equipment.    

Plasmons, polarization, and intuition

This is one of those once-every-six-months self-promotion posts about a new paper from our group.  The result is sufficiently surprising, while illustrating a generalizable idea, that I think it's worth sharing.

I've written before about plasmons, the collective "normal modes" of the electronic fluid in a metal.  Like many phenomena in condensed matter physics, there are times when it is useful to think of plasmon modes in some metal structure as generic oscillators, each analogous to a mass on a spring (only the natural frequency of the plasmon has to do with the complex dielectric function of the metal, while the natural frequency of the mass on a spring is set by the mass and the spring constant). In particular, if you take two identical oscillators, nominally of the same natural frequency \( \omega_{0} \), and you couple them together, it often makes sense to describe the coupled system in terms of two "new" normal modes "built" from linear combinations of the uncoupled modes, the symmetric ( \( \omega_{\mathrm{s}}  < \omega_{0} \) ) (the individual oscillators move in phase with each other)  and antisymmetric modes ( \( \omega_{\mathrm{as}}  > \omega_{0} \) ) (the two oscillators move \( \pi \) out of phase with each other).  In quantum mechanics we see the same idea; for example, when two 1s orbitals are coupled together, it can make more sense to think instead about "hybridized" bonding ( \(\sigma\) ) and antibonding ( \(\sigma* \) ) molecular orbitals.   As my colleagues showed almost ten years ago in this highly cited paper, plasmons can hybridize, too, and hybridization can provide real insights into the plasmonic modes of complicated structures.

In my lab, we have spent quite a bit of time over the last several years playing with and looking at the local plasmon modes that live at nanoscale gaps between lithographically fabricated Au electrodes.  In many ways, these structures look a bit like two scanning tunneling microscope tips pointing at each other.  Many other groups have made similar structures, and it has been known for a long time that placing metal tips in close proximity to each other or a tip pointing down at a very nearby metal plane leads to "tip plasmon" modes that can be useful for various spectroscopies.  In the plasmon hybridization language, the local tip modes result from the hybridization of (delocalized) surface plasmon modes of the two electrodes, thanks to their very local coupling.  For those interested in these nanogap plasmon effects, by the way, I want to point out our recent review article about this, which will appear in an issue of Phys Chem Chem Phys focusing on plasmonics.

We had lingering mysteries, however, in our own particular geometry.  For example, why did we get such good reproducibility in the resonant wavelength of the modes (always near our laser line of 785 nm), and more dramatically, why did we observe our particular polarization dependence?  It's tricky to explain what I mean without a diagram, but I'll try.  "Common sense" and intuition suggest that light polarized with the electric field across the gap between the electrodes should be best at exciting modes that are localized to the gap.  That's proven to be true in many experiments (cited in the paper).  However, in our devices we find that we get the best optical response when the light is polarized with the electric field pointing along the gap (!), and that the emitted light also is polarized along the gap.

After a series of very careful experiments and calculations (collaboration with Mark Knight of the Halas group), we know the answer.  In our system, the metal wire in which the nanogap sits has a transverse plasmon mode (because of our particular choice of material and transverse dimensions) that is well matched to our laser, and optically "bright" in the sense of having a big electric dipole coupling.  Because a given nanogap is not perfectly symmetric, the higher order, multipolar modes localized to the gap (ordinarily optically "dark" because they lack a dipole coupling) get hybridized with that bright mode.  This explains our counterintuitive polarization dependence (the dipole-active transverse piece is what couples to both the incoming and outgoing far field light), and the reproducibility of the plasmon energy (it's set largely by the wire width, not the details of the gap).  Cute stuff, and it is a good example of how even well-known physics (after all, deep down this is a matter of solving Maxwell's equations) can give interesting surprises.
 

Pomona and Harvey Mudd

It's been a busy week.  On Monday I gave a colloquium at Pomona College, and visited both there and Harvey Mudd.  It's great to see the quality of physics instruction and student research at these extremely good undergraduate institutions.  Thanks to my host, optics guru Alfred Kwok, and the other faculty with whom I met, nano-CM expert and department chair David Tanenbaum; Dwight Whittaker, who taught me about the fascinating biomechanics of exploding plants (!); and Alma Zook, who described their 1 meter telescope.  At Harvey Mudd, it was fun to visit and talk plasmonics with Peter Saeta and to meet John Townsend, the author of two books used in Rice's undergrad curriculum. 

