Phase transitions: Hiding in plain sight

Phase transitions are amazing things.  As some knob is turned (temperature, pressure, magnetic field), a physical system undergoes a sudden, seemingly discontinuous change in material properties.  This doesn't happen when looking only at one or two of the constituents of the system - only when we have a big ensemble.  In our daily experience, we are used to the control parameter being temperature, and we take particular notice of phases that have dramatically different, obvious-to-the-naked-eye properties.  Solids and liquids have completely different responses to shear (or rather, liquids lack rigidity).  Liquids and gases have vastly different densities.

It turns out that there are many more phases out there, distinguished in ways that are more subtle and harder to see.  Gadolinium is a magnet below room temperature, and a nonmagnetic metal above room temperature, but to the naked eye looks the same in both phases.  We only know that the transition is there because it has measurable consequences (e.g., you could actually see magnetic forces from a hunk of cold gadolinium).

This week, there was some media attention paid to work from David Hsieh's group at Cal Tech, where they discovered an example of a particularly subtle transition in strontium iridate (Sr₂IrO₄).  In that stuff, similar in some ways to the copper oxide superconductors (based on CuO₄ motifs), there are unpaired electrons (and therefore unpaired spins) on the iridium atoms.  Below a critical temperature (near 200 K, or about -70 Celsius), these spins somehow spontaneously arrange themselves in a subtle way that picks out special directions in the crystal lattice and breaks mirror symmetry, but is not some comparatively well-known kind of magnetic ordering.  They are only able to identify this weird "hidden" ordered phase via a particular optical measurement, since the broken symmetry of the ordered state "turns on" some optical processes that would otherwise be forbidden.  The interest in this system stems from whether similar things may be happening in the cuprate superconductors, and whether the iridates could be tweaked and manipulated like the cuprates.  Neat stuff, and a reminder that sometimes nature can be subtle:  There can be a major change in symmetry properties (i.e., above the transition temperature, the x and y directions in the crystal are equivalent, but below the transition they aren't anymore) that shows up spontaneously, but is still hard to detect.

No, dark matter probably did not do in the dinosaurs.

Resurfacing briefly in the midst of proposal writing, I feel compelled to comment on recent media coverage of Lisa Randall's new book.  The science story is an interesting one, as recounted here.   In brief:  There is some evidence of periodicity in mass extinctions due to impacts, though there are at least three hidden assumptions even in that statement.  One possible source of periodicity could be associated with the motion of the solar system (and by extension the earth) in and out of the galactic plane as the sun orbits the center of mass of the Milky Way.  Lisa Randall and Matthew Reece argued that passage through a comparatively thin disk of dark matter on the galactic plane could lead to gravitational perturbations that could lead to Oort Cloud comets getting dinked toward the inner solar system.  A neat idea, though pretty hard to test except through indirect means.  

So, could this have happened?  Sure.  Is it likely?  Well, that's much trickier to evaluate.  There are plenty of other sources of gravitational perturbations (e.g., the passage of nearby stars) that we know for sure take place.  There is no strong observational evidence right now that the would-be disk of dark matter exists, let alone whether it has the properties needed to provide a significant uptick in cometary impacts.  Lisa Randall is undoubtedly a gifted writer, and there is real science here (witness the published paper that discusses ways to test the idea), but the breathless media reaction is somewhat disappointing.

STEM education and equality of opportunity

Friday evening I went to the Katy Independent School District's Robert R. Shaw Center for STEAM, where they were having a Science Movie Night (pdf).  The science and technology policy part of Rice's Baker Institute for Public Policy had put the organizers in touch with me.  It was a very fun time.   On a night when there were two (!) homecoming high school football games next door, the movie night drew about 80 highly engaged students.  After the film, they stayed and we talked about the science of the film (what it got right and what they fudged) for another half an hour.  It was a great time.

The Shaw Center is amazing - it's a fantastic space, something like 10000 sq ft of reconfigurable maker-space, with a shop, immersive labs, and it provides a home to more than one of Katy ISD's robotics teams.  Frankly, this place rivals or exceeds the available undergrad lab facilities at many universities.  Katy is a reasonably affluent suburb of Houston, and I was floored to learn that this great science/engineering facility was built with district money, not donations or corporate sponsorship/underwriting.  This is a case where public school funding has been deliberately and consciously dedicated to providing a district-wide resource for hands-on science and engineering learning.

