Silicon nanoelectronics is a truly extraordinary achievement.

Arguably the greatest technical and manufacturing achievement in all of history is around us all the time, supporting directly or indirectly a huge fraction of modern life, and the overwhelming majority people don't give it a second's thought.  

I'm talking about silicon nanoelectronics (since about 2003, "microelectronics" is no longer an accurate description).  As I was updating notes for a class I'm teaching, the numbers really hit me.  A high end microprocessor these days (say the AMD "Epyc" Rome) contains 40 billion transistors in a chip about 3 cm on a side.  These essentially all work properly, for many years at a time.  (Chips rarely die - power supplies and bad electrolytic capacitors are much more common causes of failure of motherboards.)  No other manufacturing process of components for any product comes close to the throughput and reliability of transistors.  

The transistors on those chips are the culmination of many person-years of research.  They're FinFETs, made using what is labeled the 7 nm process.  Remember, transistors are switches, with the current flow between the source and drain electrodes passing through a channel the conductance of which is modulated by the voltage applied to a gate electrode.  The active channel length of those transistors, the distance between the source and drain, is around 16 nm, or about 50 atoms (!).  The positioning accuracy required for the lithography steps (when ultraviolet light and photochemistry are used to pattern the features) is down to about 3 nm.  These distances are controlled accurately across a single-crystal piece of silicon the size of a dinner plate.  That silicon is pure at the level of one atom out of every 10 trillion (!!).  

This is not an accident.  It's not good fortune.  Science (figuring out the rules of the universe) and engineering (applying those rules to accomplish a task or address a need) have given us this (see here and here).  It's the result of an incredible combination of hard-earned scientific understanding, materials and chemistry acumen, engineering optimization, and the boot-strapping nature of modern technology (that is, we can do this kind of manufacturing because we have advanced computational tools for design, control, and analysis, and we have those tools because of our ability to do this kind of manufacturing.)   

This technology would look like literal magic to someone from any other era of history - that's something worth appreciating.

Emergent monopoles

One of the truly remarkable things about condensed matter physics is the idea that, from a large number of interacting particles that obey comparatively simple rules, there can emerge new objects  (in the sense of having well-defined sets of parameters like mass, charge, etc.) with properties that are not at all obviously related to those of the original constituents.   (One analogy I like:  Think about fans in a sports stadium doing The Wave.  The propagating wave only exists because of the cooperative response of thousands of people, and its spatial extent and propagation speed are not obviously related to the size of individual fans.)

A fairly spectacular example of this occurs in materials called spin ices, insulating materials that have unusual magnetic properties. A prime example is Dy₂Ti₂O₇.  The figure here shows a little snipped of the structure.  The dysprosium atoms (which end up having angular momentum \(J = 15/2\), very large as compared to a spin-1/2 electron) sit at the corners of corner-sharing tetrahedra.  It's a bit hard to visualize, but the centers of those tetrahedra form the same spatial pattern as the locations of carbon atoms in a diamond crystal.  Anyway, because of some rather deep physics ("crystal field effects"), the magnetic moments of each Dy are biased to point either radially inward toward or radially outward from the center of the tetrahedron.  Moreover, because of interactions between the magnetic moments, it is energetically favored so that for each tetrahedron, two moments (shown as a little arrows) point inward and two moments point outward.  This is the origin of the "ice" part of the name, since this two-in/two-out rule is the same thing seen in ordinary water ice, where each oxygen atom is coordinated by four hydrogen atoms, two strongly (closer, covalently bound) and two more weakly (farther away, hydrogen bonding).  The spin ice ordering in this material really kicks in at low temperatures, below 1 K.  

So, what happens at rather warmer temperatures, say between 2 K and 15 K?  The lowest energy excitations of this system act like magnetic monopoles (!).  Now, except for the fact that electrical charge is quantized, there is no direct evidence for magnetic monopoles (isolated north and south poles that would interact with a Coulomb-like force law) in free space.  In spin ice, though, you can create an effective monopole/antimonopole pair by flipping some moments so that one tetrahedron is 3-out/1-in, and another is 1-out/3-in, as shown at right.  You can "connect" the monopole to the antimonopole by following a line of directed magnetic moments - this is a topological constraint, in the sense that you can see how having multiple m/anti-m pairs could interfere with each other.  This connection is the analog of a Dirac string (where you can think of the m/anti-m pair as opposite ends of an infinitesimally skinny solenoid).  

This is all fun to talk about, but is there really evidence for these emergent monopoles?  Yes.  A nice very recent review of the subject is here.  There are a variety of experiments (starting with magnetization and neutron scattering and ending up with more sophisticated measurements like THz optical properties and magnetic flux noise experiments looking at m/anti-m generation and recombination) that show evidence for monopoles and their interactions.  (full disclosure:  I have some thoughts on fun experiments to do in these and related systems.)  It's also possible to make two-dimensional arrays of nanoscale ferromagnets that can mimic these kinds of properties, so-called artificial spin ice.  This kind of emergence, when you can end up with excitations that act like exotic, interacting, topologically constrained (quasi)particles that seemingly don't exist elsewhere, is something that gets missed if one takes a reductionist view of physics.