Then on Tuesday I gave a physical chemistry seminar at UCSD, hosted by Misha Galperin.  Good conversations with Francesco Paesani and John Weare about various computational challenges, and Michael Tauber taught me about pump-probe Raman spectroscopy to understand the dynamics of charge and spin in carotenoids.  Then it was on to the physicists, visiting with Max Di Ventra, Dimitri Basov, and Ivan Schuller.  Whew!  A great visit.

Finally, at the very end of the week, I came to Boston to hit one day of the AAAS meeting, where I got a chance to hear a talk by Susan Hockfield, who spoke about the essential role of government investment in basic research.  I also got to hear a talk about geoneutrinos and a lecture by Silvan Schweber about Hans Bethe.  Good stuff.

Passing the laugh test

Some scientific claims are so outlandish that they do not pass the "laugh test".  That is, for these claims to be true it would require throwing out physical principles that have been tested in excruciating detail for decades if not hundreds of years.   Perpetual motion machines based on magnets are one example.  Another is the "EM Drive".  Depressingly, the latter has reappeared, featured in the UK edition of Wired.  The idea is that one can make a microwave resonator shaped like a truncated cone, and that somehow when this resonator is pumped with lots of electromagnetic radiation, it will experience a net thrust in one direction, despite the fact that nothing (including photons) is being exhausted.  Why is this absurd on its face?  Well, put a box around the system, and it clearly violates conservation of momentum, a principle that has been tested with extreme precision for hundreds of years.  Despite double-talk about group vs. phase velocity, reference frames, and relativistic effects, the fact remains that the theory of electricity and magnetism, upon which this device allegedly relies, does not violate conservation of momentum, and neither does the quantum version.  The reason this has come up again is that a Chinese researcher claims to have experimentally verified that the effect exists.  No offense to her particular institution, but call me when people at Beijing or USTC or Tsinghua have done this.  I won't be holding my breath.

Quantum sense of smell?

I had been meaning to write a blog post about this for some time, and the recent news article from the BBC and post by ZapperZ inspired me. There is a debate going on in the bio literature about the physical basis for the sense of smell. The traditional idea is that the olfactory receptors in your nose rely on a combination of binding motifs, including molecular shape (think lock-and-key), to identify analytes. However, it is possible for some animals such as fruit flies to tell the difference between a molecule and a deuterated version of that molecule. Since replacing hydrogen with deuterium should not affect chemical binding mechanisms (generally), and since the shapes of the molecules are the same, this would seem to suggest that something else is going on. A new paper has shown that people can also distinguish isotopic ally labeled molecules via smell, in double blind experiments.

 

The suggested candidate is inelastic electron tunneling. As I've discussed elsewhere, electrons can traverse a molecule through a second order tunneling process, and if enough energy is available to those electrons and the microscopic couplings work out right, they can leave behind a vibrational quantum of energy. In so doing, there is a kink in the current as a function of voltage, signifying the onset of this process.

I am very skeptical that true inelastic tunneling of that type is at work in your nose. First, the natural linewidth of IETS features is several times kT. At room temperature, that is several times 26 meV. The energetic difference between, e.g., the CH and CD stretch vibrations is around 125 meV. Basically, even with a laboratory setup and far higher currents than present in biological systems, and with the benefit of phase-sensitive detection, it would be very difficult if not impossible to use IETS to resolve that isotopic difference. That doesnt even take into account the complicated nature of electronic motion in biological conditions. That being said, I suppose there could be some weird physics where that vibrational frequency makes itself known through the noise in electronic motion - I am thinking along the lines of a fluctuation-dissipation effect like this one. Any mechanism has to be robust in the presence of environmental and thermal noise, and IETS is not, in my view. Still, it's a neat mystery!

 

 

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Why whiskey stones don't cool as well as ice.

While they sound like something you might find in the Skymall catalog, whiskey stones have been touted as a way to cool drinks without the annoyance of dilution that you get from melting ice cubes.  It's true that they don't dilute your beverage of choice, and you get to make jokes about having drinks "on the rocks".  However, for real physics reasons these stones just aren't that effective at cooling your drink down.  To see this, let's consider how much energy it takes to warm four of these stones from -40 \(^{\circ}\)C to room temperature (25 \(^{\circ}\)C).  Each stone is around 8 cm³, and granite has a density of 2.7 g/cm³, and the specific heat of granite is 0.79 J/gK.  Combining, that means that warming four of those stones to room temperature would take around 4400 J.

