What is localization?

Physicists love simplifying idealizations, and this is especially true in the physics of materials.  The simplest decent model for metals, for example, is the ideal Fermi gas, where we neglect the existence of atoms entirely and just model the electrons as noninteracting particles in some box.  One step up from there, the Sommerfeld model, assumes that the electrons are in a perfectly periodic crystal lattice.  In both cases, the standard semiclassical approach treats the electrons as waves but basically ignores quantum interference.  

Real conductors have defects that break the lattice periodicity, like vacancies, interstitials, impurities, grain boundaries, surfaces and interfaces, etc.  It's natural to wonder, are there major consequences to this "disorder"?  Common sense suggests that sufficiently minor or dilute disorder can't be too important.  Sure, once you break the lattice symmetry, the electronic wavefunctions can't be exactly Bloch waves anymore, but if only one atom out of 10 billion is out of place, how big a deal can it be?

In the late 1970s, a number of theorists were thinking about this problem, and they came up with some impressive insights about the role of disorder, leading to the concept of localization.  The key point to consider is whether the wavefunctions in the presence of disorder are delocalized (extending "to infinity", like plane waves or Bloch waves), or whether they are localized (decaying exponentially away from some origin region where their magnitude is large).  This idea can apply to wavefunctions for electrons, but it can also apply to other kinds of waves, including electromagnetic waves in inhomogeneous dielectric media (think light bouncing around in a cloud).  

A major result that came out of this thinking was the scaling theory of localization.  That link points to some excellent lecture notes and a couple of youtube videos by Piet Brouwer for people interested in a more technical explanation.  Intuitively, if the electronic states are exponentially localized, then making a block of material bigger should lead to the conductance of that material dropping exponentially.  Alternately, if the electronic states are delocalized, making a hunk of material larger should generally increase its conductance.  (Think about a piece of copper wire.  Now double both the length and the diameter of the wire.  The conductance \(= \sigma (\pi d^2)/(4L)\) has doubled.)  

Let's call \(g(L) = G(L)/(e^2/h)\) the (dimensionless) conductance of some hunk of material of size \(L\).  The question is, if you increase \(L\), what happens to \(g\)?  There is a scaling function \(\beta(g) \equiv d \ln g/d \ln L\) that describes this.  If \(\beta(g)\) is positive, then the system is metallic.  If \(\beta(g)\) is negative, then the system is insulating in the large size limit, a situation called strong localization.  The technical bit is figuring out what \(\beta(g)\) looks like.   (This scaling idea had many contributors, including most famously people like Anderson and Thouless)  

Remarkably, in this famous paper, the conclusion is that in 2D and 1D, any disorder at all makes \(\beta(g)\) negative.  Thus the surprising conclusion is that, for this model (with no interactions), in principle there are no 2D or 1D metals.  (The distance scale over which the conductance decays with increasing size is the "localization length", \(\xi\), and it could be very long.  That's why seeing metal-like conduction in cm-scale gated graphene or 2D electron gas samples isn't surprising or necessarily inconsistent with this.  There are many subtleties here.)  In 3D, the situation depends on the actual magnitude of \(g\), where if \(g\) starts too small, the system runs away toward localization as system size is increased.

This idea, that interference of scattered waves from disorder can lead to exponentially confined waves, is called Anderson Localization.  This is generic to waves in disordered media, as in this famous paper where it was demonstrated for light.  By the way, you can think of localization of light as an effective cavity that confines the radiation via disorder scattering, an idea which in turn led to the random laser.  Just earlier this year, people successfully demonstrated 3D Anderson localization of ultrasound.

I used google gemini to code up a toy model of Anderson localization (of light) in HTML5, where the disorder is in the form of a spatially varying index of refraction. (I used periodic boundary conditions.) If the disorder is weak (5 in toy units), all the energy dumped into the middle of the space spreads out roughly equally to fill the whole region.  However, if the disorder is strong (50 in toy units), the energy of the waves is localized near the origin for long simulation times.  Here is the model.  (No deep claims of strict accuracy here; this was quick and dirty.  To really see localization in this small play area, we'd need to \(\xi\) to be small compared to the size of the region because of the periodic boundary conditions.)

The ideas here have had a very long reach, and I'll likely write more about related physics soon.

Thermometry at the mK scale, revisited

It's been almost a decade since I last wrote about this topic, and a preprint on the arXiv this week is a good jumping off point for more discussion.

Thermometers are devices that allow us to take some physical observable and infer temperature.  I wrote about the nature of temperature 17 years ago (!!!) in a way that did not completely satisfy me or most of my readers, so maybe I should take another crack at it.  Temperature is a statistically emergent quantity (it doesn't make sense to talk about the temperature of a single particle in isolation) that tells us whether there will be a net flow of energy when a system we care about is brought into contact (able to exchange energy via microscopic degrees of freedom that we aren't tracking, like jiggling of atoms bumping into each other or emission/absorption of radiated photons) with some other system.  Temperature is closely related to the energy stored in the microscopic degrees of freedom of a system.  Our definition of \(T\) is such that there will be a spontaneous, net, averaged flow of energy from hot (a high \(T\) system) to cold (a low \(T\) system).  Two systems in contact at the same \(T\) will still exchange energy microscopically, but on average there will be no net flow, and in the absence of other complications, these systems are said to be in thermal equilibrium.

Measuring temperature is serious business with a fascinating history.  The kelvin is, as of 2019 (see, told you it was time to revisit this), defined by using the fundamental definitions of the kilogram, the meter, and the second, and by declaring that Boltzmann's constant \(k_{\mathrm{B}}\) is exactly 1.380 649 ×10⁻²³ J/K or equivalently kg m²/s²K.   In practice, there are fixed, measurable reference points that help make sure temperatures are calibrated.  For example, the triple point of water is a standard reference point at 273.16 K.  In total, there are two internationally agreed temperature scales, ITS-90 (pdf) and PLTS-2000 (pdf), that include a total of 21 reference points spanning from 0.9 mK to 1357.77 K.  

It's extremely helpful to have primary thermometers, where the physics involved in some measurable quantity are so well known that it is possible to analyze a measurement and directly pull out \(T\) based only on the data and known fundamental and numerical constants.  The preprint linked at the top of this post does an extremely careful comparison of two nanostructure-based approaches.  

Adapted from Fig. 1 from here.

