Stacked, Strained, and Twisted: Emergent Phenomena in 2D Materials

Session 1B: 12:30 PM – 3:00 PM Pacific Time on Friday, August 21.

Archana Raja*, Sinead Griffin, Frank Ogletree, Alex Weber-Bargioni

An appealing characteristic of atomically thin two-dimensional materials is the lack of dangling bonds at the surface. This allows for seamlessly building novel, artificial materials one monolayer at a time. A new degree of freedom enabled by stacking is the relative rotation between the individual monolayers. This has opened the field to unprecedented band structure engineering, of which a famous example is the discovery of unconventional superconductivity in twisted bilayer graphene just two years ago. The twist angle-dependent Moiré potential between two semiconducting monolayers like transition metal dichalcogenides have been used to demonstrate strongly correlated behavior and the potential for scalable creation of quantum emitters. Studies of the underlying lattice reconstruction and nanoscale strain modulation in such twisted systems are still at a nascent stage. The proposed symposium will bring together leaders in this new, very active field, including current users of the Molecular Foundry. The Foundry’s internal research program, both experimental and theoretical, are poised to make an impactful contribution to this emerging area of nanoscale research.

Symposium Schedule:

12:30 pm

Prof. Kwabena Bediako, Chemistry, UC Berkeley // Chemical Sciences Division, Berkeley Lab

12:55 pm

Prof. Jie Shan, Applied and Engineering Physics, Cornell University

1:20 pm

Prof. Libai Huang, Chemistry, Purdue University

1:45 pm
5-minute break
1:50 pm

Prof. Xiao-Xiao Zhang, Physics, University of Florida

2:15 pm

Prof. Andrea Young, Physics, UC Santa Barbara

2:40 pm

Prof. Kwabena Bediako, Prof. Jie Shan, Prof. Libai Huang, Prof. Xiao-Xiao Zhang, Prof. Andrea Young

Symposium Abstracts:

12:30 PM

Mapping strain in twisted bilayer graphene

Prof. Kwabena Bediako
Chemistry, UC Berkeley // Chemical Sciences Division, Berkeley Lab

Twisted bilayer graphene (TBG) displays a host of correlated electronic phases associated with the formation of flat electronic bands near an interlayer “magic angle” (MA) of 1.1 degrees. Intralayer lattice reconstruction, which involves local rotations with consequent localized strain, suppresses the formation of flat bands and symmetry breaking due to extrinsic heterostrain may have significant implications for electronic behavior at the MA. Reconstruction and strain are therefore fundamental to the electronic behavior of TBG. Yet, directly mapping the reconstruction mechanics in the MA regime has been elusive and the strain tensor fields of TBG have not been measured. In this talk, I will introduce Bragg interferometry, based on four-dimensional scanning transmission electron microscopy (4D-STEM), which is used to capture the local atomic displacements of TBG with twist angles ranging from 0.1 to 1.6 degrees. Sub-nanometer resolution allows us to image atomic reconstruction in MA-TBG and resolve twist angle disorder at the level of single moiré domains. I will show how we manage to quantitatively map the strain tensor fields and observe two distinct regimes of reconstruction that depend on the twist angle, in contrast to previous models depicting a single continuous process. I will also show how the application of heterostrain results in anisotropic saddle point strain that is particularly pronounced in MA-TBG. The findings and methodology I shall discuss in this talk establish the reconstruction mechanics underpinning the twist angle-dependent electronic behavior of TBG, and provide a new framework for directly visualizing strain and reconstruction in other moiré matter.

12:55 PM

Strong correlation physics in semiconductor moiré superlattices

Prof. Jie Shan
Applied and Engineering Physics, Cornell University

Moiré superlattices are formed by stacking two identical lattices with a small twist angle or two lattices with a small period mismatch. The flat electronic minibands afforded by moiré superlattices have led to a plethora of emergent phenomena. In this talk, I will discuss recent experiments on angle-aligned WSe2/WS2 bilayers, which exhibit remarkable correlated insulating states. We study the quantum phase diagram of the system by continuously tuning the filling of the first moiré miniband and probing the charge and magnetic order. These results demonstrate that two-dimensional semiconductor moiré superlattices are a highly tunable platform to study strong correlation physics.

1:20 PM

Twist-Angle Dependent Interlayer Exciton Diffusion in WS2-WSe2 Heterobilayers

Prof. Libai Huang
Chemistry, Purdue University

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures have emerged as a platform for designing exciton superlattices. However, our understanding of the motion of excitons in moiré potentials is still limited. Here, we investigated interlayer exciton dynamics and transport in WS2-WSe2 heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with first-principles calculations. We find that exciton motion is modulated by twist-angle dependent moiré potentials around 100 meV and deviates from normal diffusion due to the interplay between the moire potentials and strong exciton-exciton interactions. Our experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and showed exciton-population dynamics that are controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. These results form a basis to investigate exciton and spin transport in van der Waals heterostructures, with implications for the design of quantum communication devices.

1:50 PM

Ultrafast excitation of coherent spin waves in a 2D antiferromagnet/semiconductor heterostructure

Prof. Xiao-Xiao Zhang
Physics, University of Florida

The recently discovered atomically-thin magnetic crystals provide a unique playground to develop new approaches to manipulate magnetism. Rapid progresses have been made that demonstrate the potentials of utilizing 2D magnets to construct novel spintronics devices. However, their spin dynamics, which are crucial for microscopic understanding and determine the fundamental limit of spin manipulation, still remain elusive due to the difficulty to characterize these micron-sized samples with conventional microwave techniques. In this talk, I will show how we can access and probe the collective spin-wave excitations in an antiferromagnetic bilayer CrI3/monolayer WSe2 heterostructure, which allows us to extract magnetic anisotropy and exchange energy. In particular, we will demonstrate the gate tunability of magnon frequencies, which is unique for the 2D magnet system.

2:15 PM

Orbital Chern insulators in moiré heterostructures

Prof. Andrea Young
Physics, UC Santa Barbara

I will describe recent experiments probing the unusual properties of orbital magnets realized in moiré heterostructures based on graphene. These are unusual states in which time reversal symmetry is broken spontaneously by orbital currents, in the absence of spin orbit coupling that might order the spins. Remarkably, these magnets generically show quantized anomalous Hall effects at integer numbers of electrons per moire superlattice unit cell, owing to the underlying topological quantum numbers of the valley projected moiré minibands. My talk will give an overview of our experiments on these systems, including observation of high-temperature quantum anomalous Hall effects and direct measurement of the orbital magnetic moment. Remarkably, the strong dependence of the orbital moment on carrier density leads to a regime where the magnetization can be reversed using a purely electric field at fixed, finite magnetic field, which we use to demonstrate a new kind of magnetic memory.