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.

Interesting links: peer review, falsifiability

Slow blogging - I've got the usual papers plus working on finishing a really big writing project (more about that soon), combined w/ summer travel.  The posting rate will pick up again in another week and a half.  In the meantime, here are a few interesting links from the last couple of weeks.

• A thoroughly dishonest scientist (and I guess a couple of other people) were exposed as running an awful peer review scam.  More about this here.  The scam involved creating fake email addresses and identities to mask people essentially reviewing their own and friends' papers.  The worst thing about this whole mess is that it gives ammunition to the anti-science crowd who are convinced that scientific research is a corrupt enterprise - people like the person I wrote about here.
• Peter Woit has written an interesting review of a book about string theory and whether the scientific method needs to be revised to deal with "post-emprical" theory verification, whatever that means.  I haven't read the book, but the idea of post-empiricism is pretty sketchy to me.
• Natalie Wolchover has written an article about some fluid droplet experiments that show quantum-like behavior of droplets (e.g., interference-fringe-like distributions, for example).  The physics here is that the droplets are interacting with associated surface waves of an underlying fluid, and the mechanics of those waves self-consistently guides the droplets.  This is similar in spirit to Bohm's ideas about pilot waves as a way of thinking about quantum mechanics.  The authors of the fluid paper are clearly high on this idea.  These are clearly very cool experiments, but it's a huge stretch to say that they should motivate re-thinking our interpretations of quantum mechanics. 

An expression of concern about an expression of concern

There has been a big kerfluffle about Facebook conducting a mass social psychology experiment.  At heart is the issue of informed consent.  By clicking "ok" on a vaguely worded license agreement, did users really give true informed consent to participate in experiments designed to manipulate them?  The study was published in the Proceedings of the National Academy of Sciences here.  Now, in hindsight, PNAS has published an "Expression of Concern" here about whether the study was in compliance with the Common Rule regarding informed consent by human subjects.  The PNAS editors point out that as a privately funded, for-profit corporation not taking federal funding for this work, Facebook isn't technically bound by this constraint.

This is technically correct (the best kind of correct), but doesn't this have frightening implications?  Does this mean that private companies are free to perform experiments on human subjects without asking for informed consent, so long as they don't violate obvious laws like killing people?  Seems like there must be some statutes out there about human experimentation, right?  Perhaps one of my readers knows this issue....

What are universal conductance fluctuations?

Another realization I had at the Gordon Conference:  there are plenty of younger people in condensed matter physics who have never heard about some mesoscopic physics topics.   Presumably those topics are now in that awkward purgatory of being so established that they're "boring" from the research standpoint, but they are beyond what is taught in standard solid state physics classes (i.e., they're not in Ashcroft and Mermin or Kittel).  Here is my attempt to talk at a reasonably popular level about one of these, so-called "Universal Conductance Fluctuations" (UCF).

In physics parlance, sometimes it can be very useful to think about electrons in solids as semiclassical, a kind of middle ground between picturing them as little classical specks whizzing around and visualizing them as fuzzy, entirely wavelike quantum states.  In the semiclassical picture, you can think of the electrons as following particular trajectories, and still keep in mind their wavelike aspect by saying that the particles rack up phase as they propagate along.  In a typical metal like gold or copper, the effective wavelength of the electrons is the Fermi wavelength, \( \lambda_{\mathrm{F}} \sim 0.1~\)nm.  That means that an electron propagating 0.1 nm changes its quantum phase by about \(2 \pi\).  In a relatively "clean" metal, electrons propagate along over long distances, many Fermi wavelengths, before scattering.  At low temperatures, that scattering is mostly from disorder (grain boundaries, vacancies, impurities).