A Coulomb blockade thermometer consists of a series of tiny metal/insulator/metal junctions.  The energy required to move a single electron across one such junction is proportional to \(e^2/C\), where \(C\) is the capacitance of the junction structure.  When the temperature is low, that charging energy scale can exceed the thermal energy scale, \(k_{\mathrm{B}}T\), so that the conductance \(dI/dV\) of the junction near zero applied voltage is suppressed compared to its high voltage and high temperature value.   If temperature is very low, conductance is suppressed all the way to zero, and that is Coulomb blockade.  If instead you have an array of identical junctions in series, and the temperature isn't too low, there is a perturbative suppression of conductance at zero bias. Remarkably, in this regime the shape of \(dI/dV\) as a function of \(V\) is universal, independent of details, and for an array of \(N\) junctions in series, its width is \(5.44 N k_{\mathrm{B}}T/e\). (top panel of figure) 

In a single tunnel junction, it is possible to measure Johnson-Nyquist noise, the current (voltage) fluctuations that take place across the device due to thermally driven motion of the electrons in equilibrium, and the charge shot noise, the fluctuations due to the statistical variations in the arrival times of the electrons.  The theoretical expression for the noise as a function of bias voltage is known (Eq. (2) in the paper).  (bottom panel of figure).

The authors find that the two thermometric approaches are quantitatively consistent to better than 2.5% between 20 mK and 235 mK, and the biggest uncertainty comes from knowing the effective bandwidth of the noise measurement.  This is a characteristically careful, clean work from this Finnish group, who are world experts in the field.  

NAS "State of Science" 2026 address

I watched the webcast of the NAS State of Science address by outgoing NAS president Dr. Marcia McNutt.  (I did not watch the panel discussion afterward, so sorry if I missed critical pieces.)  A few thoughts on this:

• The intro music was a very classy baroque string quartet.  Hard not to think of this scene from Titanic.
• 
  The main theme was about ways to revitalize US science, and there were six main points that she wanted to emphasize, each with examples of relevant projects underway, ways to measure success, and the consequences of failure.  That's fine, and I'll relay them below with some comments, but first an overall impression:  This was largely an exercise in avoiding talking about the elephant in the room, the overt hostility toward and the attempted wanton dismantling of much of the publicly funded US research ecosystem by the executive branch.  I'm unfortunately not surprised that this was largely brushed over, given the position of the Academies (see here).  As the saying goes, I'm not mad, I'm just disappointed.  The realization that the National Academies leadership do not feel empowered to have a frank discussion about this publicly has been depressing.
• Dr. McNutt mentioned that in her previous address, she had pointed out the US vulnerability in STEM by being so reliant on international talent, and that now that other countries are heavily investing in research, the US STEM research world needs to do a better job getting US citizens in the workforce.  That's all true, but leaving out how the government leadership is explicitly trying to curtain international scholars and international collaboration seems like quite an omission.
• She mentioned in passing that industrial research in the US in the 1950s was tiny, nothing compared to the fraction of R&D it is today.  Is that actually correct?  I mean, that was the heyday of Bell Labs, IBM, GE, Westinghouse, and big research labs at companies like Ford and GM.  Much has been written about this.  
• The first big point was the need for improved relationships between universities and industry, and some examples of ways to encourage this, including relatively simple policy changes like making it easier for faculty and others to take leaves in industry.  Certainly it would be broadly good for the US research ecosystem to have more diverse forms of support, and as I've written before, major industrial sectors with lots of capital rely in the long term on trained people. 
• The second point was the need to realign the academic reward system, so that industrial/entrepreneurial/coalition-building activities are incentivized, rather than rewarding on lone-wolf PIs. That's fine, and honestly I think it's already happening to some large degree at major research universities. 
• The third point was meeting the needs of the STEM workforce, though increased interactions with industry (including, e.g., prospective industrial employers helping to define dissertation topics), co-op efforts, some training in businessy aspects (note:  the Sloan Foundation was pushing this 25 years ago.).  This is all laudable to try, but I don't see how any of this actually addresses the issue of fewer STEM workforce participation from US citizens, which is quite complicated.
• The fourth point was the need to reduce regulatory burden.  Sure, we all want to reduce bureaucratic BS.  I have to say, though, that it was genuinely baffling to me that the most Dr. McNutt had to say about the threatened OMB rule changes (apart from a passing mention early on) is that they would increase bureaucracy.  That isn't even in the top 15 problems raised by those changes.  Remember, the default position of those pushing those rules is that academics are fundamentally untrustworthy and poor stewards of public resources.  
• Fifth was the need for automated/self-driving labs.  I agree completely that advanced degree training should not be driven by the need for cheap labor to do tedious lab tasks (e.g. a zillion cell cultures or chemical syntheses).  Overall this was pretty innocuous.
• Sixth, Dr. McNutt emphasized the need to take on big challenges - researchers need to be bold and not play it safe, and peer review can be inherently biased toward incrementalism.  She gave examples of large privately endowed institutes as enabling such work (MBARI, the Allen Institute).  Apparently STAC will be proposing new multi-agency science and technology "breakthrough funds".  The argument in favor of public investment in science in this section sounded rote rather than heartfelt.  If anything, I thought knocking peer review right now at a time when OMB wants to ignore it at their pleasure was a weird position to take.

To be clear:  I don't think any of the ideas highlighted in the speech are actually bad (necessarily).  It just avoided emphasizing that publicly funded research has been incredibly beneficial, and that irreversible harm is being done.  The statement that science agencies "have seen a loss of key personnel" is the worst kind of passive voice garbage.  A hundred thousand technical personnel leaving agencies is not something that just "happened" like the weather.  Being quiet, avoiding confrontation, and only trying to work behind the scenes is not the leadership that is needed now.  (See, I can do passive voice, too.)

I will try to get back to more science posting....

Info gathering: Excellent intro undergrad lab courses and facilities?

Introductory undergraduate labs are a recurring challenge at nearly every university.  Is the purpose to teach students something about how experimental science works (formulating hypotheses, defining measurement needs, setting up equipment, acquiring and analyzing data)?  Is the purpose to emphasize and reinforce specific scientific points from the curriculum?  How structured should they be?  Where are there opportunities for interdisciplinary labs rather than traditional physics/chemistry/biology/earth sciences stovepipes?  

I'm interested in learning about US examples of outstanding introductory physics labs - both in their content/execution, and in the intro lab facilities that my readers consider to be particularly well done.  Please respond in the comments or via email.   I'd really appreciate your thoughts on this, even knowing that my blog readership is a highly biased sample.

(I tried launching a survey about undergrad physics lab instruction five years ago.  I got zero responses.  Hopefully this will be a little more successful.)

