APS March Meeting 2018, day 1

As I explained yesterday, my trip to the APS is even more scattered than in past years, but I'll try to give some key points.  Because of meetings and discussions with some collaborators and old friends, I didn't really sit down and watch entire sessions, but I definitely saw and heard some interesting things.

Markus Raschke of Colorado gave a nice talk about the kinds of ultrafast and nonlinear spectroscopy you can do if you use a very sharp gold tip as a plasmonic waveguide.  The tip has a grating milled onto it a few microns away from the sharp end, so that hitting the grating with a pulsed IR laser excites a propagating surface plasmon mode that is guided down to the really sharp point.  One way to think about this:  When you use the plasmon mode to confine light down to a length scale \(\ell\) comparable to the radius of curvature of the sharp tip, then you effectively probe a wavevector \(k_{\mathrm{eff}} \sim \2\pi/\ell\).  If \(\ell\) is a couple of nm, then you're dealing with \(k_{\mathrm{eff}}\) values associated in free space with x-rays (!).  This lets you do some pretty wild optical spectroscopies.  Because the waveguiding is actually effective over a pretty broad frequency range, that means that you can get very short pulses down there, and the intense electric field can lead to electron emission, generating the shortest electron pulses in the world.  

Andrea Young of UCSB gave a very pretty talk about looking at even-denominator fractional quantum Hall physics in extremely high quality bilayer graphene.  Using ordinary metal electrodes apparently limits how nice the effects can be in the bilayer, because the metal is polycrystalline and that disorder in local work function can actually matter.   By using graphite as both the bottom gate and the top gate (that is, a vertical stack of graphite/boron nitride/bilayer graphene/boron nitride/graphite), it is possible to tune both the filling fraction (ratio of carrier density to magnetic field) in the bilayer and the vertical electric field across the bilayer (which can polarize the states to sit more in one layer or the other).  Capacitance measurements (e.g., between the top gate and the bottom gate, or between either gate and the bilayer) can show extremely clean quantum hall data.

Sankar Das Sarma of Maryland spoke about the current status of trying to use Majorana fermions in semiconductor wire/superconductor electrode structures for topological quantum computing.  For a review of the topic overall, see here.   This is the approach to quantum computing that Microsoft is backing.  The talk was vintage Das Sarma, which is to say, full of amusing quotes, like "Physicists' record at predicting technological breakthroughs is dismal!" and "Just because something is obvious doesn't mean that you should not take it seriously."  The short version:  There has been great progress in the last 8 years, from the initial report of possible signatures of effective Majorana fermions in individual InSb nanowires contacted by NbTiN superconductors, to very clean looking data involving InAs nanowires with single-crystal, epitaxial Al contacts.  However, it remains very challenging to prove definitively that one has Majoranas rather than nearly-look-alike Andreev bound states.

In case you are interested in advanced (beyond-first-year) undergraduate labs and how to do them well, you should check out the University of Minnesota's site, as well as the ALPhA group from the AAPT.   There is also an analogous group working on projects to integrate computation into the undergraduate physics curriculum.

One potentially very big physics news story that I heard about during the day, but won't be here to see the relevant talk: [Update:  Hat tip to a colleague who pointed out that there is a talk tomorrow morning that will cover this!]  There are back-to-back brand new papers in Nature today by Yuan Cao et al. from the Jarillo-Herrero group at MIT.  (The URLs don't work yet for the articles, but I'll paste in what Nature has anyway.)  The first paper apparently shows that when you take two graphene layers and rotationally offset them from graphite-like stacking by 1.05 degrees (!), the resulting bilayer is alleged to be a Mott insulator.  The idea appears to be that the lateral Moire superlattice that results from the rotational offset gives you very flat minibands, so that electron-electron interactions are enough to lock the carriers into place when the number density of carriers is tuned correctly.  The second paper apparently (since I can't read it yet) shows that as the carrier density is tuned away from the Mott insulator filling, the system becomes a superconductor (!!), with a critical temperature of 1.7 K.  This isn't particularly high, but the idea of tuning carrier density away from a Mott state and getting superconductivity is basically the heart of our (incomplete) understanding of the copper oxide high temperature superconductors.  This is very exciting, as summarized in this News and Views commentary and this news report.  

APS March Meeting 2018

It's that time of year again:  The running of the physicists annual APS March Meeting, a gathering of thousands of (mostly condensed matter) physicists.  These are (sarcasm mode on) famously rowdy conferences (/sarcasm).  This year the meeting is in Los Angeles.  I came to the 1998 March Meeting in LA, having just accepted a fall '98 postdoctoral fellow position at Bell Labs, and shortly after the LA convention center had been renovated.   At the time, the area around the convention center was really a bit of a pit - very few restaurants, few close hotels, and quite a bit of vacant and/or low-end commercial property.  Fast forward 20 years, and now the area around the meeting looks a lot more like a sanitized Times Square, with big video advertisements and tons of high end flashy stores.

Anyway, I will try again to write up some of what I see until I have to leave on Thursday morning, though this year between DCMP business, department chair constraints, and other deadlines, I might be more concise or abbreviated.  (As I wrote last year, if you're at the meeting and you don't already have a copy, now is the perfect time to swing by the Cambridge University Press exhibit at the trade show and pick up my book :-) ).

Vibranium and its properties

Fictional materials can be a fun starting point for thinking about and maybe teaching about material properties.  Back in 2015 I touched on this here, when I mentioned a few of my favorite science fictional materials (more here, here, and here). 

With the release of Black Panther (BP), we now have much more information about the apparent properties of vibranium in the Marvel Cinematic Universe.   

Vibranium is pretty amazing stuff - like many fictional materials, it sometimes seems to have whatever properties are necessary to the story.  As a physicist I'm not qualified to talk about its putative medicinal properties mentioned in BP, but its physical properties are fun to consider.  Vibranium appears to be a strong, light, silvery metal (see here), and it also has some remarkable abilities in terms of taking macroscopic kinetic energy (e.g., of a projectile) and either dissipating it (look at the spent bullets in the previously linked video) or, according to BP, storing that energy for later release.  At the same time, Captain America's vibranium shield is able to bounce around with incredibly little dissipation of energy, prompting the Spider-Man quote at right.

In the spirit of handwaving physics, I think I've got this figured out.  

In all solids, there is some coupling between the deformation of the atomic lattice and the electronic states of the material (here is a nice set of slides about this).  When we talk about lattice vibrations, this is the electron-phonon coupling, and it is responsible for the transfer of energy from the electrons to the lattice (that is, this is why the actual lattice of atoms in a wire gets warm when you drive electrical current through the material).  The e-ph coupling is also responsible for the interaction that pairs up electrons in conventional superconductors.  If the electron-phonon coupling is really strong, the deformation of the lattice can basically trap the electron - this is polaron physics.  In some insulating materials, where charge is distributed asymmetrically within the unit cell of the crystal, deformation of the material can lead to big displacements of charge, with a corresponding buildup of a voltage across the system - this is piezoelectricity.  

