Topological Materials: From Weak to Strong Correlations

The Focus workshop TopCor22 took place from April 11-13, 2022 and brought together 145 participants (50 on-site + 95 virtual) from 19 countries. Its main aim was to foster exchange between two largely distinct communities working on the weak and strong correlation regimes of topological quantum materials, to highlight emerging unifying themes of topology across the correlation spectrum and stimulate new ideas.

The workshop featured a diverse line-up of speakers that covered a range of topics from experiment to theory, and from weak to strong correlations. For instance, Erik Sorensen and Hidenori Takagi gave illuminating updates on Kitaev spin systems, Yoichi Ando came with new data from Majorana heterostructures, Joseph Checkelsky focused on flat bands in kagome materials, Amalia Coldea introduced a method to extract information about Berry phases from quantum oscillation data, and Chandan Setty and Diego Zocco discussed the design and control of correlation-driven topological semimetals, while Leslie Schoop and Claudia Felser added a chemist’s view on materials discovery. Jennifer Cano proposed twist-tronics with topological surface states and Adrian Po reported on work in progress seeking to extend Jordan-Wigner transformations to higher dimensions. Annica Black-Schaffer gave the institute colloquium of the week, in which she pedagogically introduced concepts of topological superconductivity and discussed the emergence of phase crystals in boundary modes of high-temperature superconductors as an example of new correlation-induced phenomena. Two discussion sessions were integrated into the workshop. For these, participants were asked to hand in questions and comments in advance. The stimulating discussions, which activated many participants, revolved around these questions, as well as selected topics that came up during the workshop talks.

The discussion sessions were also a great platform for junior participants to get involved and clearly demonstrated the advantages of the in-person format. Many PhD students used the opportunity for in-depth exchanges and networking. Alexander Tyner from Northwestern University summarized his experience as follows: „As a graduate student in my penultimate year, I found it to be a wonderful opportunity to both share my research and get a better sense of the directions others in the field have taken their research during the years of relative isolation […]. In particular the discussion sessions and informal opportunities to discuss were greatly appreciated.“ Besides presentations at the poster session, a number of junior participants gave 5-minute contributed talks. They were of very high quality and nicely complemented the topics of the invited talks, by including, for instance, novel aspects of topological band theory or twisted van-der-Waals systems.

That the workshop indeed helped bridge the gap between the two communities is particularly highlighted in the two lively discussion sessions, with topics that came from both communities, such as

• To what extent does the shape/spread/topology of Wannier functions impact correlated instabilities?
• Strong correlations vs topology: when do they cooperate (in producing correlated topology), and when do they compete? Are there principles at play?
• Can parton type construction for correlated models be developed into ab initio methods for correlated topology?

The organizers came away from the workshop with the sense that continued dialogues between the communities and cross-fertilization of ideas will help explore and advance the unifying themes for electronic topology in disparate materials settings and across the spectrum of correlation strength.

The organizers thank the Institute staff, in particular Mandy Lochar for providing outstanding organizational support in these complicated times and Ronny Börner for his support with the brand new hybrid conference system, which helped to overcome the challenge of a 50 persons on-site cutoff!

Silke Bühler-Paschen, Titus Neupert, Qimiao Si