Fun with single atoms

IBM Almaden research has produced "A Boy and His Atom", a stop-motion movie where the frames are scanning tunneling microscope images of carbon monoxide molecules on the Cu(111) surface.  Here is the "Making of..." movie as well.  Fun stuff.  I was particularly amused by Andreas Heinrich's comment that "if I can get a thousand kids to join science rather than go into law school, I'd be super happy."  Amen :-)

Can Congress please not screw up the NSF? Please?

I just read this article at Science's blog, describing how Lamar Smith, chair of the House science committee, basically wants to gut peer review at the NSF and replace it with something more to the liking of the House Republicans.  This would be catastrophically bad for a large number of reasons.  If they do this, you know it's only a matter of time before they decide to undermine peer review at NIH and DOE Office of Science also.  Gahh.  Can't blog - too incoherently angry.

update:  For what it's worth, this would presumably have a hard time passing the Senate and getting signed into law by the President, though given Congress' tendency to lump zillions of unrelated bills together into giant omnibus legislation, you never know for sure.   NSF is probably not the real long-term target of these types.  Picture what would happen to research if big pharma lobbyists get to have Congress decide what NIH grants should be funded, or if big energy lobbyists do the same for DOE grants (to say nothing of de-funding anything they find politically unacceptable).

Cryogenic dark matter detection, redux

About 3.5 years ago, I posted about the technology used by the CDMS collaboration to look for dark matter using clever solid-state detectors.  These folks have some news that is, as is often the case in any novel particle detection experiment, intriguing but not yet definitive.  This paper is probably the best place to see a summary of the results.  In their CDMS II run, the team had 19 germanium-based and 11 silicon-based detectors (cooled to 0.04 K!) running for five years (2003-2008) in an old mine in Minnesota (to cut down on cosmic ray background).  This paper reports results from the Si detectors, where after a lengthy blind analysis they see three events that look interesting.  Since germanium has a higher atomic number than Si, the idea of running the two materials in parallel was to have a cross-check and provide some information about how the searched-for weakly interacting massive particles (WIMPs) might interact with ordinary matter as a function of energy.  I should also note that the collaboration is now running "SuperCDMS" with a larger mass of germanium (9 kg) since 2011, and will eventually expand up to 200 kg of Ge running in Soudan in Ontario.  It's interesting that their earlier analysis from their Ge detectors reported no candidate events (as far as I can tell), while the analysis of the Si detectors shows three candidate events.  My understanding is that this could have to do with the mass range of the WIMPs, but I would be happy if someone would provide more context in the comments below.  Either way, I think it's great to see how condensed matter physics (and in particular cool device fabrication, as in the superconducting transition-edge sensors used here) can have an impact on Big Questions like dark matter.

MOOCs and online education

Massive open online courses (MOOCs) and concerns about online education are all the rage these days at universities.  There is a growing recognition of a few key points:  The cost of undergraduate education (in the US at least) continues to increase much more rapidly than inflation; online capabilities are sufficiently advanced now that it is possible, for comparatively little investment, to distribute educational content to many thousands of people at very low cost, in principle having a major pedagogical impact (see, e.g., the Khan Academy, to say nothing of MIT's opencourseware); more than one major concern is springing up trying to guide online education at the university level (see, e.g., coursera and edX).  [Note to self:  find some demo as cool as the thermite reaction to hook people into any online course I ever teach.]  There is clearly a major sense of urgency on the part of university administrators.  To belabor an overused analogy, they are worried that the online education train is leaving the station, and they fear the consequences of getting left behind. 

All of these things are true, and I understand the concern.  However, a few points have occurred to me about this, and I'd be happy for some discussion in the comments if people are interested.

1. Many people do not really have the self-discipline to learn in an online-only environment.  I like to think I was a pretty dedicated student (no smart comments from my former classmates, please), and I'm not sure I would have the self-discipline to watch online-only lecture material and do online-only assignments for an entire semester.  Some people do have the personality for this, but I have a hunch that many of them are the same folks who really can check a book out of the library and teach themselves a new subject ab initio.  Most 18 year olds are not like that, and the peer pressure/social environment of having friends physically going to scheduled classes is a major motivator.  Bill Press, when he visited Rice and we chatted about this, pointed out that many people pay real money to take Microsoft online certification courses, and complete them at a high rate.  That's true, but it's also a particular case where the financial benefits of completing that particular course are often very clear to the student, and it's also true that there's a difference between university study and vocational training.
2. It only makes sense to develop online courses where your institution really adds value.  Does anyone think it would be a good idea for every major university to develop their own MOOC for Introductory Calculus?  We could do that, but in the end there will likely be a small handful of truly innovative, extremely well done calc courses.  The market will drive toward some kind of mix-and-match mode of operation (unless the content providers constrain things greatly).
3. The sense of urgency is not unreasonable, but early innovators don't necessarily win the day.  For example, Lycos and Alta Vista were early to the scene in "search", yet comparative latecomer google crushed them.