The point of keeping track of the quantum phase \(\phi\) is that this is how we find probabilities for quantum processes.  In quantum mechanics, if there are two paths to do something, with (complex) amplitudes \(A_{1}\) and \(A_{2}\), the probability of that something is \(|A_{1} + A_{2}|^{2}\), which is different than just adding the probabilities of each path, \(|A_{1}|^{2}\) and \(|A_{2}|^{2}\).  For an electron propagating, for each trajectory we can figure out an amplitude that includes the phase.  We add up all the (complex) amplitudes for all the possible trajectories, and then take the (magnitude) square of the sum.  The cross terms are what give quantum interference effects, such as the wavy diffraction pattern in the famous two-slit experiment.  This is how Feynman describes interference in his great little book, QED. 

Electronic conduction in a disordered metal then becomes a quantum interference experiment.  An electron can bounce off various impurities or defects in different sequences, with each trajectory having some phase.  The exact phases are set by the details of the disorder, so while they differ from sample to sample, they are the same within a given sample as long as the disorder doesn't change.  The conduction of the electrons is then something like a speckle pattern.  The typical scale of that speckle is a change in the conductance \(G\) of something like \(\delta G \sim e^{2}/h\).  Note that inelastic processes can change the electronic wavelength (by altering the electron energy and hence the magnitude of its momentum) and also randomize the phase - these "dephasing" effects mean that on length scales large compared to some coherence length \(L_{\phi}\), it doesn't make sense to worry about quantum interference.

Now, anything that alters the relative phases of the different trajectories will lead to fluctuations in the conductance on that scale (within a coherent region).  A magnetic field can do this, because the amount of phase racked up by propagating electrons depends not just on their wavelength (basically their momentum), but also on the vector potential, a funny quantity discussed further here.  So, ramping a magnetic field through a (weakly disordered) metal (at low temperatures) can generate sample-specific, random-looking but reproducible, fluctuations in the conductance on the order of \(e^{2}/h\).  These are the UCF. 

By looking at the UCF (their variation with magnetic field, temperature, gate voltage in a semiconductor, etc.), one can infer \(L_{\phi}\), for example.  These kinds of experiments were all the rage in ordinary metals and semiconductors in the late 1980s and early 1990s.  They enjoyed a resurgence in the late '90s during a controversy about coherence and the fate of quasiparticles as \(T \rightarrow 0\), and are still used as a tool to examine coherence in new systems as they come along (graphene, atomically thin semiconductors, 2d electron gases in oxide heterostructures, etc.). 

Gordon Conference thoughts

Because of travel constraints I'm missing the last day of the meeting, but here are some thoughts, non-science first:

• These meetings remain a great format - not too big, a good mix of topics, real opportunities for students and postdocs to interact w/ lots of people, chances for older researchers to play soccer and ultimate frisbee, etc.  As travel costs rise and internet connectivity improves, there are going to be sensible reasons to have fewer in-person meetings of otherwise distant participants, but there remains no substitute for a good conversation face-to-face over a coffee or a beer.
• College dorm rooms, while better than when I was a student, are still not high on ambiance.  Generic fitted and top sheets for bedding appear to be made from dryer lint.
• Food options have become progressively healthier and tastier in general.
• Mount Holyoke is a lovely campus, with very loud and happy frogs.
• A session about cuprate superconductors correlated with the literal gathering of storm clouds in an otherwise sunny week.
• About 30% of the audience got the reference (after about a 5 second delay) when, on a slide about magnetic interactions (\(J_{zz} S^{z}_{i} \cdot S^{z}_{j}\)), there was an unlabeled picture of Jay-Z.  

A couple of science thoughts (carefully brief to avoid violating the GRC policy about discussing conference talks and posters):

• Cuprate superconductors remain amazingly complicated, even after years of improving sample quality and experimental techniques. 
• Looking at driven systems is becoming very exciting.  Basically under some circumstances you can use light to flip on or off topological changes in band structure, for example.
• It remains very challenging to figure out how to think about systems with low energy excitations that don't look like long-lived quasiparticles. 

