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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)

   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.
2. 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 withmore »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.« less
3. Antiferromagnetic proximity coupling between semiconductor quantum emitters in WSe ₂ and van der Waals ferromagnets

   https://doi.org/10.1039/d0nr06632j  

   Liu, Na ; Gallaro, Cosmo M. ; Shayan, Kamran ; Mukherjee, Arunabh ; Kim, Bumho ; Hone, James ; Vamivakas, Nick ; Strauf, Stefan ( January 2021 , Nanoscale)

   van der Waals ferromagnets have gained significant interest due to their unique ability to provide magnetic response even at the level of a few monolayers. Particularly in combination with 2D semiconductors, such as the transition metal dichalcogenide WSe 2 , one can create heterostructures that feature unique magneto-optical response in the exciton emission through the magnetic proximity effect. Here we use 0D quantum emitters in WSe 2 to probe for the ferromagnetic response in heterostructures with Fe 3 GT and Fe 5 GT ferromagnets through an all-optical read-out technique that does not require electrodes. The spectrally narrow spin-doublet of the WSe 2 quantum emitters allowed to fully resolve the hysteretic magneto-response in the exciton emission, revealing the characteristic signature of both ferro- and antiferromagnetic proximity coupling that originates from the interplay among Fe 3 GT or Fe 5 GT, a thin surface oxide, and the spin doublets of the quantum emitters. Our work highlights the utility of 0D quantum emitters for probing interface magnetic dipoles in vdW heterostructures with high precision. The observed hysteretic magneto response in the exciton emission of quantum emitters adds further new degrees of freedom for spin and g -factor manipulation of quantum states.
4. Coherent helicity-dependent spin-phonon oscillations in the ferromagnetic van der Waals crystal CrI3

   https://doi.org/10.1038/s41467-022-31786-3  

   Padmanabhan, P. ; Buessen, F. L. ; Tutchton, R. ; Kwock, K. W. C. ; Gilinsky, S. ; Lee, M. C. ; McGuire, M. A. ; Singamaneni, S. R. ; Yarotski, D. A. ; Paramekanti, A. ; et al ( August 2022 , Nature Communications)

   The discovery of two-dimensional systems hosting intrinsic magnetic order represents a seminal addition to the rich landscape of van der Waals materials. CrI₃is an archetypal example, where the interdependence of structure and magnetism, along with strong light-matter interactions, provides a new platform to explore the optical control of magnetic and vibrational degrees of freedom at the nanoscale. However, the nature of magneto-structural coupling on its intrinsic ultrafast timescale remains a crucial open question. Here, we probe magnetic and vibrational dynamics in bulk CrI₃using ultrafast optical spectroscopy, revealing spin-flip scattering-driven demagnetization and strong transient exchange-mediated interactions between lattice vibrations and spin oscillations. The latter yields a coherent spin-coupled phonon mode that is highly sensitive to the driving pulse’s helicity in the magnetically ordered phase. Our results elucidate the nature of ultrafast spin-lattice coupling in CrI₃and highlight its potential for applications requiring high-speed control of magnetism at the nanoscale.
5. Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6

   https://doi.org/10.1038/s41467-022-32808-w  

   Li, Junxue ; Rashetnia, Mina ; Lohmann, Mark ; Koo, Jahyun ; Xu, Youming ; Zhang, Xiao ; Watanabe, Kenji ; Taniguchi, Takashi ; Jia, Shuang ; Chen, Xi ; et al ( September 2022 , Nature Communications)

   Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe₂, a two-dimensional quantum spin Hall insulator, and few-layer Cr₂Ge₂Te₆, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe₂manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.