So, are MOOCs really going to sweep through and destroy the modern university system?  Are physical universities going to become like specialty bookshops and online providers like Amazon?  Let me know what you think.

Book review: Alsos

I just found and read a great book, Alsos, by Samuel Goudsmit. The Alsos mission was the Allied dual scientific/military intelligence gathering expedition following the Normandy Invasion, tasked with learning the status of the German atomic program and rounding up German nuclear scientists. Goudsmit, who with Uhlenbeck helped convince people like Pauli of the usefulness of the concept of spin (the intrinsic angular momentum of particles like the electron), was a Dutch Jew, and while he was in the States working on radar, his parents were sent to a concentration camp and killed. The book is fascinating. It's split between the story of the actual mission (which discovered relatively quickly and much to the relief of all involved that the Germans never even got a nuclear pile to go critical) and an indictment of science and industry in a totalitarian regime. It is quite the cautionary tale of the politicization of scientific research and the arrogance of some physicists (Heisenberg fares particularly poorly, to the surprise of no one), told with a wry sense of humor. Highly recommended.

 

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Workshop on "Electronic Properties of Carbon-based Nanostructures"

I'm on my way back to the US from this workshop at the Universität Regensburg.  It was a fun and interesting meeting, and the quality of the invited talks was uniformly high.  The city was also very neat.  I'd had no idea that it had managed to escape (almost completely) Allied bombing during WWII, so as a result it has many buildings dating back to the Middle Ages.  

On the science side, it was particularly nice to hear some talks from and meet a number of people that whose work I've seen over the years but I'd never met face-to-face before.  For example, Steven Louie (linking to wikipedia since all of the Berkeley servers are inexplicably slow right now) spoke about ways to accurately calculate the optical properties of graphene (including electron-hole interactions properly). Philip Collins showed how it's possible to look at single-molecule biophysics (like the functioning of individual enzyme molecules) by anchoring the molecules of interest to single-walled carbon nanotubes, where the action is transduced into changes in the conductance.  Adrian Bachtold gave a nice overview of their work on optomechanics of nanotubes, which has enabled them to do mass sensing at the resolution of a single atomic mass unit (10^-27 kg) and force sensing with similarly impressive sensitivity.  Richard Berndt from Kiel discussed his group's work where they argue that light emission from STM tips shows the signature of shot noise in the current at optical frequencies.  Wolfgang Wernsdorfer, grand poobah of molecular magnetism, presented new results showing amazing control and detection of individual electronic spin lifetimes (in Tb-containing molecules).  For a spin to flip spontaneously, the molecule has to transfer angular momentum to the rest of the world somehow.  In the new experiment, this happens by dumping angular momentum into a carbon nanotube to which the molecule is anchored.  Since the allowed vibrational states of the nanotube can be controlled, this in turn tunes the spin flip rates.  Finally, Klaus Müllen gave an overview of ways to rationally synthesize, by chemical means, graphene flakes, ribbons, and other shapes.  The chemistry is just unreal.

Spin Hall physics

As I mentioned during the APS meeting, Dan Ralph presented some beautiful work (for example) on spin torque devices (where the flow of spin-polarized electrons is able to rotate the magnetization of some "free" ferromagnetic layer of material).  This spin torque business is a fairly mature idea, and the early demonstrations of this effect made use of layered structures (ferromagnet/normal metal/ferromagnet), with the current flowing perpendicular to the layers.  That is, if electrons flow from FM1, some of them are spin-polarized because of the magnetization of FM1, and those polarized electrons traverse the normal layer into FM2.  That works fine, but the most angular momentum you're ever going to transfer that way is \( \hbar/2 \) per electron, and that assumes that the electrons from FM1 are perfectly polarized.   Suppose you could do better than this.  Is there some way, for a given amount of charge current that you flow, to get more angular momentum transferred?