The ability of vibranium to absorb kinetic energy, store it, and then later discharge it with a flash, suggests to me that lattice deformation ends up pumping energy into the electrons somehow.  Moreover, that electronically excited state must somehow be metastable for tens of seconds.  Ordinary electronic excitations in metals are very short-lived (e.g., tens of femtoseconds for individual excited quasiparticles to lose their energy to other electrons).  Gapped-off collective electronic states (like the superconducting condensate) can last very long times.  We have no evidence that vibranium is superconducting (though there are some interesting maglev trains in Wakanda).  That makes me think that what's really going in involves some topologically protected electronic states.  Clearly we need to run experiments (such as scanning SQUID, scanning NV center, or microwave impedance microscopy) to search for the presence of edge currents in percussively excited vibranium films to test this idea.

Physics in the kitchen: Jamming

Last weekend while making dinner, I came across a great example of emergent physics.  What you see here are a few hundred grams of vacuum-packed arborio rice:

The rice consists of a few thousand oblong grains whose only important interactions here are a mutual "hard core" repulsion.  A chemist would say they are "sterically hindered".  An average person would say that the grains can't overlap.  The vacuum packing means that the whole ensemble of grains is being squeezed by the pressure of the surrounding air, a pressure of around 101,000 N/m² or 14.7 pounds per in².  The result is readily seen in the right hand image:  The ensemble of rice forms a mechanically rigid rectangular block.  Take my word for it, it was hard as a rock. 

However, as soon as I cut a little hole in the plastic packaging and thus removed the external pressure on the rice, the ensemble of rice grains lost all of its rigidity and integrity, and was soft and deformable as a beanbag, as shown here. 

So, what is going on here?  How come this collection of little hard objects acts as a single mechanically integral block when squeezed under pressure?  How much pressure does it take to get this kind of emergent rigidity?  Does that pressure depend on the size and shape of the grains, and whether they are deformable? 

This onset of collective resistance to deformation is called jamming.  This situation is entirely classical, and yet the physics is very rich.  This problem is clearly one of classical statistical physics, since it is only well defined in the aggregate and quantum mechanics is unimportant.  At the same time, it's very challenging, because systems like this are inherently not in thermal equilibrium.  When jammed, the particles are mechanically hindered and therefore can't explore lots of possible configurations.   It is possible to map out a kind of phase diagram of how rigid or jammed a system is, as a function of free volume, mechanical load from the outside, and temperature (or average kinetic energy of the particles).   For good discussions of this, try here (pdf), or more technically here and here.   Control over jamming can be very useful, as in this kind of gripping manipulator (see here for video).  

Rice Cleanroom position

In case someone out there is interested, Rice is hiring a cleanroom research scientist.  The official job listing is here.  To be clear:  This is not a soft money position.

The Cleanroom Facility at Rice University is a shared equipment facility for enabling micro- and nanofabrication research in the Houston metropolitan area. Current equipment includes deposition, lithography, etching and a number of characterization tools. This facility attracts users from the George R. Brown School of Engineering and the Wiess School of Natural Science and regional universities and corporations whose research programs require advanced fabrication and patterning at the micro- and nanoscale. A new state of the art facility is currently being constructed and is expected to be in operation in summer 2018. Additionally, with new initiatives in Molecular Nanotechnology, the Rice University cleanroom is poised to see significant growth in the next 5-10 years. This job announcement seeks a motivated individual who can lead, manage, teach and grow this advanced facility.

The job responsibilities of a Cleanroom Research Scientist include conducting periodic and scheduled maintenance and safety check of equipment and running qualification and calibration recipes. The incumbent will be expected to maintain the highest safety standards, author and update standard operation procedures (SOPs), maintain and calibrate processes for all equipment. The Cleanroom Research Scientist will help facilitate new equipment installation, contact vendors and manufacturers and work in tandem with them to resolve equipment issues in a timely and safe manner. Efficient inventory management of parts, chemicals and supplies will be required. The Cleanroom Scientist will also oversee personal one-to-one training of users. Additionally, the incumbent will help develop cleanroom laboratory short courses that provide lectures to small groups of students. The incumbent will also coordinate with technical staff members in Rice SEA (Shared Equipment Authority).

This week in the arxiv

Back when my blogging was young, I had a semi-regular posting of papers that caught my eye that week on the condensed matter part of the arxiv.  As I got busy doing many things, I'd let that fall by the wayside, but I'm going to try to restart it at some rate.  I generally haven't had the time to read these in any detail, and my comments should not be taken too seriously, but these jumped out at me.

arxiv:1802.01045 - Sangwan and Hersam; Electronic transport in two-dimensional materials
If you've been paying any attention to condensed matter and materials physics in the last 14 years, you've noticed a huge amount of work on genuinely two-dimensional materials, often exfoliated from the bulk as in the scotch tape method, or grown by chemical vapor deposition.  This looks like a nice review of many of the relevant issues, and contains lots of references for interested students to chase if they want to learn more.

arxiv:1802.01385 - Froelich; Chiral Anomaly, Topological Field Theory, and Novel States of Matter
While quite mathematical (relativistic field theory always has a certain intimidating quality, at least to me), this also looks like a reasonably pedagogical introduction of topological aspects of condensed matter.  This is not for the general reader, but I'm hopeful that if I put in the time and read it carefully, I will gain a better understanding of some of the topological discussions I hear these days about things like axion insulators and chiral anomalies.

arXiv:1802.01339 - Ugeda et al.; Observation of Topologically Protected States at Crystalline Phase Boundaries in Single-layer WSe2
arXiv:1802.02999 - Huang et al.; Emergence of Topologically Protected Helical States in Minimally Twisted Bilayer Graphene
arXiv:1802.02585 - Schindler et al.; Higher-Order Topology in Bismuth

Remember back when people didn't think about topology in the band structure of materials?  Seems like a million years ago, now that a whole lot of systems (often 2d materials or interfaces between materials) seem to show evidence of topologically special edge states.   These are three examples just this week of new measurements (all using scanning tunneling microscopy as part of the tool-set, to image edge states directly) reporting previously unobserved topological states at edges or surface features.

New readers: What is condensed matter physics? What is special about the nanoscale?

If you're a new reader, perhaps brought here by the mention of this blog in the Washington Post, welcome!   Great to have you here.  Just a couple of quick FAQs to get you oriented:

What is condensed matter physics?  Condensed matter (once known as "solid state) is a branch of physics that deals with the properties of matter consisting of large numbers of particles (usually atoms or (electrons+the rest of the atoms)) in "condensed" states like liquids and solids - basically the materials that make up an awful lot of the stuff you interact with all the time.  New properties can emerge when you bring lots of particles together.  See here for an example involving plastic balls, or here (pdf) for a famous essay about this general point.  Condensed matter physicists are often interested in identifying the different types of states or phases that can arise, and understanding transitions between those states (like how does water boil, or how does magnetism turn on in iron as its temperature is lowered from the melting point, or how does a ceramic copper oxide suddenly start letting electricity flow without resistance below some particular temperature).  Hard condensed matter typically deals with systems where quantum mechanics is directly important (electronic, magnetic, and optical properties of materials, for example), while soft condensed matter describes systems where the main actors (while quantum deep down like all matter) are not acting in a quantum way - examples include the jamming of grains of sand when you build a sand castle, or the spontaneous alignment of rod-like molecules in the liquid crystal display you're using to read this.

While particle physics tries to look at the tiniest bits of stuff, condensed matter hits on some of the same (literally the same concepts and math) deep ideas about symmetry, and often has direct implications for technologies that affect your daily life.   Understanding this stuff has given us things like the entire electronics industry, the telecommunications industry, and soon probably quantum computers.  