Now consider an equivalent volume of ice starting at -40 \(^{\circ}\)C.  Ordinary ice has a density of 0.917 g/cm³, and a specific heat of roughly 2.05 J/gK.  Warming four 8 cm³ ice cubes up to 0 \(^{\circ}\)C takes 2400 J.  However, converting ice from solid to liquid requires a latent heat, in this case 334 J/g.  So, just melting those ice cubes requires 9800 additional Joules.  Without even worrying about warming up the resulting water, the ice cubes are able to take up almost three times as much energy just by warming up to the melting point and melting.  So, while it's true that ice can dilute your beverage, it is much better at cooling things (if that's what you want to do), thanks to the latent heat, the energy required to change phases.

A lack of understanding of specific heats and so forth is quite common.  Even the article I linked above about whiskey stones says "Another obscure advantage of whiskey stones is they freeze quickly. Granite ice cubes are ready to go after 20 to 30 minutes in the freezer, whereas water needs hours to freeze into ice cubes."  That's not an advantage - it tells you that the heat capacity of your whiskey stones is low compared to the water equivalent.

FSP's fake CV contest

Sorry about the blogging lull.  More to come soon.

In the meantime, this is hilarious, distressing, and insightful in equal portions.  I encourage you to read each one (and the comments). 

Black holes, information, and "fire walls"

It's not my area, but I always take notice when part of the physics community is abuzz about a thought experiment that seems to expose flaws in our understanding.  At issue is whether something dramatic (involving quantum gravitational effects) takes place at the event horizon of a black hole, from the point of view of an infalling observer.  Way back when I took a general relativity course, I learned that, because of the way spacetime works, the more massive the black hole, the more mild the actual curvature of spacetime at the horizon.  Tiny blackholes = tightly curved spacetime at the horizon; galactic-mass blackholes = nearly flat spacetime at the horizon.  (The horizon is the location where, in the usual Schwarzchild treatment, the sign of the metric components flips; that's another way of saying that once you cross the horizon, classically you are inevitably going to hit the singularity.  Avoiding it is mathematically as hard as avoiding next Tuesday in flat spacetime, as my professor had said.)  So, the old-school classical picture says, a freely falling observer can cross the event horizon and not even realize it.  Moreover, classically, an observer at rest relative to the black hole outside the horizon never actually sees anything cross the horizon - from such a perspective, a clock falling toward the horizon gets progressively more red-shifted and runs slower and slower, stopping altogether at the horizon after being infinitely red-shifted.

I freely admit that I don't understand Hawking radiation beyond a handwave level.  Still, I would be grateful if someone could explain to me, even more clearly than the article linked above, what the big deal is.  Arguments about entanglement across the horizon sound almost theological to me - if it's by definition impossible to check to see if measurements inside the horizon are quantum-correlated with measurements outside the horizon, then such discussions don't seem scientifically meaningful.  

Workshop on Surface Plasmons, Metamaterials, and Catalysis

Three of my colleagues and I are helping to organize a workshop at Rice University on May 20-22, 2013.  The goal of this ARO-sponsored workshop is to explore the opportunities for chemical catalysis arising from recent advances in the fields of metamaterials and plasmonics.  The workshop will bring together scientists from the disciplines of electrochemistry, catalysis, and plasmonics, which have not traditionally had a common platform.

Topics include:

• The state of the art in plasmonics, metamaterials, and chemical catalysis
• Areas of catalysis that could benefit from enhanced optical/electromagnetic concepts
• Concepts for nanophotonic- and metamaterials-driven catalysis and heat generation
• Surface nanoengineering to merge nanophotonics and catalysis
• Quantum plasmonics
• Hot electrons driving chemistry
• Chemical sensing using nanophotonic and plasmonic concepts
• Nanophotonic characterization of catalytic structures:  Where do the reactions happen, and how fast?

The deadline for abstract submission is Feb. 14, 2013, and space is limited.  The workshop website is here:  http://PlasEnhCat2013.rice.edu  .

Please feel free to distribute this information to people that would be interested!