In a study in contrasts, my sons and I then volunteered Saturday morning at the Teachers Aid facility run by the Houston Food Bank.  At the Teachers Aid facility, teachers from qualifying schools (where 70+% of the enrollment is sufficiently low-income that they qualify for free lunches) can arrive, by appointment, and pick up basic school supplies (pencils, pens, notebooks) for their students.  In three hours we helped about 70 teachers who serve more than 3000 students.  These are teachers who chose to come in on their own time, to get basic supplies that neither their schools nor the students themselves can afford.  

It's appalling to see the divergence in basic educational opportunities between the more affluent school districts and the economically disadvantaged.  We have to do better.  Making sure that children, regardless of their background, have access to a good education should be a guiding principle of our society, not something viewed as pie-in-the-sky or politically tainted.  It amazes me that some people seem to disagree with this.

Several topics: The Nobel, self-organization, and Marcy

I'm heavily into some writing obligations right now, but I wanted to point out a few things:

• Obviously this year's Nobel in physics was for neutrino oscillations.  ZZ has thoughtfully provided two nice postings (here and here) with relevant links for further reading.  I remember the solar neutrino problem vividly from undergrad days, including one professor semi-jokingly speculating that perhaps the lack of expected electron neutrinos was because the sun had gone out.  The whole particles-oscillating-into-new-types business, famous in physics circles from the K mesons, is tough to explain to a general audience.  You could probably come up with some analog description involving polarized light....
• Here is a neat article about a record Chinese traffic jam, and it includes links to papers about self-organization.  I'm happy to see this kind of article - I think there is real value in pointing out to people that there can be emergent, organized states that result from the aggregate of simple rules.  That's the heart of condensed matter and statistical physics.
• This week buzzfeed helped break the story that Geoff Marcy, an astronomer famed for his role in the discovery of extrasolar planets and mentioned as a likely Nobel candidate, was found guilty of multiple violations of Berkeley's sexual misconduct policy.  Note to those who haven't followed this:  This is the result of a long investigation - it's not hearsay, it's essentially a conviction via a long university investigative process.  Marcy's apology letter is here.   Apparently this bad behavior had been tolerated for many years.  Berkeley's response has been, shall we say, tepid.  Despite a clear finding of years of inappropriate behavior involving students 1/3 his age, the response is "do this any more, and you'll be dismissed".  The initial response from the department head had an inexcusably awful last paragraph that implied that this whole process was hardest on Marcy, rather than on the victims.   This is terrible.   People, including departmental colleagues, are calling for Marcy to step down.  Bottom line:  There is simply no excuse for this kind of behavior, and actual sanctions must be applied when people are found through due process to have violated this level of basic social decency.

Table-top electron microscopes

A quick question in the hopes that some people in this blog's readership have direct experience:  Anyone work with a table-top scanning electron microscope (SEM) that they really like?  Any rough idea of the cost and operations challenges (e.g., having to replace tungsten filaments all the time)?  I was chatting with someone about educational opportunities that something like these would present, and I was curious about the numbers without wanting to email vendors or anything that formal.  Thanks for any information.

(Note: It would really be fun to try to develop a really low-budget SEM - the electron microscopy version of this.  On the one hand, you could imagine microfabricated field emitters and the amazing cheapness of CCDs could help.  However, the need for good vacuum and some means of beam focusing and steering makes this much more difficult.  Clearly a good undergrad design project....)

Annual Nobel speculation

It's getting to be that time of year again.  The 2015 Nobel Prize in Physics will be announced this coming Tuesday morning (EU/US time).  Based on past patterns, it looks like it could well be an astro prize.  Dark matter/galaxy rotation curves anyone?  Or extrasolar planets?  (I still like Aharonov + Berry for geometric phases, perhaps with Thouless as well.  However, it's unlikely that condensed matter will come around this year.)

On Wednesday, the chemistry prize will be awarded.  There, I have no clue.  Curious Wavefunction has a great write-up that you should read, though.

Speculate away!

DOE Experimental Condensed Matter Physics PI Meeting 2015 - Day 3

Things I learned from the last (half)day of the DOE PI meeting:

• "vortex explosion" would be a good name for a 1980s metal band.
• Pulsed high fields make possible some really amazing measurements in both high \(T_{\mathrm{C}}\) materials and more exotic things like SmB₆.
• Looking at structural defects (twinning) and magnetic structural issues (spin orientation domain walls) can give insights into complicated issues in pnictide superconductors.
• Excitons can be a nice system for looking at coherence phenomena ordinarily seen in cold atom systems.  See here and here.  Theory proposes that you could play with these at room temperature with the right material system.
• Thermal gradients can drive spin currents even in insulating paramagnets, and these can be measured with techniques that could be performed down to small length scales. 
• Very general symmetry considerations when discussing competing ordered states (superconductivity, charge density wave order, spin density wave order) can lead to testable predictions.
• Hybrid, monocrystalline nanoparticles combining metals and semiconductors are very pretty and can let you drive physical processes based on the properties of both material systems.