Room temperature superconductivity!

As many readers know, the quest for a practical room temperature superconductor has been going on basically ever since Kamerlingh Onnes discovered superconductivity over 100 years ago.  If one could have superconductivity with high critical currents and high critical fields in a material that could readily be made into wires, for example, it would be absolutely transformative to the world.  (Just one example:  we lose 5-10% of generated electricity just in transmission lines due to resistive heating.)  

One exotic possibility suggested over 50 years ago by Neil Ashcroft (of textbook fame in addition to his scientific prestige) was that highly compressed metallic hydrogen could be a room temperature superconductor.  The basic ingredients for traditional superconductivity would be a high electronic density of states, light atoms (and hence a high soundspeed for phonon-based pairing), and a strong electron-phonon coupling.  

In recent years, there have been striking advances in hydrogen-rich compounds with steadily increasing superconducting transition temperatures, including H2S (here and here) and LaH10 (here and here), all requiring very high (200+ GPa) pressures obtained in diamond anvil cells.  In those cool gadgets, tiny sample volumes are squeezed between the facets of cut gemstone-quality diamonds, and there is a great art in making electronic, optical, and magnetic measurements of samples under extreme pressures. 

Today, a new milestone has been reached and published.  Using these tools, the investigators (largely at Rochester) put some carbon, sulphur, and hydrogen containing compounds in the cell, zapped them with a laser to do some in situ chemistry, and measured superconductivity with a transition temperature up to 287.7 K (!) at a pressure of 267 GPa (!!).  The evidence for superconductivity is both a resistive transition to (as near as can be seen) zero resistance, and an onset of diamagnetism (as seen through ac susceptibility).  

This is exciting, and a milestone, though of course there are many questions:  What is the actual chemical compound at work here?  How does superconductivity work - is it conventional or more exotic? Is there any pathway to keeping these properties without enormous externally applied pressure?  At the very least, this shows experimentally what people have been saying for a long time, that there is no reason in principle why there couldn't be room temperature (or above) superconductivity.

How fast can sound be in a solid or liquid?

There is a new paper here that argues through dimensional analysis for an upper limit to the speed of sound in solids and liquids (when the atoms bump up against each other).  The authors derive that the maximum speed of sound is, to within numerical factors of order 1, given by \(v_{\mathrm{max}}/c = \alpha \sqrt{m_{e}/(2m_{p})} \), where \(\alpha\) is the fine structure constant, and \(m_{e}\) and \(m_{p}\) are the masses of the electron and proton, respectively.  Numerically, that ends up being about 36 km/s.  

It's a neat argument, and I agree with the final result, but I actually think there's a more nuanced way to think about this than the approach of the authors.  Sound speed can be derived from some assumptions about continuum elasticity, and is given by \(v_{s} = \sqrt{K/\rho}\), where \(K\) is the bulk modulus and \(\rho\) is the mass density.  Bulk modulus is given by (negative) the inverse fractional change in volume of a substance when the pressure on the substance is increased.  So, a squishy soft substance has a low bulk modulus, because when the pressure goes up, its volume goes down comparatively a lot.

The authors make the statement "It has been ascertained that elastic constants are governed by the density of electromagnetic energy in condensed matter phases."  This is true, but for the bulk modulus I would argue that this is true indirectly, as a consequence of the Pauli principle.  I wrote about something similar previously, explaining why you can't push solids through each other even though the atoms are mostly empty space.  If you try to stuff two atoms into the volume of one atom, it's not the Coulomb repulsion of the electrons that directly stops this from happening.  Rather, the Pauli principle says that cramming those additional electrons into that tiny volume would require the electrons to occupy higher atomic energy levels.  They typical scale of those atomic energy levels is something like a Rydberg, so that establishes one cost of trying to compress solids or liquids; that Rydberg scale of energy is how the authors get to the fine structure constant and the masses of the electron and proton in their result.  

I would go further and say that this is really the ultimate limiting factor on sound speed in dense material.  Yes, interatomic chemical bonds are important - as I'd written, they establish why solids deform instead of actually merging when squeezed.  It's energetically cheaper to break or rearrange chemical bonds (on the order of a couple of eV in energy) than to push electrons into higher energy states (several eV or more - real Rydberg scales).  

Still, it's a cool idea - that one can do intelligently motivated dimensional analysis and come up with an insight into the maximum possible value of some emergent quantity like sound speed.  (Reminds me of the idea of a conjectured universal bound on diffusion constants for electrons in metals.)

