The Joint Condensed Matter Seminar (JCMS) series is organised by KTH Royal Insitute of Technology, Nordita, and Stockholm University.
On Monday, September 15th, 2025 from 11.00 am to 12.00 we will host a seminar by Wojciech Jankowski from Cambridge University.
Probing higher forms in topological matter
String theory deals with extended objects, such as open and closed strings, which couple to the higher-form gauge fields proposed by Kalb and Ramond [1]. The flux of these tensor fields amounts to torsion in the quantum gravity contexts. I will present a construction of higher-tensorial Kalb-Ramond fields from single-particle Bloch states over momentum space, which I shall connect to physical observables in topological insulators [2]. I will show how the constructed gauge fields are equivalent to higher tensor Berry connection forms [3], dealing with the higher cohomologies of bundle gerbes, the generalisations of vector bundles. I will show that different classes of multigap phases and Hopf insulators [4] realise higher-form bundle gerbe invariants, with the corresponding higher Berry curvature fluxes being electromagnetically accessible through magnetoelectric and nonlinear optical responses. Finally, I will discuss many-body generalisations that admit fractionalisations of higher-dimensional many-body gerbe invariants.
[1] M. Kalb & P. Ramond, Phys. Rev. D 9, 2273 (1974). [2] W. J. Jankowski, R.-J. Slager, and G. Palumbo, arXiv:2507.22116. [3] G. Palumbo, Phys. Rev. Lett. 126, 246801 (2021). [4] W. J. Jankowski & R.-J. Slager, Phys. Rev. Lett. 133, 186601 (2024).