The answer is "yes", and the key is to leverage the spin Hall effect.  (For a good summary of spin Hall physics, see this paper by one of the progenitors of the field - I'll briefly summarize.)  In the regular Hall effect, we think about charge current flow in a plane in the presence of a perpendicular magnetic field.  The charge carriers experience a Lorentz force from the magnetic field that pushes them in the plane transverse to the direction of the (longitudinal) charge current.  Net charge of one sign piles up at one transverse edge of the sample, and net charge of the other sign piles up at the opposite edge, until the force from the resulting transverse electric field balances the Lorentz force.  (Glad to see wikipedia has fixed the figure in this article.  A few years ago they had the direction of the Lorentz force backward.)  In the spin Hall effect, we again think about current flow in a plane.  However, there is no external magnetic field.  Instead, we have the current flowing in a material with strong spin-orbit scattering (that is, in the reference frame of the moving electron, the effective charge current due to the nuclei seemingly moving by produces enough of a magnetic field in that frame to couple significantly to the spin of the electron.  Fundamentally this is a relativistic effect!).  Because of the coupling of spin to orbital motion, if the charge carriers scatter, the spins self-polarize; spin-"up" electrons will pile up on one transverse edge of the sample, while spin-"down" electrons will tend to pile up on the opposite edge.  The extent to which this happens is determined mostly by the strength of the spin-orbit coupling, which is larger in heavier atoms.

So, Ralph and coworkers have used this effect to great advantage.  Instead of the FM/N/FM layered structure, they make a structure that looks like SO/FM/N/FM, where SO is a strong spin-orbit material, such as tungsten or platinum.  They can flow a current within the plane of the SO layer.  Through the spin Hall effect, this can pump polarized spins perpendicular to the plane, into the adjacent FM layer.  (The electrical resistance vertically through the FM/N/FM stack is a way of monitoring the relative alignment of the FM layers, thanks to the giant magnetoresistance.)  This is particularly clever, because for strong SO coupling in the SO layer, thanks to the large contact area at the SO/FM interface, they can get more like 10 \( \hbar \) of angular momentum per electron flowing within the SO layer.   Fascinating to realize that these effects (because they originate from SO physics) are really dramatic experimental proof of the way electric and magnetic fields obey special relativity!

Vanadium dioxide and ionic liquids

Before posting about the APS topics I'd mentioned, I wanted to write a bit about a recent paper that has received quite a bit of attention.  Stuart Parkin's group at IBM Research in Almaden reported in Science last week that they had used an ionic liquid to alter the properties of the strongly correlated oxide VO₂.  As I'd written previously, VO₂ has a metal-insulator transition at 67 C in the bulk, and there has been much interest in using some kind of electrical means to trigger the transition from the insulating to the metallic state in such correlated oxides.  If the insulating state results from Mott physics (i.e., the on-site electron-electron repulsion in the half-filled d band is so strong that the electrons cannot doubly occupy any of the transition metal sites, and the system acts like an insulator even though it would be a metal in the absence of the Coulomb repulsion), then electronically tuning the system away from half-filling (e.g., with a gate using the field effect) could switch the system into the metallic state.  Such a MottFET would be a switch operating on principles very different from those used in conventional Si transistors, and could in principle have superior operating characteristics (for the experts, things like subthreshold swing).   In the Parkin group's work, however, they report pretty compelling evidence that the switching in their system is driven by electrochemistry rather than simple electrostatic doping.  They argue that the metallic state that they produce results from chemical reactions between the VO₂ and the gate-biased ionic liquid that pull oxygen atoms out of the VO2 film.  The evidence for this is that a film treated this way recovers the insulating state when properly exposed to oxygen.  Moreover, they can expose the device to a specific isotope, ¹⁸O.  They can then sputter material off the film and use mass spectrometry to "weigh" the fragments (secondary ion mass spectrometry), and they find evidence that ¹⁸O makes it into the film to a depth of tens of nanometers (many unit cells). 

I find three things interesting about this paper.  First, the actual science is very nice, and I like the isotope tagging/SIMS quite a bit.  Second, I found the perspective put on this by IBM (and the resulting media coverage) a bit surprising - that using liquids and ion motion was a major advance because it would allow chips that operate more like the brain (history dependence = learning, + nonvolatile state retention).   I think that's a surprising spin to put on this.  That brings me to my third point, the true significance of the paper in (part of) the CM community:  This shows that you have to be very very careful when playing with ionic liquids to avoid electrochemistry!  There are previous papers out there that show very modest response of VO₂ in some forms to ionic liquid gating (here and here, for example), and a high profile Nature paper from last summer that reports a huge response.  The present work places these prior publications in an important context, calling into question the relative importance of electrostatics vs. electrochemistry. 