DOE Experimental Condensed Matter Physics PI Meeting 2015 - Day 2

Among the things I learned at the second day of the meeting:

• In relatively wide quantum wells, and high fields, you can enter the quantum Hall insulating state.  Using microwave measurements, you can see signatures of phase transitions within the insulating state - there are different flavors of insulator in there.  See here.
• As I'd alluded to a while ago, you can make "artificial" quantum systems with graphene-like energetic properties (for example).  
• In 2d hole gasses at the interface between Ge and overlying SiGe, you can get really huge anisotropy of the electrical resistivity in magnetic fields, with the "hard" axis along the direction of the in-plane magnetic field.
• In single-layer thick InN quantum wells with GaN above and below, you can have a situation where there is basically zero magnetoresistance.  That's really weird.
• In clever tunneling spectroscopy experiments (technique here) on 2d hole gasses, you can see sharp inelastic features that look like inelastic excitation of phonons.  
• Tunneling measurements through individual magnetic nanoparticles can show spin-orbit-coupling-induced level spacings, and cranking up the voltage bias can permit spin processes that are otherwise blockaded.  See here.
• Niobium islands on a gold film are a great tunable system for studying the motion of vortices in superconductors, and even though the field is a mature one, new and surprising insights come out when you have a clean, controlled system and measurement techniques. 
• Scanning Josephson microscopy (requiring a superconducting STM tip, a superconducting sample, and great temperature and positional control) is going to be very powerful for examining the superconducting order parameter on atomic scales.
• In magnetoelectric systems (e.g., ferroelectrics coupled to magnetic materials), combinations of nonlinear optics and electronic measurements are required to unravel which of the various possible mechanisms (charge vs strain mediated) generates the magnetoelectric coupling.
• Strongly coupling light in a cavity with Rydberg atoms should be a great technique for generating many body physics for photons (e.g., the equivalent of quantum Hall).
• Carbon nanotube devices can be great systems for looking at quantum phase transitions and quantum critical scaling, in certain cases.
• Controlling vortex pinning and creep is hugely important in practical superconductors.  Arrays of ferromagnetic particles as in artificial spin ice systems can control and manipulate vortices.  Thermal fluctuations in high temperature superconductors could end up limiting performance badly, even if the transition temperature is at room temperature or more, and the situation is worse if the material is more anisotropic in terms of effective mass.
• "Oxides are like people; it is their defects that make them interesting."

DOE Experimental Condensed Matter Physics PI meeting 2015 - Day 1

Things I learned at today's session of the DOE ECMP PI meeting:

• In the right not-too-thick, not-too-thin layers of the 3d topological insulator Bi_1.5Sb_0.5Te_1.7Se_1.3 (a cousin of Bi₂Se₃ that actually is reasonably insulating in the bulk), it is possible to use top and bottom gates to control the surface states on the upper and lower faces, independently.  See here.
• In playing with suspended structures of different stackings of a few layers of graphene, you can get some dramatic effects, like the appearance of large, sharp energy gaps.  See here.
• While carriers in graphene act in some ways like massless particles because their band energy depends linearly on their crystal momentum (like photon energy depends linearly on photon momentum in electrodynamics), they have a "dynamical" effective mass, \(m^* = \hbar (\pi n_{2d})^{1/2}/v_{\mathrm{F}}\), related to how the electronic states respond to an electric bias.  
• PdCoO₂ is a weird layered metal that can be made amazingly clean, so that its residual resistivity can be as small as 8 n\(\Omega\)-cm.  That's about 200 times smaller than the room temperature resistivity of gold or copper.  
• By looking at how anisotropic the electrical resistivity is as a function of direction in the plane of layered materials, and how that anisotropy can vary with applied strain, you can define a "nematic susceptibility".  That susceptibility implies the existence of fluctuations in the anisotropy of the electronic properties (nematic fluctuations).  Those fluctuations seem to diverge at the structural phase transition in the iron pnictide superconductors.  See here.   Generically, these kinds of fluctuations seem to boost the transition temperature of superconductors.
• YPtBi is a really bizarre material - nonmetallic temperature dependence, high resistivity, small carrier density, yet superconducts.  
• Skyrmions (see here) can be nucleated in controlled ways in the right material systems.  Using the spin Hall effect, they can be pushed around.  They can also be moved by thermally driven spin currents, and interestingly skyrmions tend to flow from the cold side of a sample to the hot side.  
• It's possible to pump angular momentum from an insulating ferromagnet, through an insulating antiferromagnet (NiO), and into a metal.  See here.
• The APS Conferences for Undergraduate Women in Physics have been a big hit, using attendance as a metric.  Extrapolating, in a couple of years it looks like nearly all of the undergraduate women majoring in physics in the US will likely be attending one of these.
• Making truly nanoscale clusters out of some materials (e.g., Co₂Si, Mn₅Si₃) can turn them from weak ferromagnets or antiferromagnets in the bulk into strong ferromagnets in nanoparticle form.   See here.

