Tuesday, December 11, 2018

Rice Academy of Fellows, 2019

Just in case....

Rice has a competitive endowed postdoctoral program, the Rice Academy of Fellows.  There are five slots for the coming year (application deadline of January 3).  It's a very nice program, though like all such things it's challenging to get a slot.  If someone is interested in trying this to work with me, I'd be happy to talk - the best approach would be to email me.

Friday, December 07, 2018

Shoucheng Zhang, 1963-2018

Shocking and saddening news this week about the death of Shoucheng Zhang, Stanford condensed matter theorist who had made extremely high impact contributions to multiple topics in the field.    He began his research career looking at rather exotic physics; string theory was all the rage, and this was one of his first papers.  His first single-author paper, according to scopus, is this Phys Rev Letter looking at the possibility of an exotic (Higgs-related) form of superconductivity on a type of topological defect in spacetime.  Like many high energy theorists of the day, he made the transition to condensed matter physics, where his interests in topology and field theory were present throughout his research career.  Zhang made important contributions on the fractional quantum Hall effect (and here and here), the problem of high temperature superconductivity in the copper oxides (here), and most recently and famously, the quantum spin Hall effect (here for example).   He'd won a ton of major prizes, and was credibly in the running for a share of a future Nobel regarding topological materials and quantum spin Hall physics.

I had the good fortune to take one quarter of "introduction to many-body physics" (basically quantum field theory from the condensed matter perspective) from him at Stanford.  His clear lectures, his excellent penmanship at the whiteboard, and his ever-present white cricket sweater are standout memories even after 24 years.  He was always pleasant and enthusiastic when I'd see him.  In addition to his own scholarly output, Zhang had a huge, lasting impact on the community through mentorship of his students and postdocs.  His loss is deeply felt.  Depression is a terrible illness, and it can affect anyone - hopefully increased awareness and treatment will make tragic events like this less likely in the future.

Saturday, December 01, 2018

Late Thanksgiving physics: Split peas and sandcastles

Last week, when I was doing some cooking for the US Thanksgiving holiday, I was making a really good vegetarian side dish (seriously, try it), and I saw something that I thought was pretty remarkable, and it turns out that a Nature paper had been written about it.

The recipe involves green split peas, and the first step is to rinse these little dried lozenge-shaped particles (maybe 4 mm in diameter, maybe 2 mm thick) in water to remove any excess dust or starch.  So, I put the dried peas in a wire mesh strainer, rinsed them with running water, and dumped them into a saucepan.  Unsurprisingly, the wet split peas remained stuck together in a hemispherical shape that exactly mimicked the contours of the strainer.  This is a phenomenon familiar to anyone who has ever built a sandcastle - wet particulates adhere together.  

The physics behind this adhesion is surface tension.  Because water molecules have an attractive interaction with each other, in the absence of any other interactions, liquid water will settle into a shape that minimizes the area of the water-vapor interface.  That's why water forms spherical blobs in microgravity.  It costs about 72 mJ/m2 to create some area of water-air interface.  It turns out that it is comparatively energetically favored to form a water-split pea interface, because of attractive interactions between the polar water molecules and the mostly cellulose split pea surface.  

For a sense of scale, creating water-air interface with the area of one split pea (surface area roughly 2.5e-5 m2) would take about 2 microjoules of energy.  The mass of the split pea half I'm considering, assuming a density similar to water, is around 25 mg = 2.5e-5 kg.  So, lifting such a split pea by about it's own height requires an energy of \(mgh \sim\) 2.5e-5*9.807*2e-4 = 0.5 microjoules.  The fact that this is comparable to (but smaller than) the surface energy of the water-air interface of a wet split pea tells you that you should not be surprised that water coatings can hold wet split peas up against the force of gravity.

What I then saw, which was surprising to me, was that even as I started adding the 3.5 cups of water mentioned in the recipe,  the hemispherical split pea "sandcastle" stayed together, even when I prodded it with a cooking spoon.  This surprised me.  A few minutes of internet search confirmed that this effect is surprising enough to merit its own Nature Materials paper, with its own News and Views article. The transition from cohering wet grains to a flowing slurry turns out to happen at really high water fractions.  Neat physics, and the richness of a system as simple as grains/beads, water, and air is impressive.