Postdoc opportunities

There is a postdoc opportunity coming up in my lab to look at light emission from molecular-scale plasmonic nanostructures.  It's going to be very cool, looking at (among other things) photon counting statistics (this kind of thing), coupling plasmon-based emission to single fluorophores, all kinds of fun.  Please check it out and share with those who might be interested:  https://jobs.rice.edu/postings/24792 

In addition:  The Smalley-Curl Institute is happy to announce that they are accepting applications to 2 (two) J Evans Attwell-Welch Postdoctoral Research Associate positions.   Highly competitive, the Attwell-Welch fellowship was established in 1998 to provide Ph.D. recipients in nanosciences and nanotechnology-related fields, an opportunity to further their basic scientific research experience.

The deadline for the Evans Attwell-Welch submissions is Monday Dec 7th, 2020.  Applications containing the candidate’s resume, a two-page research project, and a letter of support from an SCI member must be emailed to sci@rice.edu before the deadline.  Only applicants sponsored by an SCI Rice faculty member will be considered.   

I would be happy to work with a potential applicant, particularly one interested in strongly correlated nanostructures and spin transport in magnetic insulators.  If you're a student finishing up and are interested, please contact me, and if you're a faculty member working with possible candidates, please feel free to point out this opportunity.   Rice University is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability or protected veteran status.

Annual Nobel speculation, + nanoscale views on twitter

It's that annual tradition:  Who do people think will win the Nobel this year in physics?  Or chemistry?  On the physics side, I've repeatedly predicted (incorrectly) Aharonov and Berry for geometric phases.  Another popular suggestion from years past is Aspect, Zeilinger, and Clauser for Bell's inequality tests.   Speculate away in the comments.

I've also finally taken the plunge and created @NanoscaleViews on twitter.  Hopefully this will help reach a broader audience, even if I don't have the time to fall down the twitter rabbit hole constantly.

The Barnett Effect and cool measurement technique

 I've written before about the Einstein-deHaas effect - supposedly Einstein's only experimental result (see here, too) - a fantastic proof that spin really is angular momentum.  In that experiment, a magnetic field is flipped, causing the magnetization of a ferromagnet to reorient itself to align with the new field direction.  While Einstein and deHaas thought about amperean current loops (the idea that magnetization came from microscopic circulating currents that we would now call orbital magnetism), we now know that magnetization in many materials comes from the spin of the electrons.  When those spins reorient, angular momentum has to be conserved somehow, so it is transferred to/from the lattice, resulting in a mechanical torque that can be measured.

Less well-known is the complement, the Barnett effect.  Take a ferromagnetic material and rotate it. The mechanical rotational angular momentum gets transferred (via rather complicated physics, it turns out) at some rate to the spins of the electrons, causing the material to develop a magnetization along the rotational axis.  This seems amazing to me now, knowing about spin.  It must've really seemed nearly miraculous back in 1915 when it was measured by Barnett.

So, how did Barnett actually measure this, with the technology available in 1915?  Here's the basic diagram of the scheme from the original paper:

There are two rods that can each be rotated about its long axis.  The rods pass through counterwound coils, so that if there is a differential change in magnetic flux through the two coils, that generates a current that flows through the fluxmeter.  The Grassot fluxmeter is a fancy galvanometer - basically a coil suspended on a torsion fiber between poles of a magnet.  Current through that coil leads to a torque on the fiber, which is detected in this case by deflection of a beam of light bounced off a mirror mounted on the fiber.  The paper describes the setup in great detail, and getting this to work clearly involved meticulous experimental technique and care.  It's impressive how people were able to do this kind of work without all the modern electronics that we take for granted.  Respect.

Rice ECE assistant professor position in Quantum Engineering

The Department of Electrical and Computer Engineering at Rice University invites applications for a tenure track Assistant Professor Position in the area of experimental quantum engineering, broadly defined. Under exceptional circumstances, more experienced senior candidates may be considered. Specific areas of interest include, but are not limited to: quantum computation, quantum sensing, quantum simulation, and quantum networks.

The department has a vibrant research program in novel, leading-edge research areas, has a strong culture of interdisciplinary and multidisciplinary research with great national and international visibility, and is ranked #1 nationally in faculty productivity.* With multiple faculty involved in quantum materials, quantum devices, optics and photonics, and condensed matter physics, Rice ECE considers these areas as focal points of quantum engineering research in the coming decade. The successful applicant will be required to teach undergraduate courses and build a successful research program.

The successful candidate will have a strong commitment to teaching, advising, and mentoring undergraduate and graduate students from diverse backgrounds. Consistent with the National Research Council’s report, Convergence: Facilitating Transdisciplinary Integration of Life Sciences, Physical Sciences, Engineering, and Beyond, we are seeking candidates who have demonstrated ability to lead and work in research groups that “… [integrate] the knowledge, tools, and ways of thinking…” from engineering, mathematics, and computational, natural, social and behavioral sciences to solve societal problems using a convergent approach.

