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1. Excited-state spin-resonance spectroscopy of V$${}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$ defect centers in hexagonal boron nitride

   https://doi.org/10.1038/s41467-022-30772-z  

   Mathur, Nikhil ; Mukherjee, Arunabh ; Gao, Xingyu ; Luo, Jialun ; McCullian, Brendan A. ; Li, Tongcang ; Vamivakas, A. Nick ; Fuchs, Gregory D. ( June 2022 , Nature Communications)

   The recently discovered spin-active boron vacancy (V$${}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$${}_B^{-}$) defect center in hexagonal boron nitride (hBN) has high contrast optically-detected magnetic resonance (ODMR) at room-temperature, with a spin-triplet ground-state that shows promise as a quantum sensor. Here we report temperature-dependent ODMR spectroscopy to probe spin within the orbital excited-state. Our experiments determine the excited-state spin Hamiltonian, including a room-temperature zero-field splitting of 2.1 GHz and a g-factor similar to that of the ground-state. We confirm that the resonance is associated with spin rotation in the excited-state using pulsed ODMR measurements, and we observe Zeeman-mediated level anti-crossings in both the orbital ground- and excited-state. Our observation of a single set of excited-state spin-triplet resonance from 10 to 300 K is suggestive of symmetry-lowering of the defect system fromD_3htoC_2v. Additionally, the excited-state ODMR has strong temperature dependence of both contrast and transverse anisotropy splitting, enabling promising avenues for quantum sensing.

    

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2. Charge state-dependent symmetry breaking of atomic defects in transition metal dichalcogenides

   https://doi.org/10.1038/s41467-024-47039-4  

   Xiang, Feifei ; Huberich, Lysander ; Vargas, Preston A. ; Torsi, Riccardo ; Allerbeck, Jonas ; Tan, Anne Marie Z. ; Dong, Chengye ; Ruffieux, Pascal ; Fasel, Roman ; Gröning, Oliver ; et al ( March 2024 , Nature Communications)

   The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS₂by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). By changing the built-in substrate chemical potential, different charge states of sulfur vacancies (Vac_S) and substitutional rhenium dopants (Re_Mo) can be stabilized. Vac$${}_{{{{{{{{\rm{S}}}}}}}}}^{-1}$$${}_S^{-1}$as well as Re$${}_{{{{{{{{\rm{Mo}}}}}}}}}^{0}$$${}_{\mathrm{Mo}}^0$and Re$${}_{{{{{{{{\rm{Mo}}}}}}}}}^{-1}$$${}_{\mathrm{Mo}}^{-1}$exhibit local lattice distortions and symmetry-broken defect orbitals attributed to a Jahn-Teller effect (JTE) and pseudo-JTE, respectively. By mapping the electronic and geometric structure of single point defects, we disentangle the effects of spatial averaging, charge multistability, configurational dynamics, and external perturbations that often mask the presence of local symmetry breaking.

    

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3. Enhanced cavity coupling to silicon vacancies in 4H silicon carbide using laser irradiation and thermal annealing

   https://doi.org/10.1073/pnas.2021768118  

   Gadalla, Mena N. ; Greenspon, Andrew S. ; Defo, Rodrick Kuate ; Zhang, Xingyu ; Hu, Evelyn L. ( March 2021 , Proceedings of the National Academy of Sciences)