The Manhattan Project and public communication

The Manhattan Project was the largest government sponsored research and development project of its time.  Some things worth noting, in light of the present US government attitude toward science:

• It's hard to overstate the role played by immigrant scientists in this story.  Szilard, Einstein, Fermi, Wigner, Teller, von Neumann, and many more.  
• I was trying to remember when the Manhattan Project became publicly known in any detail.  It turns out, within three days of the US bombing of Nagasaki, the US released a tidily written report headlined by Henry DeWolf Smyth on all the essentials, including the administrative story of how the project came to be and was managed.  That report is available in many forms, including this cute version on the internet archive and simple pdf files at DOE and Princeton.  It's an outstanding piece of clear, spare writing.  It almost boggles the mind: Here was a technical topic that the national leadership considered important for the public to understand (!), so a highly readable report was prepared and released basically immediately following public knowledge of the bombs. (!!)
• The National Academies played a pivotal role in this story.  On page 51:  "In the spring of 1941, Briggs, feeling that an impartial review of the problem was desirable, requested [presidential science adviser Vannevar] Bush to appoint a reviewing committee. Bush then formally requested F. B. Jewett, president of the National Academy of Sciences, to appoint such a committee. Jewett complied, appointing A. H. Compton, chairman; W. D. Coolidge, E. O. Lawrence, J. C. Slater, J. H. Van Vleck, and B. Gherardi."  Once upon a time, the national leadership respected the National Academies and trusted them to provide impartial, accurate scientific advice to inform policy.  Somehow I doubt that Frank Baldwin Jewett, president of the NAS at the time, was worried that the government would cut off funding to the Academy if they didn't toe the line.  (As far as I know, no one from the Roosevelt administration was taking “donations” for lucrative government contracts on the bomb, and no one from the cabinet or the Department of War were personally betting for profit on whether it would work, either, but I digress.)

Just some food for thought.

Brief items - news roundup, AI, international issues, good reading

Several items worth reading about as we head into a long weekend in the US.  Starting with news related to funding and other aspects of US government policy:

• US government taking equity stakes in some quantum information sciences companies while investing around $2B (seemingly from the Department of Commerce and the CHIPs Act resources.  (Non-paywall news story here).  This raises a number of thorny issues. 
• Some US funding agencies (NIH, NASA) are enacting restrictions (Science article here, Inside Higher Ed article here) on publishing scientific papers with non-US coauthors.  It's understandable that US funding agencies are concerned about the possibility US funds directly or effectively supporting researchers in foreign countries.  This is not that, though.  Some people making policy seem to be moving toward wanting to ban any co-authorship, but even the agencies seem confused about what they want.
• In a move that will stress out many non-US-citizens in the country, the administration is floating making people leave the US to apply for green cards (PBS article here).  This just was sort of announced yesterday, so I don't know anything about this other than on its face it sounds to me like a terrible idea for multiple reasons.  
• The AAAS is pushing for a Senate hearing on the nominee for NSF director, on the theory that this issue and the nominee at least need to be discussed in a public forum rather than coasting along without a NSB and no end in sight to interim leadership.
• It would seem that some Republican congresspeople are pushing the idea of de-funding the National Academies.  This is directly related to the issues mentioned here.  I think the National Academies should be endowed and thus not so reliant on federal funding; this would be a way to make sure that they always feel secure in delivering reports even if the customer is a part of the government and the conclusions might be something the customer doesn't want to hear.

There was a lot of AI-related news this week:

• 
  There were three papers published in Nature about using AI agents to do science (here, here, and here, with a news and views).  The first two papers are both about drug discovery research, and the third is about using AI to help write scientific software models (also medically related).  It'll be interesting to see how this progresses.  
• One of OpenAI's tools solved an Erdos problem (that's the OpenAI release) by finding a counterexample to a conjecture long thought to be true.  Here is the accompanying paper, which includes commentary by several esteemed mathematicians.  The commentary parts of the paper are very much for non-mathematicians and fascinating to read.  It seems like the AI tools are genuinely good at pulling together complex arguments, and that so far a key advantage they have is an exhaustive familiarity with the full breadth of the literature.
• Unsurprisingly, university graduates are not fans of AI.  This cartoon from this week's New Yorker is topical.  
  

 Additional suggestions that look cool but I haven't had time to actually read:

• Jeremy Levy at Pitt has posted his draft graduate quantum mechanics textbook to the arXiv - looks like an interesting take on the pedagogy of the subject for sure. 
• Dimitrii Makarov at UT Austin has posted a list of critical papers in chemical physics.  
• Samuel Braunstein at York has put up this preprint regarding the Clay Millennium Prize problem about the Navier-Stokes equations.  His main points seem at a quick read to be (1) at finite temperature, fluctuation-dissipation requires some additional noise term be present in the N-S equations; and (2) the N-S equations assume a continuum fluid, so that infinitely-short wavelength excitations are permitted, while for all real fluids wavelengths get cut off at the molecule size.  Basically the issues w/ the N-S equations come from unphysical aspects of the formulation.

What are heavy fermions?

I'm surprised that I haven't written about heavy fermions as a separate post before, so here we go. (It's a break from thinking about science and politics, anyway.)

I've written before about "effective mass" for electronic excitations in solids (wiki page here).  From classical physics, we are used to the idea that inertial mass \(m\) is the ratio between an external force \(\mathbf{F}\) and the acceleration \(\mathbf{a}\) of some object, \(\mathbf{F} = m\mathbf{a}\), which is also the rate of change of momentum, \(d\mathbf{p}/dt\).  Kinetic energy (for a nonrelativistic particle) is \(p^{2}/2m\).  Electrons in crystalline solids "feel" the lattice, so in general their kinetic energy \(\epsilon\) can be a more complicated function of their (crystal) momentum, and we can try do define an effective mass as \(1/m* \equiv d^{2}\epsilon/dp^{2}\).  So, if the kinetic energy is very weakly dependent on \(p\), this corresponds to having a very large effective mass.  TL/DR:  the periodic lattice can strongly alter how an electronic excitation accelerates in the presence of a force from, e.g., an electric field, compared to a free particle.  This isn't too surprising.  

Interestingly, in most semiconductors and metals, \(m*\) for electrons in the conduction band (or holes in the valence band) is not thaaaaat different than the free electron mass \(m_{0}\).  The lightest effective mass I know (leaving aside graphene and other Dirac systems when \(\epsilon\) is approximately linear in \(p\)) is electrons in InSb, about \(0.014 m_{0}\).  Holes tend to be a bit heavier.  Also, \(m*\) in molecular organic semiconductors like pentacene tends to be a bit larger, since hopping from molecule to molecule is comparatively weak.  There are ways to measure effective mass, including cyclotron resonance, electronic transport including Shubnikov-de Haas oscillations, magnetic susceptibility and de Haas/van Alphen oscillations, and specific heat measurements.  The electronic specific heat contribution for a metal is linear in the temperature at low \(T\), and the constant of proportionality includes the density of electronic states at the Fermi energy, which can be written in terms of \(m*\).  I've left out a lot of the complications of real anisotropic materials with complicated band structures, but generally the different measurements give consistent results. 

Therefore, it was a big surprise in 1975 when investigators found a material, CeAl3, in which the heat capacity implied an effective mass tens to hundreds of times larger than \(m_{0}\).  They knew right away that this had something to do with the very localized \(4f\) electrons of the Ce atoms.  Because those electrons are very localized, their energy is almost independent of \(p\), implying a large effective mass.  (Some heavy fermion materials also superconduct at temperatures surprisingly high given their effective masses.)