Blogs as a way to deal with bad or fraudulent science

Thanks to a colleague, in addition to the controversy discussed here, I have been following closely the saga of science-fraud.org . That website, the content of which has now been taken down (but remains available through google's cache, if you know what you're doing), had been a clearing house where an anonymous primary poster (assisted by anonymous colleagues) reported on biomedical research papers that had what looked like some serious problems with image manipulation. For context, in that research area, a common tool is a Western Blot, where stained or tagged proteins are identified by the position of bands in digital images of electrophoresis gels. Because this data is basically shown in papers just as an image, it is ripe for manipulation by unscrupulous researchers with photoshop. While it might be ok to crank up the contrast, for example, it is definitely not ok to copy and paste sections of image, or erase inconvenient bands, or duplicate images from paper to paper and pretend that they are from different proteins. That site highlighted many many very suspicious images in the literature, and outright accused a number of bio researchers of fraud - the poster contacted the deans of many of these people, as well as their institutional research integrity officers and the national Office of Research Integrity.

That blogger has now been revealed as Paul Brookes, a scientist at the University of Rochester med center. His anonymity was broken when a very angry individual spammed a large number of relevant researchers with an email claiming the Brookes was responsible for that page, which the angry person (pseudonymous, ironically), claimed was a hate site. Brookes has come under serious legal threats; at issue is whether some of the posts were libelous, since they often went well beyond pointing out suspicious figures and directly accused people, in public and to their institutions and grant agencies, of deliberate research misconduct.

There are several lessons to draw from all this, as has been pointed out both by Brookes (in a post that he took down) and others.

• There is a very serious problem with image manipulation out there in that community. It involves investigators and labs at some major places (as well as minor ones).
• Anecdotally, it looks like some journal editors in this field are reluctant to look at this issue as closely as it needs.
• Science-fraud.org did identify serious problems with a number of papers and prompted ORI investigations that uncovered research misconduct.
• There does seem to be a communal need for a better means of identifying suspicious papers, and blogs could serve this role. I'm reluctant to take page views as a direct measure of that need, though - a lot of people click on articles about Kim Kardashian's pregnancy and the latest celebrity trainwrecks, but that doesn't mean that there is a need for those articles.
• There really is no such thing as online anonymity. That's why I've never bothered to obscure my identity.
• Do not post anything online or email anything that you would be ashamed to see on the front page of the New York Times.
• Publicly accusing people of fraud is a serious business - it's not something that should be done without a lot of consideration of the consequences. The university's lawyers are also unlikely to protect a faculty member who makes public accusations like that without consulting them first.
• It does make me wonder whether there are similar issues in the physical sciences on this scale.

On a related note, getting caught up on last year's news, I see that a Columbia economist involved in Freakonomics has somehow ended up with $240k in "lost" research money, and had to write a check to the university for $13k. Wow. I'm pretty sure that I would be fired if I couldn't account for $240k of research funds, and claiming that I just wasn't good at bookkeeping would be considered a laughable response.

All in all, a rather depressing roundup. Remember, it's this kind of fraud and poor behavior that just give ammunition to the anti-science parts of society.

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Ahh, poor journalism and high energy physics. Again.

Well, apparently I should just retire.  According to this article at the National Post, "Higgs boson discovery may signal the world’s last physics experiment as scientists struggle to come up with next big question". This isn't just a case of poor headline scripting by some editor. The article itself says:

What modern physics knows about the matter in the universe (better at the end of 2012 than the beginning) is that it is basically shrapnel, strange bits that endure from an ancient explosive nativity, known as the Big Bang. What Melissa Franklin knows about modern physics (likewise better than ever, or pretty much anyone) is that it is finished. Done. Kaput.

I am going to assume that this is just a case of poor journalism, and that Prof. Franklin, the chair of Harvard's physics department, does not really think that all of modern physics is encompassed by collider-based high energy work.