Sunday, November 25, 2018

Fundamental units and condensed matter

As was discussed in many places over the last two weeks, the official definition of the kilogram has now been changed, to a version directly connected to Planck's constant, \(h\).  The NIST description of this is very good, and I am unlikely to do better.  Through the use of a special type of balance (a Kibble or Watt balance, the mass can be related back to \(h\) via the dissipation of electrical power in the form of \(V^{2}/R\).  A point that I haven't seen anyone emphasize in their coverage:  Both the volt and the Ohm are standardized in terms of condensed matter phenomena - there is a deep, profound connection between emergent condensed matter effects and our whole fundamental set of units (a link that needs to be updated to include the new definition of kg).

Voltage \(V\) is standardized in terms of the Josephson effect.  In a superconductor, electrons pair up and condense into a quantum state that is described by a complex number called the order parameter, with a magnitude and a phase.  The magnitude is related to the density of pairs.  The phase is related to the coherent response of all the pairs, and only takes on a well-defined value below the superconducting transition.  In a junction between superconductors (say a thin tunneling barrier of insulator), a dc voltage difference between the two sides causes the phase to "wind" as a function of time, leading to an ac current with a frequency of \(2eV/h\).  Alternately, applying an ac voltage of known frequency \(f\) can generate a dc voltage at integer multiples of \(h f/2e\).  The superconducting phase is an emergent quantity, well defined only when the number of pairs is large.

The Ohm \(\Omega\) is standardized in terms of the integer quantum Hall effect.  Electrons confined to a relatively clean 2D layer and placed in a large magnetic field show plateaus in the Hall resistance, the relationship between longitudinal current and transverse voltage, at integer multiples of \(e^{2}/h\).  The reason for picking out those particular values is deeply connected to topology, and is independent of the details of the material system.  You can see the integer QHE in many systems, one reason why it's good to use as a standard.  The existence of the plateaus, and therefore really accurate quantization, in actual measurements of the Hall conductance requires disorder.  Precise Hall quantization is likewise also an emergent phenomenon.

Interesting that the fundamental definition of the kilogram is deeply connected to two experimental phenomena that are only quantized to high precision because they emerge in condensed matter.


Tuesday, November 13, 2018

Blog stats weirdness

This blog is hosted on blogger, google's free blogging platform.  There are a couple of ways to get statistics about the blog, like rates of visits and where they're from.  One approach is to start from the nanoscale views blogger homepage and click "stats", which can tell me an overview of hit rates, traffic sources, etc.  The other approach is to go to analytics.google.com and look at the more official information compiled by google's tracking code. 

The blogger stats data has always looked weird relative to the analytics information, with "stats" showing far more hits per day - probably tracking every search engine robot that crawls the web, not just real hits.  This is a new one, though:  On "stats" for referring traffic, number one is google, and number three is Peter Woit's blog.  Those both make sense, but in second place there is a site that I didn't recognize, and it appears to be associated with hardcore pornography (!).  That site doesn't show up at all on the analytics page, where number one is google, number two is direct linking, and number three is again Woit's blog.  Weird.  Very likely that this is the result of a script trying to put porn spam in comments on thousands of blogs.  Update:  As I pointed out on social media to some friends, it's not that this blog is porn - it's just that someone somewhere thinks readers of this blog probably like porn.  :-)


Monday, November 12, 2018

Book review: Solid State Insurrection

Apologies for the slow updates.  Between administrative responsibilities and trying to get out a couple of important papers, posting has been a bit slower than I would like, and this is probably going to continue for a few weeks.

If you've wondered how condensed matter physics got to where it is, more in terms of the sociology of physics rather than the particular scientific advances themselves, I strongly recommend Solid State Insurrection:  How the Science of Substance Made American Physics Matter, by Joseph D. Martin.  This book follows the development of condensed matter physics from its beginnings before WWII through to what the author views as the arrival of its modern era, the demise of the Superconducting Supercollider in the early 1990s, an event strongly associated by some with critiques by Phil Anderson.  