   The negatively charged silicon monovacancy${\mathrm{V}}_{\text{Si}}^{-}$in 4H silicon carbide (SiC) is a spin-active point defect that has the potential to act as a qubit in solid-state quantum information applications. Photonic crystal cavities (PCCs) can augment the optical emission of the${\mathrm{V}}_{\text{Si}}^{-}$, yet fine-tuning the defect–cavity interaction remains challenging. We report on two postfabrication processes that result in enhancement of the$\mathrm{V}1\prime$optical emission from our PCCs, an indication of improved coupling between the cavity and ensemble of silicon vacancies. Below-bandgap irradiation at 785-nm and 532-nm wavelengths carried out at times ranging from a few minutes to several hours results in stable enhancement of emission, believed to result from changing the relative ratio of${\mathrm{V}}_{\text{Si}}^0$(“dark state”) to${\mathrm{V}}_{\text{Si}}^{-}$(“bright state”). The much faster change effected by 532-nm irradiation may result from cooperative charge-state conversion due to proximal defects. Thermal annealing at 100 °C, carried out over 20 min, also results in emission enhancements and may be explained by the relatively low-activation energy diffusion of carbon interstitials${\mathrm{C}}_{\mathrm{i}}$, subsequently recombining with other defects to create additional${\mathrm{V}}_{\text{Si}}^{-}$s. These PCC-enabled experiments reveal insights into defect modifications and interactions within a controlled, designated volume and indicate pathways to improved defect–cavity interactions.

    

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4. Magnetic field-tuned Fermi liquid in a Kondo insulator

   https://doi.org/10.1038/s41467-019-13421-w  

   Kushwaha, Satya K. ; Chan, Mun K. ; Park, Joonbum ; Thomas, S. M. ; Bauer, Eric D. ; Thompson, J. D. ; Ronning, F. ; Rosa, Priscila F. S. ; Harrison, Neil ( December 2019 , Nature Communications)

   Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$, owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only$${B}_{{\rm{c}}}\approx$$$B_c\approx$11 T. A peak in the strength of electronic correlations near$${B}_{{\rm{c}}}$$$B_c$, which is evident in transport and susceptibility measurements, suggests that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

    

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5. Measurement of the mass dependence of the transverse momentum of lepton pairs in Drell–Yan production in proton–proton collisions at $$\sqrt{s} = 13\,\text {Te\hspace{-.08em}V} $$

   https://doi.org/10.1140/epjc/s10052-023-11631-7  

   Tumasyan, A. ; Adam, W. ; Andrejkovic, J. W. ; Bergauer, T. ; Chatterjee, S. ; Dragicevic, M. ; Valle, A. Escalante ; Frühwirth, R. ; Jeitler, M. ; Krammer, N. ; et al ( July 2023 , The European Physical Journal C)

   The double differential cross sections of the Drell–Yan lepton pair ($$\ell ^+\ell ^-$$${\ell }^{+}{\ell }^{-}$, dielectron or dimuon) production are measured as functions of the invariant mass$$m_{\ell \ell }$$$m_{\ell \ell }$, transverse momentum$$p_{\textrm{T}} (\ell \ell )$$$p_{\text{T}}\left(\ell \ell \right)$, and$$\varphi ^{*}_{\eta }$$${\phi }_{\eta }^{\ast }$. The$$\varphi ^{*}_{\eta }$$${\phi }_{\eta }^{\ast }$observable, derived from angular measurements of the leptons and highly correlated with$$p_{\textrm{T}} (\ell \ell )$$$p_{\text{T}}\left(\ell \ell \right)$, is used to probe the low-$$p_{\textrm{T}} (\ell \ell )$$$p_{\text{T}}\left(\ell \ell \right)$region in a complementary way. Dilepton masses up to 1$$\,\text {Te\hspace{-.08em}V}$$$\text{Te}\text{V}$are investigated. Additionally, a measurement is performed requiring at least one jet in the final state. To benefit from partial cancellation of the systematic uncertainty, the ratios of the differential cross sections for various$$m_{\ell \ell }$$$m_{\ell \ell }$ranges to those in the Z mass peak interval are presented. The collected data correspond to an integrated luminosity of 36.3$$\,\text {fb}^{-1}$$${\text{fb}}^{-1}$of proton–proton collisions recorded with the CMS detector at the LHC at a centre-of-mass energy of 13$$\,\text {Te\hspace{-.08em}V}$$$\text{Te}\text{V}$. Measurements are compared with predictions based on perturbative quantum chromodynamics, including soft-gluon resummation.

    

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