APS March Meeting 2019

Once more, it is that time of year, when (mostly) condensed matter physicists gather in ever-increasing numbers to give and watch talks, network, try to get cool swag at the tradeshow, and generally grouse about the poor quality and high price of convention center coffee.  The March Meeting this year is in Boston for the first time since I've been going 2012.  (Strangely, I was unable to find a list of all the March Meeting sites online.  Perhaps a reader knows the last time the meeting was in Boston.)  It's my third and last year as a DCMP member-at-large, so it will be interesting to hear what comes up at the business meetings this time.   As I have done in past years, I'll do my best to write up some of what I see and give my impressions of the conference, though I may be more concise compared to previous years.

Why twisting materials is interesting

Twisted bilayer graphene is a hot topic, with 32 preprints on the arxiv using those keywords just since the beginning of the year.  It's worth explaining for non-experts, why this system, comprising two atomic layers of graphene twisted relative to each other by some angle, is so interesting. 

Let's start w/ the basics.  In the (non-relativistic) quantum world, we talk about the wavefunction \(\psi(\mathbf{r},t)\) of a system.  The Schroedinger equation describes how the wavefunction evolves with time, and by solving it we can find the particular energy levels ("stationary states") for a given problem.  The magnitude-squared of the spatial wavefunction, \(|\psi(\mathbf{r},t)|^2\) gives the probability of finding the particle in a particular place at a particular time.  

The wavefunction a free particle with a well-defined momentum \(\mathbf{p}\) can be treated as a wave with a wavevector \(\mathbf{p}/\hbar \equiv \mathbf{k}\), and therefore a wavelength \(2 \pi \hbar/|\mathbf{p}|\).   Higher momentum = shorter wavelength = the wavefunction has more closely spaced wiggles.  The kinetic energy goes like \(p^{2}/2m\), as in classical nonrelativistic mechanics.   (Note that the magnitude-squared of such a wave is constant as a function of spatial position.  That is consistent with the uncertainty principle:  Knowing the momentum precisely means that the position could be anything.)  

Take a particle and stick it in an environment where the local potential energy varies periodically in space - ideally in a system so large that we can neglect boundary effects for now.  The classic example of this is an electron in a crystalline solid.   I've talked about this kind of spatial periodicity before.  There are a couple of ways to think about this situation.  We have replaced "continuous translational symmetry" (the environment of the particle is unchanged if we consider moving the particle anywhere) with "discrete translational symmetry" (now we have to move an integer number of lattice spacings to get back to the same environment for the particle).  Mathematically, the single-particle stationary states can still be labeled by a parameter \(\mathbf{k}\), but they're Bloch waves rather than plane waves, and the energy \(E(\mathbf{k})\) is no longer necessarily proportional to \((\hbar k)^{2}\) all the time.   Physically, when the naive spatial periodicity of the single-particle state matches up with the spatial periodicity (or some harmonic) of the lattice, it makes sense that there should be deviations from what we'd see with a free particle.  The result is "band structure", ranges of energy densely filled with allowed single-particle states, separated by "band gaps", ranges of energy in which there is no way to make a single-particle state and still satisfy the Schroedinger equation with the spatially periodic potential energy.

The particular spatial periodicity of the lattice determines the form of \(E(\mathbf{k})\).  For a hexagonal lattice like single-layer graphene, it turns out that there are two "Dirac points", and that near those special values of \(\mathbf{k}\), the form of \(E(k)\) looks like what is obeyed by photons in free space (!), with energy linearly dependent on \(k\).

The key point here:  if we can tune the spatial periodicity of the potential arbitrarily, we can create interesting forms of \(E(\mathbf{k})\).  That's really a neat idea.  Carefully growing stacked multilayers of semiconductors along one direction has been used to create "minibands" for optoelectronic devices.  Starting from a 2D surface state in copper, people have been able to put down patterns of CO molecules to create spatial periodicities in 2D, creating structures that look and act like graphene, or very recently even fractals.  People have also tried doing this by patterning semiconductor structures, but it's very hard to get sufficient uniformity so that disorder isn't a problem.

