Thursday, September 24, 2020

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.

Monday, September 21, 2020

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: 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:

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.

* 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 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.

Thursday, September 10, 2020

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.

Tuesday, September 08, 2020

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?

Saturday, August 29, 2020

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.

Tuesday, August 18, 2020

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.

Wednesday, August 05, 2020

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.)