Friday, March 29, 2024

Thoughts on undergrad solid-state content

Figuring out what to include in an undergraduate introduction to solid-state physics course is always a challenge.   Books like the present incarnation of Kittel are overstuffed with more content than can readily fit in a one-semester course, and because that book has grown organically from edition to edition, it's organizationally not the most pedagogical.  I'm a big fan of and have been teaching from my friend Steve Simon's Oxford Solid State Basics, which is great but a bit short for a (US) one-semester class.  Prof. Simon is interested in collecting opinions on what other topics would be good to include in a hypothetical second edition or second volume, and we thought that crowdsourcing it to this blog's readership could be fun.  As food for thought, some possibilities that occurred to me were:

  • A slightly longer discussion of field-effect transistors, since they're the basis for so much modern technology
  • A chapter or two on materials of reduced dimensionality (2D electron gas, 1D quantum wires, quantum point contacts, quantum dots; graphene and other 2D materials)
  • A discussion of fermiology (Shubnikov-DeHaas, DeHaas-van Alphen) - this is in Kittel, but it's difficult to explain in an accessible way
  • An introduction to the quantum Hall effect
  • Some mention of topology (anomalous velocity?  Berry connection?)
  • An intro to superconductivity (though without second quantization and the gap equation, this ends up being phenomenology)
  • Some discussion of Ginzburg-Landau treatment of phase transitions (though I tend to think of that as a topic for a statistical/thermal physics course)
  • An intro to Fermi liquid theory
  • Some additional discussion of electronic structure methods beyond the tight binding and nearly-free electron approaches in the present book (Wannier functions, an intro to density functional theory)
What do people think about this?

Sunday, March 24, 2024

Items of interest

The time since the APS meeting has been very busy, hence the lack of posting.  A few items of interest:

  • The present issue of Nature Physics has several articles about physics education that I really want to read. 
  • This past week we hosted N. Peter Armitage for a really fun colloquium "On Ising's Model of Magnetism" (a title that he acknowledged borrowing from Peierls).  In addition to some excellent science about spin chains, the talk included a lot of history of science about Ising that I hadn't known.  An interesting yet trivial tidbit: when he was in Germany and later Luxembourg, the pronunciation was "eeesing", while after emigrating to the US, he changed it to "eye-sing", so however you've been saying it to yourself, you're not wrong.  The fact that the Isings survived the war in Europe is amazing, given that he was a Jew in an occupied country.  Someone should write a biography....
  • When I participated in a DOD-related program 13 years ago, I had the privilege to meet General Al Gray, former commandant of the US Marine Corps.  He just passed away this week, and people had collected Grayisms (pdf), his takes on leadership and management.  I'm generally not a big fan of leadership guides and advice books, but this is good stuff, told concisely.
  • It took a while, but a Scientific American article that I wrote is now out in the April issue.
  • Integrating nitrogen-vacancy centers for magnetic field sensing directly into the diamond anvils seems like a great way to make progress on characterizing possible superconductivity in hydrides at high pressures.
  • Congratulations to Peter Woit on 20 (!!) years of blogging at Not Even Wrong.  

Thursday, March 07, 2024

APS March Meeting 2024, Day 4 and wrap-up

Because of the timing of my flight back to Houston, I really only went to one session today, in which my student spoke as did some collaborators.  It was a pretty interesting collection of contributed talks.  

  • The work that's been done on spin transport in multiferroic insulators is particularly interesting to me.  A relevant preprint is this one, in which electric fields are used to reorient \(\mathbf{P}\) in BiFeO3, which correspondingly switches the magnetization in this system (which is described by a complicated spin cycloid order) and therefore modulates the transmission of spin currents (as seen in ferromagnetic resonance).  
  • Similarly adding a bit of La to BiFeO3 to favor single ferroelectric domain formation was a neat complement to this.
  • There were also multiple talks showing the utility of the spin Hall magnetoresistance as a way to characterize spin transport between magnetic insulators and strong spin-orbit coupled metals.
Some wrap-up thoughts:
  • This meeting venue and environment was superior in essentially every way relative to last year's mess in Las Vegas.  Nice facilities, broadly good rooms, room sizes, projectors, and climate control.  Lots of hotels.  Lots of restaurants that are not absurdly expensive.  I'd be very happy to have the meeting in Minneapolis again at some point.  There was even a puppy-visiting booth at the exhibit hall on Tuesday and Thursday.
  • Speaking of the exhibit hall, I think this is the first time I've been at a meeting where a vendor was actually running a dilution refrigerator on the premises.  
  • Only one room that I was in had what I would describe as a bad projector (poor color balance, loud fan, not really able to be focused crisply).  I also did not see any session chair this year blow it by allowing speakers to blow past their allotted times.
  • We really lucked out on the weather.  
  • Does anyone know what happens if someone ignores the "Warning: Do Not Drive Over Plate" label on the 30 cm by 40 cm yellow floor plate in the main lobby?  Like, does it trigger a self-destruct mechanism, or the apocalypse or something?
  • Next year's combined March/April meeting in Anaheim should be interesting - hopefully the venue is up to the task, and likewise I hope there are good, close housing and food options.

Wednesday, March 06, 2024

APS March Meeting 2024, Day 3

My highlights today are a bit thin, because I was fortunate enough to spend time catching up with collaborators and old friends, but here goes:
  • Pedram Roushan from Google gave an interesting talk about noisy intermediate-scale quantum experiments for simulation.  He showed some impressive data looking at the propagation of (simulated) magnons in the 1D Heisenberg spin chain.
  • In the same session, Lieven Vandersypen from Delft presented their recent results using gate-defined Ge/SiGe quantum dot arrays to simulate a small-scale version of the Hubbard model.  Looking at exciton formation and propagation in a Hubbard ladder while being able to tune many parameters, the data are pretty neat, though I have to say it seems like scaling this up to large arrays will be extremely challenging in terms of layout and tuning.  He also showed some in-preparation work on spin propagation in similar arrays - neat.
  • In a completely different session, Jacques Prost, recipient of this year's Onsager Prize, gave an interesting talk about broken symmetries and dynamics of living tissue.  This included cell motion driven by nematicity (living tissue as liquid crystal....) and how in a cylindrical environment this can lead to rotation of growing tissue.  These sorts of interactions in "active matter" can be related to how tissue grows and differentiates in living systems.
  • My colleague Gustavo Scuseria is this year's recipient of the Aneesha Rahman Prize, and he gave a good explanation of his group's recent work on using dualities to map strongly correlated models onto more tractable (polynomial-growth rather than exponential growth in problem size) equivalent weakly correlated models.
  • In a session on quantum spin liquids, Tyrel McQueen of Johns Hopkins spoke about two examples of his group's recent work.  Chemical substitution can help tune interactions in a Kitaev spin liquid candidate, and they've also examined the controlled interplay of charge density waves and magnetic order.  The talk did a great job of conveying a taste of the breadth and depth of the space of quantum magnets.
  • Lastly, Chih-Yuan Lu, recipient of this year's George E. Pake Prize, gave a very nice historical overview of the development of semiconductor electronics from the integrated circuit to the present frontiers (of gate-all-around transistors and 3D integrated NAND memory).
Two other notes not directly germane to the APS meeting:
  • The AAAS appropriations tracker shows how outlays for the coming year are shaping up for NSF and the other agencies.  </begin rant>Can someone explain to me why the conference NSF budget allocation for research ends up -8.5%, when the House pushed +0.3% and the Senate pushed -2.9%? Also, cutting the STEM education budget (which includes GRFP) by 28% seems terrible.  Griping about US STEM competitiveness and the need for developing the next-generation technical workforce, while simultaneously cutting research training resources:  Congress in action.   Once again, they feel good about supporting the authorization of doubling the NSF budget over five years, but don't actually want to appropriate the funds to do it.  </end rant>
  • Purely by random chance (ahem), I want to point to this column.

Tuesday, March 05, 2024

APS March Meeting 2024, Day 2

A decent part of today was spent in conversation with friends and colleagues, but here are some high points of scientific talks:

More tomorrow....

Monday, March 04, 2024

APS March Meeting 2024, Day 1

There is no question that the meeting venue in Minneapolis is superior in multiple ways to last year's meeting in Las Vegas.  The convention center doesn't feel scarily confining, and it also doesn't smell like a combination of cigarettes and desperation.

Here are a few highlights from the day:

  • There was an interesting session about "polar materials", systems that have the same kind of broken inversion symmetry within a unit cell as ferroelectrics; this includes "polar metals" which host mobile charge carriers.  One polar material family involving multiferroic insulators was presented by Daniel Flavi├ín, in which dielectric (capacitance) measurements can show magnetic quantum critical phenomena, as in here and here.  Both sets of materials examined, Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12, show remarkable dielectric effects due to fluctuating electric dipoles, connected to quantum critical points at B-field driven transitions between magnetic ordered states.
  • Natalia Drichko from Johns Hopkins showed Raman spectroscopy data on an organic Mott insulator, in which melting charge order is connected to spin fluctuations.
  • Pavel Volkov from U Conn discussed doped strontium titanate (STO), an example of an incipient polar metal, and looking at how polar fluctuations might be connected with the mechanism behind the unusual superconductivity of STO. 
  • The last talk of that session that I saw was Pablo Jarillo-Herrero giving a characteristically clear presentation about sliding ferroelectricity.  Taking a material like hBN and trying to stack a bilayer with perfect A-A alignment is not energetically favored - it's lower in energy if the two layers shift relative to each other by a third of a lattice parameter, resulting in an out-of-plane electric dipole moment, pointing either up or down depending on the direction of the shift.  Applying a sufficiently large electric field perpendicular to the plane can switch the system - this works on TMDs as well.  Putting a moire bilayer in the mix, and you can get some neat charge ratcheting effects
  • The session on transport in non-Fermi liquids was fun and informative.  I thought the discussion of possible intrinsic nonlinear transport in strange metals was intriguing.
  • I also saw a couple of interesting invited talks (here and here) about experiments that try to use electronic transport in adjacent layers to probe nontrivial magnetic properties of adjacent spin ices.  Very cool.
More tomorrow....

Sunday, March 03, 2024

APS March Meeting 2024 - coming soon

This week I'm going to be at the APS March Meeting in Minneapolis.  As I've done in past years, I will try to write up some highlights of talks that I am able to see, though it may be hit-or-miss.  If readers have suggestions for sessions or talks that they think will be particularly interesting, please put them in the comments.