Stacking graphene layers with some relative twist angle is a great way to create a 2D modulation with excellent uniformity over large areas (many many lattice spacings).  This 2D modulation shows up because of the Moire pattern, which gives a spatially periodic coupling between the bands in each of the layers.  By tweaking the relative angle, the spatial pattern can be tuned.  By squishing on the bilayer, in principle the strength of the coupling can be tuned.  This kind of 2D modulation should be possible in principle in twisted bilayers of all kinds of stackable materials.

The situation is even more interesting once we start thinking about electron-electron interactions.

Another way to think of bands:  Start from the atomic energy levels of the individual, isolated constituent atoms.  The electronic levels of each atom are sharply defined.  All of the 4s orbitals, say, have the same energy.  If you think about possible electronic states, the "band" made out of isolated (localized to individual atoms) 4s orbitals is very narrow in energy.  If you built up some linear combination of those 4s orbitals that had a parameter \(\mathbf{k}\), the energy \(E(\mathbf{k})\) of that state would basically be independent of \(\mathbf{k}\).  That is, the band would be "flat".   Turn on hopping between atoms, and band broadens out in energy. 

If we throw in a bunch of electrons and ask what is the lowest energy state of the many-electron system, we can often get away with mostly neglecting electron-electron interactions.  Because of the Pauli Principle, we fill up the bands from the bottom up, and very often the (single-particle kinetic + lattice interactions) energy grows very rapidly, so much so that any electron-electron interactions are not very important.   (That's what happens in the periodic table - as you go to atoms containing more and more electrons, the kinetic energy grows fast enough that e-e interactions don't really disrupt the basic hydrogen-like s-p-d-f orbital structure of energy levels.)

In the twisted bilayers, it is possible to end up with some bands that are very flat - so flat that the typical electron-electron interaction energy is comparable or large compared to the bandwidth.  In these flat band situations, electron-electron interactions can end up being very important in determining the collective many-body state of the electrons.  That appears to be what people are seeing in the experiments mentioned previously.

The bottom line:  Twisted stacking is a great, robust way to create a lateral spatially modulated potential, and therefore (within particular geometric limits) a "designer" band structure.  The resulting bands can be very flat, so that electron-electron interaction effects (apparently) can lead to remarkable many-body responses, like the onset of superconductivity or magnetism. 

More brief items

Some additional interesting links:

• Another example of emergent universal behavior, as it is demonstrated that runners at the start of a marathon seem to collectively obey hydrodynamics, like a fluid.
• The Voices of the Manhattan Project oral histories effort has a large number of interviews online.  It’s important for posterity that these were recorded before everyone involved is gone.
• Maybe massive open online courses were not, in fact, the end of the traditional model of higher education.  Who could have foreseen this?
• There are people arguing that the preprint arxiv model is a good path toward opens access.  This is definitely something I like, especially more than models involving authors paying thousands of dollars to for-profit publishers for open access journals.

Brief items

This is the absolute most dense time of the year in terms of administrative obligations, so posting is going to be a bit sparse.  In the meantime, here is a bit of interesting reading:

Scientific American has an interesting article about the fact that two independent means of assessing the Hubble constant (analysis of the cosmic microwave background on one hand; analysis of "standard candles" on the other) disagree well outside the estimated systematic uncertainties.

Kip Thorne posted a biographical reminiscence about John Wheeler on the arxiv.  I haven't read it yet, but it's in my queue.

Quanta Magazine had put up a very well done article about turbulence.  Good stuff.  I liked the animation.

Three brief book reviews

In the spirit of Peter Woit's latest post, I also wanted to offer up three miniature book reviews.

