Date: Monday, 10 March 2025
Time: 11:30 – 13:00
Venue: Seminar Room, Institute for Theoretical Physics, Philosophenweg 19
Speaker: Lucas Maisel Licerán (U. Utrecht, Henk Stoof group)

Title of Talk: Unconventional high-temperature excitonic insulators in two-dimensional topological materials

Abstract: Bound electron-hole pairs in semiconductors known as excitons can form a coherent state at low temperatures akin to a BCS condensate. The resulting phase is known as the excitonic insulator and has superfluid properties. In this talk, I will give a general introduction to the topic and then discuss our most recent work on the topic [https://arxiv.org/abs/2501.18694]. We have theoretically studied the excitonic insulator in a pair of recently proposed two-dimensional candidate materials with nontrivial band topology. Contrary to previous works, we include interaction channels that violate the individual electron and hole number conservations. These arise due to the substantial overlap of Wannier orbitals of different bands, which cannot be exponentially localized due to the nontrivial Chern numbers of the latter. Their inclusion is crucial to determine the symmetry of the electron-hole pairing, and by performing mean-field calculations at nonzero temperatures we find that the order parameter is a chiral d-wave. I will discuss the nontrivial topology of this unconventional state and give an overview of its superfluid properties. In particular, we estimate BKT temperatures between 75 K and 100 K on realistic substrates, over an order of magnitude larger than in the number-conserving approximation where s-wave pairing is favored. Our results highlight the interplay between topology at the single-particle level and long-range interactions, motivating further research in systems where both phenomena coexist. If time permits, I will outline the role of such non-number-conserving processes in other systems, particularly how they may influence superconductivity.

Looking forward to your participation!