APS March meeting, day 4 (and wrapup for me)

Due to the constraints of real life, the March Meeting was over for me yesterday afternoon, but I did see a few more interesting talks.

In the morning, I went to the session about valley polarization in transition metal dichalcogenides That's quite a mouthful, and demands some explanation.  In a number of materials (including dichalcogenides like MoS₂), the conduction band has more than one energy minimum (or valley) as a function of electron momentum.  In MoS2, there are two energetically equivalent valleys.  Achieving "valley polarization" refers to exciting electrons in only one of those valleys.  Why would you care?  Well, any way of labeling your charge carriers is potentially a means of storing and manipulating information.  In materials like these but possessing broken inversion symmetry (that is, the material has a built-in directionality due to its structure), it is possible to do clever things with circularly polarized light to populate a valley preferentially.  In materials with strong spin-orbit coupling, it is then possible to manipulate spin through valley selection, etc.  The talk by Wang Yao did a very clear job of explaining all this pedagogically, and later talks in the session were also good.

I tried to check out an invited talk on resistive memories, but the chair had let the session fall 15 minutes behind schedule in the first hour.  Note:  there is a reason for timers, and if you're going to be a session chair, you have to hold people to their allotted slots.

I did make it to John Martinis' talk about whether materials are good enough to build a superconducting quantum computer.   It sounds like there is cause for cautious optimism, but wow is it going to be a difficult engineering task.  I need to look up how surface coding is supposed to work.

Finally, I finished off my time at the meeting by going to a session on science and public policy.  Unfortunately this was a depressing way to leave things, since the general message in the end was that Congress is truly dysfunctional, with little hope for any bipartisan support for science - in part because of reflexive opposition, and in part because a significant fraction of the Republican base literally does not believe that science is a valid tool for shaping policy.

One final note for the APS planners in future years:  Please make sure that the APS webserver for the meeting site can actually handle the load.  Still, all in all, a good meeting.

APS March meeting, day 3

Yesterday was again a mix of talks (including three from my group), chatting with friends and colleagues, looking around the vendor show (nice toys, and books, including the new edition of Purcell (and Morin) in SI units, and an intriguing graduate E&M text by Zangwill intended as a replacement for Jackson that focuses more on physics than on special functions), and answering email.

One session that was particularly fun was dedicated to artificial quantum matter. This topic is again worthy of a dedicated blog post that I will write sometime soon. The basic idea for the first few talks is a simple one: we know a number of different ways to impose spatially dependent potential energies on electrons constrained to move in 2d. (Note that while it is often convenient to act like the electrons in 2d electron gas or 2d surface states are free, as always this is shorthand for the true situation, where the single particle states are really Bloch-like states that exist due to the underlying periodic potential from the atoms). For example, if you impose a hexagonal lattice of potential wells on the free electrons, you get an effective band structure that looks like that of graphene. This has been done by etching on top of semiconductor structures, and by arranging molecules on the surface of Cu (as I'd mentioned here a year ago, work by Hari Manoharan). Making deeper potential wells gives you the chance to try to create an engineered system analogous to Mott insulators.

Another flavor of artificial quantum matter was discussed by Andrew Houck. If you make a little microwave resonator (a piece of stripline, superconducting to minimize loss), and then add in a superconducting quantum bit to act as a nonlinear element, you can have an effective photon-photon interaction in the cavity. Now consider wiring up a coupled network of such cavities, where the photons feel each other in each cavity and have some hopping from cavity to cavity. This raises the possibility of making "insulating" states of photons. As the speaker said, it's condensed matter without matter. Very thought provoking.

 

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APS March Meeting, day 2

Yesterday I spent much of the meeting talking with collaborators and old friends, and seeing some invited talks at the sessions associated with some of the APS prizes. There were several really excellent talks.

