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A New Twist of Two-Dimensional Lattices

Among various solid-state materials, two-dimensional materials (2DM) offer the highest tunability because of the combined freedom from electrostatic tuning and van der Waals (vdW) stacking.  In stacks of 2DM, a new degree of freedom, namely the twist angle between the layers, plays a crucial role in their physical properties. For two layers of graphene — the simplest 2DM made from carbon, the twist angle generates an extremely rich set of flat-band physics that originates from the moiré pattern between the twisted bilayers. In this system, we discovered that the flat bands in twisted bilayer graphene are host to an unexpected many-body insulator state, as well as a remarkable superconducting state that shows multiple unconventional behaviours. These findings opened up a fast expanding field of ‘twistronics’, where twisting and other moiré effects could now be utilized to engineer new quantum materials and quantum phases.

 

In the second part of the talk, I will talk about our recent progress in achieving real-time control of twisting in 2DM. While electrostatic tuning of 2DM is a well-established method for manipulating 2DM, achieving real-time control over interfacial mechanical properties such as twisting in situ remains a frontier. Current methods, often reliant on scanning microscopes, are limited in their application scope, lacking the accessibility and scalability of electrostatic gating. Recently, we introduced an on-chip platform for 2DM with in situ adjustable interfacial properties, employing a microelectromechanical system (MEMS). This platform comprises compact and cost-effective devices capable of precise voltage-controlled manipulation of 2DM, including approaching, twisting, and pressurizing actions. We demonstrate this technology by creating synthetic topological singularities, such as half-skyrmions or merons, in the nonlinear optical susceptibility of twisted hexagonal boron nitride (h-BN). A key application of this technology is the development of integrated light sources with real-time and wide-range tunable polarization. More importantly, it paves the way of investigating moiré 2D physics with the highest reproducibility and reconfigurability. Our work extends the capabilities of existing technologies in manipulating low-dimensional quantum materials and paves the way for novel hybrid 2D-3D devices, with promising implications in condensed-matter physics, quantum optics, and related fields.

 

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