Heavy fermions, adapted from here.  (a) At high temperatures, the 

conduction  electrons are not well coupled to the unpaired local 4f 

moments.  (b) At low enough temperatures, Kondo scattering

hybridizes the f electrons with the conduction  electrons, boosting 

the carrier density.  (c) The hybridized energy-momentum relation 

is much flatter near the Fermi energy leading to a large effective mass.  

So what's the physics?  I wrote about the Kondo effect here, where "ordinary" conduction electrons scatter in a nontrivial way from local magnetic moments (such as partially filled \(4f\) states), and well below a characteristic temperature \(T_{\mathrm{K}}\), the conduction electrons hybridize with the impurities, screening out the unpaired spin.  In the heavy fermion compounds, instead of impurities, there is a whole crystal lattice of local magnetic moments. At sufficiently low temperatures, thanks to that Kondo scattering process, those otherwise localized electrons hybridize with the conduction electrons, boosting the effective density of charge carriers (see figure) and greatly increasing the effective mass.  See this figure, adapted from excellent lecture notes by Piers Coleman.  

So, two key ingredients for heavy fermions are itinerant conduction electrons and a periodic array of comparatively localized, unpaired electrons that have magnetic moments. It turns out that this combination can also be achieved in moiré lattice materials.  There are no \(f\) electrons here, but the moiré lattice can localize spins.  Apologies for not linking to all the relevant papers, but a couple of key theory results are here, here, and here, and a key experimental result is here.  The tunability of the 2D material-based systems is an excellent feature for digging down into the detailed physics.

Update:  Now some added insight from Prof. Andrew Millis:

Hi Doug:

An addendum to your very nice post on heavy fermions, to draw attention to what I think were important experimental results: Frank Steglich’s 1979  Phys. Rev. Lett. 43, 1892–1896 reporting superconductivity in CeCu2Si2 and Louis Taillefer and Gil Lonzarich’s 1988 determination of the quasiparticle mass and fermi surface in UPt3.

Prior to Steglich’s paper we knew that some rare earth/actinide intermetallics (e.g. CeAl3) had a very enhanced specific heat coefficient at low temperatures and that the entropy implied by  this specific heat was  derived from the magnetic moments of the rare earth ions. But  while it was plausible, there was no direct evidence that this enhanced specific heat was associated with heavy-mass fermions, so the physical relevance of the Kondo lattice concept remained uncertain.

Steglich observed that in CeCu2Si2 the specific heat jump at the superconducting transition (which in BCS theory is basically the same size as the electronic specific heat at Tc) was about as big as the normal state specific heat coefficient, thus showing that the spin entropy had been transmuted into something that could go superconducting. Then (I think in subsequent experiments) Steglich observed that the rest of the superconducting thermodynamics in Cecu2Si2 was also consistent with pairing of heavy mass entities. This, I believe, is what convinced everyone that the spin entropy from the rare earth moments had been converted into heavy mass electrons—in other words, that the lattice Kondo effect was real. 

A few years after this, Louis Taillefer and Gil Lonzarich’s quantum oscillation study of UPt3 (Phys. Rev. Lett. 60, 1570 ) showed indeed that the U-f electrons (which appear as local moments at higher temperatures) were included in the Fermi surface at low T and had heavy masses, providing direct experimental confirmation of the Kondo lattice concept.

Cheers

Andy Millis

NSF, National Science Board, and the politics of staying quiet

As I mentioned previously, the National Science Board was summarily fired on April 25.  The NSB nominally advises the National Science Foundation.  There have been a number of pieces written about this:

• Going back in time to 2022, this essay is interesting to read, about the history of the NSF and the NSB, and the compromises put in place with the administrative structure.  Short version: Initially there was a real tension between the Director (reporting to the President) and the NSB.  Over time, the NSB was made subordinate to the director (1968).  Senatorial confirmation of board members was waived by the Senate in 2011.  
• Many professional organizations issued statements expressing grave concern about this wholesale dismissal of the board.  This AIP news article has a summary.  The CEO of the APS wrote this, the ACS leadership wrote this, the AAS wrote this, etc.
• The presidents of the National Academy of Sciences, National Academy of Engineering, and National Academy of Medicine issued this joint statement.  That has to set some kind of record for blandness, as it somehow does not even mention that the NSB was fired.  I fully understand that the Academies have a number of federal contracts, as one of their key responsibilities is leveraging their membership to do authoritative studies, with federal agencies usually being the customers.  I have no inside knowledge, but it sure looks like they are trying to walk a line of not raising the administration's ire.  (Surely this raises the question:  If it's never acceptable to say anything that might upset the administration, then how can the objectivity of their reports relating to policy ever be trusted?)
• In contrast to the leadership, a lot of Academy membership has signed an open letter to Congress demanding the reinstatement of the board.
• Scientific American has very good reporting on this, including a no-holding-back statement by my colleague Neal Lane.
• Update:  Here is Dan Garisto's reporting in Science about letters sent by House Democrats and by Senate Democrats demanding action on this.  That article includes a statement by the fired head of the NSB, basically saying they were dismissed for defending the NSF budget from OMB.  I'm glad these letters were sent, but without the R majority signing on, I'm not holding my breath.

Meanwhile, the pace of NSF awards continues to be glacial, even compared to last year.  See this plot from Grant Witness. 

We are 7 months into the fiscal year, and obligated dollars are less than half at this time last year, and more like 27% of those at this time in "normal" year.  It's hard to look at that and not wonder whether someone is aiming for a pocket rescission, regardless of what Congress appropriated.  NSF looks like an outlier here, by the way.  As badly hit as NIH has been, their award curves look much closer to last year.

Other related things worth reading:  

• This is a sobering and interesting article about what the author describes as the anti-science movement.
• When is staying quiet effectively giving tacit support to administration policies?  There is an article in The Nation (not readable without a free signup) talking about what they term "Vichy Science".
• Update:  Essay by Holden Thorp, EIC of Science, including a link to a half-hour interview with Tim Snyder, author of "On Tyranny", about how resistance can work.  

Back to science in my next post.

Energy storage in the internal states of molecules - old and new

A science story first, then a US research ecosystem story later.

When we think about using molecules to store energy, it's usually in the context of food or fuel, so that chemical reactions take place - bonds are broken and remade, and in an exothermic reaction, the products end up with more kinetic energy (center of mass motion, molecular vibrations and rotations) than the initial reactants.  However, there are other ways that molecules can store energy.  I read about a cool example of this last week, but first I want to give tell you an old and very quantum mechanical story that I learned about in grad school when I did very low temperature physics.