Applicants should submit a cover letter, curriculum vitae, statements of research and teaching interests, and at least three references through the Rice faculty application website: http://jobs.rice.edu/postings/24582. The deadline for applications is January 15, 2021; review of applications will commence November 15, 2020. The position is expected to be available July 1, 2021. Additional information can be found on our website: http://www.ece.rice.edu.

Rice University is a private university with a strong reputation for academic excellence in both undergraduate and graduate education and research. Located in the economically dynamic, internationally diverse city of Houston, Texas, 4th largest city in the U.S., Rice attracts outstanding undergraduate and graduate students from across the nation and around the world. Rice provides a stimulating environment for research, teaching, and joint projects with industry.

The George R. Brown School of Engineering ranks among the top 20 of undergraduate engineering programs (US News & World Report) and is strongly committed to nurturing the aspirations of faculty, staff, and students in an inclusive environment. Rice University is an Equal Opportunity Employer with commitment to diversity at all levels and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We seek greater representation of women, minorities, people with disabilities, and veterans in disciplines in which they have historically been underrepresented; to attract international students from a wider range of countries and backgrounds; to accelerate progress in building a faculty and staff who are diverse in background and thought; and we support an inclusive environment that fosters interaction and understanding within our diverse community.

*http://news.rice.edu/2007/11/30/rices-electrical-engineering-and-computer-science-programs-rank-no-1/ Rice University is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability or protected veteran status.

Tenure-track faculty position in Astronomy at Rice University

The Department of Physics and Astronomy at Rice University invites applications for a tenure-track faculty position in astronomy in the general field of galactic star formation, including the formation and evolution of planetary systems. We seek an outstanding theoretical, observational, or computational astronomer whose research will complement and extend existing activities in these areas within the Department. In addition to developing an independent and vigorous research program, the successful applicant will be expected to teach, on average, one undergraduate or graduate course each semester, and contribute to the service missions of the Department and University. The Department anticipates making the appointment at the assistant professor level. A Ph.D. in astronomy/astrophysics or related field is required. 

Applications for this position must be submitted electronically at http://jobs.rice.edu/postings/24588. Applicants will be required to submit the following: (1) cover letter; (2) curriculum vitae; (3) statement of research; (4) statement on teaching, mentoring, and outreach; (5) PDF copies of up to three publications; and (6) the names, affiliations, and email addresses of three professional references. Rice University is committed to a culturally diverse intellectual community. In this spirit, we particularly welcome applications from all genders and members of historically underrepresented groups who exemplify diverse cultural experiences and who are especially qualified to mentor and advise all members of our diverse student population. We will begin reviewing applications December 1, 2020. To receive full consideration, all application materials must be received by January 1, 2021. The appointment is expected to begin in July, 2021. 

Rice University is an Equal Opportunity Employer with a commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We encourage applicants from diverse backgrounds to apply.

The power of a timely collaboration

Sometimes it takes a while to answer a scientific question, and sometimes that answer ends up being a bit unexpected.  Three years ago, I wrote about a paper from our group, where we had found, much to our surprise, that the thermoelectric response of polycrystalline gold wires varied a lot as a function of position within the wire, even though the metal was, by every reasonable definition, a good, electrically homogeneous material.  (We were able to observe this by using a focused laser as a scannable heat source, and measuring the open-circuit photovoltage of the device as a function of the laser position.)  At the time, I wrote "Annealing the wires does change the voltage pattern as well as smoothing it out.  This is a pretty good indicator that the grain boundaries really are important here."

What would be the best way to test the idea that somehow the grain boundaries within the wire were responsible for this effect?  Well, the natural thought experiment would be to do the same measurement in a single crystal gold wire, and then ideally do a measurement in a wire with, say, a single grain boundary in a known location.  

Fig. 4 from this paper

Shortly thereafter, I had the good fortune to be talking with Prof. Jonathan Fan at Stanford.  His group had, in fact, come up with a clever way to create single-crystal gold wires, as shown at right.  Basically they create a wire via lithography, encapsulate it in silicon oxide so that the wire is sitting in its own personal crucible, and then melt/recrystallize the wire.  Moreover, they could build upon that technique as in this paper, and create bicrystals with a single grain boundary.  Focused ion beam could then be used to trim these to the desired width (though in principle that can disturb the surface).

We embarked on a rewarding collaboration that turned out to be a long, complicated path of measuring many many device structures of various shapes, sizes, and dimensions.  My student Charlotte Evans, measuring the photothermoelectric (PTE) response of these, worked closely with members of Prof. Fan's group - Rui Yang grew and prepared devices, and Lucia Gan did many hours of back-scatter electron diffraction measurements and analysis, for comparison with the photovoltage maps.  My student Mahdiyeh Abbasi learned the intricacies of finite element modeling to see what kind of spatial variation of Seebeck coefficient \(S\) would be needed to reproduce the photovoltage maps.  

