Spin coherence properties of VB- centers in ultrathin hBN layers

Jessica Tournaud
Université de Montpellier – Montpellier, France
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Spin defects with optically detectable magnetic resonances in hexagonal boron nitride (hBN) are currently attracting a deep scientific interest for the deployment of quantum sensing technologies on a two-dimensional (2D) material platform [1]. Among several optically-active spin defects recently discovered in hBN, the negatively-charged boron vacancy (V_B ^– ) center stands out due to its wellestablished atomic structure and ease of creation by various irradiation methods. This defect features a spin triplet ground state whose electron spin resonance (ESR) frequencies can be interrogated by optical means and strongly depends on external perturbations such as magnetic fields, strain, and temperature [1,2]. Such properties make the VB – center in hBN a promising candidate for the design of a flexible 2D quantum sensing unit, that could be placed in atomic contact of any type of 2D material within a van der Waals heterostructure. Despite such appealing prospects, the properties of V_B ^– centers embedded in ultrathin hBN layers still remain poorly documented.

In this work, we study the optical and spin properties of V_B ^– centers in atomically-thin hBN flakes obtained by mechanical exfoliation of neutron-irradiated hBN crystals isotopically purified with ¹⁵N and ¹⁰B [3]. We first show that the ESR frequencies of VB – centers remain optically detectable in the 2D limit [4,5]. We then analyze how the spin relaxation times, T₁ and T₂, evolve with the hBN thickness under ambient conditions. While T₁ is significantly reduced in thin flakes, the spin echo coherence time T₂ is preserved and can even be improved in ultrathin films despite the nanoscale proximity of the crystal surface that usually leads to a degradation of the spin coherence properties of solid-state spin defects. This work provides important insights into the properties of V_B ^–centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

References

[1] S. Vaidya et al., Advances in Physics: X 8, 2206049 (2023).
[2] A. Gottscholl et al., Nat. Mater 19, 540-545 (2020).
[3] T. Clua-Provost et al., Phys. Rev. Lett. 131, 126901 (2023).
[4] A. Durand et al., Phys. Rev. Lett. 131, 116902 (2023).
[5] T. Clua-Provost et al., Nano Lett. 24, 12915 (2024)

Email: jessica.tournaud@umontpellier.fr