Diatomic hydrogen, H2, is the simplest molecule there is, just two electrons and two protons.  Roughly speaking, the \(1s\) orbitals of the H atoms hybridize to form \(\sigma\) bonding and \(\sigma*\) antibonding molecular orbitals.  The lowest electronic state is the two electrons in a spin singlet, \((1/\sqrt{2})(|\uparrow \downarrow\rangle - |\downarrow \uparrow\rangle)\) in the \(\sigma\) molecular orbital.  Remember, the electrons are fermions, so the electronic wavefunction has to be antisymmetric (pick up a minus sign) under exchange of the electrons. The spin singlet is antisymmetric under exchange, the \(\sigma\) orbital is spatially symmetric under exchange, so the full electronic wavefunction (product of the spin and spatial components) is appropriately antisymmetric.  

That's not all there is to it, though, as explained thoroughly here.  The protons (while being made up of quarks and gluons, etc.) are (composite) fermions, so we have to think about the quantum wavefunction that describes them, too.  There are two possibilities.  In the "para" configuration, the proton spins are in a singlet (antisymmetric), meaning that the spatial wavefunction of the protons must be symmetric under exchange.  The spatial state of the bound protons can have some orbital angular momentum \(\mathbf{L}\), and the simplest, lowest energy situation is with quantum numbers \(\ell =0\) and therefore \(m_{\ell} = 0\).  In contrast, in the "ortho" configuration, the proton spins form a triplet state (symmetric under exchange), meaning that the spatial wavefunction must be antisymmetric, \(\ell = 1\).  Approximating the H2 molecule as a rigid barbell-like rotor with some moment of inertia \(I\), then ortho molecule has a rotational energy \(\hbar^2/2I\) larger than the para case.  That works out to about 15 meV of energy per molecule.  So, para-hydrogen is the true ground state.  It turns out that the ortho/para spin isomer energy difference makes liquefying hydrogen a challenge, since the latent heat of vaporization for H2 is only 9.4 meV.  That is, every time an ortho-hydrogen molecule converts to para-hydrogen through some collisional process, it releases enough energy to kick a hydrogen molecule out of the liquid.  I learned about this in my thesis work playing around at ~ 1 mK temperatures - any H2 adsorbed or otherwise stuck in the apparatus could result in detectable long-term heating effect as it slowly converted from ortho to para.  Bottom line:  Energy can be stored in the internal states of molecules.

From Fig. 1 of this paper.

This seems very esoteric, but the idea of storing energy in some internal state of a molecule for later release shows up elsewhere.  Last week, in this article in Science, for example, the authors report a molecule inspired by aspects of DNA that can be put via UV exposure into a distorted form ("Dewar isomer") where it is metastable at room temperature (half-life of 481 days).  It can be induced to pop back into the undistorted isomer by heat, acid exposure, or via a catalyst, and when it does, each molecule releases the stored energy (2.36 eV per molecule!) into vibrations and rotations that heat its surroundings.  The stored energy density in this stuff is about 4% of the releasable energy density of gasoline, which is not too shabby.  The authors propose a system where exposure to sunlight can store energy in the molecules, and this can later be released on demand via catalyst.  They demonstrated that the heat release from enough dissolved molecules can readily boil water.  Very neat stuff.

News items and essays to read

First, some inside-baseball US funding discussion.  Apologies to my international readers, who likely don't care much about this except in the abstract.

• Breaking news:  According to journalist Dan Garisto, as of April 25, 2026, the president has fired the entire National Science Board.  The NSB helps oversee the National Science Foundation.   From the outside, it had sure looked to me like the NSB had tried verrrrrrry hard to stay on the administration's good side.  That was fraying recently, as this February article in Science included comments implying that not everyone was thrilled with the executive branch strongly shifting NSF priorities.  It sure looks like their reward for not speaking out strongly about the importance of continued support for research was apparently to be terminated with prejudice.  If any readers have first-hand knowledge of what happened, please post in the comments.
• I had already been planning to point out this article in Nature, which says that NSF funding may finally be about to flow, after a long, murky back-and-forth between the agency, Congress, and OMB.  It's worth noting that Congress had stated in their guidance that no directorate within NSF should be cut by more than 5%, while OMB has mandated (apparently) different spending levels which, among other things, would cut Mathematics and Physical Sciences by 15% and Engineering by 18%.  It sure doesn't look obvious to me, with everything else going on right now, that Congress is willing to truly push back on this.  Surrendering microscopic spending authority to OMB seems like a complete abdication of congressional authority, but what do I know.

One bit of science:

• I participated in a virtual workshop this week, Targeted Questions: Strange Metals.  I have written about strange metals before (see here and here).  The workshop was streamed, including the discussion sessions, and here is the youtube link if you are interested.

Some essays this week:

• There was a report by a Yale University committee about the erosion of trust in higher education in the US earlier this month.  It certainly spurred a lot of conversation, and it raises many important issues.  That said, it does seem to downplay the fact that there has been a decades-long campaign by some with the goal of eroding trust in higher education, as pointed out in this essay.
• A friend pointed me to this essay by Santiago Schnell about higher education in the era of AI - I found it very thoughtful, even though I don't agree with every aspect.
• Speaking of AI, this essay ("The future of everything is lies, I guess: Bullshit about bullshit machines") by Kyle Kingsbury was provocative and worth reading, again even if I don't agree with every aspect.
• Speaking of thoughtful commentary, here is an essay by a billionaire that is worth reading.

Floating magnets to sense magnetic fields

We've all seen a traditional compass.  A ferromagnetic, magnetized needle is mounted on a rotating bearing (or floated on the surface of a liquid) so that it can rotate in the \(x-y\) plane.  If there is an in-plane magnetic field \(\mathbf{B}\), the needle will rotate to align with that component of the field.  (It stops in the aligned state because of friction; otherwise it would "librate", oscillating back and forth about the field direction.)  In first-year undergrad physics, we learn a simple model of why this happens.  The magnetized needle can be modeled as a magnetic dipole \(\mathbf{m}\).  We learn that a magnetic dipole in a uniform magnetic field generates a torque \(\boldsymbol{\tau} = \mathbf{m}\times \mathbf{B}\).  If both \(\mathbf{m}\) and \(\mathbf{B}\) are in the \(x-y\) plane, any torque must be directed along \(z\), and the torque goes to zero when \(\mathbf{m} || \mathbf{B}\).  The simplest result of \(\boldsymbol{\tau} || z\) is an angular acceleration that would cause an otherwise at-rest compass needle to rotate in the plane counterclockwise about the \(z\) axis. 

Left: A compass needle out of alignment with an in-plane 
magnetic field produces a torque along \(z\), and angularly
accelerates to reorient toward the field . Right: Under the 
right circumstances, the spin angular momentum of the 
needle could also precess around the field.