From Fig. 1 of our new paper.  Panel g upper shows the local crystal
misorientation as found from electron back-scatter diffraction, while 
panel g lower shows a spatial map of the PTE response.  The two 
patterns definitely resemble each other (panel h), and this is seen
consistently across many devices.

A big result of this was published this week in PNAS.  The surprising result:  Individual high-angle grain boundaries produce a PTE signal so small as to be unresolvable in our measurement system.  In contrast, though, the PTE measurement could readily detect tiny changes in Seebeck response that correlate with small local misorientations of the local single crystal structure.  The wire is still a single crystal, but it contains dislocations and disclinations and stacking faults and good old-fashioned strain due to interactions with the surroundings when it crystallized.  Some of these seem to produce detectable changes in thermoelectric response.  When annealed, the PTE features smooth out and reduce in magnitude, as some (but not all) of the structural defects and strain can anneal away.  

So, it turns out it's likely not the grain boundaries that cause Seebeck variations in these nanostructures - instead it's likely residual strain and structural defects from the thin film deposition process, something to watch out for in general for devices made by lithography and thin film processing.  Also, opto-electronic measurements of thermoelectric response are sensitive enough to detect very subtle structural inhomogeneities, an effect that can in principle be leveraged for things like defect detection in manufactured structures.  It took a while to unravel, but it is satisfying to get answers and see the power of the measurement technique.

Materials and popular material

This past week was a great one for my institution, as the Robert A. Welch Foundation and Rice University announced the creation of the Welch Institute for Advanced Materials.  Exactly how this is going to take shape and grow is still in the works, but the stated goals of materials-by-design and making Rice and Houston a global destination for advanced materials research are very exciting.  

Long-time readers of this blog know my view that the amazing physics of materials is routinely overlooked in part because materials are ubiquitous - for example, the fact that the Pauli principle in some real sense is what is keeping you from falling through the floor right now.  I'm working on refining a few key concepts/topics that I think are translatable to the general reading public.  Emergence, symmetry, phases of matter, the most important physical law most people have never heard about (the Pauli principle), quasiparticles, the quantum world (going full circle from the apparent onset of the classical to using collective systems to return to quantum degrees of freedom in qubits).   Any big topics I'm leaving out?

Diamond batteries? Unlikely.

The start of the academic year at Rice has been very time-intensive, leading to the low blogging frequency.  I will be trying to remedy that, and once some of the dust settles I may well create a twitter account to point out as-they-happen results and drive traffic this way.  

In the meantime, there has been quite a bit of media attention this week paid to the claim by NDB that they can make nanodiamond-based batteries with some remarkable properties.  This idea was first put forward in this video.  The eye-popping part of the news release is this:  "And it can scale up to electric vehicle sizes and beyond, offering superb power density in a battery pack that is projected to last as long as 90 years in that application – something that could be pulled out of your old car and put into a new one."

The idea is not a new one.  The NDB gadget is a take on a betavoltaic device.  Take a radioactive source that is a beta emitter - in this case, ¹⁴C which decays into ¹⁴N plus an antineutrino plus an electron with an average energy of 49 keV - and capture the electrons and ideally the energy from the decay.  Betavoltaic devices produce power for a long time, depending on the half-life of the radioactive species (here, 5700 years).  The problem is, the power of these systems is very low, which greatly limits their utility.  For use in applications when you need higher instantaneous power, the NDB approach appears to be to use the betavoltaic gizmo to trickle-charge an integrated supercapacitor that can support high output powers.

To get a sense of the numbers:  If you had perfectly efficient capture of the decay energy, if you had 14 grams of ¹⁴C (a mole), my estimate of the total power available is 13 mW. (((6.02e23 *49000 eV *1.602e-19 J/eV)/2)/(5700 yrs*365.25 days/yr*86400)). If you wanted to charge the equivalent of a full Tesla battery (80 kW-h), it would take (80000 W-hr*3600 s/hr)/(0.013 W) = 2.2e10 seconds. Even if you had 10 kg of pure ¹⁴C, that would take you 180 days.

Now, the actual image in the press release-based articles shows a chip-based battery labeled "100 nW", which is very reasonable.  This technology is definitely clever, but it just does not have the average power densities needed for an awful lot of applications.

