If you haven’t already watched this, you should! A fantastic and powerful speech. Sorry for another non-physics post, but this really should be required viewing.
I can’t help being a little obsessive about the coronavirus situation in our little town of Santa Barbara. One can find a lot of data at the county level in the California state web sites, but broken down by city I could find it only in the County web page and not in a form that was quite what I wanted. So I wrote a little python script that extracts the data from the county web page, and creates plots of the total cases in the City of Santa Barbara, as well as a 7-day average of new cases reported (it is an average over the last 7 days of reports, not over a fixed week range, but that is because the county does not report every day).
The original data is here: https://publichealthsbc.org/status-reports/
I’ve automated this so that the plots automatically update daily. Be aware that this is just an amateur project, and there could be errors, for example if the County changes its format for displaying the data. This was a little bit of a fun project to do. I wrote a python script to extract the data from the county page and create the plots. Then a shell script uses git to upload the plot to github, and the web page has an embedded image link to the plot in the github repository. The shell script runs daily on my mac laptop using launchd. All the code (except the launchd xml file) is on github here: https://github.com/balents/covidSB .
I followed the #ShutDownAcademia action Wednesday, as many of my colleagues did. The unfortunate truth is that there are very few Black academic physicists. Despite traveling extensively to conferences and working at the KITP, where we have a huge influx of visitors, I have met only a handful in more than 25 years of my academic career. I sincerely hope that changes, and people like me need to do our part to make it possible.
I came across some fascinating discussions of the situation from arguably the most famous physicist, Albert Einstein. I could not track down the original sources, but there are some good online articles here and here. In an address at Lincoln University, a predominantly black university in Pennsylvania, Einstein reported said “There is a separation of colored people from white people in the United States. That separation is not a disease of colored people. It is a disease of white people. I do not intend to be quiet about it.” Einstein’s activism on the subject was not known to me, but is yet another aspect of his life that inspires.
Something that my daughter and I have been doing in the last few weeks (with no causal connection to recent events) has been to try to learn about Africa, is countries, cultures, and geography. We found an engaging series of videos on YouTube called GeographyNow, which posts 10-15 minute summaries of the countries of the world. We’ve been going through African nations one by one, and while this is not the most academic format but it is fun and both of us find it fascinating and have been impressed by the richness and cultural diversity of the continent and its peoples.
This is not the most coherent thread, but I wanted to write something. There is really no way for a privileged person like myself to do this topic justice. I just wanted to show that the injustice of the Black condition in this country is deeply felt, and that I believe it is relevant to us in physics. By improving diversity in the field, we will enrich it and I am sure improve the science.
I’ll be doing my first virtual seminar this week. I was scheduled to visit the Flatiron institute in NYC this week, and obviously that was not going to happen, so I’ll be giving the talk from the comfort of my dining table 🙂
The talk is Tuesday 12pm east coast time, so 9am for me. I’ll be talking about some recent and on-going work in which Oleg Starykh, Anna Keselman and I study how you can observe *interactions* between quasiparticles in quantum magnets from the dynamical structure factor.
Here is a link:
and yes…they quite systematically mis-spelled my name!
This is the season in which students are deciding where to go for graduate school in the US. Around the country, students have been admitted to PhD programs, passing through a remarkably selective filter. To give you an idea, we typically get something like 600-700 applicants for what we hope will be 20-30 incoming students in all fields of physics. It’s a tough competition, and those that make it are a talented bunch. After that, the shoe is on the other foot. These students invariably have multiple offers, and now *they* have to choose where to go. The deadline to respond to our offer is generally tax day, April 15 (and as far as I know this is pretty much true everywhere). So we’re getting close.
Normally physics departments invite these “prospective” students to a day-long visit of the campus, to meet professors and students, and learn more about the program. With COVID-19, we like many others canceled the visit, which would have been yesterday, and held instead a virtual visit. I spent 4.5 hours yesterday on Zoom with prospectives. It worked pretty well but I can’t help but wonder what got lost in the mix. For example, we normally hold a poster session with current students showing their work to the prospectives, there are casual events to discuss, etc. It all was pretty compressed and intensified by the last-minute change to virtual space.
So…I thought maybe it would be useful to give a few advertisements for UCSB physics, focusing on condensed matter because well that’s what I know best.
- We’ve got a big and top-notch condensed matter theory effort. Apart from myself, there are five physics faculty working in quantum matter theory: Matthew Fisher, Chetan Nayak, Cenke Xu, Andreas Ludwig, and starting this fall Sagar Vijay. But that’s not all. Microsoft Quantum has several permanent scientists that advise students: Bela Bauer, Roman Lutchyn, Parsa Bonderson. There are a number of computational theorists in other departments: Chris van der Walle, Vojtech Vlcek, Glenn Fredrickson (Glenn does polymer physics, which is classical not quantum, but recently his group has discovered they can use their polymer algorithms to simulate quantum spins or bosons!). We also have a number of theorists in soft and living matter, which isn’t quantum but neverthess we share a lot of ideas in common: Boris Shraiman, Cristina Marchetti (Cristina and I go way back – we collaborated back in the 90s!), Mark Bowick. There’s also some convergence of interest with the string/gravity group, where chaos and scrambling, SYK models, etc. are being actively studied.
- Don’t forget our postdocs! The KITP has a track record of hosting the best postdocs in the world, and it hasn’t changed. They are young, motivated, and brilliant, and they are eager to work with our students. Typically we have about a half dozen overall in condensed matter and quantum information.
- More is different: Probably the most unique feature of UCSB is all the collaborative opportunities.
- In theoretical physics at the center is the KITP: take a look here to see the 16 programs already scheduled for the next 1.5 years. Each one will bring 20-30 world-class scientists *every week* to the KITP to present their research, discuss ideas, and start new collaborations. It’s an amazing opportunity for our students to learn new things and gain valuable exposure.
- There’s Microsoft Quantum, which is next door to the KITP and 5 minutes from the Physics department.
- The new UCSB Quantum Foundry is an NSF sponsored center aimed to develop the quantum materials needed for quantum information science. I am part of “Thrust 1” whose goal is to study highly entangled matter such as spin liquids and topological superconductors. The Foundry offers collaborations, seminars, fellowships and more for students.
- I am the co-director of the international program on Quantum Materials run through CIFAR. It’s a collaborative network of currently 15 scientists (theorists, experimentalists, and sample growers) from around the world interested in the deepest questions in the field. We meet a couple of times a year with students and postdocs and visitors to discuss and chart new research directions.
- UCSB is a member of the Simons Collaboration on Ultra-Quantum Matter, which combines condensed matter, quantum information, atomic, and string theorists to study the nature of highly entangled and quantum critical matter. The Collaboration holds schools and meetings, and encourages exchanges of members.
- We are an EPiQS Theory Center sponsored by the Moore Foundation. This supports some of those great postdocs and beyond.
- That’s not all – I just got tired to typing. There are even more collaborations and multi-university efforts that students can become a part of.
- It all connects to experiment: My personal favorite part of the field is connecting to real-world measurements in the lab. I have a lot of friends at UCSB who make those measurements. Stephen Wilson in materials grows new materials all the time, and we’ve been collaborating constantly on quantum magnets, spin liquids, Mott insulators, and superconductors. Susanne Stemmer is one of the top thin film (MBE) growers in the world, and has made many advances in correlated and topological materials. Andrea Young‘s lab is bursting with new discoveries in 2d materials such as twisted bilayer graphene, which provides a rich source of phenomena for theory to tackle. We keep hiring great young experimentalists in both the physics and materials departments, in both condensed matter and amo physics, and beyond this networks such as CIFAR and EPiQS, MURI grants, and more give students even more access to exciting experiments — and arguably I am pretty good at teaching them how to connect these with theory.
- Our students are successful. The vast majority of the condensed matter theory group’s students stay in academia, and find permanent research positions.
I taught this August in a summer school in Cargèse, Corsica, which is an island in the Mediterranean that is part of France. It was an especially fun trip for me as I attended a school at the same location in 1993, as a graduate student. Other students there with me were Leo Radzihovsky, Ali Yazdani, Katherine Moler, and Dan Dessau, to name a few that come immediately to mind. They all did rather well in physics since!
Anyway, I decided to lecture about twisted bilayer graphene (TBG), which was a bit of an undertaking – I gave 6 hours of lectures over 4 days. You can find my hand-written lecture notes on this site (under pedagogy), and I’m working on a latex version (I have them for 3 out of four of the 1.5 hour lectures).