So, how sensitive of a magnetic field detector could you make using this basic approach?  This week I came upon this paper from a few years ago, which looks at this problem from the theoretical modeling side, and then this paper from last year that does the experiment.  The magnet in question is a little (21 \(\mu\)m diameter) sphere of Nd2Fe14B, a rare-earth magnet.  The authors put that inside a lead chamber with a rounded bottom, and they cool the lead down to 4.2 K, well below its superconducting transition temperature.  As a result, the sphere is magnetically levitated inside thanks to the Meissner effect, with its magnetization lying in the \(x-y\) plane.  There is some residual magnetic flux trapped in the setup that does lead to a preferred field direction.  The authors can use cleverly wound pickup coils inside the chamber to detect the orientation of the sphere, as well as apply AC magnetic fields.  The authors are primarily concerned in thinking about energy resolution of detection, because they are thinking about detecting unusual particles (e.g. dark matter, axions), but they point out that it should be possible to achieve tens of atto-Tesla per Hz\(^{1/2}\) field sensitivity per unit bandwidth - pretty wild.

But wait, there's more!  The magnetic moment of the magnetized needle originates from the spins of electrons in there.  This is gyromagnetism, so \(\mathbf{m} \propto \mathbf{S}\), the total spin angular momentum of the electrons in the magnet.  This means that in the presence of \(\boldsymbol{\tau} || z\), if mechanically possible the needle could start swinging up out of the \(x-y\) plane to project a component of \(\mathbf{S}\) along \(z\).  This is gyroscopic precession.  For macroscopic magnets, it's hard to be in the regime where this is the dominant effect, because that would require the precessional angular momentum to be small compared to \(\mathbf{S}\), and that's tough to achieve.  Maxwell (!) tried to do it in 1861 (!!), with no success.

In a very recent paper, this precessional response was finally observed, again in Nd2Fe14B microspheres.  (For a uniformly magnetized sphere of radius \(R\), the moment of inertia \(I \propto R^{5}\), and \(|\mathbf{S}| \propto R^{3}\), so it's easier to get into the precessional regime with smaller \(R\).)  This precession approach is a pathway to even higher sensitivity measurements of magnetic fields.

I think this is very cool, and it is a strong reminder that spin angular momentum is just as real as any "mechanical rotation of solids" angular momentum.  

Disorder and illumination

No, this is not another grim post about the chaotic US research funding environment.  Instead I wanted to write a bit about a good example of empiricism in experimental condensed matter physics, the use of illumination to (somewhat but not entirely mysteriously) improve electronic transport in 2D electronic systems.  

This story goes back decades, and it's all about the role of "disorder" and its effects on electronic conduction.  It's been appreciated since the 1930s that, at low temperatures so that lattice vibrations are frozen out, conduction in ordinary crystalline metals and semiconductors is limited by the charge carriers (let's work with electrons rather than holes to make discussion simpler) scattering from disorder, deviations from an infinite periodic crystal lattice.  Grain boundaries, vacancies, impurities - these all can scatter electrons that would otherwise propagate ballistically through the material, and this is often modeled as a "disorder potential", a spatially varying potential energy \(V(\mathbf{r})\).  If you want the best transport properties (longest elastic mean free path), you want \(V(\mathbf{r})\) to be small in magnitude and as smooth as possible.  This is even more important if you want to study some delicate many-body state that is expected to arise at very low temperatures - you need the disorder potential to be small compared to the energy scale of that state to avoid messing it up.

In semiconductors, where the carrier density is low and screening is therefore not as good, charged defects are particularly effective at scattering.  In modulation doping, the dopants that are the source of the charge carriers in some nearby semiconductor 2D interface or quantum well are spatially distant from where the current is going to be flowing, to minimize the scattering from those ionized donors.  

For decades it has been known that, to get the best transport properties in GaAs-based (and other) semiconductor structures, it can be good to illuminate the devices at cryogenic temperatures with a red LED.  See, for example, this paper trying to explore the upper limits of charge mobility in GaAs 2D electron gas (2DEG), where the authors say "For measurement, our samples are loaded into a 3He cryostat, where a red light-emitting diode (LED) is used to illuminate the samples for 5 min at \(T \sim \) 10 K. Following illumination, we wait for 30 min at \(T  \sim \) 10 K after the LED has been turned off before resuming the cool down to base temperature."   The qualitative explanation for this is that the photons provide enough energy to excite charge carriers, and those mobile carriers can occupy trap states, rearrange themselves, and generally set up a better screened disorder potential.  In GaAs 2DEG, the result is higher mobilities (as inferred from conductivity + Hall effect) and much cleaner fractional quantum Hall effect data, showing that the post-illumination disorder is now sufficiently weak that more delicate states can form - see Fig. 1 of this paper (arXiv version) for the before/after.  As far as I know, there is not a deep, rigorous theory of how this works, but it is known empirically.  

Fig. 2 from this paper, showing electronic magnetotransport

before/after illumination by a UV LED at low temperatures.

This preprint on the arXiv this week shows that a similar improvement in transport properties can be found in structures where graphene is encapsulated by hexagonal boron nitride (hBN).  Sandwiching graphene and other 2D materials in hBN has been known since 2010 as a way of drastically improving the charge disorder situation compared to just putting 2D materials on top of SiO2.  (That paper has 8800+ citations on google scholar btw.)  Now, it is shown that if you shine 5 eV photons (deep UV = 248 nm wavelength) on such a sandwiched structure, the already-good charge environment can become even better.  Even though that energy scale is below the band gap of hBN, the light is able to kick enough charges around to smooth out some residual disorder.  Very cool.  

FY27 Presidential budget request

To the surprise of no one at all, the 2027 presidential budget request is extremely bad for science.  Remember, this is largely a political document, and Congress does not have to follow this.  In the past year, Congress largely ignored the recommendations and appropriated a much flatter budget (though agency priorities are still set by the PBR for executive agencies).  This new request shows that Vought et al. still would prefer to kill much public funding for science.

• NSF: request to cut from $8.8B (FY26 enacted) to $4B, a 56% cut that would eviscerate the agency.
• DOE: request to cut $1.1B from $8.4B (FY26 enacted) Office of Science budget.
• NASA: request to cut $5.6B from $24.4B (FY26 enacted), including $3.7B from science programs and $1.1B from the ISS.
• Commerce: This one shocks me. Request to cut $993M from NIST's $1.184B (FY26 enacted) budget. That would be an 84% cut (!!), seemingly destroying NIST. This needs to get headlines.  Either the people making this recommendation have no idea what NIST does (seems plausible), or someone has a personal grudge against the standard kilogram. Update:  Dan Garisto on bluesky points out that the enormous cut topline number is not consistent with the budget appendix, which implies a much smaller cut.  Unclear what the answer is here - it'd be quite a goof to have a topline number that far off.
• NIH: Proposed $5.5B cut from $47.2B (FY26 enacted).
• DOD: It's very hard to tell, especially since they're proposing hundreds of billions of dollars in additional spending including for missile defense. The proposed DOD increases vastly outweigh the cuts described above. 

