When 12:00 PM - 2:30 PM Apr 27, 2023
Where 3158 HH Dow (Pod Room)
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PhD defense: "Quantum materials explored via Advanced Electron Microscopy"

Suk Hyun Sung
Hovden group

New materials are needed to unlock next generation computational capabilities that will fuel solutions spanning all areas of science—from medicine to clean-energy. Harnessing exotic quantum materials is among the most promising areas for progress; however extrinsic and thermal disorder degrade quantum behavior. Our work shows room temperature access to spatially coherent quantum states (e.g., charge density waves) and clean two-dimensional (2D) atomic confinement that can enable a paradigm shift in device logic and quantum computing.

The 2D materials has exploded since the discovery that graphene—a single layer of carbon atoms—can be exfoliated to its existence. Since then, a plethora of 2D materials has been exfoliated or synthesized, each with an exciting properties and opportunities. 

TaS2 is a layered 2D material that hosts several charge density waves (CDWs) that spontaneously break crystal symmetries, mediate metal–insulator transitions and compete with superconductivity. These quantum states are promising candidates for novel devices, and efficient ultrafast non-volatile switching. Room temperature access to spatially-coherent CDWs and clean 2D confinement could enable a paradigm shift toward quantum computing. Unfortunately, disorder in free standing 2D layers degrade correlation-driven quantum behavior, and clean 2D CDWs are near absent. 

This work introduces endotaxial engineering. We stabilize spatially-coherent, long-range ordered commensurate (C-) CDWs well above the room temperature by engineering novel endotaxial heterostructures of TaS2 (up to 150 K increase in the critical temperature). The stabilization or ordered electronic phases with 2D polytype engineering has significant implications for routes to access fragile, exotic correlated electron states. Stabilization of 2D CDWs with endotaxial engineering has further implications in condensed matter physics. Low dimensional physics is hard to experimentally study because true 2D system is rare. The endotaxial heterostructure is a potential platform for experimental demonstration of exotic 2D physics such as two-phase melting. We have preliminary results in agreement with the theoretical predictions. This work could shed light on low dimensional condensed matter physics that is experimentally hard to realize.

On the other hand, we explore interlayer twist, a tunability unique to 2D materials. Unexpected superconductivity was recently found when two layers of graphene are stacked with a specific 1.1° twist angle. Moiré pattern is formed when two layers of 2D materials are stacked with small twist angle between them. Moiré pattern results in periodically modulated out-of-plane interaction and constituent 2D crystals are periodically distorted with distinct feature in electron diffraction patterns. Therefore, twisted bilayer graphene is not a simple superposition of two layers and having atomistic description of distorted crystal is essential to understand and unveiling exotic quantum phenomena in twisted 2D materials. We introduce torsional periodic lattice distortion (PLD) as the single order parameter that describes positions of >10,000 carbon atoms. Furthermore, we experimentally quantify the torsional PLD amplitude from a single electron diffraction pattern.