Semiconductor moiré materials

Emergent color centers with accessible spins hosted by van der Waals materials have attracted substantial interest in recent years due to their significant potential for implementing transformative quantum sensing technologies. Hexagonal boron nitride (hBN) is naturally relevant in this context due to its remarkable ease of integration into devices consisting of low-dimensional materials. Taking advantage of boron vacancy spin defects in hBN, we report nanoscale quantum imaging of low-dimensional ferromagnetism sustained in Fe3GeTe2/hBN van der Waals heterostructures. Exploiting spin relaxometry methods, we have further observed spatially varying magnetic fluctuations in the exfoliated Fe3GeTe2 flake, whose magnitude reaches a peak value around the Curie temperature. Our results demonstrate the capability of spin defects in hBN of investigating local magnetic properties of layered materials in an accessible and precise way, which can be extended readily to a broad range of miniaturized van der Waals heterostructure systems. 

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Wide field imaging of van der Waals ferromagnet Fe3GeTe2 by spin defects in hexagonal boron nitride

Solid-state spin sensors have the capacity to act as quantum microscopes for probing material properties and physical processes. However, so far, these tools have relied on quantum defects hosted in rigid, three-dimensional (3D) crystals such as diamond, limiting their ability to closely interface with the sample. Here we demonstrate a versatile quantum microscope using point defects embedded within a thin layer of the van der Waals material hexagonal boron nitride. To showcase the multi-modal capabilities of this platform, we assemble two different heterostructures of a van der Waals material in combination with a quantum-active boron nitride flake. We demonstrate time-resolved, simultaneous temperature and magnetic imaging near the Curie temperature of a van der Waals ferromagnet, as well as map out charge currents and Joule heating in an operating graphene device. The straightforward integration of the hexagonal boron nitride quantum sensor with other van der Waals materials will yield substantial practical benefits for the design and measurement of 2D devices. Hexagonal boron nitride is a common component of 2D heterostructures. Defects implanted in boron nitride crystals can be used to perform spatially resolved sensing of properties, including temperature, magnetism and current. 

Quantum microscopy with van der Waals heterostructures

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology. 

Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride

Two-dimensional (2D) materials and their van der Waals (vdW) heterostructures provide powerful platforms for exploring fascinating physical phenomena and implementing intriguing applications. The recent discovery of 2D magnetic materials opens up many exciting topics in the research of 2D materials. 2D magnetic materials not only exhibit diverse properties on their own, which can be effectively controlled via external stimuli, but also can serve as a part of vdW heterostructures, creating a variety of unprecedented properties and functionalities. The field of 2D magnetism has grown rapidly in the past few years. In this review, we summarize the recent progress on the emergent phenomena in this active field. We review the well-established understanding of typical 2D magnetic materials and the vdW heterostructures and introduce the manipulations of their magnetic properties. In addition, we discuss the interplay between magnetism and topology in some recently discovered 2D materials. Moreover, we also address the valley relevant physics and the topological spin textures in 2D magnetic materials. Finally, we highlight some future interests in this promising field. 

Emergent Phenomena in Magnetic Two-Dimensional Materials and van der Waals Heterostructures

Moire superlattices of twisted nonmagnetic two-dimensional (2D) materials are highly controllable platforms for the engineering of exotic correlated and topological states. Here, we report emerging...

Jimin Qian

Papers

Direct visualization of magnetic domains and moiré magnetism in twisted 2D magnets.