Friday, March 18, 2022

APS March Meeting 2022, Day 4 and wrap-up

I gave my contributed talk this (Fri) morning, and I will head to the airport shortly, so this is the end of my March Meeting blogging.  A few highlights from yesterday:

• Konrad Lehnert gave a very nice, pedagogical talk about the possibility of detecting axionic dark matter using quantum sensing.  The super short version:  it is thought that axions if they exist can, in the presence of a large magnetic field, convert at some rate into photons with energy $\hbar \omega = m_{\mathrm{a}}c^2$.  In a microwave cavity, it is possible to detect such excess photons, and by doing clever things with "squeezing", it is possible to beat the standard quantum limit and to examine parameter space more rapidly than otherwise.  There is still a lot of room for improvement if one wants to be able to look across the whole range of potential axion masses and not have it take years and cost a gazillion dollars.  One approach using entanglement can eliminate a number of confounding factors.
• I saw two very clear talks, one by Kevin Nuckolls and one by Stevan Nadj-Perge about using STM and tunneling (and point contact) spectroscopy to examine superconductivity in magic-angle twisted bilayer and trilayer graphene, respectively.  In the former, one challenge is to decide how much of the observed gap features in tunneling are due to superconductivity, and then using the functional form of that superconducting part to consider pairing mechanisms.  It is also possible to see how band flattening increases the density of states even at angles away from the magic angle.
• In a different session, Inti Sodemann spoke about whether and how it is possible to get current rectification in semiconductors when they are illuminated by light with energy below the band gap, so that there is no absorption.  There are thermodynamic restrictions that come in - you can't get energy from nowhere, and you can't break the second law.  Thanks to Berry curvature effects, it is actually possible to have this kind of rectification under some circumstances.
• There was another extremely clear talk by N. Peter Armitage about Co-containing compounds as Kitaev spin liquid candidates.  There was some really great THz absorption data as a fn of temperature and magnetic field for CoNb2O6 that had amazing agreement with theory, and newer results looking at a more 2D system, BaCo2(AsO4)2.
• Unfortunately I was unable to attend the Kavli Symposium.  I hope to be able to watch the talks later, as these are typically of very high quality and general interest.
Closing thoughts:
• It was nice and kind of weird to finally see a good number of people in person.  Really great to catch up with old friends, though I think my conference stamina has waned since the 2019 meeting.
• When the participants skew younger, as seemed to be the case this year, the crowd definitely looks more diverse.  It would be interesting to know the demographics of the attendees.
• I don't think pre-recorded short talks work well.  The inability to ask/answer questions is a problem.
• I wonder if we will have hybrid meetings in general from now on.  There are definitely environmental impact reasons to go that way, and it would help solve the APS's problem that prior to covid the meeting had grown so large that it was difficult to plan or host.

Wednesday, March 16, 2022

APS March Meeting 2022, Day 3

Highlights are brief today, because I spent more of my time seeing talks from my group and chatting with people:

• Started the day with the Keithley Prize session, and Dan Rugar talking about the history of magnetic resonance force microscopy.   Very interesting and educational.  It is inspiring to see the evolution of a technique, from the genesis of the idea (an early paper here) to initial testing to advanced developments.
• Later I saw Marcel Franz give a very clear talk about how to try to build a topological superconductor (fully gapped with topologically protected chiral edge modes) by stacking individual cuprate layers rotated by 45 degrees with respect to each other.
• There was a neat talk by Naomi Ginsberg on her group's pump-probe interferometric technique ("stroboSCAT") that allows them to visualize and separate the diffusion of heat and the diffusion of charge in various materials.  For a review, see here.
• Later in the day I bopped back and forth a bit between the Buckley/Isakson/Onsager Prize session and a session about the BCS/BEC crossover in condensed matter systems.  It was pretty neat hearing Emmanuel Rashba speak.
Now to figure out what to see tomorrow....

Tuesday, March 15, 2022

APS March Meeting 2022, Day 2

It was a busy day today, and I saw a lot of talks, some live and in person, some live via zoom, and some prerecorded.  Some highlights:

• Started off with Nai Phuan Ong's invited talk about their recent results on thermal transport measurements in the proximate Kitaev spin liquid material $\alpha$-RuCl3.  His group performs measurements of the longitudinal thermal conductivity $\kappa_{xx}$ and the thermal Hall conductivity $\kappa_{xy}$ in this system by gluing tiny thermometers to the very delicate crystals.  They see some remarkable results, including evidence that there are heat-carrying bosonic (not fermionic) edge modes.
• In a different session, I heard Dan Ralph talk about a variety of issues involved in really properly understanding all the pitfalls that can come into interpreting the different experimental attempts to measure SO torque efficiency.
• This was followed by a nice talk by Alex McLeod about nanophotonic near-field probes of correlated materials.  The talk included a great history of the field, including this paper that I'd somehow never seen before.
• Anand Bhattacharya gave a nice presentation about his group's work on 2D superconductivity at the interface of KTaO3 with other oxides, especially the dependence on crystallographic orientation.  They have another recent paper that explains features of the gate dependence of the superconducting transition, and there is a theoretical proposal for the underlying mechanism.
• Garnet Chan spoke on a very interesting topic:  Is there an exponential quantum advantage (relative to classical computing) to be had in using quantum computers to try to solve theoretical chemistry problems such as finding the ground state of a large molecule or material?  Such an advantage requires, among other things, that classical methods exponentially poorly with the problem size, and that initial state preparation of the quantum system is not exponentially difficult.  The short answer:  it's not clear that this is the case.  (Quantum computers could still be very useful for quantum chemistry.  Here is a relevant review article.)
• Mathieu Taupin from TU Wien spoke about superconductivity at very low temperatures in the quantum critical strange metal YbRh2Si2, and also about whether these kinds of heavy fermion strange metals are "Planckian".
• I also heard Linda Ye present data showing that Ni3In, a "kagome flat band" material, is a strange metal.  In this kind of system, because of the lattice structure and its symmetry, there is particular destructive quantum interference that happens to disfavor electronic hopping between certain lattice sites - see here.  As a result, the electrons in that band tend to localize, leading to an energy band that is flat.  In this system, that band sits at the Fermi level, and strange metallicity seems to result.
Besides the talks, I also got to see and catch up with a number of friends and colleagues for the first time since the pandemic started.  The exhibition show part of the meeting has changed quite a bit. It's really amazing how big a difference three years makes in terms of the exhibitors. Now it seems like 75% of the vendors there are "quantum"-related.

Monday, March 14, 2022

APS March Meeting 2022, Day 1

My first impressions of this year's March Meeting are a bit limited, since I flew today and didn't make it to the convention center until around 4pm.  Still, a few thoughts:

• Population density at the meeting does seem lower than 2019, though that could partly be because the convention center is enormous.  Attendance also seems to skew younger this year.
• It was interesting attending a contributed session, with a mix of in-person speakers and the session chair playing pre-recorded presentations from those who could not or chose not to be present.  It seems to work ok, though the lack of Q&A for the recorded talks takes some getting used to.  In asking around, I get the impression that the full live streaming interactive Q&A approach from last year's virtual meeting was very expensive to implement, and that's one reason why the method this year is different (with live streaming only for invited talks).
• While I was getting my bearings, I popped into a session about spin transport by electrons and magnons.  I saw a talk where an interesting "spin diode" effect takes place in a thin film multilayer structure consisting of (from one side to the other) permalloy/gold/platinum/cobalt.  Driving the permalloy layer into ferromagnetic resonance can effectively pump spin into the gold and so forth, and in that direction spin current flows and is absorbed in the cobalt.  However, if one instead drives the cobalt layer to try to push a spin current the other way, the permalloy does not act like a "sink".  This can be modeled and the directionality makes sense.
• I saw a second talk that somewhat similar in spirit, in which magnons are launched into an insulating magnet using the spin Hall effect in a Pt wire, and then detected by another Pt wire via the inverse spin Hall effect.  By placing a NiFe pad between the Pt wires, it is possible to make it so that magnons transport more easily from one Pt wire to the other than vice versa.  This work is described here.
• Then I went to the APS special session about Ukraine.  I expected this to be largely about how to help displaced scholars and scientists, but it was more complex, and most of the time was a listening session for the APS president and the CEO.  The APS statements about the Russian invasion of Ukraine are here.  There are many issues.  For example, 200+ rectors of top Russian universities and institutes signed an open letter supporting the war. Should the APS still allow those places to have journal access? Faculty members there to have society membership and privileges? What is mandated as a result of sanctions?  There are many Ukrainian and Russian and Belarusian physicists in the APS, and feelings are intense.  On an historical note, the question was raised about what if anything the APS did in regard to German physicists and institutions (like the German Physical Society and the Kaiser Wilhelm Institutes) when Germany invaded Poland in 1939 - can any readers point to a record of what the APS actually did?  I spent a while googling and could find nothing.
It is good to see folks face to face, even if attendance is down.  It's been a long time.

Thursday, March 10, 2022

2022 March APS Meeting - coming soon.

I will actually be attending the 2022 March APS Meeting in Chicago next week, so look for posting to pick up as I try to be good about my annual routine of reporting some highlights.  I have been so busy in recent weeks that I really have not had time to go through the meeting program in any depth; if there are particularly exciting sessions that people want to recommend, please point them out in the comments.