It’s been too long since I last wrote. What can I say – 2020! I hope to get back to more regular posts, and mostly focus on physics. What’s new since the last time I wrote?

My group has written a few papers:

• Dynamical signatures of quasiparticle interactions in quantum spin chains – With Anna Keselman and Oleg Starykh, we managed to uncover some unappreciated physics in one dimensional Heisenberg antiferromagnetic chains. These are some of the most iconic and heavily studied models in theoretical physics, so it’s surprising to find anything new in them! Yet it turns out there are some beautiful lessons to be learned here about interactions between quasiparticles.
• Hybrid Wannier Chern bands in magic angle twisted bilayer graphene and the quantized anomalous Hall effect – With my student Kasra Hejazi and former postdoc Xiao Chen. We (really mostly Kasra!) reformulated the now-famous continuum model for twisted bilayer graphene in terms of hybrid Wannier states, which circumvents all topological obstructions, and helps to make Chern number physics more transparent.
• Heterobilayer moiré magnets: moiré skyrmions, commensurate-incommensurate transition and more – This is actually our (Kasra Hejazi, Zhu-Xi Luo and myself) second paper on physics arising from moiré patterns in magnetic two dimensional materials. Here we wanted to see how the length scales of moiré patterns interact with those of spiral magnetism and skyrmions in non-centrosymmetric magnets. It could be a powerful way of manipulating the latter.
• Frustrated Heisenberg J₁−J₂ model within the stretched diamond lattice of LiYbO₂: This is a collaboration with Stephen Wilson’s experimental group in the UCSB materials department. His student Mitchell Bordelon spearheaded this project, uncovering an interesting rare earth effective spin-1/2 antiferromagnet. My student Chunxiao Liu took the reins on the theory side and came up with a rather appealing simple model which explains most (but not all – it’s probably better to keep some mystery in the world) of the main features of Mitchell’s experiments. For the experts, the story places this compound close to the competing exchange models for “spiral spin liquids” studied for the honeycomb and diamond lattices.

Apart from these papers, of course there are more in the works, and I am really hoping to finish a few in particular that have been in the pipeline for months to years in some cases (hopefully the best papers are like fine wine and need some time to mature). We’ve been running virtual group meetings, and we have a new crop of KITP graduate fellows for fall who are attending virtually. Some new KITP postdocs in condensed matter have arrived: Wenjie Ji and Jong Yeon Lee are here in Santa Barbara and Urban Seifert is at least part-time here in spirit, while enjoying an intermediate-term position with Lucile Savary’s group at ENS Lyon. The KITP’s new Correlated program has started, and that’s been a fun meeting twice a week (but unfortunately we have to start at 8am PST to keep the Europeans in the loop Zoom/pandemic style) – great job finding interesting speakers Natasha, Lucile, George and Oskar! KITP has also tried a new initiative: short virtual conferences organized by KITP postdocs. The one last week organized by Urban Seifert and Zhu-Xi Luo was excellent!

I’ve started teaching for the fall, of course virtually – doing the first quarter of graduate condensed matter physics, i.e. solid state physics, bands, phonons, transport, etc. Virtual teaching is its own experience. Going full gaucho – I’m uploading my hand-written ipad notes from class to UCSB’s Gauchospace, and recordings of the video of the lectures to UCSB’s Gauchocast. It’s not clear where my students are. I do know that one of them is in Kazakhstan – perhaps not the most obvious place to try to get a US visa, but apparently that’s the place for this fellow. Poor guy is tuning into my lecture at midnight his time… Anyway, I’m trying/hoping to present an “updated classic” take on it, and writing my own latex lecture notes along the way. I spent quite some time learning more about Thomas-Fermi theory and figuring out how to present some of that. Now I’m on to more conventional material and it’s certainly easier going.

And today I got confirmation of receipt of my 2020 ballot: