It’s been too long since I last wrote. One tends to get busy. Quite a bit has happened since April. I went to a rock concert with my daughter for the first time. I lectured on quantum spin liquids at a summer school in Capri (nice place!). I attended a conference in Tbilisi, in the republic of Georgia – an extremely interesting destination full of ancient and modern history – and a GRC in Hong Kong which impressed upon me how far topological materials have come. I also got a glimpse of the protests there, which is inspiring in another way.

A few new initiatives are starting. I’m happy to report we’ve received a renewal of the KITP’s EPiQS grant from the Moore Foundation, to support postdocs in quantum materials theory for the coming 5 years. I’m also part of a new Simons Collaboration on Ultra-Quantum Matter, which starts in September.

Our group has put out a few new papers/preprints. I’ll give something of a quick summary:

- With Oleg Starykh, we posted a paper about the dynamical structure factor of the U(1) quantum spin liquid with a spinon Fermi surface in two dimensions, in an applied magnetic field. It’s the first time I really fully engaged with this challenging problem, described theoretically by fermions coupled to a Landau-damped gauge field, in order to try to really sort out some of its spectral properties. We found surprisingly that there should be a sharp (not infinitely well-defined) collective mode which could be sought experimentally.
- Chunxiao finally posted his encyclopedic paper (with Gábor Halász) on Z2 spin liquids on the pyrochlore lattice and proximate ordered states. This has been years in the making, but taken back seat to other more pressing things. Lots of work there – thanks Chunxiao and Gábor!
- With Urban Seifert, a student of Matthias Vojta from Dresden, who visited here earlier this year, we developed a fully quantum theory of ultra-fast laser excitation of antiferromagnets, with application to Sr2IrO4. I found this a stimulating introduction to non-equilibrium driven dynamics of quantum many body systems, a subject I hope to work much more on in the future.
- I also enjoyed a number of experimental collaborations. I’ve had a few projects with Stephen Wilson, an experimentalist in our materials department at UCSB. We just published a paper on NaYbO2, which is an effective S=1/2 triangular lattice antiferromagnet and a possible quantum spin liquid. I collaborated with Kamran Behnia’s group at ESPCI in Paris, to understand physical properties of and quantum transport through domain walls in Mn3Sn – see the paper. I helped David Hsieh’s group at Caltech understand spin correlations in the paramagnetic regime of the ferromagnetic material CrSiTe2, which they probed by nonlinear optics. Most recently, I worked with Andrea Young’s group to understand aspects of their recent discovery of zero field quantized quantum anomalous Hall effect in twisted bilayer graphene. These experiments are amazing! My Science paper with Lucile Savary, Takehito Suzuki, and Joe Checkelsky has also finally appeared. This took *a long time* to publish mainly because my experimental friends Joe and Takehito were extremely responsible and conscientious about taking time to vet and prove a sample growth recipe that could be shared and readily reproduced. Making quality materials is hard work!

Right now I’m preparing for lectures at a summer school in Cargèse, Corsica, where I’ll be teaching in just under two weeks. I decided to talk about the theory of twisted bilayer graphene, which has been moving so fast it feels rather challenging to cover!