A powerful concept in physics in general and condensed matter in particular is universality.  For example, materials built out of all kinds of different ingredients can be mechanically rigid solids; there is something universal about mechanical rigidity that makes it emerge independent of the microscopic details.  Another example:  Lots of very different systems (metallic lead; waxy crystals of buckyball molecules with some alkaline metal atoms in between; ceramic copper oxides; hydrogen sulfide gas under enormous pressure) can conduct electricity without resistance at low temperatures - why and how is superconductivity an emergent property?

What is special about the nanoscale?  Because it's about collective properties, traditional condensed matter physics often uses a lot of nice approximations to describe systems, like assuming they're infinite in extent, or at least larger than lots of physically important scales.   When you get down to the nanoscale (recall that a typical atom is something like 0.3 nanometers in diameter), a lot of the typical approximations can fail.  As the size of the material or system becomes small compared to the length scales associated with various physical processes, new things can happen and the properties of materials can change dramatically.  Tightly confined liquids can act like solids.  Colorless materials can look brilliantly chromatic when structured on small scales.  Two electrical insulators brought together can produce a nanoscale-thick metallic layer.   We now have different techniques for structuring materials on the nanoscale and for seeing what we're doing down there, where the building blocks are often far smaller than the wavelengths of light.  Investigations at the nanoscale are tied to some of the most active topics in condensed matter, and verge into the interdisciplinary boundaries with chemistry, biology, materials science, electrical engineering, and chemical engineering.   That, and it's fun.

Please browse around through the archives, and I hope you find it interesting.

Why science blogging still matters

Nature has a piece up about science blogging.  It's pretty much on target.  I'm a bit surprised that there wasn't more discussion of blogging vs. twitter vs. other social media platforms, or the interactions between blogs and formal journalism.

Photonics West

A significant piece of my research program is optics-related, and thanks to an invited talk, I'm spending a couple of days at the SPIE Photonics West meeting in San Francisco, a mix of topics from the very applied (that is, details of device engineering and manufacturing) to the fundamental.   It's fun seeing talks on subjects outside of my wheelhouse.

A couple of items of interest from talks so far today:

• Andrew Rickman gave a talk about integrated Si photonics, touching on his ideas on why, while it's grown, it hasn't taken off in the same crazy exponential way as Moore's Law(s) in the microelectronics world.  On the economic side, he made a completely unsurprising argument:  For that kind of enormous growth, one needs high volume manufacturing with very high yield, and a market that is larger than just optical telecommunications.  One challenge of Si-based photonics is that Si is an indirect band gap material, so that for many photonic purposes (including many laser sources and detectors) it needs to be integrated with III-V semiconductors like InP.  Similarly, getting optical signals on and off of chips usually requires integration with macroscopically large optical fibers.   His big pitch, presumably the basis for his recent founding of Rockley Photonics, is that you're better off making larger Si waveguides (say micron-scale, rather than the 220 nm scale, a standard size set by certain mode choices) - this allegedly gives you much more manufacturing dimensional fault tolerance, easier integration with both III-V and fiber, good integration with electroabsorption modulators, etc. One big market he's really interested in is cloud computing, where apparently people are now planning for the transition form 100 Gbs to 400 Gbs (!) for communication within racks and even on boards.  That is some serious throughput.
• Min Gu at Royal Melbourne Institute of Technology spoke about work his group has been doing trying to take advantage of the superresolution approach of STED microscopy, but for patterning.   In STED, a diffraction limited laser spot first illuminates a target area (with the idea of exciting fluorescence), and then a spot from a second laser source, in a mode that looks donut-shaped, also hits that location, depleting the fluorescence everywhere except at the location of the "donut hole".  The result is an optical imaging method with resolution at the tens of nm level.  Gu's group has done work combining the STED approach with photopolymerization to do optical 3d printing of tiny structures.  They've been doing a lot with this, including making gyroid-based photonic crystals that can act as helicity-resolved beamsplitters for circularly polarized light.  It turns out that you can make special gyroid structures so that they have broken symmetries so that these photonic crystals support topologically protected (!) modes analogous to Weyl fermions.
• Venky Narayanamurti gave a talk about how to think about research and its long-standing demarcation into "basic" and "applied".  This drew heavily from his recent book (which is now on my reading list).   The bottom line:  In hindsight, Vannevar Bush didn't necessarily do a good thing by intellectually partitioning science and engineering into "basic" vs. "applied".  Narayanamurti would prefer to think in terms of invention and discovery, defined such that "Invention is the accumulation and creation of knowledge that results in a new tool, device, or process that accomplishes a particular specific purpose; discovery is the creation of new knowledge and facts about the world."  Neither of these are scheduled activities like development.  Research is "an unscheduled quest for new knowledge and the creation of new inventions, whose outcome cannot be predicted in advance, and in which both science and engineering are essential ingredients."  He sounded a very strong call that the US needs to change the way it is thinking about funding of research, and held up China as an example of a country that is investing enormous resources in scientific and engineering research.

In condensed matter, what is a "valley", and why should you care?

One big challenge of talking about condensed matter physics to a general audience is that there are a lot of important physical concepts that don't have easy-to-point-to, visible consequences.  One example of this is the idea of "valleys" in the electronic structure of materials. 

To explain the basic concept, you first have to get across several ideas:

You've heard about wave-particle duality.  A free particle in in quantum mechanics can be described by a wavefunction that really looks like a wave, oscillating in space with some spatial frequency (\(k\ = 2 \pi\)/wavelength).  Momentum is proportional to that spatial frequency (\(p = \hbar k\)), and there is a relationship between kinetic energy and momentum (a "dispersion relation") that looks simple.  In the low-speed limit, K.E. \(= p^2/2m\), and in the relativistic limit, K.E. \( = pc \).

In a large crystal (let's ignore surfaces for the moment), atoms are arranged periodically in space.  This arrangement has lower symmetry than totally empty space, but can still have a lot of symmetries in there.  Depending on the direction one considers, the electron density can have all kinds of interesting spatial periodicities.  Because of the interactions between the electrons and that crystal lattice, the dispersion relation \(E(\mathbf{k})\) becomes direction-dependent (leading to spaghetti diagrams).  Some kinetic energies don't correspond to any allowed electronic states, meaning that there are "bands" in energy of allowed states, separated by gaps.  In a semiconductor, the highest filled (in the limit of zero temperature) band is called the valence band, and the lowest unoccupied band is called the conduction band.

Depending on the symmetry of the material, the lowest energy states in the conduction band might not be near where \(|\mathbf{k}| = 0\).  Instead, the lowest energy electronic states in the conduction band can be at nonzero \(\mathbf{k}\).  These are the conduction band valleys.  In the case of bulk silicon, for example, there are 6 valleys (!), as in the figure.

The six valleys in the Si conduction band, where the axes 

here show the different components of \(\mathbf{k}\), and 

the blue dot is at \(\mathbf{k}=0\).

One way to think about the states at the bottom of these valleys is that there are different wavefunctions that all have the same kinetic energy, the lowest they can and still be in the conduction band, but their actual spatial arrangements (how the electron probability density is arranged in the lattice) differ subtly. 

In the case of graphene, I'd written about this before.  There are two valleys in graphene, and the states at the bottom of those valleys differ subtly about how charge is arranged between the two "sublattices" of carbon atoms that make up the graphene sheet.  What is special about graphene, and why other some materials are getting a lot of attention, is that you can do calculations about the valleys using the same math that gets used when talking about spin, the internal angular momentum of particles.  Instead of being in one graphene valley or the other, you can write about having "pseudospin" up or down. 