The New Science of Strong Materials: Or Why You Don't Fall Through the Floor by J. E. Gordon.  This book is a fine, accessible (minimal math) introduction to materials science by one of the people who created the field as a distinct discipline.  The first edition came out in 1968, so it is a bit of an historical journey.  For example, the author describes how just recently people were able to achieve the first transmission electron microscopy images that directly showed dislocations.  The only way they could do it was to image a material that was actually a crystal of Pt-containing organic complexes - the Pt has high electron density for imaging, and the organic ligands keep the Pt spatially separated by a large enough distance (a few nm) to resolve in the equipment of the day.  Quite a difference from the present state of the art.  Gordon wrote in an engaging style with a dry UK wit, and clearly had a genuine fondness for wood as an amazing, versatile composite material.  Should be required reading for undergrad mechanical and civil engineers who need to get a real physical picture for stress, strain, and ways to mitigate crack propagation.  A fun read.

How to Invent Everything: A Survival Guide for the Stranded Time Traveler by Ryan North.
I won't spoil the amusing conceit that's used as a frame for this remarkable, fun collection of bite-sized bits of knowledge.   Suffice it to say that, in the event of a global collapse of civilization, this will be a handy tome to have on hand, should you need to recreate, say, agriculture or printing or distillation or the steam engine.  The recurring theme is, there are many societal and technological advancements that the human race seemed to be curiously slow to figure out (like, many tens of thousands of years slower than could have been done).  Just the kind of fun you would expect from the person who brought us Dinosaur Comics.  It does have a bit of a Randall Munroe What If vibe, but it's distinctive.

Math with Bad Drawings: Illuminating the Ideas That Shape Our Reality by Ben Orlin.
This was also very enjoyable.  Parts of it made me think that "Condensed Matter with Bad Drawings" would be a great approach, except that now it would seem hopelessly derivative.  The book takes a free-wheeling path through math in our lives, with large, healthy doses (perhaps a bit lengthy) of statistics (lies and damn lies - what different statistical quantities are telling and not telling you) and economics.  It's well done, and I particularly liked the beginning sections that explain what math really looks and feels like to a mathematician; that really resonated, and I wish I could convey even half as well that aspect of how physicists look at and think about the world around us.

"Seeing" atoms

The power of modern transmission electron microscopy (TEM) is very impressive.  Often in TEM images at high magnification, you can see what looks like the atomic lattice, but that can be a bit illusory.  Because the scattering effects of individual atoms, especially light ones like carbon, can be very slight, often those images are looking at the result from scattering off columns of atoms, with the crystalline structure of the material helping greatly to produce a clean image.  With state of the art instrumentation and processing, however, it is possible to resolve single atoms, even in atomically thin, light materials like graphene.  This image, from a new ACS Nano paper by Lee et al. from Oxford University, is a great example of what is now possible, showing the reconfiguration of carbon bonds as a nanoscale graphene constriction is modified.  Pretty eye-popping.

Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!

This week in the arxiv

Twisted bilayer graphene (TBLG): Is there anything it can't do?  Two recent papers have appeared on the arxiv that show that TBLG looks like a versatile platform for exploring the physics of electrons in comparatively flat bands.  Remember, flat bands as a function of (crystal) momentum = high effective mass = tendency toward localization = interaction effects have an easier time dominating the kinetic energy.  There was a big splash at the APS meeting last year when superconductivity was found in this system that had some resemblance to the phenomena seen in the high-T_c cuprates.

arxiv:1901.03520 - Sharpe et al., Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
In this new work the Goldhaber-Gordon group at Stanford shows that, in TBLG, at the right gate voltage (that is, the right filling of the rather flat bands), the system seems to develop spontaneous ferromagnetism, seen both through the appearance of hysteresis in the electrical resistance as a function of magnetic field, and through the onset of an anomalous Hall signature.  Through non-local transport effects (e.g., drive a current over here, measure a voltage over there) they see indications of edge currents, suggesting that topology is important here. 

arxiv:1901.03710 - Cao et al., Strange metal in magic-angle graphene with near Planckian dissipation
Another feature of strongly correlated electronic materials like the cuprate superconductors is "strange metallicity", when the temperature dependence of the electrical resistance is linear with T over a large range, in contrast with simple expectations of Fermi liquid theory.  There have been arguments that in the limit of very strong electron-electron scattering, a kind of hydrodynamics kicks in for the electrons, with universal bounds on charge diffusion (and hence the resistance).  These arguments are sometimes based on fairly exotic ideas.   Not everyone agrees with the proposed universality.  In this new paper, the MIT group shows that the TBLG system does show resistance similar in form and magnitude to this strange metallicity.