DOE Experimental Condensed Matter Physics PI meeting 2015

As they did two years ago, the Basic Energy Sciences part of the US DOE is having a gathering of experimental condensed matter physics investigators at the beginning of next week.  The DOE likes to do this (see here for proceedings of past meetings), with the idea of getting people together to talk about the current and future state of the field and ideally seed some collaborations.  I will try to blog a bit about the meeting, as I did in 2013 (here and here).

CMP and materials in science fiction

Apologies for the slower posting frequency.  Other obligations (grants, research, teaching, service) are significant right now.

I thought it might be fun to survey people for examples of condensed matter and materials physics as they show up in science fiction (and/or comics, which are fantasy more than hard SF).  I don't mean examples where fiction gets science badly wrong or some magic rock acts as a macguffin (Infinity Stones, Sankara stones) - I mean cases where materials and their properties are thought-provoking.

A couple of my favorites:

• scrith, the bizarre material used to construct the Ringworld.  It's some exotic material that has 40% opacity to neutrinos without being insanely dense like degenerate matter.
• From the same author, shadow square wire, which is an absurdly strong material that also doubles as a high temperature superconductor.  (Science goof in there:  Niven says that this material is also a perfect (!) thermal conductor.  That's incompatible with superconductivity, though - the energy gap that gives you the superconductivity suppresses the electronic contribution to thermal conduction.  Ahh well.)
• Even better, from the same author, the General Products Hull, a giant single-molecule structure that is transparent in the visible, opaque to all other wavelengths, and where the strength of the atomic bonds is greatly enhanced by a fusion power source.
• Vibranium, the light, strong metal that somehow can dissipate kinetic energy very efficiently.  (Like many materials in SF, it has whatever properties it needs to for the sake of the plot.  Hard to reconcile the dissipative properties with Captain America's ability to bounce his shield off objects with apparently perfect restitution.)
• Old school:  cavorite, the H. G. Wells wonder material that blocks the gravitational interaction.

What are your favorite examples?

Amazingly clear results: density gradient ultracentrifugation edition

Ernest Rutherford reportedly said something like, if your experiment needs statistics, you should have done a better experiment.  Sometimes this point is driven home by an experimental technique that gives results that are strikingly clear.  To the right is an example of this, from Zhu et al., Nano Lett. (in press), doi:  10.1021/acs.nanolett.5b03075.  The technique here is called "density gradient ultracentrifugation". 

You know that the earth's atmosphere is denser at ground level, with density decreasing as you go up in altitude.  If you ignore temperature variation in the atmosphere, you get a standard undergraduate statistical physics problem ("the exponential atmosphere") - the gravitational attraction to the earth pulls the air molecules down, but the air has a non-zero temperature (and therefore kinetic energy).  A density gradient develops so that the average gravitational "drift" downward is balanced on average by "diffusion" upward (from high density to low density). 

The idea of density gradient ultracentrifugation is to work with a solution instead of the atmosphere, and generate a vastly larger effective gravitational force (to produce a much sharper density gradient within the fluid) by using an extremely fast centrifuge.  If there are particles suspended within the solution, they end up sitting at a level in the test tube that corresponds to their average density.  In this particular paper, the particles in question are little suspended bits of hexagonal boron nitride, a quasi-2d material similar in structure to graphite.  The little hBN flakes have been treated with a surfactant to suspend them, and depending on how many layers are in each flake, they each have a different effective density in the fluid.  After appropriate dilution and repeated spinning (41000 RPM for 14 hours for the last step!), you can see clearly separated bands, corresponding to layers of suspension containing particular thickness hBN flakes.  This paper is from the Hersam group, and they have a long history with this general technique, especially involving nanotubes.  The results are eye-popping and seem nearly miraculous.  Very cool.