The first that really stood out was Daniel Fisher's talk on the occasion of his winning the Onsager Prize. Fisher is a statistical mechanician, and he gave a very clear all about randomness, using domain walls in random magnets as an illustrative case for his ideas. In ferromagnets, there is an energetic cost associated with having a domain wall between regions of differently oriented magnetization. That acts like a surface tension, with the system tending to try to minimize the length of such a boundary, all other things being equal. Now if you allow the magnetic coupling between neighboring spins to have a random variation, the domain walls take on funny shapes, "finding" the lowest exchange locations because that also lowers the energy cost. Fisher talked about the statistical physics of this system, including the characteristic slow, history dependent kinetics of equilibration. The tails of the distribution of exchange values are really important here. Fisher then finished up talking about evolution as a statistical mechanics problem, where instead of minimizing an energy, the system tries to maximize "fitness", which is essentially the difference between birth and death rates.

The other talks that were exceptional were those in the Buckley Prize session. The prize this year was awarded to John Slonczewski, who predicted, quantitatively, the existence of the effects of spin transfer torque, which I've indirectly discussed before. Since spin really is angular momentum, flowing a spin polarized current into a magnetized material exerts a torque on the magnetization, if the flowing spins are not aligned with M. This is a way of using currents to cause magnetic domains to precess (ferromagnetic resonance) or flip altogether. Luc Berger gave a very good talk outlining the history of this field in a very pedagogical way, harkening all the way back to work done eighty years ago. Dan Ralph in the same session spoke about their incredibly beautiful results demonstrating all of these effects with quantitative agreement with theory. Further, Ralph showed how one can pump spin currents like this and drive such systems using the spin Hall effect rather than just direct current flow. That's worthy of a blog post all of its own, which I will do sometime soon.

 

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APS March Meeting, Day 1

It's that time of year again, when I get together with thousands of my closest condensed matter physics friends to hear and give talks, swap gossip, and swill overpriced coffee. This year the action is in Baltimore, where one of my main observations after the first day is that someone needs to label the correct room lighting setting so that the project images aren't really washed out.

Real life has intruded in a couple of ways on the meeting this year for me, so my posting will likely be more brief than in past years - sorry.

Yesterday I spent most of my time in the sessions on nickelates and vanadates. The nickelates are a very interesting system, of the form RNiO3, where R is a rare earth atom. These form a family of strongly correlated oxides, where electron-electron interactions can be extremely important in determining the properties. The key is the partially filled d band from the Ni atoms, each of which is octahedrally coordinated by oxygens. Depending on the rare earth ion, the Ni-O bond angles change, and there can be two inequivalent Ni sites. Simple band structure without interactions says these should be metals, and LaNiO3 is a (correlated) metal. However, other members of the family are more complex, such as NdNiO3, which has a metal insulator transition in the bulk at around 200K, between a high T paramagnetic metal and a low T antiferromagnetic insulator - some flavor of the Mott transition. I heard a very interesting talk by Greg Fiete from UT about the possibility that one can use interactions in these materials to create new topological insulators, ones where the energy gap that makes them insulating is an interaction-based gap (as opposed to ordinary TI materials, where they are boring band insulators).

The vanadate sessions were also pretty compelling. VO2 also has a metal-insulator transition, this one at 340K, where interactions and lattice distortions both play very important roles. There were many good talks, including one about some extremely pretty work to figure out the triple point of the phase transitions between rutile and two monoclinic phases. There were also multiple talks about manipulating the transition via chemical doping and field effect approaches. Fun stuff.

Other observations so far: lots of sessions on topological materials, lots of sessions on experimental approaches to quantum bits, and lots of worried discussion of the sequester.

 

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Interesting links

Time to play catch-up.  Here are some interesting links I've seen recently.

How to get a faculty offer - a lecture by John Guttag of MIT to their comp sci graduate students.  Not everything translates to the physics/chem/materials/nano communities, but much of this is great advice.  Thanks to Jen Rexford for bringing this to my attention.

A related post from the FSP about the faculty search process. 

An editorial/blog post at Scientific American about the importance of basic research and the painful choices being faced in the US right now.  It contains some choice quotes from Marc Kastner, a great physicist and current dean at MIT.

A very weird article from the Guardian, essentially taking some secular popularizers of science to task for trying to inspire a sense of wonder.  I had no idea that inspiring a sense of wonder was entirely the purview of the clergy.

A Swiftian editorial in the Journal of Cell Science, decrying blogging efforts to point out suspicious (at least to some) figures in scientific papers.  While it's sensible to have some concerns about how blogs are used in this way, I think this editorial is way off the mark.

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.