Review about quantum coherence

Happy 2013 to all!  The Proceedings of the Royal Society A has a special issue from this past fall about the issue of decoherence in quantum mechanics.  When last I looked, the content from this issue was free (!) for download.  I need to read through the rest of it, but I enjoyed Philip Stamp's article about decoherence and the possibility of intrinsic decoherence.  The point is, in ordinary quantum mechanics the state function of a quantum system evolves with time according to the Schroedinger equation, also called unitary time evolution.  However, that seems at odds with what happens when we perform measurements - in that situation, it seems like the state of the system "collapses" into an eigenstate of a measured observable, so that coherence appears to be lost, and we don't observed, e.g., Schroedinger's cat to be in a superposition of alive and dead.  The now standard treatment says that the apparent decoherence of a quantum system of interest when we "perform a measurement" results from  the coupling of the system with many environmental degrees of freedom (a "bath"), the states of which we then "trace over".  When we do this, looking only at the system, we see what looks like decoherence of the system, but the idea is that this is an approximation of the true situation, in which the whole system + environment is still obeying the Schroedinger equation.  (This skirts other aspects of the "measurement problem", like what really picks out the particular classical states that we seem to observe.)  Intrinsic decoherence would imply either that there is something genuinely wrong with unitary time evolution (i.e., quantum mechanics is incomplete), or there are environmental degrees of freedom out there in the very fabric of the universe that are impossible to avoid, such as the quantum degrees of freedom of curved spacetime itself.   

Quantum spin liquids - neat stuff!

There is a new result in this week's issue of Nature that is very neat (and my college classmate Young Lee is the PI - small world!).  The experiment is an inelastic neutron scattering measurement that looks at a material with the unusual name herbertsmithite, and reports evidence that this material is a "quantum spin liquid".  I'll try to break down the physics here into reasonably accessible bite-sized chunks.

First, what is a spin liquid?  Imagine having a bunch of localized spins on a lattice.  You can picture these like little bar magnets.  In this case, the spins are the unpaired d electrons of the copper atoms in the herbertsmithite structure.   In general, the spins in a solid (this particular one is an insulator) "talk" to each other via the exchange interaction.  What this really means is that there are interactions between the spins so that the spins prefer a particular relative orientation to each other.  In this case, the interaction between the electron spins is antiferromagnetic, meaning that for spins on two neighboring Cu atoms, having the spins be oppositely directed saves some energy (17 meV) compared to having the spins aligned.  As the temperature is lowered, an ensemble of spins will tend to find whatever configuration minimizes the total energy (the ground state).  In a ferromagnet, that will be a state with the spins all aligned with their neighbors.  In a perfect antiferromagnet, that would be a state where the spins are all antialigned with their neighbors.  Both of these are ordered ground states, in that there is some global arrangement of the spins (with a particular symmetry) that wins at T = 0.   The problem in herbertsmithite is, because of the spatial arrangement of the Cu atoms (in a Kagome lattice), it's impossible to have every spin antialigned with all of its neighbors.  This is an example of geometric frustration.   As a result, even as T gets very low, it would appear that the spins in herbertsmithite never order, even though they interact with their neighbors very strongly.  This is an analog to the liquid state, where the molecules of a liquid clearly interact very strongly with their neighbors (they bump right into each other!), but they do not form a spatially ordered arrangement (that would be a solid).

Why a quantum spin liquid?  Two reasons.  First, I cheated in my description above.  While we can talk classically about antialigned spins, we really should say that pairs of spins want to form singlets, meaning quantum mechanically entangled antialigned states with net spin zero.  So, you can think of this spin liquid state as involving a big entangled mess of spins, where each spin is somehow trying to be entangled in a singlet state with each of its nearest neighbors.  This is very complicated to treat theoretically.  Second, the fluctuations that dominate in this situation are quantum fluctuations, rather than thermally driven fluctuations.  Quantum fluctuations will persist all the way down to T = 0. 

What's special about a quantum spin liquid?  Well, the low energy excitations of a quantum spin liquid can be very weird.  If you imagine reaching into the material and flipping one spin so that it's now energetically "unhappy" in terms of its neighbors, what you find is that you can start flipping spins and end up with "spinon" excitations that travel through the material, having spin-1/2 but no charge, and other exotic properties.  This is described reasonably well here.  Importantly, these excitations have effects that are seen in measurable properties, like heat capacity and how the system can take up and lose energy.

So what did the experimenters do?  They grew large, very pure single crystals of herbertsmithite, and fired neutrons at them.  Knowing the energies and momenta of the incident neutrons, and measuring the energies and momenta of the scattered neutrons, they were able to map out the properties of the excitations, showing that they really do look like what one expects for a quantum spin liquid. 