Black Si, protected qubits, razor blades, and a question

The run up to the new academic year has been very time-intense, so unfortunately blogging has correspondingly been slow.  Here are three interesting papers I came across recently:

• In this paper (just accepted at Phys Rev Lett), the investigators have used micro/nanostructured silicon to make an ultraviolet photodetector with an external quantum efficiency (ratio of number of charges generated to number of incoming photons) greater than 100%.  The trick is carrier multiplication - a sufficiently energetic electron or hole can in principle excite additional carriers through "impact ionization".  In the nano community, it has been argued that nanostructuring can help this, because nm-scale structural features can help fudge (crystal) momentum conservation restrictions in the impact ionization process. Here, however, the investigators show that nanostructuring is irrelevant for the process, and it has more to do with the Si band structure and how it couples to the incident UV radiation.  
• In this paper (just published in Science), the authors have been able to implement something quite clever that's been talked about for a while.  It's been known since the early days of discussing quantum computing that one can try to engineer a quantum bit that lives in a "decoherence-free subspace" - basically try to set up a situation where your effective two-level quantum system (made from some building blocks coupled together) is much more isolated from the environment than the building blocks themselves individually.  Here they have done this using a particular kind of defect in silicon carbide "dressed" with applied microwave EM fields.  They can increase the coherence time of the composite system by 10000x compared with the bare defect.
• This paper in Science uses very cool in situ electron microscopy to show how even comparatively soft hairs can dull the sharp edge of steel razor blades.  See this cool video that does a good job explaining this.  Basically, with the proper angle of attack, the hair can torque the heck out of the metal at the very end of the blade, leading to microfracturing and chipping.

And here is my question:  would it be worth joining twitter and tweeting about papers?  I've held off for a long time, for multiple reasons.  With the enormous thinning of science blogs, I do wonder, though, whether I'd reach more people.

The energy of the Beirut explosion

The shocking explosion in Beirut yesterday was truly awful and shocking, and my heart goes out to the residents.  It will be quite some time before a full explanation is forthcoming, but it sure sounds like the source was a shipment of explosives-grade ammonium nitrate that had been impounded from a cargo ship and (improperly?) stored for several years.

Interestingly, it is possible in principle to get a good estimate of the total energy yield of the explosion from cell phone video of the event.  The key is a fantastic example of dimensional analysis, a technique somehow more common in an engineering education than in a physics one.  The fact that all of our physical quantities have to be defined by an internally consistent system of units is actually a powerful constraint that we can use in solving problems.  For those interested in the details of this approach, you should start by reading about the Buckingham Pi Theorem.  It seems abstract and its applications seem a bit like art, but it is enormously powerful.  

The case at hand was analyzed by the British physicist G. I. Taylor, who was able to take still photographs in a magazine of the Trinity atomic bomb test and estimate the yield of the bomb.  Assume that a large amount of energy \(E\) is deposited instantly in a tiny volume at time \(t=0\), and this produces a shock wave that expands spherically with some radius \(R(t)\) into the surrounding air of mass density \(\rho\).  If you assume that this contains all the essential physics in the problem, then you can realize that the \(R\) must in general depend on \(t\), \(\rho\), and \(E\).  Now, \(R\) has units of length (meters).  The only way to combine \(t\), \(\rho\), and \(E\) into something with the units of length is \( (E t^2/\rho)^{1/5}\).  That implies that \( R = k (E t^2/\rho)^{1/5} \), where \(k\) is some dimensionless number, probably on the order of 1.  If you cared about precision, you could go and do an experiment:  detonate a known amount of dynamite on a tower and film the whole thing with a high speed camera, and you can experimentally determine \(k\).  I believe that the constant is found to be close to 1.  

Flipping things around and solving, we fine \(E = R^5 \rho/t^2\).  (A more detailed version of this derivation is here.)  

This youtube video is the best one I could find in terms of showing a long-distance view of the explosion with some kind of background scenery for estimating the scale.  Based on the "before" view and the skyline in the background, and a google maps satellite image of the area, I very crudely estimated the radius of the shockwave at about 300 m at \(t = 1\) second.  Using 1.2 kg/m³ for the density of air, that gives an estimated yield of about 3 trillion Joules, or the equivalent of around 0.72 kT of TNT.   That's actually pretty consistent with the idea that there were 2750 tons of ammonium nitrate to start with, though it's probably fortuitous agreement - that radius to the fifth really can push the numbers around.

Dimensional analysis and scaling are very powerful - it's why people are able to do studies in wind tunnels or flow tanks and properly predict what will happen to full-sized aircraft or ships, even without fully understanding the details of all sorts of turbulent fluid flow.  Physicists should learn this stuff (and that's why I stuck it in my textbook.)

How long does quantum tunneling take?

The "tunneling time" problem has a long, fun history.  Here is a post that I wrote about this issue 13 years ago (!!).  In brief, in quantum mechanics a particle can "tunnel" through a "classically forbidden" region (a region where by simple classical mechanics arguments, the particle does not have sufficient kinetic energy to be there).  I've written about that more recently here, and the wikipedia page is pretty well done.  The question is, how long does a tunneling particle spend in the classically forbidden barrier?  