I got into condensed matter physics starting in the early 1990s, in the post-"More is Different" era, and CMP had strongly taken on its identity as a field dedicated to understanding the states of matter (and their associated structural, electronic, and magnetic orders) that emerge collectively from the interactions of many underlying degrees of freedom.  While on some level I'd known some of the history, Prof. Martin's book was eye-opening for me, describing how solid-state physics itself emerged from disparate, fluctuating subfields (metallurgy, in particular).   

Martin looks at the battles within the APS and the AIP into the 1940s about whether it's good or bad to have topical groups or divisions; whether it's a good or bad thing that the line between some of solid-state physics and electrical engineering can be blurry; how the societies' publication models could adapt.  Some of that reads a bit like the standard bickering that can happen within any professional society, but the undercurrent throughout is interesting, about the sway held in the postwar era by nuclear and later particle physicists.  

The story of the founding of the National Magnet Lab (originally at MIT, originally funded by the Air Force before switching to NSF) was new to me.  It's an interesting comparison between the struggles to get the NML funded (and how "pure" vs "applied" its mission should be) and the rate at which accelerator and synchrotron and nuclear science facilities were being built.  To what extent did the success of the Manhattan Project give the nuclear/particle community carte blanche from government funders to do "pure" science?  To what degree did the slant toward applications and away from reductionism reinforce the disdain which some held for solid-state (or should I say squalid state or schmutzphysik)?

Martin also presents the formalization of materials science as a discipline and its relationship to physics, the rise of the antireductionist/emergence view of condensed matter (a rebranding that began in the mid-60s and really took off after Anderson's 1972 paper and a coincident NRC report), and a recap of the fight over the SSC along the lines of condensed matter vs. high energy.   (My take:  there were many issues behind the SSC's fate.  The CM community certainly didn't help, but the nature of government contracting, the state of the economy at the time, and other factors were at least as contributory.)

In summary:  Solid State Insurrection is an informative, interesting take on the formation and evolution of condensed matter physics as a discipline.  It shows the very human, social aspects of how scientific communities grow, bicker, and change.



Saturday, November 03, 2018

Timekeeping, or why helium can (temporarily) kill your iphone/ipad

On the day when the US switches clocks back to standard time, here is a post about timekeeping and its impact.  

Conventional computers need a clock, some source of a periodic voltage that tells the microprocessor when to execute logic operations, shift bits in registers, store information in or retrieve information from memory.  

Historically, clocks in computer systems have been based on quartz oscillators or similar devices.  Quartz is an example of a piezoelectric, a material that generates a voltage when strained (or, conversely, deforms when subjected to a properly applied voltage).  Because quartz is a nice material with a well-defined composition, its elastic properties are highly reproducible.  That means that it's possible to carve it into a mechanical resonator (like a tuning fork), and as long as you can control the dimensions well, you will always get very close to the same mechanical resonance frequency.  Pattern electrodes on there, making the quartz into a capacitor, and it's possible to set up an electrical circuit that takes the voltage produced when the quartz is resonantly deforming, amplifies that signal, and feeds it back onto the material, so that the quartz crystal resonator will ring at its natural frequency (just like a microphone pointed at a speaker can lead to a ringing).  Because quartz's elastic and electrical properties depend only weakly on temperature, this can act as a very stable clock, either for a computer like your desktop machine or tablet or smartphone, or in an electric wristwatch.  

In recent years, though, it's become attractive for companies to start replacing quartz clocks with microelectromechanical resonators.  While silicon is not piezoelectric, and so can't be used directly as a substitute for quartz, it does have extremely reproducible elastic properties.  Unlike piezoelectric resonators, though, MEMS resonators typically have to be packaged so that the actual paddle or cantilever or tuning fork is in vacuum.  Gas molecules can damp the resonator, lowering its quality factor and therefore hurting its frequency stability (or possibly damping its motion enough that it just can't function as part of a stable self-resonating circuit).  

The issue that's come up recently (see this neat article) is that too much helium gas in the surrounding air can kill (at least temporarily) iphones and such devices that use these MEMS clocks.  In a helium-rich environment like when filling up superconducting magnets, helium molecules can diffuse through the packaging into the resonator environment.  Whoops.  Assuming the device isn't permanently damaged (I could imagine feedback circuits doing weird things if the damping is way out of whack), the helium has to diffuse out again to resolve the problem.  Neat physics, and something for helium-users to keep in mind.