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.
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