Why should you care?  This is a great example of seeing exotic properties (like these weird spin excitations) that emerge because of the collective response of a large number of particles.  A single Cu ion or unit cell of the crystal doesn't do this stuff - you need lots of spins.  Moreover, this is now a system where we can study what this weird, highly quantum-entangled does - I think it's very very far from practical applications, but you never know.   Looks like a very nice piece of work.

Just how self-correcting is science?

In an extremely timely article in the new issue of American Scientist, Joseph Grcar looks at what fraction of publications in various disciplines are basically corrective (that is, comments, corrigenda,  corrections, retractions, or refutations).  He finds in the sciences in general the correction rate is about 1-1.5% of publications.  This is probably a bit of an underestimate, in my view, since there are new works published that are essentially soft refutations that may not be detected by the methods used here.  Likewise, some fraction of the body of published work (constituting the denominator of that fraction) has no impact (in the sense of never being cited).  Still, that's an interesting number to see. 

A nano controversy

A couple of my colleagues pointed me to this blog, that of  Raphaël Lévy at Liverpool.  Lately it has become a clearing house for information about a particular controversy in nanoscale science, the question of "stripy nanoparticles".  The ultrashort version of the story:  Back in 2004, Francesco Stellacci (then at MIT, now in Switzerland) published a paper in Nature Materials arguing that his group had demonstrated something quite interesting and potentially useful.  Very often when solution-based materials chemistry is used to synthesize nanoparticles, the nanoparticles end up coated in a monolayer of some molecule (a ligand).  These ligands can act as surfactants to alter the kinetics of growth, but their most important function is to help the nanoparticles remain in suspension by preventing their aggregation (or one of my favorite science terms, flocculation).  Anyway, Stellacci and company used two different kinds of ligand molecules, and claimed that they had evidence that the ligands spontaneously phase-segregated on the nanoparticle surface into parallel stripes.  His group has gone on to publish many papers in high impact journals on these "stripy" particles.  

However, it is clear from the many posts on Lévy's blog, to say nothing of the paper published in Small, that this claim is controversial.  Basically those who disagree with Stellacci's interpretation argue that the scanned probe images that apparently show stripes are in fact displaying a particular kind of imaging artifact.  As an AFM or STM tip is scanned over a surface, feedback control is used to maintain some constant conditions (e.g., constant AFM oscillation frequency or amplitude; constant STM tunneling current).  If the feedback isn't tuned properly, there can be "ringing" so that the image shows oscillatory features as a function of time (and therefore tip position).  

I have no stake in this, though I have to say that the arguments and images shown by the skeptics are pretty persuasive.  I'd have to dig through Stellacci's counterarguments and ancillary experiments, but this doesn't look great.

This whole situation does raise some interesting questions, though.  Lévy points out that many articles seem to be published that take the assertion of stripiness practically on faith or on very scant evidence.  Certainly once there is a critical mass of published literature in big journals claiming some effect, it can be hard as a reviewer to argue that that body of work is all wrong.  Still, if you see (a series of) results in the literature that you really think are incorrectly interpreted, what it is the appropriate way to handle something like this?  Write a "comment" in each of these journals?  How should journals respond to concerns like this?  I do know that editors at high profile journals really don't like even reviewing "rebuttal" papers - they'd much rather have a "comment" or to let sleeping dogs lie.  Interesting stuff, nonetheless.

Update: To clarify, I am not taking a side here scientifically - in the long run, the community will settle these questions, particularly those of reproducibility.  Further, one other question raised here is the appropriate role of blogs.  They are an alternative way of airing scientific concerns (compared to the comment/rebuttal format), and that's probably a net good, but I don't think a culture of internet campaigns against research with which we disagree is a healthy limiting case.