The broad idea of tuning band flatness by stacking and rotating 2d materials continues to show promise as a playground for looking at the competition between different possible ordered states.

Magnetic data storage - heat-assisted v microwave-assisted

(Ironically, given my recent missive about the importance of condensed matter beyond applications to information technology, here is a post about condensed matter in information technology.)

It may seem like solid-state drives - basically flash memory - have taken over data storage, particularly in phones, tablets, and laptops.  However, magnetic storage media, particularly in the form of hard drives, are still the main tools of choice for the cloud.  Magnetic storage is very robust and doesn't rely on charge staying put on tiny floating gates to hold your information.  Rather, in a hard drive the zeros and ones of your data are encoded as magnetic domains of particular orientation in a specially engineered thin film of material on a disk platter. 

The amount of research and engineering development that has gone into hard drives is amazing.  The read/write heads "fly" at nanometer separation over incredibly smooth magnetic surfaces.  The smaller you can make the magnetic domains and still manipulate and read them in a controlled manner, the higher the density of the information storage.  The timing and positioning stability required to store and retrieve terabytes per square inch is amazing.  Reading the data requires detecting the magnetic fields produced by the domains.  These days that's done using magnetoresistance, some component of the read head with an electrical resistance that changes depending on the local magnetic environment.  Giant magnetoresistance went from the laboratory to hard drive read heads to Stockholm for the 2007 Nobel Prize in Physics.  Its successor in read heads, tunneling magnetoresistance, now rules the roost.

On the physics side, there are many challenges for continued scaling.  Magnetic domains interact with each other.  To be reliable for storage, it's important that the magnetization \(\mathbf{M}\) of a domain remain fixed once it is set.  The energy scales associated with pinning a domain tend to scale with domain size, meaning that, all other things being equal, tinier domains are easier to flip (for writing, yay, for long term stability, boo).  In the extreme limit, thermal fluctuations can provide enough energy to reorient the magnetization, leading to superparamagnetism.  A number of years ago, IBM and others came up with ways to increase the coercive fields (and anisotropy energies) of the bits in disk media.  Still, as bits get smaller, it's more important to pin them down, but somehow still allow deliberate reorientation of \(\mathbf{M}\) for write operations.

This article spurred me to write a little about this, in part because of what it gets wrong.  Hard drive manufacturers have decided that the best plan is to pin down the bits firmly, and then during the write process, locally kick those bits hard enough to allow reorientation of \(\mathbf{M}\). 

One method that's been under consideration for a while, and is the favorite of Seagate, is HAMR - heat-assisted magnetic recording.  This requires some means of locally heating the disk media right under the write-head so that thermal energy is available to help the bit reorient.  The Seagate approach uses an embedded laser source and a plasmonic antenna to drive the local optical heating (this has nothing to do with an electrical discharge, despite the linked article at the start of this paragraph).

Western Digital is pursuing an alternative approach microwave-assisted magnetic recording (MAMR).  The idea there (video) is to use a local oscillator (one based on spin torque) to generate microwaves locally at the write head, with the frequency of those microwaves tuned to drive ferromagnetic resonance of the bit to flip it.

Tons of physics in all of this, and an enormous amount of engineering cleverness.  Magnetic data storage will be with us for a while yet.

Ask me something.

As we approach the end of another year, I realize two things:

• Being chair has a measurable impact on my blogging frequency - it's dropped off appreciably since summer 2016, though fluctuations are not small. 
• It's been almost 2.5 years since I did an "Ask me something" post, so please have at it.

