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....

Some science leading into the APS Global Physics Summit

Next week is the annual APS conference that was once the March Meeting and is now the combined March/April "Global Physics Summit".  As I've done annually, I will try to give some impressions of interesting talks that I see, hopefully at an understandable level.  This year I'm only there from Monday through late Thursday morning, so I may miss exciting things - hopefully people will still discuss such things here as has happened in past years.

A few science tidbits in the meantime:

• People sometimes time arXiv submissions to coincide with the APS meeting, and sometimes it's just coincidence.  Two preprints (here and here) popped up very recently, both experiments on interferometry and braiding of anyons in bilayer graphene.  There are many subtleties in such experiments.  The colorized electron microscope images of the devices show how sophisticated fabrication has become in these systems, where very small amounts of disorder can disrupt the fragile many-body quantum states of interest.
• On a much more classical physics note, this preprint uses some sophisticated multiscale modeling to address the question, why is ice so slippery?  A super-thin layer of water on the surface of the ice under sliding conditions is crucial, and the roles of frictional heating and heat transfer have been tricky to quantify.
• Meanwhile, across town from me at the University of Houston, Paul Chu and company have published this paper in PNAS, where they have demonstrated ambient pressure superconductivity in a mercury-based cuprate at 151 K, breaking the old ambient pressure record by 18 K (!).  The trick here has been pressure annealing.  Many superconductors, particularly the cuprates, tend to have higher transition temperatures at elevated pressures.  One idea is that pressure distortion of certain bond angles favors superconductivity in this system, and Chu et al. have been exploring the idea of cycling pressure and temperature to "lock in" the altered crystal structure.
• Moving away from condensed matter and turning to science used in the aid of history:  When Vesuvius erupted in 79 CE, the pyroclastic flow swept through Herculaneum and a nearby Roman villa, housing a library of more than 1800 now-carbonized scrolls.  Using 3D x-ray tomography, it is hoped that these scrolls may actually be read without trying to physically unroll them, prompting the Vesuvius Prize.  This effort, involving x-ray imaging and AI methods, seems to be bearing fruit.  There may be many more scrolls still buried as well.  It would be amazing if great lost works of ancient Greek and Roman literature could be recovered.
• Tangentially related to science, the arXiv is looking for a CEO - here is the position description.  It's hard to overstate the impact of the arXiv and its relations in terms of open science, and in the chaotic world of scientific publishing, it's more important than ever.
• If you need evidence of how screwed up scientific publishing is, apparently Springer-Nature has been surveying people to see how willing they would be to pay an up-front fee (e.g. $299) just for the privilege of submitting an article.  

  

RIP Tony Leggett

It's been an extremely busy time, and there are all kinds of distressing events afoot.  Talking about new science results or the funding situation can seem self-indulgent when there are ongoing global events of huge impact.  That said, it's important not to lose sight of the humanity in the global physics community.  This past weekend, Tony Leggett passed away (wikipedia page here).   

(image from UIUC)

Prof. Leggett was a soft-spoken, kind person who was also a brilliant theoretical physicist.  I was fortunate enough to first meet him back when I was a graduate student working in Doug Osheroff's lab.  Doug had discovered (along with his thesis advisors Bob Richardson and Dave Lee) the superfluid phases of the rare isotope of helium, 3He in 1972.  

[Science digression:  3He atoms are fermions - if you add up the spin angular momentum from the two protons, the neutron, and the two electrons, you end up with a net spin of 1/2.  To condense into a superfluid state, by analogy with electrons in superconductors, the 3He atoms need to pair up, and it's the pairs that condense into the superfluid.  This pairing ends up being quite complicated; the pair of 3He atoms end up having \(\ell = 1\) orbital angular momentum, and this implies that the nuclear spins of the 3He atoms in the pair have to form a triplet.  Prof. Leggett figured out a ton of the insights on this topic - see here for an early paper on this, and here for a definitive review c. 1975.]

Prof. Leggett made many contributions beyond 3He.  For example, he and others studied the problem of a tunneling particle coupled to some dissipative environment (like phonons, say), and similarly of a two-state quantum system coupled to a "bath", as in this paper with several thousand citations.  These both had close connections to the "measurement" problem in quantum mechanics - how in detail do you go from a highly quantum system (e.g., a particle tunneling out of a bound state, or a particle coherently oscillating back and forth) and end up with more classical-looking outcomes due to coupling to "baths" with large numbers of degrees of freedom?  He was interested in these kinds of foundational quantum issues all the way along (see this 1980 paper) and was still writing about them within the last couple of years.  Prof. Leggett also wrote important tutorial reviews of superfluidity and of Bose-Einstein condensation in ultracold gases.  When I got to meet him on a trip through Stanford, I was introduced to the ideas that he and Clare Yu developed about tunneling two-level systems in solids - looking at the big question of why the properties of TLS in disordered solids are so universal even though the materials can be very different at the microscopic level.  He was a great scientist while also being a kind person.

AI/ML, multiscale modeling, and emergence

I've been attending a lot of talks lately about AI/machine learning and multiscale modeling for materials design and control.  This is a vast, rapidly evolving research area, so here is a little background and a few disorganized thoughts.  

For a recent review article about AI and materials discovery, see here.  There is a ton of work being done pursuing the grand goal of inverse design - name some desired properties, and have AI/ML formulate a material that fits those requirements and is actually synthesizable.  Major companies with publicly known efforts include Google Deepmind and GNoME,  Microsoft, Meta working on catalysts, Toyota Research Institute, IBM, and I'm certain that I'm missing major players.  There are also a slew of startup companies on this topic (e.g. Periodic).  

In addition to materials design and discovery, there is enormous effort being put into using AI/ML to bridge across length and timescales.  Quantum chemistry methods can look at microscopic physics and chemistry, for example, but extending this to macroscopic system sizes with realistic disorder is often computationally intractable.  There are approaches like time-dependent DFT and DMFT to try to capture dynamics, but following dynamics even as long as picoseconds is hard.  Using microscopic methods and ML to try to compute and then parametrize force fields between atoms (for example), one can look at larger systems and longer timescales using molecular dynamics for atomic motions.  However, getting from there to, e.g., the Navier-Stokes equations or understanding phase boundaries, is very difficult.  (At the same time, there are approaches that use AI/ML to learn about the solutions of partial differential equations, so that one can, for example, compute good fluid flows quickly without actually having to solve the N-S equations - see here.) 

We want to keep coarse-graining (looking at larger scales), while maintaining the microscopic physics constraints so that the results are accurate.  There seems to be a lot of hope that either by design or by the action of the AI/ML tools themselves we can come up with descriptors that are good at capturing the essential physics as we move to larger and larger scales.  To use a fluids example, somehow we are hoping that these tools will naturally capture that at scales much larger than one water molecule, it makes sense to track density, temperature, velocity fields, surface tension, liquid-vapor interfaces, etc.  

From the always fun xkcd

One rough description of emergence is the idea that at larger scales and numbers of constituents, new properties appear for the collective system that are extremely difficult to predict from the microscopic rules governing the constituents.  For example, starting from the Schroedinger equation and basic quantum mechanics, it's very hard to determine that snowflakes tend to have 6-fold symmetry and ice will float in water, even though the latter are of course consequences of the former.  A nice article about emergence in physics is here.  

It feels to me like in some AI/ML endeavors, we are hoping that these tools will figure out how emergence works better than humans have been able to do.  This is certainly a worthy challenge, and it may well succeed in a lot of systems, but then we may have the added meta-challenge of trying to understand how our tools did that.  Physics-informed and structured ML will hopefully take us well beyond the situation in the xkcd comic shown here.