Once you start thinking of valley-ness as a kind of internal degree of freedom of the electrons that is often conserved in many processes, like spin, then you can consider all sorts of interesting ideas.  You can talk about "valley ferromagnetism", where available electrons all hang out in one valley.  You can talk about the "valley Hall effect", where carriers of differing valleys tend toward opposite transverse edges of the material.   Because of spin-orbit coupling, these valley effects can link to actual spin physics, and therefore are of interest for possible information processing and optoelectronic ideas.

About grants: What is cost sharing?

In addition to science, I occasionally use this forum as a way to try to explain to students and the public how sponsored research works in academia.  Previously I wrote about the somewhat mysterious indirect costs.  This time I'd like to discuss cost sharing.

Cost sharing is what it sounds like - when researchers at a university propose a research project, and the funding agency or foundation wants to see the university kick in funding as well (beyond obvious things like the lab space where the investigators work).  Many grants, such as NSF single-investigator awards, expressly forbid explicit cost sharing.  That has certain virtues:  To some extent, it levels the playing field, so that particularly wealthy universities don't have an even larger advantage.  Agencies would all like to see their money leveraged as far as possible, and if cost sharing were unrestricted on grants, you could imagine a situation where wealthy institutions would effectively have an incentive to try to buy their way to grant success by offering big matching funds.   

In other programs, such as the NSF's major research instrumentation program, cost sharing is mandated, but the level is set at a fixed percentage of the total budget.  Similarly, some foundations make it known that they expect university matching at a certain percentage level.  While that might be a reach for some smaller, less-well-off universities when the budget is large, at least it's well-defined.    

Sometimes agencies try to finesse things, forbidding explicit cost sharing but still trying to get universities to invest "skin in the game".  For the NSF materials research science and engineering center program, for example, cost sharing is forbidden (in the sense that explicit promises of $N matching or institutional funding is not allowed), but proposals are required to include a discussion of "organizational commitment":  "Provide a description of the resources that the organization will provide to the project, should it be funded. Resources such as space, faculty release time, faculty and staff positions, capital equipment, access to existing facilities, collaborations, and support of outreach programs should be discussed, but not given as dollar equivalents.

"  First and foremost the science and broader impacts drive the merit review, but there's no question that an institution that happens to be investing synergistically with the topic of such a proposal would look good.

The big challenge for universities are grants where cost sharing is not forbidden, and no guidance is given about expectations.  There is a game theory dilemma at work, where institutions try to guess what level of cost sharing is really needed to be competitive.   

So where does the money for cost sharing come from on the university side?  Good question.  The details depend on the university.  Departments, deans, and the central administration typically have some financial resources that they can use to support cost sharing, but how these responsibilities get arranged and distributed varies.  

For the open-ended cost sharing situations, one question that comes up is, how much is too much?  As I'd discussed before, university administrations often argue that research is already a money-losing proposition, in the sense that the amount of indirect costs that they bring in does not actually come close to covering the true expenses of supporting the research enterprise.  That would argue in favor of minimizing cost sharing offers, except that schools really do want to land some of these awards.  (Clearly there are non-financial or indirect benefits to doing research, such as scholarly reputation, or universities would stop supporting that kind of work.)  It would be very interesting if someone would set up a rumor-mill-style site, so that institutions could share with peers roughly what they are offering up for certain programs - it would be revealing to see what it takes to be competitive.  

Selected items

A few recent items that caught my eye:

• The ever-creative McEuen and Cohen groups at Cornell worked together to make graphene-based origami widgets.   Access to the paper seems limited right now, but here is a link that has some of the figures.
• Something else that the Cohen group has worked on in the past are complex fluids, such as colloidal suspensions.  The general statistical physics problem of large ensembles of interacting classical objects (e.g., maybe short-range rigid interactions, as in grains of sand, or perhaps M&Ms) is incredibly rich.  Sure, there are no quantum effects, but often you have to throw out the key simplifying assumption of statistical physics (that your system can readily explore all microscopic states compatible with overall constraints).  This can lead to some really weird effects, like dice packing themselves into an ordered array when stirred properly.  
• When an ensemble of (relatively) hard classical objects really lock up collectively and start acting like a solid, that's called jamming.  It's still a very active subject of study, and is of huge industrial importance.  It also explains why mayonnaise gets much more viscous all of the sudden as egg yolk is added.
• I'd be remiss if I didn't highlight a really nice article in Quanta about one of the grand challenges of (condensed matter) physics:  Classifying all possible thermodynamic phases of matter.   While the popular audience thinks of a handful of phases (solid, liquid, gas, maybe plasma), the physics perspective is broader, because of ideas about order and symmetries.  Now we understand more than ever before that we need to consider phases with different  topological properties as well.  Classification is not just "stamp collecting".

The new year and another arbitrary milestone

Happy new year to all!  I'm sure 2018 will bring some exciting developments in the discipline - at minimum, there will surely be a lot of talk about quantum computing.  I will attempt to post more often, and to work further on ways to bring condensed matter and nanoscale physics to a broader audience, though other responsibilities continue to make that a challenge.  Still, to modify a quote from Winston Churchill, "Writing a [blog] is like having a friend and companion at your side, to whom you can always turn for comfort and amusement, and whose society becomes more attractive as a new and widening field of interest is lighted in the mind."

By the way, this is the 1000th post on Nanoscale Views.  As we all know, this has special significance because 1000 is a big, round number.

The Quantum Labyrinth - a review

Because of real life constraints I'm a bit slow off the mark compared to others, but I've just finished reading The Quantum Labyrinth by Paul Halpern, and wanted to get some thoughts down about it.  The book is a bit of a superposition between a dual biography of Feynman and Wheeler, and a general history of the long-term impact of what started out as their absorber theory.  

The biographical aspects of Feynman have been well trod before by many, including Feynman himself and rather more objectively by James Gleick.   Feynman helped create his own legend (safecracking, being a mathematically prodigious, bongo-playing smart-ass).  The bits called back in the present work that resonate with me now (perhaps because of my age) are how lost he was after his first wife's death, his insecurity about whether he was really getting anything done after QED, his embracing of family life with his third wife, and his love of teaching - both as theater and as a way to feel accomplishment when research may be slow going.  

From other books I'd known a bit about Wheeler, who was still occasionally supervising physics senior theses at Princeton when I was an undergrad.  The backstory about his brother's death in WWII as motivation for Wheeler's continued defense work after the war was new to me.   Halpern does a very good job conveying Wheeler's style - coining pithy epigrams ("Spacetime tells matter how to move; matter tells spacetime how to curve.", "The boundary of a boundary is zero.") and jumping from topic to topic with way outside the box thinking.  We also see him editing his students' theses and papers to avoid antagonizing people.  Interesting.

From the Feynman side, the absorber theory morphed into path integrals, his eponymous diagrams, and his treatment of quantum electrodynamics.   The book does a good job discussing this, though like nearly every popularization, occasionally the analogies, similes, and metaphors end up sacrificing accuracy for the sake of trying to convey physical intuition.    From the Wheeler angle, we get to learn about attempts at quantizing gravity, geons, wormholes, and the many worlds interpretation of quantum mechanics.

It's a fun read that gives you a sense of the personalities and the times for a big chunk of twentieth century theoretical physics, and I'm impressed with Halpern's ability to convey these things without being a professional historian.  

The state of science - hyperbole doesn't help.

It seems like every few weeks these days there is a breathless essay or editorial saying science is broken, or that science as a whole is in the midst of a terrible crisis, or that science is both broken and in the midst of a terrible crisis.  These articles do have a point, and I'm not trying to trivialize anything they say, but come on - get a grip.  Science, and its cousin engineering, have literally reshaped society in the last couple of hundred years.  We live in an age of miracles so ubiquitous we don't notice how miraculous they are.  More people (in absolute numbers and as a percentage of the population) are involved in some flavor of science or engineering than ever before.

That does mean that yes, there will be more problems in absolute numbers than before, too, because the practice of science and engineering is a human endeavor.  Like anything else done by humans, that means there will be a broad spectrum of personalities involved, that not everyone will agree with interpretations or ideas, that some people will make mistakes, and that occasionally some objectionable people will behave unethically.   Decisions will be made and incentives set up that may have unintended consequences (e.g., trying to boost Chinese science by rewarding high impact papers leads to a perverse incentive to cheat.).   This does not imply that the entire practice of science is hopelessly flawed and riddled with rot, any more than a nonzero malpractice rate implies that all of medicine is a disaster.

Why is there such a sense of unease right now about the state of science and the research enterprise?  I'm not a sociologist, but here's my take.

Spreading information, good and bad, can happen more readily than ever before.  People look at sites like pubpeer and come away with the impression that the sky is falling, when in fact we should be happy that there now, for the first time ever, exists a venue for pointing out potential problems.  We are now able to learn about flawed studies and misconduct far more effectively than even twenty years ago, and that changes perceptions.  This seems to be similar to the disconnect between perception of crime rates and actual crime rates.

Science is, in fact, often difficult.  People can be working with complex systems, perhaps more complicated than their models assume.   This means that sometimes there can be good (that is, legitimate) reasons why reproducing someone's results can be difficult.  Correlation doesn't equal causation; biological and social phenomena can be incredibly complex, with many underlying degrees of freedom and often only a few quantifiable parameters.  In the physical sciences we often look askance at those fields and think that we are much better, but laboratory science in physics and chemistry can be genuinely challenging.  (An example from my own career:  We were working with a collaborator whose postdoc was making some very interesting nanoparticles, and we saw exciting results with them, including features that coincided with a known property of the target material.  The postdoc went on to a faculty position and the synthesis got taken over by a senior grad student.  Even following very clear directions, it took over 6 months before the grad student's particles had the target composition and we reproduced the original results, because of some incredibly subtle issue with the synthesis procedure that had changed unintentionally and "shouldn't" have mattered.)

Hyperbolic self-promotion and reporting are bad.   Not everything is a breakthrough of cosmic significance, not every advance is transformative, and that's ok.  Acting otherwise sets scientists and engineers up for a public backlash from years of overpromising and underdelivering.   The public ends up with the perception that scientists and engineers are hucksters.  Just as bad, the public ends up with the idea that "science" is just as valid a way of looking at the world as astrology, despite the fact that science and engineering have actually resulted in technological society.  Even worse, in the US it is becoming very difficult to disentangle science from politics, again despite the fact that one is (at least in principle) a way of looking at the world and trying to determine what the rules are, while the other can be driven entirely by ideology.  This discussion of permissible vocabulary is indicative of a far graver threat to science as a means of learning about the universe than actual structural problems with science itself.  Philosophical definitions aside and practical ones to the fore, facts are real, and have meaning, and science is a way of constraining what those facts are.

We can and should do better.  Better at being rigorous, better at making sure our conclusions are justified and knowing their limits of validity, better at explaining ourselves to each other and the public, better at policing ourselves when people transgress in their scientific ethics or code of conduct.

None of these issues, however, imply that science itself as a whole is hopelessly flawed or broken, and I am concerned that by repeatedly stating that science is broken, we are giving aid and comfort to those who don't understand it and feel threatened by it.

Finding a quantum phase transition, part 2

See here for part 1.   Recall, we had been studying electrical conduction in V₅S₈, a funky material that is metallic, but on one type of vanadium site has local magnetic moments that order in a form of antiferromagnetism (AFM) below around 32 K.  We had found a surprising hysteresis in the electrical resistance as a function of applied magnetic field.  That is, at a given temperature, over some magnetic field range, the resistance takes different values depending on whether the magnitude of H is being swept up or back down. 

One possibility that springs to mind when seeing hysteresis in a magnetic material is domains - the idea that the magnetic order in the material has broken up into regions, and that the hysteresis is due to the domains rearranging themselves.  What speaks against that in this case is the fact that the hysteresis happens over the same field range when the field is in the plane of the layered material as when the field is perpendicular to the layers.   That'd be very weird for domain motion, but makes much more sense if the hysteresis is actually a signature of a first-order metamagnetic transition, a field-driven change from one kind of magnetic order to another.   First order phase transitions are the ones that have hysteresis, like when water can be supercooled below zero Celsius.

That's also consistent with the fact that the field scale for the hysteresis starts at low fields just below the onset of antiferromagnetism, and very rapidly goes to higher fields as the temperature falls and the antiferromagnetic state is increasingly stable.   Just at the ordering transition, when the AFM state is just barely favored over the paramagnetic state, it doesn't necessarily take much of a push to destabilize AFM order.... 

There was one more clue lingering in the literature.  In 2000, a paper reported a mysterious hysteresis in the magnetization as a function of H down at 4.2 K and way out near 17-18 T.  Could this be connected to our hysteresis?  Well, in the figure here at each temperature we plot a dot for the field that is at the middle of our hysteresis, and a horizontal bar to show the width of the hysteresis, including data for multiple samples.  The red data point is from the magnetization data of that 2000 paper.  

A couple of things are interesting here.   Notice that the magnetic field apparently required to kill the AFM state extrapolates to a finite value, around 18 T, as T goes to zero.  That means that this system has a quantum phase transition (as promised in the post title).  Moreover, in our experiments we found that the hysteresis seemed to get suppressed as the crystal thickness was reduced toward the few-layer limit.  That may suggest that the transition trends toward second order in thin crystals, though that would require further study.  That would be interesting, if true, since second order quantum phase transitions are the ones that can show quantum criticality.  It would be fun to do more work on this system, looking out there at high fields and thin samples for signatures of quantum fluctuations....

The bottom line:  There is almost certainly a lot of interesting physics to be done with magnetic materials approaching the 2d limit, and there are likely other phases and transitions lurking out there waiting to be found.

Finding a quantum phase transition, part 1

I am going to try to get the post frequency back up now that some tasks are getting off the to-do list....

Last year, we found what seems to be a previously undiscovered quantum phase transition, and I think it's kind of a fun example of how this kind of science gets done, with a few take-away lessons for students.  The paper itself is here.

My colleague Jun Lou and I had been interested in low-dimensional materials with interesting magnetic properties for a while (back before it was cool, as the hipsters say).  The 2d materials craze continues, and a number of these are expected to have magnetic ordering of various kinds.  For example, even down to atomically thin single layers, Cr₂Ge₂Te₆ is a ferromagnetic insulator (see here), as is CrI₃ (see here).  The 2d material VS₂ had been predicted to be a ferromagnet in the single-layer limit.  

In the pursuit of VS₂, Prof. Lou's student Jiangtan Yuan found that the vanadium-sulphur phase diagram is rather finicky, and we ended up with a variety of crystals of V₅S₈ with thicknesses down to about 10 nm (a few unit cells).  

[Lesson 1:  Just because they're not the samples you want doesn't mean that they're uninteresting.]   

It turns out that V₅S₈  had been investigated in bulk form (that is, mm-cm sized crystals) rather heavily by several Japanese groups starting in the mid-1970s.  They discovered and figured out quite a bit.  Using typical x-ray methods they found the material's structure:  It's better to think of V₅S₈  as V_0.25VS₂.  There are VS₂ layers with an ordered arrangement of vanadium atoms intercalated in the interlayer space.  By measuring electrical conduction, they found that the system as a whole is metallic.   Using neutron scattering, they showed that there are unpaired 3d electrons that are localized to those intercalated vanadium atoms, and that those local magnetic moments order antiferromagnetically below a Neel temperature of 32 K in the bulk.  The moments like to align (antialign) along a direction close to perpendicular to the VS₂ layers, as shown in the top panel of the figure.   (Antiferromagnetism can be tough to detect, as it does not produce the big stray magnetic fields that we all associate with ferromagnetism. )

If a large magnetic field is applied perpendicular to the layers, the spins that are anti-aligned become very energetically unfavored.  It becomes energetically favorable for the spins to find some way to avoid antialignment but still keep the antiferromagnetism.  The result is a spin-flop transition, when the moments keep their antiferromagnetism but flop down toward the plane, as in the lower panel of the figure.  What's particularly nice in this system is that this ends up producing a kink in the electrical resistance vs. magnetic field that is a clear, unambiguous signature of the spin flop, and therefore a way of spotting antiferromagnetism electrically. 

My student Will Hardy figured out how to make reliable electrical contact to the little, thin V₅S₈ crystals (not a trivial task), and we found the physics described above.  However, we also stumbled on a mystery that I'll leave you as a cliff-hanger until the next post:  Just below the Neel temperature, we didn't just find the spin-flop kink.  Instead, we found hysteresis in the magnetoresistance, over an extremely narrow temperature range, as shown here.

[Lesson 2:  New kinds of samples can make "old" materials young again.]

[Lesson 3:  Don't explore too coarsely.  We could easily have missed that entire ~ 2.5 K temperature window when you can see the hysteresis with our magnetic field range.] 

Tune in next time for the rest of the story....

Very busy time....

Sorry for the light blogging - between departmental duties and deadline-motivated writing, it's been very difficult to squeeze in much blogging.  Hopefully things will lighten up again in the next week or two.   In the meantime, I suggest watching old episodes of the excellent show Scrapheap Challenge (episode 1 here).  Please feel free to put in suggestions of future blogging topics in the comments below.  I'm thinking hard about doing a series on phases and phase transitions.

Max the Demon and the Entropy of Doom

My readers know I've complained/bemoaned repeatedly how challenging it can be to explain condensed matter physics on a popular level in an engaging way, even though that's the branch of physics that arguably has the greatest impact on our everyday lives.  Trying to take such concepts and reach an audience of children is an even greater, more ambitious task, and teenagers might be the toughest crowd of all.  A graphic novel or comic format is one visually appealing approach that is a lot less dry and perhaps more nonthreatening than straight prose.   Look at the success of xkcd and Randall Munroe!   The APS has had some reasonable success with their comics about their superhero Spectra.  Prior to that, Larry Gonick had done a very nice job on the survey side with the Cartoon Guide to Physics.  (On the parody side, I highly recommend Science Made Stupid (pdf) by Tom Weller, a key text from my teen years.  I especially liked Weller's description of the scientific method, and his fictional periodic table.)

Max the Demon and the Entropy of Doom is a new entry in the field, by Assa Auerbach and Richard Codor.  Prof. Auerbach is a well-known condensed matter theorist who usually writes more weighty tomes, and Mr. Codor is a professional cartoonist and illustrator.  The book is an entertaining explanation of the laws of thermodynamics, with a particular emphasis on the Second Law, using a humanoid alien, Max (the Demon), as an effective superhero.  

The comic does a good job, with nicely illustrated examples, of getting the point across about entropy as counting how many (microscopic) ways there are to do things.  One of Max's powers is the ability to see and track microstates (like the detailed arrangement and trajectory of every air molecule in this room), when mere mortals can only see macrostates (like the average density and temperature).    It also illustrates what we mean by temperature and heat with nice examples (and a not very subtle at all environmental message).   There's history (through the plot device of time travel), action, adventure, and a Bad Guy who is appropriately not nice (and has a connection to history that I was irrationally pleased about guessing before it was revealed).   My kids thought it was good, though my sense is that some aspects were too conceptually detailed for 12 years old and others were a bit too cute for world-weary 15.  Still, a definite good review from a tough crowd, and efforts like this should be applauded - overall I was very impressed.

Taxes and grad student tuition

As has happened periodically over the last couple of decades (I remember a scare about this when Newt Gingrich's folks ran Congress in the mid-1990s), a tax bill has been put forward in the US House that would treat graduate student tuition waivers like taxable income (roughly speaking).   This is discussed a little bit here, and here.

Here's an example of why this is an ill-informed idea.  Suppose a first-year STEM grad student comes to a US university, and they are supported by, say, departmental fellowship funds or a TA position during that first year.  Their stipend is something like $30K.  These days the university waives their graduate tuition - that is, they do not expect the student to pony up tuition funds.  At Rice, that tuition is around $45K.  Under the proposed legislation, the student would end up getting taxed as if their income was $75K, when their actual gross pay is $30K.   

That would be extremely bad for both graduate students and research universities.  Right off the bat this would create unintended (I presume) economic incentives, for grad students to drop out of their programs, and/or for universities to play funny games with what they say is graduate tuition.   

This has been pitched multiple times before, and my hypothesis is that it's put forward by congressional staffers who do not understand graduate school (and/or think that this is the same kind of tuition waiver as when a faculty member's child gets a vastly reduced tuition for attending the parent's employing university).  Because it is glaringly dumb, it has been fixed whenever it's come up before.  In the present environment, the prudent thing to do would be to exercise caution and let legislators know that this is a problem that needs to be fixed.

Links + coming soon

Real life is a bit busy right now, but I wanted to point out a couple of links and talk about what's coming up.

• I've been looking for ways to think about and discuss topological materials that might be more broadly accessible to non-experts, and I found this paper and videos like this one and this one.  Very cool, and I'm sorry I'd missed it back in '15 when it came out.
• In the experimental literature talking about realizations of Majorana fermions in the solid state, a key signature is a peak in the conductance at zero voltage - that's an indicator that there is a "zero-energy mode" in the system.  There are other ways to get zero-bias peaks, though, and nailing down whether this has the expected properties (magnitude, response to magnetic fields) has been a lingering issue.  This seems to nail down the situation more firmly.
• Discussions about "quantum supremacy" strictly in terms of how many qubits can be simulated on a classical computer right now seem a bit silly to me.  Ok, so IBM managed to simulate a handful of additional qubits (56 rather than 49).  It wouldn't shock me if they could get up to 58 - supercomputers are powerful and programmers can be very clever.  Are we going to get a flurry of news stories every time about how this somehow moves the goalposts for quantum computers?    
• I'm hoping to put out a review of Max the Demon and the Entropy of Doom, since I received my beautifully printed copies this past weekend.