The future of Si: Into the fog

Today we had a visit at Rice from Mike Mayberry, the VP for the Technology and Manufacturing group at Intel.  He gave a very good talk about where semiconductor electronics is going (naturally with the appropriate disclaimers that we shouldn't buy or sell stocks based on anything he said).  The general rule is that there is a metaphorical fog out there about ten years off, beyond which it's not clear what the industry will be doing or how it will be doing it.  However, for the comparatively near term, the path is fairly well known.  In short: complementary metal-oxide-semiconductor (CMOS) based on Si is going to continue for quite a ways longer.  Device design will continue toward improved electrostatic control of the channel - we will likely see evolution from tri-gate finFET structures toward full wrap-around gates.  We are firmly in the materials-limited regime of device performance, and Intel has been playing games with strain to improve charge mobility.  They have even experimented with integration of III-V materials via epitaxy onto Si platforms.  It's pretty clear that there is a healthy skepticism about post-CMOS alternative technologies, particularly given the absurdly low cost and high volumes of Si.  Intel ships something like 4 trillion transistors per minute (!).  Other remarkable facts/figures:  Network traffic right now is around 7 exabytes (10¹⁸ bytes) per day, or the equivalent of 17000 HD movies every second, including around 204 million emails per minute on gmail, and these numbers are increasing all the time.  Amazing.

Ionic liquid gating - amazing results

I've mentioned ionic liquid gating a couple of times (here, here) before.  Ionic liquids are basically organic salts (molecular anion; molecular cation, though there are some that use, e.g., alkaline metal ions instead) that are liquid at room temperature.  The concept is simple enough:  use the ionic liquid as an electrolyte in a capacitor comprising a wire employed as a gate electrode and the surface of a sample of interest as the counterelectrode (helpfully contacted by source and drain electrodes).   Setting the bias of the gate relative to the surface drives the ions to move, building up (ionic) surface charge densities at the sample surface.   The material responds to screen what would otherwise be an enormous electric field penetrating the surface by accumulating mobile charge carriers of the appropriate species in a layer at the interface.  This approach, adopted by multiple groups, has been pushed hardest by the group of Iwasa.  The real advantage of this approach is that it allows access to electrostatically gated charge densities comparable to what can be achieved in chemical doping - on the order \( 10^{15} \) per cm² in that top layer of material.  (This is the regime that He Who Shall Not Be Named fraudulently claimed to access, but this time it's real!)

Particular highlights have included:

• Gating SrTiO₃ into metallicity and then superconductivity.
• Gating ZnO into metallicity.
• Gating ferromagnetic response in GaMnAs and Co-doped TiO₂.
• Gate-induced superconductivity in KTaO₃.

A new paper appears in Science this week with yet another impressive result.  Iwasa's team has used ionic liquid gating to push an exfoliated flake of MoS₂ into superconductivity.  Indeed, they see what looks a lot like a superconducting "dome" as a function of gated charge, with \(T_{c}\) actually decreasing above an optimal gated carrier density.   Wild.

The results just keep coming.  There are some real subtleties to the technique, though.  It's extremely important to make sure that the ionic liquid is really acting as a chemically inert electrolyte, and not inducing electrochemistry at the material surface or any other kind of chemical doping.

Anyway, this whole area is clearly one to watch in condensed matter.  Anytime you can push into a previously inaccessible regime, Nature tends to offer up surprises.

Tidbits

Now that a couple of papers are in and the never-ending list of tasks is getting shorter, hopefully blogging will pick back up. Here are a few interesting links for the (American) Thanksgiving holiday.

 

This is a paper about the subtleties and challenges of computational physics, meant to be very conversational and pedagogical for students. It was a fun read, and we should have more resources like this.

The Bad Astronomer has relocated to Slate.com. He did this just in time for the rampant speculation about NASA's pending chemistry results from the Curiosity rover. I don't think Curiosity actually killed a cat, but you never know.

The University of Houston just had a one-day symposium in honor of the twenty-fifth anniversary of the discovery of YBCO, the first superconductor with a transition temperature greater than the boiling point of liquid nitrogen. I wish I could've been there, but I had other commitments. My colleague tells me it was a very nicely done affair with many interesting talks and panel discussions. Now if only we could figure this out definitively....

It's always interesting to see a very thought-provoking paper from someone you take seriously. Miles Blencowe, a condensed matter/quantum measurement theorist at Dartmouth, argues that perturbative quantum gravity (a general approach to extending general relativity a bit into the quantum regime) can be a major source of decoherence of quantum superpositions of massive objects. This implies that the very quantum structure of spacetime acts dynamically to "collapse wavefunctions" in the language of the Copenhagen take on quantum mechanics. I need to read this more carefully.

 

 

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Physics education, again.

This video is great at pointing out much of what is wrong with high school physics education in the US.  However, I find the implication that this is not that hard to fix to be a bit tough to swallow.  More later....