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1. High-Contrast Plasmonic-Enhanced Shallow Spin Defects in Hexagonal Boron Nitride for Quantum Sensing

   https://doi.org/10.1021/acs.nanolett.1c02495  

   Gao, Xingyu ; Jiang, Boyang ; Llacsahuanga Allcca, Andres E. ; Shen, Kunhong ; Sadi, Mohammad A. ; Solanki, Abhishek B. ; Ju, Peng ; Xu, Zhujing ; Upadhyaya, Pramey ; Chen, Yong P. ; et al ( September 2021 , Nano Letters)

   null (Ed.)

   The recently discovered spin defects in hexagonal boron nitride (hBN), a layered van der Waals material, have great potential in quantum sensing. However, the photoluminescence and the contrast of the optically detected magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which limits their sensitivity. Here we report a record-high ODMR contrast of 46% at room temperature and simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold film microwave waveguide. Our results are obtained with shallow boron vacancy spin defects in hBN nanosheets created by low-energy He+ ion implantation and a gold film microwave waveguide fabricated by photolithography. We also explore the effects of microwave and laser powers on the ODMR and improve the sensitivity of hBN spin defects for magnetic field detection. Our results support the promising potential of hBN spin defects for nanoscale quantum sensing. 

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2. Néel-type skyrmion in WTe2/Fe3GeTe2 van der Waals heterostructure

   https://doi.org/10.1038/s41467-020-17566-x  

   Wu, Yingying ; Zhang, Senfu ; Zhang, Junwei ; Wang, Wei ; Zhu, Yang Lin ; Hu, Jin ; Yin, Gen ; Wong, Kin ; Fang, Chi ; Wan, Caihua ; et al ( December 2020 , Nature Communications)

   Abstract The promise of high-density and low-energy-consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. At the same time, recently discovered long-range intrinsic magnetic orders in the two-dimensional van der Waals materials provide a new platform for the discovery of novel physics and effects. Here we demonstrate the Dzyaloshinskii–Moriya interaction and Néel-type skyrmions are induced at the WTe 2 /Fe 3 GeTe 2 interface. Transport measurements show the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image Néel-type skyrmion lattice and the stripe-like magnetic domain structures as well. The interfacial coupling induced Dzyaloshinskii–Moriya interaction is estimated to have a large energy of 1.0 mJ m −2 . This work paves a path towards the skyrmionic devices based on van der Waals layered heterostructures. 

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3. Coherent dynamics of strongly interacting electronic spin defects in hexagonal boron nitride

   https://doi.org/10.1038/s41467-023-39115-y  

   Gong, Ruotian ; He, Guanghui ; Gao, Xingyu ; Ju, Peng ; Liu, Zhongyuan ; Ye, Bingtian ; Henriksen, Erik A. ; Li, Tongcang ; Zu, Chong ( June 2023 , Nature Communications)

   Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ($${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$$V_B^{-}$) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$$V_B^{-}$ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$$V_B^{-}$. We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$$V_B^{-}$to the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$$V_B^{-}$, which are important for future use of defects in hBN as quantum sensors and simulators.

    

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4. Dielectric Environment Sensitivity of Carbon Centers in Hexagonal Boron Nitride

   https://doi.org/10.1002/smll.202300144  

   Badrtdinov, Danis I. ; Rodriguez‐Fernandez, Carlos ; Grzeszczyk, Magdalena ; Qiu, Zhizhan ; Vaklinova, Kristina ; Huang, Pengru ; Hampel, Alexander ; Watanabe, Kenji ; Taniguchi, Takashi ; Jiong, Lu ; et al ( June 2023 , Small)

   A key advantage of utilizing van‐der‐Waals (vdW) materials as defect‐hosting platforms for quantum applications is the controllable proximity of the defect to the surface or the substrate allowing for improved light extraction, enhanced coupling with photonic elements, or more sensitive metrology. However, this aspect results in a significant challenge for defect identification and characterization, as the defect's properties depend on the the atomic environment. This study explores how the environment can influence the properties of carbon impurity centers in hexagonal boron nitride (hBN). It compares the optical and electronic properties of such defects between bulk‐like and few‐layer films, showing alteration of the zero‐phonon line energies and their phonon sidebands, and enhancements of inhomogeneous broadenings. To disentangle the mechanisms responsible for these changes, including the atomic structure, electronic wavefunctions, and dielectric screening, it combines ab initio calculations with a quantum‐embedding approach. By studying various carbon‐based defects embedded in monolayer and bulk hBN, it demonstrates that the dominant effect of the change in the environment is the screening of density–density Coulomb interactions between the defect orbitals. The comparative analysis of experimental and theoretical findings paves the way for improved identification of defects in low‐dimensional materials and the development of atomic scale sensors for dielectric environments.

    

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5. Revealing room temperature ferromagnetism in exfoliated Fe 5 GeTe 2 flakes with quantum magnetic imaging

   https://doi.org/10.1088/2053-1583/ac57a9  

   Chen, Hang ; Asif, Shahidul ; Whalen, Matthew ; Támara-Isaza, Jeyson ; Luetke, Brennan ; Wang, Yang ; Wang, Xinhao ; Ayako, Millicent ; Lamsal, Saurabh ; May, Andrew F ; et al ( March 2022 , 2D Materials)

   Abstract Van der Waals (vdW) material Fe 5 GeTe 2 , with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe 5 GeTe 2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe 5 GeTe 2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of vdW magnet Fe 5 GeTe 2 via quantum magnetic imaging technique based on nitrogen vacancy centers in diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe 5 GeTe 2 thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities is consistent with previously reported perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at ≈300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe 5 GeTe 2 . This work also demonstrates that the imaging technique enables rapid screening of multiple flakes simultaneously as well as time-resolved imaging for monitoring time-dependent magnetic behaviors, thereby paving the way towards high throughput characterization of potential two-dimensional (2D) magnets near room temperature and providing critical insights into the evolution of domain behaviors in 2D magnets due to degradation. 

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