It turns out that this is not a trivial issue at all.  While that's a perfectly sensible question to ask from the point of view of classical physics, it's not easy to translate that question into the language of quantum mechanics.  In lay terms, a spatial measurement tells you where a particle is, but doesn't say anything about where it was, and without such a measurement there is uncertainty in the initial position and momentum of the particle.  

Some very clever people have thought about how to get at this issue.  This review article by Landauer and Martin caught my attention when I was in grad school, and it explains the issues very clearly.  One idea people had (Baz' and Rybochenko) is to use the particle itself as a clock.  If the tunneling particle has spin, you can prepare the incident particles to have that spin oriented in a particular direction.  Then have a magnetic field confined to the tunneling barrier.  Look at the particles that did tunnel through and see how far the spins have precessed.  This idea is shown below.

"Larmor clock", from this paper

This is a cute idea in theory, but extremely challenging to implement in an experiment.  However, this has now been done by Ramos et al. from the Steinberg group at the University of Toronto, as explained in this very nice Nature paper.  They are able to do this and actually see an effect that Landauer and others had discussed:  there is "back-action", where the presence of the magnetic field itself (essential for the clock) has an effect on the tunneling time.  Tunneling is not instantaneous, though it is faster than the simple "semiclassical" estimate (that one would get by taking the magnitude of the imaginary momentum in the barrier and using that to get an effective velocity).  Very cool.

Kitchen science: insulated cups

An impromptu science experiment this morning.  A few months ago we acquired some very nice insulated tumblers (initially from causebox and then more from here).  Like all such insulated items, the inner and outer walls are made from a comparatively lousy thermal conductor, in this case stainless steel.  (Steel is an alloy, and the disorder in its micro and nanoscale structure scatters electrons, making it have a lower electrical (and hence thermal) conductivity than pure metals.)  Ideally the walls only touch at the very top lip of the cup where they are joined, and the space between the walls has been evacuated to minimize heat conduction by any trapped gas in there.  When working well, so that heat transfer has to take place along the thin metal wall, the interior wall of the cup tends to sit very close to the temperature of whatever liquid is in there, and the exterior wall tends to sit at room temperature.

We accidentally dropped one of the cups this morning, making a dent near the base.  The question was, did this affect the thermal insulation of that cup?  To test this, we put four ice cubes and four ounces of water from our refrigerator into each cup and let them sit on the counter for 15 minutes.  Then we used an optical kitchen thermometer (with handy diode laser for pointing accuracy) to look at the exterior and interior wall temperatures.  (Apologies for the use of Fahrenheit units.)  Check this out.

The tumbler on the left is clearly doing a better job of keeping the outside warm and the inside cold.  If we then scrutinize the tumbler on the right we find the dent, which must be deep enough to bring the inner and outer walls barely into contact.

The bottom line:  Behold, science works.  Good insulated cups are pretty impressive engineering, but you really should be careful with them, because the layers really are close together and can be damaged.

Recently in the arxiv - van der Waals interfaces

Sometimes when looking at the pace of results coming out of the 2D material community, I am reminded of an old joke from Tom Lehrer about super-productive people:  "It's people like that who make you realize how little you've accomplished. It's a sobering thought, for example, that, when Mozart was my age, he had been dead for two years." (See here and then listen to the whole album - National Brotherhood Week has particular resonance this year.).

Recently in the arxiv, there were two different back-to-back preprint pairs uploaded by extremely strong collaborations in the trade of creating new condensed matter systems at the interfaces of stacked van der Waals materials (systems like graphene and mica, that can be exfoliated down to atomically thin layers).  

The first pair of papers (and my apologies if I missed others) were this one and this one.  In the former, the investigators take advantage of the energies of the bands in \(\alpha\)-RuCl₃, and find that when it is layered stacked with various 2D materials (graphene, bilayer graphene, WSe₂, electrons are spontaneously transferred from the 2D materials to the \(\alpha\)-RuCl₃ (The normally empty conduction band of \(\alpha\)-RuCl₃ lies at lower energy than the top of the valence band of the 2D material.)  This leads to very high hole concentrations within the graphene (etc.), with comparatively minimal disorder, reminiscent of modulation doping, the technique used to achieve outstanding charge mobility in 2D electron and hole gases.  The latter paper is complementary to the former:  the investigators use near-field optical techniques to look at both the plasmon properties of the graphene in such structures, and can back out the optical conductivity of the now-electron-doped \(\alpha\)-RuCl₃.

The second pair of papers, this one and this one, show a whole hierarchy of insulating states that appear in moire bilayer structures made from twisted WS₂/WSe₂ bilayers.  As I've written before, putting together close but not identical lattices and/or twisting one layer relative to another leads to a moire pattern, and therefore superlattice for charge carriers at that interface.  Both groups find (the first using optical methods, the second using microwave techniques) that for a large number of rational fraction ratios between the number of charge carriers and the number of lattice sites, the system is very strongly insulating.  Each insulating state corresponds to a particular periodic arrangement of the charge carriers, trying to stay generally as far away from each other as possible to minimize their potential energy.  These can be analogous to Wigner crystals and charge density waves.

