An-Ping Li leads the Scanning Tunneling Microscopy Group and the Heterogeneities in Quantum Materials theme at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory. His research focuses on understanding and controlling the atomic structure and its correlation with electronic, magnetic, and transport properties in quantum materials through the development of advanced scanning tunneling microscopy (STM) techniques. His group has established unique capabilities, including a dilution refrigerator vector-magnet STM, a spin-polarized four-probe STM, and a scanning NV microscope. These tools have enabled major discoveries such as the identification of skyrmions in van der Waals magnets, direct evidence of spin–momentum locking in topological materials via spin chemical potential measurements, observation of single-vacancy-based nonvolatile resistive switching (NVRS), and the demonstration of single-molecule telegraphy across surfaces.
Presentation Title:
Create and Understand Quantum States through Defect Control
Presentation Abstract:
Controlling quantum states at the atomic scale is key to advancing quantum materials and technologies. Here, we present an on-surface approach to create, manipulate, and probe quantum states through defect engineering, aiming at electronic correlation, quantum sensing, and quantum simulation. We first show that charge-density-wave (CDW) defects in the intrinsic heterostructure 4Hb-TaS₂ enable precise control of electron correlation and flat bands. Two defect types, arising from layer-specific sulfur vacancies, either distort the CDW and suppress flat bands or preserve the lattice while enhancing band filling through modified interlayer coupling. Notably, individual defects can be reversibly created and erased using STM, providing deterministic, atomic-scale control of correlated states. We then employ a single nitrogen-vacancy (NV) center in diamond as a scanning quantum sensor to detect and map spin defects in hexagonal boron nitride via changes in spin relaxation (T₁). This approach enables nanoscale imaging of spin systems without direct optical access, expanding the toolkit for identifying and characterizing quantum defects. Finally, we demonstrate the creation of designer quantum states in artificial lattices, including molecular graphene and Kagome structures assembled via STM, where Dirac dispersions and flat bands can be engineered with high precision. These advances highlight defect control as a unifying strategy for engineering and understanding quantum states, opening pathways toward programmable quantum materials and hybrid quantum architectures.