James Edgar

Boron vacancies in hexagonal boron nitride (hBN) are among the most extensively studied optically active spin defects in van der Waals crystals, due to their promising potential to develop two-dimensional (2D) quantum sensors. In this letter, we demonstrate the tunability of the charge state of boron vacancies in ultrathin hBN layers, revealing a transition from the optically active singly negatively charged state to the optically inactive doubly negatively charged state when sandwiched between graphene electrodes. Notably, there is a photoluminescence quenching of a few percent upon the application of a bias voltage between the electrodes. Our findings emphasize the critical importance of considering the charge state of optically active defects in 2D materials, while also showing that the negatively charged boron vacancy remains robust against external perpendicular electric fields. This stability makes it a promising candidate for integration into various van der Waals heterostructures.

Spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of flexible two-dimensional quantum sensing platforms. Here we rely on hBN crystals isotopically enriched with either 10B or 11B to investigate the isotope-dependent properties of a spin defect featuring a broadband photoluminescence signal in the near infrared. By analyzing the hyperfine structure of the spin defect while changing the boron isotope, we first confirm that it corresponds to the negatively charged boron-vacancy center (V B-). We then show that its spin coherence properties are slightly improved in 10B-enriched samples. This is supported by numerical simulations employing cluster correlation expansion methods, which reveal the importance of the hyperfine Fermi contact term for calculating the coherence time of point defects in hBN. Using cross-relaxation spectroscopy, we finally identify dark electron spin impurities as an additional source of decoherence. This work provides new insights into the properties of VB- spin defects, which are valuable for the future development of hBN-based quantum sensing foils.

The nature of point defects in hexagonal boron nitride (hBN) is of current interest for the potential to alter its optical and electrical properties. The strong interaction between neutrons and the boron-10 isotope makes neutron irradiation a controllable way to introduce point defects in hBN. In this study, we perform Raman spectroscopy, photoluminescence, electron paramagnetic resonance (EPR), and optically detected magnetic resonance (ODMR) characterization of neutron-irradiated monoisotopic (hBN with a single boron isotope) 10B and 11B-enriched hBN crystalline flakes and a pyrolytic BN (pBN) reference sample. In h10BN and pBN, neutron irradiation produced two new Raman bands at 450 and 1335 cm−1, which could be associated with B-related vacancies or defects. The near-bandedge optical emission was also significantly impacted by the neutron irradiation. EPR measurements clarified the origin of a high-spin defect center due to negatively charged boron vacancies, which was recently reported for similar neutron-irradiated hBN crystals. The ODMR experiments further confirmed this assignment. High temperature annealing partially recovered some of the hBN vibrational and optical properties. Our results are helpful to identify the nature of defects in hBN and enable defect-engineered applications such as quantum information and sensing.

Pressure is a powerful tool for tuning the magnetic properties of van der Waals magnets owing to their weak interlayer bonding. However, local magnetometry measurements under high pressure still remain elusive for this important class of emerging materials. Here we demonstrate magnetic imaging of a van der Waals magnet under high pressure with sub-micron spatial resolution, using a two-dimensional (2D) quantum sensing platform based on boron-vacancy centers in hexagonal boron nitride (hBN). We first analyze the performances of vacancy centers in hBN for magnetic imaging under pressures up to few GPa, and we then use this 2D sensing platform to investigate the pressure-dependent magnetization in micrometer-sized flakes of 1T-CrTe2. Besides providing a new path for studying pressure-induced phase transitions in van der Waals magnets, this work also opens up interesting perspectives for exploring the physics of 2D superconductors under pressure via local measurements of the Meissner effect.

Boron vacancies (𝑉−𝐵) in hexagonal boron nitride (hBN) have emerged as a promising platform for two-dimensional (2D) quantum sensors capable of operating at atomic-scale proximity. However, the mechanisms responsible for photoluminescence quenching in thin hBN sensing layers when placed in contact with absorptive materials remain largely unexplored. In this Letter, we investigate nonradiative Förster resonance energy transfer (FRET) between 𝑉−𝐵 centers and either monolayer graphene or 2D semiconductors. Strikingly, we find that the FRET rate is negligible for hBN sensing layers thicker than 3 nm, highlighting the potential of 𝑉−𝐵 centers for integration into ultrathin quantum sensors within van der Waals heterostructures. Furthermore, we experimentally extract the intrinsic radiative decay rate of 𝑉−𝐵 defects.

Hybrid systems consisting of color centers and magnetic materials provide an appealing solid-state platform for advancing the burgeoning quantum technological revolution. Exploring novel coupling mechanisms between optically active spin defects and quantum df is directly relevant in this context. Here, we report observation and control of spontaneous magnon emission from boron vacancy centers in two-dimensional hexagonal boron nitride (hBN) to a proximal ferrimagnet in the near zero–temperature limit. The spontaneous magnon emission process is overshadowed by thermal magnon effect as temperature increases, reflecting the crossover from a spontaneous-emission–dominated magnon reservoir to a thermally occupied spin bath where absorption and stimulated processes restore balance. By increasing the spin defect density, we further present that spontaneous magnon emission into a common spin bath could help establish magnon-mediated correlations in dense hBN spin ensembles. Our results are quantitatively captured by theoretical modeling, bringing insights into understanding qubit-magnon coupling, correlated spin dynamics, and many-body physics of color centers in the quantum regime.

The negatively charged boron vacancy (V−B) in hexagonal boron nitride (hBN) is a promising quantum defect that can be used to sense pressure, temperature, and magnetic field with high spatial resolution. hBN enriched with the boron-10 and nitrogen-15 isotopes, denoted h10B15N, has good contrast and coherence for quantum sensing because nitrogen-15 has nuclear spin (1/2), reducing hyperfine interactions. Boron vacancies can be generated by neutron irradiation, which causes the transmutation of the boron-10 isotope to lithium-7. In this study, the ancillary structural, compositional, and mechanical properties of h10B15N crystals that have been subjected to neutron irradiation fluences from 1.4e16 to 8.4e17 n/cm2 were thoroughly characterized. Besides creating V−B , the process also induces other defects that generate strain in the crystal lattice. In turn, the mechanical properties of these crystals change drastically. Investigated here are the visual changes, lattice integrity, composition, crystal strain, and elastic constants (C33 and C66) to assess how these characteristics change as a function of neutron fluence.