Very cool stuff.

APS Division of Condensed Matter Physics invited symposium nominations

While no one knows right now whether the 2021 March Meeting will be in person, online, or some hybrid form next spring, now is the time to put in your nominations for invited speakers and symposia for the Division of Condensed Matter Physics.  The deadline to nominate is August 17.  The whole community benefits from high quality invited talks, so if you're in a position to do this, please think about it.

To Members of the Division of Condensed Matter Physics:

APS is now accepting invited speaker and invited symposium nominations for the March Meeting in 2021. Here is the link to the APS website for submitting nominations.

The Meeting is planned for March 15 to 19, 2021. Join more than 11,000 physicists attending, presenting, and networking at the APS March Meeting 2021. Showcase your work to a global audience of physicists, scientists, and students representing APS units and committees and explore groundbreaking research from industry, academia, and major labs.

Note that the decision regarding a virtual or in-person March Meeting will be made later this summer.

Jim Sauls
DCMP Secretary/Treasurer

Quantum Coffeehouse and other physics videos

Who doesn't need more videos to watch these days?

Erica Carlson has started a new Quantum Coffeehouse video series, including interviews with practicing physicists (including yours truly).  She had also presented a "Great Course", "Understanding the Quantum World".

I'm also a fan of Physics Girl.  I really liked her recent video with supercooled sodium acetate.

Minute Physics is truly outstanding, including their look at N95 masks and how they use electrets to gather and trap polarizable particles.

Andrew Dotson is reliably funny and insightful.

For the musically inclined, acapellascience is engaging, including their particularly timely William Rowan Hamilton.

I also have to plug my colleague Jason Hafner's channel, which netted him an on-screen appearance in the new movie Palm Springs.  Also making an appearance in the movie is Jim Freericks' edx course, Quantum Mechanics for Everyone.

Brief items - updated

Some further items of note:

• There is great anxiety and frustration over the latest pronouncement from DHS/ICE about international students in the US.  Let me give a little context.  For many years there has been a rule that international students studying in the US can take no more than 3 credits (or equivalent) per semester of purely online instruction. The point of that was to prevent many people from applying for F visas and then "studying" at online-only diploma mills while actually working. That is, it was originally a policy meant to encourage that student visas go to legitimate international students and scholars pursuing degrees at accredited universities.  In the spring when the pandemic hit and many universities transitioned to online instruction in the middle of the semester, DHS granted a waiver on this requirement.  Well, now they are trying to rescind that, and are doing so in a particularly draconian way: As written, if a university goes online-only, either from the start of the semester or even partway through due to public health concerns, the international students would face having to leave the US on short notice.   This is a terrible, stupid, short-sighted way to handle this situation, and it doesn't remotely serve the best interests of any constituency (student, university, or country).  Unsurprisingly, many many organizations are pushing back against this.  Hopefully there will be changes and/or workarounds.  UPDATE:  The administration appears to have backed down from this.  Hopefully that will stick.
• On to science.  Quanta has an article about the origins of the rigidity of glass.  The discussion there is about whether there is a kind of hidden structural order in the glassy material.  Fundamentally (as I've written previously), rigidity in any solid results from a combination of very slow timescales for atomic motion (due to lack of thermal energy available to overcome "barriers") and the Pauli principle giving a hard-core repulsion between atoms.  Still, the question of the underlying nature of glassy systems remains fascinating.
• The 2D materials experts at Columbia have shown clean fractional quantum Hall physics in a monolayer of WSe₂.  The actual paper is here.  I have yet to come up with a really nice, generally accessible write-up of the FQH effect. The super short version:  Confine charge carriers in strictly two dimensions, and throw in a large magnetic field perpendicular to the plane (such that the energy associated with cyclotron motion dominates the kinetic energy). At certain ratios of magnetic field to number of charge carriers, the charge carriers can condense into new collective states (generally distinguished by topology rather than broken symmetries like the liquid-gas or nonmagnetic/ferromagnetic phase transitions).  The fractional quantum Hall states can have all sorts of unusual properties, but the key point here is that they are fragile.  Too much disorder (like missing atoms or charged impurities), and the energy associated with that disorder can swamp out the energy savings of condensing into such a state.  It's remarkable that the material quality of the monolayer transition metal dichalcogenide (and its encapsulating boron nitride surroundings) is so high.  Seeing how FQH states evolve in this example new material system with rich band structure should be interesting.
• I feel bad for only now learning about this great series of talks about the state of the art in spintronics, trying to understand, engineer, and control the motion of spin.
• For your animal video needs, get the behind-the-scenes story about Olive and Mabel here.