Gordon Conference

I am going to be at the Gordon Research Conference on correlated electrons for the next few days. Should be fun, but blogging about such meetings is generally frowned upon (don't want to discourage people from frank discussions and showing brand new, untried stuff).  There are rules about confidentiality for these meetings.  I'll write more later in the week on other topics.

FeSe on SrTiO3: report of 100 K superconductivity

I'd heard rumors about this for a while.  I presume that the posting of this on the arxiv means that some form of this paper is in submission out there to a suitably glossy, high impact journal that requires reference citations in its abstracts.  Background:  Bulk FeSe superconducts below around 8 K at ambient pressure (see here).  Under pressure, that transition can be squeezed up beyond 35 K (see here).  The mechanism for superconductivity in this material is up for debate, as far as I know (please feel free to add a reference or two in the comments). 

These investigators have a very fancy ultrahigh vacuum system, in which they are able to grow single layer FeSe on top of SrTiO₃ (with the substrate doped with niobium in this case).  This material is not stable in air, and apparently doesn't do terribly well even when coated with some protective layer.  However, these folks have a multi-probe scanning tunneling microscope system in their chamber, along with a cold stage, so that they can perform electrical measurements in situ without ever exposing their single layer to air.  They find that the electrical resistance measured in their four-point-probe configuration drops to zero below around 100 K (as high as 109 K, depending on the sample).  One subtle point that clearly worried them:  SrTiO₃ is know to have a structural phase transition (the onset of ferroelasticity - see here) at around 105 K, so they wanted to be sure that what they saw wasn't somehow an artifact of that substrate effect.  (Makes me wonder what happens to superconductivity in the FeSe depending on the ferroelastic domain orientation underneath it.)  For the lay audience:  liquid nitrogen boils at ambient pressure at 77 K.  This would be the first iron-based superconductor to cross that threshold, a domain previously limited to the copper oxides.   Remember, if the bulk transition is at 8 K and the single layer case exceeds 100 K, it doesn't seem crazy to hope for some related system with an additional factor of three or four that takes us beyond room temperature.

Important caveats:  Right now, they have resistance measurements and tunneling spectroscopy measurements.  Because of the need for in situ measurement they don't have Meissner data.  It's also important to realize that the restrictions here (not air stable; only happens in single layer material when ultraclean) are not small.  At the same time, this is potentially very exciting, and hopefully it holds up well and can be the foundation for more exciting materials.

750th post - blog demographics

This is the 750th post since this blog's inception.  Fun facts gleaned from google analytics:

1) Unsurprisingly, the US leads in blog hits over that time, with 270,648.  In second place, the UK with 26,698.

2) According to google's tracking, over the last nine years there have been hits from every country in North, Central, and South America, as well as Europe.  In Asia, the only countries with zero hits are Turkmenistan and New Guinea.  In Africa, I'm missing about a dozen, basically the sub-Saharan region plus Somalia. 

3) In the US, the state with the least hits is South Dakota (84 visits over nine years), narrowly edging out Wyoming (88) and Alaska (91).   The states with the most hits are Texas, California, New York, and Massachusetts.

4) Most common browser, by a wide margin, is Firefox, followed by Chrome.  I like the idea that someone has read the blog on a PlayStation 3, and someone else on a PlayStation Portable.  Disappointed (and showing my age by that fact) that no one used lynx or emacs.  

5) Most-viewed post of all time was the meme contest.  Most-viewed physics posts were these on plasmons and polarons. 

Thank you all for reading!

"Seeing" chemical bonds with sub-molecular resolution

Chemists (and physicists) often draw molecular bonds as little lines connecting atoms, but actually imaging the bonds themselves is very hard.  With the advent of the scanning tunneling microscope, it's become almost commonplace to be able to image the position of atoms.  STM images the ability of electrons to enter or leave a conducting surface, and since an atom on the surface carry electrons within itself, the presence of an atom on the surface strongly modulates the STM signal.  This doesn't show anything direct about bonding between atoms, however.