Thoughts after a NSF panel

I just returned from a NSF proposal review panel.  I had written about NSF panels back in the early days of this blog here, back when I may have been snarkier.

• Some things have gotten better.  We can work from our own laptops, and I think we're finally to the point where everyone at these things is computer literate and can use the online review system.  The program officers do a good job making sure that the reviews get in on time (ahead of the meeting).
• Some things remain the same.  I'm still mystified at how few people from top-ranked programs (e.g., Harvard, Stanford, MIT, Cornell, Cal Tech, Berkeley) I see at these.  Maybe I just don't move in the right circles.  
• Best quote of the panel:  "When a review of one of my papers or proposals starts with 'Author says' rather than 'The author says', I know that the referee is Russian and I'm in trouble."
• Why does the new NSF headquarters have tighter security screenings that Reagan National Airport?  
• The growth of funding costs and eight years of numerically flat budgets has made this process more painful.  Sure looks like morale is not great at the agency.  Really not clear where this is all going to go over the next few years.  There was a lot of gallows humor about having "tax payer advocates" on panels.  (Everyone on the panel is a US taxpayer already, though apparently that doesn't count for anything because we are scientists.)
• NSF is still the most community-driven of the research agencies. 
• I cannot overstate the importance of younger scientists going to one of these and seeing how the system works, so you learn how proposals are evaluated.

Whither science blogging?

I read yesterday of the impending demise of scienceblogs, a site that has been around since late 2005 in one form or other.  I guess I shouldn't be surprised, since some of its bloggers have shifted to other sites in recent years, such as Ethan Siegel and Chad Orzel, who largely migrated to Forbes, and Rhett Allain, who went to Wired.  Steinn Sigurðsson is going back to his own hosted blog in the wake of this.

I hope this is just indicative of a poor business model at Seed Media, and not a further overall decline in blogging by scientists.  It's wonderful that online magazines like Quanta and Aeon and Nautilus are providing high quality, long-form science writing.  Still, I think everyone benefits when scientists themselves (in addition to professional science journalists) carve out some time to write about their fields.

Neutron stars and condensed matter physics

In the wake of the remarkable results reported earlier this week regarding colliding neutron stars, I wanted to write just a little bit about how a condensed matter physics concept is relevant to these seemingly exotic systems.

When you learn high school chemistry, you learn about atomic orbitals, and you learn that electrons "fill up" those orbitals starting with the lowest energy (most deeply bound) states, two electrons of opposite spin per orbital.  (This is a shorthand way of talking about a more detailed picture, involving words like "linear combination of Slater determinants", but that's a detail in this discussion.)  The Pauli principle, the idea that (because electrons are fermions) all the electrons can't just fall down into the lowest energy level, leads to this.  In solid state systems we can apply the same ideas.  In a metal like gold or copper, the density of electrons is high enough that the highest kinetic energy electrons are moving around at ~ 0.5% of the speed of light (!).  

If you heat up the electrons in a metal, they get more spread out in energy, with some occupying higher energy levels and some lower energy levels being empty.   To decide whether the metal is really "hot" or "cold", you need a point of comparison, and the energy scale gives you that.  If most of the low energy levels are still filled, the metal is cold.  If the ratio of the thermal energy scale, \(k_{\mathrm{B}}T\) to the depth of the lowest energy levels (essentially the Fermi energy, \(E_{\mathrm{F}}\) is much less than one, then the electrons are said to be "degenerate".  In common metals, \(E_{\mathrm{F}}\) is several eV, corresponding to a temperature of tens of thousands of Kelvin.  That means that even near the melting point of copper, the electrons are effectively very cold.

Believe it or not, a neutron star is a similar system.  If you squeeze a bit more than one solar mass into a sphere 10 km across, the gravitational attraction is so strong that the electrons and protons in the matter are crushed together to form a degenerate ball of neutrons.  Amazingly, by our reasoning above, the neutrons are actually very very cold.  The Fermi energy for those neutrons corresponds to a temperature of nearly \(10^{12}\) K.  So, right up until they smashed into each other, those two neutron stars spotted by the LIGO observations were actually incredibly cold, condensed objects.   It's also worth noting that the properties of neutron stars are likely affected by another condensed matter phenomenon, superfluidity.   Just as electrons can pair up and condense into a superconducting state under some circumstances, it is thought that cold, degenerate neutrons can do the same thing, even when "cold" here might mean \(5 \times 10^{8}\) K.

Gravitational waves again - should be exciting

There is going to be a big press conference tomorrow, apparently to announce that LIGO/VIRGO has seen an event (binary neutron star collision) directly associated with a gamma ray burst in NGC 4993.  Fun stuff, and apparently the worst-kept secret in science right now.  This may seem off-topic for a condensed matter blog, but there's physics in there which isn't broadly appreciated, and I'll write a bit about it after the announcement.

Piezo controller question - followup.

A couple of weeks ago I posted:

Anyone out there using a Newport NPC3SG controller to drive a piezo positioning stage, with computer communication successfully talking to the NPC3SG?  If so, please leave a comment so that we can get in touch, as I have questions.

No responses so far.  This is actually the same unit as this thing:
https://www.piezosystem.com/products/piezo_controller/piezo_controller_3_channel_version/nv_403_cle/

In our unit from Newport, communications simply don't work properly.  Timeout problems.  The labview code supplied by Newport (the same code paired with the link above) has these problems, as do many other ways of trying to talk with the instrument.  Has anyone out there had success in using a computer to control and read this thing?   At issue is whether this is a hardware problem with our unit, or whether there is a general problem with these.  The vendor has been verrrrrrrrry slow to figure this out.

The Abnormal Force

How does the chair actually hold you up when you sit down?  What is keeping your car tires from sinking through the road surface?  What is keeping my coffee mug from falling through my desk?  In high school and first-year undergrad physics, we teach people about the normal force - that is a force that acts normal (perpendicular) to a surface, and it takes on whatever value is needed so that solid objects don't pass through each other.

The microscopic explanation of the normal force is that the electrons in the atoms of my coffee mug (etc.) interact with the electrons in the atoms of the desk surface, through a combination of electrostatics (electrons repel each other) and quantum statistics (the Pauli principle means that you can't just shuffle electrons around willy-nilly).  The normal force is "phenomenological" shorthand.  We take the observation that solid objects don't pass through each other, deduce that whatever is happening microscopically, the effect is that there is some force normal to surfaces that touch each other, and go from there, rather than trying to teach high school students how to calculate it from first principles.  The normal force is an emergent effect that makes sense on macroscopic scales without knowing the details.  This is just like how we teach high school students about pressure as a useful macroscopic concept, without actually doing a statistical calculation of the average perpendicular force per area on a surface due to collisions with molecules of a gas or a liquid.  

You can actually estimate the maximum reasonable normal force per unit area.  If you tried to squeeze the electrons of two adjacent atoms into the volume occupied by one atom, even without the repulsion of like charges adding to the cost, the Pauli principle means you'd have to kick some of those electrons into higher energy levels.  If a typical energy scale for doing that for each electron was something like 1 eV, and you had a few electrons per atom, and the areal density of atoms is around 10¹⁴ per cm², then we can find the average force \(F_{\mathrm{av}}\) required to make a 1 cm² area of two surfaces overlap with each other.   We'd have \(F_{\mathrm{av}} d \sim 10^{15}\)eV, where \(d\) is the thickness of an atom, around 0.3 nm.   That's around 534000 Newtons/cm², or around 5.3 GPa.   That's above almost all of the yield stresses for materials (usually worrying about tension rather than compression) - that just means that the atoms themselves will move around before you really push electrons around.

