Abstract
When two atomically thin layers are assembled through van der Waals (vdW) stacking, new phases of matter that are absent in each constituent layer can emerge due to reconstruction of electronic structures, as exemplified by the recent discovery of rich correlated and topological phases in moiré superlattice formed by twisted graphene and transition-metal dichalcogenides [1]. In this talk, I will discuss the novel topological and geometric phases in two different types of bilayer systems with unusual stacking configurations.
The first part is devoted to a new application of the twist-angle scheme to create chiral topological superconductors from unconventional but topologically trivial superconductors, such as high Tc cuprates [2] and the recently discovered spin-triplet superconductivity in rhombohedral graphene and zirconium nitrides [3]. In particular, I will explain how the combination of a new type of large-angle moiré physics and quasi-crystalline symmetry emergent at the ‘maximal-twist’ limit provides a viable route toward non-Abelian Majorana modes.
In the second part, I will discuss the Berry phase origin behind the out-of-plane stacking ferroelectricity (SFE) observed in a wide range of aligned vdW bilayers [4], which are formed otherwise by non-polar monolayers such as hexagonal boron nitride (hBN), silicon carbide (SiC) and rhombohedral molybdenum disulfide (3R-MoS2) [5]. I will reveal a meaningful mapping between the electronic Hamiltonians relevant for SFE and the celebrated Su-Schrieffer-Heeger (SSH) model [6], which allows one to visualize the Berry phase in the non-periodic bilayer limit and develop a self-consistent explanatory framework for out-of-plane SFE.
References
[1] Eva Y. Andrei et al., Nature Reviews Materials 6, 201–206 (2021).
[2] O. Can, T. Tummuru, R. P. Day, I. Elfimov, A. Damascelli, M. Franz, Nat. Phys. 17 (4), 519-524 (2021).
[3] Benjamin T. Zhou, S. Egan, D. Kush, M. Franz, Commun. Phys. 6 (1), 47 (2023).
[4] M. Wu and J. Li, Proc. Natl. Acad. Sci. USA 118, (50) e2115703118 (2021).
[5] Dongyang Yang, Jingda Wu, Benjamin T. Zhou et al., Nature Photonics 16, 469–474 (2022).
[6] Benjamin T. Zhou, V. Pathak, M. Franz, arXiv:2309.00728 (under review at PRL)
Please contact phweb@ust.hk should you have questions about the talk.