Wilson Ho's group at UC Irvine has published another gem.  The paper is here (unfortunately behind the Science paywall), and the news release is here.  The new STM-based imaging technique, "itProbe", is based in inelastic tunneling, which I've described before.  (One advantage in being an ancient blogger - I can now refer back to my old stuff, with google helping me remember what I wrote.)  The Ho group deliberately attaches a CO molecule to their STM tip.  The CO molecule has a couple of very sharp vibrational (and "hindered translational") modes at low energies that can be seen electrically through inelastic electron tunneling spectroscopy (IETS) - basically sharp features in (the second derivative of) the tunneling current-voltage curve.  In the itProbe technique, the experimenters map out spatially what happens to those modes.  The idea is, as the CO molecule interacts with the sample close by, the precise energies of those vibrational modes shift - the environment of the CO molecule tweaks the effective spring constant for the CO's motion.  Imaging in this way, they find that maps of the inelastic signal seem to show the bonds between the atoms in an underlying molecule, rather than the atom positions themselves.  I admit I don't understand the precise mechanism here, but the images are eye-popping.  A similar idea, involving atomic force microscopy with CO attached to an AFM tip, was demonstrated before (here and here, for example).  In those experiments, the investigators looked at how interactions between the CO on the tip and the sample affected the mechanical properties of the tip as a whole.   

This is an example of a tour de force experiment that can be accomplished by long, sustained effort - the Ho group has been refining their IETS measurements for nearly two decades, and it's really paid off.  Hopefully these kinds of efforts will not become even less common as research funding seems to be focused increasingly on short time horizons and rapid changes in fashion.

Bad physics as a marker for tracking text recycling

A colleague of mine was depressed to find, in a reasonably high impact journal, a statement that magnetic nanoparticles obey Coulomb's law, and thus can be manipulated by external magnetic fields.  As far as physics goes, this is just wrong.  Coulomb's law is the mathematical relationship that says that the force between two charges is proportional to the product of their charges and inversely proportional to the distance between them.  This has nothing to do with magnetic nanoparticles. 

I was curious - where did this weird, incorrect statement come from?  I turned to google to find out.  The earliest result I can find is from this paper by Pankhurst, Connolly, Jones, and Dobson.  The paper seems quite good, and the (strange to me) Coulomb's Law language appears to be some shorthand for a physically sound description of the interactions of magnetic materials with magnetic fields.  The Pankhurst paper includes the following sentence:  "Second, the nanoparticles are magnetic, which means that they obey Coulomb’s law, and can be manipulated by an external magnetic field gradient." This is part of a paragraph that lists three virtues of magnetic nanoparticles for biological applications.  


For fun, try copy/pasting that sentence into google.  Look at how many times that sentence (indeed, that whole introductory paragraph with very minimal changes) shows up nearly verbatim in other publications.  At the risk of saying something actionable, this is plagiarism.   This tends to happen in obscure proceedings, edited book chapters, etc., rather than high impact literature.  The proliferation of shady publication houses and vanity press journals only aggravates this situation.  Very depressing.

My views on teaching "nano"

Blatant self-promotion time:  I was grateful for the invitation to write an editorial about teaching "nano" for Nature Nanotechnology.  The full text is available for free at the above link, and comments and feedback are invited below.  (As a blog reader, you get the added bonus of reading the analogy I made that was cut due to space constraints.  When I advise becoming an expert in a traditional discipline first before tackling an interdisciplinary field, I had written:  "To make a food analogy, it would be very difficult to become an expert at Korean/Mexican fusion cuisine if you did not first know Korean and/or Mexican cooking at a high level.") 

What is chemical potential?

I've been meaning to do a post on this for a long time.  Five years ago (!) I wrote a post about the meaning of temperature, where I tried to go from the intuitive understanding given by common experience (temperature has something to do with energy content, and that energy flows from hot things to cold things) to a deeper view (that flow of energy comes from the tendency of the universe to take on macroscopic configurations that correspond to the most common ways of arranging microscopic degrees of freedom - the 2nd law of thermodynamics, basically).  I wasn't very satisfied with how the post turned out, but c'est la vie.

Chemical potential is a similar idea, but with added complications - while touch gives us an intuition for relative temperatures, we have no corresponding sense for chemical potential; and the rigorous definition of chemical potential is more complicated.  (For another take on this, see this article, available in full text via google from a variety of sources.)

Let's reason by analogy with temperature.  Energy tends to flow from a system at high temperature to a system at low temperature; when systems with identical temperatures are brought into contact so that they may exchange energy (e.g., by thermal conduction), there is no net flow of energy.  Now suppose systems are able to exchange particles as well as energy.  If two systems are at the same temperature, then particles will tend to flow from the system of higher chemical potential (one of the several parameters denoted by the symbol \(\mu\)) to that of lower chemical potential.  If two systems have identical chemical potentials for a particular kind of particle, there will be no net flow of particles.  In general, particles tend to flow from regions of high \(\mu/T\) to regions of low \(\mu/T\).  The classic example of this is the case of a closed bottle of perfume in a room full of (non-perfumed) air.  The perfume molecules have a high \(\mu\) in the bottle relative to the rest of the room.  When the bottle is opened, perfume molecules will tend to diffuse out of the bottle, simultaneously lowering their \(\mu)\) in the bottle and increasing their \(\mu\) in the room.  This will continue until the chemical potentials equalize.  From the point of view of entropy, there are clearly very many more arrangements of molecules with them roughly spread throughout the room+bottle than the number of arrangements with the molecules happening to occupy just the bottle.  Hence, the universe tends toward the macroscopic configuration corresponding to the most microscopic configurations.  Bottom line:  equilibrium between two systems that can exchange particles requires equal temperatures and equal chemical potentials. 

Where this also gets tricky is that thermodynamics tells us that \(\mu\) also corresponds to the energy per particle required to add (or remove) one particle from the system at constant temperature and pressure (!).  This identity is not at all obvious from the above description, but it's nevertheless true.  This latter way of thinking about chemical potential means that when particles can couple to some "real" potential (gravitational, electrical), it is possible to tune their total \(\mu\).  The connection to the entropic picture is the idea that particles will tend to "fall downhill" (there are usually fewer configurations of the combined system that have some particles "stacked up" in a region of high potential energy with others in a region of low potential energy, than the situation when the energy gets spread around among all the particles). 

Prize season again - updated w/ Kavli winners

Once again the Breakthrough Prize and New Horizons Prize in fundamental physics are seeking nominations.  See here.  I have very mixed feelings about these prizes, given how the high energy theory components seem increasingly disconnected from experiment (and consider that a feature rather than a bug).

On a related note, the Kavli Prizes are being awarded this Thursday.  Past nanowinners are Millie Dresselhaus (2012), Don Eigler (love his current affiliation) and Nadrian Seeman (2010), and Louis Brus and Sumio Iijima (2008).   Not exactly a bunch of underachievers.  Place your bets.  Whitesides?  Alivisatos and Bawendi?

Update:  Thomas Ebbeson (extraordinary transmission through deep sub-wavelength apertures, thanks to plasmons), Stefan Hell (stimulated emission depletion microscopy, for deep subwavelength fluorescence microscopy resolution), and John Pendry (perfect lenses and cloaking).  Congratulations all around - all richly deserved.  I do think that the Kavli folks are in a sweet spot for nano prizes, as there is a good-sized pool of outstanding people that has built up, few of whom have been honored already by the Nobel.  This is a bit like the early days of the Nobel prize, though hopefully with much less political infighting (see this book if you really want to be disillusioned about the Nobel process in the early years).