Short items

The end of the calendar year has been very busy, leading to a slower pace of posting.  Just a few brief items:

• I have written a commentary for Physics Today, which is now online here.  The topic isn't surprising for regular readers here.  If I'm going to keep talking about this, I need to really settle on the correct angle for writing a popular level book about CMP.
• This article in Quanta about this thought experiment is thought-provoking.  I need to chew on this for a while to see if I can wrap my brain around this.
• The trapped ion quantum computing approach continually impresses.  The big question for me is one that I first heard posed back in 1998 at Stanford by Yoshi Yamamoto:  Do these approaches scale without having the number of required optical components grow exponentially in the number of qubits?
• Superconductivity in hydrides under pressure keeps climbing to higher temperatures.  While gigapascal pressures are going to be impractical for a long long time to come, progress in this area shows that there does not seem to be any inherent roadblock to having superconductivity as a stable, emergent state at room temperature.
• As written about here during the March Meeting excitement, magic angle graphene superconductivity has been chosen as Physics World's breakthrough of the year.

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.

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.

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.

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.

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.  :-)

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.

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. 

Imposter syndrome

If you're reading this, you've probably heard of imposter syndrome before - that feeling that, deep down, you don't really deserve praise or recognition for your supposed achievements, because you feel like you're not as good at this stuff as your colleagues/competitors, who must really know what they're doing.  As one of my grad school roommates said as a bunch of us were struggling with homework:  "Here we are, students in one of the most prestigious graduate programs in the country.  I sure hope someone knows what they're doing."  

This feeling can be particularly prevalent in fields where there is great currency in the perception of intellectual standing (like academia, especially in science).  My impression is that a large majority of physicists at all levels (faculty, postdocs, grad students, undergrads) experience this to greater or lesser degrees and frequencies.  We're trained to think critically, and driven people tend to overthink things. If you're fighting with something (some homework set, or some experiment, or getting some paper out, or writing a proposal), and your perception is that others around you are succeeding while you feel like you're struggling, it's not surprising that self-doubt can creep in.  

I'm not posting because I've had a great insight into mitigating these feelings (though here are some tips).  I'm posting just to say to readers who feel like that sometimes: you're not alone.   

Scalable materials for quantum information

There is no question that the explosive spread of electronics and optoelectronic technology in the 20th century has its foundation in the growth and preparation of high quality materials - silicon with purity better than parts per billion, single crystals cut and polished to near-atomic flatness, with exquisite control of impurity concentrations; III-V compound semiconductors for high speed transistors, LEDs, and lasers; even ultrapure SiO₂ for millions of km of ultralow loss optical fiber.

Any new electronics-based technology intended to supplant or supplement now-traditional electronic materials at scale is going to need a material platform that can credibly reach similar quality.  Many of the 2d materials have a long way to go in that regard.  However, there have been recent advances in a couple of specific systems targeted for particular forms of quantum information devices.  

arXiv:1810.09350 - Nelz et al., Towards wafer-scale diamond nano- and quantum technologies
It is possible to grow single-crystal diamond films on the 100 mm wafer scale, starting with Si substrates coated with iridium/yttria-stabilized zirconia.  There are dislocations and stacking faults, but it's getting there.  If the native defect density can be controlled and eliminated to a very fine level, and ion implantation can be used to create well-defined defects (NV centers and the like), that would be a big boost to hopes of wide-spread use and mass fabrication of quantum devices based on these systems.

arXiv:1810.06521 - Sabbagh et al., Wafer-scale silicon for quantum computing
Those who want to use electron spins in Si as quantum bits need to worry about whether nuclear spins from naturally abundant ²⁹Si.  It has now been shown that it is possible to use isotopically enriched silane made from ²⁸Si to grow epitaxial layers of material almost devoid of ²⁹Si, and that MOS devices made from this stuff can be of high quality.  It's worth noting:  Isotope separation of different Si isotopic variants of silane by centrifuge is easier than trying the same thing with, e.g, uranium hexafluoride to enrich ²³⁵U, because the percentage mass difference is considerably higher in the Si case.