I already benefited myself from the class, because it motivated me to finally work out an old idea I had. This is a derivation of the “continuum model” of Bistritzer and MacDonald (BM), which is what really started the whole TBG discovery by predicting the presence of “magic angles”. Their work is really nice, and many others have since used their formulation to elaborate or extend the treatment in many ways. I always felt though that the derivation in BM’s paper was not so intuitive, and the final result – the continuum model – is simpler than the way it was obtained. Specifically it just seems to capture the idea that locally TBG just “looks” like an untwisted bilayer with some relative shift between layers. I had the idea after reading their paper that one could derive the continuum model by turning that idea into a calculation.
There is some subtlety related to the difference between Eulerian and Lagrangian coordinate systems which stymied me initially. But since I had to teach the stuff I was motivated to spend the time and I was able to overcome that issue. The result is so simple it borders on trivial, but I like the simplicity of it. Maybe it makes the problem a bit more accessible to some people. You can read about it here: https://arxiv.org/abs/1909.01545.
Yesterday I gave a talk to the Santa Barbara Astronomical Unit – a local group of astronomy enthusiasts who meet at the Santa Barbara Museum of Natural History. I don’t do astronomy or astrophysics but it was a nice group of people curious about physics in general. I told them about Quantum Materials, giving a number of relations to astrophysics.
The group does a lot of outreach and if you want to learn about the sky and how to observe it, and live in the area, check them out at http://sbau.org/.
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!
Come this summer, I will be taking on a new role: co-director of the Canadian Institute for Advanced Research’s “Quantum Materials” program. This is a program with a 30 year history that began around the discovery of high-Tc superconductivity and managed to make Canada into a powerhouse in that area. Over the years, it evolved into a more international effort, and broadened to embrace what we now call quantum materials (arguably the name originated from the program). I’ve been a regular member for a number of years, and what is impressive about this program has been the way everyone involved comes together to really try to connect materials synthesis, experiment, and theory for greater understanding and impact. I’m not Canadian but I do very much respect the effort and I’ll be joining Louis Taillefer in the re-imagining of this program starting in July. This is not a physical institute, and the main activities are meetings to foster collaborations, and to train students and postdoctoral scientists. I’m excited about the team Louis and I are putting together, and hope we can keep up the traditions and impact of the program going forward.
You can find out more on CIFAR’s web site: https://www.cifar.ca/research/programs/quantum-materials
There’s a lot of excitement over twisted bilayer graphene (TBG), and increasingly over other sorts of moiré heterostructures from 2d materials. In TBG, most of the interest is due to the discovery of many correlation phenomena that occur at partial filling of the mini-bands nearest the charge neutrality point. Theory says that they are very flat, and have novel topological aspects to them. The flatness explains their tendency to be unstable to interactions that induce the correlated states. But the details of that flatness still matter to any theory. There are lots of models for this – how do we know what is right? It would be great to be able to actually measure the bands in experiment.
The most sensitive way to measure bands experimentally is via quantum oscillations. Semiclassically, these are oscillations as a function of doping/field/etc due to interference of electron trajectories that go in closed orbits in momentum space. So one can learn about these orbits. Quantum mechanically, this is due to Landau level formation. Anyway, many groups have seen quantum oscillations in TBG in the regime where correlated states form. But guess what? They do not really agree with the naïve explanations based on most band theories. There are many discrepancies, but the most obvious one is that even close to charge neutrality, the “magic angle” samples do not show the oscillations that would be expected from Dirac cones at the corners of the moiré Brillouin zone. The expected behavior is actually seen at larger angles. So something different happens near the magic angles that needs to be explained.
My students Kasra Hejazi and Chunxiao Liu and myself wondered if the differences might be related to topological changes that happen near the magic angles, in the “standard model” of these systems due mainly to Bistritzer and Macdonald. So these guys set out to do a fully honest calculation of the quantum oscillations. This is a variant of the Hofstadter “butterfly” problem, and the true electron spectrum is actually fractal. But they managed to subdue the beast of a challenge and work things out rather completely. Short answer: for certain angles in the “magic range” one indeed gets anomalous quantum oscillations consistent with experiment just from band structure. This is associated with additional topological transitions, or semi-classically to orbits that are not near the moiré zone corners.
This could be an explanation of the experiments! But maybe it is just a piece of the puzzle. The electron-electron interactions can and probably do modify the quantum oscillations. Maybe they could even “fix” the one dis-satisfying feature of our calculations: the anomalous oscillations only occur for a rather narrow range of angles. Maybe interactions make them more robust?
Anyway, the paper is out there – have a look: https://arxiv.org/abs/1903.11563