The (Intel) Science Talent Search - time to end corporate sponsorship?

When I was a kid, I heard about the Westinghouse Science Talent Search, a national science fair competition that sparked the imaginations of many many young, would-be scientists and engineeers for decades.  I didn't participate in it, but it definitely was inspiring.  As an undergrad, I was fortunate enough to work a couple of summers for Westinghouse's R&D lab, their Science Technology Center outside of Pittsburgh, learning a lot about what engineers and applied physicists actually do.  When I was in grad school, Westinghouse as a major technology corporation basically ceased to exist, and Intel out-bid rival companies for the privilege of supporting and giving their name to the STS.  Now, Intel has decided to drop its sponsorship, for reasons that are completely opaque.  "Intel's interests have changed," says the chair of the administrative board that runs the contest.

While it seems likely that some other corporate sponsor will step forward, I have to ask two questions.  First, why did Intel decide to get out of this?  Seriously, the cost to them has to be completely negligible.  Is there some compelling business reason to drop this, under the assumption that someone else will take up the mantle?  It's a free country, and of course they can do what they like with their name and sponsorship, but this just seems bizarre.  Was this viewed as a burden?  Was there a sense that they didn't get enough effective advertising or business return for their investment?  Did it really consume far more resources than they were comfortable allocating?

Second, why should a company sponsor this?  I ask this as it seems likely that the companies with the biggest capital available to act as sponsors will be corporations like Google, Microsoft, Amazon - companies that don't, as their core mission, actually do physical sciences and engineering research.  Wouldn't it be better to establish a philanthropic entity to run this competition - someone who would not have to worry about business pressures in terms of the financing?   There are a number of excellent, well-endowed foundations who seem to have missions that align well with the STS.  There's the Gordon and Betty Moore Foundation, the David and Lucille Packard Foundation, the Alfred P. Sloan Foundation, the W. M. Keck Foundation, the Dreyfus Foundation, the John D. and Catherine T. MacArthur Foundation, and I'm sure I'm leaving out some possibilities.  I hope someone out there gives serious consideration to endowing the STS, rather than going with another corporate sponsorship deal that may not stand the test of time.

Update:  From the Wired article about this, the STS cost Intel about $6M/yr.  Crudely, that means that an endowment of $120M would be enough to support this activity in perpetuity, assuming 5% payout (typical university investment assumptions, routinely beaten by Harvard and others).

Update 2:  I've thought about this some more, and maybe the best solution would be for a university to sponsor this.   For example, this seems tailor-made for MIT, which styles itself as a huge hub of innovation (see the Technology Review, e.g.). Stanford could do it.  Harvard could do $6M a year and not even notice.  It would be perfect as a large-scale outreach/high school education sponsorship effort.  Comments?

Science and narrative tension

Recently I've come across some good examples of multidisciplinary science communication.  The point of commonality:  narrative tension, in the sense that the science is woven in as part of telling a story.  The viewer/reader really wants to know how the story resolves, and either is willing to deal with the science to get there, or (more impressive, from the communication standpoint) actually wants to see the science behind the plot resolution.

Possibly the best example of the latter is The Martian, by Andy Weir.  If you haven't read it, you should.  There is going to be a big budget film coming out based on it, and while the preview looks very good, the book is going to be better.  Here is an interview with Andy Weir by Adam Savage, and it makes the point that people can actually like math and science as part of the plot.


Another recent example, more documentary-style, is The Mystery of Matter:  Search for the Elements, which aired this past month on PBS in the US.  The three episodes are here, here, and here.  This contains much of the same information as a Nova episode, Hunting the Elements.  It's interesting to contrast the two - some people certainly like the latter's fun, jaunty approach (wherein the host plays the every-person proxy for the audience, saying "Gee whiz!" and asking questions of scientists), while the former has some compelling historical reenactments.  I like the story-telling approach a bit more, but that may be because I'm a sucker for history.  Note:  Nice job by David Muller in Hunting the Elements, using Cornell's fancy TEM to look at the atoms in bronze.

I also heard a good story on NPR this week about Ainissa Ramirez, a materials scientist who has reoriented her career path into "science evangelist".   Her work and passion are very impressive, and she is also a proponent of story-telling as a way to hold interest.  We overlapped almost perfectly in time at Stanford during our doctorates, and I wish we'd met. 

Now to think about the equivalent of The Martian, but where the audience longs to learn more about condensed matter physics and nanoscale science to see how the hero survives.... (kidding.  Mostly.)

Nano and the oil industry

I went to an interesting lunchtime talk today by Sergio Kapusta, former chief scientist of Shell.  He gave a nice overview of the oil/gas industry and where nanoscience and nanotechnology fit in.   Clearly one of the main issues of interest is assessing (and eventually recovering) oil and gas trapped in porous rock, where the hydrocarbons can be trapped due to capillarity and the connectivity of the pores and cracks may be unknown.  Nanoparticles can be made with various chemical functionalizations (for example, dangling ligands known to be cleaved if the particle temperature exceeds some threshold) and then injected into a well; the particles can then be sought at another nearby well.  The particles act as "reporters".  The physics and chemistry of getting hydrocarbons out of these environments is all about the solid/liquid interface at the nanoscale.  More active sensor technologies for the aggressive, nasty down-hole environment are always of interest, too.

When asked about R&D spending in the oil industry, he pointed out something rather interesting:  R&D is actually cheap compared to the huge capital investments made by the major companies.  That means that it's relatively stable even in boom/bust cycles because it's only a minor perturbation on the flow of capital.  

Interesting numbers:  Total capital in hardware in the field for the petrochemical industry is on the order of $2T, built up over several decades.  Typical oil consumption worldwide is around 90M barrels equivalent per day (!).   If the supply ranges from 87-93M barrels per day, the price swings from $120 to $40/barrel, respectively.  Pretty wild.

Short term-ism and industrial research

I have written multiple times (here and here, for example) about my concern that the structure of financial incentives and corporate governance have basically killed much of the American corporate research enterprise.  Simply put:  corporate officers are very heavily rewarded based on very short term metrics (stock price, year-over-year change in rate of growth of profit).  When faced with whether to invest company resources in risky long-term research that may not pay off for years if ever, most companies opt out of that investment.  Companies that do make long-term investments in research are generally quasi-monopolies.  The definition of "research" has increasingly crept toward what used to be called "development"; the definition of "long term" has edged toward "one year horizon for a product"; and physical sciences and engineering research has massively eroded in favor of much less expensive (in infrastructure, at least) work on software and algorithms. 

I'm not alone in making these observations - Norm Augustine, former CEO of Lockheed Martin, basically says the same thing, for example.  Hillary Clinton has lately started talking about this issue.

Now, writing in The New Yorker this week, James Surowiecki claims that "short termism" is a myth.  Apparently companies love R&D and have been investing in it more heavily.  I think he's just incorrect, in part because I don't think he really appreciates the difference between research and development, and in part because I don't think he appreciates the sliding definitions of "research", "long term" and the difference between software development and physical sciences and engineering.  I'm not the only one who thinks his article has issues - see this article at Forbes.

No one disputes the long list of physical research enterprises that have been eliminated, gutted, strongly reduced, or refocused onto much shorter term projects.  A brief list includes IBM, Xerox, Bell Labs, Motorola, General Electric, Ford, General Motors, RCA, NEC, HP Labs, Seagate, 3M, Dupont, and others.  Even Microsoft has been cutting back.  No one disputes that corporate officers have often left these organizations with fat benefits packages after making long-term, irreversible reductions in research capacity (I'm looking at you, Carly Fiorina).   Perhaps "short termism" is too simple an explanation, but claiming that all is well in the world of industrial research just rings false.

News items: Feynman, superconductors, faculty shuffle

A few brief news items - our first week of classes this term is a busy time.

• Here is a video of Richard Feynman, explaining why he can't readily explain permanent magnets to the interviewer.   This gets right to the heart of why explaining science in a popular, accessible way can be very difficult.  Sure, he could come up with really stretched and tortured analogies, but truly getting at the deeper science behind the permanent magnets and their interactions would require laying a ton of groundwork, way more than what an average person would want to hear.
• Here is a freely available news article from Nature about superconductivity in H₂S at very high pressures.   I was going to write at some length about this but haven't found the time.  The short version:  There have been predictions for a long time that hydrogen, at very high pressures like in the interior of Jupiter, should be metallic and possibly a relatively high temperature superconductor.  There are later predictions that hydrogen-rich alloys and compounds could also superconduct at pretty high temperatures.  Now it seems that hydrogen sulfide does just this.  Crank up the pressure to 1.5 million atmospheres, and that stinky gas becomes what seems to be a relatively conventional (!) superconductor, with a transition temperature close to 200 K.  The temperature is comparatively high because of a combination of an effectively high speed of sound (the material gets pretty stiff at those pressures), a large density of electrons available to participate, and a strong coupling between the electrons and those vibrations (so that the vibrations can provide an effective attractive interaction between the electrons that leads to pairing).    The important thing about this work is that it shows that there is no obvious reason why superconductivity at or near room temperature should be ruled out.
• Congratulations to Prof. Laura Greene, incoming APS president, who has been named the new chief scientist of the National High Magnetic Field Lab.  
• Likewise, congratulations to Prof. Meigan Aronson, who has been named Texas A&M University's new Dean of Science.  

