Speaker:  Tomohiro Soejima (Harvard University)

Date & Time: 1pm, Nov 29, 2023.

Place: Hybrid - Room 64, 2F, Engineering Building 6/Online via Zoom

Title: Experimental observation of an incommensurate order of magic-angle twisted bilayer graphene

Abstract: 

Magic-angle twisted bilayer graphene (MATBG) hosts a wide variety of correlated insulating (CI) states, some of which are proximate to superconducting (SC) states in the phase diagram. While symmetry breaking in the flavor space has been the dominant paradigm for explaining the appearance of the CI states, there has been no direct experimental confirmation of this fact. In this talk, I will report a recent scanning tunneling microscope (STM) experiment on MATBG that unambiguously establishes the presence of incommensurate symmetry breaking order [1].We make this determination by combining atomic-resolution STM images with a newly-developed image analysis routine that precisely determines the location of all graphene unit cells within the view of the image. This data analysis allows us to carefully map out local order parameters (LOPs) of MATBG, whose transformation properties can be used to check the symmetry of the state. Using LOPs, we establish the MATBG samples break U(1) valley symmetry, excluding the theoretically-favored Kramers-Intervaley Coherent state as the ground state. In addition, the phase of these LOPs show an incommensurate pattern, establishing the presence of incommensurate order in the ground state. This observation strongly suggests the ground state is an incommensurate Kekule spiral (IKS) state [2, 3, 4], which is stabilized by heterostrain on MATBG.

[1] Nuckolls et al. arXiv:2303.00024, Nature, to appear

[2] Kwan et a., Phys. Rev. X 11, 041063

[3] Wagner et al. Phys. Rev. Lett. 128, 156401

[4] Wang et al. arXiv: 2211.02693