Very occasionally, when two surfaces are brought together, there is a force that arises at the interface that is not along the normal direction.  A great example of that is in this video, which shows two graphite surfaces that spontaneously slide in the plane so that they are crystallographically aligned.  That work comes from this paper.

As far as I can tell, there is no official terminology for such a spontaneous in-plane force.  In the spirit of one of my professional heroes David Mermin, who coined the scientific term boojum, I would like to suggest that such a transverse force be known as the abnormal force.  (Since I don't actually work in this area and I'm not trying to name the effect after myself, hopefully the barrier to adoption will be lower than the one faced by Mermin, who actually worked on boojums :-)  ).

Gravitational radiation for the win + communicating science

As expected, LIGO was recognized by the Nobel Prize in physics this year.  The LIGO experiment is an enormous undertaking that combines elegant, simple theoretical ideas; incredible engineering and experimental capabilities; and technically virtuosic numerical theoretical calculations and data analysis techniques.  It's truly a triumph.

I did think it was interesting when Natalie Wolchover, one of the top science writers out there today, tweeted:   Thrilled they won, thrilled not to spend this morning speed-reading about some bizarre condensed matter phenomenon.

This sentiment was seconded by Peter Woit, who said he thought she spoke for all science journalists.

Friendly kidding aside, I do want to help.  Somehow it's viewed as comparatively easy and simple to write about this, or this, or this, but condensed matter is considered "bizarre".  

Gravitational radiation redux + Nobel speculation

This past week, there was exciting news that the two LIGO detectors and the VIRGO interferometer had simultaneously detected the same event, a merger of black holes estimated to have taken place 1.6 billion lightyears away.  From modeling the data, the black hole masses are estimated at around 25 and 30 solar masses, and around 2.7 solar masses worth of energy (!) was converted in the merger into gravitational radiation.  The preprint of the paper is here.  Check out figure 1.  With just the VIRGO data, the event looks really marginal - by eye you would be hard pressed to pick it out of the fluctuating detector output.  However, when that data is thrown into the mix with that from the (completely independent from VIRGO) detectors, the case is quite strong.

This is noteworthy for (at least) two reasons.  First, there has been some discussion about the solidity of the previously reported LIGO results - this paper (see here for a critique of relevant science journalism) argues that there are some surprising correlations in the noise background of the two detectors that could make you wonder about the analysis.  After all, the whole point of having two detectors is that a real event should be seen by both, while one might reasonably expect background jitter to be independent since the detectors are thousands of miles apart.  Having a completely independent additional detector in the mix should be useful in quantifying any issues.  Second, having the additional detector helps nail down the spot in the sky where the gravitational waves appear to originate.  This image shows how previous detections could only be localized by two detectors to a band spanning lots of the sky, while this event can be localized down to a spot spanning a tenth as much solid angle.    This is key to turning gravitational wave detectors into serious astronomy tools, by trying to link gravitational event detection to observations across the electromagnetic spectrum.  There were rumors, for example, that LIGO had detected what was probably a neutron star collision (smaller masses, but far closer to earth), the kind of event thought to produce dramatic electromagnetic signatures like gamma ray bursts.

On that note, I realized Friday that this coming Tuesday is the announcement of the 2017 Nobel in physics.  Snuck up on me this time.  Speculate away in the comments.  Since topology in condensed matter was last year's award, it seems likely that this year will not be condensed matter-related (hurting the chances of people like Steglich and Hosono for heavy fermion and iron superconductors, respectively).  Negative index phenomena might be too condensed matter related.   The passing last year of Vera Rubin and Debra Jin is keenly felt, and makes it seem less likely that galactic rotation curves (as evidence for dark matter) or ultracold fermions would get it this year.  Bell's inequality tests (Aspect, Zeilinger, Clauser) could be there.   The LIGO/VIRGO combined detection happened too late in the year to affect the chances of this being the year for gravitational radiation (which seems a shoe-in soon).

The terahertz gap

https://commons.wikimedia.org/wiki/File:Thz_freq_in_EM_spectrum.png?uselang=en-gb

At a thesis proposal talk yesterday, I realized that I hadn't ever written anything specifically about terahertz radiation (THz, or if you're trying to market something, t-rays).   Terahertz (10¹² Hz) is the frequency of electromagnetic radiation higher than microwaves, but lower than what is traditionally labeled the far infrared.  Sometimes called "mm wave" radiation (1 THz would be a free-space wavelength of about 0.3 mm or 300 microns), THz is potentially very useful for communications (pdf, from here), imaging (here, here, here), and range detection (see here for an impressive google project; or here for an article about THz for self-driving cars), among other things.  It's also right around the frequency range of a lot of vibrations in molecules and solids, so it can be used for spectroscopy, though it's also around the energy range where water vapor in the atmosphere can be an efficient absorber.

This frequency region is an awkward middle ground, however.  That's sometimes why it's referred to as the "terahertz gap".

We tend to produce electromagnetic radiation by one of two approaches.  Classically, accelerating charges radiate electromagnetic waves.  In the low frequency limit, there are various ways to generate voltages that oscillate - we can in turn use those to drive oscillating currents and thus generate radio waves, for example.  See here for a very old school discussion.  It is not trivial to shake charges back and forth at THz frequencies, however.  It can be done, but it's very challenging.  One approach to generating a pulse of THz radiation is to use a photoconductive antenna.  Take two electrodes close together on a semiconductor substrate, with a voltage applied between them.  Smack the semiconductor with an ultrafast optical pulse that has a frequency high enough to photoexcite a bunch of charge carriers - those then accelerate from the electric field between the electrodes and emit a pulse of radiation, including THz frequencies.

The other limit we often take in generating light is to work with some quantum system that has a difference in energy levels that is the same energy as the photons we want to generate.  This is the limit of atomic emission (say, having an electron drop from the 2p orbital to the 1s orbital of a hydrogen atom, and emitting an ultraviolet photon of energy around 10 eV) and also the way many solid state devices work (say, having an electron drop from the bottom of the conduction band to the top of the valence band in InGaAsP to produce a red photon of energy around 1.6 eV in a red LED).  The problem with this approach for THz is that the energy scale in question is very small - 1 THz is about 4 milli-electron volts (!).  As far as I know, there aren't naturally occurring solids with energy level splittings that small, so the approach from this direction has been to create artificial systems with such electronic energy gaps - see here.   (Ironically, there are some molecular systems with transitions considerably lower in energy than the THz that can be used to generate microwaves, as in this famous example.)

It looks like THz is starting to take off for technologies, particularly as more devices are being developed for its generation and detection.  SiGe-based transistors, for example, can operate at very high intrinsic speeds, and like in the thesis proposal I heard yesterday, these devices are readily made now and can be integrated into custom chips for exactly the generation and detection of radiation approaching a terahertz.  Exciting times.

Lab question - Newport NPC3SG

Anyone out there using a Newport NPC3SG controller to drive a piezo positioning stage, with computer communication successfully talking to the NPC3SG?  If so, please leave a comment so that we can get in touch, as I have questions.