Anecdote 5: Becoming an experimentalist, and the Force-o-Matic

As an undergrad, I was a mechanical engineering major doing an engineering physics program from the engineering side.  When I was a sophomore, my lab partner in the engineering fluid mechanics course, Brian, was doing the same program, but from the physics side.  Rather than doing a pre-made lab, we chose to take the opportunity to do an experiment of our own devising.   We had a great plan.  We wanted to compare the drag forces on different shapes of boat hulls.  The course professor got us permission to go to a nearby research campus, where we would be able to take our homemade models and run them in their open water flow channel (like an infinity pool for engineering experiments) for about three hours one afternoon.  

The idea was simple:  The flowing water would tend to push the boat hull downstream due to drag.  We would attach a string to the hull, run the string over a pulley, and hang known masses on the end of the string, until the weight of the masses (transmitted via the string) pulled upstream to balance out the drag force - that way, when we had the right amount of weight on there, the boat hull would sit motionless in the flow channel.  By plotting the weight vs. the flow velocity, we'd be able to infer the dependence of the drag force on the flow speed, and we could compare different hull designs. 

Like many great ideas, this was wonderful right up until we actually tried to implement it in practice.  Because we were sophomores and didn't really have a good feel for the numbers, we hadn't estimated anything and tacitly assumed that our approach would work.  Instead, the drag forces on our beautiful homemade wood hulls were much smaller than we'd envisioned, so much so that just the horizontal component of the force from the sagging string itself was enough to hold the boats in place.  With only a couple of hours at our disposal, we had to face the fact that our whole measurement scheme was not going to work.

What did we do?  With improvisation that would have made McGyver proud, we used a protractor, chewing gum, and the spring from a broken ballpoint pen to create a much "softer" force measurement apparatus, dubbed the Force-o-Matic.  With the gum, we anchored one end of the stretched spring to the "origin" point of the protractor, with the other end attached to a pointer made out of the pen cap, oriented to point vertically relative to the water surface.  With fine thread instead of the heavier string, we connected the boat hull to the tip of the pointer, so that tension in the thread laterally deflected the extended spring by some angle.  We could then later calibrate the force required to produce a certain angular deflection.  We got usable data, an A on the project, and a real introduction, vividly memorable 25 years later, to real experimental work.

Drought balls and emergent properties

There has been a lot of interest online recently about the "drought balls" that the state of California is using to limit unwanted photochemistry and evaporation in its reservoirs.  These are hollow balls each about 10 cm in diameter, made from a polymer mixed with carbon black.  When dumped by the zillions into reservoirs, they don't just help conserve water:  They spontaneously become a teaching tool about condensed matter physics.

As you can see from the figure, the balls spontaneously assemble into "crystalline" domains.  The balls are spherically symmetric, and they experience a few interactions:  They are buoyant, so they float on the water surface; they are rigid objects, so they have what a physicist would call "hard-core, short-ranged repulsive interactions" and what a chemist would call "steric hindrance"; a regular person would say that you can't make two balls occupy the same place.  Because they float and distort the water surface, they also experience some amount of an effective attractive interaction.  They get agitated by the rippling of the water, but not too much.  Throw all those ingredients together, and amazing things happen:  The balls pack together in a very tight spatial arrangement.  The balls are spherically symmetric, and there's nothing about the surface of the water that picks out a particular direction.  Nonetheless, the balls "spontaneously break rotational symmetry in the plane" and pick out a directionality to their arrangement. There's nothing about the surface of the water that picks out a particular spatial scale or "origin", but the balls "spontaneously break continuous translational symmetry", picking out special evenly-spaced lattice sites.  Physicists would say they preserve discrete rotational and translational symmetries.  The balls in different regions of the surface were basically isolated to begin with, so they broke those symmetries differently, leading to a "polycrystalline" arrangement, with "grain boundaries".  As the water jostles the system, there is a competition between the tendency to order and the ability to rearrange, and the grains rearrange over time.  This arrangement of balls has rigidity and supports collective motions (basically the analog of sound) within the layer that are meaningless when talking about the individual balls.  We can even spot some density of "point defects", where a ball is missing, or an "extra" ball is sitting on top.

What this tells us is that there are certain universal, emergent properties of what we think of as solids that really do not depend on the underlying microscopic details.   This is a pretty deep idea - that there are collective organizing principles that give emergent universal behaviors, even from very simple and generic microscopic rules.  Knowing that the balls are made deep down from quarks and leptons does not tell you anything about these properties.

Anecdote 4: Sometimes advisers are right.

When I was a first-year grad student, I started working in my adviser's lab, learning how to do experiments at extremely low temperatures.   This involved working quite a bit with liquid helium, which boils at atmospheric pressure at only 4.2 degrees above absolute zero, and is stored in big, vacuum-jacketed thermos bottles called dewars (named after James Dewar).   We had to transfer liquid helium from storage dewars into our experimental systems, and very often we were interested in knowing how much helium was left in the bottom of a storage dewar.

The easiest way to do this was to use a "thumper" - a skinny (maybe 1/8" diameter) thin-walled stainless steel tube,  a few feet long, open at the bottom, and silver-soldered to a larger (say 1" diameter) brass cylinder at the top, with the cylinder closed off by a stretched piece of latex glove.   When the bottom of the tube was inserted into the dewar (like a dipstick) and lowered into the cold gas, the rubber membrane at the top of the thumper would spontaneously start to pulse (hence the name).   The frequency of the thumping would go from a couple of beats per second when the bottom was immersed in liquid helium to more of a buzz when the bottom was raised into vapor.  You can measure the depth of the liquid left in the dewar this way, and look up the relevant volume of liquid on a sticker chart on the side of the dewar.

The "thumping" pulses are called Taconis oscillations.  They are an example of "thermoacoustic" oscillations.  The physics involved is actually pretty neat, and I'll explain it at the end of this post, but that's not really the point of this story.  I found this thumping business to be really weird, and I wanted to know how it worked, so I walked across the hall from the lab and knocked on my adviser's door, hoping to ask him for a reference.  He was clearly busy (being department chair at the time didn't help), and when I asked him "How do Taconis oscillations happen?" he said, after a brief pause, "Well, they're driven by the temperature difference between the hot and cold ends of the tube, and they're a complicated nonlinear phenomenon." in a tone that I thought was dismissive.  Doug O. loves explaining things, so I figured either he was trying to get rid of me, or (much less likely) he didn't really know.

I decided I really wanted to know.  I went to the physics library upstairs in Varian Hall and started looking through books and chasing journal articles.  Remember, this was back in the wee early dawn of the web, so there was no such thing as google or wikipedia.  Anyway, I somehow found this paper and its sequels.  In there are a collection of coupled partial differential equations looking at the pressure and density of the fluid, the flow of heat along the tube, the temperature everywhere, etc., and guess what:  They are complicated, nonlinear, and have oscillating solutions.  Damn.  Doug O. wasn't blowing me off - he was completely right (and knew that a more involved explanation would have been a huge mess).  I quickly got used to this situation.

Epilogue:  So, what is going on in Taconis oscillations, really?  Well, suppose you assume that there is gas rushing into the open end of the tube and moving upward toward the closed end.  That gas is getting compressed, so it would tend to get warmer.  Moreover, if the temperature gradient along the tube is steep enough, the upper walls of the tube can be warmer than the incoming gas, which then warms further by taking heat from the tube walls.  Now that the pressure of the gas has built up near the closed end, there is a pressure gradient that pushes the gas back down the tube.  The now warmed gas cools as it expands, but again if the tube walls have a steep temperature gradient, the gas can dump heat into the tube walls nearer the bottom.  This is discussed in more detail here.  Turns out that you have basically an engine, driven by the flow of heat from the top to the bottom, that cyclically drives gas pulses.  The pulse amplitude ratchets up until the dissipation in the whole system equals the work done per cycle on the gas.  More interesting than that:  Like some engines, you can run this one backwards.  If you drive pressure pulses properly, you can use the gas to pump heat from the cold side to the hot side - this is the basis for the thermoacoustic refrigerator.