These cuts are proposed despite constant fretting that China is surpassing the US scientifically. This past year it took aggressive lobbying to ensure that Congress pushed back against these kinds of cuts. For those who favor continued public investment in science and engineering research in the US, the task of arguing against these kinds of cuts begins again now.  As I've written before, this is a marathon not a sprint, and this will be an annual exercise under this administration.

Brief science items and news

As I distract myself from work-related writing that I really need to do, here are a few science-related links:

• This article in ars technica is about this paper, in which a quantum interference experiment is performed that, in the most straightforward interpretation, involves superpositions of states where "event A preceded event B" and "event B preceded event A".  This is in the same mind-zapping vein as quantum eraser experiments.  I haven't read this in detail, but my typical takeaway from these things is two-fold:  (1) the math of the usual quantum mechanics formalism is in excellent agreement with experiment every time it's been checked, and (2) nature doesn't care about our hang-ups about interpretation, especially when phrasing questions about quantum in terms of, "what classical past history should have happened here, when we weren't actually making observations?"
• This paper in Advanced Materials is a very nice overview about application-relevant magnetic materials (e.g., rare-earth magnets like Nd2Fe14B) and where we might want to look to find new ones.  
• This youtube video is a great discussion and experimental proof of the Feynman inverse sprinkler problem.  The short version:  suppose an ordinary water sprinkler rotates clockwise when it's spewing water out of its arms, the momentum flux of the water generating a torque.  If instead you immersed the sprinkler in water and had suction bring water in through the arms, which way would it spin, if it spins at all?  I don't want to spoil it for you.  Watch the video - it's very nicely done.
• I'm not sure the link will work for everyone, but there is an article in The Atlantic called "The Shocking Speed of China's Scientific Rise".  For anyone paying the slightest bit of attention for the last decade, this must be some new definition of "shocking" with which I was previously unfamiliar.  
• Info is beginning to trickle out about the likely presidential budget request for FY27, and guess what:  some of the same folks who freak out that China might surpass the US as a science power are again going to pitch big budget cuts (article is about NIH, but it's hard to imagine other agencies won't be similarly viewed).  Congress largely pushed back against the PBR this past year, but this is an annual exercise in the US - it never stops.

AI and the practice of theoretical physics

Matthew Schwartz of Harvard has made a big recent splash, between his public Aspen talk "10000 Einsteins" a year ago about the role of AI and the future of physics, and his talk last week at the APS Global Physics summit on the same topic, and now with this essay, "Vibe Physics:  The AI Grad Student", on the website of Anthropic (producers of the AI tool Claude).

The essay talks about how Prof. Schwartz used Claude to write this paper, and he states that the AI tool functions roughly like a 2nd year grad student (one who also doesn't get tired or complain, but does need close checking and supervision).  The claim is that with this approach to doing calculations and writing papers, he was able to come out with a piece of work that would've taken literally ten times longer if done by working with a human student.  Note that he's not exactly unbiased, and he concludes his essay (on anthropic's site) saying you should spend the $20/month Claude subscription fee and it will change your life.  

There is no doubt that AI tools can speed up certain kinds of work, and there is a every hope that applying this in science will lead to increased pace of progress.  That said, right now these tools are (unsurprisingly) best at working in areas that are well-known and explored - one of my colleagues has tried applying these to really underexplored higher dimensional problems, and they're much less effective there.  The essay's claim that "LLMs are profoundly creative" is provocative.   There is also no discussion here about the cost of these tools, in financial, energy, and environmental terms.  

Still, Schwartz raises many questions about the future of the field and graduate education in general.   (His paragraph about how human beings will still be needed in science for getting experimental data, at least for a while, is really something.)  University research is not just about answering scholarly questions; it's about educating people.  Maybe some faculty will revel in writing papers without that kind of interaction, but somehow I don't think we're quite at the stage yet where we don't need to worry anymore about training experts in technical fields.  I do agree that it's good advice for everyone to pay close attention to where these capabilities are going.  We certainly live in interesting times.

APS March Meeting 2026, Day 4 and wrap-up

Since I headed home early this afternoon, I was only able to go to a couple of talks this morning.  Here are those highlights, and a couple of general observations about the meeting.

• Piers Coleman gave a very interesting talk that put me onto an experimental puzzle I'm sorry to say I had not seen previously.  Some context:  It is now well-established that one can do spin-polarized scanning tunneling microscopy, which (given certain constraints) can image magnetic contrast in conductors down to the atomic scale.  The mechanism is basically the same as tunneling magnetoresistance:  there is a difference in the density of states for spin-up and spin-down electrons, and so a spin-polarized (magnetic) tip results in a tunneling current into/out of a magnetic sample that depends on the local magnetization.  That is, the sign of the current doesn't affect the sign of the magnetic contrast.  I had missed this 2022 Science paper, where instead of a magnetic tip, the investigators used a tip made from a nanowire of SmB6.  That peculiar material is widely (though not universally) viewed as a topological Kondo insulator that can host special surface states in which the spin direction is locked to the current direction.  With that tip, they see magnetic contrast (!) that flips sign with the sign of the current (!!), which is at least hand-wavingly what you'd expect if the direction of the tunneling electron's spin is tied to the current direction.  A more recent paper does something similar with a (BiBr)4 tip (another topologically nontrivial material).  In the talk and related paper, the argument is made that something special happens to the surface states (the effect in SmB6 turns on below about 10 K) and that this tied to the condensed matter analog of axion physics.
• On a completely different note, I saw a talk by John Davis about a new, clever kind of continuously running refrigerator that has a base temperature of around 500 mK and uses only a couple of gas liters of 3He.  One can pump on liquid 3He and get down to about 270 mK in one-shot mode, or about 450 mK if recondensing the 3He gas with a heat exchanger to get continuous operation, but 3He is very expensive.  The new design works with a mixture that's mostly 4He.  After condensing, pumping on this can cool it sufficiently that the 3He phase separates and rises to the top of the liquid, and then the 3He can be preferentially pumped (and recirculated back in).  Very cute.
• Tangentially, one nice feature of conferences is that you can stumble upon facts you didn't know.  For example, during that talk, Prof. Davis mentioned, off-handedly, that in 2D turbulence as studied in things like helium films, you can end up with long-time persistent vortices, and that this is similar to how cyclonic storms persist for centuries on Jupiter.
• Regarding the meeting in general, the APS is aware that there were some AV issues, including some of the rooms having 50" monitors rather than projectors.  This was a surprise to the organizers.  I'm still not sure how much I like the merger of the March and April meetings into one super-meeting.  On the plus side, there are opportunities for cross-over events (e.g., the Kavli symposium, which I didn't see this time), and there are some financial benefits to the society via economies of scale.  Still, 14,000 attendees makes things unwieldy for sure.
• I don't understand some of the choices re the meeting website and the meeting app.  For example, people can upload their slides and make them available.  However, on the meeting website, even when you're logged in, there's apparently no way to get to them.  You can only find the files using the APS meetings app, and even then it's not trivial.  

For those at the meeting the rest of today and Friday, if there are big stories that I missed because of my travel, please feel free to discuss in the comments.

APS March Meeting 2026, Day 3

It was another eclectic day at the APS Global Physics Summit.  Here is a selection of highlights based on my stochastic sampling of talks.

• I've written before about CISS (the chirality-induced spin selection effect).  Joe Subotnik gave a neat invited talk related to this, based on something I'd never really considered.  In physics we learn about the Born-Oppenheimer approximation, which basically says that electrons are fast and nuclei are slow, so we can often solve electronic problems without worrying about nuclear motion.  In practice, as usually done, B-O theory does not strictly conserve momentum or angular momentum, so it cannot explain something like the Einstein-de Haas effect, where flipping electronic spins eventually results in actual mechanical rotation of a solid.  Similarly, ordinary Marcus theory of electron transfer doesn't worry about angular momentum conservation.   The talk focused on a recent approach (and here) that looks carefully at wavefunctions, involves the equivalent of Berry phase and quantum geometry and recaptures the key physics, and this may explain CISS.
• Javad Shabani presented his group's recent work on growing epitaxial layers of germanium substitutionally doped with gallium, at carrier densities around \(5 \times 10^{21}\) carriers per cc, basically around 1 Ga atom in each 8-atom unit cell.   This hole-dopes the material enormously.  The resulting films superconduct with a \(T_{c}\sim\) 3 K and good critical fields, and look very nice structurally.  This is potentially a route toward creating arrays of millions of epitaxially nice Josephson junctions.
• I attended the AI Town Hall, which featured Hal Finkel from DOE talking about the Genesis Mission; Rachel Burley, chief publication officer of the APS, speaking about the challenges that AI presents to all facets of journals and scientific publishing; and Sarah Demers, chair of the physics department at Yale and chair of the APS's Panel on Public Affairs, discussing the community's effort to formulate an enduring position on physics and AI in this rapidly changing landscape.  
• In the last session of the day, I attended the DCMP prize session, and it was very interesting to hear from this year's Buckley Prize winners about their journeys and what they've been doing lately.  

In addition to a lot of fun conversations, I popped in and out of a few other talks, and apologies for not covering everything.

APS March Meeting 2026, Day 2

Today was again a bit random, as I had talks for both one of my students and me, and meetings with folks.  Some highlights:

• Edoardo Baldini gave a very nice talk about exotic phases and collective excitations in van der Waals magnets.  This included using second harmonic generation microscopy and polarimetry to look at the evolution of magnetic phases in NiPS3 as a function of thickness, ending up at the monolayer which acts like a 2D XY magnet.  In the paper, they see clear evidence of a BKT transition, plus a second lower temperature ordering of some kind.
• After some AV issues (seem like quite a few of those this year), Barry Zink gave an interesting presentation about using Cr as a spin detector in spin Seebeck measurements on YIG, and looking at how the antiferromagnetism of the Cr affects the measurement (see here).  In new results, they have been adding in an intervening layer of antiferromagnetic IrMn, looking at how magnons in the IrMn affect the results.
• This was followed by a talk by Romain LeBrun all about spintronics in the GHz and THz, using hematite (Fe2O3) as an example antiferromagnetic insulator.  They see some interesting nonlinear dynamics and rectification in antiferromagnetic resonance in Fe2O3.  In NiO, they use optical excitation to drive coherent phonons, exciting a spin current, which then leads to a THz pulse when the spin current hits an inverse spin Hall detector (Pt or W).  Similar experiments in BiFeO3 show that THz generation in that multiferroic system can arise just from oscillating the ferroelectric polarization.
• Andrew Dane from IBM gave a great presentation to a more-than-packed room about their recent studies of two-level systems in qubits.  From waaaaay in the back, I learned about their use of nearby suspended electrodes to apply electric fields to try to shift the energies of some of the TLS (the ones with electric dipole moments and at the surface of the devices).  TLS drastically suppress the coherence of superconducting qubits, and understanding their origins and ways to work around them (to characterize fab processes, for example) is very important.  As I said in that post linked above, once again we see that TLS are everywhere, and they are evil.  I need to think about whether there's anything I could contribute on this.  The real highlight of the talk was the use of "percussive maintenance" (banging the side of the cryostat) to alter the not-field-tunable TLS distribution via some unknown mechanism.
• Bonking the experiment was taken to a new level in this talk about mechanoluminescence, which involved shooting the sample with an airsoft pellet gun under controlled conditions.

There were other talks as well - some fun stuff.  I also want to give a shoutout to the free-to-play vintage arcade games in the exhibit hall.  Galaga and the stand-up vector graphics Star Wars game were great consumers of my time and my quarters back in the day.

APS March Meeting 2026, Day 1

I hit a pretty random assortment of talks on my first day at the APS Global Physics Summit, after catching a very early flight to get to Denver.  Here are a few highlights:

• My colleague Hanyu Zhu gave a nice talk about the coupling between chiral phonons (vibrational excitations of atomic motion that carry net orbital angular momentum) and their coupling to electronic spins.  For example, chiral ionic motion can effectively generate enormous local magnetic fields (see here).
• I went to part of a session about magnons (quantized spin waves) and their connection to quantum information.  There was a theory talk by Silvia Viola Kusminskiy about cavity manipulation of magnons, and there was an experimental talk by Mathias Weiler about using surface acoustic waves plus magnetoelastic coupling to set up all sorts of interesting nonreciprocal magnetoacoustic devices. 
• My former postdoc Longji Cui gave a talk about molecular phononics - measuring thermal transport (by phonons) down to the single molecule level.  For a nice review of the overall topic, see here.  He then discussed extending this to measurements of polymers.
• There was a session about strange metals and the cuprates, which included a talk by Dragana Popovic about how there is evidence for persistent vortex-liquid-like phase fluctuations in these materials even into the normal state.  This was followed by Nigel Hussey showing systematic studies of the magnetoresistance in both electron-doped and hole-doped cuprates.  The upshot is that in the electron-doped materials, there is a clear anisotropic inelastic scattering rate (from spin fluctuations) that scales with the superconducting transition, implying that spin fluctuations are the "glue".  In contrast, the hole-doped system has different systematics, implying that perhaps the strange metal fraction of material is what leads to superconductivity.
• For maybe the second time in my long attendance at the meeting, I attended the APS prize session, where they present the certificates associated with the various honors.  It was very nice.

Now I just need to get some sleep and figure out what to see tomorrow....